A commonly debated issue within the electric utility industry deals with using instantaneous tripping for distribution feeders. While the instantaneous trip feature saves the downstream fuses from blowing, this mode of operation impinges on quality of service. Since the instantaneous feature in the breaker relay will de-energize an entire feeder, many engineers disable the instantaneous trip, resulting in the feeder breaker tripping only on mainline faults.
To evaluate instantaneous tripping, Long Island Lighting Co. (LILCO), Hicksville, New York, U.S., has used an extensive monitoring program to observe its distribution system characteristics under fault conditions.
Covered line conductors with HDPE are used for the majority of LILCO's 13.2-kV distribution circuits. While generally increasing reliability of service, the covering on the conductor makes it susceptible to arcing faults that can result in conductor burndown.
The Burndown Scenario An arc with high power follow creates sufficient heating to melt the conductor and cause burndown, a well-known phenomenon that is true for both bare and covered wire. The difference between bare and covered wire is that for a bare conductor, the arc will travel along the wire, away from the power source and in most cases will eventually extinguish itself. With covered wire, the movement of the arc is anchored at a discrete spot because of the covering. All of the heating action of the arc will concentrate on a small section of conductor. As the conductor loses strength, it eventually breaks apart. The burndown creates a long-time interruption because of a fault that would frequently be of short duration, such as a lightning strike. The longer interruption significantly impairs reliability and increases repair costs. In addition, burndowns are a major cause of high-impedance faults if the falling wire does not contact the neutral on its way down.
Whether it is on grass or asphalt, the failed wire may not draw sufficient current to activate a protective device because of the high impedance between the wire and true ground. This type of high-impedance fault is dangerous since the breaker can reclose on the fault, energizing the fallen line and not retripping because of the absence of sufficient fault current required to activate the relay.
The Instantaneous Trip Since burndowns depend on the magnitude and duration of the fault, the time it takes for a protective device to clear the fault is important. The substation breaker is usually controlled by a time-delay and/or an instantaneous overcurrent relay. The relay should coordinate to minimize burndowns. For the 336 kcmil, all-aluminum conductor used on LILCO's mains, the standard time-delay settings do not coordinate with con-ductor damage. If the instantaneous relay is not used, damage will likely occur to the conductor before the time-delay relay opens the breaker (Fig. 1). A temporary fault, defined as one that is cleared when power is interrupted and then restored, represents the majority of interruptions in the overhead distribution system. These temporary faults on lateral taps can be cleared by the feeder breaker before the lateral fuse blows, a practice known as fault selective feeder relaying. A disadvantage in this practice is that all customers on the feeder will experience a blink for most lateral faults. In addition, the fuse and relay breaker frequently do not coordinate properly, since a distribution feeder breaker is slow, at five cycles, compared with typical lateral fuses. For LILCO's standard lateral fuse, a 100T, the breaker and fuse will not coordinate for currents greater than 2500A. Often the fuse will blow and the feeder breaker will trip (Fig. 2).
Power Quality Since the issue of power quality becomes more important, the trend in protection has been to disable the instantaneous trip, resulting in tripping of feeder breakers only for mainline faults. The lateral fuse will operate for all lateral faults, helping with sensitive customer loads because it saves the whole feeder from a momentary interruption. Nevertheless, LILCO has decided to use the instantaneous relay because of the danger of a disabled conductor burndown. Most residential customers do not notice the extra feeder blinks because the first reclosure occurs quickly, in about 30 cycles, since no intentional delay has been added. Many residential appliances, even digital clocks and VCRs, will ride through this short interruption. To reduce momentaries while still limiting wire damages, LILCO is researching the use of microprocessor-controlled relays to delay the instantaneous operation along with the use of a smaller fuse size, which would allow some fuses to blow without tripping the whole feeder.
The Field Data Recorded fault and interruption data for a three-year period provides insight into the tradeoffs in using instantaneous relaying. The feeders, which were monitored with fault recorders, had the instantaneous relay disabled during the first year of the program and then reactivated during the next two years. With the relay reactivated, the number of breaker operations increased while wire damage and fuse operations decreased (Fig. 3). A review of the typical substation design showed two step-down transformers supplying the 13-kV feeder buses, which operated in parallel through a normally closed bus-tie breaker. The closed bus tie arrangement increases the available fault currents on feeders, especially for the first few miles out of the substation and increases the potential for wire damage on the feeder.
As a result of the data collected, LILCO has decided to use instantaneous relays along with bus-tie breakers in a normally open position. This arrangement is expected to reduce conductor burndown on most of its distribution circuits. The expectation is that the severity of voltage sags will be reduced. The quick half-second reclose will alleviate power quality problems that might ordinarily be associated with momentary interruptions.
Ron Ammon holds engineering degrees from Pratt Institute and Long Island University, where he received the BS and MS degrees, respectively. He is a program planner in LILCO's Distribution Performance Engineering organization, where he addresses customer reliability.
Thomas A. Short received the BS and MS degrees from Montana State University in 1988 and 1990, respectively. He is an analytical engineer in the Distribution Unit of Power Technologies, Inc.