THYRISTOR-CONTROLLED SERIES CAPACITOR

Series compensation Series compensation is the method of improv ing the sys tem vo l tage by connecting a capacitor in series with the transmission line.

In other words, in series compensation, reactive power is inserted in series with the transmission line for improving the impedance of the system. It improves the power transfer capability of the line. It is mostly used in extra and ultra high voltage line.

OBJECTIVES OF SERIES COMPENSATION The effect of series compensation on the basic factors, determining attainable

Maximal Power Transmission, Steady-state Power Transmission Limit, Transient Stability, Voltage Stability Power Oscillation Damping

Concept of Series Capacitive Compensation

The basic idea behind series capacitive compensation is to decrease the overall effective series transmission impedance from the sending end to the receiving end , i .e . , X in the P = (V 2 /X) sin δ relationship characterizing the power transmission over a single line.

Consider the simple two-machine model, with a series capacitor compensated line, which, for convenience, is assumed to be composed of two identical segments.

Note that for the same end voltages the magnitude of the total voltage across the series line inductance, V x = 2VX/2 is increased by the magnitude of the opposite voltage

Vc deve l oped ac ro s s t h e se r i e s capacitor and th is results f rom an increase in the line current.

Effective transmission impedance with the series capacitive compensation is

where K is the degree of series compensation,

Assuming the voltages VS = Vr = V The current in the compensated line is

Real Power Transmitted is

Reactive power supplied by the series capacitor is

VOLTAGE STABILITY: Series capacitive compensation can also be used to reduce the series reactive impedance to minimize the receiv ing-end voltage variation and the possibi l i ty of voltage collapse.

A simple radial system with feeder l ine reactance X, series compensating reactance Xc, and load impedance Z is shown .

The "nose point" at each plot given for a specific compensation level represents the corresponding voltage instability where the same radial system with a reactive shunt compensator, supporting the end voltage, is shown.

Clearly, both shunt and series capacitive compensation can effectively increase the voltage stability limit.

Shunt compensation does it by supplying the reactive load demand and regulating the terminal voltage is done by Series capacitive compensation.

ADVANTAGES OF TCSC

Use of thyristor control in series capacitors potentially offers the following little-mentioned advantages:

1. Rapid, continuous control of the transmission-line series-compensation level.

2 . Dynamic cont ro l of power f low i n se lec ted transmission lines within the network to enable optimal power-flow conditions and prevent the loop flow of power.

3. Damping of the power swings from local and inter-area oscillations.

4. Suppression of subsynchronous oscillations. At subsynchronous frequencies, the TCSC presents an inherently resist ive–inductive reactance . The subsynchronous oscillations cannot be sustained in this situation and consequently get damped.

5 Enhanced level of protection for series capacitors. A fast bypass of the series capacitors can be achieved through thyristor control when large over voltages develop across capacitors following faults.

Likewise, the capacitors can be quickly reinserted by thyristor action after fault clearing to aid in system stabilization.

6. Voltage support. The TCSC, in conjunction with series capacitors, can generate reactive power that increases with line loading, thereby aiding the regulation of local network voltages and, in addition, the alleviation of any voltage instability.

7. Reduction of the short-circuit current. During events of high short-circuit current, the TCSC can switch from the controllable-capacitance to the controllable-inductance mode, thereby restricting the short-circuit currents.

TCSC It consists of the series compensating capaci tor shunted by a Thyr istor -Controlled Reactor.

In a practical TCSC implementation, several such basic compensators may be connected in series to obtain the desired v o l t a g e r a t i n g a n d o p e r a t i n g characteristics.

Circuit Diagram

The steady-state impedance of the TCSC is that of a parallel LC circuit, consisting of a fixed capacitive impedance, XC , and a variable inductive impedance, XL(α), that is,

XL=wl, and a is the delay angle measured from the crest of the capacitor voltage (or, equivalently, the zero crossing of the line current).

The TCSC thus presents a tunable parallel LC circuit to the line current that is substantially a constant alternating current source.

BASIC PRINCIPLE: A TCSC is a series-controlled capacitive reactance that can provide continuous control of power on the ac line over a wide range.

T h e p r i n c i p l e o f v a r i a b l e - s e r i e s compensation is simply to increase the fundamental-frequency voltage across an fixed capacitor (FC) in a series compensated line through appropriate variation of the firing angle, α.

This enhanced voltage changes the effective value of the series-capacitive reactance.

The impedance of the LC network is given by

If ωL > 1/ωC, the the reactance of the FC is less than that of the parallel-connected variable reactor and that this combination provides a variable-capacitive reactance are both implied. Moreover, this inductor increases the equivalent-capacitive reactance of the LC combination above that of the FC.

