View with images and charts Study of the Design, Construction & Maintenance o f D i s t r i b u t e d Indoor Type Sub-Station CHAPTER-1 INTRODUCTION 1.1
An Electrical Sub-Station
An electrical substation is a subsidiary station of an electricity generation, transmission and distribution system where voltage is transformed from high to low or the reverse using transformers. Electric power may flow through several substations between generating plant and consumer, and may be changed in voltage in several steps. A substation that has a step-up transformer increases the voltage while decreasing the current, while a step-down transformer decreases the voltage while increasing the current for domestic and commercial distribution. The word substation comes from days before the distribution system became a grid. The first substations were connected to only one power station where the generator was housed, were subsidiaries of that power station.
Fig 1.1: Diagram of an electrical system. An Electrical Power Substation receives electric power from generating station via transmission lines and delivers power via the outgoing transmission lines. Substations are integral parts of a power system and form important links between the generating stations, transmission systems, distribution systems and the load points. Various power substations located in generating stations, transmission and
distribution systems have similar layout and similar electrical components. Electrical power substation basically consists of number of incoming circuit connections and number of outgoing circuit connections connected to the bus bars. Bus bars are conducting bars to which number of circuit connections is connected. Each circuit has certain number of electrical components such as circuit breakers, Isolators, earth switches, current transformers, voltage transformers, etc. In a Power Substation there are various indoor and outdoor switchgear and equipment. Transformers are necessary in a substation for stepping up and stepping down of a.c voltage. Besides the transformers, the several other equipment include bus bars, circuit breakers, isolators, surge arresters, Substation Earthling System, Shunt reactors, Shunt Capacitors etc . Each equipment has certain functional requirement. The equipment are either indoor or outdoor depending upon the voltage rating and local conditions In a large power System large number of Generating stations, Electrical Power Substations and load centers are interconnected. This large internet work is controlled from load dispatch center. Digital and voice signals are transmitted over the transmission lines via the Power substations. The substations are interlinked with the load control centers via Power Line Carrier Systems (PLCC). Modern Power System is controlled with the help of several automatic, semi - automatic equipment. Digital Computers and microprocessors are installed in the control rooms of large substations, generating stations and load control centers for data collection, data monitoring, automatic protection and control. 1.2
Functions of Electrical Power Substations
# Supply electric power to the consumers continuously # Supply of electric power within specified voltage limits and frequency limits # Shortest possible fault duration. # Optimum efficiency of plants and the network # Supply of electrical energy to the consumers at lowest cost 1.3
Types of Electrical Power Substations
a) Step up or primary Electrical Power substation: Primary substations are associated with the power generating plants where the voltage is stepped up from low voltage (3.3, 6.6, 11, 33kV ) to 220kV or 400kV for transmitting the power so that huge amount of power can be transmitted over a large distance to load centers. b) Primary Grid Electrical Power Substation: Such substations are located at suitable load centers along with the primary transmission lines. At primary Grid Power Substations the primary transmission voltage (220kV or 400kV) is stepped down to secondary transmission voltages (110kV) . This Secondary transmission lines are carried over to Secondary Power Substations situated at the load centers where the voltage is further stepped down to Sub transmission Voltage or Primary Distribution Voltages (11kV or 33kV).
c) Step Down or Distribution Electrical Power Substations: Such Power Substations are located at the load centers. Here the Sub transmission Voltages of Distribution Voltages (11kV or 33kV) are stepped down to Secondary Distribution Voltages (400kV or 230kV). From these Substations power will be fed to the consumers to their terminals.
Fig 1.3: Basic Substation For Transmission & Distrubtation. 1.4
Basis of Service Rendered
a) Transformer Substation: Transformers are installed on such Substations to transform the power from one voltage level to other voltage level. b) Switching Substation: Switching substations are meant for switching operation of power lines without transforming the voltages. At these Substations different connections are made
between various transmission lines. Different Switching Schemes are employed depends on the application to transmit the power in more reliable manner in a network. c) Converting Substation: Such Substations are located where AC to DC conversion is required. In HVDC transmission Converting Substations are employed on both sides of HVDC link for converting AC to DC and again converting back from DC to AC. Converting Power Substations are also employed where frequency is to be converted from higher to lower and lower to higher. This type of frequency conversion is required in connecting to Grid Systems. 1.5
Based on Operation Voltage
a) High Voltage Electrical Power Substation: This type of Substation associated with operating voltages between 11kV and 66kV. b) Extra High Voltage Electrical Power Substation: This type of Substation is associated where the operating voltage is between 132kV and 400 KV. c) Ultra High Voltage Electrical Power Substation: Substations where Operating Voltages are above 400kV is called Ultra High Voltage Substation 1.6
Based On Substation Design
a) Outdoor Electrical Power Substations: In Outdoor Power Substations , the various electrical equipments are installed in the switchyard below the sky. Electrical equipment are mounted on support structures to obtain sufficient ground clearance. b) Indoor Electrical Power Substation: In Indoor Power Substations the apparatus is installed within the substation building. Such substations are usually for the rating of 66kV. Indoor Substations are preferred in heavily polluted areas and Power Substations situated near the seas (saline atmosphere causes Insulator Failures results in Flashovers)
Fig 1.6: Indoor Substation 1.7
Based on Design Configuration
a) Air Insulated Electrical Power Substation: In Air Insulated Power Substations bus bars and connectors are visible. In this Power Substations Circuit Breakers and Isolators, Transformers, Current Transformers, Potential Transformers etc are installed in the outdoor. Bus bars are supported on the post Insulators or Strain Insulators. Substations have galvanized Steel Structures for Supporting the equipment, insulators and incoming and outgoing lines. Clearances are the primary criteria for these substations and occupy a large area for installation. b) Gas Insulated Electrical Power Substation: In Gas Insulated Substation Various Power Substation equipments like Circuit Breakers, Current Transformers, Voltage Transformers, Bus bars, Earth Switches, Surge Arresters Isolators etc are in the form of metal enclosed SF6 gas modules. The modules are assembled in accordance with the required Configuration. The various Live parts are enclosed in the metal enclosures (modules) containing SF6 gas at high pressure. Thus the size of Power Substation reduces to 8% to 10% of the Air Insulated Power Substation. c) Hybrid Electrical Power Substation: Hybrid Substations are the combination of both Conventional Substation and Gas Insulated Substation. Some bays in a Power Substation are Gas Insulated Type and some are Air Insulated Type. The design is based on convenience, Local Conditions available, area available and Cost.
Fig 1.7: Gas Insulated Substation Different Components Of Substation Complete Explanation of all the Substation Components Such as Circuit breakers, isolators, Earth Switch, Bus bars, Substation Earthling, CVT, Current Transformer, Voltage Transformer etc with Pictures. 1.8
Gas Insulated Substation
Indoor Gas Insulated Substation : Gas Insulated Substation uses sulfur hex fluoride (SF6) gas which has a superior dielectric properties used at moderate pressure for phase to phase and phase to ground. 1.9
substation Grounding or Earthling
The sole purpose of substation grounding/earthling is to protect the equipment from surges and lightning strikes and to protect the operating persons in the substation. The substation earthling system. 1.11
A distribution substation receives power from the transmission system and distributes it to an area. It is uneconomical to directly connect electricity consumers to the high-voltage main transmission network, unless they use large amounts of energy, so the distribution sub-station reduces voltage to a value suitable for local distribution.
The input for a distribution substation is typically at least two transmission or sub transmission lines. Input voltage may be, for example, 132 kV, or whatever is common in the area. The output consists of a number of feeders. Distribution voltages are typically medium voltage, between 11 and 33 kV depending on the size of the area served and the practices of the local utility. The feeders will then run overhead, along streets (or under streets, in a city) and eventually power the distribution transformers at or near the customer premises. Distribution substations may also be the points of voltage regulation, although on long distribution circuits (several km/miles), voltage regulation equipment may also be installed along the line. Complicated distribution substations can be found in the downtown areas of large cities, with high-voltage switching and backup systems on the low-voltage side. More typical distribution substations have a switch, one transformer, and minimal facilities on the low-voltage side. 1.12
Classification of Sub-Stations
There are several ways of classifying sub-station. The most Important ways of classifying them are according to (1) Service requirement and (2) Constructional features ďƒ˜ According to service requirement- A sub-station may be called upon to change voltage level or improving power factor or convert A.C power etc. According to the service requirement, sub stations may be classified into. i) Transformer sub-stations ii ) Switching sub-stations iii) Power factor correction substations iv) Frequency changer sub-stations v) Converting sub-stations vi) Industrial sub-stations ďƒ˜ According to constructional features- A sub-station has many components (e.g. circuit breakers, switches, fuses, instruments etc) which must be housed properly to ensure continuous and reliable service. According to constructional features, the sub-station are classified asi) Indoor sub-stations ii) Outdoor sub-stations iii) Underground sub-stations
Fig 1.8: Single Line Diagram Layout Plan of 400KVA Sub-Station
1.13 Elements of a Sub-Station
Substations generally contain one or more transformers, and have switching, protection and control equipment. In a large substation, circuit breakers are used to interrupt any short-circuits or overload currents that may occur on the network. Smaller distribution stations may use recover circuit breakers or fuses for protection of branch circuits. Substations do not (usually) have generators, although a power plant may have a substation nearby. A typical substation will contain line termination structures, high-voltage switchgear, one or more power transformers, low voltage switchgear, surge protection, controls, grounding (earthling) system, and metering. Other devices such as power factor correction capacitors and voltage regulators may also be located at a substation. Substations may be on the surface in fenced enclosures, underground, or located in special-purpose buildings. High-rise buildings may have indoor substations. Indoor substations are usually found in urban areas to reduce the noise from the transformers, for reasons of appearance, or to protect switchgear from extreme climate or pollution conditions. Where a substation has a metallic fence, it must be properly grounded to protect people from high voltages that may occur during a fault in the transmission system. Earth faults at a substation can cause ground potential rise at the fault location. Currents flowing in the earth's surface during a fault can cause metal objects to have a significantly different voltage than the ground under a person's feet; this touch potential presents a hazard of electrocution. 1.14
Site Selection for Sub-Stations
Sub-stations are important part of power system. The continuity of supply depends to a considerable extent upon the successfully operation of sub-stations. while selecting the site for sub-stations following factors should be considered. 1) It should be located at a proper site. As per as possible, it should be located at the centre of gravity of load. This will minimize the cost of distribution lines, the maintenance and power losses through them. 2) It should provide safe and reliable arrangement. for safety, consideration must be given to the maintenance of regulation clearances, facilities for carrying out repairs and maintenance, abnormally occurrences such as possibility of explosion or fire. 3) It should involve minimum capital cost. Sub-station conditions should be foundation a reasonable depth should be capable providing a strong support equipment. 4) The site should be selected where easy access road is available so that the operations and maintenance could be easy and less expensive. 5) Climate Conditions (Ambient air temperature). Extremities 5 to 40oC, Normal range 20 to 35oC, Ambient average annual temp 25oC, Average in any one day does not exceed 35oC, Rainfall-average annual
3000mm, Average relative humidity 50-100%, Maximum wind velocity 160 Km/hour. 1.15
General Technical Requirements of a Sub-Station
The general technical requirements of a sub station are as follows i) Economy of expenditure (i.e.) minimum capital cost & operation and maintenance cost. ii) Safety of sub-station and personnel. iii) Reliability. iv) High efficiency. v) Good working conditions. vi) Minimum losses. vii) Standards- All equipment supplied under this specification shall conform to the latest editions to the International Electromechanical Commission (I.E.C) or BS specifications. CHAPTER-2 2.1
Substation Grounding/ Earthling
The sole purpose of substation grounding/earthling is to protect the equipment from surges and lightning strikes and to protect the operating persons in the substation. The substation earthling system is necessary for connecting neutral points of transformers and generators to ground and also for connecting the non current carrying metal parts such as structures, overhead shielding wires, tanks, frames, etc to earth. Earthling of surge arresters is through the earthling system. The function of substation earthling system is to provide a grounding mat below the earth surface in and around the substation which will have uniformly zero potential with respect to ground and lower earth resistance to ensure that
To provide discharge path for lightning over voltages coming via rod-gaps, surge arresters, and shielding wires etc. .
