RGPV EX7102 UNIT II

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READING MATERIAL FOR B.E. STUDENTS

OF RGPV AFFILIATED ENGINEERING COLLEGES

BRANCH VII SEM ELECTRICAL AND ELECTRONICS

SUBJECT EHV AC AND DC TRANSMISSION

Professor MD Dutt Addl General Manager (Retd)

BHARAT HEAVY ELECTRICALS LIMITED

Professor(Ex) in EX Department

Bansal Institute of Science and Technology

Kokta Anand Nagar BHOPAL

Presently Head of The Department ( EX)

Shri Ram College Of Technology

Thuakheda BHOPAL

Sub Code EX 7102 Subject EHV AC AND DC TRANSMISSION

UNIT II

Prof MD Dutt HOD Ex Department SRCT Thuakheda Bhopal MP India

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EX 7102

RG PV Syllabus

UNIT II EHV AC AND DC TRANSMISSION

FACTS devices, basic type of controllers, series controller. Static Synchronous Series Compensator SSSC. Thyristor controlled series capacitor TCSC. Thyristor controlled series reactor TCSR . Shunt controller STATCOM, Static VAR (SVC). Series – Series controllers. Combined series shunt controller. UPFC and TCPST.

INDEX

S No Topic UNIT II Page1 FACTS devices, basic type of controllers, series controller 3- 42 Static Synchronous Series Compensator SSSC 53 Thyristor controlled series capacitor TCSC 6-94 Thyristor controlled series reactor TCSR 9-105 Shunt controller STATCOM, Static VAR (SVC). 10-136 Series – Series controllers. Combined series shunt controller

UPFC and TCPST13-18


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FACTS DEVICES, BASIC TYPE OF CONTROLLER

SERIES CONTROLLER

A.C transmission system that employ power electronic based and other static controllers to enhance controllability and to increase power transfer capability are defined as FACTS i.e Flexible Alternating Current Transmission systems.Benefits of using FACTS controllers are:-

a) They help in obtaining optimal operation by reducing power losses and improving voltage profile.

b) Due to controllability of FACTS, the power carrying capacity of lines can be increased upto thermal limits.

c) The transient stability limit is increased thereby improving the dynamic security of the system.

d) Some FACTS controllers such as TCSC can damp the Sub Synchronous Resonance SSR.

e) The problem of dynamic over voltage can be overcome by use of FACTS controllers.

TYPES OF FACT DEVICESThere are two group of FACTS devices that follow two distinctly different technical approach.The first group of FACT controllers are known as variable impedance type FACT controllers. They are

i) Static VAr compensator SVCii) Thyristor controlled Series Capacitor TCSCiii) Thyristor controlled Phase Shifting Transformer. TCPST

The second group uses self commutating static convertor operating as controlled voltage sources, The direct current in a voltage sourced convertor flows in both direction therefore the convertor valves are to be bi directional. These FACTS controllers are known as Voltage Source Convertor VSC based controllers. They are

i) Static Synchronous Compensator STATCOMii) Interline Power Flow Controllers IPFCiii) Unified Power Flow Controllers UPFC


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a) Series controller b) shunt controller c) combined series series controller d) combined series shunt controller

SERIES CONTROLLERThe series controller may be a variable capacitor, inductor or a variable frequency source. A series controller injects a variable series voltage (product of current and variable reactance in the line. A voltage in series with the transmission line can control the current flow and there by the power transfer from the sending end to the receiving


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end. When the injected series voltage is in phase Quadrature with the line current, the controller generates or consumes only reactive power. If the injected series voltage is not in phase Quadrature, the controller handle both reactive and active power. Series capacitive impedance can be decrease the overall effective series transmission impedance from sending end to the receiving end and thereby increasing the transmittable power. The inductive series compensation may be used when it is necessary to decrease the power flow in the line. However, the capacitive compensation is more commonly used some of series controllers are:-

i) Thyristor controlled Series Capacitor TCSCii) Thyristor controlled Series Reactor TCSRiii) Static Synchronous Series Compensator SSSC

