Energy-limiting Protection for Overhead Systems

Current-limiting fuses have been available for many years in padmount and overhead applications, but most utilities associate and limit their use with three-phase padmount transformers.


By Jon Spencer

Current-limiting fuses have been available for many years in padmount and overhead applications, but most utilities associate and limit their use with three-phase padmount transformers. Applications such as this typically use a bayonet-style expulsion fuse with an oil-submersible, current-limiting fuse. The combination of the two fuses provides full-range transformer protection-up to 50 kA-and reduces the energy let-through when low-impedance faults occur; this protects equipment from damaging overloads, secondary faults and failures if a low-impedance fault occurs.

This same coordination and the benefits it provides can be applied to overhead equipment protected by cutout expulsion fuses, such as pole-top transformers and capacitor banks. External current-limiting fuses can be applied with the installed cutouts to increase interrupting rating, reduce energy let-through and improve the overall installation safety when a fault occurs.

Cutouts have maximum interrupting ratings greater than bayonet-style expulsion fuses and may be sufficient for most applications. The equipment and system, however, are subjected to high-peak currents and energy let-through during fault conditions, since expulsion fuses, by design, must wait for a current zero to clear.

An advantage of current-limiting fuses is faults are modified and cleared before the first current zero, in contrast to waiting for a zero crossing interval. This capability allows current-limiting fuses to interrupt current-and limit the damage and potential for equipment failure during a fault event.

Figures 1 and 2 compare the energy let-through and peak current during a 5,000A symmetrical fault current between a cutout alone vs. a cutout with a 12K external backup fuse.

Figure 1Figure 2

The red square in Figure 1 represents the 875,000 A2 seconds of energy the cutout could let through during the first half cycle of fault current. Using a 12K external backup fuse with a cutout would limit the energy to 10,000 A2 seconds-representing a 99 percent energy reduction by adding current-limiting fuse with the cutout.

Because current-limiting fuses introduce a high resistance into the circuit before the first current peak, the peak fault current also is limited. See Figure 2. The traditional cutout fuse would be subjected to 12 kA peak current (red), but limited to 3.2 kA using the current-limiting fuse (blue).

Figure 3

Even though 5,000A fault levels possibly weren't viewed as severe and within the capabilities of a cutout device, the addition of an external, current-limiting fuse improves equipment protection and reduces the safety risks of equipment failures during fault events.

Current-limiting fuses have been available and effectively used for many years-but, for various reasons, have been limited in applications. Today's redesigned external backup, current-limiting fuses are smaller and lighter, and easier to install. Ratings also have increased to 100K, allowing utilities to increase and expand protection.

These recent improvements to external current-limiting fuses, combined with their ability to limit peak current and energy let-through, and an increasing awareness of safety for utility personnel and the general public, warrants a review of the small investment of a two-fuse protection approach for overhead applications.

About the author: Jon Spencer is an application engineer for Thomas & Betts.
Circle 160 on reader service card

Why Use Current-limiting Fuses?

By Dan Gardner

Thomas Betts

• Current-limiting fuses minimize the risk of distribution equipment failure by limiting the energy and the peak current let-through during a fault. Expulsion fuses and breakers do not limit the energy let-through and sometimes take many loops or cycles to interrupt. In addition, they don't limit the peak current during a fault.

• Current-limiting fuses interrupt high-fault currents-up to 50,000A rms symmetrical-and remove faulted equipment in areas where the available fault current level exceeds the interrupting capability of other protective devices.

• Current-limiting fuses improve component fault ratings such as with load-break elbow connectors-typically rated to 10 kA on fault close. Using a current-limiting fuse will limit the fault magnitude to a value below most component ratings and will reduce the likelihood of component damage or personal injury.

• Current-limiting fuses improve system safety. In addition to reducing the likelihood of equipment failure, current-limiting fuses address potential fire safety hazards or safety issue concerns associated with populated areas where expulsion gases are not acceptable. The noiseless operation of a current-limiting fuse also alleviates concerns for the loud noise, or bang, during fuse operation.

• Current-limiting fuses improve power quality by supporting the system voltage and reducing the blink time during faults.

• Current-limiting fuses improve coordination up to 50 kA with source side devices.

About the author: Dan Gardner, P.E., is Thomas & Betts' director of engineering, power and high-voltage.
Circle 39 on reader service card

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