transformer bonding

[j7david]

At the facility where I work one of the pump stations has several line ups of switchgear4160V and 480V. The line-up of interest dates from 1952 was last modified in 1992. Exactly what modifications were made I am not sure. This line-ups of 480V gear has two 2000A 4160V to 480/277V Δ-Y transformers, one on ether end. There is a 2500A breaker on the load side of each transformer, and in the center a 2500A breaker that when closed services as a tie breaker. The neutral and the ground bus are connected to each transformer without provision to separate it from ether.
On Wednesday we found some troubling voltage readings in the station. The readings were on the 480V equipment and observed on both the A and B sides of the 480V gear. The phase to phase reading were A? to B? 480V, B? to C? 480V, C? to A? 480V. This is normal and to be expected. The phases to ground readings were A? to ground 12V, B? to ground 277V, C? to ground 277V. The reading for B & C phase were normal and expected however the A phase reading was similar to what one would find on a Delta to Delta corner grounded system. But even on a corner grounded system one would expect 0 volts on the corner that is grounded. However our equipment as shown on our drawings and on the transformer data plates is Δ / Y. This is not a high resistance grounded setup.
The next reading was even more troubling, it was neutral to ground 277V. This reading shows unequivocally that the neutral and ground are not bonded as per the drawing that we have nor as the diagram on the transformers. (The drawings we have in the electrical shop are preliminary and not as-built) We then visually inspected the transformers as well and as the case must be, there are no connections between the ground and the neutral.
We located the offending piece of equipment that caused the initial concerns, a fan that was grounded on the A phase. This problem produced the resulting issues that were exactly as expected from such an event on a floating neutral system.
The question that I have is rather simple, how should I bond the transformers, and should they be bonded in each transformer as shown on the drawings? Or should it be only once as the theory that I have been taught would call for? Remember the neutral and the grounds are continuous between the transformers. The tie breaker does not open or close ether the neutral or the ground.
One reason that I have not already done the bonding is there are times and places were the neutral is intentionally left unbounded as in a O/R room or a steel mill electric furnace. These are rare and unusual cases and require specialized monitoring equipment to prevent personnel injuries in the event of two different grounding events occurring together. As the special monitoring equipment is not present, I am of the opinion the lack of bonding is purely a mistake that was made in the original/upgrade install and should be corrected by means of a single bond strap and not the two shown in the drawings.
I am currently waiting to hear from the City Engineering staff but am wondering about the opinion of those on this form.

 

[petersonra]

Is this thing corner grounded by chance?

 

[GoldDigger]

On Wednesday we found some troubling voltage readings in the station. The readings were on the 480V equipment and observed on both the A and B sides of the 480V gear. The phase to phase reading were A? to B? 480V, B? to C? 480V, C? to A? 480V. This is normal and to be expected. The phases to ground readings were A? to ground 12V, B? to ground 277V, C? to ground 277V.

This does not seem to add up given your description of the problem.
With 480 from line to line and 12 volts from one line to ground, there is no way you could have been reading real voltages of 277 to ground from the other two lines. This suggests that what you were actually seeing were phantom voltages read to ground in a completely ungrounded system. A small, but larger than normal, amount of leakage between the A phase and ground rather than a hard short might explain what you read. But if you put an actual current load on the wires instead of using a high-impedance meter, the various voltages have to add up (as vectors).

I will leave the question of exactly what bonding should be done to others, but with starting point that if there is a neutral which carries current the NEC does not allow the system to be ungrounded. And any ungrounded system will have to include a ground detector.

 

[jim dungar]

We located the offending piece of equipment that caused the initial concerns, a fan that was grounded on the A phase. This problem produced the resulting issues that were exactly as expected from such an event on a floating neutral system.

This sounds like a very valid explanation for the situation you found. The 'short' was probably a fairly high resistance arcing fault, which is why you were able to measure some voltage.
Remember there are ten different voltages that should be measured an recorded. (3) L-L, (3) L-N, (3) L-G, and (1) N-G

Typically in a double-ended substation, like you have, it is common to bond the X0 to ground in both transformers. Simplistically, you have two sources, you need two bonds.

Do you have ground fault protection on your mains and/or tie?
In the old days there were some pretty complicated methods for dealing with the multiple ground paths your construction presents. The last one I worked with had both transformers neutral bonded together and then used a single bonding to ground point which included a CT for measuring actual ground current.

 

[j7david]

Is this thing corner grounded by chance?

No it is a Delta Y system.

 

[j7david]

I would like to make one correction to my orginal post. the measurement from phase B and C to ground is 480V not 277 as I had stated in the post. Sorry for the misspeak.

 

[j7david]

Do you have ground fault protection on your mains and/or tie?
In the old days there were some pretty complicated methods for dealing with the multiple ground paths your construction presents.

Yes there is ground fault protection on the system which did not trip. The Mutlilin metering on both transformer A and B showed everything to be normal in every respect. That is one of the reasons that that at first it seemed so wild to get the readings that we were getting as it was in stark contrast to the what we found as we scrolled through the various measurements.

 

[GoldDigger]

Yes there is ground fault protection on the system which did not trip. The Mutlilin metering on both transformer A and B showed everything to be normal in every respect. That is one of the reasons that that at first it seemed so wild to get the readings that we were getting as it was in stark contrast to the what we found as we scrolled through the various measurements.

Keep in mind that ground detection is generally required for an ungrounded system. It does NOT depend on load current flowing to ground. Ground fault detection, on the other hand, will trip when load current is flowing through a path other than the grounded and ungrounded circuit conductors.

If there is a fault from one conductor to ground in an ungrounded system, a ground fault detector will not trip until a second fault actually sends current through the newly established ground.