3 Phase WYE Voltage imbalanced

[PCEAlabama18]

Question?
Typically when I look at phase imbalance, I am measure phase to phase. I have a customer who is complaining that the voltage from phase to ground are imbalanced. The voltage between L1-L2 is 483, the voltage between L1-L3 IS 481, and the reading between L2-L3 is 486. So the phase are within < 1%. Good there. The Phase to ground readings are L1-G 279, L2-G 297, and L3-G is 280. The L2-G stands out. The equipment only has one 1 KVA single phase transformer. Since I never considered Phase to ground for determining balanced power, this is a strange worry by the customer. Any thoughts?

 

[__dan]

If that's a utility 480 service, the main system bonding jumper has to be checked to see if there is one. Check the neutral to ground voltage to see if you can read more than 500 milliVolts on the neutral over the ground, and I would want to physically see the main bonding jumper to see if it's installed.

Also if it's a utility transformer, around here the process to to file a "voltage complaint" to get them to look at it. First thing to check would be the main bonding jumper. If that's the line to neutral Voltages, it could use a good look at it for load balance and the primary side Voltages. I would suspect the MBJ first.

 

[GoldDigger]

I think that the answer is relatively simple, with one complex caveat.
If all of the loads are connected line to line (notice that I specifically do not call it phase to phase), then a relatively small (on the scale of the system voltage) difference among line to ground voltages will not have any practical consequence.

If there are line to neutral loads (which, of course, requires a neutral!) then that degree of imbalance in line to neutral is unexpected and might be seen as the result of a compromised neutral if measured under load.

Instead, the difference in line to ground voltage you measure can be seen as a sign of a weakly grounded system. Voltages like the ones you list could be seen even on an ungrounded system as a result of capacitance to ground making it a weakly grounded system, not able to source any significant amount of current. Or it could be the result of a grounded wye source with the neutral not brought in as part of the service. And in that case the difference is unexpected but not necessarily a problem.