Understanding and solving short-term voltage stability problems

Based on actual incidents, short-term voltage instability is an increasing, but often overlooked, industry concern. A common scenario is a large disturbance such as a multi-phase fault near a load center that decelerate motor loads. Following fault clearing with transmission outages, motors raw very high current while simultaneously attempting to reaccelerate, and may stall if the power system is weak. Massive loss of load and possibly area instability and voltage collapse may follow. The authors describe actual incidents. Fast-acting generator excitation controls, fast-acting reactive power support devices (SVC, STATCOM, SMES), or fast load shedding can prevent voltage collapse. Proper analysis requires dynamic modeling of aggregated motor loads, with equivalents for distribution feeders. Power electronic based voltage support devices must be realistically modeled to determine required size, location, number, and type. Based on simulations, they conclude that voltage-sourced converter devices (STATCOM, SMES) are attractive countermeasures against load loss and voltage collapse. Factory built distribution-connected distributed devices may be cost-effective compared to larger transmission-connected devices.