The on-load tap changer（OLTC） needs to be tested for working conditions by DC or AC source testing device before the ex-factory and during the maintenance period. The OLTC AC test device has problems such as inaccurate calculation of transition resistance and other information acquisition due to the large impedance of the measuring circuit. We proposed a solution method of the loop impedance by using the wavelet transform

Fourier transform

Prony algorithm to extract the base wave component of the sampled signal

and by taking the coordinate transformation of d and q as the core algorithm. Firstly

the key points of OLTC switching was identified by the wavelet transform

and then the differences between the Fourier algorithm

Prony algorithm and the least square algorithm in extracting the fundamental wave components of the signal were compared. Finally

the equivalent impedance and transition resistance of the circuit were calculated by d and q coordinate transformation. The results show that the simulation results are basically consistent with the measured results

verifying the validity of the method

and the error of the transition resistance calculated by the Fourier algorithm is the lowest

about 104.812 5%. This method provides references for optimizing the measurement of the working parameters of OLTC equipment

and can provide ideas for the next prototype development.