Grounding a Wye Wye Transformer

[stickelec]

The application is a 25kv/480 Pad Mounted Transformer feeding a MCC (w/ Main Breaker) approximately 150 ft away. There is no Secondary OCD at the Transformer. The XO is connected to a High Resistance Ground (Post Glover) in the MCC. The Transformer HO is connected to both the POCO ground and the Plant grounding grid.

The question is: should the POCO grounding conductor be connected to the Plant grounding grid or should the two be isolated from each other? I've studied for months to find the answer and still don't have a clear understanding. Thanks

 

[Hv&Lv]

The application is a 25kv/480 Pad Mounted Transformer feeding a MCC (w/ Main Breaker) approximately 150 ft away. There is no Secondary OCD at the Transformer. The XO is connected to a High Resistance Ground (Post Glover) in the MCC. The Transformer HO is connected to both the POCO ground and the Plant grounding grid.

The question is: should the POCO grounding conductor be connected to the Plant grounding grid or should the two be isolated from each other? I've studied for months to find the answer and still don't have a clear understanding. Thanks

Normally pad mounted transformers have "bay-o-net" fuses installed from the manufacturer on the secondary side. think of the ground connections if this were an overhead application.

 

[jim dungar]

HO and XO should not be bonded 'before' (i.e. in the transformer tank) the neutral grounding resistor. Other than that I see nothing overtly wrong with HO being bonded to the plant grounding grid.

 

[Hv&Lv]

HO and XO should not be bonded 'before' (i.e. in the transformer tank) the neutral grounding resistor. Other than that I see nothing overtly wrong with HO being bonded to the plant grounding grid.

Normally pad mount transformers have a common XO/HO bushing. (I hate stating "normally" all the time, but there are always an exception or two...)

 

[stickelec]

HO and XO should not be bonded 'before' (i.e. in the transformer tank) the neutral grounding resistor. Other than that I see nothing overtly wrong with HO being bonded to the plant grounding grid.

Jim, does that mean its okay either way? Apparently its not a code issue. I'm trying to weigh out the advantages/disadvantages.

In this thread Hv&Lv mentions a "common HO/XO Bushing". In that instance, what happens if the POCO uses individual cutout-fuses and only one is closed? Doesn't the Primary voltage cross-over to the Secondary Wye?

 

[Hv&Lv]

Jim, does that mean its okay either way? Apparently its not a code issue. I'm trying to weigh out the advantages/disadvantages.

In this thread Hv&Lv mentions a "common HO/XO Bushing". In that instance, what happens if the POCO uses individual cutout-fuses and only one is closed? Doesn't the Primary voltage cross-over to the Secondary Wye?

You may want to check to see if there is an externally operated HO disconnect on this XF. SOME manufacturers offer this option as standard.
You will have to ask the POCO, because by externally I mean it accessible in the primary side of the XF.(POCO only)

 

[jim dungar]

If there is a H0-X0 connection, then your grounding resistor will effectively try to change the utility primary grid into a high resistance ground system - you will have all sorts of problems. The last install we had like this, required the transformer tank to be opened and the H0-X0 bond removed (the utility supplied the wrong transformer)

Opening fuses one-at-a-time causes problems with ferro-ressonance. The primary voltage does not "cross over to the secondary wye".

 

[stickelec]

If there is a H0-X0 connection, then your grounding resistor will effectively try to change the utility primary grid into a high resistance ground system - you will have all sorts of problems. The last install we had like this, required the transformer tank to be opened and the H0-X0 bond removed (the utility supplied the wrong transformer)

Opening fuses one-at-a-time causes problems with ferro-ressonance. The primary voltage does not "cross over to the secondary wye".

If HO and XO are connected via the common POCO and Plant grounding systems - couldn't that result in the issue you mentioned? I was on one job where the POCO did supply a Transformer with an internal bond, and when they closed the first cutout it blew-up the MCC 2000 amp Molded Case CB, blew-out the 600v cabling in the Cable Tray, and blew-holes in the sides of the Cable Tray. There engineer admitted it was due to the internal bond - they removed it during the Transformer repair.

 

[jim dungar]

If HO and XO are connected via the common POCO and Plant grounding systems...

I would guess that close to 100% of our local POCOs supplied transformers do have a H0-X0 connection.
This is why putting HRG systems on POCO service entrances is not something I usually recommend.

 

[stickelec]

I would guess that close to 100% of our local POCOs supplied transformers do have a H0-X0 connection.
This is why putting HRG systems on POCO service entrances is not something I usually recommend.

The HRG was put on the Secondary to lower the arc-flash hazard (we don't use the 277v). After the experience I mentioned, I'm a bit gun-shy when I see HO and XO bonded for any reason. It may be fine as long as the entire grounding system integrity is maintained - but as soon as a ditcher cuts it, all bets are off. I guess my question comes down to: is it safe to not bond HO and XO in an industrial setting? I really appreciate the help. Thanks

 

[jim dungar]

HRG systems do not lower the available Arc Flash Incident Energy, they nly reduced the probability of an L-G fault propagating into a three phase fault.

Bonding of H0 and X0 is not a problem on solidly grounded systems, as I said this is by far the most common POCO service available.

The issue was, you installed a resistor at X0 and then to ground, as result H0 and X0 are no longer the same point in relation to the ground grid and therefore could not be connected without creating problems.

 

[stickelec]

HRG systems do not lower the available Arc Flash Incident Energy, they nly reduced the probability of an L-G fault propagating into a three phase fault.

Bonding of H0 and X0 is not a problem on solidly grounded systems, as I said this is by far the most common POCO service available.

The issue was, you installed a resistor at X0 and then to ground, as result H0 and X0 are no longer the same point in relation to the ground grid and therefore could not be connected without creating problems.