Thyristor switched series controller

2 STATIC SYNCHRONOUS SERIES COMPENSATORA static synchronous series compensator is a device whose output voltage (injected voltage) is in Quadrature with the line current for the purpose of changing the overall reactive drop in the line. The output voltage of a device is controlled independently and is normally quite small as compare to the line voltage. The SSSC may be with storage or without storage facility. The storage system may be a battery storage or a superconducting magnetic storage device which injects a voltage vector of variable angle in series with the line.The static synchronous series compensator may be used for current control, stability improvement and fro damping oscillations during disturbances.


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SSSC without energy storage system

3. Thyristor controlled Series Capacitor TCSCThe use of thyristor control to provide variable series compensation makes it attractive to employ series capacitor in long lines. Controlled series compensation can be achieved in two ways

1. Discrete control using thyristor switched series capacitor TCSC2. GTO Thyristor controlled series capacitor GCSC

TCSC consists of a number of capacitors in series. Each shunted by a switch composed of two anti parallel thyristor as shown below

Thyristor switched series capacitor

A capacitor is inserted by turning ‘off’ the thyristor switch. Similarly it is by passed by turning ‘on’ the thyristor. If all the switches are ‘off’, the effective capacitance becomes Ceq = C/m where ‘m’ is the total number of capacitors. Similarly if all the switches are simultaneously turned ‘on’, Ceq is zero. Therefore the effective capacitance and hence the degree of series compensation are controlled in a stepped manner by changing the number of capacitors inserted in the circuit.TCSC is a mature technology available for application in EHV AC lines. Parallel combination of switched capacitors and controlled reactors provide a smooth current control range from capacitive to inductive values by switching the capacitors and controlling the current in reactor. The figure below shows linear reactor ‘L’ connected to AC source Vs through two thyristor connected in anti parallel. During positive half cycle of source voltage, T1 is turned on and during the negative half cycle T2 is turned on. For firing angle α =90˚, the source current is continuous as shown below. The circuit behaves as if the inductance L is directly connected to the source without thyristor. For α=90˚, Is a sine wave, its fundamental component If1 is same as Is and is therefore maximum. As a result, inductive reactance offered by reactor Xl=Vs/If1 is minimum. Here Vs is the rms value of source voltage and If1 is the rms value of the


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fundamental component of source current, which for α=90˚ is equal to Is( rms value of source current).

Thyristor controlled reactor

α¿90° α>90° forT1 for T2Circuit diagram and its voltage and current waveform

For firing angle α>90˚, current is discontinuous, as shown in figure b) . Its fundamental component If1 again lags Vs, by 90˚ Its fundamental component If1 decreased, therefore the inductive reactance offered by reactor (=Vs/If1) has become more. If α is further increased, fundamental component of is would be further reduced and therefore reactance offered by the reactor would be more pronounced. For firing angle α=180˚, is=0, if1=0 and theoretically, the inductive reactance offered by the reactor would be infinite. This shows that with firing angle control from α=90˚ to180˚: the effective reactance of the reactor, as seen by the source, can be regulated from its actual value Xl=2πfl when α=90˚, to an infinite value when α=180˚.


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As the fundamental component of source current lags the source voltage by 90˚, the reactor consumes no power. It draws only reactive power.

