Delays in Protection Devices and Their Effects on Selectivity: The Reaction Time Gap

Abstract

This Paper investigates how the internal delays of circuit breakers until the limitation of the fault current affect the selectivity of a low-voltage direct current (LVDC)-grid under short-circuit conditions. For conventional mechanical circuit breakers and future hybrid or solid-state devices, selectivity is, in line with AC practice, typically achieved by coordination based on their time-current trip characteristics. However, experimental short-circuit tests show that in DC-grids non-selective behavior can occur with high current slopes. To explain this effect, the delays until the fault current is effectively limited are analyzed. Based on this analysis, a critical time interval is identified and introduced as the reaction time gap. It describes the delay between the time when the circuit breaker should ideally trip immediately after detecting the fault and the time when it actually limits the current. To represent this behavior within conventional time–current characteristic curves, a measurement method for determining the relevant time intervals is proposed. A measurement-based study on several commercially available mechanical circuit breakers reveals the characteristics of short-circuit detection and current limitation, thereby confirming the existence of the reaction time gap and the resulting constraint on achievable selectivity. Possible countermeasures to overcome this reaction time gap relating to the behavior of the grid and the circuit breaker are outlined. Finally, the results and their implications for the design and protection coordination of DC-grids are discussed.