Power factor correction of large current line commutated converters using variable series capacitors

Diode rectifiers and thyristor converters are very promising as a large current power supply for high field magnets such as fusion magnets, superconducting magnetic energy storage. The objective of this work is to discuss the power factor correction of the line commutated converters using variable series capacitors. The diode rectifier can control the DC voltage with a resulting leading power factor on the AC side by controlling the series capacitance. On the other hand, thyristor converters can control the DC voltage with a lagging power factor on the AC side by controlling the firing angle. Therefore, by combining the two, the hybrid converter system can control the DC voltage of high field magnets with both leading and lagging power factors seen from the AC system. Furthermore, the reactive power compensation need can be reduced to 50 % compared to a full classical thyristor converter system with conventional shunt compensators. From the experimental results using a gate-commuted series capacitor (GCSC), the authors demonstrated the DC voltage control capability of the diode rectifier with a leading power factor, and the active and reactive power control capability of the hybrid converter system.