can it be done?

[kooman]

guys i got a question if this can be done or not, its a long story as to why i ask. I have a 10 mva sub trans, high side is 69kv and the low side is 7200/12470. My questions are, approx how much load would you think there would be if a person heated up the lowside of the trans while the highside was open? and also could the trans safely be energized from the low side with knife switches one phase at a time or would there be a chance of ferroresonance and bad things happening? thanks, maybe i will take the time later to explain the details.

[reppy007]

guys i got a question if this can be done or not, its a long story as to why i ask. I have a 10 mva sub trans, high side is 69kv and the low side is 7200/12470. My questions are, approx how much load would you think there would be if a person heated up the lowside of the trans while the highside was open? and also could the trans safely be energized from the low side with knife switches one phase at a time or would there be a chance of ferroresonance and bad things happening? thanks, maybe i will take the time later to explain the details.

New one on me,are you trying to backfeed a power plant?

[Pootnaigle]

Ummmmmmm Ferroresonance aint usually a prollem on anything less than 34.5 .The initial charging current will for sure cause some spit on single switches . but I would think there needs to be some kinda fuse protection in that primary ahead of the Transformer so just a dead blade wouldnt be suitable.

[dooghi]

Like Poot says you will get some talk when you close the switches. If the high side is open then there should not be any load except the charging current, which should be minamal. I would want some kind ofprtection on the 7200/12470 side incase something did happen. I guess the real question is why would you need to heat up a power transformer backwards and not need to have load on it. If testing there are lots of ways to do that.

Doug
If you can build it you can fix it.

[topgroove]

guys i got a question if this can be done or not, its a long story as to why i ask. I have a 10 mva sub trans, high side is 69kv and the low side is 7200/12470. My questions are, approx how much load would you think there would be if a person heated up the lowside of the trans while the highside was open? and also could the trans safely be energized from the low side with knife switches one phase at a time or would there be a chance of ferroresonance and bad things happening? thanks, maybe i will take the time later to explain the details. Is the High side wired wye? If it is you can get away with single phase switching but wouldn't it be safer and easier to simply close the breaker?


If the high side is wired delta no way in hell would I close one switch at a time. The circulating voltage would most likely blow the arrestors to rat$hit and thats if your lucky.

Is this a portable sub or something? As far as the Amperage goes maybe you can get a picture of the nameplate... Your basicly picking up the Primary and secondary coil of that 10 mva transformer,,, could get hairy.... breaking load would be downright scary with knife switches even with a loadbuster.


Why even take a chance with something that costs a million bucks? Why not use a breaker or recloser that makes and breaks all three phases with the push of a button or turn of a handle!


I'm gonna err on the side of caution and say I think its a really bad idea.

[kooman]

ok guys i will try and explain whats going on. I have sub A and sub B, sub A had a squirrel get into the breaker that connects the sub trans to the 12470 buss, blew up the breaker and knocked out the sub. we were able to tie sub A and sub B together with a tie circuit to get all the load covered. Right now the weather is nice and the load is fine on the tie but it is going to get colder and we might be maxing out the tie circuit . the new breaker wont be installed for a couple of weeks, i by passed the blown up breaker with jumpers from the knife switches to the sub trans just in case we have to have the trans for some reason. now if we have to put the trans back into service we have 3 choices that i see, number 1 would be to heat up the highside with the air break, match taps betwene 2 subs and close knife switches 1 at a time then seperate. number two would be energize trans backwards with knife switches one at a time on the low side, with the high side open, match taps and then close air break on high side then seperate. or number 3 take a complete outage (probably wont go over well). there is the problem so whats your input? Thanks

[dooghi]

Are your subs in the same sub yard, or are they at seperate locations? If the tie line can hold load now, can you isolate the 12470 buss and enegize it with no load? If you can, do you then have ocrs that isolate circuts leaving that bay? If so you could energize you trans A close the three switches were the breaker was then pick up the load with the circut ocrs and break tie as you go. Does that make sense?

[kooman]

Are your subs in the same sub yard, or are they at seperate locations? If the tie line can hold load now, can you isolate the 12470 buss and enegize it with no load? If you can, do you then have ocrs that isolate circuts leaving that bay? If so you could energize you trans A close the three switches were the breaker was then pick up the load with the circut ocrs and break tie as you go. Does that make sense?

the 2 subs are at seperate locations, the tie line is feeding all of the buss at sub A, from sub B. I have a tie line breaker and then have several breakers leaving the buss for each circuit at buss A. My opinion on the best way to do this would be to energize trans A with air break, match taps, and then close knife switches with 2 sticks and 2 guys as quickly as we can but i want some opinions as this is not a normal everyday situation.

