Enhancing Grid Strength in High-Renewable Systems: Selecting Retired Thermal Power Units Retrofit to Synchronous Condensers Based on Multi-Dimensional Evaluation Method

With the rapid development of renewable energy in China, thermal power units are facing decommissioning issues, while the power system is confronted with severe challenges such as reduced grid strength and insufficient voltage support. For power systems with multiple renewable energy stations, the short-circuit ratio at the connection points of renewable energy stations is an important indicator for measuring grid strength. Engineering requirements specify that the short-circuit ratio at these connection points should not be lower than 2.0. This study focuses on transforming retired thermal power units into synchronous condensers to improve the short-circuit ratio at renewable energy station connection points. This paper first studies the impact of thermal power unit operation, shutdown, and synchronous phasor operation on the short-circuit ratio, deriving the short-circuit ratio expressions for renewable energy stations under different states of thermal power units. It further analyzes the impact of different main transformer capacities and unit transformation capacities on the short-circuit ratio. Next, a multi-dimensional evaluation system is constructed, incorporating the change in short-circuit ratio at grid-connection points of multiple renewable energy stations (ΔMRSCRS), the main transformer capacity within short-circuit ratio enhancement range (ST), the pre-retrofit short-circuit ratio level at grid-connection points of multiple renewable energy stations (SG), and the retrofitted unit capacity (MRSCRS) to comprehensively assess the transformed thermal power units. Finally, a case analysis conducted on the modified IEEE-39 bus system using the PSASP platform verifies that operating thermal power units as synchronous condensers can significantly enhance the short-circuit ratios of multiple renewable energy sites. Given that small-capacity thermal units are approaching retirement, there is a stronger preference for retrofitting these smaller units as synchronous condensers. The multi-dimensional evaluation method proposed in this study specifically identifies small-capacity thermal units as the most suitable candidates for such retrofitting. This approach provides theoretical support for implementing synchronous condenser operation in retired thermal power units and promotes the coordinated optimization of grid security and renewable energy integration.