Abstract: The transient overvoltage harmonic signals of primary electrical equipment contain noise, leading to low convergence of harmonic amplitudes when using them to match harmonic absorption impedance. To address this, a study was conducted on methods for suppressing transient overvoltages in primary electrical equipment at hydropower plants. By introducing adaptive thresholds and optimizing wavelet basis functions in wavelets, the original transient overvoltage data from primary electrical equipment is decomposed to obtain noise-free transient overvoltage signals. Through matching the absorption impedance of transient overvoltage harmonics after decomposition, the resonance suppression of transient overvoltages in equipment is achieved. Test results show that the designed method can reduce the harmonic amplitudes to a low level based on the decomposition of noise-free transient overvoltage signals from primary electrical equipment.