Analytic formulae for estimating the motor proportion of a load model under a small disturbance

In the composite load model (CLM), which is commonly used in China, an equivalent motor and equivalent static load are used to represent all of the electrical equipment and networks connected to a load bus. Existing research has determined the typical values of electrical and mechanical parameters for load models of different load types, which improves the basis for load modeling. However, the motor proportion parameter is not the same for different load buses or at different times; therefore, obtaining the actual motor proportion is key to establishing an accurate load model. In the existing load modeling method, the motor proportion is commonly identified along with other parameters under rare large disturbances; therefore, the value of the motor proportion is fixed by the time when a large disturbance occurs. In this paper, formulae are derived to estimate the motor proportion under small disturbances, and these formulae allow the direct calculation of the motor proportion without using any optimization algorithm. The proposed estimation formulae do not rely on any parameters of the load model or the power system but instead rely only on the measurement of the voltage and active power at the steady-state points before and after a small disturbance. Because of the universality of small disturbances in power systems, estimating the time-varying motor proportion under small disturbances will be helpful for solving the time-varying problem of load models. Finally, the proposed motor proportion estimation formulae are validated by simulations, physical experiments, and field experiments.