Voltage Imbalance on Long Three Phase Feeder
Voltage Imbalance on Long Three Phase Feeder
(OP)
Folks,
I had posted a few weeks ago about a strange phase to phase voltage imbalance measured on the secondary of a three phase 12.47 x 277/480V transformer bank. We recently checked another bank further down the road and have the same issue, 208, 208 and 217. The 480 bank measured 480, 480 and 505.
My new question is, I have been made aware of a practice where the utility will actually roll the primary phases then eventually roll them back (of course watching for rotation on any other three phase banks) and I am curious if anyone has heard of this done for 12KV Distribution? I am willing to try anything at this point because we cannot seem to figure out what is going on.
Thanks in advance for the info.
I had posted a few weeks ago about a strange phase to phase voltage imbalance measured on the secondary of a three phase 12.47 x 277/480V transformer bank. We recently checked another bank further down the road and have the same issue, 208, 208 and 217. The 480 bank measured 480, 480 and 505.
My new question is, I have been made aware of a practice where the utility will actually roll the primary phases then eventually roll them back (of course watching for rotation on any other three phase banks) and I am curious if anyone has heard of this done for 12KV Distribution? I am willing to try anything at this point because we cannot seem to figure out what is going on.
Thanks in advance for the info.






RE: Voltage Imbalance on Long Three Phase Feeder
I have seen this condition so severe that phase angle sensitive Soft Starts would shut down on a phase angle error.
The cause was definitely identified as heavy neutral currents and neutral voltage drops on the primary lines.
Interestingly I understand that a wye delta transformer bank with the primary neutral connected will balance this condition. However there are issues with the loss of one phase and issues with the loss of two phases which may be more serious than the original voltage unbalance problem. I would not consider balancing voltages with a floating wye delta bank without a phase loss relay and a contactor to break the delta in the event of phase loss to avoid the aforementioned issues.
BTW earlier this week I was visiting a friend who is a distribution engineer with a large utility. They have a line that extends about 15 miles and then branches in three directions. One branch is three phases, one branch is two phases and one branch is one phase. These are difficult to load balance.
They had a wye delta bank near the point where the line branched. The customer was no longer using three phase power so the engineer had the line crew pull the fuses on two of the transformers, leaving only the single phase 120/240 service energised. When the engineer returned to the control center he found that the circuit in question now had noticably unbalanced currents when it was previously balanced. The delta bank had been doing a good job of balancing what was actually an unbalanced distribution line.
respectfully
RE: Voltage Imbalance on Long Three Phase Feeder
I've never heard of any utility transposing phases on distribution lines. I doubt that it would help much. As waross says, the phase voltage unbalance can be the result of unbalanced currents. This would result with or without phase transposition.
RE: Voltage Imbalance on Long Three Phase Feeder
RE: Voltage Imbalance on Long Three Phase Feeder
RE: Voltage Imbalance on Long Three Phase Feeder
RE: Voltage Imbalance on Long Three Phase Feeder
How about capacitor banks on the distribution circuit? Unequal capacitors on each phase could easily account for this - perhaps some blown fuses (either internal or external).
Another possibility are single-phase voltage regulators that are out of sync, either through differences in settings or regulator malfunction.
RE: Voltage Imbalance on Long Three Phase Feeder
respectfully
RE: Voltage Imbalance on Long Three Phase Feeder
RE: Voltage Imbalance on Long Three Phase Feeder