Actually for 0˚≤ α ≤ 90˚, there is no control over inductor L therefore

Xl =Vs/Is = Vs/If1 for 0˚≤ α ≤ 90˚,Or L =Vs/ωIs = Vs/ωIf1 for 0˚≤ α ≤ 90˚,

For α > 90˚, the Fourier analysis of inductor current wave form gives the fundamental component If1 as under If1= Vs/πωL [ 2π-2α+sin2α]The reactive power drawn at α=90˚ or for 0˚≤ α ≤ 90˚ isQ =VsIf1 = Vs Is = Vs²/ωLFor 90˚ ≤ α ≤ 180˚ ,Q = Vs.If1 = Vs²/πωL [ 2π - 2α +sin 2α]

At α= 90˚, reactive power drawn is maximum, when α=180˚, reactive power is zeroA simple understanding of working of TCSC can be obtained by analyzing a variable inductor connected in parallel with affixed capacitor as shown below

Working of TCSC b)capacitive operation c)Inductive operation

ZTCSC = -XC jXTCR = -j XC

j(XTCR - ¡ XC) 1 - XC

XTCR

The current through the TCR (ITCR) is given by

Î TCR = -j XC Î L = Î L j(XTCR - XC) 1 - XTCR

XC

Since the losses are neglected, the impedance of the TCSC is purely reactive. Prof MD Dutt HOD Ex Department SRCT Thuakheda Bhopal MP India

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The capacitive reactance of TCSC

XTCSC = XC

1 - XC

XTCR Just the opposite of the convention used in circuit analysis and load flow studies.The reactance of TCSC is capacitive as long as the reactance of the capacitor Xc is less than the reactance of the TCR’XTCR’. When the thyristor are blocked, the reactance of TCR is infinite and current through it is zero. For Xc < XTCR, The current through the TCR 180˚ out of phase with the line current ‘Il’ for Xc > XTCR,, The effective reactance of TCSC is negative and it behaves as an inductor. In this case the line current and the current through the TCR ‘ITCR’ are in phase.When the triggering delay angle of TCR is 180˚, the reactor becomes non-conducting and the series capacitor has its normal value. There is no difference in the performance of TCSC in this mode with that of a fixed capacitor. This operation mode is also known as ‘Waiting mode’ and is normally avoided. As the delay angle is reduced to less than conducting and the net impedance of the controller becomes inductive. The variation of TCSC reactance with firing angle α is shown below.The feasibility of fast control of thyristor enables the improvement of stability and damping of oscillations using appropriate control strategies. TCC may be used for current control, stability improvement, damping oscillation, and for limiting fault current

Variation of TCSC reactance with firing angle α


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4 Thyristor controlled series reactor TCSR

A thyristor controlled series reactor comprises of a series reactor shunted by a thyristor controlled reactor (TCR). When the triggering delay angle of TCR is 180˚, the reactor is non conducting and the uncontrolled reactor works as a fault current limiting reactor. As the delay angle of 90˚, the reactor becomes fully conducting and the net inductance value is because of the parallel combination of two inductances. Thus a smooth variable reactance control is obtained.

Thyristor – controlled series reactor are used for current control, stability improvement, damping oscillation, and for limiting fault current

5 Shunt controller STATCOM, Static VAR (SVC).

SHUNT CONTROLLERA shunt controller is a variable reactance connected in parallel to a transmission line . The controller injects current into the system at the point of connection. If the injected current is in phase Quadrature with the line voltage, the controller handles only reactive power. For any other phase angle between the current and the line voltage it handles both active and reactive power.Reactive shunt compensation can significantly increase the maximum transmittable power. The transient stability at a given power transmission level and the fault clearing time, is determined by P- δ characteristics of the post fault system. Since appropriately controlled shunt compensation can provide effective voltage support, it can increase the transmission capability of the post fault system thereby enhance transient stability.

Thus with suitable and fast controls, shunt compensation will be able to change the power flow in the system during and following dynamic disturbances so as to increase the transient stability limit and provide effective power oscillation damping.

Some basic type of shunt controllers area) Static VAR compensator SVCb) Static Synchronous Compensator STATCOMc) Static Synchronous Generator SSGd) Thyristor controlled Dynamic Brake TCDB


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STATIC SYNCHRONOUS COMPENSATORThe static synchronous compensator or simple static compensator STATCOM is a shunt connected device developed as an advanced static VAr compensator where a voltage source convertor VSC is used instead of controllable reactors and switched capacitors. The use of VSC requires self commutating devices such as GTO ,IGBT , IGCT,MCT etc, which make them costlier.

