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International Journal of Electrical and Electronics Engineering Research (IJEEER) ISSN 2250-155X Vol. 3, Issue 2, Jun 2013, 201-208 © TJPRC Pvt. Ltd.

RECOVERY OF HVDC SYSTEM AFTER FAULT WITH REACTIVE POWER INJECTON T S S S BHARAT1 & D. SESHI REDDY2 1

M. Tech Student, School of Electrical Engineering, K L University, Guntur, Andhra Pradesh, India

2

Associate Professor, School of Electrical Engineering, K L University, Guntur, Andhra Pradesh, India

ABSTRACT The paper entails “Recovery of HVDC System After Fault with Reactive Power Injection”. This involves the design of converter circuitry, gate firing units, and rectifier and inverter control systems. The complete model converts AC power into DC power by controlled rectifier and then converts this DC power into AC power by line commutated inverter, after transmitting DC power through the line. There are three basic controls namely constant current, constant extinction angle and constant ignition angle control required for prevention of large fluctuation in DC current due to variation in AC system voltage, maintaining DC voltage near rated value and prevention of commutation failure in inverter. This paper aims for conversion of power; bi-directional power flow on HVDC link and behavior of HVDC link during fault and fault cleared. When fault present on DC system it requires reactive power greatly. Hence the system is compensated by STATCOM at AC terminals was simulated in MATLAB environment, system is operating with satisfactory results after compensation.

KEYWORDS: DC Fault, HVDC Link, STATCOM, Voltage Dependent Current Limit INTRODUCTION The transmission and distribution of electrical energy started with direct current. In 1882, a 50-km-long

2-kV

DC transmission line was built between Miesbach and Munich in Germany. At that time, conversion between reasonable consumer voltages and higher DC transmission voltages could only be realized by means of rotating DC machines. In an AC system, voltage conversion is simple. An AC transformer allows high power levels and high insulation levels within one unit, and has low losses. It is a relatively simple device, which requires little maintenance. Further, a three-phase synchronous generator is superior to a DC generator in every respect. For these reasons, AC technology was introduced at a very early stage in the development of electrical power systems. It was soon accepted as the only feasible technology for generation, transmission and distribution of electrical energy. However, high-voltage AC transmission links have disadvantages, which may compel a change to DC technology: 

Inductive and capacitive elements of overhead lines and cables put limits to the transmission capacity and the transmission distance of AC transmission links.

This limitation is of particular significance for cables. Depending on the required

transmission capacity, the

system frequency and the loss evaluation, the achievable 

Transmission distance for an AC cable will be in the range of 40 to 100 km. It will mainly be limited by the charging current.

Direct connection between two AC systems with different frequencies is not possible.

Direct connection between two AC systems with the same frequency or a new connection within a meshed grid


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may be impossible because of system instability, too high short-circuit levels or undesirable power flow scenarios. Engineers were therefore engaged over generations in the development of a technology for DC transmissions as a supplement to the AC transmissions. The advantages of a DC link over an AC link are: 

A DC link allows power transmission between AC networks with different frequencies or networks, which cannot be synchronized, for other reasons.



Inductive and capacitive parameters do not limit the transmission capacity or the maximum length of a DC overhead line or cable. The conductor cross section is fully utilized because there is no skin effect. For a long cable connection, e.g. beyond 40 km, HVDC will in most cases offer the only technical solution because of the high charging current of an AC cable. This is of particular interest for transmission across open sea or into large cities where a DC cable may provide the only possible solution.



A digital control system provides accurate and fast control of the active power flow.



Fast modulation of DC transmission power can be used to damp power oscillations in an

AC grid and thus

improve the system stability.

HVDC SYSTEM CONTROL One of the major advantages of HVDC link is the rapid controllability of transmitted power through the control of firing angles of the converters. Modern converters are not only fast but also reliable and they are used for protection against line and converter faults Principle of DC Link Control

Figure 1: Steady State Equivalent Circuit of 2 Terminal DC Link

Figure 2: Schematic of a DC Link Showing Transformer Ratios The control of power in a DC link can be achieved through the control of current or voltage, form minimization of loss consideration it is important to maintain constant voltage in link and adjust the current to meet the required power. The voltage drop along a DC line is small when compared to AC line mainly because of absence of the reactive voltage drop.Consider the steady state equivalent circuit of a two terminal DC link is based on the assumptions that all the series connected bridges in both poles of a converter station are identical and have same delay angles, also the no. of series connected bridges (đ??§đ??› ) in both stations are same. The voltage sources đ??„đ???đ??Ť and đ??„đ???đ??˘ are defined by đ??„đ???đ??Ť =

