Calculation Example 2 - GE UR Series Instruction Manual

Line differential relay.

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