kW / kVAR Transducer Sensing Across 3-Phase Transformer

[EEiAM]

I am working on a project to add kW/kVAR transducers to an existing 3-phase system. There is a 13.8kV-138kV step-up transformer in this section of bus. The 13.8kV side is grounded wye and the 138kV side is configured as an open delta. All of the current transformers (C/Ts) on the 13.8kV side are being used for protective relaying. The C/Ts on the 138kV side are spare. The only potential transformer (P/T) in this section of bus is on the 13.8kV section. An easy solution would be to feed the kW/KVAR transducers with the P/T on the low side (13.8kV) of the xfmr and the C/Ts on the high side (138kV) of the xfmr. My concern is the 30 degree phase shift in voltage and current due to the wye-delta transformation.

Is there any way to compensate the circuit for this phase shift so that the kW and kVAR readings are accurate with the setup described above? Would arranging the three C/Ts on the high side of the step-up transformer in a wye configuration offset the phase shift in current (since the step-up transformer high-side windings are delta)?

Any advice or recommendations are appreciated. Thanks.

 

[dpc]

You can use auxiliary PTs to compensate for the transformer phase shift. This is generally the simplest way if you can't use CT and PTs from the same side of the transformer.

 

[waross]

Phase to neutral PTs on the 13.8 kV side will be in phase with the open delta secondary.

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

 

[EEiAM]

Thanks for the replies.

waross --> I'm sorry...I stated the transformer high-voltage side incorrectly. It is not an "open" delta. It is an ungrounded delta...3-phase at 138kV. Not sure if this makes a difference in your reply to my original post.

My objective is to ensure that the voltage and current phasing is at the same angle as it would be if both measurements were being taken from the same side of the step-up transformer.

 

[waross]

No problem. The point is, if the 13.8 PTs are connected line to neutral, they will be the same phase angles as the line to line delta phases. If you have unused PTs connected line to line, you may consider reconnecting them phase to neutral and using a meter that will accept a 69 volt input. I don't think that 69 volts is a problem with digital electronic meters.
You will be measuring only part of the transformer losses.

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