Module 2 : Current and Voltage Transformers
Lecture 6 : CT Tutorial
 

Example 5:
What will be the approximate % error if a 500:5 class C CT is connected to a secondary burden of 2.5 and the secondary current is 68A.

Answer:
For a 500/5 CT, secondary resistance Rs = 0.25
Secondary burden RB = 2.5

= 187V
Corresponding exciting current IE = 6A (fig 5.7) (approximate)
% ratio error =
                       = 8.82%

 

Example 6:
If a 300:5 class C CT (fig 6.1) is connected to a meter with resistance and secondary current in the CT is 4.5A find out the primary current, voltage developed across the meter and % ratio error. Lead wire resistance secondary resistance of a 300:5 CT

Answer:
 
,
Total secondary resistance
                                                        
Secondary voltage
                                  
                                   = 5.0265V
From Fig 5.7,
Exciting current IE for 5.265V
                                        = 0.03A (approximate)
Turns ratio N = 300/5 = 60
                  
                         = 60(4.5 + 0.03)
                         = 271.8A
Voltage across meter
                                         = 4.5V
Ratio error
                    = 0.67% 		 
 
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