To ensure safety of the operating staff by limiting voltage gradient at ground level in the substation
To provide low resistance path to the earthling switch earthed terminals, so as to discharge the trapped charge (Due to charging currents even the line is dead still charge remains which causes dangerous shocks) to earth prior to maintenance and repairs. Earth Resistance
Earth Resistance is the resistance offered by the earth electrode to the flow of current in to the ground. To provide a sufficiently low resistance path to the earth to minimize the rise in earth potential with respect to a remote earth fault. Persons
touching any of the non current carrying grounded parts shall not receive a dangerous shock during an earth fault. Each structure, transformer tank, body of equipment, etc, should be connected to earthling mat by their own earth connection. Generally lower earth resistance is preferable but for certain applications following earth resistance are satisfactory Large Power Station s– 0.5 Ohm Major Power Stations - 1.0 Ohm Small Substation – 2.0 Ohm In all Other Cases – 8.0 Ohm 2.3
Step Potential and Touch Potential
Grounding system in a electrical system is designed to achieve low earth resistance and also to achieve safe ‘Step Potential ‘and ‘Touch Potential’. a) Step Potential: Step potential is the potential difference between the feet of a person standing on the floor of the substation, with 0.5 m spacing between the feet (one step), through the flow of earth fault current through the grounding system. b) Touch Potential: Touch potential is a potential difference between the fingers of raised hand touching the faulted structure and the feet of the person standing on the substation floor. The person should not get a shock even if the grounded structure is carrying fault current, i.e, The Touch Potential should be very small.
Fig 2.3: Step Potential and Touch Potential Types of Grounding
a) Un earthed Systems: It is used no more. The neutral is not connected to the earth, also called as insulated neutral system.
b) Solid grounding or effective grounding: The neutral is directly connected to the earth without any impedance between neutral and ground. c) Resistance grounding: Resistance is connected between the neutral and the ground. d) Reactance grounding: Reactance is connected between the neutral and ground. e) Resonant Grounding: An adjustable reactor of correctly selected value to compensate the capacitive earth current is connected between the neutral and the earth. The coil is called Arc Suppression Coil or Earth Fault Neutralizer. 2.5
Different Grounding Equipment in Electrical Substation
Earthling Electrodes Earthling Mat Risers Overhead shielding wire (Earthed) H.T Metering Panels
It is used for Instantaneous/Stored Measuring and Recording Currents, Voltages and Energy accumulators (kwh, KVArh lag, KVArh lead and KVAh in forward and reverse directions) in 11KV installations.
Fig 2.6: H.T METER 11 KV SIDE 2.7
Advantages of H.T. metering panel • • • • • • • • • •
Live Parts are not directly accessible, More safety to Electrical Maintenance persons and other personals. Incoming and Outgoing by means of UG Cables. (Suitable Detachable gland plates are provided to fit different sizes of cables) Double side Earthling is provided in the panel for effective Means of earthling Panels are painted with suitable Epoxy and Enamel (standard Shades of IS & IEC standards) to suit indoor and outdoor applications Free standing, Is simple and rapid to install Having life time more than 25 years Maintenance-free live parts In conforming with IS & IEC Pad locking facility is provided with Tamper proof as per EB Standards Benefits from the experience from 1000 functional units Installed nationwide Bus-bar Arrangement in Sub-Stations
When a number of generators or feeders operating at the same voltage have to be directly connected electrically, bus-bars are used as the common electrical component. Bus-bars are copper rods or thin walled tubes and operate at constant voltage. There are different types of bus bars i) Single bus-bar system ii) Single bus-bar system with Sectionalizing iii) Duplicate bus-bar system Mimic bus materials shall be brass, bronze or copper with backed enamel finished or anodized aluminum or plastic. The mimic bus shall be attached to the panel by mechanical devices not with ad
Fig 2.8: Single Bus-bar System 2.9
H.T Switchgear with (VCB)
In such quenching, Vacuum is used as the arc quenching medium. Since vacuum offers the highest insulating strength, it has far superior arc quenching properties
than any other medium. When the contacts of the breaker are opened in vacuum (10 7 to 10-5 torr), an arc is produced between the contacts by the ionization of metal vapors. However, the arc rapidly condenses on the surface of the circuit breaker contacts, resulting in quick recovery of dielectric strength. The reader may note the salient feature of vacuum as an arc quenching medium. As soon as the arc is produced in vacuum, it quickly extinguished due to the fast rate of recovery of dielectric strength in vacuum. Technical Specification of (VCB) Sheet steel clad power coated (14 SWG), dust and vermin proof, free standing, Floor mounting indoor HT Switchgear 11kv, 50 Hz, three phase , 630 A hard drawn electrolytic copper bus-bars equipped with 1 No. 630 A, 11kv, breaking current 20 KA(3sec),making current 50KA, 50Hz, TP Vacuum circuit breaker (Fixed type) with motor operated mechanism with closing solenoid shunt releases, auxiliary contacts 5NO + 5NC and limit switch (1 NO + 1 NC) for indication â€œClosing spring chargedâ€? mechanical on/off/trip indicator.
Fig 2.9.1: V H.T Switchgear with (VCB)
Fig 2.9.2; Vacuum Circuit Breaker of 11 kv line (VCB) The term switchgear, used in association with the electric power system, or grid, refers to the combination of electrical disconnects, fuses and/or circuit breakers used to isolate electrical equipment. Switchgear is used both to de-energize equipment to allow work to be done and to clear faults downstream. This type of equipment is important because it is directly linked to the reliability of the electricity supply. The very earliest central power stations used simple open knife switches mounted on insulating panels of marble or asbestos. Power levels and voltages rapidly escalated, making open manually-operated switches too dangerous to use for anything other than isolation of a de-energized circuit. Oil-filled equipment allowed arc energy to be contained and safely controlled. By the early 20th century, a switchgear line-up would be a metal-enclosed structure with electrically-operated switching elements, using oil circuit breakers. Today, oil-filled equipment has largely been replaced by air-blast, vacuum, or SF6 equipment, allowing large currents and power levels to be safely controlled by automatic equipment incorporating digital controls, protection, metering and communications. High voltage switchgear was invented at the end of the 19th century for operating motors and others electric machines. The technology has been improved over time and can be used with voltages up to 1,100 kV.
Typically switchgear in substations is located on both the high voltage and the low voltage side of large power transformers The switchgear located on the low voltage side of the transformers in distribution type substations, now are typically located in what is called a Power Distribution Center (PDC). Inside this building are typically
smaller, medium-voltage (~15kV) circuit breakers feeding the distribution system. Also contained inside these Power Control Centers are various relays, meters, and other communication equipment allowing for intelligent control of the substation. For industrial applications, a transformer and switchgear (Load Breaking Switch Fuse Unit) line-up may be combined in one housing, called a unitzed substation or USS.
Fig 2.10.1: High voltage switchgear
Fig 2.10.2 : A section of a large switchgear panel, in this case, used to control on-board casino boat power generation.
Fig 2.10.3: Tram switchgear
Fig 2.10.4: This circuit breaker uses both SF6 and air as insulating gases; such devices are sometimes called "hybrid switchgear 2.11
Several different classifications of switchgear can be made
By the current rating. By interrupting rating (maximum short circuit current that the device can safely interrupt) o Circuit breakers can open and close on fault currents o Load-break/Load-make switches can switch normal system load currents o Isolators may only be operated while the circuit is dead, or the load current is very small. By voltage class: o Low voltage (less than 1,000 volts AC) o Medium voltage (1,000–35,000 volts AC) o High voltage (more than 35,000 volts AC)
o o o o o o o o o o o o o o o o o o o
By insulating medium: Air Gas (SF6 or mixtures) Oil Vacuum By construction type: Indoor (further classified by IP (Ingress Protection) class or NEMA enclosure type) Outdoor Industrial Utility Marine Draw-out elements (removable without many tools) Fixed elements (bolted fasteners) Live-front Dead-front Open Metal-enclosed Metal-clad Metal enclose & Metal clad Arc-resistant By IEC degree of internal separation No Separation (Form 1) Busbars separated from functional units (Form 2a, 2b, 3a, 3b, 4a, 4b) Terminals for external conductors separated from busbars (Form 2b, 3b, 4a, 4b) Terminals for external conductors separated from functional units but not from each other (Form 3a, 3b) Functional units separated from each other (Form 3a, 3b, 4a, 4b)
o o o o o o
o o o
o o o o o o
Terminals for external conductors separated from each other (Form 4a, 4b) Terminals for external conductors separate from their associated functional unit (Form 4b) By interrupting device: Fuses Air Blast Circuit Breaker Minimum Oil Circuit Breaker Oil Circuit Breaker Vacuum Circuit Breaker Gas (SF6) Circuit breaker By operating method: Manually-operated Motor-operated Solenoid/stored energy operated By type of current: Alternating current Direct current By application: Transmission system Distribution By purpose Isolating switches (disconnectors ) Load-break switches Grounding (earthing) switches
A single line-up may incorporate several different types of devices, for example, airinsulated bus, vacuum circuit breakers, and manually-operated switches may all exist in the same row of cubicles. Ratings, design, specifications and details of switchgear are set by a multitude of standards. In North America mostly IEEE and ANSI standards are used, much of the rest of the world uses IEC standards, sometimes with local national derivatives or variations.
One of the basic functions of switchgear is protection, which is interruption of shortcircuit and overload fault currents while maintaining service to unaffected circuits. Switchgear also provides isolation of circuits from power supplies. Switchgear is also used to enhance system availability by allowing more than one source to feed a load. 2.13
To help ensure safe operation sequences of switchgear, trapped key interlocking provides predefined scenarios of operation. For example, if only one of two sources
of supply are permitted to be connected at a given time, the interlock scheme may require that the first switch must be opened to release a key that will allow closing the second switch. Complex schemes are possible. Indoor switchgear can also be type tested for internal arc containment. This test is important for user safety as modern switchgear is capable of switching large currents. Switchgear is often inspected using thermal imaging to assess the state of the system and predict failures before they occur. 2.14
Current Transformer & Potential Transformer
a) Current Transformer A current transformer is the measuring device for metering and protection system. The primary current of the CT is transformed in the required secondary current of 5A or 1A. Measuring CT's have different class of accuracy i.e 0.1, 0.2S, 0.2, 0.5S, 0.5 & 1.0 and protection CT's have 5P10, 5P20 and 10P10. In the case of special protection CT's (PS class), the knee point voltage (VK) of the CT is specified along with secondary resistance and magnetizing current The construction of the CT not only depends upon the ratio but also on the short time current rating, burden and class of accuracy.
Fig 2.14: C.T & P.T Current transformers shall be cast resin insulated, 11kv dry type double core CT with Ratio 50/5, 1st core for metering, 2nd core for protection. Primary current rating = 50 amps Secondary current rating = 5 amps Standard frequency = 50 Hz Continuous thermal current rated output = 10 VA Secondary burden = 30 VA for protection Quantity of CT = 3 Nos. 15 VA for measuring. b) Potential Transformer Potential transformer shall be cast resin insulated, double pole, Potential Transformer, Ratio 11/ .11kv, class 0.5. 50 VA, (in open delta connection) Type = Magnetic type Rated secondary voltage = 110 v Rated outputs (per phase) = 25VA Power frequency voltage = 28 Kv (rms) 2.15
The manufacture shall provide internal panel wiring and connections, in accordance with the requirements. All wiring used with in the panel shall conform to the requirements of those specifications and shall be installed and tested at the factory. All wiring shall be neatly and carefully installed. Instruments, meters, control switches & protective relays shall be mounted on the front panel only. Panel output, mounting studs and support brackets shall be accurately located. 2.16
Protective relays, as specified shall be semi flush-mounted draw-out type designed for use with 5A, 50Hz, current circuits, shall be indicated type equipped with operation indicator. The protective relays should be sufficient for over current and earth protection. Protective relays shall comply ICE-255 standards. CHAPTER-3 3.1
A transformer is a device that transfers electrical energy from one circuit to another through magnetically coupled electrical conductors. A changing current in the first circuit (the primary) creates a changing magnetic field; in turn, this magnetic field induces a changing voltage in the second circuit (the secondary). By adding a load to the secondary circuit, one can make current flow in the transformer, thus transferring energy from one circuit to the other.
Fig 3.1: Transformer
Fig 3.1.2: Transformer showing the primary and secondary windings
11/ 0.415 KV Distribution Transformer
The secondary induced voltage VS, of an ideal transformer, is scaled from the primary VP by a factor equal to the ratio of the number of turns of wire in their respective windings: By appropriate selection of the numbers of turns, a transformer thus allows an alternating voltage to be stepped up - by making NS more than NP - or stepped down, by making it less. Transformers are some of the most efficient electrical 'machines', with some large units able to transfer 99.75% of their input power to their output. Transformers come in a range of sizes from a thumbnail-sized coupling transformer hidden inside a stage microphone to huge units weighing hundreds of tons used to interconnect portions of national power grids. All operate with the same basic principles, although the range of designs is wide. 3.3 Working Principle of Transformer The transformer is based on two principles: first, that an electric current can produce a magnetic field (electromagnetism), and, second that a changing magnetic field within a coil of wire induces a voltage across the ends of the coil (electromagnetic induction) . Changing the current in the primary coil changes the magnetic flux that is developed. The changing magnetic flux induces a voltage in the secondary coil.
