View Full Version : bank qeustion
AEE/linehand
11-30-2010, 08:32 PM
I have a three phase bank, 7200/12kv wye on high side with a floating neutral. low side is 240 delta with center pot producing 120v for lighting. Transformer are conventional with LA mounted on the tank. my qeustion is if you close the center tub cut out in and only the center phase cut out. how do you have measurable Voltage on the Secondary side in the lighting pot? I know the Voltage will not be true but with the high side neutral floating, how is there a return path at all for current to flow with only one cut out closed. Does it return on itself as out of cycle back feed in the center phase. do the LAs provide enough of a path. just trying to start a discussion, no baffeling with b.s. please.
topgroove
11-30-2010, 09:41 PM
Basicly you have a single phase energization of a bank with the high side floated. A tiny amount of current will flow through the lightning arrestors (milliamps). Lines of flux will develope withen the primary coils and induce a voltage on the secondary side. I would have to think your voltage would read half what it should be but the current would be negligble.
kooman
11-30-2010, 10:26 PM
not trying to change the direction of your thread, but what kva are the pots? also what rating are the arrestors? do you ground the floater to fuse in and then remove it? i allways thought it was a bad idea to put the arrestors on the load side of this type of bank unless they were phase to phase voltage rated, maybe i am wrong but that is what i allways thought.
topgroove
11-30-2010, 10:41 PM
Yes you are correct Kooman, you should allways ground the float before you close the fuses and test the secondary voltage. In this case the arrestors are connected to the line side but in this case with a single phase energization of the center pot current will flow through the primary coils and bleed out through the arrestors on the bank.
I have a three phase bank, 7200/12kv wye on high side with a floating neutral. low side is 240 delta with center pot producing 120v for lighting. Transformer are conventional with LA mounted on the tank. my qeustion is if you close the center tub cut out in and only the center phase cut out. how do you have measurable Voltage on the Secondary side in the lighting pot? I know the Voltage will not be true but with the high side neutral floating, how is there a return path at all for current to flow with only one cut out closed. Does it return on itself as out of cycle back feed in the center phase. do the LAs provide enough of a path. just trying to start a discussion, no baffeling with b.s. please.
If for some reason you only wanted to use the 1 transformer you would have to ground your H2 just like in an open delta.
With your LA attached to your transformers & having a floating neutral obviously the bottom lead of the LA is not attached to the transformer (H2) so I do not see how it could provide a return path
AEE/linehand
11-30-2010, 11:15 PM
three 25kvas, the arrestor are 10kv/8.4mcov. when you are building a new bank using brand new tubs, what purpose is there in grounding the floater before closing in the lighting pot? why not just heat it all up and check V, then open and connect service and reenergize. I under stand for the troubleshooting aspect to identify which pot is bad.
AEE/linehand
11-30-2010, 11:23 PM
If for some reason you only wanted to use the 1 transformer you would have to ground your H2 just like in an open delta.
With your LA attached to your transformers & having a floating neutral obviously the bottom lead of the LA is not attached to the transformer (H2) so I do not see how it could provide a return path
On protected tubs the LA is mounted directly on the can attached to the H1. H1 are all relavent through the wye connection and the floating neutral. New LAs always have a miliamp flow as apossed to a spark gap LA. And I understand you gotta ground the floater to have a somewhat truer Voltage. I just wanted some input to verify my own reasoning.
topgroove
11-30-2010, 11:30 PM
If for some reason you only wanted to use the 1 transformer you would have to ground your H2 just like in an open delta.
With your LA attached to your transformers & having a floating neutral obviously the bottom lead of the LA is not attached to the transformer (H2) so I do not see how it could provide a return pathRemember we're talking milliamps here... when you energize H1 on the middle pot milliamps of current will flow through the coil from H1 to H2 and since all the H2"s are connected current will flow from H2 through the coils to the H1 bushings. Its path to ground is through the arrestors. I doubt you could even light a bulb with it. I do think you could get the ellwood to light up though :D
three 25kvas, the arrestor are 10kv/8.4mcov. when you are building a new bank using brand new tubs, what purpose is there in grounding the floater before closing in the lighting pot? why not just heat it all up and check V, then open and connect service and reenergize. I under stand for the troubleshooting aspect to identify which pot is bad.
