This is the second article in a series of articles that concern new and traditional fusing philosophies for protecting transformers. The previous 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. This article covers 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.

How to Select a Transformer-Primary Fuse to Withstand Various Inrush Currents.

When an unloaded distribution or power transformer is energized, there occurs a short-duration inrush of magnetizing current which the transformer primary fuse must be capable of withstanding without operating (or, in the case of certain types of fuses, without sustaining damage to their fusible elements). A conservative estimate of the integrated heating effect on the primary fuse as a result of this inrush current is roughly equivalent to a current having a magnitude of 12 times the primary full-load current of the transformer for a duration of 0.1 second, and 25 times for 0.01 second. An example of the magnetizing-inrush current for a small overhead distribution transformer is shown in Figure 1. This example is from a laboratory test and is the highest inrush obtained from tests performed on this transformer. The inrush that occurs on any particular energization will depend, among other things, on the residual magnetism of the transformer core as well as the instantaneous voltage when the transformer is energized. Since these two parameters are unknown and uncontrollable, the fuse must be sized to withstand the maximum inrush that can occur under worst-case energization. The minimum-melting curve of the primary fuse should be such that the fuse will not operate as a result of this magnetizing-inrush current.

Figure 1. Magnetizing inrush current for a small overhead distribution
transformer (7600 V, 25 kVA, single-phase) energized at rated
voltage from source with 5-kA available fault current.
1 per-unit current = transformer rated rms full load current.

The integrated rms equivalent of the inrush current from Figure 1 is shown in Figure 2, along with the “standard” inrush points. Note that the standard inrush points are higher than the actual rms equivalent of the inrush current. As mentioned, these standard inrush points result in a conservative estimate of the magnetizing-inrush current. Sizing the transformer-primary fuse such that its minimum-melting curve is above these standard inrush points will avoid an unnecessary fuse operation, but can occasionally cause coordination problems with upstream protective devices, or it may result in compromising the protection of the transformer because of the large rating selected. On these occasions, the use of a smaller fuse rating is desirable and can be justified by using a better estimate of the heating equivalent of the magnetizing-inrush current.

Figure 2.
Figure 2. Integrated rms equivalent of magnetizing-inrush current
from Figure 1, shown with industry “standard” inrush points.

Actual magnetizing-inrush current also depends on the transformer rating and the available fault current. Because of the voltage drop across the source impedance during the inrush period, the inrush current will be less when the transformer is supplied from a weak source as compared to a strong source. Also, for small overhead distribution transformers the peak inrush current can be as high as 30 times the rated rms current, while for larger substation-type transformers the inrush peak will be lower, but the inrush duration longer. Figure 3 illustrates the maximum magnetizing inrush rms equivalent as a function of transformer size. Note that the per-unit inrush current is lower for the larger transformer sizes (actual amperes of inrush current is, of course, higher for the larger transformers). The strength of the source relative to the transformer full-load current is indicated by the ratio of the transformer full-load current to the source available fault current; a strong source will be able to supply a high fault current and will result in a lower ratio of full-load current to fault current.

The transformer-primary fuse must also be capable of withstanding the inrush current that occurs when a transformer that is carrying load experiences a momentary loss of source voltage, followed by re-energization (such as occurs when a source-side circuit breaker operates to clear a temporary fault, and then automatically recloses). In this case, the inrush current is made up of two components: the magnetizing-inrush current of the transformer and the inrush current associated with the connected loads. The ability of the primary fuse to withstand this combined magnetizing- and load-inrush current is referred to as “hot-load pickup” capability.

Figure 2.
Figure 3. Magnetizing-inrush current equivalent rms
at 0.1 second (top) and at 0.01 second (bottom), in
per unit of transformer rated current shown as a
function of transformer size (kVA rating) with
source strength indicated as a parameter; a strong
source will have a low ratio of rated load current
to available fault current.

