Series compensation of thyristor converters for superconducting magnets

Thyristor converters are used as power conditioning systems for superconducting coils. The objective of this work is to discuss the series compensation of thyristor converters using variable series capacitors. Traditional thyristor converters can control the coil voltage by changing the firing angle and then also the lagging power factor on the ac side. Similarly, the series capacitor can control the coil voltage through a unity power factor thyristor converter (α=0) with a resulting leading power factor seen from the grid. Therefore, by combining the two, both leading and lagging reactive power control, and also the active power control of the superconducting coils can be achieved. In fact, when neglecting commutation angles, the combined system reduces the total installed reactive power compensation need with 50% compared to a pure traditional system. In order to demonstrate the feasibility of the series compensation of the thyristor converter, a prototype converter using a gate-commuted series capacitor (GCSC) is developed. From the experimental results using a superconducting coil, the authors verified the leading current control capability of the series compensated thyristor converter, and also demonstrated the reactive power compensation of the combined thyristor converter system.