Transformer Neutrals 1

[WrightMA]

Can anyone provide a reference/starting point in the NEC to the tying of (2) identically sized transformers neutrals together on the secondary. These transformers distribute power through independent panels located in one enclosure and power common IT equipment. I have researched and don’t see a direct correlation in the NEC.

 

[rbulsara]

What "correlation" are you seeking? In general all transformer neutrals are required to be grounded and when they are all grounded to the same grounding electrodes (building steel) in a building, they become interconnected that way.

Key code requirement is not to ground the neutral of a service or a separately derived system at more than one point and no where downstream of its main disconnect. (You need to search for this on you own). Also avoid current through grounding path during normal operation.

If the transformers are paralleled and there is a ground fault protection on its secondary mains, you need to watch out for proper GF current sensing. It has more to do with engineering than the Code.

Perhaps, you can clarify your system/concern with more detail.

Rafiq Bulsara

http://www.srengineersct.com

 

[WrightMA]

Consider (2) transformers, fed from (2) independent UPS systems. The distribution of these (2) individual transformers is accomplished through (2)-42 circuit panels. These panels feed IT equipment with A/B (redundant) power supplies. (A) power supply is tied a panel distributing power from one transformer and (B) power supply is tied a panel distributing power from the other transformer. These panels are located in a common enclosure, the power phases (A-B-C buses) of these panels are segregated but the panels share a common neutral bus that is isolated and not tied to ground.

I do understand the engineering impacts ie. neutral currents due to phase imbalance, and necessity for like characteristics for the transformers should their load become entirely shared (paralleled).

I was curious if the NEC prohibited the transformer neutrals from be tied together on a common neutral bus.

I have read and understand the Key code requirement you mention above.

Hope this clarifies my previous post..

 

[dpc]

Agree with Rafiq. Check the NEC section on separately-derived systems, which is what transformers are. I think most of your answers will be there. The neutral from each transformer should be kept electrically separate from other neutrals, except for bonding to ground per NEC requirements.

In general, the transformers can't/shouldn't be operated in parallel.

 

[WrightMA]

dpc:

In theory - Two transformers with identical voltage ratings will be connected in parallel to serve a load. The following rating & impedance data are provided:

Transformer 1: 1000kVA, Z = 4.5%
Transformer 2: 2000kVA, Z = 6.0%

The maximum load that can be served (kVA) by the bank without overloading either transformer is:

Convert T1 impedance to T2 Base: .045j (2000/1000) = .090j

Then use equation: Load T1/Load T2 = ZT2/ZT1 or (1000kVA*.090j)/.060 = Load T2 = 1500

Load Total-Paralled = Load T1 + Load T2 = 1000kVA + 1500kVA = 2500kVA

Could secondary neutrals be connected to a common point while accomplishing the above? if the connection point is isolated from ground?

 

[davidbeach]

If you are feeding dual corded IT equipment that is dual corded for redundancy rather than for capacity you don't want any type of connection between the two transformers. You don't even want the two sets of phase busses in the same enclosure and you certainly don't want any connection between the neutrals.

 

[dpc]

WrightMA - I understand the theory pretty well. It can be done and will work electrically, but generally is not allowed by the NEC. That is what I was trying to say. It's also generally not a good idea.

Cheers,

Dave

 

[WrightMA]

gentlemen, thanks to all for the input.

Matt

 

[rbulsara]

WrithMA:

You are describing two totally different systems in two different posts!.

Parallel transformer operation is a totally different story has nothing to do with dual bus system you described in the earlier post.

I was curious if the NEC prohibited the transformer neutrals from be tied together on a common neutral bus.[/q]

The answer is Yes. For the reason I mentioned in my second paragraph of my earlier response. Plus there is no reasons to interconnect two separately derived system conductors unless they are being operated in parallel with a proper design.

I have done only data centers in last decade, and I understand your description, however you cannot interconnect the two neutrals in load centers (RDCs)as you describe. Why would you want to do that to begin with?

Isolation to ground at the RDC has no bearing. Both transformers are already grounded at their secondaries.





Rafiq Bulsara

http://www.srengineersct.com

 

[WrightMA]

rbulsara:

My 3rd post was for discussion in response to the post from dpc about paralleling.

Reason for my initial post: I discovered and error in an installation. Not my design but my problem. I was looking for valid reason to avoid systems re-work. Proper engineering dictates re-work will commence tomorrow....

Thanks again for the input.

 

[waross]

Re transformers in parallel. %imp isn't the whole story on transformers in parallel.
% impedance or transformer impedances describe the current under short circuit conditions when the entire circuit is comprised of the resistance and inductive reactance of the transformer.
Similar transformers with equal impedance often (but not always) have the same X/R ratios. If the X/R ratios are different The sum of the transformer currents may be more than the load current. This may mean overloaded transformers.

Bill
--------------------
"Why not the best?"
Jimmy Carter