Over the years, both fuse manufacturers and users have generally agreed on the use of low fusing ratios when protecting transformers against secondary-side faults and slowly evolving internal faults. This fusing philosophy is still appropriate for small three-phase power transformers used on industrial, commercial, and institutional power systems, and small-to-medium size three-phase power transformers used in utility substations. Low fusing ratios are necessary if low-magnitude faults are to be detected and quickly cleared to minimize damage to these transformers. Protection against secondary-side faults, in particular, is extremely important when one considers the criticality of the process loads served by these transformers, and the fact that replacement transformers are not readily available.

Recent field experience, as well as reports in the literature, suggests that a different fusing philosophy can be used when protecting small-kVA single-phase overhead distribution transformers. This change in philosophy is based, in part, on the realization that the majority of overhead transformer failures occur due to lightning surges and not due to secondary-side faults. It is also becoming clear that overhead transformers can be better protected against damage due to surges if the arrester is relocated to the transformer tank. Moving the arrester to the transformer tank, however, makes fuses with small ampere ratings susceptible to nuisance operations because these small fuses must pass the surge current during an arrester operation. To minimize these nuisance operations, it is necessary to increase the fuse rating on overhead transformers to withstand these surges. However, many protection engineers are concerned that the use of larger fuses may result in a reduction in the degree of protection provided against both secondary-side faults and slowly evolving internal faults. Further analysis reveals that very little protection is given up by standardizing on larger fuse ratings for these transformers, particularly if the fuses operate in a current-limiting fashion.

This article, and the four articles that follow, provide a detailed discussion of the items that must be considered when selecting a transformer primary fuse. Perhaps of interest to the reader is an expanded treatment of the inrush currents that occur when transformers are energized. Also, differences in the protection philosophy applicable to three-phase power transformers versus small-kVA overhead distribution transformers will be noted, where appropriate.

Application Factors

As a general rule, the following factors must be considered when selecting a transformer primary fuse:

System voltage;
Available fault current;
Anticipated normal loading level, including daily or repetitive peak loads, and emergency peak loads;
Transformer inrush current, including the combined effects of magnetizing-inrush current and the energizing-inrush currents associated with connected loads — following a momentary or an extended outage;
The degree of protection provided to the transformer against damaging overcurrents;
Coordination with other overcurrent protective devices; and
Protection of load-side conductors or cables against damaging overcurrents.

Voltage Rating

The maximum design voltage rating of the transformer primary fuse should equal or exceed the maximum phase-to-phase operating voltage level of the system. In the case of single-phase transformers, however, the maximum voltage rating of the primary fuse need only equal or exceed the maximum system phase-to-neutral voltage level, provided that the BIL rating and the leakage distance to ground of the fuse are sufficient for the application.

It is sometimes economically desirable to use phase-to-neutral rated current-limiting fuses to protect three-phase transformers. Fuses so rated can be used on grounded-wye / grounded-wye, grounded-wye / delta, and open-wye / open-delta transformers provided that the following conditions are met:

The probability of a primary phase-to-phase or three-phase ungrounded fault is very low.
The fault current is high enough to ensure that two fuses will operate simultaneously by melting in less than 0.2 second.
The load is predominantly grounded.
A secondary breaker is employed to interrupt overloads.

Short-Circuit Interrupting Rating

The symmetrical short-circuit interrupting rating of the transformer primary fuse should equal or exceed the maximum available fault current at the transformer location. In addition, the interrupting rating of the fuse should be chosen with sufficient margin to accommodate anticipated increases in the interrupting duty due to system growth.

Helpful Tip
When determining the required interrupting rating of the transformer primary fuse, the X/R ratio of the system at the fuse location should be considered, since certain types of fuses may have higher-than-nominal symmetrical interrupting ratings when used in applications where the X/R ratio is less than values specified in the standards. As a result, it is often possible to use a less expensive primary fuse having a lower nominal symmetrical interrupting rating. Refer to the fuse manufacturer for details.

Helpful Tip

When determining the required interrupting rating of the transformer primary fuse, the X/R ratio of the system at the fuse location should be considered, since certain types of fuses may have higher-than-nominal symmetrical interrupting ratings when used in applications where the X/R ratio is less than values specified in the standards. As a result, it is often possible to use a less expensive primary fuse having a lower nominal symmetrical interrupting rating. Refer to the fuse manufacturer for details.

Ampere Rating and Speed Characteristic

The ampere rating and speed characteristic of the transformer primary fuse should be selected to:

accommodate the normal transformer loading level, including daily or repetitive peak loads, and emergency peak loads;
withstand the magnetizing-inrush current associated with the energizing of an unloaded transformer, as well as the combined magnetizing- and load-inrush currents associated with the re-energization of a loaded transformer following a momentary or extended outage;
protect the transformer against damaging overcurrents;
coordinate with other overcurrent protective devices; and (5) protect load-side conductors and cables against damaging overcurrents.

These principles are examined in greater detail below.

Accommodate Expected Loading Levels

In general, the transformer primary fuse should be selected based on the anticipated normal loading schedule for the transformer, including daily or repetitive peak loads. The primary fuse ultimately selected should have a continuous loading capability, as differentiated from its ampere rating, equal to or greater than this highest anticipated loading level. Refer to the fuse manufacturer’s recommendations for such loading capability values.

Conditions may occur during which the transformer will be loaded far in excess of the normal loading schedule. Such emergency peak loading typically occurs when one of two transformers (in a duplex substation, for example) is compelled, under emergency conditions, to carry the load of both transformers for a short period of time. Where emergency peak loads are contemplated, the primary fuse should have an emergency peak-load capability at least equal to the magnitude and duration of the emergency peak load. It is important to remember that the primary fuse should be selected to accommodate — not to interrupt — emergency peak loads. This requirement may result in the selection of a primary fuse ampere rating larger than would be required for a similarly rated single transformer installed alone, and therefore the degree of transformer protection provided by the primary fuse may be reduced.

Helpful Tip
The continuous, daily, and emergency peak-load capability values published for solid-material power fuses employing silver fusible elements are based on 70% preload and a 30° C ambient temperature. Peak-load capability values should be increased 0.5% for each degree centigrade that the average ambient temperature is below 30° C, and decreased 0.5% for each degree centigrade that the average ambient temperature is above 30° C. The continuous and 8-hour emergency peak-load capability values for distribution fuse links employing silver fusible elements are based on a 25° C average ambient temperature. These capabilities should be increased or decreased 0.4% for each degree centigrade that the average ambient temperature is above or below 25° C. The published emergency peak-load capability values represent the maximum non-repetitive load that the fuse can carry without impairing its ability to perform properly. In no event should fuses be subjected to these loading levels more than twice per year in the case of power fuses, or more than 10 times over the life of a fuse link.

Helpful Tip

The continuous, daily, and emergency peak-load capability values published for solid-material power fuses employing silver fusible elements are based on 70% preload and a 30° C ambient temperature. Peak-load capability values should be increased 0.5% for each degree centigrade that the average ambient temperature is below 30° C, and decreased 0.5% for each degree centigrade that the average ambient temperature is above 30° C. The continuous and 8-hour emergency peak-load capability values for distribution fuse links employing silver fusible elements are based on a 25° C average ambient temperature. These capabilities should be increased or decreased 0.4% for each degree centigrade that the average ambient temperature is above or below 25° C. The published emergency peak-load capability values represent the maximum non-repetitive load that the fuse can carry without impairing its ability to perform properly. In no event should fuses be subjected to these loading levels more than twice per year in the case of power fuses, or more than 10 times over the life of a fuse link.

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