Low L-G reading on voltage measurement

[Pitt123]

Today we were measuring voltages on the secondary of a 480V/277V transformer and saw that one of the L-G readings was 268V which seemed a bit low. All other L-G were about 277V and all L-L readings were 480V.

Should we be concerned with this low reading? What would cause this one reading to be low?

Transformer secondary has EGC going to enclosure, but does not have grouding electrode conductor (GEC) from transfomer to ground grid. Is this GEC required? I always thought is was required by code for SDS's?

Should this absence of the GEC make a difference on voltage readings by leaving the neutral floating? Even with the neutral not tied to ground, I always though that we should still read 277V between L-G since we are reading from hot leg to the neutral point weather it was grounded or not. Could this reading be b/c neutral is not tied to ground?

 

[don_resqcapt19]

If the neutral is not tied to a grounding electrode, and if you are reading from an ungrounded conductor to a grounding conductor, the voltage can be almost anything.

 

[xformer]

Yes the GEC is required...

See article 250.30

 

[gar]

100401-0816 EST

Pitt123:

If the voltages are good sine waves and your meter is more accurate than say 1/2% of reading, then I do not believe the voltage measurements are possible.

Draw the vector diagram of what you describe and your result is not possible with sine waves for the voltages measured and assuming the three phases are exactly 120 degrees apart. Note: your problem reading is in error by about 3.4%.

With the above assumptions of sine waves, equal phase angles, and 480 line to line, then if one line to ground is 268 the other two, if equal, must be 281.8 V, not 277.14 .

You really need to make more measurements --- voltage, waveform, current, and phase angle of all wire combinations. What is between neutral at the transformers and ground? What are neutral to ground voltage and current?

.

 

[Pitt123]

100401-0816 EST

Pitt123:

If the voltages are good sine waves and your meter is more accurate than say 1/2% of reading, then I do not believe the voltage measurements are possible.

Draw the vector diagram of what you describe and your result is not possible with sine waves for the voltages measured and assuming the three phases are exactly 120 degrees apart. Note: your problem reading is in error by about 3.4%.

With the above assumptions of sine waves, equal phase angles, and 480 line to line, then if one line to ground is 268 the other two, if equal, must be 281.8 V, not 277.14 .

You really need to make more measurements --- voltage, waveform, current, and phase angle of all wire combinations. What is between neutral at the transformers and ground? What are neutral to ground voltage and current?

.

I agree I need to take more measurements. I believe the L-L voltages I am told were closer to 490V between all phases, but it still sounds like you are saying that if one L-G reading is low then all the L-L voltages would not be identical according to the phasors. So is it safe to say that with all L-L voltages the same at around 490V then the low L-G reading must be some sort of an error?

What about capacitive coupling for ungrounded secondary? Cant you still have the same L-L voltages and have different L-G voltages set up by capacitive coupling? Could this be the case without a neutral connection to a grounding electorde?

If we were indeed seeing a low L-G reading then we would expect to see on of the other L-L readings be low as well? Would this be a concern, or just an indications that one of the line voltages was low (most likely low L-G reading) and therefore was causing the one L-L reading to be low as well? I typically use +/- 10% as the tolerance for L-L voltage, so should the same be used for L-G values as well?

The transformer has a wye secondary which does not have its neutral connected to a GEC or grounding electrode. The transformer neutral does have an EGC leaving it tied to the enclosure which is housing a VFD. Even though this enclosure does not use a neutral or is not a panel, does it still need to follow the same rules for grounding a SDS? Does it therefore need to have its neutral tied to the grounding conductor in addition to the EGC going to the enclosure?

 

[mcclary's electrical]

I agree I need to take more measurements. I believe the L-L voltages I am told were closer to 490V between all phases, but it still sounds like you are saying that if one L-G reading is low then all the L-L voltages would not be identical according to the phasors. So is it safe to say that with all L-L voltages the same at around 490V then the low L-G reading must be some sort of an error?

What about capacitive coupling for ungrounded secondary? Cant you still have the same L-L voltages and have different L-G voltages set up by capacitive coupling? Could this be the case without a neutral connection to a grounding electorde?

If we were indeed seeing a low L-G reading then we would expect to see on of the other L-L readings be low as well? Would this be a concern, or just an indications that one of the line voltages was low (most likely low L-G reading) and therefore was causing the one L-L reading to be low as well? I typically use +/- 10% as the tolerance for L-L voltage, so should the same be used for L-G values as well?

The transformer has a wye secondary which does not have its neutral connected to a GEC or grounding electrode. The transformer neutral does have an EGC leaving it tied to the enclosure which is housing a VFD. Even though this enclosure does not use a neutral or is not a panel, does it still need to follow the same rules for grounding a SDS? Does it therefore need to have its neutral tied to the grounding conductor in addition to the EGC going to the enclosure?

Please re-read post #2. I really believe that's all we're seeing here. Read it phase to phase and move on.

 

[gar]

100412-1311 EST

Pitt123:

--- but it still sounds like you are saying that if one L-G reading is low then all the L-L voltages would not be identical according to the phasors. So is it safe to say that with all L-L voltages the same at around 490V then the low L-G reading must be some sort of an error?

No that is not what I am saying. Also note that higher L-L voltages would make the error greater on the two equal L-N voltages.

I made the assumption of all L-L voltages being 480. First, draw this triangle. Next, draw a vector 268 long from one corner to the interior of the triangle bisecting that corner. This defines the ground or neutral point, which ever you were measuring to. Both of the other voltages to this point must be equal because of the bisector above. Measure the distance from this ground point to one of the other corners of the triangle. The result is 281.8 V, but this is not the value you or someone measured. The error between the calculated values and the measurements are greater than the inaccuracy of your meter.

