Research on Commutations of Thyristor Converters Under Asymmetric Firing Control

Asymmetric firing control has the potential to be applied to thyristor converters in high-power magnet power supplies due to its ability to reduce reactive power generation. However, further analyses indicate new issues, including commutation failures, thyristor trigger designs, and others. The converter output characteristics, particularly regarding the dc voltage output and the fundamental reactive power generation, are affected by various commutation processes. In this article, first, theoretical analyses are conducted to reveal the mechanisms of commutations for different firing angle regions. Second, the output characteristics are analyzed with consideration of commutations, focusing on dc output voltage and fundamental reactive power generation. Calculations and simulations are performed to verify these analyses and evaluate the effect of reactive power reduction. The experimental results confirm the principles of asymmetric firing. Finally, the impact of asymmetric firing on relevant subsystems, including thyristors, snubber circuits, and trigger systems, is further discussed. Also, the effect of reactive power reduction is further supported by analyses of parallel converter systems.