Security Assessment of System Frequency Response

Low inertia power systems experience more extreme frequency dynamics immediately after a disturbance. A higher rate of change of frequency (ROCOF) contributes to a lower frequency nadir which could trigger load shedding schemes. Active power injections in a time frame of two seconds or less are necessary to maintain frequency stability. This paper develops an analysis of the closed-loop transfer function of a single-area power system model for frequency response (FR). This model is useful to study the frequency dynamics following a sudden load imbalance in a highly meshed system in which all the generating units are combined into a single equivalent unit with an aggregated inertia constant. The dynamics of the turbine and speed governor are included as first-order lag functions in the Laplace domain. A numeric example of the impact of system inertia and frequency controllers in the UK power system is then developed in MATLAB, and minimum values for Primary and Fast Frequency Response (FFR) power injections are calculated.