Voltage-source converters (VSCs) and derived controllers

The solid-state DC-AC power electronic converters can be classified into two categories 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 topologies, namely the conventional six-switch three-phase VSC and other multilevel topologies.

previous Switching transients in the general case

next Single-phase half-bridge VSC