Research on a Multi Frequency Common Mode Voltage Signal Injection Method

The insulation leakage current of distribution cables gradually increases with the degree of cable aging, but the leakage current with smaller amplitude will be submerged in the load current with larger amplitude, which is difficult to perceive and extract. To solve this problem, this article proposes a method based on common mode voltage signal injection, which achieves online monitoring of insulation aging of distribution cables by monitoring the coupling change characteristics of the injected signal. Firstly, a microampere level ultra-low frequency (0.01Hz-1Hz) broadband modulation voltage signal is superimposed between the appropriate neutral point and ground in the tested cable system. A high-precision common mode leakage current sensor is used to measure the common mode leakage current generated by the injected monitoring signal, and convert the weak current into an output voltage signal. Using a multi-channel acquisition and data processing device to collect output voltage signals, wavelet feature extraction is performed on the collected initial signal to obtain the fundamental frequency and phase of the square wave excitation source, as well as the amplitude and phase of the even harmonics of the output voltage. Based on this information, the microampere level current can be derived to determine the insulation capacitance value of the distribution cable.