Demands On The Current Transformer; Demands On The Pilot Wires - Siemens SIPROTEC 7SD600 System Manual

Numerical line differential protection SIPROTEC 7SD600 V3.0
5.2.5

Demands on the current transformer

Very large short-circuit currents flowing through the
protected zone in particular may cause larger transfor
mation errors due to the current transformers satura
ting. Due to the stabilization of the differential protec
tion, differences
in the transformation by the current transformers can
to a large degree be tolerated by the differential cur
rent measurement. Nevertheless, the current trans
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 grea
ter than the rated burden of the transformer. If a clo
sed iron core transformer is connected, it is neces
sary to determine the operational accuracy limit
factors in order to check that there is sufficient con
formity at the highest fault currents. The values speci
fied on the rating plate for the accuracy limit factor n
are minimum values.
The operational accuracy limit factor n' can be calcula
ted from the data of the current transformer and the
connected load using the following equation:
P ) P
N E
n + n @
P ) P
W E
whereby:
n' = Operational accuracy limit factor
n = Accuracy limit factor
P = Rated burden of the current transformer
N
P = Internal burden of the current transformer
E
P = Connected burden
W
The internal burden P is not specified on the rating
E
plate of the current transformer. It can be determined
by measuring the resistance of the secondary win
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..
Siemens AG January 1999
Preparation instructions
System manual
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 fol
lowing demands on the current transformer:
1. The current transformers may not saturate due to
the maximum steady-state through fault current:
I
kg max
n w
I
Npr
I = Maximum steady-state fault current
kd max
I = Primary rated current of the transformer
Npr
2. Any deviation of current transformer data (opera
tional accuracy limit factors) at both ends must be
within the following range (assuming the same
primary currents):
3/4 v p ' / p ' v 4/3
1 2
For detailed determination of current transformer re
quirements, see appendix A.4.
5.2.6

Demands on the pilot wires

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
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 preven
ted by using highly conductive cable sheaths and spe
cial armor (minimum possible reduction factor for
5
Order no. E50417-G1176-C069-A1
-3
).
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