During the switching process of the on load tap changers (OLTC)

each contact collides in sequence in closing and opening operation and generates vibration signal. The signal’s characteristics reflects the collision energy and stiffness of the contacts. In order to study the dynamic characteristic of the contact collision

the collision dynamic model with 4 contacts of M-type OLTC is set up firstly

and the influence of static contact stiffness on the response characteristic of the model is calculated. Then

the spring relaxation fault of the stationary contact is simulated and the corresponding vibration signal of spring with wire diameter of 0.6 mm and 1.0 mm are obtained. After that

the envelope calculation method based on Savitzky-Golay filter is proposed to extract the average envelope of vibration signal during the odd position to the even position and the opposite. Finally

the correlation coefficients of the average envelope corresponding to the static spring of the two wire diameters are calculated. The study results show that the natural frequency of the contact collision system decreases with the decreasing stiffness of stationary contact

consequently leads to a slower response speed of the contact collision and the time delay of the first peak of the vibration signals. Furthermore

the correlation coefficient of the average envelopes is only 1.6

indicating a significant variation of the vibration signal caused by the change of stiffness of the contact. The study conclusion can be used in the diagnosis of the contact condition of the OLTC.