Current Transformer Requirements - Siemens SIPROTEC 7UT613 series Manual

A.4

Current Transformer Requirements

7UT613/63x Manual
C53000-G1176-C160-2
Formula symbols/terms used (in accordance with IEC 60044-6, as defined)
K = rated symmetrical short-circuit current factor
ssc
(example: CT 5P20 → K
K' = effective symmetrical short-circuit current factor
ssc
K = rated transient dimensioning factor
td
I
= maximum symmetrical through flowing fault current
scc max (ext.
fault)
I
= CT rated primary current
pn
I
= CT rated secondary current
sn
R = secondary winding d.c. resistance at 75
ct
(or other specified temperature)
R = rated resistive burden
b
R' = R + R
b lead
T = primary time constant (net time constant)
p
V = knee-point voltage in V (r.m.s.)
k
R = relay burden
relay
with:
I
= single conductor length from CT to relay in m
ρ = specified resistance = 0.0175 Ω mm
(or other specified temperature)
A conductor cross-section in mm
The transient rated dimensioning factor K
primary time constant T
1
time of only / period, the influence of T
4
For CT's that are defined by the rated symmetrical short-circuit current factor K
the rated burden R (e.g. 5P, 10P), the effective K'
b
The minimum required K'
(required) ≤ K'
Condition: K'
ssc
A.4 Current Transformer Requirements
= 20)
SSC
= connected resistive burden
relay
2
depends on the device version and the
td
. For the devices 7UT613/63x with a required saturation-free
p
is negligible.
p
can be calculated by the formula:
ssc
(r.m.s.)
ssc
o
C
2
/m (copper wires) at 20
can be calculated by the formula:
ssc
o
C
and
ssc
539