Risk assessment and mitigation on frequency stability for operational planning

Abstract

The integration of renewable energy sources (RES) brings more uncertain active power supply into the system and reduces the system inertia, which incurs more severe frequency fluctuation and obvious heterogeneity of frequency responses in different areas. Thus, the risk of system and area-level frequency instability, represented by two indices, i.e., rate of change of frequency (RoCoF) and frequency nadir/vertex (FN/FV), requires to be assessed and mitigated in operational planning. Firstly, the cumulant­based assessment framework is proposed to efficiently assess system RoCoF and FN/FV, which includes analytical sensitivity and numerical sensitivity. Secondly, a linear sensitivity-based method with a straightforward calculation process is proposed to achieve fast and effective area-level RoCoF assessment, where two kinds of sensitivities are proposed, analytical sensitivity and integrated sensitivity. Then, the evaluated high risk of area-level RoCoF violation is mitigated by increasing inertia, and a probabilistic enhancement strategy is proposed to identify the required inertia demand for different levels of enhancement, where a small percentage of violation is allowed. Moreover, six allocation plans are proposed to distribute the area-level inertia to individual power plants in the region according to different considerations, i.e., technical feasibility and individual cost, and RoCoF performance. The multi-sensitivity and multi-interval methods are proposed to assess the risk of area-level FN/FV violation by additionally considering the impact of the RES locations and excitation system compared with the widely employed system frequency response (SFR) model, where the multi-interval method can achieve more accurate evaluation due to the additional consideration of generator frequency oscillation. The effectiveness and efficiency of the proposed frameworks are validated by numerical scenario-based simulation (SBS).