This is the third article in a series of articles that concern new and traditional fusing philosophies for protecting transformers. The first article (Unit 1) served as an introduction to the application principles that must be considered when selecting a transformer-primary fuse, in particular, the voltage rating, the short-circuit interrupting rating, and the ampere rating and speed characteristic of the fuse. The second article (Unit 2) covered how to select a transformer-primary fuse to withstand the various inrush currents it may experience in service, such as magnetizing inrush, hot-load pickup inrush, and cold-load pickup inrush. This article covers how to select a transformer-primary fuse to protect the transformer in accordance with industry-accepted through-fault protection curves.

How to Select a Transformer-Primary Fuse to Protect the Transformer in Accordance With Industry-Accepted Through-Fault Protection Curves

Fusing philosophies, as they relate specifically to secondary-fault protection, can vary significantly depending on the type of transformer under consideration. For small three-phase power transformers used on industrial, commercial, and institutional systems, and small-to-medium size three-phase power transformers used in utility substations, it is generally appropriate to use tight fusing (i.e., a low fusing ratio) to provide maximum protection to the transformer against secondary-side faults. On the other hand, for small-kVA single-phase overhead distribution transformers, larger fuse ratings are generally recommended, particularly if the arrester is mounted on the transformer tank to enhance overvoltage protection. These cases will be treated separately in this article.

Three-Phase Power Transformers and Utility Substation Transformers.

The most important application principle to be considered when selecting a primary fuse for a three-phase power transformer is that it must protect the transformer against damage from mechanical and thermal stresses resulting from secondary-side faults that are not promptly interrupted. A properly selected primary fuse will operate to clear such a fault before the magnitude and duration of the overcurrent exceed the through-fault current duration limits recommended by the transformer manufacturer or published in the standards. Curves representing these limits can be found in ANSI/IEEE Standard C37.91-1985, “Guide for Protective Relay Applications to Power Transformers,” and ANSI/IEEE C57.109-1993, “Guide for Transformer Through-Fault Current Duration.”

The degree of transformer protection provided by the primary fuse should be checked for the level of fault current and type of fault (i.e., three-phase, phase-to-phase, or phase-to-ground) producing the most demanding conditions possible for each particular application, viz., those for which the ratio of the primary-side line currents to the transformer winding currents is the lowest. For these situations, one or more of the primary fuses will “see” a proportionately lower level of current than will the windings and, as a consequence, the primary fuses must be carefully selected to operate fast enough to avoid damage to the transformer windings. Table 1 lists the ratio of per-unit primary-side line currents to per-unit transformer winding currents for three common transformer connections under a variety of secondary-fault conditions.

From Table 1, it is clear that a phase-to-phase secondary fault on a delta / delta connected transformer and a phase-to-ground secondary fault on a delta / grounded-wye connected transformer produce the most demanding conditions possible for those particular transformer connections, since the per-unit primary-side line currents are less than the per-unit transformer winding currents. Accordingly, to ensure proper transformer protection for these two situations, it is necessary to “shift” the appropriate through-fault protection curve to the left (i.e., in terms of current) by the ratio of the per-unit primary-side line current to the per-unit transformer winding current listed in Table 1. The shifted through-fault protection curve will then be in terms of the primary-side line current and, as such, will be directly comparable with the total-clearing curve of the primary fuse. For the grounded-wye / grounded-wye connected transformer, the per-unit primary-side line currents and the per-unit transformer winding currents are the same, hence the base (unshifted) through-fault protection curve applies.

Table 1.

Figure 1 illustrates the infrequent-fault incidence through-fault protection curve applicable to a grounded-wye / grounded-wye connected transformer (Curve A), as well as through-fault protection curves shifted to reflect the two situations discussed above. Curve B represents Curve A shifted to reflect the reduced level of current (0.87 per unit) flowing in two primary lines during a phase-to-phase secondary fault on a delta / delta connected transformer. Similarly, Curve C represents Curve A adjusted to reflect the reduced level of current (0.58 per unit) flowing in two primary lines during a phase-to-ground secondary fault on a delta / grounded-wye connected transformer.

Figure 1.

HELPFUL TIP
For a delta / wye connected transformer with its neutral grounded through a significant impedance, the ratio of per-unit line current to per-unit winding current for a phase-to-ground secondary fault is the same as that discussed above for a delta / grounded-wye connected transformer. However, since the impedance in the neutral limits the magnitude of the phase-to-ground fault current to levels well below the level of current which will damage the transformer, the phase-to-ground through-fault protection curve is of no concern and may be ignored. Accordingly, the base (unshifted) through-fault protection curve, applicable to multiphase secondary faults, should be used for this transformer.

