Calculation Example 2 - GE UR Series Instruction Manual

9.1 L90 CT REQUIREMENTS
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
r
KNEEPOINT VOLTAGE CHECK:
Use the procedure shown for Example Number 1 above.
9
9-2
(
0.008 Ω
=
3.75 Ω
× 2 0.26 Ω × 0.528 Ω
------------------- - = =
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
. This is less than the allowed 3 Ω,

9.1.3 CALCULATION EXAMPLE 2

GE Power Management