Memphis Light, Gas And Water (MLGW) Updates 1930s Substation with CLiP
June 25, 2014
A significant station improvement was the addition of the G&W current-limiting protectors, or CLiP fuses, which replaced the distribution feeder reactors. They have logic circuits to detect fault current levels that will exceed the rating of the connected protective devices and — using small, self-contained pyrotechnic charges — quickly open a faulted circuit.
The protectors clear faults in a time frame of a fourth of one cycle and send a command to the protective breaker to open and isolate the unfaulted phases instantaneously. The fuses are being used on the secondary of each station transformer and on each of the 10 network 12.47-kV circuits.
The five overhead circuits do not need reactors or fuses as they are isolated from the network bus by a normally open sectionalizing breaker and a 115/12.47-kV station transformer. The primary benefits of using CLiP fusing are the virtual elimination of depressed voltage during substation bus or circuit faults that exceed a preset magnitude. The resulting effect is a reduction of available fault current, a significant reduction in overall fault energy, increased feeder capacity and a reduction of arc-flash energy exposure to workers.
Problems Solved and Advantages Gained
In addition to limiting fault current and mitigating depressed voltage events, the CLiP fuse technology solved several concerns plaguing substation 3.
In recent history, a hard distribution feeder fault (network or overhead) would result in depressed voltage for some MLGW customers. The landmark Peabody Hotel elevators were famous for stopping during these types of circuit events. Once during the substation 3 construction project, an overhead circuit experienced a hard fault. The resulting depressed voltage caused a power outage at the FedExForum arena 45 minutes before a Memphis Grizzlies basketball game. Power was restored in time for the game, but MLGW reps had to explain, in depth, how the station improvements in progress would mitigate this type of event in the future.
The CLiP fuse also considerably lowers energy delivered during a bus or feeder fault. The voltage-depressing event would be cleared by CLiP fusing approximately 20 times faster than traditional breaker and reactor protection with an energy level of approximately 1% of a typical substation breaker protection. This reduction in overall energy is less damaging to connected equipment. It also significantly reduces the likelihood for potential injury to any utility workers near the fault by reduction of arc flash energy, blast and propelled debris.
Past experience at other utilities with this type of protection has shown arc flash/blast has been reduced to nonevent status in terms of worker harm.
Choosing to use CLiP fusing also turned out to be a source of significant cost savings in several ways. On the original substation 3 distribution circuits, the current-limiting reactors previously in service were consuming US$55,000 per year in power losses. The reduction in losses is projected to be valued at approximately $3.2 million over the life of the station. It is also worth noting that, prior to the installation of CLiP fuses, the feeder reactors limited the rating factor on the network circuits at about 350 A. Because the rating of the feeder CLiP fuses is 1,200 A, they are not the limitation of the feeder circuit, and the cable/duct bank becomes the limiting factor of each circuit at about 425 A. This is a gain of about 20% in the available capacity of the station, or about two additional circuits.
The Project Concludes
Substation 3 construction is in the final stages. All of the new station transformers are in place, the new control house is in operation, and the transformer and feeder CLiP fusing is in place. When construction is completed, customers of the MLGW southern electric network should enjoy the high level of reliability and power quality they have come to expect from their hometown public utility.
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