Insulation Parameter Online Measurement Technology Based on Extended Zero-Sequence Circuits for Resonant Grounding Distribution Networks

To eliminate the measurement errors of insulation parameters caused by the internal impedance of VT (voltage transformer) and the damping resistance in resonant grounding distribution networks, a new online measurement technology based on an extended zero-sequence circuit is proposed. A constant current signal with variable frequency is injected into distribution network through a VT, and the return voltage is measured from another unloaded VT. An extended zero-sequence circuit is established to ensure that the injected current and the return voltage directly act on the insulation branch to ground; thus, the influence of the VT internal impedance can be eliminated. The resonance frequency is obtained according to the phase difference between the injected current and the return voltage, and the measurement error caused by damping resistance can be eliminated. Then, the capacitance to ground and leakage conductance can be accurately calculated. A measuring device based on the proposed technology was developed, and tests were carried out on a 10-kV distribution network laboratory and an actual 10-kV substation. The test results show that the proposed method has high measurement accuracy. The measurement process does not require a power interruption, and it does not affect the normal operation of the power system.