Transformer H0 connection and BIL rating

[Amex12]

In a 230 kV system, what is the thought process for leaving the H0 of a WYE-WYE transformer ungrounded?

If the HV bushings are rated at 900 kV BIL, would this make the requirement for the H0 bushing 900 kV BIL as well since it is ungrounded?

Thanks in advance!

 

[cranky108]

I would think you might leave it ungrounded for zero sequence concerns, but you should be sure the windings are insulated for full line to ground voltage. The same goes with the H0 bushing.

Then again, be sure it is the H0 bushing you are talking about, and not the core ground.

 

[argotier]

If the neutral will be ungrounded, then you must consider a fully insulated wye, with same insulation for neutral and phase bushings (and fully insulated windings).

In the standards is not so clear, because the statement is not of the form:

When neutral ungrounded --> go fully insulated

But the reciprocal:

When neutral grounded --> lower neutral insulation is admissible (depending on type of grounding)

I can't say much about why an HV neutral would be ungrounded, as is a very rare practice in IEC world.

Only case I've heard of something like that is when two or more transformers are in parallel operation and one of the neutrals is disconnected from ground to lower the ground fault source of the system (cannot say if this is right or wrong practice). In this case the transformer HV wye is fully insulated and the neutral is connected to ground by a disconnect switch to ground or unground the neutral at will.

Hope it helps.

 

[Amex12]

@cranky108 - yes it is the H0 bushing I am talking about and thank you.

@argotier - this is interesting and likely why it was done since there are multiple transformers in parallel with the H0 ungrounded but a surge arrester connected to the H0 terminals.

Essentially what you both are saying is that my H1, H2, H3 bushings, H winding, and H0 bushing should have all the same BIL rating, say 900 kV. What about adding the surge arrester to the H0 terminals, can we consider reducing the insulation level of this bushing? What does adding a buried delta tertiary winding do?

Thanks!

 

[cuky2000]

If the H0 is ungrounded with lower than full winding BIL some protection can be achieved by selecting the appropriate surge arrester rating and MCOV.

 

[cranky108]

A buried delta is for harmonic stability, and to provide zero sequence path. Or said differently, it would add to the ground fault.

 

[RRaghunath]

Grid authorities don't like Wye-Wye with star point grounded on both sides. This is because any ground fault on secondary side distribution system of transformer will reflect on to the grid side and will be source of additional concern for the grid protection engineers.
The tertiary delta acts as a zero sequence source for faults on transformer secondary side by converting SLG fault currents in to L-L currents on primary. This way, even though the transformer primary neutral is open, the earth fault currents will still flow in secondary system.