[joe b]

Just my 2 cents which don't mean ****

I take it that these feeds are heavily loaded, but instead of taking a big chance of burning down a square block since the one trans is carrying double of its norm have yall discussed what the Hek plan c will be when just say there is a problem between switching and you lose everything and burn up something else!!!!!!

Like I said just my 2 cents

I may be missing something here and if so please explain

I see your problem and it does suck to have an outage but from the following To me that would be throwing fuel to the fire if something went wrong.... Then you would have an accidental
Outage which would be one of the options in the first place

Maybe that helps

Work safe!!!!!!

[topgroove]

ok guys i will try and explain whats going on. I have sub A and sub B, sub A had a squirrel get into the breaker that connects the sub trans to the 12470 buss, blew up the breaker and knocked out the sub. we were able to tie sub A and sub B together with a tie circuit to get all the load covered. Right now the weather is nice and the load is fine on the tie but it is going to get colder and we might be maxing out the tie circuit . the new breaker wont be installed for a couple of weeks, i by passed the blown up breaker with jumpers from the knife switches to the sub trans just in case we have to have the trans for some reason. now if we have to put the trans back into service we have 3 choices that i see, number 1 would be to heat up the highside with the air break, match taps betwene 2 subs and close knife switches 1 at a time then seperate. number two would be energize trans backwards with knife switches one at a time on the low side, with the high side open, match taps and then close air break on high side then seperate. or number 3 take a complete outage (probably wont go over well). there is the problem so whats your input? Thanks

well with option 1 heating up the high side with the air break and matching taps. you still have to check phase across the switches... Just ran into this exact situation last month. the high side on the two station transformers was 34.5 low side was 7.6/13.2kv.

We had the two feeders tied in the field at a tie switch.

when I phased across the switches at the get-away I was geting way over 1000 volts across the open switches. The impediance on the two transformers were off a bit.
We ended up doing a drop and pick.

In your case if you choose option one make dam sure you phase across the switches. You want to see less than ten percent of your working voltage across the open switches.In your case 720 volts.

Option two, I just don't like at all... way too much stuff could go terribly wrong.


Option three is by far the safest way to go .

Forgot to mention... what happens if you get a MVA or a tree gets some phases together on your sub without a breaker?

If the station transformer goes super nova your gonna be in a bad way.

[joe b]

I by far go with topgroove. Option 3 because of to many things can go wrong!!!

QUOTE=topgroove;120179]well with option 1 heating up the high side with the air break and matching taps. you still have to check phase across the switches... Just ran into this exact situation last month. the high side on the two station transformers was 34.5 low side was 7.6/13.2kv.

We had the two feeders tied in the field at a tie switch.

when I phased across the switches at the get-away I was geting way over 1000 volts across the open switches. The impediance on the two transformers were off a bit.
We ended up doing a drop and pick.

In your case if you choose option one make dam sure you phase across the switches. You want to see less than ten percent of your working voltage across the open switches.In your case 720 volts.

Option two, I just don't like at all... way too much stuff could go terribly wrong.


Option three is by far the safest way to go .

Forgot to mention... what happens if you get a MVA or a tree gets some phases together on your sub without a breaker?

If the station transformer goes super nova your gonna be in a bad way.[/QUOTE]

[dooghi]

Can you be in a harder spot or maybe a bigger rock? No good choices. Hope you do not have to do any option. It is feasible to energize then close two switches at a time still have single phasing problems, and you might want a change of shorts. Safest option dump load enegize buss pick load back up.Some wont care some wont notice then there are those that are relly goining to be ticked. A losing situtaion all around. Best advice do it safe and everyone be on the same page and everyone go home in one piece. Good luck

[kooman]

well with option 1 heating up the high side with the air break and matching taps. you still have to check phase across the switches... Just ran into this exact situation last month. the high side on the two station transformers was 34.5 low side was 7.6/13.2kv.

We had the two feeders tied in the field at a tie switch.

when I phased across the switches at the get-away I was geting way over 1000 volts across the open switches. The impediance on the two transformers were off a bit.
We ended up doing a drop and pick.

In your case if you choose option one make dam sure you phase across the switches. You want to see less than ten percent of your working voltage across the open switches.In your case 720 volts.

Option two, I just don't like at all... way too much stuff could go terribly wrong.


Option three is by far the safest way to go .

Forgot to mention... what happens if you get a MVA or a tree gets some phases together on your sub without a breaker?

If the station transformer goes super nova your gonna be in a bad way.

the tie has a breaker at both ends and the sub trans has fuses on the high side. I hope we dont have to do anything with this but i wanted to be prepared in case we do. thanks for all the help

[kooman]

I forgot to mention that we have had the 2 subs tied together thousands of times but it has allways been with both transformers sharing load and allways with a tie breaker never with single knifes.

