Study on protective devices of a distribution substation

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Study on protective devices of a distribution substation Chapter 1 Introduction 1.1 GENERAL BACKGROUND The modern society is so much dependent upon the use of electric energy that it has become a part parcel our life. The present day advancement in science and technology has made it possible to convert electric energy into any desired form. This has given electric energy a place of pride in the modern world. The importance of electric supply in everyday life has reached such a stage that it is desirable to protect the power system from harm during fault condition and to ensure maximum continuity of supply. For this purpose from reliable point of view the protective device used in power system takes an important role of this system. Since in our Bangladesh, Distribution Substation Distributed electric power to the total area of Bangladesh .So we want to study of electrical power protection scheme of Distribution Substation 1.2 PROBLEM DEFINATION A particular type of equipment used in electric power systems to detect abnormal conditions and to initiate appropriate corrective action known as protective device. Equipment applied to electric power systems to detect abnormal and intolerable conditions and to initiate appropriate corrective actions. These devices include lightning arresters, surge protectors, fuses, and relays with associated circuit breakers, reclosers, and so forth. From time to time, disturbances in the normal operation of a power system occur. These may be caused by natural phenomena, such as lightning, wind, or snow; by falling objects such as trees; by animal contacts or chewing; by accidental means traceable to reckless drivers, inadvertent acts by plant maintenance personnel, or other acts of humans; or by conditions produced in the system itself, such as switching surges, load swings, or equipment failures. Protective devices must therefore be installed on power systems to ensure continuity of electrical service, to limit injury to people, and to limit damage to equipment when problem situations develop. Protective devices are applied commensurately with the degree of protection desired or felt necessary for the particular system.

1.3 OBJECTIVES OF THIS WORK The development of any country of the world is based on electricity & without it on industry is impossible. So we should take great about reliability & stability of a power system. Protective device serve this purpose sufficiently. It is to be noted that protective device can not remove the fault, it protect the equipment from harmful damage. As the fault in the equipment in the supply system leads to disconnection of supply to a large portion of the system. If the fault part is quickly disconnected the damage caused by the fault is minimum & the faulty part can be repaired quickly & the service can be restored without further delay. Better service continuity has its own merits. As a protection of engineer, we should have to gain a best knowledge about protective device that are used in our distribution system. For a better understanding we can denote ourselves to the study of the protective device in a distribution substation. We can summaries the objective of the study in the following lines: 1. To Study of substation & substation equipment. 2. To Study of protective devices of a distribution substation. 3. To study of protective devices used in Vurulia 33/11kv distribution Sub-station. 4.

To develop suggestion for better protection.

Chapter-2 Brief description of substation and substation equipment Substation is an important part of power system. The continuity of supply depends to a considerable extent upon the successful operation of sub-station. It is therefore essential to exercise utmost care while designing and building substation. The following parts are important point which must be kept in view while laying out a substation.

2.1 WHAT IS SUBSTATION? Substation: The assembly of apparatus used to change some characteristic (e.g. Voltage, a.c to d.c, frequency, p.f. etc) of electric supply is called a substation.

Importance of substation: 1. It should be located at a proper site as far as possible it should be at the center of load. 2. It should provide safe and reliable arrangement for safety consideration must


given to the maintenance abnormal occurrence such as possibility of explosion or fire etc. For reliability, Consideration must be given for design and constriction. The provision of suitable protection gear etc. 3. It should involve minimum capital cost.

Classification of substation: According to service requirement: 1. Transformer substation. 2. Switching substation. 3. Power factor correction substations. 4. Frequency changer substations. 5. Converting substation. 6. Industrial substation. According to constructional feature the substation are classified as: 1. Indoor substation. 2. Out door substation. 3. Underground substation. 4. Pole mounted substation. 2.1.1 TRANSFORMER SUBSTATION Those substations which change the voltage level of electric supply are called transformer these substation receive power at some voltage and deliver it at some other voltage. Obviously, transformer will be the main component in such substation. Most of the substation in the power system is of type this.

2.1.2 POWER FACTOR CORRECTION SUBSTATION Those substations which improve the power factor of the system are called power factor correction substation. Such substation is generally located at the receiving end of transmission the power factor improvement equipment. 2.1.3 SWITCHING SUBSTATION These substations do not change the voltage level i.e. incoming and outgoing lines have the same the voltage. However they simple perform the switching operations of power lines.

2.1.4 FREQUENCY CHANGER SUBSTATION Those substations which change the frequency are known as change substation, such a frequency change may be rewired for industrial utilization.

2.1.5 CONVERTING SUBSTATION Those substation which change a.c power into d.c power are called converting substation. These substations which supply power with suitable apparatus (e.g. Ignitron) to supply for such purpose as traction, electroplating, electric welding. 2.1.6 INDUSTRIAL SUBSTATION Those substations which supply power to individual industrial concern is known as industrial substations.

2.1.7 INDOOR SUBSTATION For voltage upto 11kv the equipment of the substation is installed indoor because of economic consideration .However when the atmosphere is contain with impurities these substation can be erected for voltage upto 66kv . 2.1.8 OUT DOOR SUBSTATION: For voltage beyond 66kv ,equipment is invariably installed out door .It is because for such voltage the clearance between conductor and the space required for switch , circuit breaker and equipment becomes so great it is not economical to install the equipment indoor

2.1.9 UNDER GROUND SUBSTATION In thickly populated areas, the space available for equipment and building is limited and cost of land is high. Under such condition the substation is created underground. The reader may further discussion on underground substation. 2.1.10 POLE–MOUNTED SUBSTATION: This is an out door subs station with equipment installs over head on H-pole or 4-pole structure. It is cheapest from of substation for over voltage not exceeding 11KV (or 33 KV in the some case. Electric power is almost distributed in localities through such substation. For complete discussion on pole mounted substation. 2.2 EQUIPMENTS OF DISTRIBUTION SUBSTATION The following equipments are essential for a distribution sub-station: 1. Transformer 2. Power Transformer 3. Instrument Transformer 4. Current Transformer (C.T) 5. Voltage Transformer (P.T). 6. Isolator 7. Insulator 8. Line Support 9. Metering and Indicating Instrument 10. Bus bars 11. Power Factor Improvement Device. 12. Voltage Regulator 13. Earthing 2.2.1 WHAT IS TRANSFORMER? Transformer: A transformer is an electrical device that transfer energy from one circuit to another by magnetic coupling with no moving parts. A transformer comprises two or more coupled windings, or a single tapped winding and, in most cases, a magnetic core to concentrate magnetic flux. An alternating current in one winding creates a time varying magnetic flux in the core, which induces a voltage in the other windings. Transformers are

used to convert between high and low voltages, to change impedance, and to provide electrical isolation between circuits.

Figure 2.1 Transformer. 2.2.2 POWER TRANSFORMERS A power transformer is used in a substation to step-up or step-down the voltage. Except at the power station, all the subsequent substations use step-down transformers to gradually reduce the voltage of electric supply and finally deliver it at utilization voltage. The modern practice is to use 3-phase transformer in substation, although 3-phase bank of transformer can also be used. The use of 3-phase transformer (instead of 3 single phase bank of transformers) permits two advantages. Firstly, only one 3-phase load tap changing machine can be used. Secondly, its installation much simpler than the three single phase transformers. The power transformer is generally installed upon lengths of rails fixed on concrete slabs having foundations 1 to 1.5m deep. For rating unto 10MVA, naturally cooled, oil immersed transformers are used. For higher ratings, the transformers are generally air blast cooled.

Figure 2.2 Three phase power transformer. 2.2.3 INSTRUMENT TRANSFORMERS: The lines in substations operate at high voltages and carry current of thousands of amperes. The measuring instruments and protective devices are designed for low voltage (generally 110) and currents (about 5A). Therefore, they will not work satisfactorily if mounted directly on the power lines. The function of these instrument transformers is to transfer voltages or currents in the power lines to values which are convenient for the operation of measuring instruments and relays. There are two types of instruments. 1. Current Transformer (C.T). 2. Potential Transformer (P.T). 2.2.4 CURRENT TRANSFORMER (C.T) A current transformer in essentially a step-up transformer which steps down to a known ratio. The primary of this transformer consist of one or more turns of thick wire connected in series with the line. The secondary line consist of a large number of turns of fine wire and provides for the measuring instruments and relays a current which is a constant fraction of the current in the line. Suppose a current transformer rated at 100/5A is connected in the line to measure current. if the in the line is 100A, then current in the secondary will be 5A. Similarly, if current in the line is 50A, then secondary of C.T. will have a current of 2.5A. Thus the C.T. under consideration will step down the line current by a factor of 20. 2.2.5 VOLTAGE TRANSFORMER It is essentially a step down the voltage to a known ratio. The primary of this transformer consist of a large number of turns of fine wire connected across the line. The secondary winding consist of a few turns and provides for measuring instrument and relay a voltage which is a known fraction of the line voltage. Suppose a potential transformer rated at 66KV/110V is connected to a power line. If line voltage is 66KV, then voltage across the secondary will be 110V.