The integrated heating effect on the transformer-primary fuse as a result of the hot-load pickup current is equivalent to a current having a magnitude of between 12 and 15 times the primary full-load current of the transformer for a duration of 0.1 second. Here again, the minimum-melting curve of the fuse (adjusted for preload) should exceed the magnitude and duration of the combined inrush current. This is again a conservative estimate of the inrush, and the advantages of using a smaller fuse can sometimes be obtained by recognizing certain mitigating factors. First, this hot-load inrush phenomenon is usually not applicable to transformers serving predominantly industrial loads such as a factory. Because the manufacturing processes will generally require equipment and machines to be restarted in an orderly sequence, not all of the loads are immediately restarted as soon as power is restored. Also in industrial applications, the transformers are typically sized to carry continuously the maximum expected demand load with the result that they are often actually loaded to perhaps only half or less of rated power. Second, as with magnetizing inrush, the ability of the source to supply current, limited by the source impedance, will limit the magnitude of the hot-load inrush current.

The final type of inrush currents to which the transformer-primary fuse will be exposed are long-duration overcurrents that occur due to the loss of load diversity following an extended outage (30 minutes or more). These long-duration overcurrents are referred to as “cold-load pickup.” The cold-load pickup phenomenon is typically associated with utility distribution transformer loading practices, where the transformers are often sized for the average peak load rather than the maximum expected peak load, thereby exposing the transformers to overcurrents of up to 30 minutes duration following re-energization. This phenomenon occurs since large electrical loads such as air conditioners, refrigerators, and electric heaters are thermostatically controlled and cycle on and off at random times relative to one another such that only a fraction of total possible load is connected to the system at any one time. After an extended loss of power, many more of the thermostatically controlled devices will be outside of their respective set-point limits with the result that, as soon as power is restored, the thermostats will demand power for their controlled equipment.

To avoid a nuisance operation of the transformer-primary fuse, it must be capable of withstanding the magnetizing-inrush current of the transformer superimposed on the transient overcurrent associated with picking up cold, the expected overload current associated with the total kVA connected. The time-integrated heating effect of the cold-load current profile on thermally responsive devices, such as fuses, is usually represented by the following equivalent multiples of transformer nominal rated load current:

6 × nominal load current for one second;
3 × nominal load current for up to 10 seconds; and
2 × nominal load current for up to 15 minutes.

The ability of the transformer primary fuse to withstand the combined magnetizing- and load-inrush current associated with an extended outage is referred to as its cold-load pickup capability. Here again, the cold-load inrush will be ameliorated by the source impedance and, if the source is weak, use of a smaller fuse rating may often be justified. Equivalent rms current representing the cold-load inrush that can be expected from small distribution transformers (50 to 225 kVA) is illustrated in Figure 4. Standard cold-load pickup points are also illustrated. For larger transformers (up to 1000 kVA) the cold-load pickup profile differs slightly only in the short time region (less than 0.1 seconds) from that illustrated in Figure 4; this is because the difference in the magnetizing-inrush current with transformer size has a greater influence when the inrush current is integrated over this short time period.

Helpful Tip
In contrast to residential loads, transformers applied in industrial, commercial, and institutional power systems are usually sized to accommodate maximum peak demand load without being overloaded. As a result, these transformers are often actually loaded to only a small fraction of their rated power — perhaps only one-half or less. For this reason, and the requirement for an orderly re-starting of equipment, the combined magnetizing- and load-inrush currents associated with the energizing of these transformers following an extended outage is no more severe than the inrush currents encountered under hot-load pickup conditions. Accordingly, cold-load pickup need not be considered when selecting the ratings and settings of primary fuses for transformers applied on industrial, commercial, and institutional power systems.

Helpful Tip

In contrast to residential loads, transformers applied in industrial, commercial, and institutional power systems are usually sized to accommodate maximum peak demand load without being overloaded. As a result, these transformers are often actually loaded to only a small fraction of their rated power — perhaps only one-half or less. For this reason, and the requirement for an orderly re-starting of equipment, the combined magnetizing- and load-inrush currents associated with the energizing of these transformers following an extended outage is no more severe than the inrush currents encountered under hot-load pickup conditions. Accordingly, cold-load pickup need not be considered when selecting the ratings and settings of primary fuses for transformers applied on industrial, commercial, and institutional power systems.

Figure 4.
Figure 4. Equivalent rms of combined magnetizing-
and cold-load pickup inrush current, in per unit of transformer
rated current, for small transformers serving residential loads.
Industry “standard” cold-load pickup points are shown. Source strength
is indicated as a parameter; strong sources will have low ratios of
rated load current to available fault current.

Go to Unit 3