Procedure for Distribution Circuit Paralleling and Load Transfer?

[dpc]

I'm looking for feedback on utility operating procedure for momentary paralleling of two distribution feeders fed from two different substations. This would typically involve closing a normally open group-operated air break switch that is at the end of two feeders. The purpose is to temporarily parallel the feeders then open one of substation breakers to put all the load on one feeder. And then the subsequent re-transfer back to the normal condition.

So my questions relate to standard procedures for attempting to match voltages at the tie point and operation of the substation load tap changer or regulators:

Prior to paralleling:
Place one or both LTCs or regulators in manual?
Run both to neutral or to the same tap?
Any attempt to measure/match actual voltages at the remote normally open switch prior to closing?

Same questions would apply to a re-transfer procedure.

I'm curious what procedures the various utilities represented here might be following and any issues that have come up.

Thanks in advance,

Dave

 

[wareagle]

Utilities I know would just put the Regs on non auto or manual and transfer. There was no attempt to adjust the voltage, but the subs were far apart. Yours may be close together may need to consider that.

 

[DTR2011]

I have done it with both LTCs in manual mode (with tap verification). Also it may be useful to disable the 51G element during the transition and re enable afterwords. Return the LTC to Auto after the switching.

If it is possible and practical it never hurts to measure the voltage. It depends on what is available. A lot of the voltage hot sticks I have seen have an analog gauge and it is being read from quite a distance. One would expect ~Zero volts across the open tie.

 

[racobb]

Some prefer to put the LTC's or Feeder Voltage Regulators on neutral and some prefer to lock them down in the position they are on. As long as they are not bypassing them, it doesn't matter to me. I always recommend that they also cut them off to make sure they do not move in manual. Siemens had a problem with some of their regulator controls and they would sometimes still step even though they were in manual. I just cut them all off!

The other obvious check is to make sure that the 2 circuits were "phased in" when the switch was installed. Generally they are.

I don't worry about disabling the 51G if we are doing 3 phase gang switching as opposed to single phase hookstick switches since it is generally faster to trip if there is a problem.

I always disable reclosing on both circuits while switching.

Alan

 

[racobb]

Forgot to add that it all "assumes" that the substations are fed from the same transmission line or 2 lines that are in phase. Some transmission lines of different voltages are 30 degrees out of phase to start with. In our area, one utility has 44 kV and 115 kV that are 30 degrees out of phase.

Alan

 

[dpc]

Thanks for the feedback. We find some differences in how this procedure is done and I'm trying to make sure I'm not overlooking something.

One concern is that the voltages at the open switch can be different even if the LTC taps are set the same. This can result in some large current flows that sometimes come as a surprise to the linemen.

Cheers,

Dave

 

[racobb]

In rare cases when we have had long/loaded rural lines, I have checked and matched voltages closely on both sides of the tie switch. Usually it is not a problem on short feeders in urban areas.

Alan

 

[DTR2011]

"One concern is that the voltages at the open switch can be different even if the LTC taps are set the same. This can result in some large current flows that sometimes come as a surprise to the linemen."

This is especially true if the LTC's / Transformers do not share the exact same ratio. Different manufacturers, ages, specs, etc.

If you are writing a procedure for an existing utility, they generally have a feel for how their system works. If in doubt, or if there is a new piece of equipment coming into play, measuring the voltage across the open switch with a digital hot stick meter is your safest bet. Treat it as a commissioning test and document the results of the LTC positions at both ends. Adjust as necessary. This generally adds a comfort factor to the linemen.

 

[bacon4life]

When preparing to open a switch, the crew can use a hot stick ammeter to measure current, then have the dispatcher remotely raise or lower tap positions to minimize current at the switch. We have lots of slightly different ratios and DETC settings, so setting the OLTC to the same voltage is much better for us than using tap position.

 

[stevenal]

http://www.sandc.com/products/switching-overhead-distribution/loadbuster-tool.asp