2.2.6 ISOLATOR Isolator is a disconnecting switch, which operate under no load condition. It has no any specified current breaking capacity or current making capacity. Isolator is not even used for breaking load current.

Figure 2.3: Isolator. In some case isolators are used for breaking charging currents of x-mission lines, Isolators are used in addition to circuit breakers while opening a circuit. The circuit breaker is opened first. Then isolator While closing a circuit the isolator are necessary on the supply side of circuit breakers in order to ensure isolation of the circuit breaker form live parts for the purposes of maintenance. The operating mechanism is manual plus one of the following: i. Electrical motor mechanism. ii. Pneumatic mechanism. 2.2.7 INSULATOR The insulator serves two purposes. They support the conductor and confined the current in the conductors. The most commonly used material for the manufacture of Insulator porcelain: There are several kinds of insulator (e.g. pin type, suspension type, post insulator etc.) and their use in the sub-station will depend upon the service requirement. For example, post

Insulator is used for bus bars. a post insulator consists of a porcelain body, cast iron cap and flagged cast iron base. The hole in the cap is threaded so that bus bars can be directly bolted to the cap. Types of line insulation: a. Pin type insulators.

d. Shackle insulator.

b. Suspension type insulators.

e. Stay insulator:

c. Strain insulators.

f. Guy insulator

Pin type insulators: Pin type insulators are used for transmission and distribution of electric power voltage up to 33KV.

. Figure 2.4 Pin type insulators. SUSPENSION TYPE INSULATORS For high voltage i.e. beyond 33KV transmission line, Suspension type insulators used. This type insulator consists of a number of porcelain discs connected in series by the metal links in the form of strength. The conductor is suspended at the bottom end of this string while the other end of the string is secured to the cross-arm of the tower. Each unit or discs is designed for 11KV. The number of discs in series would obviously depend upon the working voltage.

Figure 2.5 Suspension type insulators. STRAIN INSULATORS When there is a dead end of the line or there is corner or sharp curve, the line is subjected to greater tension. In order to relieve the line of excessive tension, strain insulators are used. For low voltage lines shackle insulators are used as strain insulators. For high voltage transmission lines, strain insulator consists of an assemble of suspension insulators. The discs of strain insulators are used in vertical plane.

Figure 2.6: Strain insulators GUY INSULATOR In Distribution Substation guy insulators are used in low voltage distribution. STAY INSULATOR For low voltage lines, the stays are to be insulated from ground at a height not less than 13 meters from ground.

SHACKLE INSULATOR Such insulators can be used either in a horizontal position or in a vertical position. They can be directly fixed to the pole with a bolt or to the cross-arm. The conductor in the groove is fixed with a soft binding wire.

Figure 2.7: shackle insulator 2.2.8 LINE SUPPORTS The supporting structure for overhead line conductors are various types of poles and tower called line supports. Classification of line supports: 1) Wooden poles. 2) Steel tower. 3) Reinforce concrete (RCC) poles. 4) Steel tubular pole. WOODEN POLES Wooden poles used for low voltage distribution purpose. The wooden poles generally tend to rote below the ground level, causing foundation failure. STEEL TUBULAR POLE It is used instead of wooden pole in urban area or town for increasing vision satisfactory. It is also stronger than the wooden pole. Such poles are generally used for distribution purpose in the cities. In BPDB steel tubular poles are used distribution system.

REINFORCE CONCRETE (RCC) POLES RCC poles have greater mechanical strength, longer life and permit longer spans than steel poles; they require little maintenance and have good insulating properties. In BPDB, RCC poles are used in 11KV and 33KV transmission systems. STEEL TOWER

For long distance transmission line at higher voltages, steel towers are invariably employed. Steel tower have greater mechanical strength, longer life can withstand most severe climatic conditions and permit the use of longer spans. In BPDB steel towers are used in single circuit and double circuit transmission line, which has about 132KV and 230KV.


There are several metering and indicating instrument (e.g. ammeters, volt meters energy meters etc.) install in a substation to maintain watch over the circuit quantities. The instrument transformer are invariably used with them for satisfactory operation

2.2.10 BUS-BAR: When a number of generator or feeders operating at the same voltage have to be directly connected electrically, bus-bar are used as the common electrical component. Bus-bars are copper rods or thin walled tubes and operated at constant voltage. Thus electrical bus bar is the collector of electrical energy from one location. The selection of any bus bar system depends upon the following: 1. Amount of flexibility required in operation. 2. Immunity from total shut-down. 3. Initial cost of the installation. 4. Load handled by the bus bar.

Classification of bus bar: 1. Single bus bar system. 2. Sectionalized bus bar. 3. Duplicate bus bar. 4. Ring bus bar. 5. One and half breaker arrangement. Arrangement of different types of bus bar and is advantages and disadvantages: SINGLE BUS BAR: C CB

CB Isolator

Isolator Single bus


CB Transform er

Transforme r

Figure 2.8: Single Bus bar. Advantages: 1. It is cheapest arrangement as only one circuit breaker for each outgoing circuit breaker is required. 2. The relaying on this system is simple. It should be noted that in this system the relaying on each of the circuit and the bus bar is only required. 3. Due to the absence of the transfer breaker and disconnections, the operation has become simple. For de-energizing a circuit only the associated circuit breaker is to be opened. 4. The maintenance cost, which is only dependent upon the number of breakers, will be appreciably low for a single bus bar system. Disadvantages: 1. The biggest disadvantages of this system is complete shut-down of the line in case of a bus bar fault. 2. It is not possible to have any regular maintenance work on the energized bus bar.

3. When a breaker on any circuit of a single bus bar system fails, the will be complete shutdown of the station , for however re-energizing first the effected circuit breaker is disconnected from the bus bar with the help of isolator . 4. For maintaining or repairing a circuit breaker, the circuit is required to be disconnected from the bus bar. 5. If any stage, a circuit is required to be added to the existing single bus bar arrangement, SINGLE BUS BAR SYSTEM WITH SECTIONALISATION:





Isolato rs CB





Figure 2.9: Single Bus bar system with Sectionalisation Advantages: 1. In this system, only one additional breaker will be needed, thus its cost in comparison to single bus bar system will not be much. 2. The operation of this system is as simple as that of single bus bar. 3. The maintenance cost of this system is comparable with the single bus bar. 4. For maintaining or repairs of the bus bar only one-half of the busber is required to be deenergized and possibility of complete shut-down is thereby avoided. 5. It is possible to utilize the bus bar potential for the line relays. Disadvantages: 1. On the bus bar fault, one half of the station will be switched off. 2. For regular maintenance also, one of the bus bar is required to be de-energized. 3. For maintaining or repairing a circuit breaker, the circuit is required to be isolates from the bus bar.

MAIN AND TRANSFER BUS ARRANGEMENT: Figure 2.10 Double bus bar systems with one circuit breaker per circuit Advantages: CB CB 1. It ensures supply in case of bus fault, in case of any fault in one of the bus, the circuit can be transferred to the transfer bus.

Main bus bar

2. The circuit breaker can be maintained with uninterrupted supply as the load can be transferred to the other bus through the bus coupler circuit breaker. Transfer bus bar 3. It is easy to connect the circuit from either bus.


4. The maintenance cost of substation decreased. 5. The bus potential can be used for relays.

Bus coupler


CB or expanded CB TheCB bus is maintained by transferring all ofCB the circuit to the transfer or auxiliary B bus depending upon the remote back up relays and breaker for eliminating faults of the

circuit’s. During this connection a line fault on any pf the circuits of the bus would shut down Transformer the entire circuit. CB

RING BUSBAR: Advantages: 1. It provides double feed to all the feeders at minimum cost. 2. At the time of failure of the circuit breaker of bus section only the effective circuit goes out of service while the heal by circuits are not affected . 3. The arrangement is quite economical as the number of breakers used is nearly the same as that of a single bus bar system. Disadvantages: 1.

The circuit has to be energized while the maintenance of the bus is carried out,

although it may be possible to arrange tripping of supply to the concerned feeder. 2. It is necessary to supply potential to relays separately to each of the circuit. 3. The operating of any section of the breaker may cause overloading of the circuits because power can flow in one direction only. 5. It is difficult to add any new circuit to the ring 2.2.11 POWER FACTOR:

Power factor: The cosine angle between voltage and current in an a c circuit is known as power factor. Causes of low power factor: Low power factor is undesirable for economic point of view. Normally, the power factor of the whole load on the supply system in lower than 0.8 .The following are the causes of low 1. Most of the c motor is of induction type which have low lagging power factor. This motor work at a power factor which is extremely small on light load (0.2 to 0.3) and rises to 0.8 or 0.9 at full load power factor: 2. Arc lamps, electric discharge lamp and industrial heating furnaces operate at low lagging power factor. 3. The load on the power system is varying, being high during morning and evening and low at other times. During low load period, supply voltage is increased which increases the magnetization current. This results in the decreased power factor. Methods of Power factor improvement: Normally, the power factor of the whole load on a large generating station is in the region of 0.8 to 0.9. However, sometimes it is lower in such cases it is generally desirable to take special steps to improve the power factor. This can be achieved by the following equipments a. Static capacitor. b. Synchronous condenser. c. Phase advancers. (A) STATIC CAPACITOR The power factor can be improved by connecting capacitor in parallel with the equipment operating at lagging power factor. The capacitor draws a leading current and partly or completely neutralizes the lagging reactive component of load current. This raises the power factor of the load. For three phase loads, the capacitor can be connected in delta or star as shown in figure. Static capacitors are invariably used for power factor improvement in factories.