[topgroove]

I forgot to mention that we have had the 2 subs tied together thousands of times but it has allways been with both transformers sharing load and allways with a tie breaker never with single knifes.

Kinda curious Kooman,, how is the high side wired? inside, inside inside inside outside outside? (Delta)


Or one bushing on each phase hot and the others to ground? (wye)

If the high side is delta, single phase switching is out of the question.

[T-Man]

Could you get a switch fuse unit for say 24.9 and wire the three single blades with URD cable and terminate in the SFU and do the same on the station buss then close the three blades on each side and close the SFU gang style? You could prop the SFU up on ties temp to get the cable into it. Just another hair brain Troubleshooter talking.

[kooman]

Kinda curious Kooman,, how is the high side wired? inside, inside inside inside outside outside? (Delta)


Or one bushing on each phase hot and the others to ground? (wye)

If the high side is delta, single phase switching is out of the question.

to be honest i am not 100% sure if it is delta or wye on the high side, i allways thought transmission voltage was delta, so i assumed it was delta, but i will have to check into it. the bushings on the high side of the sub have just the 3 phases, and the 3 low side ones as well, and the static wire bonds into the ground grid.

[topgroove]

Ok that makes sence.. with only three bushings on the high and low side, its definatly wired wye-wye.

[kooman]

Ok that makes sence.. with only three bushings on the high and low side, its definatly wired wye-wye.

so your saying the transmission is wye voltage then? i guess i really never gave it much thought, i just kinda assumed transmission voltages were delta. thanks

[topgroove]

so your saying the transmission is wye voltage then? i guess i really never gave it much thought, i just kinda assumed transmission voltages were delta. thanks

Probebly the most confusing subjects ever...... Transmission Voltage can be un-grounded wye or delta.... Its all how the Generator or substation transformer is wired. On the generation end of things a Generator can be wired star or what we call wye, or it can be wired delta. All depends on the speed of the turbine. for a slower spinning turbine a delta is way more effective. at higher speeds a wye is the way to go.


it gets kinda complicated and hard to explain. most generators produce 13.2kv or around that and get stepped up to 115kv. from there it can be stepped up or down .


At this point it really dosen't matter much it its un-grounded wye or delta. Only way to tell is to read the nameplate at the substation transformer.


Inside the station it gets easy... just look how the station transformer is wired,

[Orgnizdlbr]

Option 3..... so someone gets a chaffed ass cuzz you have an outage, they'll get over it.

If option 3 is totally out of the question, Close the high side breakerm match voltage, phase it in and tie.....

[Highplains Drifter]

I agree with Orgnizedbr, Option three if it takes your company this long to get a breaker, I bet it would seem forever to get a replacement tx.

[kooman]

I wanted to thank everyone for the input on this subject, its one of those situations that just doesnt come up everyday. I looked today at the sub trans and it is delta on the high side and of course wye on the low side, also checked both imps. and they match allmost perfectly. Like i said we have tied the two together hundreds of times over the years but allways with a switch and allways with both sharing the load, never one phase at a time with one trans having all the load. Hopefully the load will be fine on the tie circuit until we get the new breaker installed. thanks again

[Pootnaigle]

After a full explanation I bleve using an OR temporarily to tie all that stuff together would be the easiest way. It would also help when ya had to seperate it. Just make sure its big enough

[lewy]

With both transformers being fed from 69 kv is better than if the high side is different. Did you just loose the breaker on 1 feeder coming out of the station? Do your stations have automatic tap changers? If so not hard to match voltages. I am assuming the 2 stations have been tied together in the past. We parallel stations all of the time, unless I am missing something the only way to handle the extra load is by feeding the second transformer from the high side, just leave that one breaker racked out until changed. As far as tieing them together as long as they are in phase & the voltages are matched should not be a problem. We never had problems closing the only time we noticed a small arc was when we were breaking between stations fed from different voltages 44kv vs 115 kv the 115 station would try & hog all of the load.

[rob8210]

Well since these stations have been tied together in the past they certainly will tie together again. How many feeders are there out of the station with the defective breaker? Is it possible to install temporary reclosers just outside the station? Or even 3 phase load interrupter switches? Are there any power fuses on the low side of the station? I worked at one utility years ago that had a substation with no breakers just power fuses . It was fed 44kv and stepped down to 4kv. As long as there is some kind of fuse protection on the low side a fault on the low side feeders will not likely damage the transformer.

