Impedance-Based Characterization of Power System Frequency Response

This paper presents an impedance-based noninvasive method for the characterization of power system frequency response in real time in the absence of a transient event. The proposed method measures transfer function from the injected active power to the frequency at the point of interconnection for the estimation of system inertia, primary frequency response, and the speed of the primary frequency control of the system. The socalled frequency response transfer function FR(s) is measured by injecting perturbations in the active power output of a battery energy storage system. The measured response of FR(s) can also predict the frequency nadir and ROCOF following a transient event. The relationship between FR(s) and the system impedance response as seen from the point of interconnection is also derived in this paper; it can be used for the control design of frequency support services by inverter-coupled generation and storage. The proposed methodology is demonstrated on a modified IEEE 9-bus system with 25% penetration of wind and PV generation.