"Rolling" Aux CT Circuit 2

[Mbrooke]

Why would you wire an aux CT circuit like this? Would the CTs not saturate? How would readings still be accurate?

 

[davidbeach]

Probably not wired that way. And if it is, all of the meters always measure the same zero sequence current.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[Mbrooke]

I swear the prints show it wired that way.

What exactly will the meters read day to day?

 

[davidbeach]

The meters will always read the same. The aux CTs all have the same primary current, so the meters all have to see the same thing. The main CTs will be driven into saturation since the only current that can flow in the secondary of them is the zero sequence current in the power circuit; everything else is forced through the excitation branch. Not sure how real CTs would respond to that, but it would certainly heat the CTs considerably.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[cranky108]

Are you measuring unbalance current?

 

[stevenal]

In my experience, when the engineers screw it up the techs will do what is needed to make it work. The corrections then don't always make it back to the master drawings.

 

[jghrist]

Maybe it's supposed to be a delta-wye conversion wired like the attachment?

 

[davidbeach]

Sure, that works, provides phase-phase currents and each ammeter would show a different current. For balanced loading they would all read the same, but they would show any unbalance in the loading.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[Mbrooke]

Why are CTs sometimes connected in delta?


I agree something is off with the original. Unless they are trying to measure only zero sequence currents?

 

[RRaghunath]

I have seen aux CTs used in metering circuit when the main CTs are protection class and hence cannot protect the meters against the flow of large magnitude currents during faults in the feeder. Of course, the main CTs are connected in Star.
Other than that, Aux CTs are also used with old/obsolete relays meant for transformer differential for vector group correction.

 

[cranky108]

CT's were connected in delta to align the currents of a differential with those of the other side of the transformer, and to filter out zero sequence current.
Normally used on the wye side of a delta-wye transformer.

New relays can make this alignment, and filter out zero sequence internally.

Some types of metering also use delta connected metering, with the assumption that there is no single phase loading.