đ?&#x;‘ đ?&#x;? đ?›‘

đ??§đ??› đ??„đ??Żđ??Ť đ??œđ??¨đ??Źđ?›‚đ??Ť

(1)


Recovery of HVDC System after Fault with Reactive Power Injecton

Edi =

3 2

nb Evi cosÎłi

Ď€

203

(2)

Where đ??„đ??Żđ??Ť and đ??„đ??Żđ??˘ are line - to – line in the valve side windings of the rectifier and inverter transformer respectively. These voltages can be obtained as đ??„ đ??Żđ??Ť =

đ???đ??Źđ??Ť đ??„đ??Ť

đ??„đ??Żđ??˘ =

đ???đ??Šđ??Ť đ??“ đ??Ť

đ???đ??Źđ??˘ đ??„đ??˘

(3)

đ???đ??Šđ??˘ đ??“đ??˘

Where đ??„đ??Ť and đ??„đ??˘ are the AC voltages of converter buses on rectifier. đ??“đ??Ť And đ??“đ??˘ are the off nominal tap ratios on the rectifier and inverter side. Combing equations (1) (2) & (3) we get đ??„đ???đ??Ť = đ??„đ???đ??Ť =

đ??€đ??Ť đ??„đ??Ť đ??“đ??Ť đ??€đ??˘ đ??„đ??˘ đ??“đ??˘

đ??œđ??¨đ??Ź đ?›‚đ??Ť

(4)

đ??œđ??¨đ??Ź đ?›„đ??˘

(5)

�� � �

Constants

đ??„đ???đ??˘ Can also be written as đ??„đ???đ??˘ =

đ??€đ??˘ đ??„đ???đ??˘ đ??“đ??˘

đ??œđ??¨đ??Ź đ?›ƒđ??˘ + đ?&#x;?đ??‘ đ??œđ??˘ đ??ˆđ???

(6)

Where đ??‘ đ??œđ??˘ =

đ?&#x;‘đ??§đ??› đ?›‘

đ??— đ??œđ??˘ , đ??‘ đ??œđ??Ť =

đ?&#x;‘đ??§đ??› đ?›‘

đ??— đ??œđ??Ť

(7)

đ??— đ??œđ??Ť And đ??— đ??œđ??˘ are the leakage reactance’s of the converter transformer in the rectifier and inverter station. The steady state current đ??ˆđ??? in the DC link is obtained as đ??ˆđ??? =

đ??„đ???đ??Ť −đ??„đ???đ??˘ đ??‘đ??œđ??Ť +đ??‘đ??? −đ??‘đ??œđ??˘

(8)

Substitute equations (4) & (5) in (8) đ??ˆđ??? =

đ??€ đ??„ đ??€ đ??Ť đ??„đ??Ť đ??œđ??¨đ??Ź đ?›‚đ??Ť − đ??˘ đ??˘ đ??œđ??¨đ??Ź đ?›„đ??˘ đ??“đ??Ť đ??“đ??˘

đ??‘đ??œđ??Ť +đ??‘đ??? −đ??‘đ??œđ??˘

(9)

As the denominator is small, even small changes in the voltage magnitudes đ??„đ??Ť and đ??„đ??˘ can result in large changes in the DC current, if the control variables held constant. The current control at the rectifier station under normal condition 

The increase of power in the link is achieved by reducing đ?›‚ đ??Ť which improves the power factor, at rectifier for higher loadings and minimizes the reactive power consumption



The inverter can now be operated at minimum đ?›„ , thereby minimizing the reactive power consumption at the inverter also.



The operation at minimum excitation angle at the inverter and current control at the rectifier results in better voltage regulation than the operation with minimum delay angle at the rectifier and current control at the inverter


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

The currents during line faults are automatically limited with rectifier station in current control

FAULTS ON DC SIDE OF THE SYSTEM Usual faults that occur on the DC side of the system are (i) pole – to – ground fault, (ii) pole – to – pole short circuit, or (iii) a ground fault at the DC bus, smoothing reactor or failure of insulation resulting in a ground fault. They may be a temporary short circuit due to a lightning stroke on one of the DC over lines which are usually self – clearing after a few milliseconds. The fault current cannot be extinguished by itself until the current is brought down to zero and the arc is deionised. As soon as the fault occurs, DC voltage reduces and becomes zero; the rectifier current tends to rise and the inverter current tends to fall to zero. As such the rectifier is blocked and inverter phase is advanced beyond 900 to rectifier action to maintain sufficient voltage. Normal converter controls will not be adequate to reduce the fault current to zero and special control is employed to drive both converters into the inverter mode. Thus, the energy stored in inductor and capacitors is cleared quickly. This is done speedily by sensing the fault and making the rectifier go into the inverter mode. After fault clearing, both converters are brought to the normal mode VAR Management Reactive power (VARs) is required to maintain the voltage to deliver active power (watts) through transmission lines. When there is no enough reactive power, the voltage sags down and it is not possible to push the power demanded by loads through the lines. Reactive power supply is necessary in the reliable operation of AC power systems. Several recent power outages worldwide may have been a result of an inadequate reactive power supply which subsequently led to voltage collapse.

