Lifetime-Based Selection Procedures for DC Circuit Breaker Varistors

Varistors are widely used in dc circuit breaker (DCCB) application as fault energy absorption elements, which is not an application the varistors are originally designed for. The fault energy stress adversely impacts the varistor lifetime, thus affecting DCCB reliability and performance. This article presents two methods for choosing the appropriate varistor type and arrangement in a DCCB by linking 1) varistor lifetime and 2) DCCB component-and system-level parameters. The first method uses a set-based design approach with random Latin hypercube sampling to narrow down viable solutions. In comparison, the second method relies on a point-based design approach and proposes an integer programming problem to optimize varistor lifetime. Both procedures are applied to the selection of varistors in 1) a 12 kV DCCB rated for 2-kA nominal current and a peak fault current of 15 kA and 2) a reduced-scale 1.2-kV DCCB hardware prototype, using publicly available varistor data. The article shows the possibility of design for varistor lifetime in DCCB applications, and concludes that both selection procedures provide consistent, reasonable, and verifiable results. However, the results also pinpoint the significant gaps and call for future work on impulse stress versus lifetime relationships for DCCB varistors.