Slide 1
5. Concepts of SVC Voltage ControlThe performance of SVC voltage
control is critically dependent on several factors, including the
influence of network resonances, transformersaturation, geomagnetic
effects, and voltage distortion. When SVCs are applied in
series-compensated networks, a different kind of resonance between
series capacitors and shunt inductors becomes decisive in the
selection of control parameters and filters used in measurement
circuits.Dynamic Characteristics voltagecurrent characteristic of
the SVC
voltagereactive-power characteristic of the SVC.
Linear range
The slope can be changed by the control system in
thyristor-controlled compensators,whereas in the case of saturated
reactor compensators, the slope is adjusted by the series
slope-correction capacitors.The slope is usually kept within 110%,
with a typical value of 35%. Although the SVC is expected to
regulate bus voltage, that is, maintain a flat voltage-current
profile with a zero slope, it becomes desirable to incorporate a
finite slope in the V-I characteristicsSteady-State
CharacteristicThe steady-state V-I characteristic ofthe SVC is very
similar to the dynamic V-I characteristic except for a deadband in
voltageVoltage control by SVC
A simplified block diagram of the power system and SVC control
system;
a phasor diagram of the ac system for the inductive SVC current;
and (c)characteristics of the simplified power system and the
SVC
Equation 2 represents the power-system characteristic or the
system load line. An implication of Eq. 2 is that the SVC is more
effective in controlling voltage in weak ac systems (high Xs) and
less effective in strong ac systems (low Xs).Advantages of the
Slope in the SVC Dynamic CharacteristicThe SVC slope1.
substantially reduces the reactive-power rating of the SVC for
achieving nearly the same control objectives;2. prevents the SVC
from reaching its reactive-power limits too frequently;
3. facilitates the sharing of reactive power among multiple
compensators operating in parallel.Reduction of the SVC Rating
Prevention of Frequent Operation at Reactive-Power Limitssmall
change in the system load line (from a small variation, E2 E1, in
the no-load equivalent system voltage, as viewed from the SVC bus)
maycause the SVC to traverse from one end of the reactive-power
range to the other end to maintain constant voltage. The
reactive-power limits of the SVC are reached more frequently if the
ac system tends to be strong, that is, when the slope of the system
load line is quite small.Load Sharing Between Parallel-Connected
SVCs
Consider two SVCs, SVC1 and SVC2, connected at a system bus as
depicted in Fig. 5.4(a). The two SVCs have the same ratings but the
reference voltages, Vref, of the two control characteristics differ
by a small amount, . In practice, is small, although it is not
zerotwo SVCs in parallelwith difference in the reference-voltage
setpoints without current droop
two SVCs in parallel with current droop and with difference in
the reference-voltagesetpoints
Design of the SVC Voltage Regulatortwo alternative ways of
modeling the voltage regulator exist: the gaintime-constant form
the integratorcurrent-droop form.In the gaintime-constant
representation, the voltage regulator is expressedby the following
transfer function:
Basic elements of SVC voltage-regulation control with TSC.
Simplistic Design Based On System Gain
an integratorwith susceptance-droop feedback;
an integrator with current-droop feedback
The block diagram of an SVC-compensated power system is shown It
is assumed that1. the change in system voltage DV caused by the SVC
is small;2. the SVC bus voltage is very close to the nominal-rated
voltage, that is, VSVC 1 pu; and3. the variations in the SVC
reference voltage are also quite smallA block diagram of the system
voltage controller incorporating an SVC
simplified block diagram of the system voltage controller for V
Vrated
The following simplifications are made:1. The voltage- and
current-measurement systems are considered identical.2. The TSC
switchings are ignored, and the droop effect of the capacitive
current is merged with Vref.3. The only variable considered is the
inductive current IL, which reduces the system bus voltage .The
effect of constant-capacitive SVC current on the SVC bus voltage is
incorporated in V0. The influence of any power-system disturbance,
Vz, is neglected.Effect of the system short-circuit level on the
SVC response time
EFFECT OF NETWORK RESONANCES ON THE CONTROLLERRESPONSE
single-line diagram of an SVC-compensated system and
(b)impedance-versus-frequency characteristics for an
SVC-compensated system
Comparision of three sysstem
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From the foregoing studies, the following are illustrated1. The
SVC response, in general, becomes faster with the increase in
transient gain of the voltage regulator.2. If the gain is made very
large, the SVC response may become oscillatoryor even unstable.3. A
sluggish SVC response for a strong system becomes faster as the
system strength deteriorates. If the regulator gain is optimized
for a highsystem strength, the SVC response may become unstable for
a weak system, implying that the SVC regulator gain should always
be optimized for the weakest system state to ensure the stable
response for any variation in system strength. The only
repercussion of this strategy is that the SVC response will become
slower as the system strength increases. However, this problem can
be resolved through a variable gain strategy.Effect of power-system
characteristics on the SVC transient response
Choice of transient gain
Estimate of gain1. identify the weakest network state
corresponding to the worst contingency;2. determine the ESCR0 and
Fr0 from impedance-versus-frequency studies;3. calculate the
transient-gain limit from eigenvalue studies; and4. choose the
regulator gain as half of the gain limit obtained
previously.Certain Features of the SVC ResponseAs soon as a fault
occurs, the response of the SVC is detWhen the fault is cleared,
certain overvoltage is experience detrmined by its transient
gainMethods for Improving the Voltage-Controller Response(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 existing network states based on breaker-status
signalsThe Nonlinear Gain
The Gain Supervisor
Gain supervisor check stability or oscillationsThe connection of
gain-supervisor control to the SVC voltage-controlsystem (V
Vrated).
1. block diagam of gain supervisor2.1. i/p signal2. o/p from
pulse detector3. o/p from pulse discriminiator4. signal prop to the
gain reductionInput Filter This is a bandpass filter with its
center frequency tuned to the frequency 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.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.Behavior
of the SVC voltage controller: (a) without gain-supervisor
controland (b) with gain-supervisor control
Effect of 2nd harmonics
Causes of 2nd Harmonic Distortion
Reactor/ Transformer Switching Near an SVC
transient response
TCR Balance Control
