A new approach to the capacitor-commutated converter for HVDC-a combined commutation-capacitor of active and passive capacitors

This paper introduces a new approach to the capacitor-commutated converter (CCC) for HVDC. A small-rated three-phase voltage-source PWM converter is connected between a series commutation capacitor and thyristor rectifier through matching transformers. The PWM converter acts as auxiliary commutation-capacitor for the thyristor rectifier while the series capacitor acts as the main commutation capacitor. The capacitance, which is the sum of the small-rated active and series passive capacitors, is variable, so that stable commutation is obtained. In CCCs commutation failure occurs when the AC bus voltage is recovered whereas the proposed combined commutation-capacitor can achieve successful commutation for both rapidly decreasing and increasing AC bus voltages. The basic principle of the proposed active-passive capacitor commutated converter is discussed in detail. Then, constant margin-angle control with a constant firing angle of the thyristor converter is proposed using a function generator block. Digital simulation demonstrates the novelty and effectiveness of the proposed active-passive capacitor commutated converter