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Fast diagnosis of commutation failure in HVDC systems can improve the level of operation in the power systems. It is also the base to realize the prevention of commutation failure. By application of wavelet transform, the rapidity and correctness of faults diagnosing are enhanced. AC voltage and DC voltage in inverter side are taken as the signals for wavelet analysis. Wavelet coefficient d6 changed suddenly when fault occurred. This paper studied a diagnosing method based on the variance of wavelet coefficient d6. With moving sample windows, the sampling signals in the windows are analyzed with wavelet and the variance of wavelet coefficient d6 in real time. Therefore, the fault diagnoses are realized in real-time. The functions are realized in MATLAB/Simulink. The results of simulations show that the method can output the alarm signals after about half cycle when fault occurred. It can also tell out the AC fault without CF and DC line fault with CF fault. Above all, the diagnosing method using wavelet analysis ameliorates the rapidity and correctness of faults diagnosing.