Calculation Example 2 - GE L90 Instruction Manual

Ur series line current differential system
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10.1 CT REQUIREMENTS
3.
This gives a total burden of:
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:
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
R
0.75
CT
0.2 VA
R
----------------- -
0.008
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
+ R
= 0.008 + 0.52 = 0.528 , which is less than the allowed 0.6 , which is OK.
r
l
CT
tan
78
4.71
.
4.71
1
0.75
4.71
1
0.75
L90 Line Current Differential System
2
0.26
0.528
0.75
0.008
0.52
L
1500 2000
400
300 V
0.26
0.008
271.26 V (< 300 V, which is OK)
0.52
0.008
291.89 V (< 300 V, which is OK)
0.6
at the 5 A rated current
1500 2000
400
300 V
0.26
0.008
271.26 V (< 300 V, which is OK)
0.52
0.008
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

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