Masahiro Takasaki

Enhancement of continuous operation performance of HVDC with self-commutated converter

This paper describes a converter control scheme to enhance continuous operation performance of HVDC system with voltage source type self commutated (SC) converter. The continuous operation of SC converter can be achieved by suppressing converter overcurrents and AC overvoltages in the case of AC line faults including phase failure. Development of a new control scheme has been accomplished by experiment using an AC/DC power system simulator in CRIEPI and the EMTP (Electro Magnetic Transients Program) analysis. At first, we show that converter overcurrents and AC overvoltages are caused by lack of control information to static limiter and insufficient control performance to negative-sequence current of SC converter. Next, an adequate converter control scheme is proposed which is composed of an ACRs (automatic current legulator) adjustable dynamic limiter to suppress the converter overcurrents, and the limit logic of SC converter to suppress AC overvoltages during phase failure.

Control and protection scheme of HVDC system with self-commutated converter in system fault conditions

For extending self-commutated converter application to future trunk power systems, it is important to develop a stable operation scheme as well as to realize substantial cost reduction through coordinated system and control design. Suppression controls of converter overcurrent and dc overvoltage in various system fault conditions are essential in order to ensure stable operation and cost reduction of HVDC systems with voltage source type self-commutated converters. Converter control and protection schemes which include such suppression controls have been developed, employing CRIEPIs ac/dc Power System Simulator test and EMTP analysis. This paper first discusses the cause of converter overcurrent at ac system faults, considering the effect of PWM pulse number and converter control speed. Continued operation has been achieved by adding a new overcurrent suppression scheme to the converter control. In the case of a dc line grounding fault, the selection of the grounding circuit constant and the adoption of a high-speed converter control practically ensure the reduction of dc overvoltage while suppressing converter overcurrent. The converter block and restart sequence after a dc fault, which is coordinated with dc circuit breaker operation, enables stable recovery of HVDC transmission as fast as the usual line-commutated HVDC system.