Study on the Operation of MMC-STATCOM under Unbalanced Grid Voltage

The introduction of renewable energy into power systems has been increasing, and it is expected to reach 40 % of generation power by 2030 around the world. The rise of renewable energy is bringing technical problems such as reduced grid inertia or grid instability. To overcome such problems, compensators (STATCOMs) can play an important role. The STATCOMs contribute to the stabilization of the power systems by compensating for three-phase balanced reactive currents. Therefore, the number of installations of the STATCOM has increased as a means of improving power quality since the 80 MVA GTO STATCOM was introduced to the Inuyama switching station of the Kansai Electric Power Co., Inc. in 1991 [1]. Recently, STATCOMs applying Modular Multi-level Converter (MMC) technology have been commercialized. For the MMC-STATCOM, several papers regarding the balancing control of capacitor voltages have been published [2]~[4]. This paper describes the operation of an MMC-converter configuring a delta connection for the STATCOM under unbalanced AC system voltages. Under the unbalanced conditions, when the MMC-STATCOM outputs the three-phase balanced reactive current, it is possible to balance the capacitor voltages of each phase using the circulating current flowing in the delta connection. The authors have proposed a method of controlling unbalanced currents in the three phases of the STATCOM in addition to controlling the circulating current for current reduction in the previous paper [5]. In this paper, the amplitude of the arm currents is investigated since the maximum current affects to the converter design factors such as the current rating.