Protection of 35kV delta connected Wind Farm transformers with fuses

Internal fuse protection of wind farm transformers has been difficult due to their typical size and configuration (Gt1000 kVA, 24.9 kV-34.5 kV delta). With no suitable 37 kV current-limiting (CL) fuses available, smaller transformers have used ldquomatched meltrdquo coordination between oil submersible 34.5 kV expulsion fuses and 23 kV CL fuses. This works because line-to-line faults, higher than the expulsion fuse can interrupt, are interrupted by two CL fuses (one in each phase) melting in the first current loop and sharing the voltage. To fuse larger transformers, CL fuses must work when melting in more than one loop. This paper discusses a project to investigate the ability of certain CL fuses to do this. While series sharing at melting times of more than one or two loops proved to be unlikely, single fuses were found to be capable of interrupting 37 kV up to currents that did permit voltage sharing.