POWER TRANSMISSION AND DISTRIBUTION AUTOMATION

Automation provides options for improving the reliability of the distribution supply. An auto-loop automated distribution configuration is a popular way to improve reliability on a normally radial circuit. These systems automatically reconfigure a distribution system: we do not need outside intervention or communications.


In the three-recloser loop example in Figure 1.18, a normal sequence of operations for a fault upstream of recloser R1 is: (1) breaker B1 senses the fault and goes through its normal reclosing cycle and locks open; (2) recloser R1 senses loss of voltage and opens; and (3) recloser R2, the tie recloser, senses loss of voltage on the feeder and closes in.

Since R2 can be switching into a fault, normally it is set for one shot; if the fault is there, it trips and stays open.

We can add more reclosers to divide the loop into more sections, but coordination of all of the reclosers is harder. Consider a five-recloser loop (Figure 1.19). Each feeder has two normally closed reclosers, and there is a normally open tie-point recloser.


If feeder #1 is faulted close to the substation, breaker B1 locks out, recloser R1 opens, and the tie recloser closes. Now, we have a long radial circuit with the station breaker in series with four reclosers  that is a lot to try to coordinate.

To ease the coordination, some reclosers can lower their tripping characteristics when operating in reverse mode. So, in this example, recloser R2 would drop its pickup setting. R2 sees much lower fault currents than it usually does, and we want it to trip before recloser R3 or R4.

For a fault between B1 and R1, the five-recloser loop responds similarly to a three-recloser loop: (1) breaker B1 locks out, (2) R1 opens on loss of voltage, (3) recloser R2 drops its trip setting, and (4) R3 senses loss of voltage on feeder 1 and closes in.

For a fault between R1 and R2, the sequence is more complicated: (1) recloser R1 locks out, (2) recloser R2 drops its trip setting and goes to one shot until lockout, (3) R3 senses loss of voltage on feeder 1 and closes in (and closes in on the fault), and (4) R2 trips in one shot due to its lower setting.

In a variation of this scheme, utilities use sectionalizers instead of reclosers at positions R2 and R4. Sectionalizers are easier to coordinate with several devices in series.

Remotely controlled switches are another option for automating a distribution circuit. The preferred communication is radio. Remotely controlled switches are more flexible than auto-loop schemes because it is easier to apply more tie points and we do not have to worry about coordinating protective equipment.

Most commonly, operators decide how to reconfigure a circuit. Even if a circuit is automated, doing another step of sectionalizing within the isolated section can squeeze out better reliability. Brown and Hanson (2001) show that manually sectionalizing after automated switches have operated can reduce SAIDI by several percent.

As with other sectionalizing, crews should decide on a case-by-case basis whether to sectionalize. Auto-loops will not necessarily help with momentary interruptions. Automation turns long-duration interruptions into momentary interruptions.

To help with momentary interruptions, consider the following enhancements to automation schemes:

• Line reclosers — As part of an automated loop, line reclosers significantly improve momentaries; automated switches do not. Using single-phase reclosers helps interrupt fewer customers.

• Tap reclosers — Use reclosers on long lateral taps. Consider singlephase reclosers on three-phase taps. These will interrupt fewer customers.

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