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Power electronic control in electrical systems 311

It is clear from these results that the SVC is an effective system controller which
may be used to provide voltage regulation at the point of connection and to improve
substantially the voltage quality in power systems. In most three-phase applications
the SVC uses symmetrical voltage control (average value of 3 phases). However, the
SVC can also be used to provide compensation to unbalanced three-phase loads and,
at the same time, to restore voltage balance. This is achieved with a more sophisti-
cated SVC control module.

8.5 Thyristor-Controlled Series Compensator (TCSC)

This section presents time domain simulations of the TCSC circuit and its imple-
mentation in PSCAD/EMTDC v3.04 for PCs. Two examples are presented:
1. Firstly, the basic model of the TCSC is implemented in PSCAD/EMTDC in order
to identify its main characteristics.
2. Secondly, a test system representing a transmission network with an embedded
TCSC system is implemented to show the effectiveness of the TCSC to provide
line impedance compensation.
The TCSC is a key member of the FACTS family of power electronic controllers that
provides smooth, rapid and continuous adjustment of the transmission line imped-
ance. Figure 8.25 shows the equivalent circuit representation of the basic TCSC
scheme, where a TCR is placed in parallel with a fixed capacitor. The controlling
elements are the two back-to-back thyristors, connected in series with the linear
reactor.
The TCSC has three fundamental modes of operation as follows:
1. thyristor-blocked mode
2. thyristor-bypassed mode
3. thyristor operating in phase-controlled mode.

In thyristor-blocked mode, the current through the TCR is zero and the TCSC
functions as a capacitive reactance X C . In thyristor-bypassed mode, the thyristor
valves are fired with no delay and the TCSC has small inductive impedance.
When the thyristor operates in phase-controlled mode, the value of the firing angle

Fig. 8.25 TCSC basic scheme.