Capacitor Bank connected to TV of the autotransformer

[PiotrM]

Hi everyone,

There is a new idea in my company to connect Capacitor Banks (CB) to TV of the autotransformer. This is of course for power grid improvement (400 kV power grid).

I know about similar solutions in other countries, but there CB are connected directly to the power grid (as a CB field in power substation).

Do you have any experience in maintain such combination? Are there any problems with autotransformer?

Regards,
PiotrM

 

[bacon4life]

We used to have a synchronous condenser connected to the tertiary of a 230/115 kV auto-transformer. Eventually we removed the synchronous condenser and installed much larger capacitor banks on a separate breaker in part because the tertiary did not have enough MVA capacity.

A point of discussion may be the fault exposure to the transformer. I was told, but don't quite understand why, that the tertiary is more likely to be damaged by a close in fault that the other windings. Also, would taking the transformer off line for a capacitor bank fault be acceptable?

Would you ever need the cap banks when the transformer was off line?

 

[PiotrM]

Hi everyone,

bacon4life - thank you for your response.
I have to tell you, that our CB-autotransformer combinations are very new. So, we don't have much experience in using it. That's why I would know about similar solutions in other companies or countries.

I think that TW is the weakest from all windings because of insulation level. Other question is voltage changing. We did some theoreticaly calculations. But you know how it is with something new.

So, I'm really interesting in your solutions.
Regards,
PiotrM

 

[JensenDrive]

The circuit you are creating is a series LC circuit. You have to think about ferroresonance issues. I do not know how big a deal this is. I would research the matter a bit.

Also, trip the capacitor bank whenever you trip the transformer. Don't let the capacitor bank discharge through the xfmr windings. If you do, the capacitor bank is effectively applying DC to the xfmr winding, which means it rapidly saturates and current becomes quite high. I recently ran into a case where a capacitor bank discharged through a station service transformer, and the xfmr fuses were blown.

 

[prc]

It was a good old practice to connect synchronous condensers to tertiary of transformers to compensate the load/line reactance.Later it was changed to capacitor banks and now more with capacitor/reactor combinations.The fault level on tertiary depends on the impedance of tertiary with primary or secondary .In auto transformers this can be adjusted by changing the physical dispositions of various windings with respect to core.Any level of tertiary fault current withstand level can be built in to tertiary at the time of design.

In case your transformer was ordered /designed only for stabilising purpose,but not for loading,then the effect of a 3L fault on tertiary is to be checked by using H-L,H-T and L-T impedances.Make sure that manufacturer has designed the tertiary for loading.If it is only for stabilising purpose,designer may not have given any cooling ducts in winding, a must for loading.

Ferroresonnace was never reported for this application.Regarding Jensendrives comment-how a capacitor can discharge in to an unearthed delta winding?I could not understand how it can saturate transformer core.Of course,there will be a charge in winding soon after switching of,whether capacitor or inductor load.This will be dissipated through stray capacitance to earth.To avoid this,normally a star connected (with neutral earthed) electro-magnetic PT will be connected to terminals of the transformer tertiary terminals.