Module 2 : Current and Voltage Transformers
Lecture 6 : CT Tutorial
  Review Questions
  Note: Use fig 5.7 wherever necessary.
1. A circuit has a 1200:5 C 400 CT with characteristics as shown in fig 5.7. The maximum symmetrical fault for which the
  associated relays are to operate is 17,800 Amp. Find out the approximate % error if the secondary burden is:
(1) 2 (2) 4
2. The secondary burden in a relay circuit is 4. The overcurrent relay set to pick up at 3 Amp and CT ratio is 400:5.
  Calculate the minimum primary current to just operate the relay?
3. Find out the maximum secondary burden that can be used and still maintain rated accuracy at 20 times rated
  symmetrical secondary current if a 1200:5 C 400 CT is connected on 800/5 tap.
4. If the secondary burden of 1200:5 C 400 CT connected to 300:5 tap is 2, find out. Find out the maximum symmetrical
  fault current that may be applied to this CT without exceeding 10% ratio error.
5. Let the primary fault current in the primary side of a CT be given by
 
(a) Show that this equation can be rearranged as follows.

For u(t) where u(t) is the unit step function.
(b) Consider the simplified equivalent circuit of CT as shown in fig 6.2
 
    
  Let be the angle as shown in fig 6.3 and . Then show that secondary reflected current components for the primary are given by

 Primary Fault current

 Secondary reflected current

1)

  (Sine Component)

 

2)

  (Cosine Component)

 

3)

  (d c component)

 

 
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