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5 - 9
Siemens AG ⋅ January 1999
Very large short-circuit currents flowing through the
protected zone in particular may cause larger transforĆ
mation errors due to the current transformers saturaĆ
ting. Due to the stabilization of the differential protecĆ
tion, differences
in the transformation by the current transformers can
to a large degree be tolerated by the differential curĆ
rent measurement. Nevertheless, the current transĆ
formers must have a minimum degree of conformity
at the largest short-circuit arising in the application.
If the differential protection is connected to linearized
current transformers at both line ends, it is essential
to ensure that the overall connected load is not greaĆ
ter than the rated burden of the transformer. If a cloĆ
sed iron core transformer is connected, it is necesĆ
sary to determine the operational accuracy limit
factors in order to check that there is sufficient conĆ
formity at the highest fault currents. The values speciĆ
fied on the rating plate for the accuracy limit factor
are minimum values.
The operational accuracy limit factor ' can be calculaĆ
ted from the data of the current transformer and the
connected load using the following equation:
ȀĂ +Ă Ă @Ă
P
N
Ă )ĂP
E
P
W
Ă )ĂP
E
whereby:
' = Operational accuracy limit factor
= Accuracy limit factor
P
N
= Rated burden of the current transformer
P
E
= Internal burden of the current transformer
P
W
= Connected burden
The internal burden P
E
is not specified on the rating
plate of the current transformer. It can be determined
by measuring the resistance of the secondary winĆ
ding The internal burden cannot be calculated from
the rated burden. Current transformers with rated
burden equal to or greater than the connected burden
and with a sufficiently large accuracy limit factor
should be used for the 7SD600..
Implementation of sets of transformers with widely
varying responses is not permitted, e.g. a linearized
transformer at one end of a line and a closed iron core
transformer at the other end.
The line differential protection 7SD600 makes the folĆ
lowing demands on the current transformer:
1. The current transformers may not saturate due to
the maximum steady-state through fault current:
I
kd max
= Maximum steady-state fault current
I
Npr
= Primary rated current of the transformer
2. Any deviation of current transformer data (operaĆ
tional accuracy limit factors) at both ends must be
within the following range (assuming the same
primary currents):
3/4 v
1
'/
2
' v 4/3
For detailed determination of current transformer reĆ
quirements, see appendix A.4.
The measuring principle of the 7SD600 requires a
symmetrical pilot wire pair between the two stations.
The rated voltage must correspond to the voltages
that may be induced by faults. It must be at least 500
V. The symmetry of the pilot wires must comply with
Post Office requirements for telephone operation
(symmetrical wire/wire at 800 Hz:10
-3
).
Pilot wires as cable connections must always be
checked for high-voltage interference. The wires of
the protection cable must be able to cope, not only
with the load of the currents to be transmitted and
their fault-induced voltage peaks, but also with any
external stresses.
The greatest electrical danger for pilot wires cables
occurs in high-voltage systems during a short-circuit
with earthing. The fault current induces a longitudinal
voltage in the pilot wires running parallel to the high-
voltage line.
The induced voltage in the pilot wires can be prevenĆ
ted by using highly conductive cable sheaths and speĆ
cial armor (minimum possible reduction factor for
ȀĂ wĂ
I
kgĂmax
I
Npr
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