Voltage Imbalance

[Wedoca]

this is gonna sound really stupid .....but here it goes ..

when we talking about voltage imbalance in a three phases system, I see everybody are refering to the phase to phase voltage imbalance .. but how about phase to ground voltage ??? is there a recommended range of % difference for phase-ground voltage???

 

[waross]

Phase to ground imbalance is most often a symptom of a high resistance grounding path and unequal phase to ground impedances.
There are many reasons and many variables, but a phase to ground imbalance often indicates a poor or failing ground connection and/or phase to ground leakage through insulation resistance and/or capacitive leakage.
Phase to ground unbalances of more than a few percent may be normal for the system but should be investigated.

Bill
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"Why not the best?"
Jimmy Carter

 

[Wedoca]

what are some of the possible damages that can be cause by this voltage imbalance ???

 

[erickench]

Well first of all a motor does not have a neutral connection
therefore the phase currents add up to zero. The voltages however become unbalanced because the neutral is not grounded.

Eric Kench, P.E.

 

[waross]

There are other reasons for unbalanced phase voltages, some more serious than others. I have had serious problems with unbalanced line to ground/neutral voltages and the unequal phase angles that are often the result of unequal phase to neutral voltages.
Damage to equipment; That depends to a great extent on the reason for the unbalanced voltages and the type of equipment that is installed. You may also have balanced phase to neutral voltages but unbalanced phase to phase voltages.
Equipment may be damaged by high common mode voltages, high voltages from one phase to ground and phase angle errors. If a wye delta transformer bank has the primary neutral connected to the source, it will be subject to overheating and possible burnout if the phase to neutral voltages are not equal, or if the phase angles are not equal.
Unbalanced line to neutral voltages are a symptom of a number of issues, some serious, some not so serious.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jcarrr]

The circumstances have really not been well characterized.

Do nearby motors exhibit the same charactersitics?

The service or on-premises transformer may be delta.

If one phase is consistently .866 of the phase to phase voltage from ground and the other two are .5 of the phase voltage from ground, then he has a service or delta transformer with the center of one winding at ground/neutral

Not bad, just not what he expects.

 

[waross]

That is a valid observation jcarrr. Often called four wire delta. No problem to equipment at all.

Bill
--------------------
"Why not the best?"
Jimmy Carter