Accidental open delta= damaged electronics?

[TexasMaster]

No better way to title this. A call I'm sent in to follow up on goes as follows.
The original call: I.t. Man at an hvac wholesale warehouse (with much office space btw) has low voltage alarm going off on a server (108v or so). We have someone out to find the voltage low on one phase coming from poco's center tapped 240 delta. Poco responds promptly while my guy is still there and the i.t. Man is to witness what happens next. While troubleshooting he pulls a single fuse on a bank of 3 oh xfmrs for about 10mins while he did what?... We aren't sure. He puts the fuse back in, reads 120, 120, 208 to ground. Poco believes underground is bad. At this time the place is closing down and no one notices anymore problem than they already had.
Next morning they are calling us- it has hit the fan- breakers are tripped on a couple of a/c's and they have an in house tech who is finding control board after board burned up (visually the varistors are burned on.. I think 8 Mitsubishi units!) that is the only damage out of the whole building (as if that's not enough... they are claiming $20k worth of a/c control boards buy the end of the day) but nothing else, no compressor damage in these units, no damage to the server that was on line and nobody else had a computer zapped. Now I arrive for a second look with the poco and look at the service point, a doughnut box they had to open up, and we find they have brought in 4/0 al for a 400 amp svc 225' from the transformers!!!! So at least the voltage drop is solved! Mind you these have been our first really hot days in awhile and the building has had some office additions in recent years, but how does this just show up now?
The real question is why the damage to these boards? I don't think the single fuse pull on the poco was a good idea, but shouldn't the system have worked like an open delta for that 10min or so? I believe the theory is the voltages would remain the same but the capacity would reduce. And if a surge or a spike occurred isn't the varistor just the part to stand up to that? Can anybody wrap a head around this? Thanks

 

[mbrooke]

Are the primaries connected in delta or floating wye? What voltage? Sounds like Ferroresonance might have taken place. POCO should know better then to leave the bank running like that.


In a closed delta primary bank pulling one fuse does not equal an open delta if one fuse protects 2 cans. Rather you end up with 50%-100%-50% voltages. An open delta is when power is disconnected to only 1 can rather than 2. A grounded wye primary will function as an open delta when one fuse is pulled but not a floating wye or delta bank only protected with 3 cutouts (1 for every corner).

 

[TexasMaster]

Are the primaries connected in delta or floating wye? What voltage? Sounds like Ferroresonance might have taken place. POCO should know better then to leave the bank running like that.


In a closed delta primary bank pulling one fuse does not equal an open delta if one fuse protects 2 cans. Rather you end up with 50%-100%-50% voltages. An open delta is when power is disconnected to only 1 can rather than 2. A grounded wye primary will function as an open delta when one fuse is pulled but not a floating wye or delta bank only protected with 3 cutouts (1 for every corner).

I payed little attention to the poco's primary, but will look into it. I believed it was a cutout for each of three xfmrs, 50kva for 120/240 and 2- 25kva. He pulled cutout over a 25. Ferroresonance is very interesting reading btw thank you. Lots of mov damage seen in these cases I've read about.

 

[mbrooke]

I payed little attention to the poco's primary, but will look into it. I believed it was a cutout for each of three xfmrs, 50kva for 120/240 and 2- 25kva. He pulled cutout over a 25. Ferroresonance is very interesting reading btw thank you. Lots of mov damage seen in these cases I've read about.

Welcome If its a closed 3 phase bank very good chance the primary is floating wye or delta, 2 connections prone to ferroresonance. In any case opening 1 cutout only was a bad idea. Under load there is less risk, but as load drops off such as when a business is closing the risk goes way up.


POCO did a poor job handling this is they left a cutout open for so long. The rule is always, always pull all cutouts. Never leave one open. Even where ferroresonce is not a risk such as a wye grounded-wye grounded transformer, if that unit is 3 phase inductive heating can take place in the tank with an open fuse.

 

[TexasMaster]

I've swung back by. Looks to be the Floating wye primary. No clue their primary voltage though. 240 center tap delta secondary

 

[ritelec]

Not highjacking. Floating wye primary? Never heard of it so a google search shows it to be that the neutral is not grounded (I think).

brought this post up also

http://forums.mikeholt.com/showthread.php?t=134233 (importance of floating neutral on primary wye)

But never explained why it was important.


Interesting story. Ever find out the cause?
Who will be responsible for $20K worth of circuit boards?

 

[mbrooke]

Not highjacking. Floating wye primary? Never heard of it so a google search shows it to be that the neutral is not grounded (I think).

brought this post up also

http://forums.mikeholt.com/showthread.php?t=134233 (importance of floating neutral on primary wye)

But never explained why it was important.


Interesting story. Ever find out the cause?
Who will be responsible for $20K worth of circuit boards?

There was a huge thread on this a while back. Anyway boiling it down what I wrote. You can have an ungrounded wye primary for a closed delta secondary. There are pros and cons to both:


With a grounded wye primary to pros are; 1. the ability to run the bank open wye delta when one fuse is pulled. The bank will seamlessly transition to it when the fuse is pulled or phase opens (technically you wouldn't even be able to tell inside the building) and the bank will only have 58% of its available capacity. 2. Ferroresonance risk is eliminated.


However 1 major con exists: You now have a major artificial neutral point out on the line. The bank will try and balance the line with current circulating in the secondary. The big issue in this comes with faults. Faults out on the lines (such as a phase to neutral fault) can blow the bank fuses. A 10T or 15K fuse link will often blow before the lateral fuse or recloser opens. The end result is nuisance fuse blowing from faults, even temporary ones that clear with reclosing.

As a result the solution is to leave the wye floating. The nuisance blowing issue is eliminated, but now ferroresonance may be an issue. To combat the issue it is not uncommon for poco crews to tie the transformer neutral to the line neutral during opening and closing of cutouts and then removing that jumper when done. Some will actually go with a fuse cutout using a solid blade. The cutout is closed when switching open or closed any primary fuses then opening the blade when done. The cutout is placed in between the transformer's bank neutral and the multi grounded neutral strung to the pole. In your case your poco should have jumped the primary to the pole neutral if it was present.


Of note, if the line does not have a neutral to tie into or the primary is delta, ferroresonance can be prevented by putting a load on the secondary.

 

[TexasMaster]

Not highjacking. Floating wye primary? Never heard of it so a google search shows it to be that the neutral is not grounded (I think).

brought this post up also

http://forums.mikeholt.com/showthread.php?t=134233 (importance of floating neutral on primary wye)

But never explained why it was important.


Interesting story. Ever find out the cause?
Who will be responsible for $20K worth of circuit boards?

If I can truly understand the instability they experienced during the time that cutout was down... I'll be pointing at the poco. They have given it to me to investigate, so we can possibly prevent loss again

 

[mbrooke]

If I can truly understand the instability they experienced during the time that cutout was down... I'll be pointing at the poco. They have given it to me to investigate, so we can possibly prevent loss again

If that pole has an MGN automatically they should be held liable. I would never allow for such.

 

[TexasMaster]

So I think we've arrive at: 1 cut out was pulled on a floating wye primary (I doubt he grounded during this time), the building was ramping loads down, as they closed.

Can any one tell me what hit these mov's in the circuit boards that let the smoke out of them?