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 61869, 3.4.209:
V
sal
≥ K
d
· K
ssc
·I
sn
·(R
ct
+ R
b
)
●
or for the minimum required rated accuracy limit factor acc. to IEC 61869,
3.4.208, as follows:
n
n
≥ K
d
· K
ssc
·
R
ct
+ R
b
R
ct
+ R
bn
= K
d
· K
ssc
·
P
ct
+ P
b
P
ct
+ P
bn
The relation between both methods is given as follows:
V
sal
= n
n
·(
P
bn
I
sn
+ 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
For devices out of the platform Easergy MiCOM 30, the input CT burden R
rel
is
less than 20 mΩ, independent of the set nominal current (1 A or 5 A). Usually this
relay burden can be neglected.
The rated knee point voltage V
k
according to IEC 61869, 3.4.217 is lower than
the secondary accuracy limiting voltage V
sal
according to IEC 61869, 3.4.209. 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
:
P634
2 Technical Data
2-26 P634/EN M/R-42-A // P634‑311‑653
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