Deltaforce
Senior Member
- Location
- India
- Occupation
- Electrical Engineer
Utility mv capacitors switching than lv capacitors switching likely voltage transients
May also check neighbors same issue
As utility switch mv capacitors same time
ABB VFD delta high leg
- Thread starter wncjefftino
- Start date
May also check neighbors same issue
As utility switch mv capacitors same time
wncjefftino
Member
- Location
- Asheville, NC
- Occupation
- electrician
I'll have to check with the neighbors and see if they are experiencing any issues
neemarnick
New User
- Location
- NJ, USA
- Occupation
- Electrical Engineer
Hey, I'm new here and found this thread looking for some info on some totally different VFD issues with 240 High leg delta, but I noticed a pattern in here and haven't seen any resolution. I hope this info helps and saves someone a lot of time and aggravation.
I've seen the issue you're describing where there's a slight voltage imbalance but either no current or very low current on one leg. Every time I've seen zero current on one leg that showed voltage, there was a blown primary fuse on a pole somewhere and the transformers were backfeeding and making it appear there was power where there wasn't.
I've also seen (twice now) where there is a slight voltage imbalance, but a large current imbalance that drives VFD's and especially soft starts crazy showing phase loss. Both times there was a primary line break and the POCO re-routed just one phase to feed from another circuit. The first time we were chasing our tails and nobody knew what was going on with it, other than the load imbalance was weird but the first POCO guy said it was within spec and must be a plant problem. We finally got someone to come out that understood what I was telling him and then he realized what they had done. I can only assume the third phase got rerouted to a different primary transformer and the three phase loads didn't like it.
The second time I saw the same thing, even after explaining the exact scenario I thought we had, I had to argue with the POCO guys telling me it must be the site's transformer that's bad and there was no problem on their end, until I asked them how they made their repairs 6 hours ahead of schedule to get the power back on. Then they made another phone call, came back over and apologized and told me it would be back to normal in another hour.
I've seen the issue you're describing where there's a slight voltage imbalance but either no current or very low current on one leg. Every time I've seen zero current on one leg that showed voltage, there was a blown primary fuse on a pole somewhere and the transformers were backfeeding and making it appear there was power where there wasn't.
I've also seen (twice now) where there is a slight voltage imbalance, but a large current imbalance that drives VFD's and especially soft starts crazy showing phase loss. Both times there was a primary line break and the POCO re-routed just one phase to feed from another circuit. The first time we were chasing our tails and nobody knew what was going on with it, other than the load imbalance was weird but the first POCO guy said it was within spec and must be a plant problem. We finally got someone to come out that understood what I was telling him and then he realized what they had done. I can only assume the third phase got rerouted to a different primary transformer and the three phase loads didn't like it.
The second time I saw the same thing, even after explaining the exact scenario I thought we had, I had to argue with the POCO guys telling me it must be the site's transformer that's bad and there was no problem on their end, until I asked them how they made their repairs 6 hours ahead of schedule to get the power back on. Then they made another phone call, came back over and apologized and told me it would be back to normal in another hour.
wncjefftino
Member
- Location
- Asheville, NC
- Occupation
- electrician
I can confirm that the larger transformer is shared with a house and the smaller transformer is only us. The next 2 business have the same setup. One is a food coop and the other a restaurant. They each have a large and small transformer on their poles.
I though a saw a post about feeding the 208V input through 2 coils of a line reactor. This did not work. It didn't matter with 1, 2, or 3 coils. Today I had voltages of AB 255, AC 255, BC 236 volts.
What would power monitoring look like?
I though a saw a post about feeding the 208V input through 2 coils of a line reactor. This did not work. It didn't matter with 1, 2, or 3 coils. Today I had voltages of AB 255, AC 255, BC 236 volts.
What would power monitoring look like?
ptonsparky
Tom
- Occupation
- EC - retired
wncjefftino
Member
- Location
- Asheville, NC
- Occupation
- electrician
Would a contractor or service that would provide a monitor, would that show if the power company or a piece of equipment in the building is responsible for the issue?
Elect117
Senior Member
- Location
- California
- Occupation
- Engineer E.E. P.E.
You can contact some engineering firms in the area for suggestions. You can install the monitors if you are qualified to do electrical work. They are usually CTs and a line connection. Or some run on a battery. After some time, you take them out and download the data. Let an engineer look at it and decipher what they think is going on. Event settings might be difficult for initial troubleshooting but there are some basic settings you can use.
Memory is also an issue. If you have too many events you can sometimes overwrite (ill advised) or it will just stop recording. So maybe after a week pull it and download the data. Review it and then put it back in.
Some utilities offer a power monitor service or review on their own if you complain.
Memory is also an issue. If you have too many events you can sometimes overwrite (ill advised) or it will just stop recording. So maybe after a week pull it and download the data. Review it and then put it back in.
Some utilities offer a power monitor service or review on their own if you complain.
wncjefftino
Member
- Location
- Asheville, NC
- Occupation
- electrician
Thank you
Elect117
Senior Member
- Location
- California
- Occupation
- Engineer E.E. P.E.
I should include that the customer or person having the issue needs to write down the times. And the monitor should have the right time set. So when a issue happens, the engineer can look back for it in the events / recording.
synchro
Senior Member
- Location
- Chicago, IL
- Occupation
- EE
Were those voltages measured with an averaging type meter? If so, then the ratio of the BC voltage to the AB and AC voltages is the same as you noted in post #67 at the link below when using an averaging meter. And such consistency of voltage ratios, in spite of a 4% change in their absolute voltages, could provide some useful information.
ABB VFD delta high leg
Maybe I missed something, but didn’t the op say the phase loss moved with the changing of the phase? Sounds like it’s in the drive, possibly surge protection on the front end blown and open. From the OP's post #1: "I moved the wire on A and the no current "moved" to the new location, C. "...
forums.mikeholt.com
Last edited:
wncjefftino
Member
- Location
- Asheville, NC
- Occupation
- electrician
I don't remember. I carry both now. I believe I grabbed my avg. meter for convenience's sake. If the ratio remains the same this might indicate a internal problem not utility. Is my thinking correct? I checked tightness from the load side of the main. I ought to check the line side.
A 75kVA and 25kVA open delta has a total capacity of 86.6 kVA or about 208 Amps 3 ph. 240.
What is the peak demand of the service over the last 18 months?
You mentioned in one comment that there is a 400A sub panel:
Considering the NEC calls out larger wire and panels that the utility uses, to me it still seems a 25 kVA and 75kVA may be a bit undersized, with other customers on them.
The load combined with the mismatching of transformer winding impedance is my guess the cause of the imbalance you see.
I would look at the peak demand, and run a load calc per 220.87, then ask the utility to replace the 25 kVA with one sized to your peak demand, or if they are willing provide a full delta.