Page 314 - Power Electronic Control in Electrical Systems

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302 Transient studies of FACTS and Custom Power equipment

information for SVC control and protection purposes. The measured signals are
conditioned to provide suitable control to the other blocks of the control system.
Voltage regulator. The voltage regulator performs the closed loop voltage control.
The difference between the voltage reference and the voltage measured at the point of
connection of the SVC is fed as the control error signal to a PI (proportional-integral)
regulator that provides the total SVC susceptance reference required to minimize the
error.
Power oscillation damping. When a power modulation control circuit is included in
the SVC controller, this high-level control function utilizes the power system
response as the input and acts on the voltage regulation to provide damping for slow
electro-mechanical swings in the power system.
Allocator. This block has the function of converting the susceptance reference from
the voltage regulator into specific information which is then processed in order to
determine the number of reactive banks that must be switched on and the required
firing angle.
Linearizer. The linearizer converts the susceptance from the allocator to a firing
angle a. To maintain the same control response over the entire SVC operating range,
the angle a is determined as a non-linear function of the susceptance reference order.
This function is normally given as a table that is derived from the following formula
sin (pa)
1 X L B(a) ˆ a ‡ (8:1)
p
where B(a) is the susceptance of the TCR fired at the angle a.
Hunting detection and gain adjustment. The stability controller supervises the
operation of the voltage controller. Unstable operation (hunting), which may take
place during weak system operating conditions, will be detected and the gain of the
PI controller would be reduced by half to try to achieve stable operation.
TSC±TCR firing units. These units compute the angles a and generate firing pulses
for the TSC and TCR thyristor valves.
Other control elements that can be added to the control circuit are the DC and
reactive power controllers. With the addition of these elements the control system
becomes more robust and efficient; however, its complexity increases considerably.
Then, it is necessary to design the control system according to application require-
ments bearing in mind simplicity, efficiency and reliability. To illustrate the design
and implementation of the SVC control system, a simple single-phase circuit is
selected, where the SVC is connected between the source and the load. The FC/
TCR topology is used as shown in the test system in Figure 8.14.
The aim of the SVC in this application is to provide voltage regulation at the point
of connection, following load variations. Initially the SVC is operated in open-loop
mode and for this condition, the power exchange between the SVC and the AC
system should be zero. When breaker Brk is closed, the load is increased and the
voltage at the load point experiences a voltage sag of nearly 16%. When the load is
increased, the SVC controller operation changes to closed-loop mode in order to
adjust the SVC effective impedance X SVC so that it injects capacitive current into the
system to restore the voltage back to the target value.
The SVC parameters have been determined according to the compensation
requirements for the case when the second load is connected. Based on the reactive