If ωC - 1/ωL = 0 a resonance develops that results in an infinite-capacitive impedance an obviously unacceptable condition.

I f , however , ωL < 1/ωC , the LC combination provides inductance above the value of the fixed inductor.

In the variable-capacitance mode of the TCSC, as the inductive reactance of the va r i ab l e i n duc t o r i s i n c rea sed , t he equivalent-capacitive reactance is gradually decreased.

The min imum equ iva lent -capac i t i ve reactance is obtained for extremely large inductive reactance or when the variable inductor is open-circuited, in which the value is equal to the reactance of the FC itself.

The behavior of the TCSC is similar to that of the parallel LC combination.

The difference is that the LC-combination analysis is based on the presence of pure sinusoidal voltage and current in the circuit, whereas in the TCSC, because of the voltage and current in the FC and thyristor-controlled reactor (TCR) are not sinusoidal because of thyristor switchings.

TCSC OperationMODES OF TCSC OPERATION: BYPASSED-THYRISTOR MODE BLOCKED-THYRISTOR MODE PARTIALLY CONDUCTING THYRISTOR, OR VERNIER, MODE

BYPASSED-THYRISTOR MODE: In this bypassed mode, the thyristors are made to fully conduct with a conduction angle of 180deg.

Gate pulses are applied as soon as the voltage across the thyristors reaches zero and becomes positive , resulting in a continuous sinusoidal of flow current through the thyristor valves.

The TCSC module behaves l ike a paral le l capacitor–inductor combination.

However, the net current through the module is inductive, the reactor is chosen to be greater than that of the capacitor.

Also known as the thyristor-switched-reactor (TSR) mode, the bypassed thyristor mode is distinct from the bypassed-breaker mode, in which the circuit breaker provided across the series capacitor is closed to remove the capacitor or the TCSC module in the event of TCSC faults or transient over voltages across the TCSC.

This mode is employed for control purposes and also for initiating certain protective functions.

Whenever a TCSC module is bypassed from the violation of the current limit, a finite-time delay, T Delay , must elapse before the module can be reinserted after the line current falls below the specified limit.

BLOCKED-THYRISTOR MODE: In this mode, also known as the waiting mode, the firing pulses to the thyristor valves are blocked.

If the thyristors are conducting and a blocking command is given, the thyristors turn off as soon as the current through them reaches a zero crossing.

The TCSC module is thus reduced to a fixed-series capacitor, and the net TCSC reactance is capacitive.

In this mode, the dc-offset voltages of the capacitors are monitored and quickly discharged using a dc-offset control without causing any harm to the transmission-system transformers.

PARTIALLY CONDUCTING THYRISTOR, OR VERNIER, MODE: This mode allows the TCSC to behave either as a continuously controllable c a p a c i t i v e r e a c t a n c e o r a s a continuously controllable inductive reactance.

It is achieved by varying the thyristor-pair firing angle in an appropriate range.

However, a smooth transition from the capacit ive to induct ive mode is not permitted because of the resonant region between the two modes.

A variant of this mode is the capacitive-vernier-control mode, in which the thyristors are fired when the capacitor voltage and capacitor current have opposite polarity

This condition causes a TCR current that has a direction opposite that of the capacitor current, thereby resulting in a loop-current flow in the TCSC controller.

The loop current increases the voltage across the FC , effectively enhancing the equivalent-c a p a c i t i v e r e a c t a n c e a n d t h e s e r i e s -compensation level for the same value of line current.

Based on the three modes of thyristor-valve operation, two variants of the TCSC emerge:

Thyristor-switched series capacitor (TSSC), which permits a discrete control of the capacitive reactance.

Thyristor-controlled series capacitor (TCSC), which offers a continuous control of capacitive or inductive reactance. (The TSSC , however , i s more commonly employed.)

STATCOM

Introduction

Single Line Diagram of STATCOM

What is STATCOM

STATCOM or Static Synchronous Compensator is a power electronic device using force commutated devices like IGBT, GTO etc. to control the reactive power flow through a power network and thereby increasing the stability of power network.

STATCOM is a shunt device i.e. it is connected in shunt with the line.

A Static Synchronous Compensator (STATCOM) is also known as a Static Synchronous Condenser (STATCON).

It is a member of the Flexible AC Transmission System (FACTS) family of devices.

Existing System In power system, real power and reactive power is controlled by the voltage and phase angle di f ference between the sending end and receiving end respectively.

Impedance of the transmission line can also be used to control real and reactive power.

