Calculation Example 1; Calculation Example 2 - GE L90 Instruction Manual

9.1 CT REQUIREMENTS
To check performance of a class C400 ANSI/IEEE CT, ratios 2000/1800/1600/1500 : 5 A connected at 1500:5, and where:
• 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 No. 10.
BURDEN CHECK:
ANSI/IEEE class C400 requires that the CT can deliver 1 to 20 times the rated secondary current to a standard B-4 burden
(4 Ω or lower) without exceeding a maximum ratio error of 10%.
The maximum allowed burden at the 1500/5 tap is
0.75 Ω
R
=
CT
0.2 VA
R ----------------- -
=
r
( ) 2
5 A
×
R 2 75 m
=
L
Therefore, the Total Burden
which is OK.
KNEEPOINT VOLTAGE CHECK:
The maximum voltage available from the
The system X/R ratio
=
The CT Voltage for maximum phase fault is:
14000 A
---------------------------------- -
V
=
ratio of 300:1
The CT Voltage for maximum ground fault is:
12000 A
---------------------------------- -
V
=
ratio of 300:1
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 identical parameters as for Example
Number 1.
BURDEN CHECK:
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:
15 VA
Burden ----------------
=
( ) 2
5 A
The total Burden = R
9
r
KNEEPOINT VOLTAGE CHECK:
Use the procedure shown for Example Number 1 above.
9-2
(
0.008 Ω
=
3.75 Ω
× × 0.26 Ω 0.528 Ω
------------------- - 2
= =
1000 m
0.75 Ω
R R R
= + + =
CT r L
(
CT 1500 2000
=
tan 78° 4.71 .
=
× ( ) × (
4.71 1 0.75 0.26
+ + +
× ( ) × (
4.71 1 0.75 0.52
+ + +
0.6 Ω at the 5 A rated current
=
= 0.008 + 0.52 = 0.528 Ω, which is less than the allowed 0.6 Ω, which is OK.
+ R
l
L90 Line Differential Relay
⁄ ) 4 × 3 Ω
1500 2000 . Now,
=
0.008 Ω 0.52 Ω 1.28 Ω
+ + =
⁄ ) 400 ×
300 V .
=
0.008 Ω )
271.26 V (< 300 V, which is OK)
=
0.008 Ω )
291.89 V (< 300 V, which is OK)
=
9 APPLICATION OF SETTINGS

9.1.2 CALCULATION EXAMPLE 1

. This is less than the allowed 3 Ω,

9.1.3 CALCULATION EXAMPLE 2

GE Multilin