Certain Features of the SVC Response

Concepts of SVC Voltage Control

5.3 EFFECT OF NETWORK RESONANCES ON THE CONTROLLER RESPONSE

5.3.5 Certain Features of the SVC Response

Figures 5.15 and 5.19 illustrate some interesting characteristics of the SVC response. As soon as a fault occurs, the response of the SVC is determined by its transient gain, not by the power-system parameters. Note that the initial decay of the SVC susceptance following the fault takes the same amount of time irrespective of the system strength. The rate of SVC-susceptance decay is equal to the transient gain.

When the fault is cleared, certain overvoltage is experienced. Although the SVC eventually controls this overvoltage, it has no influence on the first over- voltage peak, irrespective of how fast the control may be, because the initial overvoltage peak is determined by the resonance in the ac system. Even if the TCR is fired instantaneously, it will not affect the initial response of the ac resonant system.

The subsequent stabilization of the SVC voltage and TCR susceptance will, however, be dependent on the ac system characteristics, as discussed already in previous sections of this chapter. The SVC response is faster with weak ac systems and relatively slower with strong systems, as evident from Figs. 5.15 and 5.19. However, the SVC controller is more susceptible to instability with the weak systems.

EFFECT OF NETWORK RESONANCES ON THE CONTROLLER RESPONSE 177 5.3.6 Methods for Improving the Voltage-Controller Response

In SVC applications in which large variations in network short-circuit power are anticipated, the voltage-regulator gain is optimized for the weakest network state or the worst contingency to ensure a fast, stable response under such con- ditions. If this gain is maintained constant, even when the network assumes a normal configuration with a significantly enhanced short-circuit power, a much slower response will result. Nevertheless, it is, desirable for the SVC to have a fast response over the entire range of network configurations. The different methods available for improving the SVC response are discussed in the follow- ing text.

5.3.6.1 Manual Gain Switching This method involves predetermining the optimal regulator gains for different system-operating conditions and allowing the operating personnel to manually switch the gains according to the exist- ing network states based on breaker-status signals. Higher gains can thus be used with stronger systems, resulting in a fast response. The drawback of this switching procedure is that the manual selection of gains cannot keep pace with the dynamically changing network conditions. In instances of large, sudden changes taking place within the network, this manual procedure may result in control instability. Even if the procedure were initiated automatically, it might not always be practical or possible to determine the optimum gains for numer- ous network-operating conditions. A block diagram of the control system incor- porating gain switching is given in Fig. 5.21.

5.3.6.2 The Nonlinear Gain This gain function introduces an enhanced gain to provide a fast response in case of large voltage fluctuations. In this scheme, the

V_ref

−

−

+ ++

Other Power-System Disturbances

Power System X_s

Slope K_SL

Variable Gain

SVC Voltage Regulator

SVC Variable Susceptance Additional

Control Signals (f, P, q) E

V

External Signals

B_max

B_min + Σ

Σ

Σ

±

± B_SVC − ~I_SVC

Figure 5.21 The SVC voltage controller with variable gain (V −∼Vrated).

178 CONCEPTS OF SVC VOLTAGE CONTROL

V_ref −

+ PI

Regulator V

B_ref e

V_e Σ

Figure 5.22 Nonlinear gain in the SVC voltage regulator.

variable gain block shown in Fig. 5.21 incorporates a nonlinear function, as depicted in Fig. 5.22. If the error voltage, DV, at the output of the referenced summing junction is less than a preset small value,ε, a unity gain is offered by the nonlinear function. Then, the response of the voltage regulator corresponds to small signals. IfDV is large and exceedsε, the gain increases continuously to give a fast response. If placing emphasis on a one-sided operation (overvolt- age or undervoltage control) is desired, a larger gain on one side of the error signal can easily be built into the nonlinear characteristics. An example case demonstrating the effectiveness of nonlinear gain is given in Fig. 5.23.

5.3.6.3 Bang-Bang Control This is a limiting case of the nonlinear gain in which the TCR and

/

or the TSCs switch between their off and on states.

This control is highly sensitive to the initiating disturbance and, hence, must be carefully preevaluated for all possible disturbances to avoid any instability.

5.3.6.4 The Gain Supervisor This [15], [16] is an automatic gain-control scheme that provides a continuous control of the regulator gain over a wide range of system-operating conditions, ensuring a consistently stable response in all situations. The gain supervisor implements an optimal gain based on nor- mal system configuration and continuously monitors the output of the voltage regulator for any sustained or growing oscillations. Such oscillations result if any degradation in the system strength occurs while the regulator still oper- ates with a high gain. The supervisor then repeatedly reduces the gain until the oscillations cease, thus maintaining control stability. As this gain super-

9 Volts

Volts

×10⁰ 6 3 0

×10⁰9 6 3 00

1 ms ×10² ms

1 2 3 4 5

With Nonlinear Gain

Without Nonlinear Gain

Figure 5.23 An example of the SVC response improvement with a nonlinear gain.

