Droop control strategy of MMILC based on energy fluctuation in sub-module capacitor

In hybrid AC/DC microgrid, interlinking converter (ILC) can balance power flow between AC and DC microgrid, thus stabilizing AC frequency and DC voltage in islanding mode. As a topology of ILC, modular multilevel interlinking converter (MMILC) has advantages of good harmonic characteristics, high quality waveforms and low switching loss. A two-stage droop control strategy for MMILC is proposed. Firstly, based on the energy fluctuation characteristics of the sub-module capacitance, a new unified droop characteristic of AC and DC side for MMILC is established. Then, a mathematical relation between DC voltage and equivalent sub-module capacitor voltage is given to adjust power reference according to deviation of dc voltage. Also the dead zone and an allowable minimum operation range of the droop characteristic are set. Finally, the control performance of MMILC in different load profiles and overload conditions are simulated. The simulation results show that the proposed two-stage droop control strategy of MMILC can automatically compensate the power to both AC/DC sides in different load profiles. Under overload conditions, MMILC can instantaneous block the active power transmission, which validates the correctness and availability of the control strategy.