VOLTAGE DEPENDENT CURRENT LIMIT The low DC voltages in the link are mainly due to the faults in the AC system on the rectifier or inverter side. The low AC voltage due to faults on the inverter side can result in persistent commutation failure because of the increase of overlap angle. In such cases it is necessary to reduce the DC current in the link until the condition that led to the reduce DC voltages are relieved. Also the reduction of current relieves those valves in the inverter which are overstressed due to continuous current flow in them. If the low voltage is due to fault on rectifier side AC system the inverter has to operate at very low power factor causing excessive consumption of reactive power, which is also undesirable. Thus it becomes it becomes useful to modify the control characteristics to include voltage dependent current limits.

Figure 3: Control Characteristics Including VDCOL This is shown in above figure 3 which also shows current error characteristics to stabilize the mode when operating with DC current between đ??ˆđ???đ?&#x;? and đ??ˆđ???đ?&#x;? . The characteristic đ??œđ??œ ′ and đ??œ ′ đ??œ ′′ show the limitation of current due to the


Recovery of HVDC System after Fault with Reactive Power Injecton

205

reduction in voltage. The DC current is reduced đ??ˆđ???đ?&#x;? from đ??ˆ ′ đ???đ?&#x;? linearly and maintained at đ??ˆ ′ đ???đ?&#x;? below the voltageđ??•đ???đ?&#x;? . The inverter characteristics also follow the rectifier characteristics to maintain the current margin except đ??Ąđ???′′ which is due to lower limit imposed on the delay angle of the inverter

CONTROL OF VARs USING STATCOM The Static Synchronous Compensator (STATCOM) is a shunt device of the Flexible AC Transmission Systems (FACTS) family using power electronics to control power flow and improve transient stability on power grids. The basic voltage-sourced converter scheme for reactive power generation is shown schematically, in the form of a single-line diagram in figure 4 From a DC input voltage source, provided by the charged capacitor Cs, the converter produces a set of controllable three-phase output voltages with the frequency of the ac power system. Each output voltage is in phase with, and coupled to the corresponding ac system voltage via a relatively small (0.1-0.15 p.u.) tie reactance.

Figure 4: Reactive Power Generation by Rotating a Voltage-Sourced Switching Converter By varying the amplitude of the output voltages produced, the reactive power exchange between the converter and the ac system can be controlled in a manner similar to that of the rotating synchronous machine. That is, if the amplitude of the output voltage is increased above that of the ac system voltage, then the current flows through the tie reactance from the converter to the AC system and the converter generates reactive (capacitive) power for the ac system. If the amplitude of the output voltage is decreased below that of the ac system, then the reactive current flows from the AC system to the converter, and the converter absorbs reactive (inductive) power. If the amplitude of the output voltage is equal to that of the AC system voltage, the reactive power exchange is zero.

MATHEMATICAL MODELING OF STATCOM The node at which the STATCOM is connected is connected is represented as a PV node which may change to a PQ node in the event of limits being violated. In such case the generated/absorbed reactive power would correspond to the violated limit. Contrary to the SVC, the STATCOM is represented as a voltage source for the full range of operation, enabling a more robust voltage support mechanism. An alternative way to model the STATCOM in a Newton – Raphson power flow algorithm is described in this section. It is a simple and efficient model based on the use of a variable voltage source, which adjust automatically in order to achieve a specified voltage magnitude. In this case the node at which the STATCOM is connected is a controlled node where the nodal voltage magnitude and the nodal active and reactive powers are specified while the source voltage magnitude is handled as a state variable.


