UNIT-VIIStatic Series Compensators

(1)Variable impedance type series compensators.

(a) GTO Thyristor-Controlled Series Capacitor (GCSC)(b) Thyristor-Switched Series Capacitor (TSSC) (c) Thyristor-Controlled Series Capacitor (TCSC)

(2)Switching converter type series compensators. Static synchronous series compensator (SSSC)

GTO Thyristor-Controlled Series Capacitor (GCSC)

It consists of a fixed capacitor in parallel with a GTO thyristor (or equivalent) valve (or switch) that has the capability to turn on and off upon command.

Fig. (a) Basic GTO-Controlled Series Capacitor, (b) principle of turn-off delay angle control, and (c) attainable compensating voltage waveform

• The objective of the GCSC scheme is to control the ac voltage vc across the capacitor at a given line current i. Evidently, when the GTO valve, sw, is closed, the voltage across the capacitor is zero, and when the valve is open, it is maximum. For controlling the capacitor voltage, the closing and opening of the valve is carried out in each half-cycle in synchronism with the ac system frequency.

• The GTO valve is stipulated to close (through appropriate control action) whenever the capacitor voltage crosses zero. (Recall that the thyristor valve of the TCR opens whenever the current crosses zero.)

• When the valve sw is opened at the crest of the (constant) line current (γ = 0), the resultant capacitor voltage vc will be the same as that obtained in steady state with a permanently open switch. When the opening of the valve is delayed by the angle γ with respect to the crest of the line current, the capacitor voltage can be expressed with a defined line current, i(t) = I cos ωt, as follows:

sinsin1)(1)( t

Cdtti

Ctv

t

C

The amplitude of fundamental capacitor voltage can be expressed as a function of γ

)2sin121()(

CIVCF

where γ is the amplitude of the line current, C is the capacitance of the GTO thyristor controlled capacitor, and ω is the angular frequency of the ac system.

Fundamental component of the series capacitor voltage vs. the turn-off delay angle γ.

This impedance can be written as )2sin121(1)(

C

X C

In a practical application the GCSC can be operated either to control the compensating voltage, VCF(γ), or the compensating reactance, XC(γ). In the voltage compensation mode, the GCSC is to maintain the rated compensating voltage in face of decreasing line current over a defined interval Imin<= I <=Imax as illustrated in Figure (a1).In this compensation mode the capacitive reactance XC, is selected so as to produce the rated compensating voltage with I= Imin, i.e., VCmax = XC Imin. As current Imin is increased toward Imax, the turn-off delay angle γ is increased to reduce the duration of the capacitor injection and thereby maintain the compensating voltage with increasing line current.

• In the impedance compensation mode, the GCSC is to maintain the maximum rated compensating reactance at any line current up to the rated maximum. In this compensation mode the capacitive impedance is chosen so as to provide the maximum series compensation at rated current, XC = Vcmax/Imax, that the GCSC can vary in the 0 <= XC(γ) <= XC range by controlling the effective capacitor voltage VCF(γ), i.e., XC(γ) = VCF(γ)/I.

Thyristor-Switched Series Capacitor (TSSC)

• The operating principle: the degree of series compensation is controlled in a step-like manner by increasing or decreasing the number of series capacitors inserted. A capacitor is inserted by turning off, and it is bypassed by turning on the corresponding thyristor valve.

• A thyristor valve commutates "naturally," that is, it turns off when the current crosses zero. Thus a capacitor can be inserted into the line by the thyristor valve only at the zero crossings of the line current.

• Since the insertion takes place at line current zero, a full half-cycle of the line current will charge the capacitor from zero to maximum and the successive, opposite polarity half-cycle of the line current will discharge it from this maximum to zero.

• As can be seen, the capacitor insertion at line current zero, necessitated by the switching limitation of the thyristor valve, results in a dc offset voltage which is equal to the amplitude of the ac capacitor voltage. In order to minimize the initial surge current in the valve, and the corresponding circuit transient, the thyristor valve should be turned on for bypass only when the capacitor voltage is zero. With the prevailing dc offset, this requirement can cause a delay of up to one full cycle, which would set the theoretical limit for the attainable response time of the TSSC.

Thyristor-Controlled Series Capacitor (TCSC)

It consists of the series compensating capacitor shunted by a TCR. In a practical TCSC implementation, several such basic compensators may be connected in series to obtain the desired voltage rating and operating characteristics. This arrangement is similar in structure to the TSSC and, if the impedance of the reactor, X1, is sufficiently smaller than that of the capacitor, XC, it can be operated in an on/off manner like the TSSC.

Basic TCSC Scheme

• However, the basic idea behind the TCSC scheme is to provide a continuously variable capacitor by means of partially canceling the effective compensating capacitance by the TCR.

sin2)(

)()()(

LL

CL

LCTCSC

XX

XXXXX

Damping effects of TCSC

Applications of variable series compensation -TCSC

• Power flow control • Enhancing transient stability • Damping of power swings • Sub-synchronous resonance damping

TCSC at a Substation

Static Synchronous Series Compensator(SSSC)

The SSSC is one of the most recent FACTS devices for power transmission series compensation. It can be considered as a synchronous voltage source as it can inject an almost sinusoidal voltage of variable and controllable amplitude and phase angle, in series with a transmission line. The injected voltage is almost in quadrature with the line current. A small part of the injected voltage that is in phase with the line current provides the losses in the inverter.

Most of the injected voltage, which is in quadrature with the line current, provides the effect of inserting an inductive or capacitive reactance in series with the transmission line. The variable reactance influences the electric power flow in the transmission line. The basic configuration of a SSSC is shown in Fig.

SSSC (a)WITH OUT STORAGE and (b)WITH STORAGE

• A static synchronous Series Compensator operated without an external energy source as Reactive Power with output voltage is in quadrature with and fully controllable independently of the transmission line current for the purpose of increasing or decreasing the overall reactive voltage drop across the transmission line and thereby controlling the electric power flow.

• The SSSC FACTS device can provide either capacitive or inductive injected voltage compensation, if SSSC-AC injected voltage, (Vs), lags the line current IL by 90º, a capacitive series voltage compensation is obtained in the transmission line and if leads IL by 90º, an inductive series compensation is achieved.

Theory of the SSSC

• Figure 1 shows a single line diagram of a simple Transmission line with an inductive transmission reactance, XL, connecting a sending end voltage source, and a receiving end voltage source, respectively.

• The expression of power flow is given by

Where Xeff is the effective total transmission line reactance between its sending and Receiving power system ends, including the equivalent “variable reactance” inserted by the equivalent injected voltage (Vs) (Buck or Boost) by the SSSC-FACTS Compensator.