Transformer connection

[Sungate]

We have Wind generators connected to 500V/36kv wye/wye step up transformers, grounded on generator side and ungrounded on MV side. Due unbalanced LV side phase-neutral voltages was decided to install additional transformer to compensate harmonics and to balance the voltage. This transformer is 10A, 500v/500v wye grounded/delta shuld be connected by short but oversized cables (250mcm) to withstand fault current is 25ka in generator circuit, protected by line MV fuse.
I am doubt to use these cables for 10A tranny, that connected by 10A/25A breaker to the WTG bus.
Does anyone meet that kind of problem?

 

[7anoter4]

If the cable is connected between generator busbar and the breaker you need 250 MCM copper. This is required in order to avoid overheating during the short circuit process till Generator Breaker will trip [as this damaged cable may produce busbar damage in turn].
If the cable would be XLPE insulated it could withstand 250 Cdegrees[482 Fdegrees] for a short time.
If the tripping time of the generator breaker will be 1 sec. then 250 MCM copper will reach 482 Fdegrees in this time.
The cable connecting the breaker to the transformer will be thinner being protected by this breaker in less than 0.1 sec. But will still required 125 MCM to withstand the short circuit.
As this cable is less important you may reduce the conductor cross section area to the minimum required for
10 A [it depends upon lay-out way: free air, conduit, tray, underground and so on].
Regards

 

[Sungate]

Thanks.
So we need 125mcm for cable for connecting the breaker to the transformer because it should withstand 0.1 sec to fault current before the breaker trip.

But transformer winding itself made from small wires, as I assume 12 awg as well, and 10kva transformer has 100kva capacity for 5 sec - for load current. If we use only 12 awg conductors in downstream circuit, this cable could get burn during this time (0.1 sec) or transformer burn first if we will enlarge cable cross section to 125mcm, especcialy in case of breaker fault, before line fuse trip the circuit.
I do not know transformer cabability to withstand to the fault current, but appreciate if you can clarify how can we keep transformer protected in the worst case.
Thanks

 

[7anoter4]

First of all the 482 Fdegrees is not the fuse temperature of the copper which is 1084 Cdegrees [1983 Fdegrees] but is the maximum temperature that the XLPE insulation withstands and after then may be in use again.
Second, the short circuit current of 25 KA is calculated for a metallic short circuit and usually does not occur.
The transformer also, has its own impedance which could reduce dramatically the current if the short circuit is some where in the windings and not on the primary terminals.
But I think that a current-limiting fuse would solve the problem, almost entirely. You may use also 6 AWG conductor in order to be always on the safety side.
Instead [on series with the breaker] you may provide a current-limiting fuse that is designed
to open at one-half cycle or less of it's rated “peak let-through current.” See:

http://www.netaworld.org/files/ItemFileA190.pdf

Regards

 

[apowerengr]

Y-gnd to Y???? It sounds like you need to ground the MV Y connection to correct your imbalanced voltage issues.

 

[Sungate]

It was not possible due the resonance/harmonics in ungrounded MV system. MV Y grounding of all transformers in parallel circuit has dropped total system-ground impedance and provided path to ground of harmonic currents, that result overvoltages and protection misfunction. We still looking to put cap banks on, to investigate if it can reduce a resonance issue.