An Online Measuring Method for Tangent Delta of Power Cables Based on an Injected Very-Low-Frequency Signal

The deterioration of cable insulation is a widely acknowledged problem, and detection lays the foundation for maintenance and overhaul. The measurement of Tangent delta (tan $\boldsymbol {\delta }$ ) at very low frequency (VLF) works well on cable insulation detection, nevertheless the conventional method could only be conducted offline, where the power outage is usually required, interrupting the normal operation. To break thus a limitation, an advanced online method is proposed, and tan $\boldsymbol {\delta }$ would be calculated based on the measurement of an artificially injected VLF voltage and that of the VLF leakage current through cable insulation. To coordinate with this method, an injecting source, with the comprehensive self-protection and the by-pass channel for power-frequency signals, is developed to safely inject a VLF voltage to the power system, and a monitoring system, with an accuracy better than $\pm 0.3~\boldsymbol {\mu }\text{A}$ , is established to obtain the injected voltage’s and leakage current’s waveforms, which would be used to further calculate tan $\boldsymbol {\delta }$ with the cross-correlation algorithm. In addition, this method, as well as developed devices, is applied on several cables in China Electric Power Research Institute, and the measuring results keep consistent with the actual conditions, which verifies the validity.