The performance of bulk power systems is analyzed using computer models that represent the systems’ behavior under normal or abnormal conditions. As the load changes during the day, the generators follow, and the power flows can be calculated. However, for historical reasons, most load models are based on time-invariant, voltage-dependent polynomial load representations. The loads may be represented as constant impedance (Z), constant current (I), or constant power (P). It has been found recently that these models no longer provide a true representation of the dynamic response of the load, particularly in view of the increased penetration of residential air conditioners in recent years. The problem comes about because the load models do not capture the electromechanical behavior of the motors or the load protection in the moments after a fault occurs on the power system. A Fault Induced Delayed Voltage Recovery (FIDVR) event is the phenomenon whereby system voltage remains at significantly reduced levels for several seconds after a transmission, subtransmission, or distribution fault has been cleared. The effect is thought to be caused by the stalling of highly concentrated induction motor loads with constant torque. These motors stall in response to low voltage. The stalled motors draw more reactive power from the grid, and that holds down the local voltage. A vicious circle is created. After several seconds of being stalled, motor protection devices begin to act and trip the motors to save prevent them from overheating. As a result, there will be a large decrease in the load on the power system, with a potential secondary effect of high system voltage. That response is particularly likely if the protection response is slower than the voltage regulation response of the system.