CT error. (Power Systems Protection)

Discussion in 'Homework Help' started by Jess_88, Feb 17, 2013.

  1. Jess_88

    Thread Starter Member

    Apr 29, 2011
    Hey guys,

    Just going through my lecture notes and I'm a little confused.
    The notes say

    What I'm having trouble understanding,
    (1) What is 'ek' on the graph?
    (2) I understand how to get Vs and the knee voltage (Vk) from various equations, but how would this help fined the Ie?

    I'm usually given information such as;
    Fault Current (If)
    Full Load Current (I,fl)
    CT ratio
    Lead, Relay, and CT Resistance.

    Could someone please;
    (1) Explain the steps in my notes
    (2) Maybe give me some equations pertaining to the information I'm usually given,
    (3) If possible point me in the direction of example problems pertaining to this topic.

    Thanks so much for the help guys :)
  2. Jess_88

    Thread Starter Member

    Apr 29, 2011
    Maybe I need to elaborate on my current understanding.
    There was a question similar to this that I no longer have access to.
    I could work out the Knee Voltage but I had to draw the graph, and from that determine the excitation current at that point.
    I know that at Vk, a 50% increase in Ie will result in a 10% increase in Vs.
    Also, is 'Is', the relay current setting?
    Is there a source of information and practise questions relating to this?