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Figure 5: VSC Connected to the AC Network via a Shunt Transformer Based on the representation given in figure 5 the following equations can be written đ??ˆđ??Żđ??‘ = đ??˜đ??Żđ??‘ đ??•đ??Żđ??‘ − đ??•đ??Ľ đ??˜đ??Żđ??‘ =

đ?&#x;? đ??™đ??Żđ??‘

(10)

= đ??†đ??Żđ??‘ + đ??Łđ?? đ??Żđ??‘

(11)

The active and reactive powers injected by the source may be derived using the complex power equation ∗ ∗ ∗ đ??’đ??Żđ??‘ = đ??•đ??Żđ??‘ đ??ˆđ??Żđ??‘ = đ??•đ??Żđ??‘ đ??˜đ??Żđ??‘ đ??•đ??Żđ??‘ − đ??•đ??Ľâˆ—

(12)

Taking the variable voltages sources to be đ??•đ??Żđ??‘ = đ??•đ??Żđ??‘ đ??œđ??¨đ??Źđ?›‰đ??Żđ??‘ + đ??Łđ??Źđ??˘đ??§đ?›‰đ??Żđ??‘ , and after performing some Complex operations the following active and reactive power equations are obtained đ???đ??Żđ??‘ = đ??•đ??Żđ??‘ 2đ??†đ??Żđ??‘ − đ??•đ??Żđ??‘ đ??•đ??Ľ đ??†đ??Żđ??‘ đ??œđ??¨đ??Ź đ?›‰đ??Żđ??‘ − đ?›‰đ??Ľ + đ?? đ??Żđ??‘ đ??Źđ??˘đ??§ đ?›‰đ??Żđ??‘ − đ?›‰đ??Ľ

(13)

đ???đ??Żđ??‘ = − đ??•đ??Żđ??‘ 2đ?? đ??Żđ??‘ − đ??•đ??Żđ??‘ đ??•đ??Ľ đ??†đ??Żđ??‘ đ??Źđ??˘đ??§ đ?›‰đ??Żđ??‘ − đ?›‰đ??Ľ − đ?? đ??Żđ??‘ đ??œđ??¨đ??Ź đ?›‰đ??Żđ??‘ − đ?›‰đ??Ľ

(14)

SIMULATION OF TEST SYSTEM The figure 6 shows the configuration of a HVDC in which the converts AC power into DC power by controlled rectifier and then converts t his DC power into AC power by line commutated inverter, after transmitting DC power through the line

Figure 6: Layout of a HVDC System When a line – to – line fault occurs on a DC link at time (t) = 0.1 to 0.3 seconds the voltage of the system is shown below in figure 8

Figure 7: Fault on DC Link


Recovery of HVDC System after Fault with Reactive Power Injecton

207

Figure 8: Three Phase Voltages during Fault As per the observation from Figure 8 there is dip in voltage of 80 kv and system is delivering power at the constant of 300 kv. The reactive power requirement is 1000 MVAR. Hence 1000 MVAR capacity of STATCOM is placed in the AC system to compensate reactive power demand in the system.

Figure 9: HVDC System with STATCOM

Figure 10 : Recovery of the System After Fault with Reactive Power Injection As we simulate the system with STATCOM as shown in Fig (9). The simulation results are present in Fig (10). At the time of fault voltage profile maintained constant after compensation.

CONCLUSIONS The study of the simulation gives an idea about the basic concept and operation of HVDC link, first as rectifier mode of operation and after converting the power from AC to DC and next the Inverter mode of operation by converting power from DC to AC. when a fault occurs on transmission line proper reactive power compensation is discussed by use of STATCOM. The detailed modelling of STATCOM is done using MAT-LAB simulation results are found satisfactory.


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REFERENCES 1.

Sarkar Aratrik, Buch Surahi, C. B. Bhatt, “Simulation and Analysis of HVDC Transmission line,” Nirma University, 2012.

2.

E. W. Kimbark, “Direct Current Transmission,” John Wiley, 1971.

3.

J. Ainsworth, “The Phase-Locked Oscillator: A New Control System for Controlled Static Converters,” IEEE Trans. On power Apparatus and Systems. IEEE std., 1968.

4.

K. R. Padiyar, “HVDC Power Transmission System,” New Age International Publishers, 1991.

5.

P. C. Sen., “Power Electronics,” Tata McGraw Hill Company Ltd.,New Delhi, 1991.

6.

P. S. Kundur, “Power System Stability and Control,” McGraw Hill, Inc.New York, 1994

7.

C. L .Wadhwa, “Electrical Power Systems”, Wiley Eastern Ltd, New Delhi,

8.

N.G.Hingorani and L.Gyugyi, “Understanding FACTS”, IEEE PES, Sponsor, Standard Publishers Distributors New Delhi, 1999.

21 recovery of hvdc system full  

The paper entails “Recovery of HVDC System After Fault with Reactive Power Injection”. This involves the design of converter circuitry, gate...