Fig 3.3: An ideal transformer An ideal transformer is shown in the adjacent figure. Current passing through the primary coil creates a magnetic field. The primary and secondary coils are wrapped
around a core of very high magnetic permeability such as iron so that most of the magnetic flux passes through both the primary and secondary coils.
The voltage induced across the secondary coil may be calculated from Faraday's law of induction which states that:
where Vs is the instantaneous voltage Ns is the number of turns in the secondary coil and ÎŚ is the magnetic flux through one turn of the coil. If the turns of the coil are oriented perpendicular to the magnetic field lines, the flux is the product of the magneti flux density B and the area A through which it cuts. The area is constant, being equal to the cross-sectional area of the transformer core, whereas the magnetic field varies with time according to the excitation of the primary. Since the same magnetic flux passes through both the primary and secondary coils in an ideal transformer,the instantaneous voltage across the primary winding equals
Taking the ratio of the two equations for Vs and Vp gives the basic equation for stepping up or stepping down the voltage
Ideal power equation
Fig 3.5: The ideal transformer as a circuit element If the secondary coil is attached to a load that allows current to flow, electrical power is transmitted from the primary circuit to the secondary circuit. Ideally, the transformer is perfectly efficient; all the incoming energy is transformed from the primary circuit to the magnetic field and into the secondary circuit. If this condition is met, the incoming electric power must equal the outgoing power:
giving the ideal transformer equation
Transformers normally have high efficiency, so this formula is a reasonable approximation. If the voltage is increased, then the current is decreased by the same factor. The impedance in one circuit is transformed by the square of the turns ratio For example, if an impedance Zs is attached across the terminals of the secondary coil, it appears to the primary circuit to have an impedance of (Np/Ns)2Zs. This relationship is reciprocal, so that the impedance Zp of the primary circuit appears to the secondary to be (Ns/Np)2Zp.
The simplified description above neglects several practical factors, in particular the primary current required to establish a magnetic field in the core, and the contribution to the field due to current in the secondary circuit.
Models of an ideal transformer typically assume a core of negligible reluctance with two windings of zero resistance When a voltage is applied to the primary winding, a small current flows, driving flux around the magnetic circuit of the core. The current required to create the flux is termed the magnetizing current; since the ideal core has been assumed to have near-zero reluctance, the magnetizing current is negligible, although still required to create the magnetic field. The changing magnetic field induces an electromotive force (EMF) across each winding. Since the ideal windings have no impedance, they have no associated voltage drop, and so the voltages VP and VS measured at the terminals of the transformer, are equal to the corresponding EMFs. The primary EMF, acting as it does in opposition to the primary voltage, is sometimes termed the "back EMF This is due to Lenz's law which states that the induction of EMF would always be such that it will oppose development of any such change in magnetic field. 3.7
a) Leakage flux
Fig 3.7: Leakage flux of a transformer: The ideal transformer model assumes that all flux generated by the primary winding links all the turns of every winding, including itself. In practice, some flux traverses paths that take it outside the windings. Such flux is termed leakage flux, and results in leakage inductance in series with the mutually coupled transformer windings. Leakage results in energy being alternately stored in and discharged from the magnetic fields with each cycle of the power supply. It is not directly a power loss (see "Stray losses" below), but results in inferior voltage regulation causing the secondary voltage to fail to be directly proportional to the primary, particularly under heavy load. Transformers are therefore normally designed to have very low leakage inductance. However, in some applications, leakage can be a desirable property, and long magnetic paths, air gaps, or magnetic bypass shunts may be deliberately introduced
to a transformer's design to limit the short-circuit current it will supply.Leaky transformers may be used to supply loads that exhibit negative resistance such as electric arcs mercury vapor lamps and neon signs or for safely handling loads that become periodically short-circuited such as electric arc welders. Air gaps are also used to keep a transformer from saturating, especially audiofrequency transformers in circuits that have a direct current flowing through the windings. Leakage inductance is also helpful when transformers are operated in parallel. It can be shown that if the "per-unit" inductance of two transformers is the same (a typical value is 5%), they will automatically split power "correctly" (e.g. 500 kVA unit in parallel with 1,000 kVA unit, the larger one will carry twice the current).
Effect of frequency
Transformer universal EMF equation If the flux in the core is purely sinusoidal the relationship for either winding between its rms voltage Erms of the winding , and the supply frequency f, number of turns N, core cross-sectional area a and peak magnetic flux density B is given by the universal EMF equation:
If the flux does not contain even harmonics the following equation can be used for half-cycle average voltage Eavg of any waveshape: The time-derivative term in Faraday's Law shows that the flux in the core is the integral with respect to time of the applied voltage. Hypothetically an ideal transformer would work with direct-current excitation, with the core flux increasing linearly with time. In practice, the flux would rise to the point where magnetic saturation of the core occurs, causing a huge increase in the magnetizing current and overheating the transformer. All practical transformers must therefore operate with alternating (or pulsed) current. The EMF of a transformer at a given flux density increases with frequency. By operating at higher frequencies, transformers can be physically more compact because a given core is able to transfer more power without reaching saturation and fewer turns are needed to achieve the same impedance. However, properties such as core loss and conductor skin effect also increase with frequency. Aircraft and military equipment employ 400 Hz power supplies which reduce core and winding weight. Conversely, frequencies used for some railway electrification systems were much lower (e.g. 16.7 Hz and 25 Hz) than normal utility frequencies (50 â€“ 60 Hz) for historical reasons concerned mainly with the limitations of early electric traction motors. As such, the transformers used to step down the high over-head line voltages (e.g. 15 kV) are much heavier for the same power rating than those designed only for the higher frequencies. Operation of a transformer at its designed voltage but at a higher frequency than intended will lead to reduced magnetizing current; at lower frequency, the magnetizing current will increase. Operation of a transformer at other than its design
frequency may require assessment of voltages, losses, and cooling to establish if safe operation is practical. For example, transformers may need to be equipped with "volts per hertz" over-excitation relays to protect the transformer from over voltage at higher than rated frequency. One example of state-of-the-art design is those transformers used for electric multiple unit high speed trains particularly those required to operate across the borders of countries using different standards of electrification. The position of such transformers is restricted to being hung below the passenger compartment. They have to function at different frequencies (down to 16.7 Hz) and voltages (up to 25 kV) whilst handling the enhanced power requirements needed for operating the trains at high speed. Knowledge of natural frequencies of transformer windings is of importance for the determination of the transient response of the windings to impulse and switching surge voltages.
An ideal transformer would have no energy losses, and would be 100% efficient. In practical transformers energy is dissipated in the windings, core, and surrounding structures. Larger transformers are generally more efficient, and those rated for electricity distribution usually perform better than 98%. Experimental transformers using superconducting windings achieve efficiencies of 99.85%. The increase in efficiency from about 98 to 99.85% can save considerable energy, and hence money, in a large heavily-loaded transformer; the trade-off is in the additional initial and running cost of the superconducting design. Losses in transformers (excluding associated circuitry) vary with load current, and may be expressed as "no-load" or "full-load" loss. Winding resistance dominates load losses, whereas hysteresis and eddy currents losses contribute to over 99% of the no-load loss. The no-load loss can be significant, so that even an idle transformer constitutes a drain on the electrical supply and a running cost; designing transformers for lower loss requires a larger core, good-quality silicon steel or even amorphous steel for the core, and thicker wire, increasing initial cost, so that there is a trade-off between initial cost and running cost. (Also see energy efficient transformer. Transformer losses are divided into losses in the windings, termed copper loss and those in the magnetic circuit, termed iron loss. Losses in the transformer arise from: a) Winding resistance Current flowing through the windings causes resistive heating of the conductors. At higher frequencies, skin effect and proximity effect create additional winding resistance and losses. b) Hysteresis losses Each time the magnetic field is reversed, a small amount of energy is lost due to hysteresis within the core. For a given core material, the loss is proportional to the frequency, and is a function of the peak flux density to which it is subjected. c) Eddy currents
Ferromagnete materials are also good conductors and a core made from such a material also constitutes a single short-circuited turn throughout its entire length. Eddy currents therefore circulate within the core in a plane normal to the flux, and are responsible for resistive heating of the core material. The eddy current loss is a complex function of the square of supply frequency and inverse square of the material thickness. Eddy current losses can be reduced by making the core of a stack of plates electrically insulated from each other, rather than a solid block; all transformers operating at low frequencies use laminated or similar cores. Magnetostriction Magnetic flux in a ferromagnetic material, such as the core, causes it to physically expand and contract slightly with each cycle of the magnetic field, an effect known as magnetostriction. This produces the buzzing sound commonly associated with transformers, and can cause losses due to frictional heating. Mechanical losses In addition to magnetostriction, the alternating magnetic field causes fluctuating forces between the primary and secondary windings. These incite vibrations within nearby metalwork, adding to the buzzing noise and consuming a small amount of power. Stray losses Leakage inductance is by itself largely lossless, since energy supplied to its magnetic fields is returned to the supply with the next half-cycle. However, any leakage flux that intercepts nearby conductive materials such as the transformer's support structure will give rise to eddy currents and be converted to heat. There are also radiative losses due to the oscillating magnetic field, but these are usually small. Dot convention It is common in transformer schematic symbols for there to be a dot at the end of each coil within a transformer, particularly for transformers with multiple primary and secondary windings. The dots indicate the direction of each winding relative to the others. Voltages at the dot end of each winding are in phase; current flowing into the dot end of a primary coil will result in current flowing out of the dot end of a secondary coil.
3.10 Equivalent circuit The physical limitations of the practical transformer may be brought together as an equivalent circuit model (shown below) built around an ideal lossless transformer. Power loss in the windings is current-dependent and is represented as in-series resistances Rp and Rs. Flux leakage results in a fraction of the applied voltage dropped without contributing to the mutual coupling, and thus can be modeled as reactances of each leakage inductance Xp and Xs in series with the perfectly coupled region. Iron losses are caused mostly by hysteresis and eddy current effects in the core, and are proportional to the square of the core flux for operation at a given frequency. Since the core flux is proportional to the applied voltage, the iron loss can be represented by a resistance RC in parallel with the ideal transformer.
A core with finite permeabilityrequires a magnetizing current Im to maintain the mutual flux in the core. The magnetizing current is in phase with the flux; saturation effects cause the relationship between the two to be non-linear, but for simplicity this effect tends to be ignored in most circuit equivalents. With a sinusoidal supply, the core flux lags the induced EMF by 90째 and this effect can be modeled as a magnetizing reactance (reactance of an effective inductance) Xm in parallel with the core loss component. Rc and Xm are sometimes together termed the magnetizing branch of the model. If the secondary winding is made open-circuit, the current I0 taken by the magnetizing branch represents the transformer's no-load current. The secondary impedance Rs and Xs is frequently moved (or "referred") to the primary side after multiplying the components by the impedance scaling factor (Np/Ns)2.
Fig 3.10: Equivalent Circuit Transformer equivalent circuit, with secondary impedances referred to the primary side The resulting model is sometimes termed the "exact equivalent circuit", though it retains a number of approximations, such as an assumption of linearity. Analysis may be simplified by moving the magnetizing branch to the left of the primary impedance, an implicit assumption that the magnetizing current is low, and then summing primary and referred secondary impedances, resulting in so-called equivalent impedance. The parameters of equivalent circuit of a transformer can be calculated from the results of two transformer tests: open-circuit test and shortcircuit test.
