GE UR Series L90, Instruction Manual, Page: 606 / 846

10-2 L90 Line Current Differential System GE Multilin
10.1 CT REQUIREMENTS 10 APPLICATION OF SETTINGS
10
(EQ 10.3)
3. This gives a total burden of:
. (EQ 10.4)
4. This is less than the allowed 3

, which is OK.
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:
(EQ 10.5)
4. The CT voltage for maximum ground fault is:
(EQ 10.6)
5. The CT will provide acceptable performance in this application.
10.1.3 CALCULATION EXAMPLE 2
To check the performance of an IEC CT of class 5P20, 15 VA, ratio 1500:5 A, assume the following values:
• maximum I
fp
= 14 000 A
• maximum I
fg
= 12 000 A
• 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:
(EQ 10.7)
The total Burden = R
r
+ R
l
= 0.008 + 0.52 = 0.528

, which is less than the allowed 0.6

, which is OK.
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:
(EQ 10.8)
4. The CT voltage for maximum ground fault is:
(EQ 10.9)
5. The CT will provide acceptable performance in this application.
R
CT
0.75

R
r
0.2 VA
5 A
  2
------------------ 0.008

R
L
2 75 m
3.75

1000 m
--------------------
 
2 0.26
 
0.528

Total Burden R
CT
R
r
R
L
0.75

0.008

0.52

1.28

CT 1500 2000
  
400

300 V
78

tan 4.71
V
14000 A
ratio of 300:1
-----------------------------------
4.71 1
 
0.75 0.26 0.008
    
271.26 V (< 300 V, which is OK)
V
12000 A
ratio of 300:1
-----------------------------------
4.71 1
 
0.75 0.52 0.008
    
291.89 V (< 300 V, which is OK)
Burden
15 VA
5 A
  2
---------------- 0.6

at the 5 A rated current
CT 1500 2000
  
400

300 V
78

tan 4.71
V
14000 A
ratio of 300:1
-----------------------------------
4.71 1
 
0.75 0.26 0.008
    
271.26 V (< 300 V, which is OK)
V
12000 A
ratio of 300:1
-----------------------------------
4.71 1
 
0.75 0.52 0.008
    
291.89 V (< 300 V, which is OK)