Unipolar and bipolar degradation of low voltage Metal Oxide Varistors

Metal Oxide Varistors (MOVs) are widely used in power distribution networks and electric loads and appliances in order to protect against lightning surges and transients overvoltages. Degradation and failure of the MOVs due to lightning surges are one of the concerns of surge protection designers that usually lead to overdesigned or ineffective surge protection. In this paper, more than 100 low voltage MOVs are used to determine the effect of Degradation on their Energy Absorption Capabilities (EACs). First, the MOVs in different groups are degraded by different number of unipolar and bipolar impulse current surges and changes in their electrical parameters are measured and presented. Then, the MOVs are subjected to EAC tests until their destruction, in order to determine the effects of degradation on EAC. The test results show that EAC decreases with degradation due to surges. Unipolar surges degrade MOVs more than bipolar ones. However, the amount of degradation is highly dependent on the MOV and can be different in various MOVs. Furthermore, it is shown that degraded MOVs have longer Time-to-Failure (TtF) compared to that of new MOVs.