Power fuses are a generally accepted means of protecting power transformers in distribution substations.
The primary purpose of a power fuse is to provide interruption of permanent faults. Fusing is an economical alternative to circuit switcher or circuit breaker protection.
Fuse protection is generally limited to voltages from 34.5 kV through 69 kV, but has been applied for protection of 115-kV and 138-kV transformers. To provide the greatest protective margin, it is necessary to use the smallest fuse rating possible.
The advantage of close fusing is the ability of the fuse unit to provide backup protection for some secondary faults. For the common delta-wye connected transformer, a fusing ratio of 1.0 would provide backup protection for a phase-to-ground fault as low as 230% of the secondary full-load rating.
Fusing ratio is defined as the ratio of the fuse rating to the transformer full load current rating. With low fusing ratios, the fuse may also provide backup protection for line-to-ground faults remote to the substation on the distribution network.
Fuse ratings also must consider parameters other than the full load current of the transformer being protected. Coordination with other overcurrent devices, accommodation of peak overloadings, and severe duty may require increased ratings of the fuse unit. The general purpose of the power transformer fuse is to accommodate, not interrupt, peak loads.
Fuse ratings must consider the possibility of nuisance trips if the rating selected is too low for all possible operating conditions.
The concern of unbalanced voltages in a three-phase system must be considered when selecting fusing for power transformer protection. The possibility of one or two fuses blowing must be reviewed. Unbalanced voltages can cause tank heating in three-phase transformers and overheating and damage to three phase motor loads.
The potential for ferroresonance must be considered for some transformer configurations when using fusing. Fuses are available in a number of tripping curves (standard, slow, and very slow) to provide coordination with other system protective equipment.