3-Phase CT's Wired in Delta

[DMG_1]

Can anyone out there give me reasons why 3-Phase CT's may be wired in Delta as opposed to Wye? I understand that Delta provides a 30 degree phase shift but do not see why Delta would be required or what purpose it serves.

 

[jim dungar]

Maybe because we don't care about the voltages when we are dealing with CTs.

But, almost every CT circuit I designed used a Wye connection so I could ground one of the CT leads and only need to pull 4 conductors.

What type of device are these CTs feeding? Is a differential current being monitored?

 

[DMG_1]

Nothing specific. I am starting to get involved with protective relaying so I am researching PT's and CT's. Just want to make sure I know what drives a particular wiring configurations for the PT's and CT's. I found a good website for the PT's which defined which types of systems would require which PT wiring configuration. Please see the below (you will need to copy and paste into your browser):

Using Potential Transformers - Continental Control Systems, LLC

Contents 1 Overview 1.1 Scaling 2 Equipment 2.1 Current Transformers 3 Potential Transformer Circuits 3.1 Three-Wire Delta Service 3.2 Four-Wire Wye Service 3.3 Three-Wire Wye Service (No Neutral) 4 Configuring the PT Ratio 4.1 WattNode for LonWorks – Option PT 4.2 WattNode Modbus 4.3 WattNode...

ctlsys.com

 

[synchro]

Delta connected CT's are typically used on the secondary side of a delta-wye transformer when protecting it with an electromechanical differential relay. The delta CT's are basically a wye-to-delta transformer that "undoes" the delta-wye of the power transformer. The delta CT outputs are compared differentially with the outputs of wye configured CT's on the primary side using a resistive bridge. And so with appropriate scaling, the outputs of the delta CT's on the secondary will match those of the wye connnected CT's on the primary as long as there's no fault in the transformer.
With digital programmable relays, delta connected CT's would not be necessary in this application because their operation can be emulated using mathematical operations when using wye connected CT's.

Delta connected CT's will reject zero sequence currents, which may or may not be desirable depending on the application. A digital relay can be programmed to remove zero sequence currents from wye connected CT outputs. But it can't reconstruct any zero sequence currents that may be present when it's fed by delta connected CT's.

Delta connected CT's will reject triplen harmonic currents, which again may or may not be desirable depending on the application.

 

[jim dungar]

Most relays can accept 2 leads per CT, however for most relaying applications CTs are connected in wye in North America. It is not uncommon to find the CTs grounded as close to the CTs as possible.

 

[Hv&Lv]

Post #4…

All ours were delta on one side for the exact reason Syncro stated.
Once we changed to SEL 87 relays, we changed them all to wye.