Renewable Energy Integration in Power System: Clarification on Stability Indices

The continuing increase of renewable energy integration in power grids presents new challenges for system operators. These challenges emanate from converter-based renewable energy sources (RES), mainly wind and solar photovoltaic. These sources are asynchronously connected to the grid, and have limited provision of ancillary services. Essential power system services pertain to inertia and strength. Practical methodologies to evaluate and quantify these services are mandatory. Also clear requirements ought to be set for secure integration of RES.This paper presents a study on the rotational inertia and strength services of electric power grids integrating RES. The aim is to characterize how these services affect the stability indices, and synthesize requirements for secure RES integration. Methodologically, wind power plants are integrated to the standard IEEE 9-bus test system, with different locations and penetration ratios. Steady state voltage and transient frequency stability indices are evaluated. The results of this paper are twofold: clarifications on stability indices are made, then the inter-relations of these system services as inertia and system strength, are highlighted. The results are further confirmed by two industrial softwares: PSS/E and DigSILENT-PowerFactory, used as a test bench for comparison.