Figure 2.11 Power factor improvements by Static capacitor

(B)SYNCHRONOUS CONDENSER: A synchronous motor takes a leading current when over excited and therefore, behaves as a capacitor. And over excited synchronous motor running on no load is called 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. (C) PHASE ADVANCERS: Phase advancers are used to improve 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 90째. 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 simply an a.c. exciter. The phase advancer is mounted on the same shaped as the main motor and is connected in the rotor circuit of the motor. It provides exciting ampere turn to the rotor circuit at slip frequency. By providing more ampere turns then required, the induction motor can be make to operate on leading power factor like an over excited synchronous motor. The electrical energy is almost exclusively generated, transmitted and distributed in the form of alternating current. Therefore the question of power factor immediately comes into picture. Most of the loads are inductive in nature and hence have low lagging power factor. The low power factor is highly undesirable as it causes an increase in current, resulting in additional losses of active power in all the elements of power system from power station generator down to the utilization devices. In order to ensure most favorable conditions for a supply system from engineering and economical standpoint, it is important to have power factor as close to unity as possible. 2.2.12 VOLTAGE REGULATOR

A voltage regulator is an electrical regulator designed to automatically maintain a constant voltage level. It may use an electromechanical mechanism, or passive or active electronic components. Depending on the design, it may be used to regulate one or more AC or DC voltages. With the exception of shunt regulators, all voltage regulators operate by comparing the actual output voltage to some internal fixed reference voltage. Any difference is amplified and used to control the regulation element. This forms a negative feedback servo control loop. If the output voltage is too low, the regulation element is commanded to produce a higher voltage. For some regulators if the output voltage is too high, the regulation element is commanded to produce a lower voltage; however, many just stop sourcing current and depend on the current draw of whatever it is driving to pull the voltage back down. In this way, the output voltage is held roughly constant. The control loop must be carefully designed to produce the desired tradeoff between stability and speed of response.

Figure 2.12 Voltage regulator 2.2.13 EARTHING: The word ‘earth’ or ‘ground’ means many different things to many electrical engineers. In an electrical installation these words can be used to mean either the protective conductor in a mains cord; the common bonding network of the building; the earth mass electrodes of the lightning protection system, or the conductor of the mains supply that is connected to an earth mass electrode at the distribution transformer. Method of Earthing

The useful method of earthing is to join the exposed metal to earth via continuity conductors connected to an electrode buried in the ground. Three elements required for earthing systems are Earth conductor, Earthing lead and Earth electrode. Earth conductor: This is the part of earthing system, which joins or bonds together all the metal parts of an installation. The earth conductor shall have a short tine capacity adequate for the fault current which can floe in the grounding conductors for the operating time of the system. The following table gives the minimum size of copper circuit conductor: Minimum cross-sectional area of the copper Earth conductors in relation to the area of associated phase conductors: Table -2.1 Cross-sectional




phase Minimum cross-sectional area of the

conductors (mm2) Less than 16

corresponding Earth conductors (mm2) Same as cross-sectional area of the phase

16 or greater but Less than 35 35 or greater

conductor but not less then 14 SWG. 16 Half of the cross-sectional area of the phase conductor.

Earthing lead: Earthing leads are the link, which provides connection between the earth conductors and the earth electrode. Earth electrodes: The Earth electrodes shall as far as practicable into permanently moist soil p-referable below associated ground water table. The resistance of earth electrodes shall be not more then one ohm. Some important thing for the Earth electrodes: -Copper rod shall have a minimum diameter of 12.7, GI pipes shall have a minimum diameter of 50mm Copper plates shall not be less then 600mm,6000 in size, with 6mm thickness. Chapter-3 Brief description of protective devices of a distribution substation

LIST OF PROTECTIVE DEVICES 1) Fuse 2) High rupturing capacity (H.R.C.) cartridge fuse 3) Relay 4) Circuit Breaker 5) Lightning Arrester 6) Auto Reclosur 7) Isolator 8) Earthing 9) Current Limiting Reactor 10) Insulator

3.1 FUSE A fuse is a short piece of metal, inserted in the circuit which is melt when excessive current flow through it thus breaks the circuit. The fuse element generally made of materials having melting point & conductivity. It is inserted in series with the circuit to be protected under normal condition it carries normal current without over heating. But abnormal condition the increasing current flow through the fuse, produce high temperature and then fuse element melt & disconnecting the circuit.

Figure 3.1: Drop out fuse Properties of a fuse element: The function of a fuse is to carry the normal current without overheating but when the current exceeds its normal value; it rapidly heats up to melting point and disconnects the circuit protected by it.

The fuse element should have the following desirable characteristics: 1. High conductivity. 2. Low melting point. 3. Least deterioration due to oxidation. 4. Low coast e.g. lead, tin, copper. 3.1.1 IMPORTANT TERM OF FUSE ELEMENT Current rating of fuse element: It is the current which the fuse element can normally carry with over heating or melting. It is depends upon the temperature rise of the contacts of the fuse holder, fuse material and the surrounding of the fuse.

Symmetrical Current

Cut off Current

Zero Current Fault Occurs Arcing Time

Pre Arcing time time

Total operating time

Figure 3.2: Cut off characteristics of fuse Fusing Current: It is the minimum current at which the fuse element melt and thus disconnects the circuits protect by it. It value will be more than the current rating of the fuse element. Mathematically it represent by the following I=kd3/2 Where, I = fusing current d = Diameter of the wire. And

K= Fuse constant. Its value depends upon the metal of which the fuse element is made. Sir W.H. Preece found for the different materials the value of k given in the table Table: 3.1 Sl. No. 1 2 3 4

Material Copper Aluminum Tin Lead

d in cm 2530 1873 405.5 340.6

Value of K d in mm 80 59 12.8 10.8

Fusing factor: It is the ratio of minimum fusing current rating of fuse element. Fusing factor =Minimum fusing current/current rating of fuse. Its value always more than one. The fusing factor is two fore semi enclosed or rewirable fuse. Pre –arching time: It is time between the commencement of fault and the instant when cut-off occurs. The pre-arching time is very low, typically 0.001 second. Cutoff current : It is the maximum value of fault current reached before the fuse melts. The cut off value depends upon a. Current rating of fuse b. Value of prospective current c. Asymmetry of short circuit current. Prospective current: It is the r.m.s. value of the first loop of the fault current obtained if the fuse is replaced by an ordinary conductor of negligible resistance. Arcing time: This is the time between the end of pre-arching time and instant when the arc is extinguished. Operating time: It is the sum of pre-arcing and arcing time. It may be operating time of a fuse is generally quit low (say 0.002sec) as compared to a circuit breaker (say 0.2sec.). Breaking capacity: It is the r.m.s. value of a.c. component of maximum prospective current that a fuse can deal with a rated service voltage.

3.1.2 TYPE OF FUSE 

Low voltage fuse a. Semi-enclosed rewirable fuse b. High rupturing capacity cartridge fuse c. H.R.C. fuse with tripping device.


High voltage fuse a. Cartridge type fuse. b. Liquid type fuse. c. Metal clod fuse.

3.1.3 DESCRIBE DIFFERENT TYPES OF FUSE Low voltage fuses a) Semi enclosed rewirable fuse Semi enclosed rewirable fuse are made up to 500A rated current, but their breaking capacity is low about 4000A on 400V service. The use of this type of fuse is domestic and lighting loads and used where of fault current are to be interrupted. It consist of 1. A base and 2. A fuse carrier. The base is porcelain and carries the fixed contacts to which the incoming and outgoing phase wires are connected. The fuse carrier is also of porcelain hold the fuse element between its terminals. b) High rupturing capacity (H.R.C.) cartridge fuse Filling powder

Fuse Element

Outer Element Fuse link contct

Brass end plate


Figure 3.3: High rupturing capacity (HRC) Cartridge fuse

The H.R.C. fuse consist of a heat resisting ceramic body having metal end cap to which is welded silver current-carrying element .The space within the body Surrounding the element is completely packed with a filling powder. The filling material may be chalk, plaster of pairs, quartz or marble dust and acts as an arc quenching and cooling medium. Under normal condition, the fuse element is at a temperature below its melting point, it carries normal current without overheating. When fault occurs the current increases and the fuse element melts. Advantage of High rupturing capacity (H.R.C.) cartridge fuse i)

They are capable of clearing high as well as low fault currents.


