Metal Oxide Varistor Design Optimization and Main Breaker Branch Switch Control of a Progressively Switched Hybrid DC Circuit Breaker

In this article, metal oxide varistor (MOV) design optimization and switching control in the main circuit breaker (MCB) branch of a progressively switched hybrid dc circuit breaker (DCCB) is presented. A progressively switched hybrid DCCB can achieve faster fault isolation with reduced peak fault current magnitude and transient recovery voltage compared with a regular hybrid DCCB due to its modified switching strategy. Consequently, thermal stress on the semiconductor devices in MCB is significantly reduced. Analytical model of the system dynamics during fault isolation with progressive switching is derived to demonstrate the switching scheme's effect on the energy-absorbing component, MOV, during turn-off process. Derived analytical model in conjunction with the displacement curve of the fast mechanical switch of the hybrid DCCB is utilized to optimize the components of the main circuit breaker branch to reduce MOV degradation through asymmetric energy dissipation. A model of the circuit breaker is built in PSCAD to validate the performance of the proposed optimization method in a 10-kV/250-A system with four stage progressive switching. Additionally, a low voltage system model at 380 V is developed in PLECS for two stage progressive switching that works as the basis of experimental validation. This includes both lookup table-based MOV model and device thermal model for junction temperature estimation. Experimental results are provided for a 380-V system to demonstrate reduced fault current peak in a progressive switching and near uniform energy absorption in optimally selected MOVs.