A STATCOM based on voltage source convertor is shown in figure below. From a given input of DC voltage, voltage source convertor produces a set of 3 phase ACout put voltages, each in phase with and coupled to the corresponding AC system voltage through a relatively small reactance. The reactance is provided by either an interface reactor or the leakage inductance of a coupling transformer. By suitable control, the phase and magnitude of the AC voltage injected by VSC can be controlled. The output control is independent of the system voltage.

A STATCOM is comparable to a synchronous condenser which can supply variable reactive power and regulate the voltage of the bus to which it is connected. Te Ac voltage is directly proportional to the DC voltage Vdc across the capacitor. If any energy source, a battery or a rectifier, is present on the DC side, the voltage Vdc can be held constant. The self commutated switches GTO are switched on and off once in a cycle. The conduction period of each switch is 180˚. The switches are synchronized to the supply voltage ‘V’. If the line voltage ‘V’ is in phase with the convertor output voltage ‘E’ and has the same magnitude, no current flows into or out of the compensator. Thus there is no exchange of reactive power with the line. If the convertor voltage is increases, the voltage difference between ‘V’ and ‘E’ appears across the leakage reactance of the step down transformer. As a result, a leading current( leading by V) is drawn and the compensator behaves as a capacitor, generating reactive power.. On the other hand if V> E, then the STATCOM draws a lagging current, behaves as a reactor and absorbs reactive power. Thus compensator operates like a synchronous machine.A STATCOM has many technical advantages over SVC they are

i) Faster responseii) Requires less space as bulky components such as reactors are not required.iii) Modular and reloadableiv) Can be interfaced with real energy sources such as battery , SMESv) The reactive current can be maintained therefore superior performance is

achieved during low voltage conditions. Even if the reactive current can be increased under transient condition if the devices are rated for transient overloads.


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STATCOM

A STATCOM is controlled reactive power source. It provides voltage support by generating or absorbing reactive power at the point of coupling without the need of large external reactors or capacitors banks. Thus STATCOM may be used for voltage control, reactive power compensation and damping oscillations.

STATIC VAR COMPENSATOR SVCThe static Var compensator SVC is first generation FACTS controllers. It is a variable impedance device in which the current through a reactor is controlled by back to back connected thyristor . These thyristor valves are rated for lower voltages as the SVC is connected to the transmission line through a step down transformer or through the tertiary winding of a power transformer. The location of SVC is important in determining its effectiveness. They should be located at load centre or midpoint of transmission line.There are two type of SVC

1. Fixed Capacitor- Thyristor Controlled Reactor (FC-TCR)2. Thyristor Switched Capacitor – Thyristor Controlled Reactor (TSC-TCR)

The second type of SVC is more flexible, requires smaller reactor and hence generates less harmonics.Figure below shows a static VAR compensator. It is a shunt connected combination which includes a separate thyristor controlled or thyristor switched reactor for absorbing reactive power and thyristor switched capacitor for supplying the reactive power.

Static VAR compensator


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The TCR and TSC are connected on the secondary side of a step down transformer. The TSC is switched in using two thyristor connected back to back at the instant in a cycle when the voltage across valve is minimum and positive. This results in minimum switching transient. The current in a TCR can be continuously varied from zero to maximum by phase control in which the firing angle α is varied from 180˚ to 90˚. The harmonics in SVC are generated by the TCR. Neither TSC or TSR generates harmonics. The TCR current contains odd harmonics. Tuned and high pass filters are also used in parallel which provide capacitive reactive power at fundamental frequency. To limit the harmonics entering the system, some of the fixed capacitors are connected as series tuned filters. To reduce the harmonics further, a twelve pulse configuration of TCR should be used.The use of SVC improves transmission capacity and steady state limit. SVC can be used for stability improvements both during small and large disturbances. Its use can also damp the sub synchronous oscillations. The cost of a SVC is lesser as compared to a STATCOM.