3.11 Types of Transformer A wide variety of transformer designs are used for different applications, though they share several common features. Important common transformer types include: a) Autotransformer
Fig 3.11.1: An autotransformer with a sliding brush contact An autotransformer has a single winding with two end terminals, and one or more terminals at intermediate tap points. The primary voltage is applied across two of the terminals, and the secondary voltage taken from two terminals, almost always having one terminal in common with the primary voltage. The primary and secondary circuits therefore have a number of windings turns in common. Since the volts-per-turn is the same in both windings, each develops a voltage in proportion to its number of turns. In an autotransformer part of the current flows directly from the input to the output, and only part is transferred inductively, allowing a smaller, lighter, cheaper core to be used as well as requiring only a single winding . However, a transformer with separate windings isolates the primary from the secondary, which is safer when using mains voltages. An adjustable autotransformer is made by exposing part of the winding coils and making the secondary connection through a sliding brush giving a variable turns ratio.Such a device is often referred to as a variac Autotransformers are often used to step up or down between voltages in the 110-117-120 volt range and voltages in the 220-230-240 volt range, e.g., to output either 110 or 120V (with taps) from 230V input, allowing equipment from a 100 or 120V region to be used in a 230V region. b)
Fig 3.11.2: Three-phase step-down transformer mounted between two utility poles For three-phase supplies, a bank of three individual single-phase transformers can be used, or all three phases can be incorporated as a single three-phase transformer. In this case, the magnetic circuits are connected together, the core thus containing a three-phase flow of flux. A number of winding configurations are possible, giving rise to different attributes and phase shifts One particular poly phase configuration is the zigzag transformer used for grounding and in the suppression of harmonic currents. c)
Fig 3.11.3: Leakage transformer A leakage transformer, also called a stray-field transformer, has a significantly higher leakage inductance than other transformers, sometimes increased by a magnetic bypass or shunt in its core between primary and secondary, which is sometimes adjustable with a set screw. This provides a transformer with an inherent current limitation due to the loose coupling between its primary and the secondary windings. The output and input currents are low enough to prevent thermal overload under all load conditionsâ€”even if the secondary is shorted. Leakage transformers are used for arc welding and high voltage discharge lamps (neon lights and cold cathode fluorescent lamps which are series-connected up to 7.5 kV AC). It acts then both as a voltage transformer and as a magnetic ballast. Other applications are shortcircuit-proof extra-low voltage transformers for toys or doorbell installations. d) Resonant transformers A resonant transformer is a kind of leakage transformer. It uses the leakage inductance of its secondary windings in combination with external capacitors, to create one or more resonant circuits Resonant transformers such as the Tesla coil can generate very high voltages, and are able to provide much higher current than electrostatic high-voltage generation machines such as the Van de Graaff generator One of the applications of the resonant transformer is for the CCFL inverter. Another application of the resonant transformer is to couple between stages of a superheterodyne receiver where the selectivity is provided by tuned transformers in the intermediate-frequency amplifiers. e) Audio transformers Audio transformers are those specifically designed for use in audio circuits. They can be used to block radio frequency interference or the DC component of an audio signal, to split or combine audio signals, or to provide impedance matching between high and low impedance circuits, such as between a high impedance tube (valve) amplifier output and a low impedance loudspeaker or between a high impedance instrument output and the low impedance input of a mixing console.
Such transformers were originally designed to connect different telephone systems to one another while keeping their respective power supplies isolated, and are still commonly used to interconnect professional audio systems or system components. Being magnetic devices, audio transformers are susceptible to external magnetic fields such as those generated by AC current-carrying conductors. "Hum" is a term commonly used to describe unwanted signals originating the "mains" power supply (typically 50 or 60 Hz). Audio transformers used for low-level signals, such as those from microphones, often include shielding to protect against extraneous magnetically coupled signals. f) Instrument transformers Instrument transformers are used for measuring voltage and current in electrical power systems, and for power system protection and control. Where a voltage or current is too large to be conveniently used by an instrument, it can be scaled down to a standardized, low value. Instrument transformers isolate measurement, protection and control circuitry from the high currents or voltages present on the circuits being measured or controlled.
Fig 3.11.4: Current transformers, designed for placing around conductors A current transformer is a transformer designed to provide a current in its secondary coil proportional to the current flowing in its primary coil. Voltage transformers (VTs), also referred to as "potential transformers" (PTs), are designed to have an accurately known transformation ratio in both magnitude and phase, over a range of measuring circuit impedances. A voltage transformer is intended to present a negligible load to the supply being measured. The low secondary voltage allows protective relay equipment and measuring instruments to be operated at a lower voltages. Both current and voltage instrument transformers are designed to have predictable characteristics on overloads. Proper operation of over-current protective relays requires that current transformers provide a predictable transformation ratio even during a short-circuit.
3.12 Classification Transformers can be classified in many different ways; an incomplete list is: By power capacity: from a fraction of a volt-ampere (VA) to over a thousand MVA; By frequency range: power- audio or radio frequency; By voltage class: from a few volts to hundreds of kilovolts; By cooling type: air-cooled, oil-filled, fan-cooled, or water-cooled; By application: such as power supply, impedance matching, output voltage and current stabilizer, or circuit isolation; By purpose: distribution, rectifier, arc furnace amplifier output, etc.; By winding turns ratio: step-up, step-down, isolating with equal or nearequal ratio, variable, multiple windings.
3.13 Construction Cores:
Fig 3.13.1: Laminated core transformer showing edge of laminations at top of photo a) Laminated steel cores Transformers for use at power or audio frequencies typically have cores made of high permeability silicon steel. The steel has a permeability many times that of free space and the core thus serves to greatly reduce the magnetizing current, and confine the flux to a path which closely couples the windings. Early transformer developers soon realized that cores constructed from solid iron resulted in prohibitive eddycurrent losses, and their designs mitigated this effect with cores consisting of bundles of insulated iron wires. Later designs constructed the core by stacking layers of thin steel laminations, a principle that has remained in use. Each lamination is insulated from its neighbors by a thin non-conducting layer of insulation. The
universal transformer equation indicates a minimum cross-sectional area for the core to avoid saturation. The effect of laminations is to confine eddy currents to highly elliptical paths that enclose little flux, and so reduce their magnitude. Thinner laminations reduce losses, but are more laborious and expensive to construct. Thin laminations are generally used on high frequency transformers, with some types of very thin steel laminations able to operate up to 10 kHz.
Fig 3.13.2: Laminating the core greatly reduces eddy-current losses One common design of laminated core is made from interleaved stacks of E-shaped steel sheets capped with I-shaped pieces, leading to its name of "E-I transformer". Such a design tends to exhibit more losses, but is very economical to manufacture. The cut-core or C-core type is made by winding a steel strip around a rectangular form and then bonding the layers together. It is then cut in two, forming two C shapes, and the core assembled by binding the two C halves together with a steel strap. They have the advantage that the flux is always oriented parallel to the metal grains, reducing reluctance. A steel core's eminence means that it retains a static magnetic field when power is removed. When power is then reapplied, the residual field will cause a high inrush current until the effect of the remaining magnetism is reduced, usually after a few cycles of the applied alternating current. Over current protection devices such as fuses must be selected to allow this harmless inrush to pass. On transformers connected to long, overhead power transmission lines, induced currents due to geomagnetic disturbances during solar storms can cause saturation of the core and operation of transformer protection devices. Distribution transformers can achieve low no-load losses by using cores made with low-loss high-permeability silicon steel or amorphous (non-crystalline) metal alloy.
The higher initial cost of the core material is offset over the life of the transformer by its lower losses at light load. b) Solid cores Powdered iron cores are used in circuits (such as switch-mode power supplies) that operate above main frequencies and up to a few tens of kilohertz. These materials combine high magnetic permeability with high bulk electrical resistivity. For frequencies extending beyond the VHF band, cores made from non-conductive magnetic ceramic materials called ferrites are common. ome radio-frequency transformers also have movable cores (sometimes called 'slugs') which allow adjustment of the coupling coefficient (and bandwidth) of tuned radio-frequency circuits. Toroidal cores
Fig; Small toroidal core transformer Toroidal transformers are built around a ring-shaped core, which, depending on operating frequency, is made from a long strip of silicon steel or permalloy wound into a coil, powdered iron, or ferrite. A strip construction ensures that the grain boundaries are optimally aligned, improving the transformer's efficiency by reducing the core's reluctance. The closed ring shape eliminates air gaps inherent in the construction of an E-I core. The cross-section of the ring is usually square or rectangular, but more expensive cores with circular cross-sections are also available. The primary and secondary coils are often wound concentrically to cover the entire surface of the core. This minimizes the length of wire needed, and also provides screening to minimize the core's magnetic field from generating electromagnetic interference. Toroidal transformers are more efficient than the cheaper laminated E-I types for a similar power level. Other advantages compared to E-I types, include smaller size (about half), lower weight (about half), less mechanical hum (making them superior in audio amplifiers), lower exterior magnetic field (about one tenth), low off-load losses (making them more efficient in standby circuits), single-bolt mounting, and greater choice of shapes. The main disadvantages are higher cost and limited power
capacity (see "Classification" above). Because of the lack of a residual gap in the magnetic path, toroidal transformers also tend to exhibit higher inrush current, compared to laminated E-I types. Ferrite toroidal cores are used at higher frequencies, typically between a few tens of kilohertz to hundreds of megahertz, to reduce losses, physical size, and weight of switch-mode power supplies. A drawback of toroidal transformer construction is the higher labor cost of winding. This is because it is necessary to pass the entire length of a coil winding through the core aperture each time a single turn is added to the coil. As a consequence, toroidal transformers are uncommon above ratings of a few kVA. Small distribution transformers may achieve some of the benefits of a toroidal core by splitting it and forcing it open, then inserting a bobbin containing primary and secondary windings. c) Air cores A physical core is not an absolute requisite and a functioning transformer can be produced simply by placing the windings near each other, an arrangement termed an "air-core" transformer. The air which comprises the magnetic circuit is essentially lossless, and so an air-core transformer eliminates loss due to hysterics in the core material. The leakage inductance is inevitably high, resulting in very poor regulation, and so such designs are unsuitable for use in power distribution. They have however very high bandwidth, and are frequently employed in radio-frequency applications, for which a satisfactory coupling coefficient is maintained by carefully overlapping the primary and secondary windings. They're also used for resonant transformers such as Tesla coils where they can achieve reasonably low loss in spite of the high leakage inductance. 3.14
Fig 3.14 : Windings are usually arranged concentrically to minimize flux leakage.
Cut view through transformer windings. White: insulator. Green spiral: Grain oriented silicon steel. Black: Primary winding made of oxygen-free copper. Red: Secondary winding. Top left: Toroidal transformer. Right: C-core, but E-core would be similar. The black windings are made of film. Top: Equally low capacitance between all ends of both windings. Since most cores are at least moderately conductive they also need insulation. Bottom: Lowest capacitance for one end of the secondary winding needed for low-power high-voltage transformers. Bottom left: Reduction of leakage inductance would lead to increase of capacitance. The conducting material used for the windings depends upon the application, but in all cases the individual turns must be electrically insulated from each other to ensure that the current travels throughout every turn. For small power and signal transformers, in which currents are low and the potential difference between adjacent turns is small, the coils are often wound from enamelled magnet wire, such as Formvar wire. Larger power transformers operating at high voltages may be wound with copper rectangular strip conductors insulated by oil-impregnated paper and blocks of pressboard. High-frequency transformers operating in the tens to hundreds of kilohertz often have windings made of braided Litz wire to minimize the skin-effect and proximity effect losses. Large power transformers use multiple-stranded conductors as well, since even at low power frequencies non-uniform distribution of current would otherwise exist in high-current windings. Each strand is individually insulated, and the strands are arranged so that at certain points in the winding, or throughout the whole winding, each portion occupies different relative positions in the complete conductor. The transposition equalizes the current flowing in each strand of the conductor, and reduces eddy current losses in the winding itself. The stranded conductor is also more flexible than a solid conductor of similar size, aiding manufacture. For signal transformers, the windings may be arranged in a way to minimize leakage inductance and stray capacitance to improve high-frequency response. This can be done by splitting up each coil into sections, and those sections placed in layers
between the sections of the other winding. This is known as a stacked type or interleaved winding. Both the primary and secondary windings on power transformers may have external connections, called taps to intermediate points on the winding to allow selection of the voltage ratio. In power distribution transformers the taps may be connected to an automatic on-load tap changer for voltage regulation of distribution circuits. Audiofrequency transformers, used for the distribution of audio to public address loudspeakers, have taps to allow adjustment of impedance to each speaker. A centertapped transformer is often used in the output stage of an audio power amplifier in a push-pull circuit. Modulation transformers in AM transmitters are very similar. Certain transformers have the windings protected by epoxy resin. By impregnating the transformer with epoxy under a vacuum, one can replace air spaces within the windings with epoxy, thus sealing the windings and helping to prevent the possible formation of corona and absorption of dirt or water. This produces transformers more suited to damp or dirty environments, but at increased manufacturing cost. 3.15
Fig 3.15 : Cut-away view of three-phase oil-cooled transformer. The oil reservoir is visible at the top. Radiative fins aid the dissipation of heat. High temperatures will damage the winding insulation. [Small transformers do not generate significant heat and are cooled by air circulation and radiationof heat. Power transformers rated up to several hundred kVA can be adequately cooled by natural convective air-cooling, sometimes assisted by fans. In larger transformers,
part of the design problem is removal of heat. Some power transformers are immersed in transformer oil that both cools and insulates the windings. The oil is a highly refined mineral oil that remains stable at transformer operating temperature. Indoor liquid-filled transformers are required by building regulations in many jurisdictions to use a non-flammable liquid, or to be located in fire-resistant rooms. Air-cooled dry transformers are preferred for indoor applications even at capacity ratings where oil-cooled construction would be more economical, because their cost is offset by the reduced building construction cost. The oil-filled tank often has radiators through which the oil circulates by natural convection; some large transformers employ forced circulation of the oil by electric pumps, aided by external fans or water-cooled heat exchangers. Oil-filled transformers undergo prolonged drying processes to ensure that the transformer is completely free of water vapor before the cooling oil is introduced. This helps prevent electrical breakdown under load. Oil-filled transformers may be equipped with Buchholz relays which detect gas 4evolved during internal arcing and rapidly de-energize the transformer to avert catastrophic failure.Oil-filled transformers may fail, rupture, and burn, causing power outages and losses. Installations of oil-filled transformers usually includes fire protection measures such as walls, oil containment, and fire-suppression sprinkler systems. Polychlorinated biphenyls have properties that once favored their use as a coolant, though concerns over their environmental persistence led to a widespread ban on their use. Today, non-toxic, stable silicone-based oils, or fluorinated hydrocarbons may be used where the expense of a fire-resistant liquid offsets additional building cost for a transformer vault. Before 1977, even transformers that were nominally filled only with mineral oils may also have been contaminated with polychlorinated biphenyls at 10-20 ppm. Since mineral oil and PCB fluid mix, maintenance equipment used for both PCB and oil-filled transformers could carry over small amounts of PCB, contaminating oil-filled transformers. Some "dry" transformers (containing no liquid) are enclosed in sealed, pressurized tanks and cooled by nitrogen or sulfur hexafluoride gas. Experimental power transformers in the 2 MVA range have been built with superconducting windings which eliminates the copper losses, but not the core steel loss. These are cooled by liquid nitrogen or helium. Construction of oil-filled transformers requires that the insulation covering the windings be thoroughly dried before the oil is introduced. There are several different methods of drying. Common for all is that they are carried out in vacuum environment. The vacuum makes it difficult to transfer energy (heat) to the insulation. For this there are several different methods. The traditional drying is done by circulating hot air over the active part and cycle this with periods of vacuum (Hot Air Vacuum drying, HAV). More common for larger transformers is to use evaporated solvent which condenses on the colder active part. The benefit is that the
entire process can be carried out at lower pressure and without influence of added oxygen. This process is commonly called Vapour Phase Drying (VPD). For distribution transformers which are smaller and have a smaller insulation weight, resistance heating can be used. This is a method where current is injected in the windings and the resistance in the windings is heating up the insulation. The benefit is that the heating can be controlled very well and it is energy efficient. The method is called Low Frequency Heating (LFH) since the current is injected at a much lower frequency than the nominal of the grid, which is normally 50 or 60 Hz. A lower frequency reduces the affect of the inductance in the transformer and the voltage can be reduced.
