Online Frequency Security Assessment Based on Analytical Model Considering Limiting Modules

Due to the increasing implementation of high voltage direct current (HVDC) and the integration of renewable resources, frequency stability problems in power systems are drawing greater attention in recent years. It has become necessary to carry out online frequency security assessments to ensure the safe operation of power systems. Considering the low time-efficiency of simulation-based methods, analytical models, such as the frequency nadir prediction (FNP) model, are more suitable for online assessment, which requires calculating the worst frequency deviation under various contingencies. Based on the FNP model, the FNP-L model for online frequency security assessment is proposed in this paper. The proposed model implements security assessment by calculating and checking the frequency features, including the nadir time and frequency, followed by contingencies. The effect of the governor, including nonlinear constraints, is approximated into polynomial functions so that the results are obtained by solving multiple polynomial equations. Case studies are carried out using the New-England 39-bus system and a regional power grid, which shows that the proposed model could achieve both high speed and high accuracy, and can therefore be applied in online security assessment.