Closing switches

[bahrra]

Heres a problem I have been working on and really can't find any data on. Under what conditions can you close a switch. My company has certain guidelines that we follow when closing a switch. Our maintenance departmeht claims we are destroying the switches ( and using there budget) by the way we switch them. I thought you could close any switch at any time as the stress exerted on the switch was when opening. I found nothing in ANSI and manufacturers seem to give contradictory answers. Before I begin I am going to give a basic primer of switching (as I understand it) so we are talking the same language.

De-energizing an overhead line
Open breaker at one end then de-energize with an Air Break switch (horn gap or arcing horn)

De-energizing a line with cable
Open Loop current switch (vacuum bottle) then de-energize with the breaker.


Re-energizing any line
Close breaker then close switch (air break or loop current). Since both sides of the switch are energized this only picks up loop current. This is not picking up load as I hear it commonly refered to.

Picking up load
By picking up load, I mean closing a switch that will energize a load that is de-energized. This only happens after a fault. The fault drops a substation, we sectionalize the line and close an Air Break switch to energize the sub. We do this so rarely now as we have breakers almost everywhere. My company had a policy not to use Loop current switches to pick up load. The funny thing was one of the rare places we had that a fault would drop a whole sub, we routinely picked up the sub with a loop current switch. It violated the internal policy but we would do it everytime.

Some of the contradictory statements from manufacturers are:

"The interrupter(vacuum bottle) is not in the circuit during closing. The switch can be closed quickly with a load."

"An airbreak switch should not be closed into a load."

One of the problems with the manufacturer statements was the engineering group did not differentiate 'load pick up' from 'loop current pickup'. I like the first statement because if its true for all vacuum bottle switches then we can treat our loop current switches as air break switches when closing. We have decades of experience closing air break switches to make up loops and pick up load. The second statement I dont like because it says we've been doing it wrong for decades.

Any standards, papers or personal experience in this matter would be greatly appreciated.

 

[rempman]

Even though you did not state it specifically I must assume you are referring to Transmission Class switches (55kV and above).

We have what I might call 3-1/2 versions of transmission line switches on our system. Arcing horns (for droping line charging current), Vacuum bottle switches (for breaking loop current), Combination - Joslyn LSI and Turner (to have the capabiltiy of performing both). The 1/2 are KPF high speed breaks that have some limited capacity to drop loop current and charging current.

When using a switch with arcing horns we remove all load including the transformer bank(s) prior to operating the switch. Prior to adding load we charge the line with the switch.

Load is dropped and picked up in the stations. For disturbution subs the feeder breakers are opened first then the switch on the high side of the bank is opened. With the reverse occuring when picking up load.

I hope this helps.

 

[bahrra]

thanks Rempman. You were correct, I was talking about transmission switches. I've since been told by one manufacturer that their switches with arcing horns can pick up load.

 

[subtech]

You didn't state specifically what your maintenance people are complaining of, but I'd be willing to bet that the complaints involve burned switch contacts, stuck or welded "arcing whiskers" and such. I've been in substations for several years now,(construction, operation, and maintenance) I have found a common problem among those who carry out switching operations.This may or may not apply to your particular situation, but let's see. Many times, common air break switches are not operated with the "gusto" that most switch manufacturers recommend. In fact, I have on several occasions witnessed linemen intentionally operating switches slowly so they can "entertain" themselves (and possibly others who may be nearby) with the drawn out arc that occurs as well as the accompanying sound. It only takes a few of these intentional "slow" operations to pit and damage contacts, arcing horns and other parts of the switches. Make sure that your switchmen are operating such air break switches with the necessary force and speed. Also, make sure that arcing horns or "whiskers" are adjusted properly. Now, I'm not recommending that switchmen attempt to "rip the operator from the steel", but to quote from a assembly and maintenance manual from a well known switch manufacturer here in the U.S., "Switches should be operated briskly and with deliberation. This ensures positive engagement of the current carrying parts of the switch during closing, and efficient operation of the arc breaking components during opening". It has been my experience over the years that switches get assembled and installed, then forgotten. Make sure that at the very least your most critical switches get CLA'd(Cleaned, Lubed, and Adjusted) at least once every three years. If a switch is very stiff and hard to operate, even the most muscular of your switching personnel will not be able to operate your switches properly. When adjusted and maintained properly, gang operated switches in the 161KV class should be relatively easy to operate for the average sized lineman. If it takes a bloke that is 6'6" and 275 lbs. to switch, it's time for some serious maintenance. Gang operated switches for 230KV and up of course take more effort to operate just because of their sheer size and the mass that must be moved, but using "cheater pipes" or "extensions" on switch handles(yes, I've seen it done, and more than once) and similar methods should never be necessary. Some of the problems your maintenance dept. may be having could very well be self induced! I'm not pointing any fingers here, I'm just telling you what I've found more than once over the years.
As far as the electrical properties of the switch goes, where I work, we always have the breaker pick up and drop actual loads or line charging current if lines are relatively long. Otherwise, we will break relatively small charging currents and such with the switches. I believe this is pretty much in line with most utilities policies here in the middle of the U.S. Although it is true that arcing horns or "whiskers" will become scarred and pitted over time due to breaking arcs from these small charging currents, most arcing components on switches I've seen in the last 20 to 25 years are replaceable just for that reason. Please keep in mind that I'm referring to standard, air break switches, both single pole disconnect, and gang operated 3 ph. that do not have "load break" capability, 69KV and up. To me, it sounds as though your rules for switching are not that much different from other places I've been.But, remember to CLA, and "operate with attitude!"

 

[stevenal]

Your switches should have ratings for load making and fault making capability, see ANSI/IEEE C37.30. The information should also be on the nameplate according to the same standard. But why pick up load, or loop current or even line charging current with an air switch when you have a breaker that can do the job? I would always, where possible, open breakers first and close them last.