Hello,
I'm new here.
I joined because I was searching for an answer to my question and found some threads on here that sounded like they answered it, but they didn't.
But at the very least it sounded like there might be someone on here that has experience with this issue, and can help.
So that is what I'm hoping.
My name is Wayne. I work for a high voltage engineering/construction/commissioning/maintenance contractor in the Houston, TX area.
I have been in this business for 26 years, and now supervise a High Voltage maintenance crew at a petro-chem facility.
This place has nine 138KV substations, and tons of distribution.
There is only one of the substations that has this issue (that I've seen anyway).
Lets call it the "Main Sub" just because I don't want to share company info that I might not have permission to share.
This is a typical 4-GCB, 2-Transformer ring bus.
The secondary voltage is 12.47KV (I will refer to this as 15KV to save time).
There are two 15KV buses with a tie breaker in between.
On each bus there is a small cogen that feeds into the bus, and a power factor correction cap bank, and multiple pairs of distribution feeders.
Each pair of distribution feeder have a feeder on Bus-A and a parallel feeder on Bus-B.
5 of these AB feeder sets go to in plant 15KV MTM switchgear setups, which also have a tie.
So here is the issue.
When you close the tie at the Main Sub, the load on TRA and TRB balances out as you would expect and there is zero neutral current and zero ground current.
But if you then pick a pair of feeders and go downstream to that gear and close that tie, now you have a significant amount of neutral current.
It varies with the different sets of feeders, one is 60A, two others are around 40A.
And I have tried this multiple times, and every time the neutral current is exactly the same, as in it doesn't fluctuate.
It's different depending on which set of feeders you are on, but if Feeder pair #1 is 60A then that pair is always 60A, etc.
You see this neutral current on the downstream Main breakers, the Main Sub feeder breakers, and the Main Sub Main Breakers.
We have had to disable neutral current trips temporarily every time we do this, because the amplitude is near pickup level on the 50N in some cases.
Ok here is where it gets weirder (to me anyway), if you are in this initial configuration (with upstream and downstream ties closed and neutral current present), and you then open a downstream main (either one) the neutral current goes away (in all places where it was shown before).
If you are in this initial configuration and you open the Main sub Tie breaker, it goes away.
If you are in this initial configuration and you open one of the Main Sub Main breakers, it goes away.
It is only there when the Mains and tie at the Main sub are closed, and also the mains and tie downstream are closed.
Now someone (a plant tech) closed the Main Sub tie, and a 480V tie 2 levels downstream (skipping the 15KV SWGR in the middle) and there was no neutral current.
But maybe the 480V transformers had something to do with that.
I'd like to try this on another set of lower voltage downstream ties to compare at some point.
Oh I also forgot one thing, there is a 5000hp motor fed off of the Main Sub Bus-B (across the line, no VFD), but no similar motor on Bus-A
Also when this neutral current was present we checked every feeder at the Main Sub and only the ones mentioned above showed the neutral current.
Also on the initial set of feeders, we actually clamped all three phases of the 15Kv cable at the incoming side of the downstream SWGR, and it showed the 60A.
So it is real unbalance, not a CT issue or something like that.
These are fairly long cable runs from the Main sub to each downstream SWGR, and some of them have been spliced.
So my initial thought was possibly there was a bad connection somewhere, and when the loads had parallel sources, they preferred one over the other a bit on that phase, causing an unbalance which would equate to neutral current.
I'm trying to think if I've left anything out.
At one point I thought it might be a bad connection on a vacuum bottle on the downstream tie, but we tested it and it's very low and balanced (around 30 micro-ohms each phase).
Also if that were the problem it wouldn't go away when you open the main.
Some of the downstream feeder pairs haven't been tested yet because they have auto transfer schemes that are not easily disabled, so you can't just close a tie and not also open a main, which we've already proven at several other sets doesn't create the neutral current.
Oh, also I put an ammeter on the actual ground from the HRG on both of the substation transformers, and read maybe 10 amps, but it was always 10 amps, regardless of the tie breaker position.
I never saw it change. I think the main breakers at the Main Sub don't look at neutral current, only ground current, and I believe it is connected as a residual ground (ie. the non-polarity wires from all three phase CT's sum through the ground CT input on the relay). So it's measuring unbalance, just like a calculated neutral current would anyway.
On the initial SWGR we saw this issue on (the one with 60A neutral current when the ties are closed), each main at that SWGR shifted the A-phase amps when the ties were closed.
A-phase on one main went up by around 10A and A-phase on the other main went down by the same amount. They were so lightly loaded (maybe 100A per bus) before, and so well balanced it stood out more at that SWGR. It might have been the same way at other SWGR's but I didn't notice. But obviously if B&C do not change, but A does, it will measure neutral current because of the unbalance. But why the shift?
And why only in that particular configuration.
The fact that we see it on every pair of feeders from this particular sub, but not at any other sub, makes me think the problem is at the Main Sub.
Maybe a mismatch in impedance on one phase of one of the transformers, but if that were the case you think just closing the Main Sub tie alone would create this scenario, but it doesn't.
I would greatly appreciate any insight anyone might have to offer.
This has made it very difficult to get any maintenance done downstream of this particular substation.
We have been looking into this and experimenting as time permits off and on for about 6 months now.
We even installed a switch at the first set of SWGR to temporarily disable the 50N settings.
