The system impact of air conditioner under-voltage protection schemes

This paper presents the results of simulating and evaluating an under-voltage protection scheme, which takes stalled air-conditioner (a/c) units offline so that the slow voltage recovery phenomena can be prevented in areas heavily loaded with AC motors during summer peak periods. First, a three-feeder test-bed was used to quantify the effectiveness of the protection scheme and the sensitivity of the under-voltage relay settings. Then, two real system events from the Western US power grid were simulated to evaluate the area-level impact of the protection scheme proposed by Southern California Edison. The study demonstrates that after 75% or more of the stalled a/c units are disconnected, the feeder voltage recovers in a few seconds, much quicker than the tens of seconds that standard thermal relays need to trip the stalled motor offline. The drawback is that after the voltage recovered, it settles at a higher value than prior to the faults because a large proportion of load is shed. Therefore, the coordination among the capacitor bank operation is recommended to suppress the overvoltage caused by tripping of the AC motor load.