April 7, 2003
Overcurrent Protection of Transformers — Traditional and New Fusing Philosophies for Small and Large Transformers.
This is the fourth 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. The third article (Unit 3) covered how to select a transformer-primary fuse to protect the transformer in accordance with industry-accepted through-fault protection curves. This article covers how to select a transformer-primary fuse to coordinate with both secondary-side and primary-side overcurrent protective devices.
How to Select a Transformer-Primary Fuse to Coordinate With Both Secondary-Side and Primary-Side Overcurrent Protective Devices
In addition to protecting the transformer against faults, internal or otherwise, it is also important that the primary fuse coordinate with overcurrent protective devices on both the primary side and the secondary side of the transformer. The following sections describe how proper coordination is achieved both between the primary fuse and secondary-side protective equipment, and between the primary fuse and source-side protective devices.
Coordination Between the Primary Fuse and 480/277Y-Volt Secondary-Side Overcurrent Protective Devices.
Coordination between the transformer primary fuse and the feeder protective device is typically checked for the level of fault current and for the type of fault (i.e., three-phase, phase-to-phase, or phase-to- ground) producing the most demanding conditions possible for the transformer in each application. From the standpoint of coordination, the most demanding conditions possible are those where the per-unit line current on the primary side of the transformer is greater than the per-unit line current on the secondary side of the transformer. For this situation, the primary-side device carries more current, relatively, than does the secondary-side overcurrent protective device. Accordingly, an allowance must be made before checking for proper coordination between the two devices. Table 1 lists the ratio of per-unit primary-side line current to per-unit secondary-side line current for the same transformer connections and types of secondary faults discussed earlier.
TABLE 1 — Relationship Between Per-Unit Primary-Side Line Current and Per-Unit Secondary-Side Line Current for Various Types of Secondary Faults:
For a phase-to-phase secondary fault not involving ground on a delta / grounded-wye connected transformer, the per-unit primary-side line current in one phase is the same as that resulting from a three-phase secondary fault, while the secondary-side line current is only 0.87 per unit of the three-phase secondary fault-current value (hence, the ratio, as listed in Table 1, is 1.0 * 0.87, or 1.15). To compensate for the line-current differential inherent to the delta / grounded-wye connected transformer, it is generally recommended that a 15% margin in terms of current (or an equivalent margin in terms of time) be maintained between the total-clearing curve of the feeder protective device and the minimum-melting curve of the primary fuse. This is illustrated in Figure 1 for a low-voltage feeder circuit breaker.
| Helpful Tip |
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| The only exception to this recommendation is Class E-2 high-voltage industrial control equipment, where the 15% current margin is not required since the point of influence of this margin (where the curves for this device and the primary-side device are the closest to each other) occurs at approximately 20 seconds, before which time a medium-voltage phase-to-phase ungrounded fault would likely have propagated to ground. This current margin is therefore not required to ensure proper coordination for faults involving ground in this type of equipment. |
The only exception to this recommendation is Class E-2 high-voltage industrial control equipment, where the 15% current margin is not required since the point of influence of this margin (where the curves for this device and the primary-side device are the closest to each other) occurs at approximately 20 seconds, before which time a medium-voltage phase-to-phase ungrounded fault would likely have propagated to ground. This current margin is therefore not required to ensure proper coordination for faults involving ground in this type of equipment.
Occasionally, it may be deemed necessary to coordinate the transformer primary fuse with a main secondary-side protective device. In this case, the primary fuse will operate to protect the transformer against a fault located between the transformer and the main secondary protective device and will further serve as a backup to the main device — operating in the event the main secondary protective device either fails to operate due to a malfunction, or operates too slowly due to incorrect (higher) ratings or settings.