EFFECT OF NETWORK RESONANCES ON THE CONTROLLER RESPONSE 179

V_ref

−

+

++

Other Power System Disturbances

Additional Control Signals

(f, P, q) E

V −

External Signals +

Control Instability Detection

Gain Supervision

Control B_min

B_max Σ

Σ Power System

Xs

Slope X_SL

Variable Gain

SVC Voltage Regulator

SVC Variable Susceptance Σ

±

± B_SVC − ~I_SVC

Figure 5.24 The connection of gain-supervisor control to the SVC voltage-control system (V−∼V_rated).

visor is effective over a broad range of system-operating conditions, it has found widespread use in recent SVC installations.

The gain supervisor is connected in the SVC control system as shown in Fig.

5.24. It is a 3-terminal device, with one terminal used for instability detection and the other two used as the input and output, respectively, of the variable gain or voltage-control amplifier. The gain supervisor [15] comprises the following five main elements [as depicted in Fig. 5.25(a)]:

1. the input filter;

2. the level detector;

3. the pulse discriminator;

4. the integrator; and

5. the voltage-control amplifier (variable gain).

Input Filter This is a bandpass filter with its center frequency tuned to the fre- quency of the unstable controller mode. It thus allows the supervisor to respond only to the controller instability frequency, not to other system instabilities.

Level Detector This unit detects the presence of any oscillations. It compares the filtered voltage-regulator output with a preset level and generates pulses of duration equal to the time in which the input signals exceed the reference level. The magnitude of the preset level determines the sensitivity of the gain supervisor.

180 CONCEPTS OF SVC VOLTAGE CONTROL

Pulse Discriminator The unit deletes certain erroneous pulses emitted by the level detector (such pulses do not imply an unstable operation). These unwanted pulses are generated when, for instance, there is a sudden change in the regulator output in response to a step change in the bus voltage. A fixed number of pulses are eliminated in a predefined time interval to avoid an unnecessary reduction in the regulator gain.

Integrating Unit This unit integrates the total number of pulses emitted by the pulse discriminator and maintains this output until such time that the integrator is reset. The integrator output constitutes a multiplication input to the voltage- controlled amplifier.

Figure 5.25(b) illustrates the various signals in the gain supervisor [15]. Note that the pulse discriminator deletes two pulses generated by the pulse detector.

Figure 5.26 demonstrates how an unstable case with constant-regulator gain can be stabilized with a gain supervisor.

The gain supervisor is an effective device as long as the controller-instability frequency is far removed from the electromechanical-oscillation frequencies of the power system. It may be noted that the electromechanical-mode frequen- cies will not be given a path through the input filter and that the gain super- visor ensures control stability under degraded-system conditions but does not automatically guarantee optimal regulator gain during such conditions. It thus functions as a protection device. In system contingencies where optimal gains need to be selected to ensure a continually fast response, modifications in the gain supervisor or adoption of alternate methods are necessary.

One way of achieving optimal gains is through the use of integrator gain optimizers [16], [17]. Once a gain reduction has been initiated by the gain super- visor, these optimizers continuously adjust the gain to give the best response for those system conditions. They cause a small change in the voltage reference and examine the response of the voltage regulator. Based on the magnitude of the first undershoot, the gain is modified and another test is conducted. This process of adjusting the gain continues until either the original gain value has been restored or the gain has been optimized for the new prevailing system con- ditions. If the latter, the gain-optimization procedure is restarted after a certain delay period.

5.3.6.5 Series-Dynamic Compensation The voltage-regulator response can be improved by installing one or more dynamic compensator in series with the regulator [5], as shown in Fig. 5.27. This series compensator provides a phase lead to counteract the phase lags and delays that are inherent in the SVC voltage-control loop, including the ac system. As discussed previously, the delay introduced by the ac network is dependent on both the system strength and the SVC operating point. Figure 5.28 depicts the gain-and-phase character- istics of the open-loop controller-transfer function, B_SVCR

/

^BSVC0, without the series-dynamic compensation. These plots are obtained for a specified set of voltage-regulator parameters for two power systems, one of which is moder-

EFFECT OF NETWORK RESONANCES ON THE CONTROLLER RESPONSE 181

Input Filter

Level Detector

Pulse Discriminator

Voltage-Controlled Amplifer Integrating Unit

∼∼∼

(a)

(b)

1

2

3

4

Figure 5.25 (a) A block diagram of the gain supervisor and (b) the gain-supervisor signals: (1) input signal; (2) output from the pulse detector; (3) output from the pulse discriminator (two pulses deleted); and (4) the signal proportional to the gain reduction.

ately weak with ESCR₀ c0.7 pu and the other has ESCR₀ c1.6 pu. For each power system, the transfer-function characteristics are plotted for two levels of SVC susceptance outputs,B_SVC. A low-average TCR conduction ofB_TCRc0.3 pu corresponds to B_SVC (c1 − B_TCR) c0.7 pu, which is depicted by the solid curve. On the other hand, a high-average TCR conduction of B_TCR c 0.8 pu corresponds to BSVC c0.2 pu, which is depicted by the dashed curve.

