System faults can sometimes occur on the electric grid due to a variety of environmental conditions and result in protective relays isolating problem areas such that voltage returns to normal conditions. Traditionally, the voltage recovers to nominal within a second after the fault is cleared, but there have been instances of delayed voltage recovery following faults on the electric system, especially during the summer season. These fault induced delayed voltage recovery (FIDVR) events have been attributed to air conditioner (A/C) units when their compressor motors stall as a result of the momentary low voltage. During this stalled condition, the compressors’ consumption of reactive power radically increases which prevents system voltage from recovering immediately and it is held until the A/C load trips itself off via internal thermal protection. In some cases, data captured on the electric grid from phasor measurement units (PMUs) has revealed instances where voltage recovery was delayed for up to 50 seconds.
The Western Electricity Coordinating Council (WECC) has been continuously investigating FIDVR events and in 2006 its members from Bonneville Power Administration (BPA), Southern California Edison (SCE), and Electric Power Research Institute (EPRI) tested 27 residential split-phase A/C units to evaluate their dynamic performance. Among other performance characteristics, it was determined that these units typically stall between 60% and 70% nominal voltage which is well before they are disconnected due to power contactor dropout at 53% voltage. It was also discovered that these single-phase compressor motors began stalling rather quickly, normally within 3 cycles. Ultimately, this A/C unit research was utilized by the Model Validation Working Group (MVWG) to develop and validate the A/C motor model.