A voltage sag-based procedure for determining transient stability models of conventional loads

Conventional linear load models for stability studies use an exponential relationship to represent active and reactive power consumed as a function of voltage. Typical composite loads are modeled as constant power, current, impedance, or a combination of the three. There is a need for better procedures to produce more exact composite load models for use in stability studies. This paper discusses a method for extracting composite load characteristics using disturbances created by transmission voltage sags. Voltage sags, which are particularly frequent during thunderstorms, present the opportunity to refine stability load models for power systems. Fault and pre-fault power flow data are decomposed into active and reactive components, and are then further examined to extract the nonlinear load component. With the nonlinear loads removed from the data, the voltage variation of the remaining linear load active and reactive powers can be observed and the best stability models determined. The entire procedure is illustrated using actual data for a distribution feeder in Austin, TX, and the results match well with commonly cited references.