Power quality is more than a buzz word for electric utilities, who are faced with serving ever increasing loads that consist of sensitive control equipment. Voltage sags or blinks can cause equipment to malfunction, costing customers time and money.
Georgia Power Co. encountered this problem with a large manufacturing customer in Atlanta. At the site, the customer operated its headquarters and a pilot assembly line to test new assembly line processes where computers and other control equipment are used. Reliable service was never a problem until the area was targeted for a shopping mall and other commercial development. The continuous construction in the area exposed underground feeders to frequent dig-ins and faulted circuits. Expulsion fuses operated when exposed to the faults, resulting in breaker operations on feeders parallel to the manufacturing customer's circuit. These breaker operations resulted in voltage dips on the customer's line.
As part of its underground development, Georgia Power had installed live-front pad-mounted switchgear with expulsion-type power fuses. Because the problem was not associated with trouble at its facility, the customer requested a dedicated substation transformer bank to serve its load. Since the cost for a separate bank would require an investment of more than US$1 million, Georgia Power was eager to find another solution.
The Expulsion Fuse Under normal operating conditions, the fuse carries full load current. When a fault occurs and the fuse melts, current continues to flow through particles of the vaporized element and ionized gases. Heat from the arc burns the remaining portion of the element and releases large quantities of gas from the surrounding tube wall. Although the arc may be extinguished when the alternating waveform of current passes through its zero value of amplitude, the arc may re-ignite as the recovery voltage rises across the ends of the melted fuse link. Eventually, final extinction occurs and the fault is removed from the system. Although expulsion fuses require at least a half cycle to clear a fault, the process may require several cycles, during which time the voltage at the fuse drops to zero. Since all of the power at the substation flows to the fault, the voltage drops at the bus. This drop in bus voltage, affecting all other circuits that are tied to the bus, produces negative effects on motor contactors, electro-mec hanical relays, high-intensity discharge lamps, adjustable-speed motor drives and programmable logic controllers.
The Current-Limiting Fuse It has been standard practice to provide sufficient impedance in the neutral to limit current on solidly grounded systems to avoid severe disturbances if a fault is not cleared quickly. The duration of a voltage dip due to the operation of an expulsion fuse, therefore, presents a potential problem for the utility. When a fault occurs at a transformer, although the expulsion fuse can isolate the fault and de-energize the transformer, the time to clear is in the order of at least one-half cycle, resulting in a voltage dip at the transformer and all parallel feeders for as long as 8 msecs to 12 msecs.
Current-limiting fuses overcome this problem by virtue of their ability to significantly increase resistance when exposed to high-fault currents. In contrast to expulsion fuses, a current-limiting fuse will reduce the fault duration and will support system voltage during the clearing process. The current-limiting fuse is constructed with a silver ribbon element surrounded by fine granular silica sand housed in a fiberglass tube. For low fault currents, the fuse time-current characteristic is similar to that of an expulsion fuse. When high fault currents flow, the silver ribbon element quickly heats and vaporizes. The high temperature of the resulting arc melts the sand, forming a glass-like structure, which restricts the arc and produces an increase in its resistance. This resistance changes the circuit power factor to near unity, so that the time at which the current crosses zero is the same as the system voltage. This circumstance allows the fuse to clear the fault before the natural pre-fault current zer o and, typically, within a half cycle. Although the voltage drops to zero for 1 msec to 4 msec before the fuse element melts, the time is not long enough to affect equipment on parallel feeders. During the fault clearing process, the fuse and the system combine to produce an arc voltage that supports system voltage on the parallel feeders.
Changing to the Current-Limiting Fuse Investigating the possibility for replacing the expulsion fuses with current-limiting fuses, Georgia Power provided Cooper Power Systems of Waukesha, Wisconsin, with the expulsion fuse to evaluate its end fittings. After specifications were developed for voltage, current and time-current characteristics, samples of the replacement fuse were successfully field tested with only minor modifications required for its use. After the tests, more than 60 100A X-Limiter hinge fuses were installed at critical areas served by the substation. No modifications were necessary to install the fuses in the switching cubicle, which can remain energized at all times. Because of the interchangeability of these fuses with expulsion fuses, Georgia Power can still use expulsion fuses in the future if it chooses to do so.
From a cost standpoint, the current-limiting fuse proved to be inexpensive, and its installation required no alterations to the cubicle. In addition, the utility has the option for using direct-buried cable instead of encasing cable in concrete for protection from mechanical damage. If cable is accidentally cut by a dig-in, the chances have been diminished for affecting service to critical customers. The program has improved the quality of service and has saved Georgia Power at least US$900,000.
Since the installation of the X-Limiter current-limiting fuses, all low-current faults have been cleared with no customer complaints. With calculated available fault currents of more than 5000A at the bus, and with short lines, the expectation is that it is only a matter of time before the fuses will be subjected to high fault current duty.
Craig Price is a distribution engineer with Georgia Power Co. He is a graduate of the Southern Polytechnic State University in Marietta, Georgia with the BS degree in electrical engineering technology. He has had more than 18 years of experience in distribution engineering.
Comparing Voltage Characteristics for Expulsion and Current-Limiting Fuses To illustrate the difference between the expulsion fuse characteristic and the current-limiting fuse, the left figure shows that the voltage of the expulsion fuse at location B collapses to zero and remains at zero for 8.1 msecs until the current was interrupted. At this point, the system was subjected to a transient recovery voltage. Voltage at locations C and D decrease, compared to A, the system voltage, due to the fault current induced voltage drop across the source impedance. This voltage drop may be enough to interrupt sensitive industrial loads.
The figure on the right shows that the voltage at the current-limiting fuse location B collapses to zero for only 1.2 msecs and then rises to an overvoltage of 1.85 times peak system voltage at B. Voltages at C and D will be supported at close to system voltage, A. The fuse induced overvoltage exceeds normal voltage for about 1 msec and the fault is cleared in 2.5 msecs, illustrating the advantage of the current-limiting fuse in supporting system voltage.