HELPFUL TIP

For a delta / wye connected transformer with its neutral grounded through a significant impedance, the ratio of per-unit line current to per-unit winding current for a phase-to-ground secondary fault is the same as that discussed above for a delta / grounded-wye connected transformer. However, since the impedance in the neutral limits the magnitude of the phase-to-ground fault current to levels well below the level of current which will damage the transformer, the phase-to-ground through-fault protection curve is of no concern and may be ignored. Accordingly, the base (unshifted) through-fault protection curve, applicable to multiphase secondary faults, should be used for this transformer.

Although the through-fault protection curves are only a guide, they are recommended as a criterion against which to measure the degree of transformer protection provided by the primary fuse. To meet this criterion for high-magnitude secondary-side faults, the total-clearing curve of the primary fuse should pass below the point (historically called the ANSI Point) on the appropriate through-fault protection curve at the current level corresponding to the maximum three-phase secondary-fault current as determined solely by the transformer impedance (i.e., an infinite source is assumed).

Another aspect of transformer protection involves low-current overloads. Low-voltage protective devices such as circuit breakers and current-limiting fuses are designed to provide overload protection for the transformer by operating at currents only slightly larger than their minimum-pickup settings or ampere ratings. In contrast, medium-voltage fuses are not intended to provide overload protection. Accordingly, the total-clearing curve of the primary fuse will cross the through-fault protection curve at some low level of current. Because the primary fuse does not provide overload protection for the transformer, this should not be a concern; however, efforts should be made to keep the current value at which the two curves intersect as low as possible to maximize protection for the transformer against secondary-side faults.

The through-fault protection curve for a delta / grounded-wye connected transformer can be used to illustrate these principles for primary-side fuses. See Figure 2. The total-clearing curves for primary fuses with a fusing ratio of 1.0, 1.5, or 2.0 all pass below the ANSI Point of the delta / grounded-wye connected transformer’s through-fault protection curve. The total-clearing curve for primary fuses with a fusing ratio of 2.5 or 3.0 pass completely above and to the right of the through-fault protection curve and, thus, would not provide any protection for the transformer for a phase-to-ground secondary fault. Since the object of primary fusing is to provide protection for the transformer against all types of secondary faults, primary fuses having total-clearing curves that pass above the ANSI Point would be considered unacceptable.

Figure 2.

The primary fuse having the lowest fusing ratio of the three fuses that pass beneath the ANSI Point will provide the maximum protection for the transformer against secondary faults located between the transformer and the secondary-side overcurrent protective device — as well as maximum backup protection for the transformer in the event the secondary-side overcurrent protective device fails to operate, or operates too slowly due to an incorrect (higher) rating or setting.

From Figure 2, it may be seen that a primary fuse with a fusing ratio of 1.0 will provide protection for a delta / grounded-wye connected transformer against phase-to-ground secondary faults producing currents as low as 230% of the full-load current of the transformer, as reflected on the primary side. When the fusing ratio is 2.5, however, protection for the transformer is provided only when secondary faults produce primary-side line currents exceeding 670% of the transformer full-load current.

The results of published studies [1] [2] [3] indicate that under arcing conditions, secondary-switchboard and other nearby faults on 480/277Y-volt circuits may have magnitudes as low as 38% to 40% of the maximum available phase-to-ground fault current at the point of the fault. This corresponds to 290% of the full-load current of the transformer in Figure 2, as seen by the primary fuse. Hence, a primary fuse with a fusing ratio of 1.0 will provide protection for the transformer against an arcing phase-to-ground fault, since the primary fuse will operate at as low as 230% of the full-load current of the transformer. A primary fuse with a fusing ratio only slightly higher than 1.0, though, may have a total-clearing current in excess of 290% of the full-load current of the transformer, and thus may not provide protection for the transformer against a phase-to-ground fault under arcing conditions. A primary fuse with a fusing ratio only slightly higher than 1.0 will, however, protect the transformer against permanent or metallic phase-to-ground secondary faults as well as other types of secondary faults, including arcing phase-to-ground secondary faults that escalate to multiphase secondary faults.

HELPFUL TIP
You can determine if the primary fuse will protect against an arcing secondary-side fault by referring to Table 2 which lists primary-side line current values for various types of secondary-side faults and for various transformer connections and impedances, expressed in percent of the transformer full-load current. The desired protection is obtained if the current value at which the total-clearing curve of the primary fuse intersects the transformer through-fault protection curve is less than the values shown in Table 2.

