Tips & News - November 2014

“Just like everyone else, we’re driven by SAIDI, SAIFI, MAIDI, and MAIFI. So, we’re taking a hard look at our systemand figuring out what we can do to improve those numbers,” points out Robert Cheney, Team Leader of Test Lab, Alabama Power. Strategy The most obvious approach is to add fuses, reclosers and switches to the line, but there are limits to the number and types of devices that can be added. Sectionalizers are another option and they can provide a number of benefits to a utility. Fuses: Consider the simplest of line interrupting devices: The fuse. They are inexpensive anddo not require any special training to replace, but have limitations when used as a sectionalizing device. If a line has a 40-amp fuse, downstream from a 70-amp fuse, downstream from a 100-amp fuse, the appropriate fuse(s) should blow in response to a fault current. But if the short circuit is too high, they are all going to blow, cutting power to many meters. Reclosers: For time coordinated devices, like reclosers, there are are a number of timing issues to consider. First, there is a little bit of error in the timing of any device. Second, there is mechanical operating time (the interval between tripping-initiation and fault clearance) to consider, as well. John Thorne, Senior Engineer, Alabama Power explains, “Let’s say you allow 160 milliseconds for operating time of a single recloser. If I have three reclosers strung out along a line, those times add up. The first only needs 160ms to operate. The second needs 320 ms and the third needs 480 ms to operate. That’s half of a second. Back at the substation, you have to decide how long a transformer can safely experience a fault current before the breaker should trip. If I set the breaker to operate after one second and each device takes 0.16 seconds, there are only so many devices that I can fit under that one second time limit.” ALTERNATING SECTIONALIZERS & RECLOSERS aLaBaMa POWeR, see previous page

At Alabama Power, line devices and protection schemes can usually handle three or four, time-coordinated devices on a feeder, while still maintaining proper separation between the tripping curves. However, if you use a combination of both sectionalizers and reclosers you can add more devices to the line. Sectionalizers: Thorne continues, “A lot of times we would prefer to install a time-coordinated interrupting device, but we can’t because we need to maintain an acceptable separation between the operating curves. A sectionalizer allows us to add more devices.” The Alternating Approach A sectionalizer has to open while the line (or line section) is de- energized. If you have a recloser just outside a substation and three sectionalizers downstream, you have to coordinate the four devices so the furthest sectionalizer opens during the first recloser operation. The second sectionalizer opens during the second recloser operation and the closest sectionalizer opens during the third recloser operation. Cheney has a better idea. “Outside of the substation, we put in a recloser and a sectionalizer that is programmed to open during the third open interval. This provides two opportunities for the line to clear temporary faults, before the sectionalizer operates.” “Further out we install another recloser, and then another sectionalizer. Those twosectionalizers arenot directlyunderneath each other, so we can maintain that third-open interval operating characteristic. By alternating sectionalizers and reclosers, we can double the number of devices on the line,” continues Cheney. In this arrangement, there is no opportunity to automatically clear a temporary fault. So, it is not a great plan, either.

FIGURE 1 - Hubbell’s three-phase sectionalizer is relatively inexpensive. And, it is a good way to prevent ferro-resonance problems for industrial customers with three-phase service.

FIGURE 2 - PRS Sectionalizer with Polymer Cutout

FIGURE 3

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