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Power electronic control in electrical systems 197
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The reactances are chosen such at i AC ˆ 1p:u: and the natural frequency is given
by n ˆ X C /(X S ‡ X T ) ˆ 3:6p:u: In case (a), the amplitude of the oscillatory com-
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ponent of current is exactly equal to i AC . In case (b), the oscillatory component
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has the amplitude ni AC and much higher current peaks are experienced. The
capacitor experiences higher voltage peaks and the supply voltage distortion is
greater.

6.3Voltage-source converters (VSCs) and derived
controllers

The solid-state DC±AC power electronic converters can be classified into two cat-
egories with respect to the type of their input source on the DC side being either a
voltage- or a current-source:

1. Voltage-source converters or else voltage-source inverters (VSIs): the DC bus
input is a voltage source (typically a capacitor) and its current through can be
either positive or negative. This allows power flow between the DC and AC sides
to be bidirectional through the reversal of the direction of the current.
2. Current-source converters (CSCs) or else current-source inverters (CSIs): the DC
bus input is a current source (typically an inductor in series with a voltage source,
i.e. a capacitor) and its voltage across can be either positive or negative. This also
allows the power flow between the DC and AC sides to be bidirectional through
the reversal of the polarity of the voltage.
The conventional phase-controlled thyristor-based converters can only be current
source systems. The modern converters based on fully controlled semiconductors can
be of either type. In most reactive power compensation applications, when fully
controlled power semiconductors are used, the converters then are voltage-source
based. However, the conventional thyristor-controlled converters are still used in
high power applications and conventional HVDC systems.
In the following sections, we discuss first the half-bridge and the full-bridge single-
phase VSC topologies. It is important to understand the operation principles of these
two basic converters to fully understand and appreciate all the other derived topo-
logies, namely the conventional six-switch three-phase VSC and other multilevel
topologies.

6.3.1 Single-phase half-bridge VSC

Let us consider first the simplest and basic solid-state DC±AC converter, namely the
single-phase half-bridge VSC. Figure 6.23 shows the power circuit. It consists of two
switching devices (S 1 and S 2 ) with two antiparallel diodes (D 1 and D 2 ) to accom-
modate the return of the current to the DC bus when required. This happens when
the load power factor is other than unity. In order to generate a mid-point (O)to
connect the return path of the load, two equal value capacitors (C 1 and C 2 ) are
connected in series across the DC input. The result is that the voltage V dc is split into
two equal sources across each capacitor with voltage of V dc /2. The assumption here is