Arrester issues on URD system

[Special ED]

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..

[BigClive]

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.

[Special ED]

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.

[BigClive]

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.

[Special ED]

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..

[Trbl639]

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!!!

[Special ED]

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.

[BigClive]

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. :D

[wtdoor67]

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.

[Special ED]

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.



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.

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."

[Special ED]

NAh Danny no answers yet..

[topgroove]

I can gaurantee if a dummy load was placed on the load side of the utility mains they'ld stop blowing arrestors. A clasic over voltage caused by circulating voltage is developing withen the transformer windings.

[backtrk]

Circulating voltage, or current? Would circulating current cause a rapid increase in voltage to blow an arrester before a fuse? I always understood that a dummy load was needed to mitigate the issues of ferrorenance, and circulating current was a product of an imbalance in impedances. Am I way off base?

Could there be an issue with the system grounds being tied together? This seems to be the place to monitor during switching. Sounds like something is being induced into the system ground to me.

[topgroove]

The root of the problem is that unless you have a gang opperated switch or a switch gear. Its impossible to close all three switches at the same time. Maybe the customer should install a three phase padmount switchgear upstream of their transfer switch. a dummy load in the switch gear would help with the arrestors blowing at the overhead bank.

[backtrk]

If this is the case I absolutely agree.

[Divemaster]

My men and myself have been pondering with this all week long.
Number one: we don't like the idea of no visible contact to ground. Our in house electrician also added: a ground is a ground and a neutral is a neutral, they may be connected together but they are still considered seperate.
Number two: We also discussed the problem with the customer saying that nothing has been changed on their side. Is that a prooven fact? I was taught by one of our old timers "Never trust anyone!".
Number three: When they first tie into you system the generators have to be synced into your system otherwise really big noises could happen. I would venture to say that the genset would also be synced into your system when they re-tie into your system, but for any un-Godly reason they weren't, that could cause some extremely high circulating currents not to mention voltages to occur.
Number four: Does this customer have it's own substation, and does the transformer have a LTC. In my experience, they might have the voltages on the generator set to tie in with a sub at full load. When the sub looses the load the LTC should tap down to where the nominal voltage is set. Now if the voltage on the generator drops down under the load put on it, and it doesn't match the voltage at the transformer, there is a good chance the syncing voltage might be enough to put high circulating current on your powerlines and blow the arrestors. Another question for this question is: what is the proximity of the Genset to your transformer if this is the case? This can make a big difference.
We don't think that changing arrestors is the answer, changing arrestors would just appease a college educated idiot and not actually remedy a problem.
We personally don't think ferro resonance is the problem but we have been wrong before.
Just tossing out our thoughts, Good Luck!

[topgroove]

Intresting point but I doubt this customer is a cogen...there is a transfer switch that isolates utility mains from generator. Its one or the other! there is no way to have both feeds at the same time.

[NMSA122-G]

You said this is an airport.... Could there be some damage from lightning somewhere that has not been found (burned up grounds, etc.).
Our airport has had to shut down runways to repair holes that were made in the concrete after a lightning storm.

Just a dumb thought.

[topgroove]

Also Imagine the capacitive charge all that lighting balast holds:eek: must be incredible. the quick changeover from Generator to Mains must be incredible capacitive charge!

[nsmith@sunflower.net]

Maybe the generator and your system phase angles are out too many degrees? When the power dropsand the generator picks up everything is fine but when it throws the other way it isnt in sync with each other and it is too much for everything. Just another thought but the grounding would be another avenue to pursue.

[Highplains Drifter]

Maybe the generator and your system phase angles are out too many degrees? When the power dropsand the generator picks up everything is fine but when it throws the other way it isnt in sync with each other and it is too much for everything. Just another thought but the grounding would be another avenue to pursue.



The harmonics?

[Hambone]

The only thing I do not understand here is that everything has been fine for 20 yrs and now there are problems? I have read everyone's answers, but it's been awhile since we heard from special ed. Is the problem fixed?

[Troubleman72]

hey special ed are the arresters on the riser that are blown or in padmount ?? if the riser ones are is it new cable or old ?? if old cable the new arrester are blowing because the fault rating is lesser . you might need to go to a arresters that are made for risers the fault rating are higher than the disturbution ones..
thats all i can come up with .

[Troubleman72]

hey special ed . what does the generator feed is it a building with alot of computers in it ?? if that is the case . you have and hormonics problem

[Hambone]

hey special ed . what does the generator feed is it a building with alot of computers in it ?? if that is the case . you have and hormonics problem

Hey 72, I bet there a lot of lineman that don't know what harmonics is about. And also you will see on inside wiring that the neutral is starting to be a bigger conductor than the hot-legs.

[Highplains Drifter]

I never had to ground my bucket truck to the system neutral until i came to work for pike for the first time back in 1991. We did not have to put it on with a shot-gun just with your rubber gloves. Well i hung the truck ground on the yellow hook of my tool board and and went up to cover the neutral and forgot about the ground hanging on the hook and went up to cover primary and started hearing sounds like the primary was getting together but it was not real loud and a was looking around and then looked down in front of my bucket and seen the truck ground laying against the primary but was'nt really raising any hell. So i backed off the primary with the bucket and threw the f-ing cable down to the ground and never took it up with me again, whether pike liked it or not. It caught the grass on fire around the out-riggers and never screwed the truck up and to this day i am so thankful that no-one was touching the truck but everyone had over-shoes on and i still wonder if the over-shoes would have saved some-ones life.




Hey 72, I bet there a lot of lineman that don't know what harmonics is about. And also you will see on inside wiring that the neutral is starting to be a bigger conductor than the hot-legs.


For someone that takes a grounded ground into the zone and then has the gull to tell he had a fire at the out riggers, why don't you explain harmonics to us lineman. :rolleyes:

[Hambone]

For someone that takes a grounded ground into the zone and then has the gull to tell he had a fire at the out riggers, why don't you explain harmonics to us lineman. :rolleyes:

Oh boy, So I have the balls to tell my story about being a human being and making a mistake and go look up harmonics yourself.You might find it has something to do with computers.

[Highplains Drifter]

Oh boy, So I have the balls to tell my story about being a human being and making a mistake and go look up harmonics yourself.You might find it has something to do with computers.

No I undrstand it completely form the transmission side. Work a 500 kv sub sometime and you might learn how it is dealt with. Go ahead and look it up and you might find something to do with line work.

[Hambone]

No I undrstand it completely form the transmission side. Work a 500 kv sub sometime and you might learn how it is dealt with. Go ahead and look it up and you might find something to do with line work.

www.cpccorp.com/harmonic.htm.... Try this site

[MI-Lineman]

I've learned about "harmonics" but I've never (or never realized?) seen'em? Are you guys sayin it's #8 in hams site?

"Resonance that produces over-current surges. In comparison, this is equivalent to continuous audio feedback through a PA system. This results in destroyed capacitors and their fuses and damaged surge suppressors which will cause an electrical system shutdown."