보조 밸브를 사용한 MMC 기반 HVDC용 밸브 성능 시험회로

Abstract

This paper deals with a valve performance test circuit for MMC based HVDC power station. The HVDC system enables asynchronous connections between different systems with different voltage and frequency each other, and is much more advantageous than the AC system, especially in terms of cost and efficiency in long distance transmission. As the demand for large-scale DC power systems based on solar power and ESS increases, the HVDC systems have been more widely adopted to the power transmission system with its high efficiency. HVDC system is divided into LCC type and VSC type. Among them, the VSC type HVDC system has been actively researched because of its advantages, such as an ability of independent control of reactive and active power, fast transient response characteristics using PWM techniques, and small sized filter, etc. 3-phase MMC based HVDC system has 6 arms. Each arm consists of hundreds of submodules and one inductor for preventing rapid current change and reducing harmonics. It is necessary for the MMC based HVDC system to obtain highly qualified reliability before installation. Therefore, IEC-62501 provides clauses for the performance test of valve section of MMC based HVDC system. In order to satisfy this clause, it is necessary to provide a valve current having DC offset component and the 2nd harmonic component which is very similar to the real arm current of the MMC based HVDC system. Several test circuits have been suggested to provide the required current, however, unfortunately, the conventional test circuits can not perform valve performance test. This paper proposes a new test circuit that can provide the required valve current with DC component as well as the 2nd harmonic component. The circuit operation of the proposed test circuit is described, and simulation and experimental results show the usefulness of the proposed scheme.