3.16 Terminals Very small transformers will have wire leads connected directly to the ends of the coils, and brought out to the base of the unit for circuit connections. Larger transformers may have heavy bolted terminals, bus bars or high-voltage insulated bushings made of polymers or porcelain. A large bushing can be a complex structure since it must provide careful control of the electric field gradient without letting the transformer leak oil.
Fig 3.17: Image of an electrical substation in Melbourne, Australia showing 3 of 5 220kV/66kV transformers, each with a capacity of 185MVA A major application of transformers is to increase voltage before transmitting electrical energy over long distances through wires. Wires have resistance and so dissipate electrical energy at a rate proportional to the square of the current through the wire. By transforming electrical power to a high-voltage (and therefore lowcurrent) form for transmission and back again afterward, transformers enable economic transmission of power over long distances. Consequently, transformers have shaped the electricity supply industry, permitting generation to be located remotely from points of demand. All but a tiny fraction of the world's electrical
power has passed through a series of transformers by the time it reaches the consumer. Transformers are also used extensively in electronic products to step down the supply voltage to a level suitable for the low voltage circuits they contain. The transformer also electrically isolates the end user from contact with the supply voltage. Signal and audio transformers are used to couple stages of amplifiers and to match devices such as microphones and record players to the input of amplifiers. Audio transformers allowed telephone circuits to carry on a two-way conversation over a single pair of wires. A balloon transformer converts a signal that is referenced to ground to a signal that has balanced voltages to ground. such as between external cables and internal circuit .The principle of open-circuit (unloaded) transformer is widely used for characterization of soft magnetic materials, for example in the internationally standardized Epstein frame method 3.18
Specifications of Transformer
3 Phase oil immersed, naturally air cooled Transformer VDE/ICE/DIN/BSS standard with 3 HT Bushing and 4 LT Bushings arranged on tank top, conservator, oil level indicator, drain and filling valves, filling valves, lifting lugs with first filling of oil in Transformer. Technical Data of Transformer • • • • • • • •
• • • •
Rated capacity KVA : No of phase : Frequency : Normal transformation ratio : Rated H.T voltage : Rated H.T Side Current : Maximum H.T voltage : Rated L.T Voltage : to phase Rated L.T Side current : Maximum L.T Voltage : Basic insulation level : Installation : M. Type : Type of winding :
• • • •
Type of cooling : Coolant : Direction of normal power : Rated power frequency
400 KVA 3 (Three) 50 Hz 11/0.415kv at no load 11 kv phase to phase 26.25 amps 12.5 kv phase to phase 0.415 kv phase 695.62 A 0.5 kv phase to phase 78 kv High voltage Indoor Oil immersed Double wound of high Conducting copper ONAN Oil HT-LT
Withstand voltage : Vector group : Neutral to be brought out i) H.T : ii) L.T : o Impedance voltage at 75 C & at Normal ratio and rated freq- : Tapping range i) H.T :
28 kv Dyn-11 Nil Yes 5%
• • •
5 tapping ±2.5%, ±5%, ±7.5% ii) LT : Nil Maximum temperature rise over : 80/100ok 40o C ambient when transformer is working at full load & tap changer is at middle position in HV/LV winding. Regulation : OFF LOAD Regulation at rated load : 0.941 (at p.f=1) Av. Reactance’s : 4.9%
• • • • •
Av. Impedance’s No Load losses Full load losses Efficiency at full load Accessories
: : : : :
5.0% 770 W 4220 W 99 (at p.f=1) 1) Conservator 2) Silica gel breather 3) Thermometer 4) 3 H.T Bushing 5) 4 L.T Bushing
Characteristics related to the technology and transformer
This list is not exhaustive: • Choice of technology The insulating medium is: - Liquid (mineral oil) or - Solid (epoxy resin and air) • For indoor or outdoor installation • Altitude (<= 1,000 m is standard) • Temperature (IEC 60076-2) - Maximum ambient air: 40 °C - Daily maximum average ambient air: 30 °C
utilization of the
- Annual maximum average ambient air: 20 °C For non-standard operating conditions, refer to “Influence of the Ambient temperature and altitude on the rated current”
Substation ventilation is generally required to dissipate the heat produced by transformers and to allow drying after particularly wet or humid periods. However, a number of studies have shown that excessive ventilation can drastically increase condensation. Ventilation should therefore be kept to the minimum level required. Furthermore, ventilation should never generate sudden temperature variations that can cause the dew point to be reached. For this reason: Natural ventilation should be used whenever possible. If forced ventilation is necessary, the fans should operate continuously to avoid temperature fluctuations. Guidelines for sizing the air entry and exit openings of substations are presented hereafter. 3.21 Calculation methods A number of calculation methods are available to estimate the required size of substation ventilation openings, either for the design of new substations or the adaptation of existing substations for which condensation problems have occurred. The basic method is based on transformer dissipation. The required ventilation opening surface areas S and S’ can be estimated using the following formulas:
and where: S = Lower (air entry) ventilation opening area [m²] (grid surface deducted) S’= Upper (air exit) ventilation opening area [m²] (grid surface deducted) P = Total dissipated power [W] P is the sum of the power dissipated by: The transformer (dissipation at no load and due to load) The LV switchgear The MV switchgear
H = Height between ventilation Opening mid-points [m]
Fig 3.21: Natural ventilation This formula is valid for a yearly average temperature of 20 °C and a maximum altitude of 1,000 m. It must be noted that these formulae are able to determine only one order of magnitude of the sections S and S', which are qualified as thermal section, i.e. fully open and just necessary to evacuate thermal energy generated inside the MV/LV substation. The practical sections are of course larger according to adopted technological solution. Indeed, the real air flow is strongly dependant: • •
on the openings shape and solutions adopted to ensure the cubicle protection index (IP): metal grid, stamped holes, chevron louvers,... on internal components size and their position compared to the openings: transformer and/or retention oil box position and dimensions, flow channel between the components, ... and on some physical and environmental parameters: outside ambient temperature, altitude, magnitude of the resulting temperature rise.
The understanding and the optimization of the attached physical phenomena are subject to precise flow studies, based on the fluid dynamics laws, and realized with specific software. Example:
Transformer dissipation = 7,970 W
LV switchgear dissipation = 750 W MV switchgear dissipation = 300 W The height between ventilation opening mid-points is 1.5 m. Calculation: Dissipated Power P = 7,970 + 750 + 300 = 9,020 W
Ventilation opening locations To favors evacuation of the heat produced by the transformer via natural convection, ventilation openings should be located at the top and bottom of the wall near the transformer. The heat dissipated by the MV switchboard is negligible. To avoid condensation problems, the substation ventilation openings should be located as far as possible from the switchboard
Fig 3.22: Ventilation opening locations
3.23 Type of ventilation openings To reduce the entry of dust, pollution, mist, etc., the substation ventilation openings should be equipped with chevron-blade baffles. Always make sure the baffles are oriented in the right direction
Fig 3.23: Chevron-blade baffles
3.24 Temperature variations inside cubicles To reduce temperature variations, always install anti-condensation heaters inside MV cubicles if the average relative humidity can remain high over a long period of time. The heaters must operate continuously, 24 hours a day all year long. Never connect them to a temperature control or regulation system as this could lead to temperature variations and condensation as well as a shorter service life for the heating elements. Make sure the heaters offer an adequate service life (standard versions are generally sufficient). 3.25
Temperature variations inside the substation
The following measures can be taken to reduce temperature variations inside the substation: Improve the thermal insulation of the substation to reduce the effects of outdoor temperature variations on the temperature inside the substation. Avoid substation heating if possible. If heating is required, make sure the regulation system and/or thermostat are sufficiently accurate and designed to avoid excessive temperature swings (e.g. no greater than 1 °C). If a sufficiently accurate temperature regulation system is not available, leave the heating on continuously, 24 hours a day all year long. Eliminate cold air drafts from cable trenches under cubicles or from openings in the substation (under doors, roof joints, etc.).
Substation environment and humidity
Various factors outside the substation can affect the humidity inside. • Plants
Avoid excessive plant growth around the substation. Substation waterproofing The substation roof must not leak. Avoid flat roofs for which waterproofing Is difficult to implement and maintain. Humidity from cable trenches Make sure cable trenches are dry under all conditions. A partial solution is to add sand to the bottom of the cable trench.
Pollution protection and cleaning Excessive pollution favors leakage current, tracking and flashover on insulators. To prevent MV equipment degradation by pollution, it is possible to either protect the equipment against pollution or regularly clean the resulting contamination. CHAPTER-4 DISTRIBUTION PANEL 4.1 L.T Distribution Panel Sheet steel clad powder coated (16SWG), dust and vermin proof, free standing, floor mounting type, 415V, 50Hz, indoor type low Tension Switchgear with 1000A hard drawn copper bus-bars, TPN&E equipped with.
Fig 4.5.1: 400 KV L.T Distribution Panel. 4.2
Metering at low voltage allows the use of small metering transformers at modest cost. Most tariff structures take account of MV/LV transformer losses. 4.3 LV installation circuits A low-voltage circuit-breaker, suitable for isolation duty and locking off facilities, to: o Supply a distribution board o Protect the transformer against overloading and the downstream circuits against short-circuit faults.
Different types of substation Substations may be classified according to metering arrangements (MV or LV) and type of supply (overhead line or underground cable). The substations may be installed: - Either indoors in room specially built for the purpose, within a building, or - An outdoor installation which could be : - Installed in a dedicated enclosure prefabricated or not, with indoor Equipment (switchgear and transformer - Ground mounted with outdoor equipment (switchgear and transformers) Pole mounted with dedicated outdoor equipment (switchgear and transformers) Prefabricated substations provide a particularly simple, rapid and competitive choice.
Conception: A typical equipment layout recommended for a LV metering substation. Remark: the use of a cast-resin dry-type transformer does not need a fire protection oil sump. However, periodic cleaning is needed.
