Siemens siprotec 7SA6, Manual

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Functions
6-242
7SA6 Manual
C53000-G1176-C156-2
6.17 Fault Location
Measurement of the distance to fault in the event of a short circuit is an important sup-
plement to the protection functions. The availability of the line for transmission of en-
ergy in the system can be increased by a more rapid determination of the fault location
and repair of any resultant damage.
6.17.1 Method of Operation
Starting Conditions The fault location function in the Distance Protection 7SA6 is a function which is inde-
pendent of the distance measurement. It has a separate measured value memory and
dedicated filter algorithms. The short-circuit protection merely has to provide a start
command to allow the selection of the valid measuring loop and the best suited time
interval for the storage of the measured signals.
The fault location function can be triggered by the trip command of the short-circuit
protection, or also by each fault detection. In the latter case, a fault location calculation
is also possible if a different protection device clears the fault. In the case of a fault
outside of the protected feeder, the fault location output cannot always be correct, be-
cause the measured values may be distorted by for instance an intermediate infeed.
Determination of
the Fault Location
The measured value pairs of fault currents and fault voltages (in intervals of 1/20 AC
cycle) are stored in a circular buffer and frozen shortly after the trip command is issued
before any distortion of the measured values occurs due to the opening of the circuit
breaker even with very fast circuit breakers. Filtering of the measured values and the
number of impedance calculations are automatically adapted to the number of stabi-
lized measured value pairs in the determined data window. If a sufficient data window
with stabilized values could not be determined, the alarm “
Flt.Loc.invalid
” is is-
sued.
The evaluation of the measured values in the short-circuit loops is carried out after the
short-circuit has been cleared. Short-circuit loops are those, which caused the trip. In
the event of tripping by the earth fault protection, the three phase–earth loops are eval-
uated.
With the memorized and filtered measured values, at least three pairs of results for R
and X are determined according to the line equation. With the pairs of results, the av-
erage values and standard deviations are calculated. After elimination of “deviants”
which are recognized by their large deviation from the standard deviation, a new av-
erage is calculated. This average for X is the fault reactance which is proportional to
the distance to fault. If several loops were evaluated, the loop with the smallest reac-
tance is valid. In this manner, the fault on the protected feeder is in any event deter-
mined during multiple faults or in the event of tripping by only the earth fault protection.
Output of the Fault
Location The fault location function issues the following results:
•
the short-circuit loop which was used to determine the fault reactance,
•
the reactance X per phase in
Ω
primary and
Ω
secondary,
•
the resistance R per phase in
Ω
primary and
Ω
secondary,
•
the distance to fault d in kilometres or miles of the line proportional to the reactance,
converted based on the set line reactance per unit line length,