According to IEEE 242, clause 126.96.36.199 which specified to transformer fuse protection, following points should be considered for fuse selection.
188.8.131.52.1 Inrush points
In selecting fuses for transformer protection, the following practices are recommended to avoid nuisance fuse operation:
a) When a transformer is energized, magnetizing inrush current flows through the fuses.
When selecting the current rating, the minimum-melting TCC (adjusted for pre-load, ambient temperature, and, if applicable, damageability) should lie to the right of the magnetizing inrush points. The rules of thumb for these points are 12 times the full-load current of the transformer at 0.1 s and 25 times at 0.01 s for unloaded transformers.
b) Magnetizing inrush currents may be slightly higher when transformers carrying load current are subjected to a momentary interruption. In addition, under these circumstances, fuses will have been carrying the load and will melt slightly faster than when at room temperature. As a rule of thumb, the integrated heating effect of this inrush should be considered as that of a current having a magnitude of 10 to 14 times the full-load current for a duration of 0.1 s and 25 to 28 times the full-load current for a duration of 0.01 s. This effect is referred to as hot-load pickup.
c) Cold-load pickup can be a concern if fuses are sized based on load diversity, i.e., the utility practice of sizing transformers, conductors, and protective devices such as fuses to handle the highest normal load current to be expected, rather than the entireconnected load. This practice can cause problems in re-energizing a circuit after a prolonged (e.g., 4 h) interruption. In this case, all air conditioners or all electric heating devices start up at the same time rather than randomly. This condition may result in a current profile that is equivalent to six times the transformer full-load current for 1 s, three times for 10 s, and two times for 15 min. It is unusual for coldload pickup to be a problem on industrial and commercial power systems where the diversity of many small air conditioners is not a factor and where large loads are energized one at a time following a prolonged power interruption. However, if all load on a transformer is picked up at once by closing the primary or main secondary switching device, a similar inrush will occur. If this event is expected, the fuse should be selected so that its minimum-melting curve is to the right of the points mentioned in 184.108.40.206, plus any allowances recommended by the manufacturer for a safety zone.
d) For proper coordination, the secondary protection system’s total-clearing characteristics
(e.g., secondary fuses or breakers) converted to the primary side should also lie to the left of the primary fuse’s minimum-melting curve.
e) Considering the above inrush points, comparison of the steep minimum-melting TCC of a current-limiting fuse and the curves of an expulsion fuse shows which one should give better protection than the other for faults on the secondary side of the transformer. This comparison is particularly true for arcing phase-to-ground faults where the fault current may be as low as 40% of the current for a bolted fault.
220.127.116.11.2 Through-fault protection
A primary fuse selected for transformer protection should be able to carry overload currents safely. In addition, it should be large enough to coordinate with secondary-side devices. The upper limit for the size of fuse selected is a function of the NEC rule that the rating not exceed three times the transformer’s full-load rating. Fuses can provide excellent protection to the transformer for such faults when properly selected. Chapter 11 discusses transformer protection in detail. Because its zone of protection extends only through secondary-side protective devices, the primary fuse should be selected on the basis of the infrequent fault protection curve. Frequent faults on the lines emanating from the secondary bus are to be cleared by feeder protective devices.
18.104.22.168.3 Overload protection
Precise transformer overload protection is commonly required. One means is the use of internal circuit breakers. Another method for overload protection is available for pad-mounted transformers, which commonly use bayonet expulsion fusing. Rather than using conventional copper and tin bayonet fuse elements, which do not react to the transformer oil temperature, optional eutectic elements may be used to provide both overcurrent and overload protection.
Unlike conventional fuse elements, the eutectic element senses both the thermal effect of current flow and the oil temperature rise. The oil temperature is due to core and coil losses combined with ambient temperature conditions. The eutectic element melts at a predetermined temperature and opens the circuit. Thus, the transformer insulation system is protected from extreme temperatures that significantly reduce its operating life.
22.214.171.124.4 Two-fuse concept protection
The two-fuse concept is commonly applied to pad-mounted transformers. This protection scheme consists of a partial-range current-limiting fuse in series with an expulsion fuse housed in a bayonet through-wall holder. The partial-range current-limiting fuse is sized to clear high-current, low-impedance internal faults, and its field accessibility is optional. Conversely, the field accessible expulsion fuse is sized to clear lower current, external secondary faults and/or sustained overloads. Using the two-fuse concept provides assurance that transformers with internal faults remain permanently isolated and allows differentiation between internal and external faults. Typically, overcurrent conditions that transformers experience are of lower magnitude and only involve replacement of the accessible expulsion fuse element assembly.
Furdermore as per ABB switchgear manual, for protecting transformers, selectivity by making the melting times match of low-voltage fuses and HV fuses is required to ensure that the low-voltage fuses respond first. This is taken into consideration in Table below.