Detection and Location Approach to Incipient Grounding Defect in Transformer Winding Based on Sequence Voltage Ratios at PV Inverter Switching Frequency

Abstract

Silicon Carbide (SiC) power devices are used increasingly in photovoltaic (PV) inverters, with the high switching frequency (fs) and voltage slew rate. For the SiC- based-inverter-fed transformer, the ageing of groundwall insulation is aggravated and the grounding defect should be reliably detected at incipient stage. In this paper, a novel approach based on negative and zero sequence voltage components at fs is proposed to realize comprehensive online monitoring of incipient grounding defect. At fs, the equivalent sequence circuit of the PV distribution network is built and the mechanism and characteristics of sequence voltage components are investigated in scenario of the high-resistance grounding defect. The effects are distinguished caused by defect from imbalanced operation. Two sequence voltages ratios are developed to identify the occurring phase and to evaluate the defect location and severity. The MATLAB/Simulink model and the SiC-based- inverter platform are built for simulation and online experimental validation, respectively. The proposed online approach can sensitively detect and locate the defect with grounded resistance above 1 kΩ from each winding. Due to its noninvasiveness nature and convenient implementation, the operating reliability of the PV distribution network can be enhanced effectively.