Fig 22.214.171.124: Typical arrangement of switchgear panels for LV metering 4.6 Service connections and equipment interconnections a) At high voltage Connections to the MV system are made by, and are the responsibility of the utility - Connections between the MV switchgear and the transformers may be: - By short copper bars where the transformer is housed in a panel forming part of the MV switchboard - By single-core screened cables with synthetic insulation, with possible use of plug-in type terminals at the transformer
b) At low voltage Connections between the LV terminals of the transformer and the LV switchgear may be: • Single-core cables • Solid copper bars (circular or rectangular section) with heat-shrinkable insulation •
c) Metering Metering current transformers are generally installed in the protective cover of the power transformer LV terminals, the cover being sealed by the supply utility • • • •
Alternatively, the current transformers are installed in a sealed compartment within the main LV distribution cabinet The meters are mounted on a panel which is completely free from vibrations Placed as close to the current transformers as possible, and Are accessible only to the utility
Fig4.5.2: Plan view of typical substation with LV metering Substation lighting Supply to the lighting circuits can be taken from a point upstream or downstream of the main incoming LV circuit-breaker. In either case, appropriate over current protection must be provided. A separate automatic circuit (or circuits) is (are) recommended for emergency lighting purposes.
Operating switches, pushbuttons, etc. are normally located immediately adjacent to entrances. Lighting fittings are arranged such that: • Switchgear operating handles and position indication markings are adequately illuminated • All metering dials and instruction plaques and so on, can be easily read
Materials for operation and safety According to local safety rules, generally, the substation is provided with: • Materials for assuring safe exploitation of the equipment including: • Insulating stool and/or an insulating mat (rubber or synthetic) • A pair of insulated gloves stored in an envelope provided for the purpose • A voltage-detecting device for use on the MV equipment • Earthling attachments (according to type of switchgear) • Fire-extinguishing devices of the powder or CO2 type • Warning signs, notices and safety alarms: • On the external face of all access doors, a DANGER warning plaque and prohibition of entry notice, together with instructions for first • aid care for victims of electrical accidents.
4.9 Outdoor substations without enclosures These kinds of outdoor substation are common in some countries, based on weatherproof equipment exposed to the elements. These substations comprise a fenced area in which three or more concrete plinths are installed for: • A ring-main unit, or one or more switch-fuse or circuit-breaker unit(s) • One or more transformer(s), and • One or more LV distribution panel(s).
Fig 4.9: Outdoor substations without enclosures
Pole mounted substations
Field of application These substations are mainly used to supply isolated rural consumers from MV overhead line distribution systems. Constitution In this type of substation, most often, the MV transformer protection is provided by fuses. Lightning arresters are provided, however, to protect the transformer. General arrangement of equipment As previously noted the location of the substation must allow easy access, not only for personnel but for equipment handling (raising the transformer, for example) and the maneuvering of heavy vehicles.
Fig4.10: Pole-mounted transformer substation 4.11
Keeping ourselves at par with cutting edge technology, we offer a wide range of superior quality electrical isolators. These are available in various types such as horizontal / vertical type, tilting type / double break and rotating type. Compact in design, flame retardant and longer working life are some of its renowned features. We offer a range of isolators, (G.O.D) and H.T. Air Break Switch (G.O.D) to our customers. • The range is available with us in following types : • Horizontal / vertical type, single break • Tilting type / double break • Rotating type • Single/Double break consisting of hot dip galvanized channel iron base • Original post type HT insulators • Copper or copper alloy high pressure heavily tinned contacts • Plated milled steel oaring horns capable of breaking the magnetizing Current complete with horizontal connecting bar G I Pipe as down rod levers • Coupling and operating handle with locking arrangements in voltages up to and including 33KV line switchyards Current up to and including 1600 •
Amps manufactured as per IS 9921 & 1818
Drop Out Fuse We Provide an extensive range of supreme quality HT drop out fuses. Our range is made with qualitative raw materials and well known for its effective performance and longer operation life. These are widely used for system protection as the secondary back-up against faults. To prevent the faults our drop out fuse installed high voltage
Overhead Line Material We offer a wide range of overhead line material HT< insulator. These are made from precision-engineered raw materials. Our range is hugely demanded in electrical industry. We can provide our range in various KV as per the specifications of our customers. Our range is available in various specifications starts from 1.1 KV up to 33 KV.
Air Break Switch Our range of air break switch is extensively used in 11 KV and 33KV network distribution transformer substation. In an air switch the interruption of the circuit occurs in air. In this process air is the insulation medium between contacts. These are well known for their efficient performance and need no maintenance. Our range is easy to install and really user friendly.
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Gang Operator Switch
Horn Gap Fuse
Single Break Isolator Tilting Type
Double Break with Rotating Type
Combine G.O.D. & D.O.F.
Combined & Separate CT's / PT's 4.12
Automatic PFI Plant
The low power factor is mainly due to the fact that most of the loads are inductive and, therefore, take lagging currents. In order to improve the power factor, some device taking leading power should be connected in parallel with the load. One of such device taking leading power draws a leading current and partly or completely neutralizes the lagging reactive component of load current. This raises the power factor of the load. Power Factor Improvement System [Normally, the power factor of the whole load on a large generating station is in the region of 0.8 to 0.9. However, some times it is lower and in such cases it is generally desirable to take special steps to improve the power factor. This can be achieved by the following equipment. 1) Static Capacitor 2) Synchronous condenser 3) Phase advancer. 1. Static Capacitor The power factor can be improved by connecting capacitors in parallel with the equipment operating at lagging power factor. The capacitor (generally known as static capacitor) draws a leading current and partly or completely neutralizes the lagging active component of load current. This raises the power factor of the load. 2. Synchronous condenser A Synchronous motor takes a leading current when over â€“excited and, therefore, behaves as a capacitor. An over-excited synchronous motor running on no load is known as synchronous condenser. When such a machine is connected in parallel with the supply, it takes a
leading current which partly neutralizes the lagging reactive component of the load. thus the power factor is improved. 3. Phase advancer Phase advancers are used to improved the power factor of induction motors. The low power factor of an induction motor is due to the fact that its stator winding draws exciting current which lags behind the supply voltage by 90 o. If the exciting ampere turns can be provided from some other a.c source, then the stator winding will be relieved of exciting current and the power factor of the motor can be improved. This job is accomplished by the phase advancer which is simple an a.c exciter. The phase advancer is mounted on the same shaft as the main motor and is connected in the rotor circuit of the motor. It provides exciting ampere turns to the rotor circuit at slip frequency. By providing more ampere turns than required, the induction motor can be made to operate on leading power like an over-excited synchronous motor.
Fig 4.12: Automatic PFI Plant 4.13
Specification of PFI Plant
Specification of 300 KVAR Automatic PFI Plant Sheet steel clad powder coated (16 SWG), dust and vermin proof, free standing, floor mounting, 415V, 50 Hz, 300 KVAR indoor type Automatic Power Factor Improvement Plant, comprising: 2 Nos. 100 KVAR Bank of TP dry type Power Capacitor with built-in discharge resister.
1 Nos. 50 KVAR Bank of TP dry type Power Capacitor with built-in resister. 1 Nos. 25 KVAR Bank of TP dry type Power Capacitor with built-in resister. 1 Nos. 12.5 KVAR Bank of TP dry type Power Capacitor with built-in resister 1 Nos. 12.5 KVAR Bank of TP dry type Power Capacitor with built-in resister. Origin : Electronic on, Germany. 1 No. Automatic power factor correction Relay Origin : Micro/ Epos, Germany. 5 Nos. TP Air Contactors of adequate rating Origin : LG, Korea / GE, Span. 18 Nos. HRC Fuses with base of adequate rating 5 Nos. Indicating Lamps 1 Set Control fuses Manufactured by : Energy Pac Engineering Ltd. Bangladesh. CHAPTER-5 5.1
discharge discharge discharge discharge
EARTHING AND CABLES
Earthling, The process of connecting the metallic frame ( i.e. non-current carrying part) of electrical equipment or some electrical part of the system ( e.g. neutral point in star-connected system, one conductor of the secondary of a transformer etc) to earth is called grounding or earthling. It is strange but true that grounding of electrical system is less understood aspect of power system. It is a very important ,if grounding is done systematically in the line of the power system, we can effectively prevent accidents, and damage to the equipment of the power system and at the same time continuity of supply can be maintained. Grounding or earthling may be classified as (1) Equipment of ear thing (2) System earthling. Equipment grounding deals with earthling the non-current-carrying metal parts of the electrical equipment. On the other hand, system grounding means earthling some part of the electrical system e.g. earthling of neutral point of star-connected system in generating stations and substations. 5.2 Sub-Station Earthling The earthlings of all equipment shall be in accordance with the IEEE Recommendation No. 80:1976-Guide for safety in alternating current sub-station grounding the British standard code of practice CP1013:1965 or other approved standard. The contract includes the provision of materials and Installation of the complete Sub-station earthling system, including the earth connections of all equipment supplied under this contract, earth connections to panel and any other auxiliary equipmentâ€™s commissioning and testing of the earthling system.
In electricity supply systems, an earthling system defines the electrical potential of the conductors relative to that of the Earth's conductive surface. The choice of earthling system has implications for the safety and electromagnetic compatibility of the power supply. Note that regulations for earthling (grounding) systems vary considerably between different countries. A protective earth (PE) connection ensures that all exposed conductive surfaces are at the same electrical potential as the surface of the Earth, to avoid the risk of electrical shock if a person touches a device in which an insulation fault has occurred. A functional earth connection serves a purpose other than providing protection against electrical shock. In contrast to a protective earth connection, a functional earth connection may carry a current during the normal operation of a device. Functional earth connections may be required by devices such as surge suppression and electromagnetic-compatibility filters, some types of antennas and various measurement instruments. Generally, the protective earth is also used as a functional earth, though this requires care in some situations.
Fig 5.2 : Ear thing System 5.3
Ear thing System
The earthling system shall be based on soil resistively of 1 ohm-meter for wet soil and 10 ohm-meters for dry soil. Earthling points shall be provided such that the combined resistance of the earth network and earthling points does not exceed 1.0 ohms under any climatic conditions. The operating mechanisms of isolators, earth switches and circuit breaker are not integral with the circuit breaker shall be connected to the earth system by a branch entirely separated from that employed to earth their buses. The branch is to be installed such that the connection would pass beneath where an operator would stand, so as to minimize step potential. Connections to plant and equipment shall be made using the earthling terminals specified. When a strip has to be drilled to fit an earth terminals the diameter of the hole shall not be greater than half width of the strip. Joints in earthling strip shall employ chemical welding or high compression joints or clamps. 5.4
Equipment to be Connected the Earthling
1) Switchgear All metal parts including any relays, instruments, etc. Mounted on the switchboard, shall be connected to a copper earth bar which runs along the full length of the switchboard. The crosssection of the bar shall be sufficient to carry the rated short-time withstand current of the switchgear for five seconds. The frame of the draw-out circuit breakers shall be connected to the earth bar through a substantial plug type contact. 2) Low voltage panels Earth metal of switchboards, fuse boards, and distribution boards shall be bounded together and earthed to the main station earthling system. Earthling connections shall be carried out in bare copper strip having a 3 second rating not less than 20KA. 3) Control Panels Each control panel shall be provided with a copper earth bar of not less than 80sq. mm. cross section and arranged so that the bar of adjacent panel can be joined together to from a common bar The common earthling bulbar of control and relay panels shall be connected to the main station earthling systems via a copper earthling connection of not less than 80sq.mm. 5.5
In electrical power distribution a bulbar is a thick strip of copper or aluminum that conducts electricity within a switchboard distribution board substation or other electrical apparatus. Bus bars are used to carry very large currents, or to distribute current to multiple devices within switchgear or equipment. For example, a household circuit breaker panel board will have bus bars at the back, arranged for the connection of multiple branch circuit breakers. An aluminum smelter will have very large bus bars used to carry tens of thousands of amperes to the electrochemical cells that produce aluminum from molten salts. The size of the bulbar is important in determining the maximum amount of current that can be safely carried. Bus bars can have a cross-sectional area of as little as 10 mmÂ˛ but electrical substations may use metal tubes of 50 mm in diameter (1,963 mmÂ˛) or more as bus bars.
Fig 5.5: 1500 ampere bus bars within a power distribution rack for a large building
Design and placement
Bus bars are typically either flat strips or hollow tubes as these shapes allow heat to dissipate more efficiently due to their high surface area to cross-sectional area ratio. The skin effect makes 50â€“60 Hz AC bus bars more than about 8 mm (1/3 in) thick inefficient, so hollow or flat shapes are prevalent in higher current applications. A hollow section has higher stiffness than a solid rod of equivalent current-carrying capacity, which allows a greater span between bulbar supports in outdoor switchyards. A bulbar may either be supported on insulators, or else insulation may completely surround it. Bus bars are protected from accidental contact either by a metal earthed
enclosure or by elevation out of normal reach. Neutral bus bars may also be insulated. Earth bus bars are typically bolted directly onto any metal chassis of their enclosure. Bus bars may be enclosed in a metal housing, in the form of bus duct or bus way, segregated-phase bus, or isolated-phase bus. Bus bars may be connected to each other and to electrical apparatus by bolted or clamp connections. Often joints between high-current bus sections have matching surfaces that are silver-plated to reduce the contact resistance. At extra-high voltages (more than 300 kV) in outdoor buses, corona around the connections becomes a source radio-frequency interference and power loss, so connection fittings designed for these voltages are used. Bus bars are typically contained inside of either a distribution board or bus way.