Flexible AC Transmission Systems (FACTS) are used to improve the power transfer capability of transmission line.

Applications

Reactive power capability and fault ride through for wind farms

Compensation of weak transmission lines to remote areas

Compensation of starting current for large motors

Active harmonic filteringProtection of week transmission lines

Flicker control of fluctuating loads

Historical Background

Since 2005,  Texas, has been operating an ABB-supplied STATCOM on its 138 kV power system.

The world’s largest and most powerful STATCOM, entered service on Aug. 19, 2011, in the 500-kV Dongguan substation China.

Hyosung Heavy Industries installation a 400-Mvar STATCOM at Shinyoungju and Shinchungju substations of Korea Electric Power Corp on October 29, 2018 .

STATCOM in India

On 6th June 2018 Siemens has commissioned 400 kV Synchronous STATCOM solutions at Power Grid Corporation's substation in Rourkela, O r i s h a

STATCOM CONTROL

Vsh>Vt Capacitive CompensationVsh<Vt Inductive Compensation

Components

STATCOM has the following components: 1)      A Voltage Source Converter, VSC

The voltage-source converter is used to convert the DC input voltage to an AC output voltage.

PWM Inverters using Insulated Gate Bipolar Transistors ( IGBT ) : I t uses Pulse Width Modulation (PWM) technique to create a sinusoidal waveform from a DC voltage source with a typical chopping frequency of a few kHz.

2)      DC Capacitor DC Capacitor is used to supply constant DC voltage to the voltage source converter, VSC.

3)      Inductive Reactance A Transformer is connected between the output of VSC and Power System. Transformer basically acts as a coupling medium. In addition, Transformer neutralize harmonics contained in the square waves produced by VSC.

4)      Harmonic Filter Harmonic Filter attenuates the harmonics and other high frequency components due to the VSC.

Voltage-Sourced Converters There are two basic categories of self commutating converters:

Current-sourced converters Voltage-sourced converters

Principle

By varying the amplitude of the output voltages produced Vo

The reactive power exchange between the converter and the ac system can be controlled in a manner similar to that of the synchronous machine.

If the amplitude of the output voltage (Vo) is increased above that of the ac system(V), then the current flows from the converter to the ac system, and the c o n v e r t e r g e n e r a t e s r e a c t i v e power(capacitive)

It the amplitude of the output voltage (Vo) is decreased below that of the ac system(V), then the reactive current flows from the ac system tie the convertor , and the converter absorbs reactive power(inductive)

If the amplitude of the output voltage is equal to that of that of the ac system voltage, the reactive current absorb is zero.

The practical converters are used for STATCOM are. Single phase H-bridge, or three phase two level, multipulse inverter, multilevel inverter etc.

The direct current in a voltage-sourced converter flows in either direction, the converter valves have to be bidirectional, and also, since the dc voltage does not reverse, the turn-off devices need not have reverse voltage capability; Such turn-off devices are known as asymmetric turn-off devices.

Thus, a voltage-sourced converter valve is made up of an asymmetric turn-off device such as a GTO with a parallel diode connected in reverse.

However, for high power converters, provision of separate diodes is advantageous

Working

Working of A single phase STATCOM

Working T1 and T2 (based on say GTOs) are switched on and off once in a cycle.

The conduction period of each switch is 180± and care has to be taken to see that T1 is off when T2 is on and vice versa.

The diodes D1 and D2 enable the conduction of the current in the reverse direction.

The charge on the capacitors ensure that the diodes are reverse biased.

The voltage VPN = Vdc/2 when T1 is conducting (T2 is off) and VPN = -Vdc/2 when T2 is conducting (and T1 is off).

When E1 > V , the STATCOM draws a capacitive reactive current, whereas

It is inductive if E1 < V . At the instant when T1 is switched on and Ir is inductive, the current (Ir) flowing through the circuit is negative (as it is a lagging current) and flows through T1 (as iT1 is negative of Ir).

After 90±, the current through T1 becomes zero and as Ir rises above zero and becomes positive, the diode D1 takes over conduction. Similar events occur when T2 turns on and off.

On the other hand, when Ir is capacitive, the current Ir is positive at the instant of turning on T1 and flows through the diode D1.

After 90±, the current reverses its sign and flows through T1.

At the time of switching off T1, the current through it is at its peak value.

SINGLE.PHASE FULL.WAVE BRIDGECONVERTER OPERATION

Waveforms

Modes of Operation

Advantages of a STATCOM over a SC The advantages of a STATCOM over a SC are:

(a) The response is much faster to changing system conditions.