They have high speed of operation.


They provide reliable discrimination.


They require no maintenance.


They provide reliable discrimination.


They permit consistent performance.

vii) They do not deteriorate with age. Disadvantage of High rupturing capacity (H.R.C.) cartridge fuse i) Heat produced by the arc may affect the associated switches. ii) They have to be replaced after each operation. c) H.R.C. fuse with tripping device Some times H.R.C. fuse provided with a tripping device .When the fuse blow under fault condition the tripping device cause the circuit breaker to operate. The body of the fuse is of ceramic materials with a metallic cap rigidly fixed at each end. These are connected by a number of silver fuse elements. At one end is a plunger, which under fault condition electrically connected through a fusible link, chemical charge and a tungsten wire to the other end of the cap as shown. Plunge r

Chemical Charge

Weak link Tungsten Shunt

Silver fuse element

Figure 3.4: H.R.C Fuse When a fault occurs the silver fuse elements are the first to be blown out and then current is transferred to the tungsten wire. The weak link in series with the tungsten wire gets fused and causes the chemical charges to be detonated. This forces the plunger outward to operate the circuit breaker. Low voltage H. R.C. fuses may be built with a breaking capacity of 1600A to 3000A at 440v. HIGH VOLTAGE FUSES Some type of the high voltage fuses are: a) Cartridge type High voltage cartridge fuse are used up to 33Kv with breaking capacity of about 8700A at that voltage. Rating of the order of 200A at 6.6kv and 11kv and 50A at 33kv are available. In this device there are two fuses element in parallel; one of low resistance (silver wire) and other of high resistance (tungsten wire). Under normal condition, the low resistance element carries the normal current. And a fault condition the low resistance element blown out and the high resistance element reduce the short circuit current and finally breaks the circuit. b) Liquid type The liquid type of fuse are used for circuit up to about 100A rated current on system up to 132kv and may have breaking capacities of the order of 6100A. it consist of glass tube filled with carbon tetrachloride solution and sealed at both with brass caps .The fuse wire is sealed at one end of the tube and the other end of the wire is held by a strong phosphor bronze spiral spring fixed at the other end of the glass tube. When the current exceeds the limit the fuse wire is blown out. 3.1.4 ADVANTAGES & DISADVANTAGES OF LOW VOLTAGE FUSE Advantage: 1.

Low cost.


They require no maintenance.


They have high speed.


They are cheaper then other circuit interrupting devices of equal breaking capacity.

Disadvantage: 1.

This type of fuse has a low –breaking Capacity and hence cannot be used in circuit of fault level.


There is a possibility of renewal by the fuse wire of wrong size.


They have to be replaced after each operation.


Heat produced by arc may affect the associated switches.


The fuse operates of a lower current than originally rated.

3.2 RELAY In a power system consisting of generator, transformer, transmission and distribution circuits. It is inevitable that sooner or later some failure will be occurring some where in the system. When a failure occurs on any part of the system it must be quickly detected and disconnected from the system. There are two principle reasons for it firstly if the fault is not clear quickly. It may cause unnecessary interruption of service to the customers. Secondly rapid disconnection of fault apparatus limits the amount of damage to it and prevents the effects of fault form spreading in to the system. The detection of fault and disconnection of a faulty section or apparatus can by relays in conjunction with C.B.

Figure 3.5: Protective Relay

A protective relay is a device that detects the fault and initiatives the operation of the circuit breaker to isolate the defective element from the rest of the system. The protective relay should have the following quantities I) Selectivity

ii) Speed

iii) Sensitivity

iv) Reliability

v) Simplicity

VI) Economy

Selectivity: It is the ability of the protective system to select correctly that part of the system in trouble and disconnect the faulty part without disturbing the rest of the system. Sensitivity: It is the ability of the relay system to operate with low value of actuating quantity. Sensitivity of relay is a function of the volt –amperes input to the coil of the relay necessary to cause its operation. The smaller value of the volt –amperes input is the more sensitive relay. Reliability: It is the ability of the relay system to operate under the pre-determine conditions. Without reliability the protection would be simple rendered largely ineffective and could even become a liability. Simplicity: The relay system should be simple so that it can be easily maintained. The simpler the protection system, the greater will be its reliability. Economy. The most important factor in the choice of a particular protection scheme is the economic aspect. Speed: The high speed relay system decreases the possibility of development of one type of fault into the other more sever type. 3.2.1 BASIC PRINCIPLE OF RELAY Bus bar

trip coil





Figure 3.6: construction of relay

This diagram shown one phase of 3-phase system for simplicity. The relay circuit connections can be divided into three parts. 1.

First part is the primary wiring of a current transformer. Which is connected in series with the line to be protected.


Second part consists of secondary winding of C.T. and the relay operating coil.


The third part is tripping circuit which may be either ac or dc.

When a short circuit occurs of on the transmission line, the current flowing in the line increasing to an enormous value this results in a heavy current flow through the relay coil causing the relay to operate by closing its contacts. This in turn closes the trip circuit of the breaker, making the circuit breaker open and isolating the faulty section from the rest of the system. 3.2.2 IMPORTANT TERMS OF THE RELAY Pick up current: It is the minimum current in the relay coil at which the starts to operate. Pick up current =Rated secondary current of CT ×Current setting Current setting: It is often desirable to adjust the pick – up current to any required value. This is known as current setting and is usually achieved by the use of taping on the relay operating coil. Plug – setting multiplier (P.S.M) : P.S.M = =

Fault current in relay coil Pick –up current Fault current in relay coil Rated secondary current CT ×Current setting

Time –setting multiplier: A relay is generally provided with control to adjust the time of operation. This adjustment is known as time – setting multiplier. 3.2.3 TYPES OF RELAY According to the measurement the relay may be classified as follows 1.

Over current relay


Over voltage relay.


Under current relay


Under voltage relay.

According to the structure the relay can be classified as follows: 1.

Induction type relay


Attracted armature type relay


Balance beam relay


Salient and plunger type relay [Impedance].


Gas operated relay [Buchholz] .


Induction disk relay [Electromagnetic]


Rectifiers relay


Moving coil and moving iron relay [Electromagnetic].


Electro dynamic relay.


Static electronic circuit measurement relay.


Microprocessor Based digital static relay


Directional and reverse power relay.

Functional relay types: 1.

Induction type reverse power relay.


Induction type over current relay.


Differential relay


Distance relay


Tran slay scheme



Attracted armature type relay:

The schematic arrangement of an attracted armature type relay. It consists of a laminated electromagnetic M, cussing a coil C and a pivoted laminated armature . The armature is balanced by a counter weight and consist a pair of spring contract fig at the end. It is basically a single actuating quantity relay.

Fig 3.7: Attracted armature type relay Under normal operating condition the current through the relay coil C is such that counter weight holds the armature in the position shown. However when a short circuit occur, the current through the relay coil increases sufficiently and the relay armature is attracted upward. The contacted on the relay armature bridge a pair of stationary contact attached to the relay frame. This complete the trip circuit which results in the opening of the circuit breaker and therefore in the disconnection of the faulty circuit. Distance or impedance Relay: The operation of this relays discussed so far dependent upon the magnitude of current of power in the protective circuit. However there is another group of relays in which the operation is govern by the ratio of applied voltage to current in the protective circuit such relays are called distance relays. The relay will operate when the ratio V/I are less than a predetermined value.

Fig 3.8: Distance or Impedance Relay Types of distance relay:

1. Definite distance relay which operates instantaneously for fault up to a predetermined distance from the relay. 2. Time distance relay in which the time of operation is proportional to the distance of fault from the relay point. A fault nearer to the distance relays are produced by modifying either than a fault farther away from the relay. Definite distance relay: Figure shows the schematic arrangement of a definite distance relay. The relay is designed that the torques produced by the electromagnets are in the opposite direction. Under normal operating conditions, the pull due to the voltage element is greater than of the current element. Therefore, the relay contacts remain open. However, when a fault occurs in the protected zone, the applied voltage to the relay decreases whereas the current increases. The ratio of voltage to current falls below the predetermined value. Therefore, the pull of the current element will exceed that due to

Figure 3.9: Definite distance type impedance relay The voltage element and this causes the beam to till in a direction to close the trip contacts. The pull of the current element is proportional to I2 and that of voltage element to V2 consequently the relay will operate when, Κ1V 2 < Κ2 I 2 V2 K or , 2 < 2 I K1 V I


K2 K1

or , Z


K2 K1

or ,

Induction disc relay:

Electromagnetic induction relays operate on the principle of induction motor and are widely used for protecting relaying purpose involving a.c. quantities. They are not used in d.c. quantities owing to the principle of operation. An induction relay essentially consists of pivoted aluminum is placed in two alternating magnetic fields of the same frequency but displaced in time and space. The torque is produced in the

Fig 3.10: Shaded pole construction Disc by the interaction one of the magnetic fields with the currents inducted in the disc by the other. Induction type relay: This relay has two four or more electromagnetic in stator .This is energized by the relay coils. The rotor consists of a hollow metallic cylindrical cup. The rotor is free to rotate in the gap between the stationary iron and the electromagnets. In this type of relay, the eddy currents are produced in the metallic cup. This current interacts with the flux produced by the other electromagnetic and torque is produced. The theory is similar to that of the disc type induction relay. Balance beam relay: The schematic arrangement of a balance relay. It consists of an iron armature fastened to a balance beam. Under normal operating conditions, the current through the relay coil in such that the beam is healed in the horizontal position by the spring. When the fault condition the current through the relay coil becomes greater than the pickup value and the beam is attracted to close the trip circuit. This causes the opening of the circuit breaker.

