Delta connected current transformers CTs for protection and relaying (ELECTRICAL POWER PE EXAM 2022)

[@brimmed]

I signed up for a PE review class through a university cuz that's the only way my work would reimburse it. The quality difference is so extreme that I'm probably gonna end up signing up for your class to review any topics too hahaa

[@electricalpereview]

Happy to help. We just started the May - July 2022 Spring On-demand Semester last week. You can sign up at www.electricalpereview.com

[@libertarianbastard]

I really appreciate you taking the time to create and post these videos. You do a great job explaining these topics. I've recommended your course and vids to all of the junior engineers in my office.

[@electricalpereview]

Thanks for the kind words, I'm glad these videos are helpful. Feel free to request certain topics in the comments.

[@XiaoGeng-p4d]

I wanted to ask why the delta current is lagging the phase by 30 degrees? From the drawing, it appears to be a standard DAB connection (The polarity of A is connected to the non polarity of B), and assuming a ABC phase sequence, then the delta side should lead the wye side by 30 degrees.

[@electricalpereview]

Good observation! If this was a DAB connected delta deltawye transformer, you would be correct. In this case we have just three current transformers that are delta connected. When we sum up any of the phase currents to calculate the line current leaving the delta connection using Kirchoff's Current Law, the resulting current leaving the delta connection will lag by 30 degrees.

[@JamesBowlesaz]

Is circulating current in the delta negligible?

[@electricalpereview]

In this instance, yes. The circulating delta currents are all displaced by 120º and result in the line current leaving the delta (and entering the relay) being larger by a factor of times the square root of three.

[@coltonbutts96]

Why would you ever want to wire CTs in a delta manner?

[@joeprice6483]

Great question, most protection scenarios would utilize a wye-connected CT secondary. However, if you had an electromechanical relay device or panel, generally they need both delta and wye-connected CTs for transformer differential protection (assuming a delta-wye or wye-delta 3 phase transformer) to offset both the root 3 magnitude and phase shift change through the transformer. It may also be useful in other unique power system protection applications.