Voltage Stability Simulations Using Detailed Models Based on Field Measurements

Doctoral thesis, 1992

This dissertation deals with voltage stability simulations, including dynamic load models, on-load tap-changer control system dynamics and generator reactive power output capacity limits. The simulations are based on sequential solving of ordinary load flow cases augmented with dynamic models. A dynamic load model, that describes load areas including many home heating appliances, is derived from field measurements. The most impressive result is the power recovery, in the range of minutes, due to the thermostat controlled radiators in the system, after a voltage reduction. It is proved in the dissertation that for voltage stability studies electrical heating cannot be seen as constant impedance. The influence of tap-changer control system dynamics is demonstrated for cascaded tap-changers on different voltage levels, and principles for tap-changer settings are discussed. Both the maximum power transfer capability and the load device power consumption are affected by the tap-changer actions. Generator stator and rotor current limiter models are used in order to take into account the rapidly decreasing reactive power output capability at lower voltages. The load dynamics and tap-changer control system dynamics, as well as generator reactive limits, have a significant impact on the voltage collapse, stability margins, power transfer limits and other criteria related to the voltage stability phenomenon.