Estimating the frequencies of load buses and their effects on the critical clearing time

The models used to assess transient stability are in general very simple and damping effects as frequency dependent loads are commonly neglected. Aiming the incorporation of these effects in the stability analysis, a model with frequency dependent loads is proposed. The influence of these damping effects on the critical clearing time, especially in cases in which the system loses synchronism after the first swing, is also shown. In order to implement this load model it is necessary to preserve the network structure and also to know the frequencies at the load buses. In this work it is shown that the mean of generators frequency and the COA frequency are not good estimates of the frequency at load buses. It was also observed that the load frequencies are related to electrical characteristics instead of mechanical ones. Based on this observation, a new methodology to estimate the frequencies at the load buses is proposed. The obtained results are encouraging, and the new methodology allows determination of the system frequency islands, extending the concept of coherency, already known for generators, to load buses.