The method for establishing coordination between the transformer primary fuse and the main secondary protective device is essentially the same as that described previously for a feeder circuit breaker or fuse, except for the handling of the current margin (or equivalent time margin) for the phase-to-phase secondary fault not involving ground on a delta-wye connected transformer. For this particular fault, the point of influence of the 15% current margin (or equivalent time margin) typically occurs at a relatively low current (and long duration) for low-voltage circuit breakers and low-voltage current-limiting fuses. The probability of occurrence of a low-magnitude long-duration phase-to-phase secondary fault not involving ground located between the feeder protective devices and the main secondary protective device is extremely remote. Such low-magnitude long-duration faults typically occur on a feeder some distance from the transformer, and thus are cleared by the feeder protective device. Accordingly, it is not necessary to maintain the 15% current margin (or equivalent time margin) when coordinating low-voltage main secondary current-limiting fuses with the primary fuse. For medium-voltage circuit breakers, the point of influence of the 15% current margin (or equivalent time margin) occurs at a very high current — on the order of the maximum three-phase secondary fault-current level. Accordingly, this margin must be retained when coordinating medium-voltage main secondary circuit breakers with the primary fuse.
Since main secondary circuit breakers or fuses typically have high ampere ratings or settings, difficulties are sometimes experienced in simultaneously obtaining protection for the transformer against secondary-side faults in accordance with the through-fault protection curves discussed earlier, and complete coordination between the primary fuse and the main secondary protective device. If this situation is encountered, it is recommended that the ampere rating or settings of the main secondary protective device be investigated to see if a reduction is possible, rather than accepting a larger than necessary primary fuse ampere rating, which would result in reduced transformer protection.
This point is illustrated in Figure 2 for a low-voltage main secondary circuit breaker, wherein a transformer-primary fuse does not coordinate with the main secondary circuit breaker over the full range of applicable currents. Coordination between the two devices has not been obtained with the short-time pickup current of the main secondary circuit breaker set at 12,000 amperes (4X), and with the short-time delay setting on the “Maximum.” Clearly, by reducing the short-time pickup setting from 4X to 3X or even 2.5X, and by reducing the short-time delay setting from “Maximum” to “Minimum,” coordination between the main secondary circuit breaker and the primary fuse will be obtained. (The time-current curve for the main secondary circuit breaker adjusted to reflect lower short-time pickup and short-time delay settings is illustrated by solid lines.) Lack of complete coordination of the type illustrated in Figure 2 can frequently be corrected by making such adjustments.
| Helpful Tip |
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| If it is not practicable to reduce the ampere rating or settings of the main secondary-side protective device, as discussed in the example above, incomplete coordination between the primary-side protective device and the main secondary-side device should be accepted in order to obtain better transformer protection. Even if these circumstances are encountered, coordination will typically be given up over only one or two very small ranges of current. |
Coordination Between Primary-Side and Source-Side Overcurrent Protective Devices.
After the transformer primary fuse has been selected to provide the maximum degree of protection for the transformer and to coordinate with secondary-side protective devices, it is necessary to consider coordination with source-side protective devices. To achieve coordination with a source-side protective device, the total-clearing time of the primary fuse must be less than the minimum-melting time of a source-side fuse, or the minimum-operating time of a source-side relay, for all currents up to the maximum available fault current at the location of the primary fuse. In establishing such coordination, no adjustments must be made to the total-clearing curve of the primary fuse.
Certain adjustments, however, must be made to the minimum operating time-current curves of source-side protective devices. Specifically, the minimum response curves for source-side relays must be adjusted for overtravel and tolerance, and minimum-melting curves of source-side power fuses must be adjusted to reflect the assumed prefault load, elevated ambient temperature and, for certain types of fuses, damageability.
Earlier it was recommended that the smallest practicable ampere rating or setting for the primary fuse be selected in order to maximize transformer protection. Such a selection will also greatly facilitate coordination with source-side protective devices since the lower total-clearing time-current curve associated with the primary fuse will more easily fit below the time-current curve of the source-side protective device.
If difficulties in coordination with source-side protective devices are encountered, the ratings of the primary fuse should be restudied to verify that the smallest practicable ampere rating has indeed been selected. This may involve a reconsideration of the ratings and settings of the secondary-side protective devices with which coordination was previously obtained.