Voltage Imbalance on Long Three Phase Feeder 1

[vtpower]

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.

 

[waross]

Measure the neutral current at several places down the feeder. Any neutral current will cause some voltage drop. When you draw your vector diagram, and add the appropriate vector at the wye point to represent the voltage drop on the neutral, the effect will be to shift the wye point away from the geometric center of the diagram. This will cause phase shifts and unbalanced voltages to neutral on the primary phases.
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

 

[jghrist]

Rolling the phases (phase transposition) maintains a balanced impedance for the line. Sequence impedances are based on fully transposed lines. See

http://helios.acomp.usf.edu/~fehr/carson.pdf

for an explanation and the effects of not transposing phases.

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.

 

[vtpower]

Thanks for the responses. I had checked the line balancing a few weeks ago and the current un balance was very minimal. Also, the line to neutral voltages are all equal very well balanced. It is only the line to line voltages that being affected, which I believe should only be due to the voltage vectors phase angle being shifted, (not all angles are 120deg)

 

[apowerengr]

vt-I reread your original post - is this bank delta-wye or wye-wye?

 

[vtpower]

The bank is wye-wye.

 

[dpc]

If the line is long enough, the inherent difference in the inductance for the three phases could be responsible, but it would have to be pretty long.

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.

 

[waross]

One possible cause is a neutral current causing phase shifts. Then, farther down the line, a set of single phase voltage regulators equalise the line to neutral voltages, but do not change the phase angles. Past the single phase regulators you will have equal voltages to ground but shifted phase angles and unequal line to line voltages.
respectfully

 

[apowerengr]

VT - look for a cap bank upstream of the vregs with a blown fuse that did not drop out. The capacitive imbalance could cause the phase shift and the neutral currents others have described with the vregs making up the l-n voltages. You stated the l-n voltages are balanced and the feeder load amps are balanced but are the vreg position indicators running in the same positions? As to your earlier question yes some distribution utilities used to string with transpositions (tramps) in long feeder lines but found it was too hard to troubleshoot and made reconductoring difficult so the practice has been discontinued for the most part.

 

[tinfoil]

Many utilities use 3 single-phase voltage regulators with separate controls. One of those could be a bit off.