Abstract: Study on the Optimal Program for Routine Testing of 220 kV Three-Winding Transformers Abstract: The operational conditions of transformers directly impact the safety and stability in power systems, as a critical equipment. Routine testing is essential to ensure normal transformer operation. However, traditional testing methods are facing challenges including complex wiring configurations, low efficiency, and substantial safety risks in present. This paper proposes an optimal program for routine testing of the transformers, which significantly enhances testing efficiency and safety through rationalized test sequencing, minimized wiring modifications, and streamlined procedures. Case study elaborates the operation flow and operational benefits of this optimal program, and the method is applied to 220 kV three-winding transformers during outage maintenance.