Proposal and Digital Analysis of HVDC Transmission Systems Incorporating Hybrid Diode and Thyristor Rectifiers and Voltage Source Inverters

Abstract

Many researchers have proposed the application of diode rectifiers in the HVdc transmission systems, the major objective of which is cost reduction. However, simple replacement of thyristor converters at the rectifier station with diode ones while keeping line-commutated thyristor inverters on the other end will cause several problems which degrades HVdc system performance to a level that the HVdc system becomes impractical. The major problems are: (1) dc line overvoltage upon startup of the system, (2) large fault current upon occurence of dc line grounding fault, and (3) commutation failure of the inverters. To overcome these problems while almost keeping the cost reduction merit, the authors discuss the possible combination of diode rectifiers, line commutated thyristor converters and the voltage source converters. Series connection of diode and thyristor converters in the rectifier stations (which is called diode-thyristor hybrid rectifier station scheme) will solve problems (1) and (2). The use of voltage source converters in the inverter stations will remove problem (3). This combination will be a cost-effective solution when there is no need for bi-directional power flow and when the receiving end network needs the ability of voltage source inverters.