Digital Signal Processing Reference
In-Depth Information
j
X
Q
j
X
Q
Z
PQ
R
F
Z
PQ
K'
K
R
F
K
Z
m
K'
Z
m
P
R
P
R
Fig. 11.21 Impedance measured for an intermediate short-circuit: a fault out-of-zone, pre-fault
current flowing into the line b fault in-zone, pre-fault current flowing backwards
It is commonly known that the impedance measured by the relay includes
positive sequence impedance of the line section PK and the fault resistance R
F
,
which is almost always seen as a complex impedance vector
R
F
. The reactance
part of
R
F
may be negative or positive, depending on the pre-fault current direc-
tion. The resulting impedance
Z
m
becomes either decreased or increased and may
fall in-zone for some external faults (over-tripping, Fig.
11.21
a) or out-of-zone for
some faults requiring fast tripping (malfunction, Fig.
11.21
b).
As one can see, distance protection relay may face serious problems with
discrimination of faulted section of the feeder. It is to be stressed that the majority
of contemporary distance relays do not apply any sophisticated algorithms for
distance-to-fault determination with elimination of the reactance effect, such as
those used in impedance-based fault locators. Common practice is just appropriate
shaping of the relay characteristic (expanding the curve in R direction), which is
good enough for most cases but cannot eliminate the problem completely. Here the
FL technique is proposed to enhance features of traditional distance relays. Below
the fuzzy protection scheme developed as well as its testing with the signals
obtained from EMTP-ATP simulation of faults are described.
The new protection developed is based on fuzzy concepts as well as adaptive
shaping of the relay impedance characteristic for the first zone in dependence on
prevailing power system conditions. Figure
11.22
shows the block scheme of the
proposed approach.
The scheme from Fig.
11.22
operates as follows. First, using selected algo-
rithms the current and voltage samples are processed (block 1) in order to deliver
the criteria values: reactance X (a measure of distance-to-fault) and additional
variables R
0
F
ðÞ
, I
L
, and I
0
that are used for adaptation of the relay operating
characteristic. All the criteria are fuzzified in blocks 2, 3, whereas the fuzzy
numbers are obtained in the way illustrated in Fig.
11.23
. The left and right
borders of the triangle MFs as well as the peak point are calculated according to:
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