Fig 3.11: Balance beam relay

Gas operated (buchholz) relay:

Figure 3.12: Gas operated (buchholz) relay Buchholz relay is a gas – actuated relay installed in oil immersed transformer for protection against all kinds of fault .It used to give an alarm in case incipient. When fault is disconnect the transformer from this supply in this system .It is usually installed in the pipe connecting the conservator to the main tank. It is use for excess of 750KVA. Rectifier relay:

The moving coil relays are used with rectifier relays in such relays the quantities to be measured are rectified and then feed to the moving coil unit. The rectifier relay is not possible to against the high measuring speed but faster then the mechanical relays. Since moving coil has a very small mass.

Figure 3.13: Rectifier relay for one and two quantity Permanent magnet moving coil type relay: In this type relay the coil is free to rotate in the magnetic field of a permanent. The actuating current flows through the coil. The torque is produced by the interaction between the field of the permanent and field of the coil.

Directional over current relay: The Directional power relay is unsuitable for use as a directional protective relay under short circuit conditions. When a short circuit occurs the system voltage for to a low value and there may be insufficient torque developed in the relay to cause it Operation. This difficulty is overcome in the directional over current relay, which is designed to be almost independent of system voltage and power factor.

Figure 3.14: Induction directional over current relay Under normal operating condition, power flows in the normal direction in the circuit protected by the relay. Therefore, directional power relay does not operate, thereby keeping the over current element unenergized. However, when a short circuit occurs, thereby keeping the over current element rotates to bridge the reverse direction. Should this element rotate and the moving contact attached to it closes the trip circuit. This operates the circuit breaker which isolates the faulty section.

Static Relay

Figure 3.15: Block diagram of a static relay-simplified Static













electronic/magnetic/optical or other components, without mechanical motion of components. However additional electromechanical relay units may be used in output Stage as auxiliary relays. A protective system is formed by static relays and electromechanical auxiliary relays. Figure illustrates the essential components in a static relays. The output of CT’s of PT’s or transducers is rectified in rectified in rectifier. The rectified output is fed to the measuring unit. the measuring unit compromises comparators, level detectors, filters logic circuits. The output is initiated When input reaches the threshold value. The output of measuring unit is amplified by an Amplifier. 3.3 CIRCUIT BREAKER During the operation of power system, it is often desirable and necessary to switch on or off the various circuits under both normal and abnormal conditions. In earlier days this function used to be performed by a switch and a fuse placed in series with the circuit. However such a means of control presents two disadvantages. Firstly when a fuse blows out, it takes quite some time to replace it and restore supply to the customers. Secondly a fuse cannot successfully interrupt heavily fault currents that result from faults on modern high voltage and large capacity circuits. With this disadvantage of power system the lines and other equipment operate at very high voltage and carry large currents. The arrangement of switches along with fuse cannot serve the desired function of switchgears in such high –capacity

circuits. This necessitates employing a more dependable means of control such as is obtained by the use of circuit breakers.

Figure 3.16: Circuit breaker

A circuit breaker is a piece of equipment, which can 1.

Make or break a circuit either manually or by remote control under normal condition.


Brake a circuit automatically under fault condition.


Make a circuit either manually or remote control under fault conditions.

Circuit breaker are mechanically designed to close or open contact members, thus closing or opening an electrical circuit under normal and abnormal condition. 3.3.1


Trip coil

C.T. Moving

Fixed contact

Open Close

Handle automatic mechanism for opening and closing the C.B.

Figure 3.17: Basic operations of circuit breaker The figure represents an elementary schematic diagram of CB. It consists of fixed contact and a sliding contact in to which mores a moving contact. The end of the moving contact is attached to a handle which can be manually or it can be operate automatically with the help of a mechanism which has trip coil energized by the secondary of the current Transformer generally called current transformer. The power supply is brought to the terminals the e.m.f induced of the C.B. Under normal working condition the e.m.f induced in the secondary winding of CT is not sufficient to energize the trip coil fully for the operation. But under fault condition the abnormally high value of current on the primary circuit of CT induced a sufficient e.m.f in the secondary circuit to energized the trip coil so as to recluse the handle mechanism which open the C.B. 3.3.2


The power dealt by the circuit breaker is quite rage and sere as an important link between the consumers and supplies. The following are the necessary requirements for a circuit breaker 1. It must be safely interrupt the normal working current as well as the short circuit current. 2. After occurrence of fault the switchgear must isolate the faulty circuit as quickly as possible i.e. keeping the daily to minimum. 3. It must have been sense of discrimination. i.e. in systems where an alternate arrangements have been made for continuity of supply it should isolate the only faulty circuit without effecting the healthy one. 4. It should not operate when the over current flows under healthy condition. 3.3.3


Arc voltage: It is the voltage that papers across the contacts of circuit breaker during the arcing period.

Restriking voltage : It is the transient voltage that appears across the contact of on near current zero during arcing period.

Opening time: The time internal lapsed from the energization of the trip coil to the instant of contact separating is called opening time; it depends on the magnitude of fault current. Arching time: The time from separation of contact to the extinction of the are is called the arching time; it depends on the magnitude of fault current as well as the voltage available to maintain the arc and upon the mechanism used for extinguishing the arc. Total Break time: The sum of the opening time and arching time called the total break time. 3.3.4


A circuit breaker may be called upon to operation under all conditions. However major duties are imposed on the circuit breaker when there is a fault on the system in which is connected under fault condition a circuit breaker is required to perform the following three duties. 1.

It must be capable of opening the faulty circuit and breaking the fault condition.


It must be capable of carrying fault current for a short time while another circuit breaker is clearing the fault.


It must be capable of being closed on to a fault.

According to the duties mentioned above, the circuit breakers have three ratings.


Breaking capacity.


Making capacity


Short –time rating.

Breaking capacity: It is the current (r.m.s.) that a circuit breaker is capable of braking at given recovery voltage and under specified conditions. Breaking capacity is expressed as the breaking current. Which are determined from following method. Let at the instant of separation of contacts. A.C. component of short circuit current wave = x D.C. component of short circuit current wave = y Now symmetrical breaking current

= r.m.s. value of a.c. component of short circuit current

= x/√2 A symmetrical breaking current

= r.m.s. value of the total current (A.C & D.C). 2

 x   + y2  2

= 

The breaking capacity is generally expressed in terms of MVA and is equal to the product of rated breaking current in kA amp, rated voltage in and a factors which depends upon the number of phase (1-for single phase and √ 3 for three phase) Hence the breaking capacity for a 3 phase CB whether Symmetrical or Asymmetrical = 3 ×V × I ×10 −6 MVA Where I am the rated breaking current in amperes and V is the rated service line voltage in volts. Making capacity: The peak value of current during the first cycle of current wave after the closure of circuit breaker is known as making capacity. In other words the making current is equal to the maximum value of asymmetrical current. By √2 to convert this value we must multiplication factor becomes √2 ×1.8=2.55 Making capacity =2.55×Symmetrical breaking capacity.

Short –time ratting: It is the period for which the circuit breaker is able to carry fault current while remaining closed with the CB in its normal condition for three seconds. If the ratio of symmetrical breaking current to normal current is less then 40 for 1second otherwise. Normal current rating: It is the r.m.s. value of current, which the circuit breaker is capable of carrying continuously of its rated frequency under specified condition .The only limitation in this case is the temperature rise of current carrying parts. 3.3.5


The Circuit breakers can classify that’s follows. 1. Interrupting Medium

2. Operation

3. Service

4. Action

5. Contacts

6. Tank construction

7. Method of Control

8. Mountings.

Interrupting Medium: 1. Air circuit breaker

2. Oil circuit breaker

3. Air blast circuit breaker

4. Magnetic circuit breaker

Service: a. Indoor circuit breaker. b. Outdoor circuit breaker. Contact 1. Wedge circuit breaker

2. Butt circuit breaker

3. Bayonet circuit breaker

4. Laminated circuit breaker

5. Deon grid circuit breaker Operation 1. Gravity close circuit breaker 2. Gravity control circuit breaker 3. Horizontal break circuit breaker Action 1. Non automatic circuit breaker


Automatic circuit breaker

Method of control 1. Remote control

4. Electrical

2. Pneumatic circuit breaker

a. Motor circuit breaker

3. Manual circuit breaker

b. Solenoid circuit breaker

Mountings a. panel mounting circuit breaker

b. Remote for panel circuit breaker

c. Rear of panel circuit breaker.

