A Voltage Rise Mitigation Strategy under Voltage Unbalance for a Grid-Connected Photovoltaic System

Abstract

­­The voltage rise due to the injection of power from a photovoltaic (PV) system into the power system network is a major problem for high penetration of PV generation. It is also observed that the voltages at the point of common coupling (PCC) are usually unbalanced due to the unbalanced loads in the system. One popular method to solve the voltage rise problem is by injection of reactive power to adjust the power factor. However, this is uneconomic because the PV owner may be charged by the utility for the injected reactive power. On the other hand, although some works have been done on unbalanced voltage compensation, their objective is purely to improve the power quality and has nothing to do with the voltage rise problem. Therefore, in general, the voltage rise and voltage unbalance are treated as two unrelated issues. In this thesis, it will be revealed that voltage unbalance usually contributes significantly to the maximum line-to-line voltage which is the criterion for voltage rise or overvoltage. Therefore, the voltage rise over the limit is in fact caused by two factors; the reverse power flow and the voltage unbalance. With this understanding, it is proposed in this thesis that cancellation of voltage unbalance by negative-sequence current injection should be done first to reduce the maximum line-to-line voltage. And if this measure helps to bring the voltage back within the limit, then no further action is needed. Otherwise, additional reactive power injection and/or active power curtailment may be necessary.