AC-DC Voltage Source Converter

AC-DC Voltage Source Converter:

A brief introduction The converter that we have studied is a "line commutated converter". The voltages required for commutation (turning off a device and switching on another) are obtained from the AC side. On the other hand, a Voltage Source Converter (VSC) uses a voltage source (or a capacitor whose voltage is maintained constant) on the DC side and switches with turn off capability. These switches can be turned on or off at will, if the voltage on the dc side is positive. The schematic of a single phase voltage source converter is shown below.

The voltage on the AC side is related to the DC side voltage depending on the switch positions:

s1 and s2 ON, s3 and s4 OFF : Vac = Vdc, idc = iac

 s3 and s4 ON, s1 and s2 OFF : Vac = - Vdc, idc = - iac

s1 and s4 ON, s3 and s2 OFF : Vac = 0, idc = 0

 s2 and s3 ON, s1 and s4 OFF : Vac = 0, idc = 0

However, two switches in a leg (e.g. s1 and s3 in the figure above) should not be turned on simultaneously (why ?)

The direction of current on the dc side can be bidirectional, although dc voltage has to be unidirectional. The switches can be implemented using power electronic devices as shown below -- a device like a Gate Turn Off Thyristor (GTO) in parallel with a reverse connected diode. Note that devices like Gate Turn Off Thyristors can be switched off by a gate signal - a capability which an ordinary thyristor lacks. Devices like Insulated Gate Bipolar Transistor also may be used in a VSC instead GTOs.