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Is the changeover switch breaking the generator cleanly before your side kicks in? It's an odd scenario. Some kind of ferroresonance style transient could still be a factor.
That's a tricky one.
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Arrester issues on URD system
Got a 7200/12470 Grd Y system fed from the local utility here for a private customer through some CTs and a transfer switch for the customers generators.
The promblem comes when they do a monthly test of the transfer switch and run the generators. They cut over to the generator power and off the utility feed during the test and all is well. But when they switch back to utility it blows arresters all over the place.
Had a big pow wow with engineers and inspected the entire system for abnormalities and all I could find was utility feed was a 3 phase 4 wire system. The generator feed was a 3 wire system with no neutral hook up on the generators at all. This system is in an airport and Im told the all grounds and neutrals are made up to with ground grid for the strips which Im very sure are adiquate.
This system has been in place and worked fine for 20+ years and now all this is happening. And cant seem to find the source of the problem. I nor my company has the proper meters to monitor the system during the tests to see any voltage spikes or anything. Ferroresonance has been discussed and dismissed. As well as the harmonics of the system.
The arresters blowing are 10kv some distribution some riser. Some porcelin, some poly. Theres more hands in this system then just mine so some work done is iffy. Engineers are saying we need to go through and install 15kv MOV arresters and discard all the 10kv stuff. And allow me to clarify that the arresters dont blow like you would normally see them. the bottoms stay intact and they blow out the sides and usually buck phases in the process. (Was standing next to a 3 phase pad mount during a test and it bucked all 3 phases and bulged out the cabinet something fierce! lol)
Anyone have any ideas or suggestions? This is a crucial system that has to maintain its reliability at all times but the engineers and myself are stumpped..
Is the changeover switch breaking the generator cleanly before your side kicks in? It's an odd scenario. Some kind of ferroresonance style transient could still be a factor.
That's a tricky one.
Portable defibrillators were first invented to save the lives of linemen. Where's yours?
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Yeah the change over or transfer switch operates great. Im the one who brought up ferroresonance in the round table discussion and when I was asked to explain it the engineers quickly put a stop to my explanation to state what they feel it is causing the issue. Thats bout the time I said I must not be needed here since they asked for my input then discarded it so promptly.
Like I said this system has been in place for four round 20 years and had no problems till now. Im just curious if the ground grid was compromised or broke in an area and that is what is causing the ferroresonance that I suspect.
The claim that we need to change out all the arresters to 15kv MOV arresters seems unvalid to me. The MOV arresters blow when the SiC arresters that dont tollerate over voltages as well stay in tact. And 98% of these 3 phase padmounts are live fronts as well.
Im waiting for the engineers to finish bumping heads and tell me what they want me to do next. I feel they need to hook up testing equipment and monitor the system during the test cycle for over voltages as well as checking the system ground.
But unless Im missing something any help would be appreciated.. I got my company and the customer coming to me wanting answers I simply can not give and then they goto an engineer and they (more than one from more than one company) take em in diffrent directions so the customer dont know what to think and they are merely along for the ride while we try to figure this out.
Technically speaking it's the engineers job to sort this out, so it might be worth just playing dumb and let them take all the responsibility.
They should be looking at an electrical plan and doing some measurements then saying, aha, that's what it is.
In the meantime if they want to change out the arrestors to 15kV ones then just humour them and think of your wages.
Portable defibrillators were first invented to save the lives of linemen. Where's yours?
www.bigclive.com
The customer wants answers and they are wanting them from multiple sources such as me, the engineers, as well as the equipment manufacturers such as cooper systems, catapillar (generators), ect.
This is a major contract for my company and the owners have put me on the spot. Thats why I've had to take part in the discussions and voice my opinion. As a lineman of course Im thinkin of my wages and I'll change all 90 arresters 3 times if thats what they want.. But this is an unusual situation and the cause is still yet to be determined and I'm constantly asked what I think or "Whats wrong?" I speak of what I know and tell them to seek out the information from the engineers but at the same time I dont want them looking at me like "To hell with this dumbass... Lets get a lineman in here that knows whats going on.." Not only would we lose a customer but it would be to a non-union outfit...
I just want to make sure Im doing or done all I can and if theres something I may have missed or if anyone has had a similar situation and what was the outcome..
I agree that monitoring should be done when this takes place, but personally I'd check the ground Grid...sounds like it might be compromised, IMHO
As far as the arrestors, I don't think that is the solution, but humor em, and make some extra jingle!!!
Old Lineman Never Die......We Just Don't Raise Our Booms As Often
Testing the ground grid was brought up but its an international airport and they are reluctant to shut down the ramp and one strip to conduct the ground tests plus its all in concrete. It can be done on concrete but they would rather do it on dirt. Good luck with the dirt. lol
The 3 wire system on the generators is what peaked my curiousity. With no neutral lug to be had and them relying on the ground grid I told them thats the first place I would investigate after checking the chang over switch and monitoring the system with power quality metering on the primary side.
That struck me as odd too.
I wonder if there's an issue with the ground and one of the phases has found a better reference to earth meaning that the others are at the full phase ot phase voltage with reference to your own earth. That could cause an instance where any residual capacitive charge on the wires when it switched back to your supply could cause a transient that could just tip the arrestors over the edge.
Not sure. Would need to actually see a drawing and test results.
Maybe you can do an earth test next time a volcano ash cloud floats by.
Portable defibrillators were first invented to save the lives of linemen. Where's yours?
www.bigclive.com
I would suspect some form of ferroresonance or neutral problem.
What changes have been made in the customers' side since this problem started?
Is it too costly or impractical to install a complete neutral on the customers' part?
As Topgroove said, single phase switching sometimes contributes to a problem such as this.
Have arresters been changed out over the years perhaps getting a bad batch from the manufactor?
Changed arrester brands?
Just because a guy has an engineering degree doesn't mean much. Look for one within the power co. or wherever, who has a substantial amount of experience dealing with distribution problems. There's some out there.
We had a 4 KV system once that exited the sub. with a buried neutral for a few hundred feet and then it went overhead. A nearby fence would "bite" once in awhile and the grass even caught on fire. An overhead neutral was installed bypassing the buried neutral and solved the problem. Why the neutral was buried while the phases were overhead I don't know. Could have been an old original 2400 delta system or something.
Just thought I'd give you a few ideas maybe.
The customer claims nothing has changed at all. Yet theres so many hands in this theres no tellin for sure. I just know that the things I maintain such as the transformers, wire, and switches are all the same.Originally Posted by wtdoor67
There is a neutral in the conduit tied to the utility feed. The emergency back up generator does not have a neutral connection on it they just have it tied to the ground grid of the airport ( so we are told.. Theres no way to confirm this since its all in concrete..)
Single phase switching shouldn't be an issue. All switches are gang operated.
We have used cooper arresters. Had some that were installed incorrectly by god only knows who.. They had the brackets for the porcelin arresters so the took them and put them on poly arresters about half way up on the skirts. No one would fess up to that one..
The one engineer throwing out the suggestions of arresters is an engineer for the power company. I've worked on the power company property over 10 years and know that what hes suggesting is no their standards that he claims they are. They use 9kv MOV arresters on all 7200/12470 Y URD. With riser arresters at the take off..
The other engineer well Im under the impression hes just along for the ride.. lol Dont know where hes from.. But when they answer questions they throw out them big words simple men like me have trouble with and the customers dont even try to follow. They just say "Your the professionals you know what your doing take care of it."
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