HELPFUL TIP

You can determine if the primary fuse will protect against an arcing secondary-side fault by referring to Table 2 which lists primary-side line current values for various types of secondary-side faults and for various transformer connections and impedances, expressed in percent of the transformer full-load current. The desired protection is obtained if the current value at which the total-clearing curve of the primary fuse intersects the transformer through-fault protection curve is less than the values shown in Table 2.

Table 2.

Small-kVA Overhead Distribution Transformers.

In the not to distant past, the fusing philosophy applied to small-kVA single-phase overhead distribution transformers was similar to the fusing philosophy applied to larger transformers, that is, the smallest practical fuse link rating was used, subject only to loading considerations. Recently, however, this philosophy has changed to one of protecting the system from a failed transformer and protecting against catastrophic failure of the transformer rather than protecting the transformer itself from overcurrents. This shift in philosophy is due in large part to the realization that most overhead distribution transformer failures occur due to lightning-induced surges and not secondary-side faults.

One way to provide better overvoltage protection for the transformer is to relocate the arrester from the cross-arm and mount it directly to the transformer tank. This location eliminates 3 to 4 feet of lead connecting the arrester to the transformer tank, which reduces the L×di/dt voltage surge seen by the transformer when the arrester operates.

Moving the arrester to the transformer tank, however, makes small-rated fuse links susceptible to nuisance operations because these small links must pass the surge current during an arrester operation. Therefore, to provide better overvoltage protection without increasing nuisance fuse operations, it is necessary to increase the fuse link rating to withstand these surges. The only apparent downside to the use of larger fuse ratings is a reduced level of overload, secondary-fault, and internal-fault protection provided for the transformer. However, further analysis reveals that very little protection is given up by standardizing on larger fuse ratings. Consider the following:

Overload protection for the transformer is difficult to justify as the economics of overhead distribution systems necessitate the loading of transformers significantly beyond their nameplate ratings.
Where covered secondary conductor is used, secondary faults are rare. With covered conductor the possibility of faults due to mid-span tree, animal, or human contact is significantly reduced.
The rare faults that do occur on the secondary conductor will generally not sustain the arc. These faults tend to be self clearing due to the low 120/208-volt driving voltage. In addition, when aluminum secondary conductor is used, faults tend to burn back the conductor which further helps to extinguish the arc.
A bolted fault at the service drop of the nearest house will be low enough in magnitude, due to the impedance of the conductor, that a primary fuse — even one with a low fusing ratio — would not likely detect the fault.

Perhaps a greater concern than secondary-fault protection is the need to protect overhead distribution transformers from catastrophic failure due to internal faults that begin as low-current faults, and then quickly escalate to the full available fault current. One recent paper [4] studied internal transformer faults and concluded that this type of fault signature is indeed common. Interestingly, the authors also concluded that small rated fuses in general, and even surge-tolerant fuses with dual-element melting characteristics in particular, are no better at detecting internal winding faults than are larger rated fuses having the same lightning surge-withstand characteristics. In addition, fuse limiters or backup current-limiting fuses were found to be are very effective at minimizing the energy into the faulted transformer, and are recommended when surge tolerant fuses are used. Lastly, pressure relief devices are clearly recommended to limit tank pressure during an evolving fault to minimize stresses on the transformer tank should a high-current fault develop.

REFERENCES

J. R. Dunki-Jacobs, “The Effects of Arcing Ground Faults on Low-Voltage System Design,” article reprinted from the May/June 1972 issue of IEEE Transactions on Industry and General Application.
J. R. Dunki-Jacobs, “State of the Art of Grounding and Ground Fault Protection,” article reprinted from the 1977 Conference Record of the IEEE 24th Annual Petroleum and Chemical Industry Conference, September 13-14, 1977, Dallas, Texas, Catalog No. 77CH1229-4-lA.
L. E. Fisher, “Resistance of Low-Voltage Alternating Current Arcs,” IEEE Transactions on Industry and General Applications, Vol. IGA-6, November/December 1970, pages 607-616.
J. M. Lunsford and T. J. Tobin, “Detection of and Protection for Internal Low-Current Winding Faults In Overhead Distribution Transformers,” presented at the IEEE Power Engineering Society 1996 Summer Power Meeting, July 28 — August 1, 1996, Denver, Colorado.

Go to Unit 4