R
l
One-way lead resistance from CT to relay
R
rel
Resistive burden of relay’s CT input
T
p
Primary time constant (primary system time constant)
ω
(System) angular frequency
X
p
/R
p
Primary impedance ratio (system impedance ratio):
X
p
R
p
=
ω
·
T
p
K
d
Dimensioning factor for the CT
K
emp
Relay specific, empirically determined dimensioning factor for the CT
2.13.2
General Equations
The current transformer can be dimensioned
●
either for the minimum required secondary accuracy limiting voltage acc.
to IEC 60044
‑
1, 2.3.4:
V
sal
≥
K
d
·
K
ssc
·
I
sn
· (
R
ct
+
R
b
)
●
or for the minimum required rated accuracy limit factor acc. to
IEC 60044
‑
1, 2.3.3, as follows:
n
n
≥
K
d
·
K
ssc
·
Rct
+
R
b
Rct
+
Rbn
=
K
d
·
K
ssc
·
Pct
+
P
b
Pct
+
Pbn
The relation between both methods is given as follows:
V
sal
=
n
n
· (
Pbn
Isn
+
I
sn
·
R
ct
)
The actual secondary connected burden R
b
is given as follows:
●
For phase-to-ground faults:
R
b
= 2 ·
R
l
+
R
rel
●
For phase-to-phase faults:
R
b
=
R
l
+
R
rel
The wire lead burden is calculated as:
R
l
=
ρ
⋅
l
A
●
ρ
= specific conductor resistance
(e.g. for copper 0.021
Ω
mm²/m = 2.1
⋅
10
-8
Ω
m, at 75°C)
●
l = wire length
●
A = wire cross section
The MiCOM Px3x input CT burden R
rel
is less than 20 m
Ω
, independent of the set
nominal current (1A or 5A). Usually this relay burden can be neglected.
The rated knee point voltage V
k
according to IEC 60044
‑
1-am1, 2.3.12 is lower
than the secondary accuracy limiting voltage V
sal
according to IEC 60044
‑
1,
2.3.4. It is not possible to give a general relation between V
k
and V
sal
, but for
standard core material the following relations applies:
●
V
K
≈
0.85
⋅
V
sal
for class 5P CTs, and
●
V
K
≈
0.75
⋅
V
sal
for class 10P CTs, respectively.
Theoretically, the specifications of the current transformer could be calculated to
avoid saturation by inserting its maximum value, instead of the required over-
dimensioning factor K
d
:
P631
2 Technical Data
2-26
P631/EN M/R-11-C // P631-310-650
Summary of Contents for P631
Page 2: ......
Page 4: ......
Page 7: ...Changes after going to press...
Page 8: ......
Page 16: ...P631 Table of Contents 8 P631 EN M R 11 C P631 310 650...
Page 56: ...P631 2 Technical Data 2 28 P631 EN M R 11 C P631 310 650...
Page 236: ...P631 3 Operation 3 180 P631 EN M R 11 C P631 310 650...
Page 246: ...P631 4 Design 4 10 P631 EN M R 11 C P631 310 650...
Page 266: ...P631 5 Installation and Connection 5 20 P631 EN M R 11 C P631 310 650...
Page 276: ...6 8 Configurable Function Keys P631 6 Local Control HMI 6 10 P631 EN M R 11 C P631 310 650...
Page 548: ...P631 10 Commissioning 10 10 P631 EN M R 11 C P631 310 650...
Page 568: ...P631 12 Maintenance 12 8 P631 EN M R 11 C P631 310 650...
Page 570: ...P631 13 Storage 13 2 P631 EN M R 11 C P631 310 650...
Page 572: ...P631 14 Accessories and Spare Parts 14 2 P631 EN M R 11 C P631 310 650...
Page 576: ...P631 15 Order Information 15 4 P631 EN M R 11 C P631 310 650...
Page 582: ...P631 A2 Internal Signals A2 4 P631 EN M R 11 C P631 310 650...
Page 608: ...P631 A4 Telecontrol Interfaces A4 18 P631 EN M R 11 C P631 310 650...
Page 637: ......