SVC Control Components and Models
4.6 ADDITIONAL CONTROL AND PROTECTION FUNCTIONS
The basic voltage regulator is usually augmented by certain special control and protection functions [3]–[9], [13]–[15] described in this section. Modeling of these functions is dependent on the nature of study, such as fundamental-fre- quency stability simulation or electromagnetic-transient program (EMTP) sim- ulation studies.
4.6.1 The Damping of Electromechanical Oscillations
Auxiliary control of SVC based on supplementary control signals, such as bus frequency, line current, line active
/
reactive power, and generator-rotor velocity, is often employed to enhance the damping contributed by SVC to electrome- chanical or power oscillations, thereby attaining much higher levels of stable power transfer. Details of these controls are presented in Chapter 6.The relevant auxiliary signal is measured, computed, and processed through
ADDITIONAL CONTROL AND PROTECTION FUNCTIONS 129 a controller that is similar in configuration to that of a power-system stabilizer.
The auxiliary controller typically includes one or two lead-lag stages, a gain, and a washout-transfer function. The output of the auxiliary controller may be applied at the reference-summing junction, as shown in Fig. 4.16(a) for IEEE Basic SVC Model 1 at the location labeled “Other Signals.” The auxiliary con- troller modifies the voltage reference only when a change occurs in the auxiliary control signal. In the steady state, this contribution is zero.
In another variant of the supplementary controller, the output of the auxil- iary controller is applied to the voltage-regulator output, thereby bypassing the voltage regulator itself. This application is either to expedite the SVC response by avoiding the time constants inherent in the voltage regulator or to trans- form the controller for the exclusive purpose of damping the electromechanical oscillations. If the power oscillations exceed a threshold, then a discontinuous damping signal is applied that provides a “bang-bang” kind of control. The voltage-regulator output is disabled in this case.
The strategy of applying the auxiliary-control signal to the output of voltage regulator is also adopted in some schemes for damping the subsynchronous oscillations in series-compensated lines.
4.6.2 The Susceptance (Reactive-Power) Regulator
The SVC requires a substantial reactive-power reserve capacity to improve sys- tem stability. In the event of a disturbance, the fast-voltage regulator control uses a significant part of the reactive-power range of the SVC to maintain a pre- specified terminal voltage. If the SVC continues to be in this state, not enough reactive-power capacity may be available for it to respond effectively to a sub- sequent disturbance. A slow susceptance (or var) regulator is provided in the control system that changes the voltage reference to return the SVC to a preset value of reactive-power output, which is usually quite small. Other neighboring compensating devices, such as mechanically switched capacitors or inductors, can then be employed to take up the required steady-state reactive-power load- ing.
The susceptance regulator shown in Fig. 4.22 [11] compares the voltage-reg- ulator output susceptanceBrefwith a setpointBset[8]. TheBsetis usually chosen to be the fundamental-frequency reactive-power contribution of the permanently connected harmonic filters of the SVC. In these cases, the Bset corresponds to a near-floating-state operation of the SVC. The error signal is transmitted through an integral control, and a corrective voltage contribution is applied to the voltage-reference junction. This control operates in a time period of tens of seconds or minutes; hence it does not interfere with the fast voltage regulation or auxiliary control of the SVC. The susceptance regulator may be inhibited during large disturbances to permit full voltage control for system restoration.
The operation of the susceptance regulator is illustrated in Fig. 4.23 [5], [13].
Let the SVC initially be at the steady-state operating point 1, which marks the intersection of the system load line and SVC V-I characteristics. If a sudden
130 SVC CONTROL COMPONENTS AND MODELS
(b)
(c) (a) Σ
−
−− +
+
+ Σ Measuring
Circuit
Measuring Circuit
Thyristor- Susceptance
Control Mech.
Equip.
Control Logic Susceptance
Regulator
∆VSR
Voltage Regulator
Slope
Transmission Voltage
KSL
Ve Vmeas
Bref Vref
Imeas
Berror
Bset
ISVC
Berror ∆VSR
∆VSR
min
∆VSR
max
s KSR
Mech.
Equip.
Control Logic
To/From Capacitor Banks,
Reactors, and LTC Transformers Bref
Figure 4.22 The susceptance regulator and mechanical-equipment switching: (a) the general structure; (b) the susceptance regulator; and (c) the mechanical equipment switching.
disturbance occurs in the system, reducing the SVC bus voltage by DVT, the SVC moves rapidly to operating point 2 by the action of the voltage regulator.
This operating point is described by the intersection of the new system load line and the SVCV-Icharacteristic.
If this decrease in voltage persists for some time, the susceptance regulator, through its slow integrator action, will modify the SVC reference voltage by DVSR and bring the SVC steady-state operating point to 3. Now, although the SVC voltage has been reduced below the desired reference, the SVC reactive-
ADDITIONAL CONTROL AND PROTECTION FUNCTIONS 131
2
1 System Load Line
Fast Slow
3 ∆VT
VT
IC IC 1 IQr
IL 0
Inductive Capacitive
Figure 4.23 Operation of the SVC susceptance regulator.
power range is still available for coping with any system contingency. The neighboring var sources may then be switched to raise the SVC voltage to the desired value.
