AC-DC system power flow control by PWM converter

Developments in gate/base turn-off semiconductor devices with high power handling capabilities, principally the gate turn-off thyristor (GTO), have led to compact self-commutating converter circuits being widely used for applications such as AC drives, where control of the AC voltage and frequency at the motor terminals are needed. The converter operates as an inverter to supply power from the DC source to the motor, or with reversed power flow as a rectifier for braking. Additionally, pulse width modulation (PWM) can be employed for variation of the AC voltage and improving the generated AC voltage and current waveforms. Operation is again in two quadrants, but power reversal is by reversal of the direct current, not the voltage, which is much more convenient. The use of such a converter is considered as an alternative to that controlled by firing delay for long term bidirectional control of power flow between live AC and DC systems