Several retrofillings have been performed by different utilities in China. However, there is still insufficient understanding of the retrofilling procedures for minimising residual mineral oil and on the effect of the temperature rise of transformers. Following new detailed procedures, an out‐of‐service S11‐M‐315/10 (10 kV 315 kVA) and a new S13‐M‐400/10 (10 kV 400 kVA) distribution transformers were selected to retrofill their mineral oil with FR3, a soybean‐based natural ester. A method was proposed to predict the residual mineral oil content, which uses the kinematic viscosity at 60°C. This method fills a gap in the testing of residual mineral oil content. The mass ratios of residual mineral oil were 1.24% for the S11‐M‐315/10 transformer and 2.05% for the S13‐M‐400/10 transformer, based on the procedures in this study. The fire point of the mixed oil after retrofilling was 352°C, which meets the K ‐class less‐flammable requirements, indicating a proper control of residual mineral oil. After retrofilling, the time of reaching the limits of the 1.5 times rated load was 30% longer than that of the mineral oil transformer. Two retrofilled transformers have been operating in good conditions since November 2019 for the S11‐M‐315/10 transformer and December 2020 for the S13‐M‐400/10 transformer.

Temperature distribution in transformer windings is a crucial factor that influences the safe operation, and lifespan of an insulating system in an oil‐immersed power transformer. Thermal hydraulic network modelling (THNM) has been the subject of numerous studies because of its lower computation cost as compared to a computational fluid dynamics (CFD) simulation. This study proposes improvements to THNM, with primary focus on non‐uniformity of temperatures in the entry and exit vertical oil ducts, and the feasibility of using a piecewise fitting formula for Nusselt number (Nu) of convection heat transfer coefficient in transformer windings. The deviation of hot spot temperature was 0.3°C, and the location of hot spot for CFD and THNM was 34th disc and 35th disc, respectively. The calculation time of CFD and THNM was about 34 h and 35 s under the same computer hardware situations. This provides an acceptable THNM method for the rapid prediction of transformer hot spot temperature and location at low costs for digital twin.

Geomagnetically induced current (GIC) flowing through power transformers causes core saturation, leading to local hot-spot overheating, abnormal vibration, increased noise, and even irreversible transformer damage and forced outage. To prevent catastrophic GIC-induced failures, relay protection must isolate transformers in a timely and reliable manner, yet GIC has long been a critical challenge interfering with the correct operation of conventional transformer protection systems. This paper studies transformer excitation current characteristics under GIC impact, establishes a transformer model with extreme GIC injection, analyzes differential protection performance under varying GIC magnitudes, proposes a modified differential protection method, and verifies its effectiveness via simulations and experiments. Results demonstrate that GIC distorts transformer excitation current, introducing a second harmonic into differential current to trigger differential tripping blocking. The distinct second-harmonic features between magnetizing inrush and GIC injection enable reliable selective unblocking of harmonic restraint. To enable the differential protection to operate correctly under different GIC injection conditions, the operating threshold shall be adjusted according to the transformer parameters after the harmonic blocking is deactivated. The proposed strategy can potentially disable the harmonic blocker during GIC events, allowing relays to trip on demand, which is critical for ensuring transformer safety under extreme conditions.