The impact of line protection operate time on circuit breaker wear

The main elements of a fault clearance system in power transmission networks are a protective relay and a circuit breaker. The relay operate time and the circuit breaker interrupting time comprise the fault clearing time, which is important from the power system perspective. Achieving ultra-high-speed fault clearing time has been an open objective for many decades, mostly due to the need for improvements in power system stability. The relay operate time was reduced to a few milliseconds back in 1976, but improvements in the circuit breaker interrupting time, from near 2 cycles (40 ms in 50Hz systems) to ¾ cycle has not yet been achieved, despite promises to be commercially viable by 1981. Without the forthcoming circuit breaker improvements, full focus became placed on the relay operate time, where a “need for speed” became the dominant goal when designing new protection algorithms (nowadays in different relay technologies). Contrary to common opinion, in [1] it is shown that reduction in relay operate time does not directly improve the fault clearing time and has even lesser impact on the power system stability, due to the complex physics behind the circuit breaker current interruption process. 

 In this paper another important aspect of the interaction between relays and circuit breakers is analysed. Circuit breakers are designed to interrupt fault currents only if the interruption process is not initiated in a time shorter than the half cycle period; which implies that a protection operate time of less than half cycle can negatively affect circuit breakers, which is a fact that is neglected in the “need for speed” approach. The reason is the possible high level of direct current component in the fault current, which is hard to interrupt and prolongs the time till the next zero-crossing instant. In such a case a circuit breaker should be derated, alternately, the type testing process should be repeated, to ensure proper operation. 

 In this paper a new methodology to evaluate circuit breaker wear as a function of the relay operate time, is proposed. The methodology is based on type test data of a commercial circuit breaker that is used worldwide. It considers the circuit breaker opening time (mechanical time) and minimum arcing time for different conditions. The circuit breaker wear is calculated as a value proportional to the area below the fault current from the moment of opening its contacts until the zero-crossing instant, when the fault current is interrupted.  As a reference scenario, a near half cycle relay operate time is considered as the fastest operate time that does not violate limitations of the circuit breaker design. The circuit breaker wear is compared to the scenario where an ultra-high-speed relay operates in 1.5 ms to 2.5 ms. The calculated reduction in the number of circuit breaker operations can be used by network operators to better estimate circuit breaker maintenance intervals.