Earthing the Neutrals of Y-Y Power Transformers 1

[ilangineer]

I have the following scenario (in South Africa): a utility power transformer (132/33 kV) is star/delta with an NECRT on the secondary side (limiting to 300 A). I must install a star/star transformer downstream for 33/11 kV supply to a load. What must I do with the two neutral points of my transformer?

I initially want to put an NER connected to the secondary star point and leave the primary star point open...

What are the opinions out there?

 

[inpran]

I agree with your plan mostly. However, I have the following questions.
1. What is the need of a NER at the 11 kV end?
2. Is it not possbile for you to use a detla-star transformer in stead of a star-star? Cause the phase shift casued by the upstream transformer will not getting nullified at the load end with the use of a star-star transformer.

 

[ilangineer]

Thanks for the quick feedback. The NER at the 11 kV side is to limit the earth fault to 300 A on the 11 kV network.

We must use a star/star to avoid any phase displacement, since there is a possibility that other transformers can be swithced in parallel with this one.

 

[Marmite]

It sounds very similar to a typical UK network. The 11kV star point should be grounded solidly or via a resistor. The 33kV star point can be left floating,or you may choose to install a neutral VT which would give you the possibility of fitting neutral displacement protection which would be particularly useful if your substation is fed via and overhead line and there is a possibility of other transformers operating in parallel with it.
Regards
Marmite

 

[pwrtran]

33kV winding - neutral should be solid ground, no floating. Otherwise you may get voltage unbalance on the 11kV side

11kV winding - grounded via a reactor or a resistor if you do want to limit the L-G fault.

 

[Marmite]

Yes, if there are phase-neutral connected 11kV loads then I agree with pwrtran, the 33kV neutral should be grounded. If they are phase-phase only, then no need to ground the 33kV neutral.
Regards
Marmite

 

[odlanor]

I am sorry for my stupid question:
.....a utility power transformer (132/33 kV) is star/delta with an NECRT on the secondary side (limiting to 300 A). ....
What is NECRT?
I guess,33kV delta is the secondary. It is not?

 

[Marmite]

Odlanor, it's not an acronym I'm familiar with either. I took it to mean earthing transformer. Perhaps neutral earthing current reducing transformer?
Regards
Marmite

 

[prc]

May be stupid question- When the 33 Kv star is solidly grounded, then also earthing transformer is required ?

 

[odlanor]

Ilanginee,prc
A scenario (in Brazil): a utility power transformer (138/69 kV) is star(solid grounded)/delta with zig-zag transformer on the secondary side (limiting to (3< X0/X1 < 10). downstream transformer will 69/13,8kV delta/star solidly grounded).