wind turbine/ transformer circut breakers

[pittinvt]

Hello,
This is my first post.
I am trying to integrate a wind turbine into a diesel genset power system.
The turbine is 480v/285a induction machine. So I have a 400a breaker for it
(285*1.25=356).
Downstream of the turbine is a 300kva 480/208 dry type xfmer.
As I understand the NEC, Table 450.3(B) says that I have to have overcurrent protection for the primary of the xfmer. I have sized this to be 500A (300000/831=361*1.25=451).
My questions is this: I have a 400a breaker on one end and then a 500a on the other end. This just doesn't seem right.
Can anyone help?
Thanks very much!

 

[augie47]

Keep in mind Art 450 is for transformer protection and Art 240 for conductor protection so your overcurrent protection will have as much to do with the conductor size as the transformer.
The 125% on the transformer primary is based on no secondary protection. For proper secondary protection, the primary can be as high as 250%.
The only problem I can visualize with your arrangement would be the transformer inrush current might cause the 400 amp breaker to trip, but I don't see a scenario where you could increase the size unless allowed by the generator manufacturer and even then you would need to size the conductors accordingly.
If the breakers don't trip, I see no problem.
As mentioned, unless we are looking at separate structures or other mitigating circumstances you don't need the 500 amp breaker.

 

[pittinvt]

Thanks!
I'm sorry, but why again wouldn't I need primary xfmer protection?
Art 450 says that if it is above 9 amps I need primary protection.
Also, is there anything in the NEC about bi-directional power flow? I found something dealing with electric vehicles, but nothing for wind (Art 690.9 (B) does mention that power xfmers that have sources on both sides needs protection on both the primary and the secondary. I realize that 690.9 deals with photovoltaics).
I have been told by someone with past expereince dealing with wind-diesel integration that no one uses overcurrent protection for the xfmers off of wind turbines. That doesn't seem right to me.
Thanks again for your time and sage advice.

 

[Cold Fusion]

... I am trying to integrate a wind turbine into a diesel genset power system. ...

First: The NEC is a terrible design guide. Especially if you are doing anyting other than cookie cutter projects.

A couple of questions:
Is your generator significantly larger than the wind turbine?

Is the generator 208V?

Does the system interconnect with a utility?

When you say the turbine is an "induction machine, would that be an "asynchronous induction alternator" or maybe even a "Double Fed Induction Alternator"? Or is it a synchronous alternator? Or a DC machine feeding an inverter? Or something entirely different?

Do you have the wind turbine generator impedances?
*******************
You are dealing with coordination/synchronization issues.. Given the limited information we have, I'd start with the control scheme/coordination, then synchronization, then worry about meeting the NEC legal issues.

Augie - Don't snap at me (that would be a pun) for putting the NEC last. If it doesn't work, there is no need to worry about the NEC

cf

 

[augie47]

Thanks!
I'm sorry, but why again wouldn't I need primary xfmer protection?

You do need primary protection. I was simply stating, barring other unknown factors, you generator 400 amp breaker could be your transformer protection

First: The NEC is a terrible design guide. ....................
Auggie - Don't snap at me (that would be a pun) for putting the NEC last. If it doesn't work, there is no need to worry about the NEC

cf

never at you..too much respect
and I agree, NEC is not a design tool.

 

[skeshesh]

As the guys brought up there are multiple issues involved - in my humble opinion the xfmr inrush is the more immediate and greatest contributor (although its obviously impossible to be certain without more details). If you know the make, type, rating of the xfmr you can find the in-rush curves online or call the mfg if they dont have them posted and compare the graphs with the time-current characteristics of the circuit breaker you will use to see if its coordinated.

 

[Cold Fusion]

...the xfmr inrush is the more immediate and greatest contributor ...

No question that you could well be right. However, there are a couple of intertied issues:

1. If the gen is an induction alternator, it won't self start (unless there is something weird going on that I don't know about). It will have to be tied to the gen to start. That means, the transformer will have to be backfed energized. Unless, of course, it isn't hooked up the way I think it is. Which could well be cause the OP sounded like the gen was maybe 208 - and that is a bit surprising.

2. If the Wind Turbine Gen does have to direct energize the transformer, it may well be soft enough that the voltage will sag, limit the current, and eventually stagger back to it's feet upon energizing the transformer. That is why I asked about the wind gen impedances.

... compare the graphs with the time-current characteristics of the circuit breaker you will use to see if its coordinated.

3. Then there is the issue of coordination. As you suggested, get (or estimate) the inrush curves. There are CB trip units available that will allow the transformer energization to be under the trip curve. It might be a bit tricky, but generally one can coordinate anything (with enough money

cf