Sympathetic tripping
kooman
11-30-2010, 11:34 PM
three 25kvas, the arrestor are 10kv/8.4mcov. when you are building a new bank using brand new tubs, what purpose is there in grounding the floater before closing in the lighting pot? why not just heat it all up and check V, then open and connect service and reenergize. I under stand for the troubleshooting aspect to identify which pot is bad.well its going to take someone smarter than me to explain exactly how and why it happens, but i have seen a few different times over the years of blowing fuses while trying to fuse in the bank without a ground on the float. usually its on larger kva banks and higher voltages, but it happens. there was one time we had a 225kva bank that we went through more than one fuse trying to get the bank energized, two guys with two sticks and a box of fuses, (we were all young and dumb.) :D
On protected tubs the LA is mounted directly on the can attached to the H1. H1 are all relavent through the wye connection and the floating neutral. New LAs always have a miliamp flow as apossed to a spark gap LA. And I understand you gotta ground the floater to have a somewhat truer Voltage. I just wanted some input to verify my own reasoning.
how is your la Ground to the pot? to the pole ground? to the Mainline neutral?
if you have redundant grounding that could be the problem.... and the high side neutral may not then be truely floating... but I might be misunderstanding where your going with this....
Edge
topgroove
11-30-2010, 11:46 PM
three 25kvas, the arrestor are 10kv/8.4mcov. when you are building a new bank using brand new tubs, what purpose is there in grounding the floater before closing in the lighting pot? why not just heat it all up and check V, then open and connect service and reenergize. I under stand for the troubleshooting aspect to identify which pot is bad.The reason for grounding the float while testing voltage is because with a bank with a float your nuetral literally floats withen the center of your wye vector. Its the customers load that keeps that vector perfect,(well almost perfect). I've seen a bank of 500 KVA 13.2KV wye to 4.8kv delta expload like a space shuttle launch before. The float was left ungrounded and the load side fuses were open. the crew closed the first line side door and then the second ( they heard a strange sound) when they closed the third door the bank exploded. Looked like a sceene out of hollywood. I was five sections away and it looked like the flames were 65 feet high:D
topgroove
11-30-2010, 11:57 PM
how is your la Ground to the pot? to the pole ground? to the Mainline neutral?
if you have redundant grounding that could be the problem.... and the high side neutral may not then be truely floating... but I might be misunderstanding where your going with this....
Edge He was just kicking around a little theory Edge:) not that you'ld ever close one cutout and start poking around with a tester in real life.:D
AEE/linehand
12-01-2010, 07:35 AM
So if the LAs were mounted on the highside of the cut-out, there would be no Voltage because no primary current would flow through the wye? and yes theory.
topgroove
12-01-2010, 07:59 AM
yes... if the arrestors were on the arm attached to the line side of the cutouts no current could flow.
wtdoor67
12-01-2010, 08:59 AM
When pots started being stocked with the arrester mounted on the pot PSO's instructions were to take the arresters off the pots and arm mount then on a 3 pot Wye/Delta setup. This was to prevent the secondary from backfeeding the other 2 pots and possibly subjecting the arresters to an overvoltage. I think I did read an instruction pertaining to this from a manufactor also. I expect it wouldn't be a good idea but I know it's not always gonna hurt any thing. Probably cause trouble for some troubleman.
I did see a bank built contrary to these instructions and nothing happened when I closed it it. Another crew had built the bank and I suggested to the foreman we move the arresters. He refused because another crew had built it.
kooman
12-01-2010, 02:39 PM
we allyway try and ground the float on wye/delta before energizing, but is this necessary to do before opening the cutouts too or just closing? i guess i havent thought about it before.
wtdoor67
12-01-2010, 04:19 PM
at about the 20 KV level and up.
We've beat this subject around quite a bit before on this board. You start getting into Ferroresonance problems sometimes if you don't ground it when closing or opening. This mostly applies to no load situations such as a newly built bank.
Some co's even mount a grounding cutout especially for this problem.
topgroove
12-01-2010, 05:18 PM
that's exactly what we do here. we have the most problems with 13.2 kv wye to 4.8 kv delta ratio banks. most of the older installations don't have a cutout though.
Remember we're talking milliamps here... when you energize H1 on the middle pot milliamps of current will flow through the coil from H1 to H2 and since all the H2"s are connected current will flow from H2 through the coils to the H1 bushings. Its path to ground is through the arrestors. I doubt you could even light a bulb with it. I do think you could get the ellwood to light up though :D
"yes... if the arrestors were on the arm attached to the line side of the cutouts no current could flow."
Again if you are floating the neutral the grounded end of the LA cannot be connected to the H2 whether you connect you LA to the pot or above on the switch.
The only experience I have with Deltas is on 8 kv primary & we never used a 4th cutout
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