Transformer "Grounded" Through an Arrestor

[davidbeach]

I think I've run into this more than just this once, where a wye-delta transformer is installed with the wye side H0 or X0 bushing connected to an arrestor rather than either a grounding impedance or a solid ground. For now I've got one to consider how to represent in our OneLiner model. We don't own the transformer, but it connects directly to our system with the wye side facing us; at 230kV. For the most part, modeling it as an ungrounded wye is reasonable, for most faults the arrestor doesn't conduct and the wye remains ungrounded. But for SLG faults very close to that transformer the arrestor will conduct and the transformer suddenly starts contributing ground fault current, potentially significant amounts of current.

Have others faced similar situations? How do you model it? Do you change from ungrounded for remote faults to solidly grounded for close-in faults? Do you have any good fault records to show how it really works?

Thanks.

 

[cranky108]

Thanks for asking that question.

I do have one at a distribution level, and I model it as ungrounded, but that likely does not help much.

So for you to change your model, do you have a method that is easy, or just load a different model?

I have a few cases where I might want to use a different model, but don't want to maintain such a number of models. Is there a way to use a conditional model, and not the several models?

I guess one way would to have it in the model twice, one of each, and only have one in service at a time, like you would do with a mobile. Or one could use a switch to turn it one and off. But you would need to remember to change it when needed.

 

[waross]

Will it work to model it as the worst case?
I understand that while many faults will draw less current than a bolted fault, the bolted fault current is used as a maximum.
Or can you model it as an unloaded, grounded transformer in parallel with the ungrounded transformer. The error due to magnetizing current may be insignificant in comparison to the grounded fault current.
Footnoted of course.


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

 

[crshears]

I don't know if this will help, as I'm no mathematician, but I'd describe this as "tethering," visualized something like a dog on a short chain that's clipped to a peg dead smack in the centre of a yard shaped like an equilateral triangle; as long as the dog stays within the length of the chain, the chain has no effect, but stretch the chain out and its hard circumferential limit is reached.

Now as to how a mathematician would model such a thing I have no idea...

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[stevenal]

I believe Aspen can handle series compensated lines with arresters, so they should have the knowledge to handle this. They are also very customer oriented. Suggest giving Sherman the challenge.

 

[Mbrooke]

How do you know that the arrester conducts for single line to ground fault? What is the voltage rating of said arrester?


BTW, you are not alone, this is not unheard of. I know of several wye ungrounded, delta, wye grounded setups with the primary H0 grounded through an arrester.

 

[marks1080]

Model as grounded for close in single phase faults. Model as ungrounded for close in phase faults. Model as ungrounded for all remote faults.

I'm guessing you have a (line?) protection on the other end? That can probably cover a lot of your remote fault scenarios too.

 

[davidbeach]

I'll bring it up with Aspen at some point, they're working on a bunch of other stuff that I'm beta testing for them that seem to be more widely useful.

Modeling it both ways in parallel and selecting the right one might be the best option.

No line protection at our end, yet anyway. That transformer is presently connected directly to our bus with a two span "line" and a high-side breaker at the transformer. Probably been that way for over 50 years. But breakers and line relays will be making an appearance in the next year or two.

It caught my attention when the other party went from essentially ungrounded in their model to solidly grounded and then that made it into the shared regional model. Trying to correlate fault currents between our model and the regional model I was seeing ground faults on our bus stick out like a sore thumb but the phase fault currents matched.

I don't have any close-in ground fault records to compare against the model, so I don't know for sure if the arrestor ever actually conducts or not. It's a 108kV arrestor on a 230kV system so I'm picturing it operating but don't have any math to back that up. If the arrestor is seeing approximately 3V0, a fault doesn't have to be very far way (less than 25% of the shortest line) for 3V0 to be below 108kV. A hard close-in fault after I get new line relays would provide a conclusive answer.

 

[cranky108]

If the issue only appears for less than 25% of the shortest line, then likely you might only want it in your model, and not include it in the regional model.

I believe I have a like issue with a small (as in no one outside my company cares) induction generator with parallel capacitor shunt bank.
For a close in fault, it won't produce much fault current. But for a fault on the next feeder, it should still have some voltage, and should produce some fault current.

We as utilities are likely going to have to have more than one model (or things to check on and off) as wind and solar farms get bigger.