Circulating current suppressing method based on active damping control of MMC considering low-frequency oscillation

In order to analyze and suppress the low-frequency circulating currents of modular multilevel converter (MMC), a transient mathematical model for the sum of capacitor voltages and the circulating current of arm is established. The model reveals the principle of low-frequency oscillation under abnormal conditions. And then, the analytical expressions of oscillation frequency and the resonance point of 2nd circulating current without circulating current control are derived. In order to inhibit both low-frequency oscillation and 2nd circulating current, a circulating current suppressing method (CCSM) based on active damping control is proposed, which is simple without decoupling and coordinate transformation. Finally, the simulation and experimental results show that this method can suppress the low-frequency oscillation in transient state as well as 2nd and 2nd above circulating currents in steady state simultaneously. © 2015, The editorial office of Transaction of China Electrotechnical Society. All right reserved.