I'm really confused. In a powerpoint presentation by an engineer referencing Post Glover, he states that a disadvantage of a solidly-grounded system is "severe flash hazard". He also states that an advantage of an HRG is "no flash hazard". Is that just a sales-pitch without sound engineering? I have numerous other papers having similar statements. I am not trying to be argumentative - I just don't understand. It just seems like 5 amps between L-G is much less damaging than 20,000 amps.

 

[jim dungar]

I'm really confused. In a powerpoint presentation by an engineer referencing Post Glover, he states that a disadvantage of a solidly-grounded system is "severe flash hazard". He also states that an advantage of an HRG is "no flash hazard". Is that just a sales-pitch without sound engineering? I have numerous other papers having similar statements. I am not trying to be argumentative - I just don't understand. It just seems like 5 amps between L-G is much less damaging than 20,000 amps.

It is primarily a sales pitch.
Almost all of their literature talks about faults to ground, there is hardly anything about faults L-L.

A HRG system will limit the current of the first L-G fault only, it does nothing for the second fault.
Assume you are opening a cover on a panel. You hear a nut fall off and go bouncing around inside of your equipment. It shorts line to ground, and the HRG does what it is supposed to do limit the 'arc'. However say the nut falls L-L, the HRG will do nothing and you will be exposed to the full available AFIE. Or say that you have a motor 'go to ground' and again the HRG does what it is intended to do, limit the ground current, and your facility continues to run until 'it is convenient for a shut down', now some electrician is making a voltage measure ment on and their probe slips, again yu have a L-L fault and the HRG will do nothing to reduce the AFIE.

Don't get me wrong, I do recommend HRG systems as one method to reduce the likely hood of an arc flash occuring, its just when one happens you better be wearing the correct PPE.

 

[stickelec]

I agree 100% the HRG does not lower the AFC but as you mention it does lower the L-G fault current (which is the most common type of fault) and it can reduce the chance of a L-G propagating to a L-L. L-L faults remain as damaging as ever. Its just a small step toward reducing one aspect of a known hazard.

Before we get too far away - I would really like to understand the good/bad of bringing the POCO ground into the Plant ground. It especially seems bad bringing their static-line into the Plant.

My second question relates to your earlier comment that the Primary won't cross-over to the Secondary (bonded HO-XO). If the Secondary OCD is open and one leg of the Primary is closed, why won't the one Primary leg energize all three legs on both Primary and Secondary? Probably a dumb question but I've chewed on this for years since I witnessed the incident I mentioned. Thanks

 

[jim dungar]

My second question relates to your earlier comment that the Primary won't cross-over to the Secondary (bonded HO-XO). If the Secondary OCD is open and one leg of the Primary is closed, why won't the one Primary leg energize all three legs on both Primary and Secondary? Probably a dumb question but I've chewed on this for years since I witnessed the incident I mentioned. Thanks

Yes, if you have a single transformer core assembly with one winding energized (L-G), the flux in the core could cause voltages to appear across all of the other windings. However, the primary voltage stays on the primary, and the secondary voltage stays on the secondary. This is one of the reasons at least one POCO in this area uses Tri-plex cores for all of their wye-wye padmount transformers.

 

[stickelec]

Yes, if you have a single transformer core assembly with one winding energized (L-G), the flux in the core could cause voltages to appear across all of the other windings. However, the primary voltage stays on the primary, and the secondary voltage stays on the secondary. This is one of the reasons at least one POCO in this area uses Tri-plex cores for all of their wye-wye padmount transformers.

Thanks Jim. With that said, how do you suppose the incident I mentioned occurred? The Secondary obviously saw Primary - that's the only thing that would explain the vast damage in the MCC building.

Both the POCO engineer and the Transformer Mfg engineer agreed it was the internal HO-XO bond... and apparently the way the common-bushing was (or was not) properly grounded (whatever that means).

The POCO paid to replace our gear and cabling. The Mfg took the Transformer back to the factory to check the condition of the Secondary (I assumed because it was stressed in the ordeal). Thanks

 

[Hv&Lv]

Yes, if you have a single transformer core assembly with one winding energized (L-G), the flux in the core could cause voltages to appear across all of the other windings. However, the primary voltage stays on the primary, and the secondary voltage stays on the secondary. This is one of the reasons at least one POCO in this area uses Tri-plex cores for all of their wye-wye padmount transformers.

Just thought I would add this pic...
It's the inside of a three phase wye-wye pmt.

 

[jim dungar]

Thanks Jim. With that said, how do you suppose the incident I mentioned occurred? The Secondary obviously saw Primary - that's the only thing that would explain the vast damage in the MCC building.

Both the POCO engineer and the Transformer Mfg engineer agreed it was the internal HO-XO bond... and apparently the way the common-bushing was (or was not) properly grounded (whatever that means).

The POCO paid to replace our gear and cabling. The Mfg took the Transformer back to the factory to check the condition of the Secondary (I assumed because it was stressed in the ordeal). Thanks

It was not just the H0-X0 bond, it was the addition of the HRG on your side.

 

[stickelec]

It was not just the H0-X0 bond, it was the addition of the HRG on your side.

Wow! Please explain - I sure don't understand now.

 

[jim dungar]

The installation I was asked to look at was experiencing voltages of about 1000V (2X nominal 480) as their HRG system was being commissioned (primarily as the resistor was removed from the circuit and when the pulsing circuit was tested). As we started to conduct a full investigation and analysis (focusing on zero sequence currents), it was discovered there were 'standard' internal H0-X0 connections at the two POCO service transformers. At this point the analysis was stopped, and the internal bond was removed. The result was the over voltage situation disappeared and the facility has operating correctly for the past 4 years.