Switching hazards associated with leading power factor

[dlineh]

I would appreciate any advice on a potentially hazardous situation we have been experiencing during switching operations. While breaking a station tie on a 23kv system utilizing three single 900 amp solid blade disconnects we are experiencing some undesireable failures on several different feeders. We are breaking station ties between two stations fed by two different transmission voltages (140kv and 238kv)on a distribtion voltage of 23kv using the S&C Loadbuster tool. On a great deal of occassions the tool has failed, sometimes totally destroying and melting the tool. We have worked extensively with S&C and it is not the fault of the tool or lack of maintenance. Could the leading power factor create some type of transient that would cause the tools failure? Would some type of gang operated switch resolve this issue? If so what would you reccommend? So far the injuries have been minor as a result of the switching failures. The regulators at the station have been addressed before switching. The "obvious" switching hazards have been adddressed and we are still experiencing the same problem without any pattern. The switching operations failures are never consistant. Thanks for any advice.

 

[davidbeach]

Define "Failure". How leading? How much load flow?

 

[Jebb]

What is the current flow at the disconnects before opening? Does it happen when you pull the first disc??? What system type delta or wye???

 

[dpc]

It's really difficult to know how much current you will be interrupting in situations like this. It's possible there could be a high magnitude of circulating current due to voltage difference between the two system.

If this tie switch is remote from both substations, there is really no ideal way of setting the voltage regulators for opening the tie, since the actual voltage and phase angle at the switch is unknown.

What is current rating of the Loadbuster? It sounds like the current be broken is just more than the tool can handle.

I don't see that leading power factor (where exactly?) is going to the cause of this problem.




"The more the universe seems comprehensible, the more it also seems pointless." -- Steven Weinberg

 

[ScottyUK]

I have some notes on the problems of switching leading PF loads in the old Metropolitan Vickers Switchgear Handbook - will see if I can scan a page or two at work and upload them.


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If we learn from our mistakes I'm getting a great education!

 

[davidbeach]

Wow, the OP logged into the site to ask the question and has never been back to even begin to clarify what the heck it is that he is asking, and now 5 days late there is a flurry of activity? Why don't we let this drop until the OP at least has enough interest to even define what he means when he says that there have been failures? Normal arcing of a MV switch might be a "failure" to someone unfamiliar with normal switching. Maybe he knows better, maybe the switch welds shut, maybe it blows up, maybe it just opens and continues to arc in the fully open position. We don't know. Is it .98 leading power factor or is it purely capacitive current? We don't know. It is all speculation at this point.

 

[dpc]

I should clarify my earlier post - highly lagging and leading currents are more difficult to interrupt than in phase currents. So if the switch is trying to break only capacitor current, it could be part of the problem. But I was assuming you were referring to a slightly leading pf. You might check the ratings of the Loadbuster for power factor limitations.

"The more the universe seems comprehensible, the more it also seems pointless." -- Steven Weinberg

 

[wareagle]

I don't think your problem leading power factor. You are tying your 140/23 kv sub to your 238/23 kv sub thru your 23 kv distribution system. You have a power flow from the 238 kv system to the 140 kv system using the 23 kv feeder. I had this happen when we tied a 115kv/12 kv sub to a 230/12 kv sub thru a 13 kv system and burned the entire feeder down. The transmission Dept had warned about this possibility but it did not get passed on the the control center. So much for communications.

 

[dlineh]

Hello,

I apologize for not keeping my posting current. I was ill for a couple of days and have been playing catch up since. I have attached a file with a photo of one of the many failed tools. This one obviously is a catastophic failure, most have not been as bad but a few have been worse. I will attempt to answer some of the posted questions but first I would like to Thank everyone for their advice. I have worked with S&C to find a solution to this ongoing problem and they have been excellent. It has been concluded the problem must be in the system, not the tool. The S&C Loadbuster has been in use for more than 50 years and is used around the world. The tool is capable of breaking 900 amps. Our loadbusters are maintained on a regular basis as per manufacturers recommendations. It is difficult to answer the question of ampereage through the switch at time of opening because of the large number of switching failures. Please remember we are breaking a station parallel not dropping load but load transfer. The problem only occurs on our 23kv system fed by the two different transmission voltages. The problem does not exist on our 13.2 kv (phase to phase) system. Both are wye systems. I hope this along with the attched pictures will be of some assistance. Thanks again.

 

[dlineh]

2nd attempt at attaching photo!!

 

[PHovnanian]

I think wareagle is right. This sounds more like circulating current between the two systems. It may appear to be a "leading power factor" problem due to phase angle differences involved.

Some measurements of these currents and/or analysis of the network might suggest a way to minimize the problem.