An Improved Leakage-current-based Online Monitoring Method of Transformer Insulation by Injecting Asymmetric Voltages at Photovolatic Inverter Switching Frequency

The leakage current is an effective indictor to evaluate electric asset condition in photovoltaic (PV) system. The PV-inverter-connected transformer bears large electrical and thermal stresses, so the insulation ageing is critical to monitor online. In this paper, an improved common-mode (CM)-leakage-current-based method is proposed, capable of separately quantifying the ageing severity of GW insulation in different phases. The key lies in asymmetric voltage injection by extra control of PV inverter switch actions. The changes in magnitude and phase angle of CM leakage current at the switching frequency are caused. In combined use with sequence voltages, insulation capacitance parameter can be calculated and the ageing phase can be identified. Online experimental results have validated that the proposed method is effective for two monitoring targets. Compared with the traditional leakage-current-based methods, the proposed improved method reduces the hardware cost and improves the measurement accuracy. The implementation has economic and efficient advantages. It is the first time that both ageing severity and phase of GW insulation are monitored using one leakage current sensor.