Transformer Grounding

[Viceng]

Hello, I have a question regarding neutral bushing ratings that I was hoping someone could help me with.

Scenario: 16MVA transformer 138kV solidly grounded Wye - 13.8kV delta. the HV phase bushings are 550kV BIL while the neutral is 15kV 400A, 95kV BIL.

This neutral bushing seems quite small...Is this low rating typical with solidly grounded configurations?

Does a large ground impedance affect this neutral bushing rating? That is would a large ground impedance cause a neutral shift affecting this neutral bushing?

Thanks in advance.

 

[cranky108]

15kv is typical in the US for solidly grounded transformers.

Yes the ground impedance does dictate the rating of the H0 bushing. For a floated neutral the bushing would be 550kv BIL.

 

[Viceng]

Thanks for the reply.

I am still unsure why a high ground impedance would impose a larger H0 bushing requirement.

The potential difference between the H0 bushing and ground would be the current through the neutral multiplied by the resistance of the cable from the winding to ground (quite small and not varying). I think a larger ground resistance would mean a larger potential difference from the location of the fault to the ground connection of the transformer but does this come into play in calculating bushing ratings?

Cheers.

 

[waross]

The higher the impedance of the grounding system, the higher the voltage that will be seen between the H0 connection and the transformer tank in the event of a ground fault on the system.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LionelHutz]

I would expect that the neutral of this transformer connects to the neutral of the power system. The neutral of the power system is then grounded somewhere upstream of this transformer. Because of this, the neutral can be at a slightly different potential then the transformer case ground.

If the grounding was high impedance then the neutral voltage could reach 80kV during faults and could be some voltage between 0V and 80kV during normal operation.

 

[jghrist]

There is a difference between high impedance grounding and a high ground impedance. In a high impedance grounded system, there is a high impedance connected between the transformer neutral and the local grounding electrode. In this case, a high voltage can be present between the grounding electrode (and transformer tank) and the neutral bushing. If the grounding electrode has a high impedance to remote earth, but there is a direct connection from the neutral bushing to the grounding electrode, then there won't be a high voltage between the grounding electrode (and transformer tank) and the neutral bushing. In this case, reduced insulation can be used.

 

[Viceng]

Again, thank you all for the replies.


Waross and LionelHutz, I should have clarified that I was speaking of the physical impedance of the ground system (remote earth) not the impdeance of the neutral connection to ground as Jghrist has pointed out.

jghirst, I appreciate the input. I was hoping that was the case.

 

[LionelHutz]

I kind of guessed that you were talking about the resistance of the ground, not purposely adding a grounding resistance.

You are talking about the primary connection of the transformer. The 4-wire power system you connect this transformer to will likely have the neutral grounded some place upstream. This transformer will have it's H0 bushing connected to this neutral which is grounded some distance away and likely not grounded at the transformer. This transformer also will have the case (tank) grounded at the location it is installed. So, there could be some potential difference or a lot of potential difference between the neutral and the ground at the transformer location, the magnitude of which depends on the ground impedance between the 2 sources of grounding.

I was meaning before that if the power system had a high impedance neutral ground then the neutral voltage at the site of the transformer could be 80kV compared to ground.

 

[jghrist]

The H0 bushing should be grounded locally. Transmission systems are not single-point grounded.

 

[cranky108]

Even for a resistive grounded H0, the bushing would not need to be the full rating of the phases, except where the impedance is very high, or the H0 is ungrounded.
But it is not just the bushing that needs to be full rating, but also the transformer windings.

The typical method of transformer grounding that I have seen is to connect the H0 to the transformer case, and then to the station ground grid.

But I have also seen ungrounded wye's to block zero sequence.