MMC 기술기반 능동필터를 활용한 전력품질 개선에 관한 연구

Abstract

최근 전력 전자 소자의 발전과 대용량 전력변환 장치 사용 증대로 인한 비선형 부하의 증가로 국내 발전 플랜트(석탄 및 복합화력)의 소내 전력 계통에 고조파가 포함되는 현상이 늘고 있다. 특히 발전소에서 필수적으로 사용하는 대용량 인버터 모터 및 사이리스터(Thyristor)를 사용한 여자시스템과 대표적인 환경설비인 전기집진기(Electrostatic Precipitator) 그리고 해수전해염소주입설비(Seawater Electro-Hypochlorination System)의 사용은 다수의 저차수 고조파를 발생시켜 전압과 전류의 여러 가지 왜곡을 일으키고 있으며, 이러한 고조파는 전력 손실의 증가와 전력품질 및 효율을 저하시키고, 여러 주요 부하 설비의 수명을 단축시키며 전자 제어장치 및 보호계전기 등의 비 동작 또는 오동작을 유발시키는 주요 원인으로 부각되고 있다.

일반적으로 고조파를 제거하는 방식으로는 부하의 운전 방식 및 환경에 따라 커패시터와 리액터를 직렬 연결하여 지정된 주파수의 공진현상을 이용하여 특정 차수의 고조파를 흡수하는 수동형 고조파 필터(Passive Power Filter)와 50차수 이하의 여러 차수의 고조파전류를 실시간으로 분석, 전압형 컨버터를 적용하여 반대 방향의 파형을 발생시켜 고조파전류를 상쇄시켜주는 능동형 고조파 필터(Active Power Filter)가 있다. 그러나 일반 형태의 전압형 컨버터를 적용한 고조파 필터를 활용하여 발전소 내의 전력품질을 향상시키기에는 실시간 제어기의 안정적인 운용 및 소자의 사용전압 한계성 극복이라는 어려움이 있으며, 고속 스위칭으로 인한 과다손실 및 능동필터가 고조파 발생원으로써 계통에 악영향을 미치는 부작용이 따르기도 한다. 이에 본 연구에서는 기존의 일반형 능동필터 방식의 단점들을 보완한 고조파 저감(Harmonic Mitigation) 및 역률 개선 기능을 갖춘 모듈러 다중레벨(5레벨) 컨버터(Modular Multi-Level Converter) 기반의 능동형 고조파 필터를 설계, 제작하여 발전소 주요 고조파 발생원 중의 하나인 전기집진기 및 해수전해염소주입설비에 병렬로 연결하고, 발생하는 고조파 제거 및 기타 전력품질 개선을 위한 실증 시험을 통하여 MMC형 능동필터의 성능을 확인하였다.

Approximately 5% of the total generated power is used for residential load in the standard coal-fired power plant which is capable for 612[MVA], 525[MW] generation in Korea. Most of the residential loads are inductive operating at a low power factor of 77 ~ 78%. In general, several problems are caused by the low power factor operation of the 6.9kV power system of the power plant. The starting failure problem occurs due to the instantaneous voltage drop when large capacity motors start. Consequently, the load loss of the auxiliary transformer increases due to the enlarged reactive power. Therefore, the power generation efficiency can be poor. Recently, the technology of the power electronics has been rapidly developed, and the large capacity power converters are applied to power plants diversely. However, all the medium-voltage and low-voltage devices incorporating power converters are nonlinear loads and they produce large harmonics on the electric power network. For instance, static frequency converter, which adjusts motor speed with thyristor valves and generator exciter system that regulates the voltage and reactive power of the generator, is a typical medium-voltage nonlinear load. Electrostatic Precipitator and Seawater Electro-Hypochlorination System are typical low-voltage nonlinear loads. As the number of equipment generating harmonics exceeding a limit of total harmonic distortion(THD) in a power plant increases, the electric power quality gets worse. Particularly, the harmonics cause malfunction of electronic control units and protection relays. This leads to a frequent breakdown of main power generation facilities and, shortens a lifespan of the equipment. In order to avoid the aforementioned problems, the proper functional equipment is necessary. Generally, there are three types of equipment supplying reactive power to the system for improvement of low power factor and prevention of voltage drop. One of them is the capacitor banks including capacitors connecting with series reactors. Another is static var compensator(SVC) controlled by thyristor valves and the other is static var generator(SVG) which consists of insulated gate bipolar transistor(IGBT) components. Regarding equipment for harmonic elimination, there are two types. One is the shunt passive power filter(SPPF) that absorbs harmonic current of a certain order. Another is the shunt active power filter(SAPF) that eliminates harmonic currents of orders up to 50th in real time. A 2 level voltage-type converter is applied to the SAPF in order to analyze the harmonics from nonlinear loads in real time. SAPF generates currents in the opposite direction waveform in order to give an offset to the harmonic currents from the nonlinear loads. However, various unintended matters shall happen in applying the typical type of power factor correction devices or active filter to the power plants. Utilization of power factor correction equipment in the power plant is dangerous, especially when an excessive harmonic current flows into the grid. It may cause a very severe trouble due to the resonance phenomenon. Using 2 level converter type active filters in power plant can lead to abnormal voltage problems which influence generating facilities and increase the loss due to ultra fast and frequent switching of IGBT.

Therefore, this paper, proposes an active filter based on the modular multi-level converter(MMC) which overcomes the flaws of typical power factor correction devices and harmonic filters. The prototype of the MMC active filter(1kV 100A) is proposed with the control algorithms and techniques. In addition, the result of the reliability verification test through PS-CAD and RTDS and HIL(Hardware in-the Loop) is described. Finally, the prototype of MMC active filter is installed at the Seawater Electro-Hypochlorination System load in the power plant, and the harmonic elimination test is properly carried on according to the instruction suggested. As a result, all of the MMC active filter tests satisfy the performance index of the active filter, and 5th, 7th, 11th, 13th, and 17th harmonic currents generated from the nonlinear load are reduced to below the current distortion limits of individual harmonic order according to IEEE 519 standard. It is also possible to develop a ultra high voltage MMC active filter which is applicable to high voltage direct current(HVDC) transmission line by developing the MMC technology with connecting more and more sub-modules in series in order to increase the operating voltage. Furthermore, it is an excellent solution to solve the power quality problems caused by the rapidly growing of the distributed power system.