3.3.6. DISCRIPTION OF DIFFERENT TYPES CIRCUIT BREAKER AND THERE RATING: Air –break Circuit Breaker: Air break circuit breaker is used dc circuit’s ac circuit up to 12 KV. Air breaker circuit breaker are generally indoors type and installed on vertical panels or indoor draw-out type switchgear. Ac air breaker Circuit Breaker are widely used indoor medium voltage and low voltage switchgear.  Typical reference values of rating of air –breaker Circuit Breaker are 460V









 Typical rating of dc Air –break C.B. 1500V, 10KA continuous, 80KA breaking.  Typical rating of low voltage air –breaker ac circuit breakers Normal current rating



Rated voltage



Breaking current



p.f.=0.15 Miniature C.B. These are used extensively in low voltage domestic, commercial and industrial applications. They replace conventional fuse and combine the features of a good HRC

fuse and a good

switch for normal operation it is used on a switch during over load or faults it automatically trip off.

Typical rating of MCB Current rating: 5,10,15,20,30,40,50,60Amp also 0.5,1,2,2.5,3,3.5,6,7,7.5,8,10,12,25,35,45,55Amp Voltage rating 240V/415V AC, 50V/11V DC Rupturing capacity AC

: 3Ka at 415V


: 3Ka at 50V (non inductive) 1KA at 110 V (non inductive)

Air blast circuit breaker: Air blast CB are used in high voltage from 11KV for to1100KV various application. They offer several advantages such as faster operation, suitability for repeated operation, auto-reclosure, unit type multi –break construction, modest maintenance, etc. air blast circuit breakers are especially suitable for railways and are furnaces,

Where the breaker operates repeatedly . Air blast circuit breakers are used for interconnected lines and important lines where rapid operation is desired. In air blast circuit- breaker high pressure air is forced on the arc through a nozzle at the instant of contact separation. The ionized medium between the contacts is blown away by the blast of the air. After the arc extinction the chamber is filled with high pressure air, which prevents restrike.

Figure3.18: Flow of air around contacts, in air-blast circuit breakers

Typical rating of Air blast circuit breaker is: Table-3.2










3 cycle


40KA, 50KA

2.5 cycle


40KA, 50KA, 63.5KA

2 cycle

Plain break oil circuit breaker: A Plan break oil circuit breaker involves simple process of separating contacts under the whole of the oil in the tank. There is no special system for arc control other than the increasing length cause by the separation of contact the arc extinction occurs when a certain critical gap between the contacts is reached.

Fig 3.19: Plain break oil circuit breaker Sulphur Hexafluoride (SF6 ) C.B : Sulphur Hexafluoride CB is used in high voltage up to 245 KV.Figure shows the parts of a typical SF6 circuit breaker. In the closed position of the breaker , the contact remain surrounded by SF6 gas at a pressure of about 2.8 kg/cm2.When the breaker operates, the moving contact is pulled apart and an arc is struck between the contacts. The movement of the moving contact is synchronized with the opening of a valve which permits SF 6 gas at

14kg/cm2 pressure from the reservoir to the arc interruption chamber. The high pressure flow of SF6 rapidly absorbs the free electrons in the arc path to form immobile negative as charge carriers. The result is that the medium between the contacts quickly builds up high dielectric Strength and causes the extinction of the arc .After the breaker operation, the valve is closed by the action of a set of springs.

SF6 Gas inlet

Insulated rod for operating moving member moving member contact

arcing horn fixed member

arc chember Gas outlet

Fig 3.20: Sulphur Hexafluoride (SF6) C.B. Minimum oil C.B.:

In minimum oil CB, dielectric oil is used as an arc quenching medium and dielectric medium. For voltage up to 36kv, minimum OCB are generally enclosed in draw out type Mattel clad switch. For 36kv, 7.2kv, 12kv36kv,72.5kv,145kv,245kv,420kv.

Figure 3.21: Minimum oil Circuit Breaker Minimum oil circuit breakers have the following demerits: 1.

Short contact life.


Larger arcing time for small currents.


Frequent arching time for small currents..


Possibility of Explosion


Prone to restricts

Vacuum circuit breaker: Vacuum CB are used in medium voltage. Voltage 11KV to 36KV The range of vacuum switching device includes vacuum interrupted 3.6/7.2/12/36KV for indoor metal clad. Vacuum conductors rated 1.2/3.6/7.2KV for outdoor porcelain housed.

Figure 3.22: Cut away view of vacuum circuit breaker

Fig shows the parts of a typical vacuum circuit breaker. It consists of fixed contact, moving contact and arc shield mounted inside a vacuum chamber. When the breaker operates, the moving contacts separates from the fixed contacts and the arc is contact The production of arc is due to the ionization of metal ions and depends very much upon the material of contacts. The arc is quickly extinguished because the metallic vapour, electrons and ions produced during arc are diffused in a short time and seized by the surfaces of moving and fixed members and shields.

3.3.7 SELECTION OF CIRCUIT BREAKER Two of the circuit breaker rating which require the computation of SC circuit are rated momentary current and rated symmetrical interrupting current. Symmetrical SC current is obtained by using sub transient reactance for synchronous machines. Momentary Current (r.m.s.) is then calculated by a multiplying the symmetrical momentary current by a factor of 1.6 to account for the present of dc offset current. Symmetrical to be interrupted is computed by using sub transient reactance for synchronous generators and transient reactance for synchronous motor –induction motors are neglected. The dc off-set value to be added to obtain the current to be interrupted is accounted for multiplying the symmetrical SC current by factors as tabulated below: Table -3.3 Circuit Breaker Speed

Multiplying factor

8 Cycles or slower


5 Cycles


3 Cycles


2 Cycles


If SC MVA (explained below) is more than 500, the above multiplying factors are increased by 0.1 each. The multiplying factor for air breakers rated 600 lower is 1.25. The current that a circuit breaker can proper at to the operating voltage over a certain range. i.e. Rated Voltage Amperes at operating voltage =Ampere at rated voltage ×Operating Voltage

Of course operating voltage cannot the maximum design value. Also no matter low the voltage is the rated interrupting current. Over this range of voltage .the product of operating voltage and interrupting current is constant. It is therefore logical as well as convenient to express the circuit breaker rating in terms of....MVA that can be interrupted defined as Rated interrupted MVA(3-phase)capacity = √ 3 × Vline(rated) × Iline(rated)

Where V (line is in KV and I (line) is in KA. Thus instead of computing the SC current to be interrupted. We compute three phase SC MVA to be interrupted

Where, SC MVA (3 phase) = √ 3× per unit values on a three –phase basis. SC MVA (3phase) =V per fault x Isc x (MVA) base Obviously, rated MVA interrupting capacity of a circuit breaker is to be more than (or equal) the SC MVA required to be interrupted for the selection of circuit breaker for a particular location. We must find the maximum possible sc MA to be interrupted with respect to type and location of fault through rate is generally the one exception is an L-G (line to ground) fault close to synchronous generator. In a simple the fault location which gives the highest sc MVA may be obvious but in larger system various problem location bust be tried to obtain the highest sc MVA requiring.

3.4 LIGHTNING ARRESTER: In order to protect the over voltage are surge voltage a protecting device is used which is called a lighting arrester. It is a most important protecting device of power system. Lighting arrester consists by a spark gape in series with non-linear resistor. Its upper terminal connects the power circuit and lower terminal are grounded. Types of lightning arrester 1.

Road gap lightning arrester.


Valve type lightning arrester.


Horn gap lightning arrester.


Oxide film lightning arrester.


Multi gap lightning arrester.


Electrolyte lightning arrester.


Expulsion type lightning arrester. 8.

Burke lightning arrester.

Describe different type of lightning arrester: 3.4.1 ROD GAP LIGHTNING ARRESTER This is the simplest form of surge divider consisting of two of road with ends facing- other. One connected to line and the second connected to earth. These are usually connected across the bushing of various equipment .A typical rod gaps across a transformer bushing.

To avoid cascading across the insulator surface of very step fronted waves, the rod gap should be set to break down of about 20% below the impulse spark over voltage of the insulation of the pint where it is insulated. To protect the insulator from the arc, the distance between the rod gap and insulator should be more than one third of rod gap length. The spark over take place at very high voltage due to lightning surges but it cannot flash over at usual power supply voltage .The difficulty with the gap arrester is that once the spark having taken place may continue for some time even at low supply voltage .To avoid if a current resistance is used in series with the or which limit the current to such an extent that it is sufficient to maintain the arc. Another difficulty is that the rod gaps are liable to be damaged due to high temperature of the arc, which may cause the rods to melt .The performance of rod gap is badly affected due to climate and also the polarity of the surge.

