NCEES 102

[kris7o2]

Hi everyone, does anyone know what the drawing would like for this problem? I am not certain I understand what the problem means when they mention auxiliary CTs or how these would be connected to the differential relay.

A differential relay circuit connected to protect a delta-wye transformer using current transformers (CTs) must be balanced with properly connected auxiliary CTs to allow for the 30° phase shift and the turns ratio. If this is not accomplished, which of the following is most correct?
(A) The circuit could cause the relay to be picked up under non-fault conditions.
(B) The American national standard for designating terminals HI and Xl on wye-delta transformers would not be followed.
(C) A positive-sequence voltage drop from Bushing HI to the neutral would lead the positive sequence voltage drop from Bushing Xl to the neutral by more than 30°.
(D) The delta winding must be located on the high-voltage side of the transformer.

 

[akyip]

Attached are some of my notes from various textbooks and other practice exams regarding how the CTs are connected for 3-phase delta/wye transformers.

• The CTs on the Y side of a transformer are connected in delta.
• The CTs on the delta side of a transformer are connected in Y.
• The reason for the above 2 connections is that one line current from the transformer delta side is compared with the difference of two line currents from the transformer Y side.
• All this is to correctly account for the 30-degree phase shift between the transformer delta and transformer Y sides, and that the relay operates correctly.

The first wiring diagram attachment is from an electrical protection book that I have: Fundamentals of Power System Protection by Y.G. Paithankar (Amazon link - Fundamentals of Power System Protection: Y.G. Paithankar, S.R. Bhide: 9788120341234: Amazon.com: Books )

The others are from these practice exams that I have:

• Cram for the PE Power Exam Vol. 3
• Electrical PE Review Practice Exam (from Zach Stone)
• A.S. Graffeo PE Power Practice Exam