Medium Votlage Tap on secondary of transformer

[living2learn]

I have a 1750kVA, 480D:12000V fuel cell step up transformer that supplies some 12kV switchgear with a main circuit breaker in a supervised location. The 480D primary is protected by a circuit breaker. The switchgear is 10' away from the transformer. I want to tap the line side of that circuit breaker and feed some new switchgear. The new switchgear feed will consist of 40' of 400A wire. This wire will tap the existing medium voltage bussing that is rated for 1200A.

When I read the NEC it mentions 240.101 (B) Feeder Taps. Conductors tapped to a feeder shall be
permitted to be protected by the feeder overcurrent device where that overcurrent device also protects the tap conductor. That tells me that the feeder OCPD must also protect the tap...the nearest OCPD is the transformer primary circuit breaker sized at 2500A.


Q1) 400A 15kV wire tapped with 100A wire must change the relays 51 settings to 100A?
Q2) I am having difficulties interprating NEC 450 becuse my transformer secondary is over 600V. I know the wire will not be damaged by the fuel cell because it cannot produce enough energy to damagage it. Therefore is my 2500A primary transformer protection enough?

 

[collies99]

If I look at this from a load current point of view, rated secondary current at 84A and primary at 2105A based on former rating, Therefore the smallest conductor must be protected, which it is. As long as it is radial from the fuel cell it should pass inspection.

 

[rcw retired EE]

Your description is confusing. Use HV & LV instead of primary and secondary. Do you have:

480V fuel cell feeding delta LV winding of 1750 kVA transformer through a 2500A 480V breaker,
12 kV HV winding connects to 12 kV switchgear with a 1200A bus and breaker, OCP setting unknown,
New tap from 12 kV bus will feed new switchgear by connecting between transformer and exisitng switchgear. Cable rated 400A at 12 kV.

Is that correct?

The 2500A 480V breaker is equivalent to a 100A breaker on 12 kv side, (173A for single phase load and ground fault, assuming a 12 kV Wye winding).

If the fuel cell is the only possible source of power for the 12 kV switchgear, the tap is protected by the 2500A breaker. If either 12 kV switchgear is conneced to another power source, like a utility service, that could possilby feed the new cable then you need to look at protection from the 12 kV side.

 

[1capybara]

NEC Table 450.3A seems to require both primary and secondary protection.

 

[living2learn]

single line diagram.

 

[iceworm]

A couple of curiosity questions"
1. Is the xfm a 480D/12kVY, fed with 330V and the secondary voltage is 8250V?

2. What's with the 1200A secondary (HV side) CB? As mentioned, at 12kV the xfm FLA is 84A.

3. No other connected generation or utility? If so, this is pretty interesting installation.

Now for your questions:
First, as I'm sure you know, the NEC is not a design guide.

My personal opinion is that once you get even slightly out of the norm, the NEC is clueless. The code panel writers don't even to try to cover other than the cookie cutter stuff. My method is to make it safe, operate as intended, and reliable - in that order. The AHJ rarely cares if the install works, they just don't want it to start a fire. And you are out of the norm - you're on your own. You get to show it is safe.

Q2)
NEC 450 is about transformer protection. It does not cover conductor protection. For that you have to go to 240

You are correct, the 450 tables do not cover your installation. Aparently the code panel did not have GSU transformers in mind when they wrote 450.1, "This article covers installation of ALL transformers." However, I would consider the transformer protected by the 125% primary CB.

The secondary conductors are a different problem. According to your one-line, the secondary conductors are not protected at all. For your installation, 240.4.F and 240.21.C.1 specifically say the xfm primary cb does not protect the secondary conductors.

Q1)
The guiding principal for taps is 240.21. Your new concuctors are a tap of a tap - specifically prohibited.

For transformer secondary taps, see 240.21.C. And determine if you are a "Supervised Industrial Installation" 240.92 (See definitions, 240). And maybe the install qualifies for "outside unlimited length" (240.21.C.)

Maybe you could use 240.100, engineering supervision along with the appropriate studies to show all the conductors were protected. Maybe you could say the source is so soft that a 100A load on the secondary sill shut down the inverter.

Here are some of the comments I'd be prepared to answer:
1. Is it 10' or 30' to the first secondary disconnect. 30' is long unless this is an outside unlimited length tap.

2. Where is your secondary conductor protection (the 30' of 500kcmil)? That 1200A CB isn't protecting anything.

2. If the 30' is an outside unlimited length tap, you only get one disconnect. And that tap you want to put in makes two.

3. No. You can't make a tap of a tap.

4. How come you don't connect the new tap to the xfm secondary? (240.21.C)

5. Have you considered a 100A fused secondary disconnect ahead of the switchboard - and the new feeder?


I know I'm not helping much and I appologize for that. But it's the best I can do from my side of your monitor.

ice

Harmless flakes working together can unleash an avalanche of destruction

 

[davidbeach]

It's been a while since I've had to deal with the NEC, but it had (probably still has) a requirement in the 700's that when there are sources on both sides of a transformer then both sides must be considered as both the primary and the secondary. Primary rules apply to both sides and secondary rules apply to both sides. There absolutely has to be something on the high side of the transformer doing transformer protection.