Distribution boards split the electrical supply into separate circuits at one location.
Fig 5.7 : Two hot busbars are visible in this distribution board, traveling vertically from the main circuit breaker at top to feed the rows of breakers below it.
Bus ducts Bus ways, or bus ducts, are long bus bars with a protective cover. Rather than branching the main supply at one location, they allow new circuits to branch off anywhere along the route of the bus way.
Fig 5.8.1: The bus bars contained within are visible in this opened bus way, above the arrows at left and traveling horizontally at right. This bus way section was used in a fire test of a fire stop system, achieving a 2 hour fireresistance rating
Fig 5.8.2: Bus duct penetration, awaiting fire stop
Electrical conduit and bus duct in a building at Texaco Nanticoke refinery in Nanticoke 5.9
Represent of Supply to Metering
The different methods of MV service connection, which may be one of four types: • Single-line service • Single-line service (equipped for extension to form a ring main) • Duplicate supply service • Ring main service • General protection at MV, and MV metering functions • Protection of outgoing MV circuits • Protection of LV distribution circuits
Fig 5.9: Consumer substation with MV metering 5.10
The 0.38kv neutral of the 11/0.415 kv transformer shall incorporate provision for the scheduled current transformers and shall be directly connected to the main station earthling system. The connection shall be formed of twin conductors and shall be capable of carrying 20 KA for five seconds. 5.11 Ear thing Materials 2 Sets Earthling Block 2 Nos. Copper earthling block with required Nos. of Âź inch. Hole for ECC connection. Size- 254mm x 40mm x 10mm. One for transformer neutral and another for all substation equipments ECC connections. 4 Sets Earth Electrode 1.5 inch dia GI Pipe buried up to the depth of 80 fit in the damp soil including 2 SWG copper wire from bottom of GI pipe up to ground level. 150-rft Earthling lead: 2 SWG copper wire through 1 inc dial PVC pipe ( if required) from earthling block to earth electrode. 100 raft Earth Continuity Conductor: Installation of 2 SWG copper earth continuity conductor running from the bodies of substation equipment to the earthling block. 5.12 H. T Cables Three core plain aluminum conductor, PVC insulated, field limiting conducting layers over each conductor and each core insulation, concentric shields of aluminum over individual cores, three cores laid up PVC belted flat wire armoring with helical steel taping and PVC sheathed of size 3-core 70 rm cables type â€“NYSEYGbY 170 A at 30oC ambient temperature. This cable shall be connected to the DESA supply source and terminated in RMV unit of H.T panel of the sub-station. 5.13 L.T Cables Single core plain aluminum conductor, PVC insulated single core laid up, filled with rubber and PVC sheathed of size 4 x 630 rm rated at 800 A at 30 o C ambient temperature in air, for termination between L.T panel of the sub-station and Mainbus Rising. This cable shall be installed in concrete trench.
Fig 5.13 : LT & HT Cable 5.14 Maintenance of Sub-station Effective performance of a sub-station is mainly dependent Of proper maintenance. It includes I) Routine checking, servicing, cleaning of H.T switchgear and its components. II) Routine checking, servicing, cleaning of VCB. III) Routine checking, testing and adjustment of protective relays. IV) Routine checking of space heater. V) Routine checking, testing and inspection of transformer VI) Quarterly insulation testing of transformer oil and daily checking of oil level. Breakdown capacity of transformer oil must be at the range of 20-28 KV. If it falls under the specified range it can be increased by centrifuging method. Transformer oil should be maintained at a constant level. VII) Routine checking, servicing of L.T switchgear and its components. VIII) Routine checking of PFI panel and its accessories. The power factor must be maintained at a range of 0.90 to 1.00 ( Unity) X) Routine checking of earth resistance of the system. The earth resistance must be maintained at below 1 ohm, at any condition CHAPTER-6
Procedure for the establishment of a new substation From Installation Guide
Large consumers of electricity are invariably supplied at MV. On LV systems operating at 120/208 V (3-phase 4-wires), a load of 50 kVA might be considered to be “large”, while on a 240/415 V 3-phase system a “large” consumer could have a load in excess of 100 kvA. Both systems of LV distribution are common in many parts of the world. As a matter of interest, the IEC recommends a “world” standard of 230/400 V for 3-phase 4-wire systems. This is a compromise level and will allow existing systems which operate at 220/380 V and at 240/415 V, or close to these values, to comply with the proposed standard simply by adjusting the off-circuit tapping switches of standard distribution Tran formers. The distance over which the energy has to be transmitted is a further factor in considering an MV or LV service. Services to small but isolated rural consume rare obvious amplest. The decision of a MV or LV supply will depend on local circumstances and considerations such as those mentioned above, and will generally be imposed by the utility for the district concerned. When a decision to supply power at MV has been made, there are two widelyfollowed method of proceeding The power-supplier constructs a standard substation close to the consumer’s premises, but the MV/LV transformer(s) is (are) located in transformer chamber(s)inside the premise, close to the load center ,The consumer construct send equips his own substation on his own premises, to which the power supplier makes the MV connection In method no. 1 the power supplier owns the substation, cable transformer(s) transformer chamber(s), which he has unrestricted access.
The transformer chamber(s) is (are) constructed by the consumer (to plans and regulations provided by the supplier) and include plinths, oil drains, fire walls and ceilings, ventilation, lighting, and earthling systems, all to be approved by the supply authority. The tariff structure will cover an agreed part of the expenditure required to provide the service. Whichever procedure is followed, the same principles apply in the conception and realization of the project. The following notes refer to procedure no. 6.2
Before any negotiations or discussions can be initiated with the supply authorities, the following basic elements must be established: 6.3
Maximum anticipated power (kVA) demand
Determination of this parameter is described in Chapter A, and must take into account the possibility of future additional load requirements. Factors to evaluate at this stage are: • •
The utilization factor (ku) The simultaneity factor (ks) Layout plans and elevations showing location of Proposed substation
Plans should indicate clearly the means of access to the proposed substation, with dimensions of possible restrictions, e.g. entrances corridors and ceiling height, together with possible load (weight) bearing limits, and so on, keeping in mind that: • The power-supply personnel must have free and unrestricted access to the MV equipment in the substation at all times • Only qualified and authorized consumer’s personnel are allowed access to the substation 6.5 Degree of supply continuity required The consumer must estimate the consequences of a supply failure in terms of its duration: • •
Loss of production Safety of personnel and equipment
The type of power supply proposed, and define: o
The kind of power-supply system: over headline or underground-cable network
Service connection details: single-line service, ring-main installation, or parallel feeders, etc.
Power (kVA) limit and fault current level
The nominal voltage and rated voltage (Highest voltage for eqp) Existing or future, depending on the development of the system. Metering details which define:
The cost of connection to the power network
Tariff details (consumption and standing charges) Implementation
Before any installation work is started, the official agreement of the power-supplier must be obtained. The request for approval must include the following information, largely based on the preliminary exchanges noted above: • Location of the proposed substation
• • • • •
Single-line diagram of power circuits and connections, together with earthling-circuit proposals Full details of electrical equipment to be installed, including performance characteristics Layout of equipment and provision for metering components Arrangements for power-factor improvement if required Arrangements provided for emergency standby power plant (MV or LV) if eventually required Commissioning
When required by the authority, commissioning tests must be successfully completed before authority is given to energize the installation from the power supply system. Even if no test is required by the authority it is better to do the following verification tests: • • • • • • • •
Measurement of earth-electrode resistances Continuity of all equipotent a earth-and safety bonding conductors Inspection and functional testing of all MV components Insulation checks of MV equipment Dielectric strength test of transformer oil (and switchgear oil if appropriate), if applicable Inspection and testing of the LV installation in the substation Checks on all interlocks (mechanical key and electrical) and on all automatic sequences Checks on correct protective-relay operation and settings
It is also imperative to check that all equipment is provided, such that any properly executed operation can be carried out in complete safety. On receipt of the certificate of conformity (if required): • Personnel of the power-supply authority will energize the MV equipment and check for correct operation of the metering • The installation contractor is responsible for testing and connection of the LV installation When finally the substation is operational: • The substation and all equipment belongs to the consumer • The power-supply authority has operational control over all MV switchgear in the substation, e.g. the two incoming load-break switches and the transformer MV switch (or CB) in the case of a Ring-Main Unit, together with all associated MV ear thing switches • The power-supply personnel has unrestricted access to the MV equipment • The consumer has independent control of the MV switch (or CB) of the transformer(s) only, the consumer is responsible for the maintenance of all substation equipment, and must request the power-supply authority to isolate and earth the switchgear to allow maintenance work to proceed. The power supplier must issue a signed permit-to-work to the consumers maintenance
personnel, together with keys of locked-off isolators, etc. at which the isolation has been carried out. CHAPTER-7 7.1
Substation design in the old and modern substations today
Much of the 20th century focused on developing new technologies that would increase capacity, availability and limit maintenance, as well as addressing the issues of size, speed and automation. Some of these developments and innovations led to the launch in the 1960s of gas insulated switchgear (GIS). This smaller and compact switchgear reduced the dimensions of a conventional air innovations have resulted in the current numerical control and protection systems, incorporating multiple functions and tasks, that communicate systems via digital technology. For some time utilities have been able to remotely operate and control substations without the need for on-site personnel. Pre-engineered, A hundred years is nothing compared with the length of time man has been roaming the earth. When ABB manufactured substation about 100 years ago, who would have guessed what a typical substation would be like today. circuit breakers used were bulky requiring constant supervision frequent maintenance.
Fig7.1.1 : Different types of single-line configurations (a) double bulbar (b) double plus transfer bulbar (c) 1Â˝-breaker and (d) 2-breaker. a and b focus on maintenance whereas c and d cover both maintenance and fault aspects. pre-fabricated and modularized substations are available in various AIS and GIS configurations, enabling short delivery times and a high quality of installation. When the building of electricity systems started in earnest some 100 years ago, the network wasnâ€™t particularly reliable. The circuit breakers were mechanically and electrically very complicated and required frequent maintenance. Outages due to maintenance were the norm rather than the exception. The invention of the disconnect or switch certainly helped to increase the availability of these electrical
networks. The single-line configurations used were such as to surround the circuit breakers by many disconnect or switches so that adjacent parts of the switchgear were kept in service while maintenance was carried out on the breakers. These ideas led to the double bulbar and double plus transfer bulbar schemes (Fig. 1a and Fig. 1b). In addition to maintenance considerations, single-line configurations were chosen to limit the consequences of primary faults in the power system (e.g. if the ordinary circuit breaker failed to open on a primary fault on an outgoing object, or if a fault occurred on the bulbar). For the configurations shown in Fig. 1a and Fig. 1b, these types of faults will lead to the loss of all objects connected to the bus bar. To limit these consequences while still retaining the maintenance aspects, 1Â˝ - breaker and 2 - breaker single-line configurations, Fig. 1c and Fig. 1d, were introduced. Todayâ€™s breakers require less maintenance than their predecessors. In fact, SF6 circuit breakers have a maintenance interval (where the primary components need to be taken out of service) of 15 years. Open air disconnect or switches on the other hand still retain a Maintenance interval of about four to five years in areas where there is little or no pollution. Substantially more frequent maintenance is required if the switch is located in areas with natural (i.e., sand or salt)or industrial pollution.