Thank you,
Wayne
I'm new here.
I joined because I was searching for an answer to my question and found some threads on here that sounded like they answered it, but they didn't.
But at the very least it sounded like there might be someone on here that has experience with this issue, and can help.
So that is what I'm hoping.
My name is Wayne. I work for a high voltage engineering/construction/commissioning/maintenance contractor in the Houston, TX area.
I have been in this business for 26 years, and now supervise a High Voltage maintenance crew at a petro-chem facility.
This place has nine 138KV substations, and tons of distribution.
There is only one of the substations that has this issue (that I've seen anyway).
Lets call it the "Main Sub" just because I don't want to share company info that I might not have permission to share.
This is a typical 4-GCB, 2-Transformer ring bus.
The secondary voltage is 12.47KV (I will refer to this as 15KV to save time).
There are two 15KV buses with a tie breaker in between.
On each bus there is a small cogen that feeds into the bus, and a power factor correction cap bank, and multiple pairs of distribution feeders.
Each pair of distribution feeder have a feeder on Bus-A and a parallel feeder on Bus-B.
5 of these AB feeder sets go to in plant 15KV MTM switchgear setups, which also have a tie.
So here is the issue.
When you close the tie at the Main Sub, the load on TRA and TRB balances out as you would expect and there is zero neutral current and zero ground current.
But if you then pick a pair of feeders and go downstream to that gear and close that tie, now you have a significant amount of neutral current.
It varies with the different sets of feeders, one is 60A, two others are around 40A.
And I have tried this multiple times, and every time the neutral current is exactly the same, as in it doesn't fluctuate.
It's different depending on which set of feeders you are on, but if Feeder pair #1 is 60A then that pair is always 60A, etc.
You see this neutral current on the downstream Main breakers, the Main Sub feeder breakers, and the Main Sub Main Breakers.
We have had to disable neutral current trips temporarily every time we do this, because the amplitude is near pickup level on the 50N in some cases.
Ok here is where it gets weirder (to me anyway), if you are in this initial configuration (with upstream and downstream ties closed and neutral current present), and you then open a downstream main (either one) the neutral current goes away (in all places where it was shown before).
If you are in this initial configuration and you open the Main sub Tie breaker, it goes away.
If you are in this initial configuration and you open one of the Main Sub Main breakers, it goes away.
It is only there when the Mains and tie at the Main sub are closed, and also the mains and tie downstream are closed.
Now someone (a plant tech) closed the Main Sub tie, and a 480V tie 2 levels downstream (skipping the 15KV SWGR in the middle) and there was no neutral current.
But maybe the 480V transformers had something to do with that.
I'd like to try this on another set of lower voltage downstream ties to compare at some point.
Oh I also forgot one thing, there is a 5000hp motor fed off of the Main Sub Bus-B (across the line, no VFD), but no similar motor on Bus-A
Also when this neutral current was present we checked every feeder at the Main Sub and only the ones mentioned above showed the neutral current.
Also on the initial set of feeders, we actually clamped all three phases of the 15Kv cable at the incoming side of the downstream SWGR, and it showed the 60A.
So it is real unbalance, not a CT issue or something like that.
These are fairly long cable runs from the Main sub to each downstream SWGR, and some of them have been spliced.
So my initial thought was possibly there was a bad connection somewhere, and when the loads had parallel sources, they preferred one over the other a bit on that phase, causing an unbalance which would equate to neutral current.
I'm trying to think if I've left anything out.
At one point I thought it might be a bad connection on a vacuum bottle on the downstream tie, but we tested it and it's very low and balanced (around 30 micro-ohms each phase).
Also if that were the problem it wouldn't go away when you open the main.
Some of the downstream feeder pairs haven't been tested yet because they have auto transfer schemes that are not easily disabled, so you can't just close a tie and not also open a main, which we've already proven at several other sets doesn't create the neutral current.
Oh, also I put an ammeter on the actual ground from the HRG on both of the substation transformers, and read maybe 10 amps, but it was always 10 amps, regardless of the tie breaker position.
I never saw it change. I think the main breakers at the Main Sub don't look at neutral current, only ground current, and I believe it is connected as a residual ground (ie. the non-polarity wires from all three phase CT's sum through the ground CT input on the relay). So it's measuring unbalance, just like a calculated neutral current would anyway.
On the initial SWGR we saw this issue on (the one with 60A neutral current when the ties are closed), each main at that SWGR shifted the A-phase amps when the ties were closed.
A-phase on one main went up by around 10A and A-phase on the other main went down by the same amount. They were so lightly loaded (maybe 100A per bus) before, and so well balanced it stood out more at that SWGR. It might have been the same way at other SWGR's but I didn't notice. But obviously if B&C do not change, but A does, it will measure neutral current because of the unbalance. But why the shift?
And why only in that particular configuration.
The fact that we see it on every pair of feeders from this particular sub, but not at any other sub, makes me think the problem is at the Main Sub.
Maybe a mismatch in impedance on one phase of one of the transformers, but if that were the case you think just closing the Main Sub tie alone would create this scenario, but it doesn't.
I would greatly appreciate any insight anyone might have to offer.
This has made it very difficult to get any maintenance done downstream of this particular substation.
We have been looking into this and experimenting as time permits off and on for about 6 months now.
We even installed a switch at the first set of SWGR to temporarily disable the 50N settings.
Thank you,
Wayne
