Step-up transformers

[ReliabilityPF]

I have a configuration of step-up transformer with a 750 KVA generator 400V. The first transformer 400KVA is a D-Y1 4160/400-231V, the second transformer 500KVA, D-Y1 23000/4160-2402V. The main reason for this configuration is for distribution on 23KV to feed different small substation, neutral voltage and terminal are not used. My question is, should the Neutral terminal be grounded for both transformer or only one of them should be solidly grounded?

 

[RRaghunath]

There is confusion with regard to the vector group:
First Step up transformer - Expect Delta to be on generator side (400V) and Star on 4160V side (this makes Vector group to be YNd)
Second Step up transformer - Expect Delta to be on 4160V side and Star on 23000V side (YNd)
The capital letter is always to be used for HV while writing vector group.
If my understanding as above is correct, Neutral of the 4160V winding of the first transformer is to be connected to earth.
Likewise, neutral of 23kV winding of the second transformer also is to be earthed.

The above arrangement ensures that earth reference is available at all voltage levels -
* at Generator neutral for 400V,
* at first transformer for 4160V and
* at second transformer for 23kV.

 

[Kiribanda]

I believe that your 400kVA generator neutral is solidly grounded because it is LV 400V.
Therefore, in my opinion you can have the connection shown in the attachment.
When you do it, 23kV system will be a DELTA system possibly ungrounded depending on the transformers used on your 23kV system.
which you have not given.
I have few questions here.
(a) Your generator is 750kVA whereas your transformers are only 400kVA.
(b) Could your 750kVA generator supply all the reactive power required by the two transformers?

 

[ReliabilityPF]

Hello thanks for your comments, to clarify, neutral of generator solidly grunded, first step-up transformer Dy1 4160/400-231, second step-up transformer Dy1 23000/4160-2402, the output of second transformer feeds an 3,5mille aereal 23KV line that feeds 3 operation transformers Dy1 23000/400-231 where the neutral is grounded. The porpouse of the two step- up transformer is to distribute in 23KV for the 3 operation transformer. 4160V and 23KV are not used for loads. hope is more clear. All loads are feed with 400V.

 

[ReliabilityPF]

 

[Kiribanda]

After looking into your SLD, I donot support the above connection because your 23kV line is overhead and 3.5miles long.
Operating such an overhead line ungrounded is not good specially looking from its GF detection & tripping.
Therefore, my advice is to purchase a 1MVA/ 400V-23kV/ Dyn1 step-up transformer and connect your overhead line to its 23kV side.
Then any GF can be detected from the 23kV wye side and the line is not DELTA ungrounded now.

 

[davidbeach]

If the transformers don't actually already exist, get YDY transformers. That way you can have a ground source in each direction. But if this is always radial, the generator is the only possible source for the whole system, then absolutely put the delta side on the generator side and the grounded wye side on the load side.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[ReliabilityPF]

Ok it's clear that we will have a high risk for the infrastructure in case of a fault, so can we manage an isolation monitor system or zero sequence relay scheme? Purchasing a new transformer is unprobable.

 

[RRaghunath]

Going by the SLD, your 4160V and 23kV systems are unearthed systems.
You can have Residual Over voltage protection at these two voltage levels and wire it to trip the respective breakers, to protect the two systems against earth faults.
The protection shall be set sensitive and with a 1s delay (just to prevent maloperation during transients).