Odd Potential Transformer Readings 1

[mmt019]

I am using a 4200:120V three-phase P.T. and connecting it Line-to-Line on the Primary in an open delta configuration. The secondary side is an open delta as well. When I put a voltmeter on it, I am reading 120V across each phase (Vab, Vbc, Vca). But when I am reading line to ground, I get 306V, 215V and 303V (Vag, Vbg, Vcg). Why am i getting these readings phase to ground? Its causing the meter to read 9000V-Vab,Vbc, but Vca is 4160V. Any suggestions on what the problem is?

 

[scottf]

It sounds like a floating ground problem to me.

To where are you actually measuring to, physically.

 

[mmt019]

I am only taking three wires (X1, X2, X3) to the meter in a open delta configuration. I am not even connecting ground to the meter. The Square D meter read (Vab-11,000V, Vbc-9000V, Vac-4160V). I put a fluke meter on the secondary terminals and it read 118V across any phases of the secondary terminals.

 

[buzzp]

It sounds as thought the problem is with your meter. Maybe it can be set-up differently. In any case, Ill bet there is a ground wire going to this meter. This ground is referenced inside to the measuring circuit causing erroneous readings. Obviously, everything else is ok since the fluke (isolated from ground) reads the line-line voltages just fine. I would go over the spec sheet for the SqD unit with this is mind and you will likely find a problem. It may be that the set-up of this meter can be changed by the user to get the correct readings. However, the chances are just as good that the meter needs to be changed to something that can work on an open delta (no ground-or at least no safety ground referenced to the internal circuitry).

 

[busbar]

It’s likely the floating {ungrounded} secondaries are being excited by primary voltage, particularly if a 10MO meter is used for measurements. It is a serious misapplication to omit secondary grounding called for in ANSI/IEEE Std C57.13.3 ...Grounding of Instrument Transformer Secondary Circuits and Cases

This information is usually also included in instrument-transformer manufacturer’s literature. Fortunately transmission voltages were not involved.

 

[busbar]

Try shunting the meter leads with a 180K?, 2-watt resistor.

 

[skiier]

Where's the gnd on the secondary side of your t/f. Common sense says that if u r properly gnded you will read 120V, 120V, and 0V on the secondary side of your t/f. Would it not be wise to gnd the center point? By the equal line to line readings (120V) you measure you tell us that you have the polarity of the windings right. But why isn't it gnded?
What u r measuring is stray voltages off the core and harmonics and junk but I don't think you are measuring anything useful. Your meter is probably ok. Have you tried your meter lately in a known live cct?
I'm not quite up on line voltage monitoring but it seems to me that it would be difficult to monitor anything with an open delta style t/f. 120, 120, and 0 only tells me 2/3 of the information - or am I missing something here?

 

[jbartos]

Suggestion: One-line diagram for the power distribution should be followed. There just may be some irregularity or malfunction in the system. Then, all the measurements are performed correctly, and the system needs to be troubleshooted and repaired.

 

[jwerthman]

It's not surprising that you are getting those voltages to ground with the connection that you have to the meter. With no reference connection to ground, the entire metering system is floating with respect to ground and the line-to-ground reading will be all over the map, as you are seeing. An open delta VT connection will not work if you want to measure line-ground voltages. You must have line-to-ground connected VT's to meter this correctly.