Abstract
Microgrid (MG) is generally developed at utility terminal which contains lots of unbalanced loads and distributed generations (DGs). The interaction between MG and the unbalance loads or DGs will degrades the control performance of interfaced inverter in MG and dramatically leads to MG voltage unbalance. In this paper, a negative-sequence compensation based three-phase voltage unified correction strategy is proposed. While MG operates in islanded mode, a positive virtual impedance control is used to eliminate the negative voltage resulted from the negative-sequence current, and then a positive-sequence voltage control loop and negative-sequence control loop are used to improve the inverter control performance. While MG operates in grid-tied mode, the inverter operates as a negative-sequence current source to compensate the negative-sequence currents of loads to guarantee the point of common coupling (PCC) voltage balance. By using the proposed strategy, the voltage control performance of inverter can be improved; the MG power quality can be enhanced significantly. Simulation and experimental results verify the effectiveness of the proposed method.
Highlights
MG is a network which consists of distributed generations (DGs), loads, energy storage system (ESS), etc
While MG operates in grid-tied mode, both utility grid failure and unbalanced load would result in three-phase unbalanced voltage
In China power quality standards ‘GB 15543—2008’, if the unbalance voltage is resulted from the power grid, the voltage unbalanced factor (VUF) should be less than 2%; if the unbalanced voltage is result from the loads, the VUF should be less than 1.5%
Summary
MG is a network which consists of distributed generations (DGs), loads, energy storage system (ESS), etc. After the analysis of the control system in parallel inverters and the operating mode of MG, a unified threephase voltage correction strategy (UTVCS) based on negative-sequence compensation is proposed in this paper. This strategy is applied in the ESS inverter. While MG operates in grid-tied mode, the ESS inverter operates in negative-sequence current control mode to compensate the negative current of loads and to guarantee voltage balance. If the unbalanced voltage is resulted from the utility failure, MG switches its operation mode to islanded mode In this case, the MG voltage depends on the output voltage of ESS inverter. The MG voltage is still balanced in three-phase system
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