Multi-source Ground Fault / Modifified Differential Groud Fault

[JBinCA]

Greetings All,

I'm consulting on an 480V 3ph 4W ~1.5 MVA system that powers a building and small data center on a university campus in the U.S.

There are three power sources to this system, each with an independent bonding jumper. Each on a different SWBD. The system wasn't designed that way, but rather had additional sources added over the years. Different eras, different manufacturers, and as some of you might have guessed, no one took into account the bonding at the existing sources when they went to add a new one. Switching is accomplished via 3 pole ATS's that are open transition only. So the bottom line is that I have multiple bonding jumpers in the system at all times.

I am aware of the need for a Modified Differential Ground Fault (MDGF) / Multi-Source Ground Fault (MSGF) scheme to properly resolve neutral currents flowing in the ground path and ground currents flowing in the neutral path. I am also aware that if this system were installed new, all at once, and from the same MFG, it could be accomplished with LV BKR 4 Wire protection, and looping the neutral sensor circuits. All 3 of the main source BKRs have or can be fitted with shunt trips.

I understand how to analyze the problem (and the solution), but I'm not sure what limits there might be on the devices I can use to solve this problem. Can I use a scheme that would employ split-core CTs paralleled into a single phase digital TOC relay with a definite time curve as the protective device? Is there a standard or code reference that would prohibit me from going down this path.

I'm hoping some of you SWGR / SWBD gurus can give me some guidance on that. I've got UL 1053 coming my way, but I have not idea what's in there.

Thanks and Regards,

JB

 

[rbulsara]

You need 4 pole ATSs. The rest is unnecessary complication.

Rafiq Bulsara

http://www.srengineersct.com

 

[JBinCA]

Rafiq,

Thanks for the reply. 4 Pole ATS's would certainly solve the problem.

Three factors have us leaning away 4 pole ATSs.

1. One of the 3 pole ATS's is integrally installed into the Main SWBD. Even if we could find a 4 pole unit to fit into the enclosure, the needed outage time is far beyond what the customer can tolerate.

2. The Data center does not have significant capability to operate on extended outages. Outages will have to be measured in minutes.

3. A Sensor / CT and relay solution should be much cheaper.

I appreciate your input.

JB

 

[rbulsara]

JB:
You seem to know the problem and solution. Your question is more related to physically fitting something in existing gear, I would say you start with asking the OEM of the board and a good contractor.

As for assisting further to deal with this "mess", it falls way beyond the my limits of investing time on a free forum. I am also a consultant like you, after all.

Hope you understand.

Rafiq Bulsara

http://www.srengineersct.com

 

[JBinCA]

Rafiq,

I definitely understand limiting your time investment.

My question is really about what equipment can but used for "Modified Differential Ground Fault (MDGF) / Multi-Source Ground Fault (MSGF)" in 480/277 systems.

It appears that the answer is equipment listed to UL 1053. The best I can determine, the LV GF relaying systems listed to UL 1053 are paired with product specific sensors because they are rated in primary amps. They also have pick-ups limited to 1200A and delays limited to 1 sec except where marked "not suitable for service entrance".

So, if I'm correct, A GE IAC relay, or West C09 with protection class CTs could function in LV applications, but they aren't listed for LV applications and are not allowed under US NEC.

Glad for anyone to agree or shoot holes in that analysis.

Regards,

JB

 

[rbulsara]

I would talk to the OEM of the relays you are interested in.

Rafiq Bulsara

http://www.srengineersct.com

 

[wcarter]

JBinCa,

The 3 pole ATS's can be done. Though I am familiar with only one manufacturer that is capable of this. Though where I am unsure of the time table and how everything is connected, IE power sources, etc, I cannot comment if it is feasible. Also, where I work for this company, and how it is done is proprietary, I cannot give you the simple solution here. Though we would utilize inline CT's that would require breaking of the bus. Also how the grounds are connected would be a concern.

If you would like the company to contact, I can post that if you still have this issue (Due to the fact that I cannot post contact information).

 

[alehman]

Schneider has a document with a number of examples of MDGF configurations. I think there's even a 3-source example. I don't know if this would fit your situation. I'm sure other have similar relays. Talk to your local representative.

Alan
“The engineer's first problem in any design situation is to discover what the problem really is.” Unk.

 

[wcarter]

Schneider has done as many as 8 sources, though I think more have come up recently. With 3 and 4-pole ATS's and such.

What surprises me is they are the leader in MGDF cleanly. At least from what I have seen. I would think others would be in this part of the industry. Other manufacturers have MGDF, but the largest scheme I've seen is 3 sources.