[kooman]

Well since these stations have been tied together in the past they certainly will tie together again. How many feeders are there out of the station with the defective breaker? Is it possible to install temporary reclosers just outside the station? Or even 3 phase load interrupter switches? Are there any power fuses on the low side of the station? I worked at one utility years ago that had a substation with no breakers just power fuses . It was fed 44kv and stepped down to 4kv. As long as there is some kind of fuse protection on the low side a fault on the low side feeders will not likely damage the transformer.

i know they will tie together i just didnt know how much bark we might get tieing together one phase at a time with knife switches. the number of feeders out of that sub is 6.the only fuse protection betweene the switches and the trans are the high side fuses. its looking like we will get lucky and be ok until the breaker gets installed

[topgroove]

i know they will tie together i just didnt know how much bark we might get tieing together one phase at a time with knife switches.

http://stagingstatic.dstar.org/uploads/sdni.jpg
Thats the problem in a nutshell... closing the blades one at a time.
Phase to ground voltage on a bank connected in wye-delta. One phase was previously closed, the second primary phase was closed at 21 ms on the plot shown above
Wye-delta bank overvoltages are the result of one of the following phenomena:

Neutral Shift - Neutral shift is primarily a 60 Hz phenomenon which occurs when there is unequal load on the secondary phases and only one primary phase is disconnected from the source. The open-phase condition and the unbalanced secondary load, combined with the transformer’s delta secondary connection, cause a large shift of the banks neutral-to-ground potential. A large voltage, as high as 2.65 times the normal phase-ground voltage, can appear between the bank’s open phase and ground (feeder neutral) if the load imbalance is sufficiently severe.
Ferroresonance - Ferroresonance is a complex nonlinear interaction between the saturation characteristics of the transformers and a capacitance. Most often, the capacitance involved is the internal winding-ground capacitances of the transformers. For ferroresonance to occur, there must be little or no load on the transformer bank. This is in contrast to the neutral shift overvoltage for which a significant load must be present.
Saturation-Driven Neutral Instability - Saturation-driven neutral instability is characterized by bursts of high frequency voltage oscillations which occur when the second phase of a bank is closed during energization. It is caused by abrupt saturation of the transformers during inrush, in turn causing the neutral voltage to oscillate. These neutral voltage oscillations are magnified on the phase which has not yet been energized. Extreme over-voltages, possibly exceeding the transformer BIL, can occur.

[lewy]

There have been some technical stuff brought that I have never had to consider, but our system operators or station electricians might have to deal with them. I know that we make parallel between stations all of the time & we try to have as many gang operated switches as possible, but that is not always the case. For us the bigger issue has always been breaking parallel as compared to making.

[topgroove]

Alot of Lineman don't have to deal with things like this Lewy. Whenever you deal with delta wye banks you really have to expect the un-expected. Single phase switching is just one of them. I bet there's tons of guys here that never worked delta in there whole career.

[lewy]

Alot of Lineman don't have to deal with things like this Lewy. Whenever you deal with delta wye banks you really have to expect the un-expected. Single phase switching is just one of them. I bet there's tons of guys here that never worked delta in there whole career.

I would be one of them. A lot of people think our 44kv is delta because there is no neutral, but it really is wye.

[Bighorn Ape]

Top groove, are you really a lineman? I've read your posts for three years. You gotta be the smartest lineman in buffalo. I want to be that guy someday. Share your secrets with a stupid apprentice.

[topgroove]

Top groove, are you really a lineman? I've read your posts for three years. You gotta be the smartest lineman in buffalo. I want to be that guy someday. Share your secrets with a stupid apprentice.
Yes I'm really a Lineman, allthough I've been working in the trouble department the last few years.

Trust me.... there aint nothen special bout me... There's a ton of lineman here that I'm not worthy to carry there hooks:o. I know just enough to keep me out of trouble.

[rob8210]

I have done parallels on 7200/12000 switching one phase at a time, it generally doesn't bark that much when making parallel and its a great practice to loadbust the opens when breaking parallel. Although I have done it without a loadbuster ( all I heard was whats that, when I asked). The most memorable was just before the big blackout in 2003. The last phase opened with a small arc kind of greenish in color. Of course the loading was generally under 200 amps. Oh yeah and years ago we would break parallels on 27.6 kv if the indifference was less than 100 amps

[Viperexaf]

bighorn, just ask alot of questions on subjects your not sure or don't know about, soak everything up like a sponge and always remember what you've learned, alot of experienced lineman come to this site and most have been here since it opened in 97, alot of good info and good answers to questions that either you will run across in the field or may not have seen just yet, so don't look at yourself as a stupid apprentice, cause your only gonna be as good as you want to be and if you seek the knowledge to make yourself a better lineman. my journeyman lineman always told me yrs ago that knowledge is power. so get all you can get and never stop learning

[HIVOLTS]

Could you do it? Yes, but why take the chance. You have no protection on the lowside of the transformer because you jumpered out the failed breaker. I assume the high side of the transformer is protected by fuses. If you place this transformer in service and have a problem down line you risk burning everything down before the fuses blow and failing the transformer as well. Is there no where to install an ocr or some other recloser on a pole outside the sub temporarily?