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178 Power electronic equipment

be very beneficial, include the control of AC voltage near conventional HVDC
converter terminals, the minimization of transmission losses resulting from local
generation or absorption of reactive power, and the suppression of subsynchronous
resonance. Some types of compensators can also be designed to assist in the limita-
tion of dynamic overvoltages.
In later parts of the chapter some widely used thyristor based controllers, namely
the TCR, the thyristor-controlled transformer (TCT), and the TSC are introduced.
We then discuss the conventional switch-mode voltage-source converters (VSCs).
Some new topologies incorporating solid-state technology to provide multilevel
waveforms for high power applications are also presented. Finally, the chapter
discusses applications of such technology in energy storage systems, HVDC power
transmission systems and active filtering.

6.2 Thyristor-controlled equipment

6.2.1 Thyristor-controlled reactor (TCR)

In this chapter, the IEEE terms and definitions for the various power electronic based
controllers are used throughout.
Thyristor-controlled reactor (TCR) is defined as: a shunt-connected thyristor-
controlled inductor whose effective reactance is varied in a continuous manner by
partial conduction control of the thyristor valve.
Thyristor-switched reactor (TSR) is defined as: a shunt-connected, thyristor-
switched inductor whose effective reactance is varied in a stepwise manner by full-
or zero-conduction operation of the thyristor valve.

6.2.1.1 Principles of operation of the TCR
The basis of the TCR is shown in Figure 6.1. The controlling element is the thyristor
controller, shown here as two back-to-back thyristors which conduct on alternate
half-cycles of the supply frequency. If the thyristors are gated into conduction
precisely at the peaks of the supply voltage, full conduction results in the reactor,
and the current is the same as though the thyristor controller were short-circuited.
The current is essentially reactive, lagging the voltage by nearly 90 . It contains a

small in-phase component due to the power losses in the reactor, which may be of the
order of 0.5±2% of the reactive power. Full conduction is shown by the current
waveform in Figure 6.2(a).
If the gating is delayed by equal amounts on both thyristors, a series of current
waveforms is obtained, such as those in Figure 6.2(a) through (d). Each of these
corresponds to a particular value of the gating angle a, which is measured from the
zero-crossing of the voltage. Full conduction is obtained with a gating angle of 90 .

Partial conduction is obtained with gating angles between 90 and 180 . The effect of

increasing the gating angle is to reduce the fundamental harmonic component of the
current. This is equivalent to an increase in the inductance of the reactor, reducing its
reactive power as well as its current. So far as the fundamental component of current
is concerned, the TCR is a controllable susceptance, and can therefore be applied as a
static compensator.