The insulation parameters of the power grid to ground are measured by the tuning method

The tuning method is suitable for measuring the ground parameters of systems where the neutral point is grounded via an arc suppression coil, but not applicable to ungrounded neutral point systems. Its measurement principle involves injecting a current signal with continuously varying frequency from the secondary side of the Potential Transformer (PT), measuring the returned voltage signal, and identifying the system's resonant frequency.

During the frequency sweeping process, each injected heterodyne current signal corresponds to a returned voltage value, based on which insulation parameters of the distribution network such as ground capacitance, ground conductance, detuning degree, and damping rate are calculated. When the frequency of the injected current signal matches the resonant frequency, parallel resonance occurs in the system, and the amplitude of the returned voltage on the secondary side reaches its maximum.

Once the resonant frequency is determined, the ground parameters of the distribution network system can be calculated accordingly. The specific principle is illustrated in Figure 1: a variable-frequency current signal is injected from the secondary side of the PT, and by varying the signal frequency, the relationship between the injected signal and the returned voltage signal is measured to find the resonant angular frequency ω₀ of the distribution network.

The equivalent equivalent circuit of the injected signal at resonance is shown in Figure 2:

The advantage of the tuning method lies in that it does not require accurate measurement of the returned voltage value. It only needs to identify the resonant frequency injected when the returned voltage reaches its maximum, and then the grid parameters can be accurately calculated.