LA's for Delta Xfmr

[tctctraining]

I read somewhere before that LA size for a transformer if its delta with same voltage level would be higher than wye configuration by the factor of 1.73.
in my case I need to specify LAs for a 400 Y/34500V delta transformer on delta side. which feeds a cable and evenually ends up to hydro poles.
I appreciate anyfeed back on this.

Thanks

 

[jghrist]

The voltage to ground that the arresters are exposed to depends on the grounding of the 34.5 kV system. If the system is alway a solidly multigrounded 34.5/19.9 kV system, then this would govern the arrester rating. If this is a generator step-up transformer, and the hydro system can be left without a grounding source (for instance if a feeder breaker trips at a source substation), then full line-to-line voltage must be used.

 

[tctctraining]

This is for generator step-up transformer, so according to Canadian electrical code book arrester for 44KV must be used!

 

[tctctraining]

Maximum Elbow type LA I can get is 24.4 MCOV for 34.5 KV system. Both Elastimold and Hubble only offer this type and size..
Would this be sufficient for ungrounded Delta side of a transformer?
according to "jghrist" it needs to be atleast 34.5 MCOV!!!

 

[Kiribanda]

tctctraining,

Since there is a possibility of operating the 34.5 kV system ungrounded even for some duration, then the MCOV of the arresters should be either equal to 34.5 kV or higher. (If there is no possiblity at all then you can use arresters with MCOV = 24.4 kV)

Since there are no arresters with MCOV=34.5 kV or higher for nominal system voltage of 34.5 kV, you should go to the next rating which is

1)Arrester rated voltage = 48 kV
2)MCOV=39 kV

which is normally suitable for a system with nominal system voltage 46 kV. It will be little expensive.

 

[jghrist]

If the system can be operated ungrounded, then separable connectors cannot be used. I don't think there are any available that are rated for an ungrounded 34.5 kV system.

 

[tctctraining]

Maximum rating for bushing type La is for 35Kv class: 24.4 KV

http://www-public.tnb.com/util/docs/PC_SA_full.pdf

How can a LA be exposed to line voltage on delta config?

 

[davidbeach]

A ground fault on one phase of a system that is not effectively grounded will cause the voltage to ground on the two unfaulted phase to approach line-line voltage.

 

[jghrist]

The problem is not the delta configuration of the transformer winding. The problem is the lack of a ground source on the 34.5 kV system.