With 490 V L-L the error would be even greater.

Really you should start with measuring all L-L voltages, hope they are stable, and measure all L-N voltages. Do these produce results that can be verified with a vector diagram? If not, then why? Repeat with L-G voltages.

.

 

[rcwilson]

The OP mentioned a VFD enclosure. Is this an isolating transformer feeding a VFD per NEC 250.21 A (2)? Is the purpose of the transformer to minimize circulating currents by isolating the VFD from ground?

The secondary will be floating with the L-G voltages determined by the VFD input circuits which probably have some line-ground voltage protection or sensing.(surge arrestors, R-C snubber circuits or ground detectors). If the VFD is not on line, the voltages are determiend by the capacitive coupling of the phase wiring and windings to ground.

With an ungrounded wye connection you can get just about any combination of L-G voltages as the system floats around. The vectors don't have to add up vectorially unless you have three simultaneous readings. As you move the meter from one phase to another the "neutral" point of the ungrounded system can shift because it is floating. Depending on the size of the system that is connected at the time of your measurement, the meter itself can have an effect on the L-G voltages because it will change the impedance to ground.

 

[Pitt123]

100412-1311 EST

Pitt123:



No that is not what I am saying. Also note that higher L-L voltages would make the error greater on the two equal L-N voltages.

I made the assumption of all L-L voltages being 480. First, draw this triangle. Next, draw a vector 268 long from one corner to the interior of the triangle bisecting that corner. This defines the ground or neutral point, which ever you were measuring to. Both of the other voltages to this point must be equal because of the bisector above. Measure the distance from this ground point to one of the other corners of the triangle. The result is 281.8 V, but this is not the value you or someone measured. The error between the calculated values and the measurements are greater than the inaccuracy of your meter.

With 490 V L-L the error would be even greater.

Really you should start with measuring all L-L voltages, hope they are stable, and measure all L-N voltages. Do these produce results that can be verified with a vector diagram? If not, then why? Repeat with L-G voltages.

.

I drew the vectors and see what you mean. Even if we are looking at capacitive coupling we should see all the vectors add up (assuming we are looking at them all at the same time) If this is not capacitve coupling and the neutral is grounded then if we see one low L-G reading we can expect one of the L-L readings to be low as well? Do I have this correct?

The OP mentioned a VFD enclosure. Is this an isolating transformer feeding a VFD per NEC 250.21 A (2)? Is the purpose of the transformer to minimize circulating currents by isolating the VFD from ground?.

Yes this is an isolation transformer but it is intended to be grounded. Does it still need to follow same rules as you would have with a SDS transformer?

Generally speaking does any grounded transformer feeding something besides a panel still need to follow the rules of grounding a s SDS transformer

The secondary will be floating with the L-G voltages determined by the VFD input circuits which probably have some line-ground voltage protection or sensing.(surge arrestors, R-C snubber circuits or ground detectors). If the VFD is not on line, the voltages are determiend by the capacitive coupling of the phase wiring and windings to ground.

With an ungrounded wye connection you can get just about any combination of L-G voltages as the system floats around. The vectors don't have to add up vectorially unless you have three simultaneous readings. As you move the meter from one phase to another the "neutral" point of the ungrounded system can shift because it is floating. Depending on the size of the system that is connected at the time of your measurement, the meter itself can have an effect on the L-G voltages because it will change the impedance to ground.

If although the transformer neutral is not connected to a grounding electrode and we measure between one of the phases and the EGC that is tied to the neutral, then we still sould read 277 or so and be balanced on all phases. If the EGC is only tied to the transformer frame and not bonded to the neutral then we could have a capacitively coupled situation and read different voltages.

I believe though the EGC from the transformer is tied to the transformer neutral and therefore shouldnt we be reading a hard L-N voltage on the three phases even though the neutral of the transformer is not connected to a grounding electrode?

 

[gar]

100413-1329 EST

Pitt123:

I drew the vectors and see what you mean. Even if we are looking at capacitive coupling we should see all the vectors add up (assuming we are looking at them all at the same time) If this is not capacitive coupling and the neutral is grounded then if we see one low L-G reading we can expect one of the L-L readings to be low as well? Do I have this correct?

Yes.

Try the measurements to the transformer neutral, and/or the EGC that you believe is connected to the transformer neutral. Also we have to be assuming sine waves or vectors do not work.

If you do not think the neutral gets connected to earth somewhere, then your measurement might introduce errors, but that is unlikely with any large transformer and the associated wiring capacitance.

Do a voltage measurement from the neutral to earth. If there is moderate voltage between neutral and earth and there are harmonics that are significant, then the voltage errors might have an explanation.

.

 

[Pitt123]

100413-1329 EST
If you do not think the neutral gets connected to earth somewhere, then your measurement might introduce errors, but that is unlikely with any large transformer and the associated wiring capacitance.
.

Even if the neutral is not connected to earth but the EGC is connected to the neutral and we measure between the EGC and one of the phases shouldn't all the L-G readings be the same assuming balanced incoming voltage? In other words we are not dealing with capacitance coupling in this case for these measurements?

Can anyone tell me what the code says as far as a grounded wye transformer used as a SDS with something other than a panel such as a VFD or other type enclosure? Do the same rules of grounding still apply in regards to grounding electrodes and system bonding jumper?

 

[rcwilson]

Good points, Gar.

Your system sounds like an SDS and it should be grounded. Is this a delta-wye transformer? Could it be wye-wye? At 480 V it most likely is delta-wye but if it had a wye primary with one of the primary voltages a little low, you would read the same low voltage L-N or L-G on the corresponding secondary phase. But the vector math would have to still add up.