Local Measurement-Based Protection Coordination System for a Standalone DC Microgrid

A reliable protection coordination system between the power electronic converters and solid-state device-based fast protection unit in a standalone dc microgrid supplied by a wave energy converter (WEC) and an energy storage unit is presented. A system model with solid-state circuit breakers is developed in Piecewise Linear Electrical Circuit Simulation (PLECS) to analyze the steady-state operation as well as faulted conditions. The effect of both resistive and constant power loads on a current-limited bus controlling converter behavior during an overload event is analyzed theoretically. Also, the variation in the WEC power output and its effect on bus voltage during overload are investigated. The analysis has led to a novel dual current-voltage feedback-based protection coordination system. The method does not require any communication between the solid-state circuit breakers and converters and eliminates the need for adaptive updating of the overcurrent threshold with varying renewable generation output. Bus capacitor discharge during a short-circuit is analyzed theoretically as well to formulate an instantaneous trip threshold selection utilizing device gate driver functionality. The proposed protection coordination system ensures rapid fault isolation and continued power delivery to the healthy segments. An SiC-based bidirectional, solid-state dc circuit breaker prototype is designed and fabricated, which has been used to implement the proposed protection method with a 380-V dc bus-based system.