Figure 3.23: Rod gap lightning arrester 3.4.2 HORN GAP LIGHTNING ARRESTER This was one of the earliest type of surge diverts to be developed and is still used to certain a low voltage lines an low voltage lines an account of its great simplicity. It consists two horn shaped pieces of metal separated by small air gap, and connected in shunt between each conductor and earth. The distance between two electrodes is such that the normal voltage between the line and earth is insufficient to jump the gap, but abnormally high voltage will

break down the gap and so find a path to earth. The arc thus formed by reason of heated air an electromagnetic action will rise up arc. Usually a choking coil consisting of several turns of bare copper wire is connected in the line

Figure 3.24: Horn gap Lightning arrester Between the arrester and the apparatus to be protect to reflect travailing waves back an to the horns. 3.4.3 EXPULSION TYPE LIGHTNING ARRESTER It consists of 1. A tube made of fiber, which is a very effective gas evolving material. 2. An isolating spark gap and 3. An interrupting spark gap inside the fiber tube. During operation arc due to the impulse spark over inside fiber tube case some fibrous materials of the tube volatized in the form of gas, which is expelled through a event form the bottom of the tube, thus extinguishing the arc just link in the circuit breakers. Since the gases generated have to be expelled. One of the electrodes is hollow and the diverted is open of its lower end.

Figure 3.25: Expulsion type lightning arrester 3.4.4 THYRITE LIGHTNING ARRESTER Thyrite arrester is most common and is mostly used for the protection against high dangerous voltages. If operates on the fact that thirties ,a dense inorganic compound of ceramic nature has high resistance decreasing rapidly from high value to low value for current of low value to the of high value. The current increase 12.6 times a doubling the voltage. It consists of discs of 15 cm diameter and 19mm thickness both the sides are metal sprayed so as to give electrical contact between consecutive discs. These discs assembled inside the gal-aged porcelain container. When lighting takes places, voltage is raised and break down of the gap occurs .The resistance falls to very low value and wave is discharged to earth, after the surge has passed the thirties again comes back to its original position.

Figure 3.26: Thyrite lightning arrester


It operates and the fact certain chemicals have the property to change rapidly from a good conductor to almost perfect insulator when slightly heated. For example lead per oxide that has a specific resistance of 25 ohm per mm cub of normal temperature, become red lead of about 1500c and has specific resistance of the order of 600 mega- ohm per mm cub. It consists of 2.4 mm plants of lead per oxide with a thin porous coating of lethargies arranged in column and enclosed in tube of diameter of about 6 cm and height of 5 cm per kv of rating. Out of the two leads of the arrester upper is connected to the line, where the lower is connected not the earth. The tube contains a series spark gap. When an over –voltage occurs an arc passed through the series spark gap and additional voltage is applied to the pellet column and a discharge takes places.

3.4.6 ELECTROLYTE ARRESTER It operate an the fact that a thin film of aluminum hydroxide deposit on the aluminum plates immersed in electrolyte acts as a high resistance to low voltage but a low resistance to voltage above a critical value. Voltage more than 400V a puncture and flow of current earth. 3.4.7 VALUE TYPE LIGHTNING ARRESTER This type of lightning arrester is very cheap effective and robots an its therefore extensively used now a day for high voltage systems. This consists of a number of flat discs of a porous material stacked are above the other and separated by thin mica ring. As the materials of the discs is not homogeneous and conducting materials has also been added, therefore the glow discharge occurs between the discs of the of over voltage. This discs are arranged in such a way that normal voltage may not caused the discharge to occur. The mica ring provides insulation during normal operation. 3.4.8 MULTIGAP ARRESTER Multigap Arrester consist of a series of metallic (generally alloy of zinc) a cylinders insulated from one another and separated by small intervals of air gaps the first cylinder in the series is

connected to the line and the other to the ground through a series resistance. The series resistance limits the power arc.

Figure 3.27: Multigap Lightning arrester Under normal condition the point be is at earth potential and the normal supply voltage is unable to break down the series gap on the occurrence of an over voltage the break down of series A to B occurs. The heavy current after breakdown will choose the state through path to earth via the shunted gap B and C. Instead of the alternative path through the shunt resistance when the surge is over the arc B to C go out and any power current flowing the surge is limited to resistance which are now in series. 3.5 AUTO RECLOSUR

Many fault an overhead transmission lines are transient in nature. Statically evidence shows that about 90% of faults an overhead transmission lines are caused by lightning or by passing of object near or through lines. These condition results in arcing faults and the arc in the fault can be extinguished by the simultaneous opening of circuit breakers on both ends of the lines or a one end of the line. Since the cause of transient faults mentioned above disappears after a short time the circuit breaker can be recluse as soon as the arc in fault has been extinguished. The auto reclosure trips open three times when circuit has any fault. Operating sequence of auto Reclosure Fault occurs

Breaker Trips If fault persist

Trips open

Breaker Recloses

If fault is cleared

Remain Closed

Fig 3.28: Sequence of auto reclosure 3.6 ISOLATOR Isolator is a disconnecting switch which operates under no load conditions; it has no specified current breaking capacity. Isolator is not even used for breaking current. In some case isolator are used for breaking charging current of transmission lines. Isolator is used in addition to circuit breaker is opened first then isolator. While closing a circuit the isolator is necessary on the supply side of circuit breakers in order to ensure isolation of the circuit breaker from live parts of the purpose of maintenance .The operating mechanism manual plus one of the following. a.

Electrical motor mechanism


Pneumatic mechanism.

Types of construction of Isolators 1.

Vertical break type


Horizontal break type, either center-break or double –break


Vertical pantograph type.

The vertical pantograph type design is preferred for rated voltage of 4200 KV and above .The other types of designs are used from 12 to 420 KV. Isolator does not have breaking and making capacity .Rating of the isolator is similar to the corresponding terms of high voltage A.C circuit breakers.

Figure 3.29: Isolator 3.7 EARTHING Earthling or Grounding is most important and simple protection of electrical system earthling is mainly two types. 1.

Alternator and Transformer neutral earthling.


Non current carrying / metallic body.

3.7.1 ALTERNATOR AND X-FORMER NEUTRAL EARTHLING Alternator and X-former neutral is earthling directly or body by resistance or inductance. These earthing reduced or minimize traveling wave, any surge voltage and unbalanced voltage. 3.7.2 NON CURRENT CARRYING / METALLIC BODY Metallic body means the body of motor , generator , x-former, metal tank tower and pole earthling .This earthling protects any abnormal current as a result protective relay and fuse operate easy . Earthling Electrode The earthling electrodes are two types. 1. Pipe electrode 2. plate electrode 1. Pipe electrode It is steel pipe which diameter 1.5 inch to 4 inch and length 9 inches. 2. Plate electrode

It is steel or cost iron which size 3’×2’×1/2’.This plate is placed in the earth at 10’ from the earth surface. 3.8 CURRENT LIMITING REACTOR In order to limit the short circuit current to a value which the circuit breakers can handle additional reactance known as reactors .Which is connected in contain with the system at suitable points .A reactor is coil of number of turns designed to have a large inductance as compared to its ohmic resistance .It may be added that due to small resistance of reactors there is very little change the efficiency of the system. Location of Reactors Short circuit limiting reactor may be connected as 1. In series with each generator 2. In series with each feeder and 3. In bus-bar. 3.8.1GENERATOR REACTOR When the reactors are connected in series with each generator they are known as generator reactors .Which is shown in below.

Figure 3.30: Generator reactor 3.8.2 FEEDER REACTOR

When the reactors are connected in series with each feeder they are known as feeder reactors .which is shown in below. Since most of the short circuit occur on feeders a large no of reactor are used for such circuits.

Figure: Feeder reactor

Figure 3.31: Feeder Reactor

3.8.3 Bus-bar Reactor When the reactors are connected in the bus bar they are known as bus bar reactors. Which is shown in below .The two above methods of locating reactors suffer from the advantage that there is considerable voltage drop and power loss in the reactor even during normal operation? This disadvantage can be overcome by locating the reactors in the bus-bars

Fig 3.32: Bus-reactor 3.8.4 TIE BAR REACTOR:







Comparing the ring system with tie bar system, its clear that in the tie bar system, there are effectively two reactors in series between sections so that reactors must have approximately half the reactors of those used in a comparable ring system. Another advantage of tie-bar system is that additional generators may be system


connected requiring

to the changes

in the existing reactors. Figure 3.33: Tie bar-reactor 3.9 INSULATOR The insulator provides necessary insulation between line conductors and supports and prevents any leakage from conductors to earth. This is achieved by securing line conductors to supports with the help of insulators. Safety factor of insulator = Puncture strength รทFlash over voltage It is desirable that the value of safety factor is high.

