NEC Article 450 // Transformers Vaults
Transformer protection consists of both overload protection and short circuit protection. Overload protection is usually accomplished via proper selection of the secondary overcurrent protective device.
NEC Article 450 gives specific primary and secondary overcurrent device ratings that may not be exceeded. These vary depending upon the accessibility of the transformer to unqualified persons and the impedance of the transformer.
The smallest protective device that allows the rated full-load current of the transformer gives the best practical overcurrent protection.
Short circuit protection involves comparison of the transformer damage curve per IEEE Std. C57.109-1993 with the primary overcurrent device time-current characteristic. In general, the damage curve must be to the right and above the primary overcurrent device characteristic.
Another constraint on the primary overcurrent device is that it must be capable of withstanding the inrush of the transformer without tripping (and without damage for currentlimiting fuses).
An example time-current characteristic showing protection for a 1000 kVA 13.2 kV Delta:
480 Y/277 V, 5.75%Z dry-type transformer is shown in Figure 1. The transformer is protected with a 65E current-limiting primary fuses and a 1200 A electronic-trip secondary circuit breaker. As can be seen from the figure, the fuses do withstand the inrush without damage since the inrush point is to the left and below the fuse minimum melt curve.
The transformer is protected from short circuits by the primary fuses. The secondary circuit breaker provides overload protection at the full-load current of the transformer. Note that the primary fuse and secondary circuit-breaker characteristics overlap for high fault currents; this is unavoidable and is considered acceptable. Note also that the fuse curve and the transformer damage curve overlap; this is unavoidable but these should overlap at the lowest current possible.
For currents below the fuse/transformer damage curve overlap the secondary circuit breaker must protect the transformer.
The lower the point of overlap, the more likely the fault is an external fault on the load side of the secondary circuit breaker and therefore greater chance the secondary circuit breaker will effectively protect the transformer for faults in this region.
Also note that the transformer damage characteristic is shown twice. Because transformer is a delta-wye transformer, a ground-fault on the secondary side of the transformer will result in only 57.7% of the maximum three-phase primary fault current while one secondary winding experiences the full fault current.
This is illustrated in Figure 2, as well as the corollary for delta-delta transformers.
The damage characteristic has therefore been shifted to 57.7% of its published value to account for secondary line-to-ground faults. Also, the shifted curve has another, more conservative curve shown; this is the frequent-fault curve and is applicable only to the secondary overcurrent device since faults between the transformer secondary and the secondary overcurrent protective device should not be frequent.
Differential protection for transformers is very effective for transformer internal faults.
If differential protection is supplied it is the primary protection for internal faults and will operate before the primary overcurrent device. The primary overcurrent device serves as a backup protective device for internal faults in this case.
Reference: System Protection – Bill Brown, P.E., Square D Engineering Services