6. Series – Series controllers. Combined series shunt controller UPFC and TCPST

It is a combination of two or more separate series controllers with each series controller connected in a transmission line or in a multi-line transmission system. All the controllers connected in series are controlled in a coordinated manner. Another variation of a series series controller is the inter – line power flow controller IPFC. This is recently introduced 1998 controller having a combination of two or more static synchronous compensator as shown in figure. The SSSCs are coupled through common DC link.With this arrangement, in addition to providing series reactive compensation, any convertor can be controlled to supply real power to the common DC link from its own transmission line .Thus real power can be made available from the under utilized lines and can be used by other lines.Consider an IPFC scheme consisting of two back to back DC to AC, each compensating a transmission line by injecting a series voltage as shown in figure.

INTERLINE POWER FLOW CONTROLLERProf MD Dutt HOD Ex Department SRCT Thuakheda Bhopal MP India

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AN IPFC ARRANGEMENT

The two synchronous voltages V1c and V2c represent the two convertors. X1 and X2 are he reactance’s of Line 1 and line 2 respectively. For clarity both the sending end and receiving end of two lines are assumed to be constant with fixed magnitudes and fixed angles; resulting in identical transmission angles ( δ1 and δ2) for the two systems. The two line reactance’s and the compensating voltages are also assumed to be identical. Although in actual the two system could be different with different voltages, impedances and angle. System 1 is selected as the prime system for which free controllability of both real and reactive power is stipulated.

PHASOR DIAGRAM OF A SYSTEM WITH IPFC

Figure above shows the Phasor diagram showing V1s , V1c, I1, V1x (Voltage across reactance X1) and the injected compensating voltage V1c with controllable magnitude ( 0 ≤ V1c ≤ V1cmax) and angle ( 0 ≤ P1 ≤360˚). The rotation of Phasor V1c with angle P1 varies both the magnitude and angle of V1x and results in the change in real power and reactive power.


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Thus it manages the overall real and reactive power management of a multi line transmission system and therefore optimizing the capability of the transmission system. The IPFC arrangement essentially requires the rigorous maintenance of the overall power balance at the DC terminals by appropriate control action and real power transfer. The IPFC together with independent controllable series reactive compensation of each transmission line helps as follows.

1. Reduce the burden of overloaded line by real power transfer.2. Compensate for resistive line drops and the corresponding reactive power

demands.3. Increase the effectiveness of the overall compensating system against dynamic

disturbances.

COMBINED SERIES SHUNT CONTROLLERA combined series shunt controller has separate series and shunt controllers whose operation is coordinated. The series controller injects voltage in series with the line voltage and the shunt controller injects current into the location of the controller.

UNIFIED POWER FLOW CONTROLLER UPFCIn the UPFC, the shunt and series controllers are unified. A unified power flow controller is a combination of static synchronous compensator (STATCOM) and a static synchronous series compensator (SSSC). The STATCOM and SSSC are coupled by a common DC link as shown in figure. The DC link allows bi- directional flow of real power between the series output terminals of the SSSC and the shunt output terminals of STATCOM. UPFC is controlled to provide concurrently active and reactive series line compensation. It is able to control, simultaneously or selectively . all the parameters affecting power flow in the transmission line that is transmission line voltage , impedance and angle, therefore the active power and reactive power flow in line. The UPFC may also provide independently controllable shunt reactive compensation.Conceptually, the UPFC is generalized synchronous voltage source represented by voltage Phasor Vc with controllable magnitude Vc ( 0 ≤ Vc ≤ Vcmax) and angle ( 0 ≤ P1 ≤360˚) in series with the transmission line. Convertor 2 (SSSC) provides the main function of the UPFC by injecting the voltage Vc with controllable magnitude and phase in series with the line through a transformer. The transmission line current flowing through this voltage results in reactive and real power exchange between it and the AC system.