The properties of the insulator are following: 1. High mechanical strength. 2. High electrical resistance of insulator material. 3. High relative permittivity of insulator material. 4. High ratio of puncture strength to flash over. 5. The insulator material should be non-porous; free from impurities and cracks otherwise the permittivity will be lowered. Type of insulators: 1. Pin type insulator

3. Strain insulator

2. Suspension type insulator

4. Shackle insulator

Describe different type insulator 3.9.1 PIN TYPE INSULATOR The pin type insulator are used for transmission and distribution of electrical power at voltage up to 33 KV .Beyond operating voltage of 33 KV ,the pin type insulators become too bulky and hence uneconomical .For pin type insulator the value of safety factor is about 10.

Figure 3.34: Pin type Insulator 3.9.2 SUSPENSION TYPE INSULATOR This type of insulator is not economical beyond 33KV. For high voltage it is a usual practice to use of suspension type insulator .They consist of number of porcelain disc connected in series by metal link in the form of string .The conductor is suspended at the bottom end of this string while the other end of the string is secured to the cross arm of the tower .Each disc is design for low voltage, say 11 KV.

Figure 3.35: Suspension type insulator

3.9.3 STRAIN INSULATOR When there is a dead end of the line or there is corner or sharp curve ,the line is subjected to great tension .in order to relive the line of excessive tension , strain insulators are used .the disc of strain insulator are used in vertical plane .

Figure 3.36: Strain insulator 3.9.4 SHACKLE INSULATOR In early day, the shackle insulators were used as strain insulators but now days they are frequently used for low voltage distribution lines .Such insulator used either horizontal position or vertical position.

Figure 3.38: Shackle type insulator Chapter-4

Study of protective devices used in khagrachari 33kv/11kv distribution Substation 4.1 INTRODUCTION: We have visited 33/11KV Khagrachari distribution substation. Here we study the single line diagram and different protective devices. There is one 33KV incoming line come from Hathazari Grid Substation. Here used: 01) Two (02) Power Transformers. 02) Three (03) Lightning Arresters 03) Four (04) Isolators 04) Three (03) C.Ts 05) Two (02) SF6.C.Bs, One (01) O.C.Bs., six (06) 630 V.C.Bs. 06) One (01) Buscupler 07) Two (02) Station Transformers. 08) One (01) Potential Transformers. 09) Two (02) A.C.Rs, There are two transformers T1, T2 of 33/11KV, 10MVA with four main feeders. 1. Feeder number one go to PANKHAIYA PARA. 2. Feeder number two go to STADIUM. 3. Feeder number three go to POLICE LINE. 4. Feeder number four go to CANTONMENT. 4.2 RATING OF THE KHAGRACHARI SUB-STATION Table 4.1: Transformer Rating (T1, T2) Power

10 MVA Current


Primary current 175 A

Primary voltage 33 KV

Secondary current 575A

Secondary voltage 11KV


0.9 lagging

Weight Weight of oil 5300 Kg Weight of core12102 Kg

Table A: Station Transformer


125 KVA Current



Primary current 3.78 A

Primary voltage 33 KV

Secondary current 312.5A

Secondary voltage 0.4KV

Weight of oil 2600 Kg Weight of core 6000 Kg

Table B: Oil circuit breaker:



Normal Current=630 A Breaking Current=25 KA

Time Making time=5 s

Capacity Voltage=36 KV

Opening time=0.045 s

Making Current=62.5 KA Table C: The table of the rating of Khagrachari Sub-station:

SL. NO 01 02 03 04 05 06

Name of CB


SF6(33KV) O.C.B(33KV) A.C.R(33KV) A.C.R(11KV) V.C.B(11KV) Isolator (33KV)

1250Amp 880Amp 630 Amp 630 Amp 630Amp 1250 Amps


Isolator (11KV)

1250 Amps




Lightning Arrester


Station Transformer

10MVA,33/11KV 36KV 125KVA,33/0.4KV

C.T Ratio 400:5 300/5 200/5 400/5

Relay setting O/C


3.75 Amps 3.75 Amps 190 Amps 250 Amps 200Amps

2.5 Amps 2.5 Amps 40 Amps 50 Amps 120 Amps


Figure 4.1: Single line diagram of Khagrachari Sub-station

Fig: Office of the Executive Engineer of BPDB, Khagrachari.

Fig: Khagrachari 33/11KV Sub-station.

Fig: Khagrachari 33/11KV Sub-station.

Fig: Khagrachari 33/11KV Sub-station.

Chapter-5 Suggestions for better protection 5.1 INTRODUCTION: Much attention has been given to the use of PLCs in substation and distribution automation applications in recent years. Innovative engineers and technicians have been actively seeking new applications for PLCs in substations. PLCs have an important place in substation automation and their use in substation application will grow. As the use of PLCs in substation automation application increases, and the demand for substation and distribution automation increases, utility engineers are seeking ways to implement application. With deregulation, utilities are decreasing engineering staff levels. Utility engineers are required to field more projects with fewer available resources. 5.2 APPLICATION FOR PLCS IN SUBSTATION AUTOMATION: There are many applications for PLCs in substation automation, distribution automation. As utility engineers become more familiar with the capability of PLCs and PLC manufactures develop new substation specific products, the number and type of potential applications continues to increase. 1. Protection and control a) Circuit breaker Lockout b) Protective relay interface c) Dynamic protective relay setting for dynamic station topology 2. Automatic Switching a) Emergency Load Shedding b) Station maintenance C) Automatic transfer schemes d) Load sectionalizing e) Automatic reclosing schemes f) Automatic service restoration g) Circuit breaker control interlocking h) Feeder automation and fault recovery

3. Voltage regulation Management a) LTC (Load Tap Changer) Control b) Voltage regulator control c) Capacitor control 4. Transformer management a) Parameter monitoring and alarming b) Real Time modeling c) Interface to existing transformer monitors 5. Automation system diagnostics a) Power apparatus health monitoring b) PLC and communications self monitoring 6. Maintanence and safety a) Maintenance lock-out management b) Automatic circuit isolation control 7. Remote control 8. Demand control 9. Generator synchronization 5.3 BENEFITS OF USING PLCS IN SUBSTATION AUTOMATION: Reliability a large installed base, extensive support resources and low costs are some of the benefits of using PLCs as a basis for substation automation. PLCs are extremely reliable. They have been developed for application in harsh industrial environments. They are designed to operate correctly over wide temperature ranges and in very high electromagnetic noise and high vibrations environments. They can operate in dusty or humid environments as well. The large installed base of PLCs offers the advantages of reduced costs, readily available and low cost spare parts and trained personnel to work on PLCs. The large installed base also allows the manufactures more opportunity to improve design and offer new products for more varied applications. In many, if not all, applications PLCs offers lower cost solutions than traditional RTUs. They offer lower cost solutions than traditional electromechanical control relay systems for

automated substation applications. With the lower cost solutions PLCs based systems offer in substation and distribution automation application along with the other benefits, it is no surprise that there is so much interest in the application of PLCs in substation. 5.4 CONCLUSION: The use of PLCs (Programmable Logic Controller) in substation and distribution automation application has grown in recent years. The economics of PLCs based solultions mean that substation automation can be applied even more widely. This will help the utilities respond to the challenges presented by deregulation. As the use of PLCs in substation increases, the criteria for selection of control system will become an extremely important factor in the success of PLCs substations automation. Chapter 6 Discussion and conclusion 6.1 DISCUSSION: Electricity is the basic necessity for the economic of a country. The industrial development and the increase of living standard of people are directly related to the more use of electricity. Without it is not possible to drive industrial machine, pump for irrigation and possible to develop living slandered of people. It is quit impossible for us to study on protective devices in a distribution substation. So in short we have tried to discuss the main components of the power protection system of the distribution substation. It has seen that fuses, circuit breakers, relays, lightning arresters, isolators, ear-thing and current limiting reactors are used as protective devices of distribution substation system protection. 6.2 CONCLUSION:

We visited Khagrachari distribution sub-station then studied very carefully the single line diagram and got an idea about the different element of the protective devices. We have study the power protection scheme, protection zone, primary and back up protection and different type of protective devices used in the distribution sub-station. Also our target was to collect information about protective devices and develop suggestions for better protection of distribution substation. The use of PLCs (Programmable Logic Controller) in substation and distribution automation application has grown in recent years. The economics of PLCs based solutions mean that substation automation can be applied even more widely. This will help the utilities respond to the challenges presented by deregulation. Though, we think this thesis will be very helpful for better protection in a distribution substation. BIBLIOGRAPHY 1.



V.K. MEHTHA “PRINCIPLE OF POWER SYSTEM” Published by – S. CHAND & COMPANY LTD New Delhi, Third Edition – 1998.








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