182 CONCEPTS OF SVC VOLTAGE CONTROL Voltage-

Regulator Output

Voltage- Regulator

Output Voltage- Regulator

Gain (fixed)

Voltage- Regulator

Gain Lower

Higher Time

Time Time

Time (a)

(b)

Figure 5.26 Behavior of the SVC voltage controller: (a) without gain-supervisor con- trol and (b) with gain-supervisor control.

The low TCR conduction (B_SVCc0.7 pu) results in a greater phase lag (lower phase margin) relative to the high-average TCR conduction (B_SVC c 0.2 pu).

This effect is even greater if the power system is very weak (ESCR₀ c 0.7 pu). Thus, to maintain system stability, a greater need exists to provide phase compensation under such weak network configurations.

V_meas

V_ref

K_R 1 + sT_R V_e

− Σ SC (s )

B_SVCR +

B_{SVC 0}

Figure 5.27 The SVC voltage regulator with series-dynamic compensation.

EFFECT OF NETWORK RESONANCES ON THE CONTROLLER RESPONSE 183

−200 0 200 50 25 0

−25

−200 0 200 50 25 0

−25

Gain (dB)Phase (deg.)

Open-Loop Transfer Function

1 10 100 1000

Frequency (Hz)

1 10 100 1000

Frequency (Hz) K_R= 20, T_R= 0.2 s (K_T= 100 pu / s)

F_{r 0}= 110 Hz, ESCR₀= 1.6 pu F_{r 0}= 80 Hz, ESCR₀= 0.7 pu Crossover Frequency

at Gain = 0 dB

B_SVC= 0.7 pu B_SVC= 0.3 pu

Phase Margin

= Phase at Crossover Frequency

(a) (b)

Figure 5.28 Gain-and-phase margins for a simple SVC voltage regulator.

The series-compensation device offers a phase lead in the range of gain crossover that occurs typically between frequencies of 10 to 20 Hz. As the phase lag offered by the ac network is not a constant but, instead, varies with TCR conduction, the series-compensating device is designed to be nonlinear for providing adequate compensation over the entire range of SVC operations. An example of a series compensator and its corresponding gain-and-phase charac- teristics is depicted in Fig. 5.29. It is seen that, corresponding to a lower TCR conduction (which implies higherB_SVC), the compensator provides an increased phase but with a diminished gain value.

The influence of the nonlinear series compensator on the transient perfor- mance of the system is demonstrated in Fig. 5.30. This figure demonstrates the SVC responses for the two levels of SVC susceptance outputs:B_SVCc0.2 pu and B_SVC c 0.7 pu, corresponding to two settings of the voltage-regulator transient gain K_T. All the responses are depicted both with and without the series-dynamic compensation. It is seen that the compensator helps stabilize the low-TCR-conduction cases and provides enhanced damping with high-TCR- conduction cases. The series-dynamic compensator is more effective when the KT is small.

5.3.6.6 ac-Side Control Filters Another approach that has been employed successfully in many SVC installations for counteracting the adverse interaction

184 CONCEPTS OF SVC VOLTAGE CONTROL

B_SVC0= 0.2 pu

B_SVC0= 0.7 pu

1 10

−100 0 100 0 20

−20

100 1000

Phase (deg.)Gain (dB)

Frequency (Hz) (b) 1 + sT1

1 +sT₂

−

B_SVCR ⁺ sK_R

1 +sT_{R −} +

− 1

B_TOT B_max

B_max T₁₌ 0.011 s

T₂₌ 0.007 s

K_R= 1.3 s T_R= 0.004 s

B_SVC0 Σ Σ

(a) where

Figure 5.29 (a) A block diagram and (b) transfer function (DBSVC0

/

^{DBSVCR) of a}

nonlinear series-dynamic compensator for the SVC voltage regulator.

between the SVC voltage regulator and the low-order network resonances is the installation of a notch filter on the ac side of the voltage-measurement system.

This filter is tuned to the critical-resonant frequency in the 80–100 Hz range and is designed to provide minimum phase lag at 60 Hz to not significantly slow the response of SVC. The notch filter blocks the network-resonant frequency that otherwise translates to a lower frequency from the demodulation effect of the voltage transducer, and after circulating through the voltage-regulation loop, the frequency reenters the network in such a way that it causes resonance and consequent instability. Thus the effect of the notch filter is to increase the stability margin of the voltage-regulation loop.

The system loads exercise a significant damping influence on the network