A Novel Insulation Parameter Online Measuring Technique Based on Two Voltage Transformers for Distribution Networks

To improve the measurement accuracy of ground insulation parameters in neutral point ungrounded distribution networks, a new online measuring technique based on two voltage transformers is proposed. The characteristic frequency current signal is injected at the open triangular side of the zero-sequence voltage transformer in the distribution network, and it is equivalent to the current source. The return characteristic voltage signal is measured at the open triangular side of another no-load zero-sequence voltage transformer. The ground insulation parameters are measured online based on the relationship between injected current and return voltage. According to the circulation loop of the characteristic signal, the injected current flows through the insulation parameters branch and the measured voltage is equal to the voltage of the insulation parameter branch. The voltage transformer internal impedance and harmonic elimination resistance do not affect the injected current and measured voltage, which increases the measurement accuracy. To prove the effectiveness and advantages of the method, the insulation parameter measuring device is developed and tested in a 10-kV distribution network laboratory and an actual 10-kV substation. The experiment and field test results show that the proposed method has high measurement accuracy. The measurement process can be realized without power interruption.