Small Generators - Large Transformer Inrush 1

[living2learn]

I was hoping I could get some help on this subject.

I am designing a new 480V high impedance, 2500kVA generator paralleling generator system that connects 2 generators to a 12kV bus via seperate 2500kVA Delta/wye-low impedance grounded transformers. The 12kV system feeds 10-1000kVA D-Y transformers.

I am concerned about transformer inrush. I have requested the nameplate information for each transformer - assuming 12x for now.

My approach.
I figured out how much current the generators can put out from the x''d (.1 pu) which is roughly 30kA per generator. I am assuming the transformers will not be energized until both generators are connected to the bus at full voltage. I added the GSU xfmr and reflected the generators on the 12kV side and the available current after the GSU transformer is 750A for a total of 1500A.
I know that the transformers will want 577A (12kV) of inrush current (assuming 12x fla - waiting on info) 1000kVA/(12kV*1.73) * 12 =577A per xfmr * 10 = 5773A for all 10 transformers.
I know the transformers will still get energized even though they don't get teh full current, but will take additional time.

I am kinda stuck here...I am pretty sure the generators control system, excitation will play a huge role in this model, also the 480V gen braker, generator thermal damage curve, MV SEL-751A relays will also have to be programmed to accomadate the inrush currents.

Any additional documents, rules of thumbs, etc... would be greatly appreciated. I am working on getting ATP to try and build this model, but any other insights would be great.

 

[QBplanner]

first, I don't know where you get this 12x from. second I don't really understand your problem.
the sequence I understand based on my limited knowledge is you will energize the transformer from grid side first and hoop up your generator later.
I am not sure I understand your problem.

 

[waross]

Or connect the transformer to the generator and then start the generator. It depends on where you will close when synchronizing.

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

 

[itsmoked]

12X could actually reach about 20x too.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[QBplanner]

20x yes everything is possible.
There is an IEEE paper talks about the guidance of estimateing Trx inrush. I recall it is typically between 4~6 times. with POW it can be further reduced.

 

[ters]

Would a shorter version of this question be:

There is no grid. Two 2.5MW 480V generators connect to a 12kV bus via step-up transformers and can be synchronized to each other. The 12kV bus feeds ten 1000kVA transformers which for some reason cannot be staggered but have to be energized all at once. The question is: can the two generators survive inrush without tripping?

If the is the question, then I suppose it will also depend on what the prime mover is (suppose a diesel?)

 

[davidbeach]

The magnitude of inrush does, in part, depend on source impedance. The 12x or 20x values are based on a "stiff" system. One a weak system the magnitude will be lower but the duration will be longer. But if you can't separate the transformers, can you separate the other side of the transformer? If so, have all the transformers connected to the first unit to be started and let it bring them all up from zero to nominal voltage; no inrush, then synch the other unit.

 

[living2learn]

Thanks for everyones responses, I was trying to add as much detail as possible (devils in the details). Anyways, not my design just had a friend request if it would work, so I thought I would challange myself and see what I can find and learn.
Starting the gens like davidbeach and waross suggested would work great, but not allowed. (not my design)

QBPlanner - No grid. Black start. I say 12X because the last transformer of this size, built by Cooper was 12X. "Electrical Transmission and Distribution Reference Book -1950 westinghouse" suggest 5-8 for that size, but that book is also 60 years old and new DOE requirements for transformers have come out in recent years.

I understand the stiffness of the source depends on how much current the transformer will recieve during inrush. Is there any equations for estimating the duration and magnitude of current?

for any transformer the transient current depneds upon teh magnituede of teh supply volate at the instant the xfmr is energized. I think first in need to figure out the flux in the magnetic circuit and try to get the current from there...

 

[QBplanner]

If you black start generator first and energize the transformer, technically, I don't see any major issues. As long as the generator runs no load, all the generator can see is a fault like event happenes at the generator terminal. With no real power output, all your can get is a big swing. I would hardly believe the generator will be out of synchronisum. Is that what you worry about? Or maybe I misunderstood you.

 

[ters]

I have recently been involved in the commissioning of a 3MW diesel generator which we tested un inrush using a 2MVA transformer (a larger one was not available). Your case will be more challenging than this, but in our case we had no issues except we had problems with differential protection - had to set the restrained element higher and introduce some more time delay to avoid nuisance trips. But this was partly also due to CTs not hot having the same characteristics on both sides.

 

[MCF59s]

Living2Learn, I'm in a similar situation on a current project. In my case, we have (2) 2500kW/3125kV DGs at 4160V, paralleled on a common bus. There is about 18,500kVA of installed substation TX capacity on the bus (2.96x DG capacity). I understand the transformers will eventually be fully energized (though it may take longer than normal) and the maximum inrush factor on each one will be reduced because the gen plant is not as stiff a source as the grid.

I wanted to know how much of a voltage sag to expect and how long the plant would take to recover (which is a function of the AVR) so I reached out to DG manufacturers to model the system at start-up but didn't get much help. In a similar project, a colleague of mine was successful in field measuring a 50V drop on a 4160V gen bus after unloaded transformers were connected. Voltage recovery took about 0.8s. In his case, there was about 10,500kVA of connected TX capacity on a 1500kW/1875kVA single DG system. This is 5.6x the DG capacity. I don't know that this is an adequate rule of thumb but it should give you a higher level of confidence in your design since you're at 2.0x TX to DG capacity (10,000kVA Transformers to 5,000kVA Gens). One caveat is that this all depends on how your AVRs are programmed to operate.

Now, about the TX inrush factor, I recently got confirmation from Square D about inrush factors on their MV TXs:
VPI Types
9.0x - 1000kVA, 2400V through 13,800V
8.7x - 1500kVA and larger, 2400V through 13,800V

Cast Coil Types
10.3x to 10.5x - All Voltages
9.2x to 9.6x - All Voltages

Still no solution yet but I'm checking out the Transient Stability Module on ETAP.

 

[dpc]

For dry type transformers, inrush can be pretty high. I have had nuisance trips with instantaneous trip set at 10X FLA for a 5 MVA cast-coil unit. We had to increase it to 13X. But as David Beach said, the inrush will be limited by the source impedance of the generators.

 

[living2learn]

Thanks everybody for your comments.

MCF59s: thanks for your feedback. We are hitting the switch on this installation in a few months. I modled this using EMTP-ATP, but not to sure about the results because a lot of the really important system parameters are not currently known. I do feel a little more confident now...

Never thought of expressing it as DG vs TX capacity. great thought.

I am installing some nice fluke meters 1750's during the commissioning of this generator job and will upload the results. Should be fun...