Hybrid injection strategy of sub-module voltage ripple suppression for MMC under low frequency operation

Abstract

Energy crisis and climate changes have brought great challenges to traditional power generation and transmission. Nowadays, the power transmission system of VSC-HVDC attracts more and more attention. The reactive and active power can be separately controlled, which greatly improves the flexibility of power transmission. Moreover, the harmonic content of this system is extremely low. As an important topology of HVDC system, modular multilevel converters (MMC) have significant advantages. However, there is a problem of excessive circulating current between phases in MMC, which easily causes the fluctuation amplitude of the capacitor voltage in each sub-module to be extremely large, or even unstable. An improved method to suppress the circulating current is proposed in this paper. In order to balance the voltage of each capacitor, the average capacitor voltage control of the outer loop is used. Meanwhile, an independent controller of voltage balance and a controller of arm voltage balance are applied. In order to eliminate the circulating harmonics of currents, a controller of quasi-PR is adopted for the inner control loop. Then, a simulating model of MMC system was established to prove the effectiveness of the proposed method. Finally, an experimental prototype was built and the corresponding tested results are given. It is proved that the proposed method has obvious advantage on suppressing harmonic circulating current and capacitor voltage balance control.