Effect Of Arcing Behavior Characteristics On Welding And Electrical Endurance Of Relay Contact

The reliability of power switch directly influences the whole power system, in which the electrical contacts play a very important role for the reliable operation of switchgear. Monitoring and prediction on residual electrical endurance of electrical contact materials is an important work for the performance of power switches and stabilization of electrical systems.Previous methods of life prediction of electrical contact include dependence of erosion volume on current amplitude, surface roughness, effective contact distance, contact resistance, showering arc waveforms and so on. These methods, however, could not reflect the real-time electrical endurance of the contacts. A new try is presented to diagnose the life of electrode material with the waveform-sequence characteristics of the breaking/making current and voltage.In this thesis, a set of direct current source is designed, which uses the technique of three-phase bridge semi-control. A PC-DAQ data acquisition system is built with the technique of LabVIEW virtual instrument. The system is applied to sample, display, store and analyze the current and voltage waveforms obtained from the contact endurance tests. Data analyzing is performed under the resistive, lamp and inductive load at 14VDC respectively with the contact materials AgSnO2, AgNi10 and AgNi0.15 in time-domain and frequency-domain.As a result, using the diagnosis technique of electrical contact life with the waveform of making and breaking current and voltage, it is easy to observe the short-time welding phenomena during the contact breaking operations. And this is available for the synthetical diagnosis on the reliability and the residual electrical life of the contact. The breaking current duration changes randomly with the increase of operation and the short-time welding may occur suddenly or randomly without previous cumulated increase of arc duration. The quantity of electric charge and arcing energy strongly depend upon the breaking current duration and show little correlation with the operation number. Therefore, the breaking current duration is the most important index in breaking current waveforms. In the frequency domain, the wavelet transform which can show the time-frequency characteristics of breaking current is used to indicate how the spectrum changes with the arcing current and operation number, modulus maxima and center of power spectrum are extracted. As a result, the wavelet transform modulus maxima are not closely correlated with the operation number and the center of power spectrum is relatively stable.