An interesting set of Voltage Measurements 2

[davidbeach]

I was at a project site on Friday and saw some interesting voltage measurements that I would be interested if anybody could explain. Now, I know these aren't mathematically possible, but I saw them, other saw them, and they had been reported the day prior, which is why I went to the site; so nobody need respond that there was an error in the readings.

The relevant portion of the system begins with a 480V delta transformer winding. The three phase from this transformer are fed into some 480V switchgear. Also connected to the switchgear, but not running (and breakers open) are three generators which can be run parallel to the utility, that's why the delta winding. Connected to the 480V bus are a set of 277V-120V grounded-wye/grounded-wye VTs (yes, they should be 480V-120V and that will be corrected but that shouldn't matter for this). The output of the VTs are connected to a CAT/ISO control module, an Eaton Power Meter, and an SEL-351 relay. The control module has only phases A and C connected plus the neutral of the VTs. The Eaton Power Meter has the three phase voltages input and has a separate ground connection. The SEL-351 relay has all three phases and neutral connected. All three devices have a VT ratio of 2.31 programmed in. The SEL relay and the Eaton Power Meter report about 281V line to ground/neutral for all three phases as could be expected. So far, so good.

Now the problem; the CAT/ISO control module reports 148V secondary/342V primary. OK, that's easy - a bad module. Well, if only it were that easy. Two different Fluke DMMs were used to make measurements (I didn't get the model numbers, both had calibration stickers indicating calibration in 2006) and this is where it gets very interesting. Measuring on the bus of the 480V gear, the Flukes read 485V to 490V across each of the three phase pairs - completely reasonable, and an indication that the meter isn't wildly off. They also read between 340V and 345V phase to ground. On the secondary of the VTs, the Fluke meters read 210V to 215V phase to phase and between 145V and 150V phase to neutral.

I had the thought that on this ungrounded system, the Fluke meter was somehow causing a neutral shift, but a couple of tests tend to disprove that. One test was having both Fluke meters used at the same time, one A to ground and one C to ground, and they both read about 348V. Another test was measuring the voltages at the terminals on the rear of the SEL relay while watching the metering display on the front of the relay. With a VT ratio of 2.31 programmed into the relay, and a metering display of 281V phase to neutral (primary), the relay thinks that it has 122V applied to the terminals. Measuring across the same terminals with the Fluke meter, the fluke read 148V and the metering display on the relay did not change.

Any ideas? The voltages listed above were all read from the devices indicated by more than one person. One oscillographic record was trigged and downloaded and it showed that there was between 119[°] and 121[°] between phases, so that isn't the cause either.

 

[waross]

There is a misunderstanding regarding the terms grounded and neutral.
The power transformer secondary is delta. There is no neutral.
The VTs are connected in wye. The star point is grounded but the system (power transformer secondary) has no neutral and no ground.
If one phase of the power transformer secondary were to go to ground the voltage on the VT on the grounded phase would be zero. the voltage on the other VTs would rise to phase voltage.
Although the star point is grounded it is a floating neutral as far as the ungrounded system (power transformer) is concerned.
Respectfully

 

[alehman]

waross,
I understand all that. David said [the PT's] "were connected grounded-wye/grounded-wye when I arrived and during the testing the primary ground was lifted, but that didn't change anything."

My thought was that your last analysis made sense only for the condition where the PT 'start point' is not grounded. Maybe it also is true with the PT's grounded. I'll have to think about that.

 

[waross]

Hi alehman;
Grounding the VTs will not make any difference unless the system neutral is grounded. The system is delta and there is no neutral to ground.
In this case grounding the VT wye point is for safety only. There is not return path to the transformer delta winding and so the ground connection does not make any difference to the voltages. It references the VT voltages to ground but it doesn't affect the VT voltages.
respectfully

 

[davidbeach]

It now makes perfect sense that lifting the ground on the VT primary wye-point would have no effect. When the only ground reference in the system is the VT primary wye-point, it can't tie anything down, particularly if the transformers are going in and out of saturation. Lifting that connection didn't do anything because the connection wasn't doing anything in the first place.

waross, semantics perhaps, but I wouldn't go so far as to say that there is no neutral with respect to the power transformer. There is certainly no physical manifestation of the neutral, but the mathematical concepts still apply and a system neutral exists; that point where the potential to the phase voltages is minimized.

arcflash99, I should correct some of my terminology, it is really an offset ground that you have, not an offset neutral. In an ungrounded system, ground can exist anywhere, usually within the phase triangle, but not always (yours is). Neutral is where ground would be if all the couplings to ground were perfectly balanced.

 

[waross]

Hi David;
[quote[There is certainly no physical manifestation of the neutral[/quote]
That's what I was trying to say.
I suspect that hysteresis rather than saturation is the culprit. If the instrument connected to the VTs has non linear loading characteristics it may also be a factor.
Respectfully

 

[rbulsara]

Just a thought:

Why ground the primary side star point of VT? Specially when the primary side is ungrounded delta. Does this not effectively 'try' to act as a grounding transformer?

What happens if you "unground" the primary star point of the VTs?

 

[waross]

Hi Rafiq;
(I hope that's right)
With the VT primarys grounded, you have a very high impedance grounded system. The impedance being the impedance of the VTs.
A load will cause the impedance to drop but even with a load, I think the impedance may be beyong the normal range of grounding impedances.
With no faults on the system there will be no return path and no circuit for the ground connection back to the power transformer.
David states in his {17 Dec 06 14:01} post that the ground connection was removed with no result.
See also the first paragraph of David's "19 Dec 06 1:30" post.
Respectfully

 

[alehman]

Keep in mind that system capacitances mean you always have a high impedance grounded system, with or without the PT neutral being connected. It's not unusual to see a few amps of ground fault current in an 'ungrounded' system. Whether the capacitive charging current is comparable to the PT magnetization current is questionable. Balance of the capacitances is unknown but should be somewhere close. If the system 'neutral' tends to be offset substantially from ground, saturation of the PT's may come in to play if their neutral point is grounded. It's good practice to sense only line-to-line voltage on 'ungrounded' systems.

 

[rbulsara]

waross:

Yes, Rafiq is right. I find all comments interesting, but
I just think that, its best left ungroudned on an ungrounded system. Plus just an observation, use of two VTs connected in broken delta is more common for voltage sensing and even for metering.

Merry Christmas and Happy New Year all.

 

[davidbeach]

Open delta, not broken delta. Broken delta is three windings in delta with one corner not connected so that the sum of the three voltages can be measured, used to determine 3V₀.

 

[rbulsara]

Thanks david.

 

[waross]

This is a little off topic but it may be of interest to those of you who are familiar with broken delta connections with metering transformers. This was a little bigger scale.
I was called into a glass bottle factory years ago.
They used a very large transformer to boost the heat in the glass furnace. (Yes, molten glass conducts electricity and electric resistance heating is often used for either primary heat, or supplementary heat in the glass melting furnace.)
They temperature of the transformer was disproportionately high for the load. The transformer was wye/delta with circulating currents. There were 4 electrodes connected to each phase.
We removed the jumper bus that connected "A" phase winding to "B" phase winding. We left two electrodes connected to the finish of "A" phase and two electrodes connected to the start of "B" phase. The circulating currents then circulated in the glass melt doing useful work and the transformer temperature dropped dramatically.
Respectfully

 

[BJC]

Could it be just a setting on the Iso cpntrol module?
Do you have a link to the manual on the thing?
If it thought it was looking at a 600 Volt system it would behave just like it is.