A novel capacitor voltage fluctuation suppression method for the low-frequency operating stage of MMC driven pumped storage system

The pumped storage power plant, which has the energy conversion and storage capacity, is an important solution for improving the power grid absorption ability to the unstable and fluctuating new energy from wind farms, as well as photovoltaic power stations. The modular multilevel converter (MMC) is a competitive topology for driving the pumped storage motor due to the advantages of high output power level and low output current harmonics. However, the voltages of the sub-module capacitors fluctuate largely when the MMC is operating at low-frequency stage, which would lead to an abnormal start-up process or an unstable low-speed running state of the variable speed pumped storage system. In order to suppress the low-frequency voltage fluctuation of the MMC sub-module capacitors, this paper proposes a charge channel topology including in-phase and inter-phase channel between the sub-modules for transferring the capacitor charge. Then, the reverse fluctuating fundamental component and third harmonic circulating component, as well as the synchronizing fluctuating second harmonic component, of the sub-module capacitor voltages could be counteracted, respectively. Eventually, the start-up operation performance, as well as the low-speed operation performance of the variable speed pumped storage system are enhanced greatly. The effectiveness of the proposed low-frequency capacitor voltage fluctuation suppression method is verified through simulation results.