Calculation Example 2 - GE UR Series L90 Instruction Manual

10.1 CT REQUIREMENTS
3. This gives a total burden of:
This is less than the allowed 3 , which is OK.
4.
The following procedure verifies the kneepoint voltage.
1. The maximum voltage available from the
2. The system X/R ratio
3. The CT voltage for maximum phase fault is:
14000 A
---------------------------------- -
V
ratio of 300:1
4. The CT voltage for maximum ground fault is:
12000 A
---------------------------------- -
V
ratio of 300:1
5. The CT will provide acceptable performance in this application.
To check the performance of an IEC CT of class 5P20, 15 VA, ratio 1500:5 A, assume the following values:
• maximum I = 14 000 A
fp
• maximum I = 12 000 A
fg
• impedance angle of source and line = 78°
• CT secondary leads are 75 m of AWG 10.
The IEC rating requires the CT deliver up to 20 times the rated secondary current without exceeding a maximum ratio error
of 5%, to a burden of:
The total Burden = R
The following procedure verifies the kneepoint voltage.
1. The maximum voltage available from the
2. The system X/R ratio
3. The CT voltage for maximum phase fault is:
14000 A
V ---------------------------------- -
ratio of 300:1
4. The CT voltage for maximum ground fault is:
12000 A
---------------------------------- -
V
ratio of 300:1
10
5. The CT will provide acceptable performance in this application.
10-2
0.75 
R
CT
0.2 VA
0.008 
R ----------------- -
r
  2
5 A
3.75 
  ------------------- -
R 2 75 m
L
1000 m
Total Burden R R R
CT r
CT
tan 78 4.71 .
    
4.71 1 0.75
    
4.71 1 0.75
15 VA
Burden ----------------
  2
5 A
= 0.008 + 0.52 = 0.528 , which is less than the allowed 0.6 , which is OK.
+ R
r l
CT
tan 78 4.71 .
    
4.71 1 0.75
    
4.71 1 0.75
L90 Line Current Differential System
 0.26  0.528 
2
0.75  0.008  0.52 
L
   400 
1500 2000 300 V
0.008  
0.26 271.26 V (< 300 V, which is OK)
0.008  
0.52 291.89 V (< 300 V, which is OK)
0.6  at the 5 A rated current
   400 
1500 2000 300 V
0.008  
0.26 271.26 V (< 300 V, which is OK)
0.008  
0.52 291.89 V (< 300 V, which is OK)
10 APPLICATION OF SETTINGS
1.28 
.
.

10.1.3 CALCULATION EXAMPLE 2

.
(EQ 10.3)
(EQ 10.4)
(EQ 10.5)
(EQ 10.6)
(EQ 10.7)
(EQ 10.8)
(EQ 10.9)
GE Multilin