Features of fuses operating in medium voltage instrument transformers protection

The problem of protection of 6–35 kV voltage instrument transformers (VTs) of using fuses is associated with the lack of domestic manufacturers of fuses capable of effectively protecting VTs, especially when small overload currents flow. Purpose. Establishing the characteristics of fuse operating when protecting medium voltage instrument transformers 6–35 kV with cast insulation and determining the technical parameters of improved fuses that provide effective protection of instrument transformers when small overload currents flow. Methodology. Analytical and experimental methods were used to study the processes of fuse operating when protecting medium voltage instrument transformers. An analysis of the operating modes of 6–35 kV networks was carried out, the features of fuse operation at low overload currents were investigated, and laboratory experiments were performed to measure currents, voltages, and operating times. Based on the results obtained, criteria for selecting the parameters of improved fuses were formulated to increase the efficiency of transformer protection. Findings. The results of the conducted experimental studies and oscillograms from operating power facilities confirmed the problem of disconnecting small overload currents of VTs by fuses. It was experimentally established that when fuses protecting VTs switch off small overload currents, the fuse-link burns out, but the VT power circuit is not interrupted and voltage remains at the terminals of its secondary winding. Originality. Criteria for determining the parameters of fuses to increase the efficiency of VT protection are proposed and the feasibility of creating an improved fuse for VTs with a rated fuse-link current In  0.08–0.1 A, which ensures operating for 1 hour at a current I1h  0.15 А, is substantiated. Practical value. Ways of solving the problem of VT protection are proposed by improving the designs of fuses in order to improve their protective characteristics in the range of small overload currents.