Small Generators - Large Transformer Inrush
Small Generators - Large Transformer Inrush
(OP)
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.
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.






RE: Small Generators - Large Transformer Inrush
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.
RE: Small Generators - Large Transformer Inrush
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Small Generators - Large Transformer Inrush
Keith Cress
kcress - http://www.flaminsystems.com
RE: Small Generators - Large Transformer Inrush
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.
RE: Small Generators - Large Transformer Inrush
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?)
RE: Small Generators - Large Transformer Inrush
RE: Small Generators - Large Transformer Inrush
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...
RE: Small Generators - Large Transformer Inrush
RE: Small Generators - Large Transformer Inrush
RE: Small Generators - Large Transformer Inrush
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.
RE: Small Generators - Large Transformer Inrush
RE: Small Generators - Large Transformer Inrush
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...