(b) It does not contribute to short circuit current.(c) It has a symmetric lead-lag capability.(d) It has no moving parts and hence the maintenance is easier.

(e) It has no problems of loss of synchronism under a major disturbance.

Comparison of SVC and STATCOM

The power exchange between the STATCOM and the ac system.

Any combination of real power generation or absorption with var generation or absorption is achievable if the STATCOM is equipped with an energy-storage device of suitable capacity.

With this capability, extremely effective control strategies for the modulation of reactive- and real-output power can be achieved to improve the transient- and dynamic-system-stability limits.

The power exchange between the STATCOM and the ac system.

In practice, the semiconductor switches of the converter are not lossless, so the energy stored in the dc capacitor is eventually used to meet the internal losses of the converter.

In this way, the converter absorbs a small amount of real power from the ac system to meet its internal losses and keep the capacitor voltage at the desired level.

The power exchange between the STATCOM and the ac system.

Basic Control Approaches A static (var) generator converter comprises a large number of gate-controlled semiconductor power switches (GTO thyristors).

The gat ing commands for these devices are generated by the internal converter control in response to the demand for reactive and/or real power reference signal(s).

The reference signals are provided by the external or system control, from operator instructions and system variables, which determine the functional operation of the STATCOM

Internal control

The internal control is an integral part of the converter.

Its main function is to operate the converter power switches so as to generate a fundamental output voltage waveform with the demanded magnitude and phase angle in synchronism with the ac system

Main functions of the internal converter control

The internal control computes the magnitude and phase angle of the required output voltage from Iqref provided by the external control

I t generates a set of coord inated t iming waveforms ("gating pattern"), which determines the on and off periods of each switch in converter corresponding to the wanted output voltage.

These timing waveforms have a defined phase relationship between them, determined by the converter pulse number.

Internal control can be achieved by Indirectly : via contro l l ing the capac i tor voltage(which in turn is controlled by the angle of the output voltage) or

D i r e c t l y : b y i n t e r n a l v o l t a g e c o n t r o l mechanism(e.g PWM) of converter in which case the dc voltage is kept constant ( by control of the angle)

Indirect control

The inputs to the internal control are: the ac system bus voltage, v, the output current of the converter, io, and the reactive current reference, Iqref.

Basic control scheme for the voltage-sourced converter type var generatorcontrolling the reactive output by the variation of the dc capacitor voltage ("indirect" output voltage control).

Indirect control Voltage v operates a phase-locked loop that provides the basic synchronizing signal, angle θ.

The output current, io, is decomposed into its reac t i ve and rea l componen t s , and t he magnitude of the reactive current component, 1oq , i s compared to the react ive current reference, lqref.

The error thus obtained provides, after suitable ampl i f icat ion , angle a , which def ines the necessary phase shift between the output voltage of the converter and the ac system voltage needed for charging (or discharging) the storage capacitor to the dc voltage level required.

Accordingly, angle a is summed to θ to provide angle θ+α , which represents the des i red synchronizing signal for the converter to satisfy the reactive current reference.

Angle θ+α operates the gate pattern logic (which may be a digital look-up table) that provides the individual gate drive logic signals to operate the converter power switches.

Indirect control

Direct Control The input signals are again

the bus voltage, V, the converter output current, io, and the reactive current reference Iqref plus the dc voltage reference Vdc

This dc voltage reference determines the real power the converter must absorb from the ac system in order to supply its internal losses.

Basic control scheme for the voltage-sourced converter type var generatorcontrolling the reactive output by internal voltage (magnitude and angle) control at a sustained dc capacitor voltage ("direct" output voltage control).

Direct Control

The converter output current is decomposed into reactive and real current components.

These components are compared to the external reactive current reference (determined from compensation requirements) and the internal real current reference derived from the dc voltage regulation loop.

After suitable amplification , the real and reactive current error signals are converted into the magnitude and angle of the wanted converter output voltage , from which the appropriate gate drive signals , in proper relat ionship with the phase locked loop provided phase reference, are derived.

Direct Control

ln this case the internal real current reference would be summed to an externally provided real current reference that would indicate the desired real power exchange (either positive or negative) with the ac system.

The combined internal and external real current references (for converter losses and active power compensation), together with the prevailing reactive current demand, would determine the magnitude and angle of the output voltage generated, and thus the real and reactive power exchanged with the ac system.

Direct Control

Control characteristics of a STATCOM

the STATCOM can supply both the capacitive and the inductive compensation

and is able to independently control its output current over the rated maximum capacitive or inductive range irrespective of the amount of ac-system voltage.

That is, the STATCOM can provide full capacitive-reactive power at any system voltage