A new approach to the capacitor commutated converter with three-phase voltage-source PWM converter for HVDC

摘要

The thyristor rectifiers for HVDC transmission systems have the following drawbacks: i) sensitivity to commutation failure, and ii) consumption of reactive power. One way to overcome these drawbacks is the use of capacitors connected between the thyristor rectifier and ac bus voltage. The thyristor rectifier with these commutation capacitors is called capacitor commutated converter (CCC). The CCC makes it possible to improve commutation failure performance and reduce the reactive power consumption. Recently, CCCs have been reconsidered for practical uses, and research and development task is being energetically pursued. An increased commutation margin-angle is obtained when the ac bus voltage is decreased. When the ac bus voltage is increased, however, commutation failure occurs. Here, if we can realize variable capacitance for the commutation capacitors, successful commutation is achieved with both increased and decreased ac bus voltage under a firing angle near 180°, and this results in the reduced reactive power consumption. This paper introduces a new approach to the capacitor commutated converter, in which an active capacitor consisting of a three-phase PWM converter serves as the commutation capacitor. The authors have named it the active-capacitor commutated converter (ACCC). The capacitance of the proposed active commutation capacitor is variable, so that we can overcome the drawback of CCCs mentioned above. For the inverter operation in the CCC, constant commutation margin-angle control is used. The authors propose a constant commutation margin-angle control method using a function generator block. The basic principle of the proposed active-capacitor commutated converter is discussed in detail. The validity and effectiveness of the ACCC with the proposed constant commutation margin-angle control is confirmed using digital computer simulation.