Unity power factor control and harmonic current reduction of thyristor converters using variable series capacitors

Thyristor converters will have high reliability for large current DC power supply system used for high field magnets such as superconducting magnetic energy storage (SMES) and fusion devices. However, the thyristor converters control the DC voltage with a lagging power factor on the ac side. Especially, the lower DC voltage control requires the larger reactive power compensation. The authors carry out the experimental study on the leading power factor control of a thyristor converter using variable series capacitors. Combined with a pure thyristor converter operating with a lagging power factor, the power factor correction can be achieved. Additionally, the authors also investigate the harmonic current reduction by the effect of the combined thyristor converter system from the demonstration results and the simulation results. From the demonstration results of the 6 pulse combined thyristor converter system, the combination between the leading and lagging phase angles is selected to around 15 degree, the total harmonic distortion of the AC current can be reduced to that of the 12 pulse pure thyristor converter system case. Similarly, from the simulation results of the 12 pulse combined converter system, the combination between the leading and lagging phase angles is selected to 7.5 degree, the total line current waveform can be improved as similar waveforms of the 24 pulse pure thyristor converter system, which facts verified the harmonic current reduction by the effect of the combined thyristor converter concept.