voltage anomaly

[hilltop]

I have a prepelxing voltage anomaly:
I have a 3 phase 120/208V panel fed by a 20KW UPS loaded at 75%
A=27A
B=35A
C= 25A
amperage is seen during the day when most loads are running.

Voltage at panel is between 118V and 120V normally on each phase to nuetral

I attached a voltage monitor [did not measure currrent] to the load side of the UPS [at feed to panel served by UPS] for 5 days- during this time period I only recorded a single 13 minute voltage anomaly where:
A phase went to 128.8V
B phase went to 102.9V
C phase went to 128.5 volts
All simultaneously.
After 9 minutes B phase went back to normal range C phase sagged at 104V for 3 minutes and A phase still surged for 3 more minutes

All known loads connected are lab type equipment, no 3 phase loads...some 208V loads [double tub panel approx. 80 circuits being used]

Any body want to guess what may be happening here??????????

 

[brian john]

Having current readings would be helpful.

The old, DID YOU CHECK ALL CONNECTIONS?

What about your metering connections?

Any UPS alarms?

Does the UPS have a monitoring system where you can check the history?

 

[LarryFine]

In theory, at least, the only explanation of the high readings would be a neutral issue.

 

[steve66]

In theory, at least, the only explanation of the high readings would be a neutral issue.

I'm thinking neutral issue also. Two voltages are high, while one is low. It is possible the L-L voltages were all still 208.

I'm wondering where you attached the meter's neutral. If it was at the panel, does the panel even have a neutral wire ran to it, or is it a 3 wire panel? I'm thinking it may be a 3 wire panel because you didn't mention anything about any 120 volt loads.

 

[Hameedulla-Ekhlas]

Good Question,
I am agree with others completely. But one thing else also please consider . As per your below statement

I have a prepelxing voltage anomaly:
I have a 3 phase 120/208V panel fed by a 20KW UPS loaded at 75%
A=27A
B=35A
C= 25A

There is only voltage drop and raise problem. So, see you have mentioned 3phase 120/208V panel is being fed by a 20kW UPS

Check the connected load. Are they all completely resistivity because if there is reactive power is needed the voltage voltage will drop due to reactive power which is needed.

We know that.

Q = Vr/x *(vs - Vr)

your connected load must needs also Var per your connected equipment load.


when Q is needed and Q raises the voltage drops and per load requirement it raises and drops.

I am not sure about the answer but please once consider it.

 

[hilltop]

So we have researched events that happended at the same time as the anomaly...a power scrubber was plugged in a receptacle that is served by this panel and that is when we saw the power sags and surges.

Still, would this type of animaly be normal when this type of load is applied???

 

[gar]

100204-1604 EST

hilltop:

Draw a circle of radius 120. From the center draw three radial lines from the center and equally spaced in angle (120 deg). The radial end points of the three lines are your Y 120 V legs. From two of these 120 V legs draw circles of 129 radius. These two circles intersect the third radial line at a point 16.4 radially from the original circle center (first circle drawn). Subtract 16.4 from 120 and the result is 103.6. Quite close to your 102.9 .

Note I assumed each leg voltage was 120 when balanced and I rounded your 128.8 to 129.

Basically you have a high resistance neutral and an unbalanced load shifting the neutral point used as the neutral reference point.

.

 

[hilltop]

Thank you,
Yes the loads are slightly out of balance...thank you for lesson.

It appears the equipment connected to this panel is experiencing millivolt spikes approx. every 2 minutes....it is looking like the instrument itself is the cause and effect...should know soon.....