Durability Analysis of Metal Oxide Varistor under Direct Current Switching Surges

Abstract

The circuit-break switch of a direct current (DC) system will have to dissipate any stored residual line inductance energy during the switch-off period. Therefore, it is common to make use of a metal oxide varistor (MOV) mounted in parallel with a DC switch, in order to prevent arc damage by absorbing and transferring stored residual line inductance energy around the switch. However, the parallel-connected surge protecting MOV should not fail before the expected lifetime operation of the DC switch. Under repeated DC switching surge conditions, a zinc oxide (ZnO) based varistor degrades and its energy absorption capability reduces over time. Fast consecutive switch-off operations are not common in circuit-break DC switch applications, thereby allowing sufficient time for excess absorbed energy by the MOV to be dissipated as heat to ambiance. However, it is not well understood how varistor DC switching surge durability could be modeled, for this specific type of energy handling capability (EHC). In this work, a novel method to model the MOV durability or resiliency to DC switching surges with time intervals in-between for sufficient cooling is proposed. The obtained model will then serve as an average numerical durability indicator that could assist designers in predicting the expected lifetime of the parallel connected surge protecting MOV. It will also be a more effective way of comparing this specific type of degradation or EHC between different manufacturers with similar specifications.