4.6.3 The Control of Neighboring Var Devices
Mechanically switched capacitors (MSCs), mechanically switched reactors (MSRs), and LTC transformers are some of the major neighboring reactive- power devices that constitute the overall static var system and thus need to be controlled appropriately. Some of the switching strategies are
1. switching MSCs subsequent to sensing of the short circuit;
2. switching MSRs with overload capability for voltage regulation;
3. switching MSCs and MSRs to bring back the SVC to nearly its reactive- power setpoint; and
4. controlling the tap changing on LTC transformers.
All these switchings are slow and may be deliberately delayed for allow-
132 SVC CONTROL COMPONENTS AND MODELS
U Vmax U Vmin
In B
A A B
1 0 Vmeas
0 0
t2 t1 t
t Delayed Off Comparator
Block TSCs; Set Bref to Zero;
Deactivate Voltage Regulator Deactivate Susceptance Regulator
UVmax= 0.69 pu, t1= 30 ms UVmin= 0.60 pu, t2= 200 ms Figure 4.24 The undervoltage strategies.
ing fast control devices, such as an SVC, to act. The process of mechanical equipment switching is illustrated conceptually in Fig. 4.22(c) [11].
4.6.4 Undervoltage Strategies
A logic is incorporated in the SVC control to block the SVC during instances of severe undervoltage, such as faults. If the SVC continues to operate under these conditions, the voltage regulator will act to make the SVC output immensely capacitive, which may lead to excessive overvoltages as soon as the fault is cleared. One such model of undervoltage strategies [8] is illustrated in Fig. 4.24.
There,Vmeas represents the SVC bus voltage on HV side in parts per unit, and the limitsU Vmin andU Vmax specify the permissible maximum and minimum levels of the undervoltage.
When a 3-phase–rectified voltage drops below a thresholdU Vmin (typically 0.6 pu), in case of a 3-phase fault, the following actions are initiated:
1. the voltage regulator is disabled (Bref is clamped to zero), 2. all the TSCs are blocked, and
3. the susceptance regulator is inhibited.
After a prespecified time (about 30 ms) of ac voltage recovery to a second setpoint U Vmax, typically 0.69 pu (higher than the first setpoint U Vmin), the TSCs are permitted to deblock, and theBref clamp is withdrawn. After an addi- tional prespecified time (about 170 ms), the susceptance regulator is reinstated.
The undervoltage strategy is designed to become effective mainly during 3- phase faults, not during single line-to-ground faults. The discrimination between the two types of faults is implemented through the choice of threshold voltage.
4.6.5 The Secondary-Overvoltage Limiter
The control function provided by the secondary-overvoltage limiter ensures that the secondary (low-voltage) side of the SVC coupling transformer does not
ADDITIONAL CONTROL AND PROTECTION FUNCTIONS 133
Vmeas
Xtrafo ISVC
Seclimhv
Secvresp KI Vlim max
Vlim min
To Voltage-Regulator Limits
∆Bmax
+
+
+ 1
− s
Vlim max = 0
Vlim min = −550 MVAR Σ
Σ
∫
Seclimhv = 22.1 kV Xtrafo = 14.2 %
KI set for time constant of ∼300 ms where
Figure 4.25 The secondary-overvoltage limiter.
exceed the design limits during an abnormal situation when the SVC output is highly capacitive and the voltage regulator is inoperative.
The secondary-overvoltage limiter [8] shown in Fig. 4.25 comprises a closed- loop integral control, the output of which modifies the susceptance-output limits BmaxandBmin of the main voltage regulator shown in Figs. 4.15 and 4.16 [11].
The time constant of this slow-acting limiter typically is chosen as 300 ms, which is much longer than that incorporated in the main voltage control. The operation of this limiter therefore does not adversely affect the fast performance of the main voltage controller.
4.6.6 The TCR Overcurrent Limiter
The special control function provided by the TCR overcurrent limiter restricts the TCR current during periods of high voltage, such as those caused by load rejections, to prevent any damage to the thyristors. This function is also closed loop integral in nature, and it alters the firing angle to restrain the current to a preset level. The time constant of this controller is chosen so that it does not interact with other fast-acting controls. A value of 100 ms is typical. This overcurrent limiter can be modeled by the time-varying magnitude of Bmin in Figs. 4.16 and 4.17.
4.6.7 TCR Balance Control
The TCR balance-control function monitors and restricts the magnitude of dc flow in the TCR reactors during situations of large 2nd harmonic ac voltage distortion. This control function essentially acts through the TCR firing angle;
it is discussed in Section 5.4.3.
4.6.8 The Nonlinear Gain and the Gain Supervisor
A nonlinear gain is incorporated into the SVC voltage regulator to speed up the response during large disturbances. In many cases, a gain supervisor is installed to ensure a fast, stable response during changing values of system
134 SVC CONTROL COMPONENTS AND MODELS
strength (short-circuit levels) [3], [15], [41]. (Both features are discussed in Chapter 5.) The nonlinear gain is modeled in transient-stability simulations, but the need for an appropriate model of a gain supervisor may arise only during the electromagnetic-transient program (EMTP) simulations.