Transformer differential, CT in the delta 1

[tem1234]

Hi,

We have a transformer delta-delta configuration with differential protection.

The CT on the secondary side of the transformer are in the delta of the transformer, so the current measured isn't the same as the line current.

I've made a sketch if it's not clear.

So i want to know to do accurate differential protection with these ct, since the current in the delta is not the line current. We know that current in delta will be the line current divide by 1.73 with a angle of +30 deg vs the line current.

Is it better to add CT somewhere else? We can put CT in the switchgear that is 500 ft away, so the differential protection will aso take the cables.

Thanks

 

[marks1080]

Can you reconfigure the CT to WYE?

 

[jghrist]

We know that current in delta will be the line current divide by 1.73 with a angle of +30 deg vs the line current.

This is only true if the currents are balanced. Otherwise, you will have to somehow subtract the delta secondary currents to compare with the primary.

It's better to put the CTs in the remote switchgear. If the differential trips for a cable fault, this is a good thing.

 

[stevenal]

Your CT secondaries seem to be open with no relay connections. Are they connected delta or wye? Ultimately it's the current at the relay that counts. Any decent microprocessor based transformer relay will be able to handle what ever phase shift you end up with.

 

[tem1234]

Thanks jghrist, that was one of my question. The current is suppose to be balance, but a lot of fault can be line to ground, so unbalance. I don't know how it will react in this situation.

The CT are connected in wye on the primary and secondary since it's a microprocessor relay. We only put the winding configuration in the relay (Dd0). We can connect the ct in delta if we want, or change the configuration in the relay, but i'm not sure if it will give good result for every unbalance situation!

 

[stevenal]

Once you have the relay set right all situations are covered, but it doesn't sound like you do. Suggest you speak with the relay manufacturer, hire a protection engineer, or both.

 

[tem1234]

stevenal,

The CT are placed as the drawing attached (red circle represent a CT).

The line current isn't the same as the current in the delta. So i was wondering if there's anything to do with it, or use other CT outside the delta is the only solution? Maybe for 3 phases fault it's possible to make it work, but for 2 phases fault i'm not sure???

 

[jghrist]

How are the CT secondaries connected? You can set the relay to compensate for secondary line CTs connected in delta, but this is not your connection. Will the relay current from line CTs connected in delta be the same as the relay current from CTs in the delta connected in delta? That appears to be the case for a phase-phase fault.

 

[potteryshard]

Connect the LV CTs in delta, just like the secondary delta transformer winding. Bring out the 'corners', again just like the secondary delta winding. That will give you LV line current (divided by CT ratio) will it not?

 

[tem1234]

CT secondary are connected in wye. At the beginning people didn't think that the CT were inside the Delta. So they connected it as they usually do.

potteryshard, maybe this will work, we will have to verify.

Thanks

 

[stevenal]

No need to reconnect the CTs if you can program the relay to compensate. If you can figure which way to wire the delta properly, you should be able set the relay configuration. And if you get it wrong, the relay will indicate operating current under load conditions.

 

[jghrist]

I'm not so sure that you can compensate through the relay for wye connected CTs installed in the delta. I think Potteryshard is correct. If you connect the CTs in delta and bring out the corners, it will be equivalent to wye connected CTs in the lines.

Unless these are three single phase transformer connected in delta, I wonder where the CTs are located. It would be difficult to install CTs in the delta of a three-phase transformer.

 

[wolf60]

If you use microprocessor relay it does not matter if you have Delta or Wye connection.Only what you have to do is configure the relay the correct way.It is a smart relay so it knows what to do and calculate the current.Basicaly microprocessor relay do math operations in order to take decision to trip. It is not like electromechanical relays where they have direct electromagnetic comparison.

 

[jghrist]

If you use microprocessor relay it does not matter if you have Delta or Wye connection.Only what you have to do is configure the relay the correct way.It is a smart relay so it knows what to do and calculate the current.Basicaly microprocessor relay do math operations in order to take decision to trip. It is not like electromechanical relays where they have direct electromagnetic comparison.

But I've never seen relay instructions that say how to compensate connections of CTs located in the delta instead of in the line, i.e. in the bushings of a three-phase transformer.

 

[tem1234]

I'll ask to relay manufacturer if the relay can acheive this. But maybe we will include the cable in the differential zone anyway to have a better protection.

It' a 3 phase transformer, but with 2 possible connection at the secondary, wye or delta, with different voltage.

If you put the jumper to have the delta connection, the CT are inside the delta, so a strange connection.

thanks for your help

 

[stevenal]

Probably an unusual enough connection that it won't show up in an IM. Mathematically, it's the equivalent of a delta wye transformation, so the relay should be able to compensate. Draw out the phasors to see what the shift is, and program the relay accordingly. Probably also need to adjust the programmed CT ratios by a sqrt(3) factor.

 

[odlanor]

I guess to make CT connection inside delta winding , means you must have 3 single-phase transformer connected in delta.

 

[JBinCA]

I'm agreeing with Stevenal on all points made:

Many modern multiprocessor relays will compensate for the phase shift automaticaly. If your relay does not do this, you will have to compensate for the phase shift by CT secondary connection.

Also agree that hiring a protection engineer would help.

Finally - agree that you can read operate current on modern relays or measure it in ancient ones. This will validate your configuration once you have it installed.

Regards,

JB