25 MVA transformer protection

[Jay Petersen]

Hello, I am an electrical power engineer with a strong design experience in Low and Medium voltage systems. Basically everything that is governed by the NEC. Presently working on some schematic designs for a fuel cell system that will connect to the grid at 69kv. Eventually there will be a high voltage engineer on this project, but I am working on some of the up-front design.
There will be a new 69kv primary, 13.8kv secondary, 25mva transformer to connect at the utility. Primary config may be wye or delta at utility's final decision. Secondary is wye. I can find typical IEEE protection schemes and understand what should be shown. Question is the physical location of the secondary main breaker. My experience would put the secondary breaker close to the transformer. Conceptual layouts given to me at the start of the project don't show this "local" secondary main. These documents have a piece of switchgear about a 1/4 mile away (collection switchgear for fuel cells) Am I correct in thinking there should be a stand alone main local to the transformer ? Any other thoughts on this are welcome.

JP

 

[7anoter4]

If I well understood the 13.8 kV Switchgear is located 1300 ft. from the transformer.
From the transformer terminals up to this switchgear a connection of-let's say 2*3*500 MCM copper
conductor cable[see for instance Det.8 NEC Table 310.60(C)(85)]will run underground.
A Short-Circuit and Ground-Fault Protection shall be provided as per NEC 240.92(C)(1).
It is recommended to provide a breaker between transformer terminals and cable terminations.
You'll need an overcurrent protection as per NEC 240.100 also.

 

[ElectricRob]

I hang out in the NESC world, so there may be other considerations if the NEC applies to your application, but...

It sounds like the only thing connected to the 13.8 kV side of the transformer will be the fuel cell system. In the event of a fault between the transformer and the 13.8 kV switchgear, your objective is to clear that fault. In this case, it shouldn't matter if the transformer is also de-energized when the fault is cleared.

Assuming that you will have a 69 kV breaker between the utility interconnection and the transformer, I would consider using that 69 kV breaker to protect from secondary faults (install an overcurrent relay that is connected to CT's at the transformer secondary bushings). Installing a relay to trip the 69 kV breaker for a 15 kV fault would be a fraction of the cost of installing a 15 kV breaker to protect a quarter-mile of line.

 

[Jay Petersen]

This is in the NESC world ... which is why I don't know much common practice. Not in a building at all, low side feeder is running down the road in a ductbank. Good idea, I did have a fleeting thought about relaying on the secondary to avoid the extra swgr. JP

 

[stevenal]

I'm assuming you mean MVA rather mVA? Expensive 25 MVA transformers are generally equipped with differential protection. So differential protection operates for an internal transformer fault, and the 69 kV breaker operates. With no 13.8 breaker, the faulted transformer is still energized by the fuel cell. Even if you have protection at the fuel cell end, it will probably not see past the transformer impedance to operate in a timely manner for a 69 kV winding fault. I would go with a 13.8 breaker at the transformer. The other possibility is to send a trip signal by a communication link to the fuel cell main when differential relaying operates.

How much available fault current will a fuel cell provide?

 

[bacon4life]

Rob's suggestion OK if the local utility will go for it. Depend who owns which portion, they might require a low side breaker.

 

[Jay Petersen]

Yes, 25MVA. I have 87t 50/51p,n,q Backup 50/51p,n,q The fuel cells put out 4x their FLA for exactly 1 cycle, then they shut down. So that would seem quick enough.