Design study on series compensated thyristor converters for large scale SMES

Superconducting magnetic energy storage (SMES) will be one of the feasible options for daily load leveling and power stabilization. Since the stored energy of SMES is proportional to the square of current, the rated current for superconducting coils will be selected to at least over 10 kA. Thyristor converters are very promising as a large current power conditioning system for high field magnets. The authors proposed the feasibility of the series compensation of thyristor converters applied to power conditioning system for superconducting coils. Using variable series capacitors, the thyristor converters can control the DC coil voltage with a resulting leading power factor. Combined with a classical thyristor converter operated with a lagging power factor, the combined converter system enables the power factor correction of the thyristor converters. The authors developed a prototype combined thyristor converter system using a gate-commuted series capacitor (GCSC), and verified the active power control capability of a superconducting coil with unity power factor. Additionally, both leading and lagging reactive power control capabilities of the combined system were also demonstrated. The authors also carried out a conceptual design of the 60-MW combined thyristor converter system applied to 360-MWh SMES for daily load leveling. From the results, the required capacity of the series compensators becomes 52 MVar to provide 60 MW of the rated output power. This capacity corresponds to 50% of the required capacity for the pure classical thyristor converter system with conventional shunt compensators.