Fig7.1.2: Innovative switchgear modules with the disconnecting function either built on or integrated into the circuit breaker. a) Combined. b) PASS. c) Compass. d) Compact. Even though disconnecting switches, or rather a disconnecting function, are needed, their maintenance requirements are simply not practical, let alone economical. A number of innovative switchgear concepts for air insulated substations (AIS) have effectively made the traditional open-air disconnecting switch redundant. The disconnecting function has either been built onto or integrated into the breaker. This not only increases the availability of the substation, but it helps to reduce its footprint by about 50%. The impact of going from a traditional solution, for example a 1Â˝-breaker solution for a 400 kV AIS with circuit breakers and disconnecting
switches, to a solution using a Combined (disconnecting circuit breaker) is shown in Fig. 3. The advantages of a reduced footprint include lower costs for land acquisition and preparation, the retrofitting of existing substations is easier, and the environmental impact, because of less material and therefore pollution, is considerably reduced. Instrument transformers today Instrument transformers pass on information about the primary current and voltages to the secondary equipment (protection, control and metering). Historically these transformers were large apparatus composed of insulation materials, copper and iron. They were also used to power the electromechanical secondary equipment. Nowadays, the numerical type of secondary equipment gets its operating power from a separate power supply (i.e., battery). In addition, thanks to the emergence of fiber-optic technology the old large instrument transformers can be replaced by fibre-optic sensors that give information about primary currents and voltages. These values are transformed into digital fiberoptic signals, which are fed to the secondary equipment. Replacing traditional instrument transformers with optical sensors will further reduce the switchgear footprint and lower costs, while at the same time providing secondary equipment that is more flexible and secure. Invisible substations Not only has the technology behind substations changed dramatically in the last 100 years, but so too has their appearance. Many substations were originally built on the outskirts of cities or large towns, so appearances were not all that important. However, many of these substations have since been swallowed up by the urban expansion of the past few decades. Many who live near them find both the appearance and the acoustic pollution, caused by the humming of power transformers, unpleasant. To solve this problem, substations have been placed in buildings that are in surroundings, and have therefore become â€œinvisibleâ€?. A reduced footprint: a 40% to 50% reduction for indoor AIS solutions and 70% to 80% reduction for indoor GIS solutions, has greatly simplified this process. Locating equipment indoors increases the substation availability and reliability as the risk of primary failures, due to animals and atmospheric or industrial pollution, is significantly decreased for AIS and totally eliminated for GIS. Additionally, remote supervision of the building is possible, which helps increase the substation rounding interval. The substations are also protected against burglaries, and the irritable humming noise is greatly reduced. Underground GIS substations, making the substation truly invisible, have been implemented in city centers around the world where substations at ground level are not permitted. Two important considerations engineers must take into account when constructing new substations in urban areas are size and safety. Real estate prices mean the space required for these substations must be kept to a minimum, and higher standards for personal safety apply for substations in populated areas. To meet these specific requirements in and around cities, as well as adapting to individual requirements, ABB has developed a concept, known as the Urban concept, for compact indoor substations up to 170 kV. Exclusively innovative systems from ABBâ€™s current product portfolio are used for indoor installations within this concept. Both air- energize - March 2008 - Page 28 insulated and SF6 -insulated modules can be used, depending on the actual requirements of the specific installation. Pre-fabricated indoor substations.
A pre-fabricated substation allows for quick and easy on-site installation, something that shortens the total project time and minimizes disturbances to neighbors. At the sometime, the quality of the supply is higher due to complete factory testing before shipping. One example is Malted, a type of distribution substation consisting of pre-fabricated modules that are factory tested before shipping, and with a transformer size of up to 16 MVA. Primary and secondary cabling between the modules is prepared in a way that allows for rapid connection. On-site assembly and testing only takes one week, after which the substation is ready. Its footprint, of the order 100 m2, is less than 30% of an outdoor AIS substation. The substation .consists of three main modules:
Fig7.1.3 : A MALTE prefabricated substation. a) An old substation. b) New substation. c) interior of the new substation with a power transformer in the middle, high-voltage to the right, and mediumvoltage and secondary equipment on the left. • A power transformer module consisting of the main power transformer, a prefabricated foundation that also acts as an oil-pit, walls and a roof • A high-voltage (HV) module which is equipped with a removable compact 52,5 kV circuit breaker. This module requires no foundations as it is hinged onto the side of the power transformer Module • A medium voltage (MV) module whose indoor switchgears are mounted in cubicles. In this module relay, control and auxiliary AC/DC equipment for the entire substation is included. Like the HV module, it is also hinged onto the transformer module. Replacing traditional instrument transformers with optical sensors further reduces the switchgear footprint while at the same time providing secondary equipment that is more flexible and secure. As well as its small footprint and quick assembly time, Malted, when compared to the traditional solution, offers: higher
availability because the equipment is indoors; lower maintenance and rounding costs; the substation, including its foundations, can be quickly dismantled and moved; it is environmentally friendly; and finally, it is personnel and third-party safe. Substation secondary system Like its primary counterpart, substation secondary systems have also changed a lot over the years. For example, the days of manual operation have been replaced by a more sophisticated form of information management. The secondary system in a modern substation is used for: • Primary s y s tem protect ion and supervision • Local and remote access to the power system apparatus • Local manual and automatic functions • Communication links and interfaces within the secondary system • Communication links and interfacing to network management systems All of these functions are performed by a substation automation system (SAS) which contains programmable secondary devices, known as intelligent electronic devices (IEDs), for control, monitoring, protection and automation. Typical characteristics of an IED include: • It can be used for one or more switchgear bays • It contains independent protection functionality for each feeder • It performs high-speed calculations in real-time, which will trigger a trip signal if necessary • The IED is intended as a combined protection and control device, but it can just as well function as a separate control or protection device • It can communicate with all other IEDS
Fig.7.1.4: The structure of a modern control and protection system To increase SAS reliability and availability, the protection part may be duplicated to provide a redundant system. For full redundancy, all IEDs and the supporting system (like the power supply) should be duplicated, to ensure that the two systems can work independently of each other. Future substation power handling equipment will Be even more integrated and compact, while Measuring functions and all of the secondary Functions will be done sing fiber optics. A pre-fabricated distribution substation not only allows for quick and easy on-site installation, but the quality of the supply is higher. Pre-fabrication The prefabrication and pre-testing of substation automation equipment is fast becoming the norm for a modern substation. The system is delivered in sections containing MALTE, a pre-fabricated distribution substation not only allows for quick and easy on-site installation, but the quality of the supply is higher. all the required functions for a part of the primary system, and these sections are then simply connected together via an optical fiber. Pre-fabrication has many advantages such as:
• The total costs can be kept lower due to optimized manufacturing and testing • The quality is higher because the module has been fully tested in-house and is shipped with all the wiring intact • Because much of the assembly and testing is completed before shipping, the time spent on-site is considerably reduced • Pre-fabrication is suitable for both “green field” and retrofit projects. Communication Effective and fast communication between IEDs is essential in an SAS. Numerical communication had been used for many years in substations, but a lack of standardized protocols limited the efficiency of SAS and restricted the mixing of IEDs from different suppliers. This problem was overcome by active participation and supports for the IEC in the development of a standard for substation communication, known as the IEC 61850-communication standard. Modern substations are generally remotely operated, and communication between the substation and the remote control center is via a wide area network (WAN). Nowadays, new overhead lines or power cable connections are equipped with optical fiber to enable protective system communication and for the WAN.
Fig 7.1.5 : EHV Switchgear. A look into the future The last 100 years have seen the economy move from the industrial age to the information age. A host of fascinating ideas, in particular the World Wide Web, have changed how many people and companies live and work. For example, the availability of the internet to means that customer contact is greatly simplified and faster. Projects can be executed using a common database assessed by both parties. Future substation power handling equipment will be even more integrated and compact, while measuring functions and all of the secondary functions will be done using fiber-optics. In the future, substation power handling equipment will be even more integrated and compact, while measuring functions and all of the secondary functions will be done using fiber optics. In other words,
tons of porcelain, copper and iron will be superseded by just a few fiber-optic connections. This will further speed up the delivery process, reduce the substation footprint, and make it more environmentally friendly. Conclusion And Future Work Electricity is the basic necessity for the economic development of a country. For the Industrial, commercial and domestic use reliable power supply is required. For this purpose the sub-station system of PDB / PGCB/ DPDC/ REB has been studied in this dissertation work. Sub-Station is an important and integral part of power distribution system. In this project we have studied different essential elements of sub-station, such as H.T and L.T switchgear, circuit breaker, transformer, relays, lightning arresters, isolators, earthling, C.T & P.T and other protective devices. From this study, it is clear that adequate numbers of distribution sub-station should be installed in the distribution network for greater reliability of supply and increased system stability. Sub-Stations have to be designed and installed according to the prevailing and future demands of the consumers. One of the common problems is that incoming voltage is not always available. This impedes the supply & distribution system. By manipulating tap changer voltage level can be maintained. We have found this study very useful and helpful for better understanding of design and installation of indoor type distribution sub-station. Sub-Station is an important installation at the consumer end. This is very essential for reliable and stable supply of electric power to the consumer. Sub-stations provide the electricity need of different consumers by different feeders. Each feeder carries a particular amount of load of a certain area. Feeders are used for load management, as we see load shedding program specially in the evening peak load period. If reliable and stable power supply is available to the consumers, industrial, commercial, and agricultural sectors will definitely be developed. The economic growth will be increased, and the country will be developed as well. Our Internship topic is Study & Analysis of Electrical Design, Construction & Maintenance of Distrubtation Indoor Type Sub-Station. From our analysis we notice following points: Operating cost is low in comparison to the power Substation. Fuel can be used for Transformer such as coal. Efficiency is better. It requires Shortly Space needed for a Distrubtation of Power .
Any Sub Station can be controlled by three ways i) Analog ii) Microprocessor Based Control iii) Digital Direct Control. In control are most important. In modern system above all control from a single room where only one operator is needed. Because total Sub Station controlled by microprocessor which is easy. PFI control is most important because load vary time to time. Protection is most important because damaged equipment needs time for repairs and replacement. By adequate protection, the damage can be eliminated or minimized. Protection of generators is the most complex and elaborate. It should not be shut off as far possible because that would result in power shortage and emergency. Finally, This article clearly explains the Different types of substations that exist in the power system and it explains about the stepping up, grid 2and distribution substations, indoor and outdoor substations, HV, EHV substations, AIS, GIS and HIS substations.. FUTURE WORK In this circuit the differential concept has concept has been shown that can be used as s function of a relay. But the relay built in the circuit is not capable to use in practical field. Again for the unavailability of the low range transducer (low range CT, resistor) variable DC source has been used in the circuit.
PHOTOGRAPH OF SUB-STATION.
PHOTOGRAPH OF A 400 KVA SUB-STATION.
Fig A: Internal Equipments of Substation.
Fig B : Switchgears
Fig C : LT & HT Metering Panel.
Fig D : Transformer Action
Fig E : Automatic PFI Internal.
Fig F : Metering Panel.
Fig G : LT Joint With Bas-bar & Backer.
Fig H : To Connected HT & LT Cable With Transformer. REFERENCES 
Elements of electrical power station design By â€“ Professor M.V. Desh Pande (INDIA)
“A TEXT BOOK OF ELECTRICAL TECHNOLOGY” Volume (I & II) by B.L THERAJA & A.K. THERAJA, S. Chand & Company Ltd .Design and Installation
Protective Relating By – N. Chernobrovov
Power System Protection By – The Electric Council Correspondence (UK )
Bangladesh Power Development Board.gov. Elements of Electrical Power Station Design (Third Edition) By M.V. Deshpande 
Principles of Power System (Third Edition) By V.K.Mehta, Rohit Mehta
 Alternating Electric Current Generator Tesla's generator that produces alternations of 15000 per second or more. 
British Pattern GB 20069 Improvements in Apparatus for Controlling the Application or Use of Electric Currents of High Tension and Great Quantity in 1893 on espacenet.com
Lin Jiming et al., Transient characteristics of 1100 kV circuit-breakers International Symposium on International Standards for Ultra High Voltage, Beijing, Juliet 2007.
 Robert W. Smeaton (ed) Switchgear and Control Handbook 3rd Ed., Mc Graw Hill, new York 
“Sub-station Design & Equipment” by P.V GUPTA, P.S GUPTA (1998) Dhanpat Rai & Sons.
SUNIL S.RAO (2004) “Switchgear & Protection” Khan publisher Testing Commissioning Operation & Maintenance of Electrical Equipment” by S.RAO (1984) Khan Tech. Publications.
External links The basic functions of LV switchgear IEC TC17: Switchgear & ControlgearTechnical Committee, on tc17.iec.ch IEEE Switchgear Committee, on ewh.ieee.org
This article incorporates information from the German Wikipedia. Course ETHZ, handouts_2.pdf, on eeh.ee.ethz.ch Retrieved from "http://en.wikipedia.org/wiki/Switchgear" Categories: Electric power systems components British Pattern GB 20069 Improvements in Apparatus for Controlling the Application or Use of Electric Currents of High Tension and Great Quantity in 1893, on espacenet.com Lin Jiming et al., Transient characteristics of 1 100 kV circuit-breakers, International Symposium on International Standards for Ultra High Voltage, Beijing, Juillet 2007. Robert W. Smeaton (ed) Switchgear and Control Handbook 3rd Ed., Mc Graw Hill, new York 1997 WORKING SIDE Design and Installation of 500 KVA Indoor type Sub-stationâ€? in Dhaka University. Design and Installation of 450 KVA Indoor type Sub-station in Pacific Telecom ( BD) Ltd, Bay Tower Dhaka. Supply And Installation of 11/0.415 KV, 250 KVA Indoor type Sub-station in Multi Dyeing & Fabrics (Pvt) Ltd. Rupnagar, , Mirpur, Dhaka. Forest City 50KVA Khilkhet,Dhaka.
Fig L1 : Substation Single Line Diagram.
Fig L2 : Indoor Type Substation Layout Plane
Fig L3 : Electrical Circuit Diagram of MDB.