230/115 kV power transformer neutral terminations

[Power0020]

a 230/115 kV power transformer has two neutral points brought out through oil/oil bushing similar to phase connections.

The cable compartment interface is similar to phase ones though the neutral points will be solidly earthed and no HV is supposed to appear on these points.

To save some money, Utility proposes to use "dummy" terminations just for oil sealing, or to ask the transformer manufacturer to modify the compartment base plate to contain a LV cable connection (oil-sealing gland?) enabling to terminate a 240 sq.mm cu LV grounding cable.

I have not been dealing with this type of connections before, any clue?

 

[FPelec]

We generally use LV cables for grounding the neutral of ATRs and shunt reactors; however according to IEC 60076-3, the neutral bushing should have an highest voltage for equipment (Um) equal or higher than 17.5 kV, allowing for applied voltage testing at 38 kV or above.
As a consequence we use standard MV (oil to air) bushings for neutral, and connect a couple of LV cables from neutral bushing terminal to the ground (see attached imagage).


Si duri puer ingeni videtur,
preconem facias vel architectum.

 

[Power0020]

Thanks FPelec.

I am asking about the connection of oil compartment cable into neutral with a LV cable instead of HV cable.(though the neutral is solidly earthed)

Sketch attached.

 

[bacon4life]

During transformer insulation testing, you need the ability to lift the neutral connection to ground and apply 10 kV test voltage to the winding. We have medium voltage bushings on all of our 230/115 kV transformers.

 

[prc]

Normally such neutral ends are taken out at the top cover through plain oil to air bushings and connected to station earth grid through copper strips or galvanized steel trips over pin insulators on tank side. It cannot be taken out through plain cable glad as at the time of testing at factory over voltages will appear at this point and suitable bushing to cater this voltage level(min 28kV AC) is to be provided.

 

[davidbeach]

All of ours have medium voltage bushing with a bare ground conductor connected to the pad at the top of the bushing. It's necessary to be able to disconnect as mentioned above, so you can't have a direct cable connection through a gland in the transformer housing.

 

[Power0020]

I agree about that a MV bushing or terminal should be provided. However, the manufacutrer provided a 115 kV rated bushing, cable compartment and associated cover plate. Don't really know why we have to provide a 115 kV plug termination for a neutral point?

 

[bacon4life]

Did your specification call for the ability to run the transformer with a floating neutral? One nearby utility purchases 115/12/12 kV ungrounded wye/delta/grounded wye transformers. On these transformers, the 115 kV winding is fully insulated with a 115 kV neutral bushing.

 

[Power0020]

No, spec ask for solidly earthed

115 kV ungrounded wye? Never saw that.

 

[Mbrooke]

@Power0020, they do make wye ungrounded, delta, wye grounded transformers for a zero degree phase shift. Although I am more accustomed to seeing delta zig-zag where a zero degree phase shift is needed with a symmetrical neutral reference.

 

[Power0020]

@Mbrooke, I wonder about running on 115 kV ungrounded, faults will cause very high overvoltages on healthy phases.

 

[FPelec]

Power0020,
the most effective solution is to ask to the transformer manufacturer to modify the neutral bushing, installing a MV oil-to-air bushing. Which kind of neutral bushing was required in the spec?
By the way 110 (or 115) kV sub-transmission networks in central Europe (typically Austria) often used resonant neutral grounding, so that the neutral is always fully insulated.


Si duri puer ingeni videtur,
preconem facias vel architectum.

 

[prc]

If you are sure that system is solidly grounded, you can use LV cable for earthing the neutral cable pot head. Probably 110 kV neutral take out is provided to take care of over voltages during factory testing.