The real power can freely flow in either direction between AC terminals of the two convertors. Since a synchronous voltage source is able to generate only the reactive power, the real power exchanged is supplied by one of the buses. The real power exchanged at the AC terminals is converted into DC power which appears at the DC link.The basic role of convertor 1 (STATCOM) is to supply or absorb the real power demanded by convertor 2 at the DC link to support the real power exchange resulting


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from the series voltage injection. Whereas there is a direct path for the real power through convertor 1 and 2 back to the line, the corresponding reactive power exchanged is supplied or absorbed locally by convertor 2.

UNIFIED POWER FLOW CONTROLLERIn addition, convertor 1 can also generate or absorb controllable reactive power, if needed and hence provide independent shunt reactive compensation for the line. The reactive power exchange of convertor 1 is independent of the reactive power exchanged by convertor 2. Thus each convertor can independently generate or absorb reactive power at its own Ac output terminal. There can be no reactive power flow through the DC link. UPFC are employed for controlling active and reactive power, voltage control, damping oscillations and limiting faults current.

THYRISTOR CONTROLLED PHASE SHIFTING TRANSFORMER

A special form of 3 Phase regulating transformer is realized by combing a transformer that is connected in series with a line to a voltage transformer equipped with a tap changer. The windings of the transformer are so connected that on its secondary side, phase Quadrature voltages are generated. The secondary voltages of the voltage transformer are fed into the secondary windings of the series transformer. Thus the


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addition of small ,phase Quadrature voltage components to the phase voltages of the line creates phase shifted output voltages without any appreciable change in magnitude. A phase shifted transformer is therefore able to introduce a phase shift in line.

The Phasor diagram is shown in figure shows the phase shift realized without appreciable change in magnitude by the injection of phase Quadrature voltage components in a 3 phase system .When a phase shifting transformer with on load tap changer is used, controllable phase shift is obtained. Phase shifting transformers have been in use since 1930’s for control of power flow in transmission lines in steady state. In spite of their low MVA capacity, these phase shifting transformers can exercise a significant real power control. A promising use of these devices is in creating active power regulation on selected lines and securing active power damping.

Controlled shunt compensation increases transient stability by increasing or maintaining the transmission line voltage during the accelerated swing of the disturbed machine. Controlled series reactive compensation improves transient stability by increasing the power transmission during the first swing by reducing the effective line impedance. The ability of the phase shifting transformer or the phase angle regulator to maintain the maximum effective transmission angle during the first swing can also be used effectively to increase the transient stability limit. The phase shifting transformer can provide a substantial increase in the transient stability margin. The increase in stability margin is proportional to the angular range and which in turn depends on the VA rating of the phase shifting transformer.

The modification of voltage magnitudes and or their phase by adding a control voltage is an important concept and forms the basis of some of the new FACTS devices.

By using electronic controllers, the operation of phase shifting transformer can be made fast which enables dynamic regulation of power flow and improvement of power flow system stability and dynamic security.

Both the conventional thyristor based and the GTO based phase angle regulator inject a voltage between the given bus and the controlled line. The major difference is that whereas the thyristor based regulator obtains the voltage to be injected from appropriate taps of the regulating transformer, the GTP based regulator generates this voltage from DC supply. Therefore, the function of thyristor based regulator is that of an on load tap changer, selecting the proper tap and injecting the thus obtained voltage to the line. The function of GTO based regulator is to generate the required voltage and to inject it in series with the line just as the thyristor based regulator. Thus the injected voltage need not be realized through electromagnetic winding arrangements; instead by using high speed semiconductor switches such as GTO thyristors, voltage source invertors (VSI) phase shifted components are produced.


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PHASE SHIFTING TRANSFORMER AND ITS PHASOR DIAGRAM


Engineering

RGPV EX7102 UNIT II