Assessing Frequency Nadir of Power Systems under Power Ramp Disturbances

As the penetration rate of new energy continues to increase, the inertia of the power system decreases, which deteriorates the system’s frequency response. Conventionally, frequency nadir is an important metric concerned by power systems operators. Existing studies of frequency nadir mainly focus on power step disturbance. However, realistic disturbances appear in various forms, such as power ramps. How to deal with these disturbances is rarely investigated. This paper aims at filling this gap. Firstly, based on the simplified models (named unified transfer function structure) of generation devices, an equal energy method is proposed to obtain the equivalent step disturbance of any given disturbance, promising the frequency nadir of power systems under the equivalent step disturbance will be the same as that under the given disturbance. Then, the empirical formula of the frequency nadir, which is suitable for the step disturbance, can be used. Furthermore, the method is applied to assess the frequency nadir in power systems where a power ramp disturbance occurs to illustrate the analysis process in detail. Finally, the effectiveness of the method is verified by simulation.