Elements of Fuzzy Logic in Protective Relays - Digital Signal Processing in Power System Protection and Control - page 242

(a) (b)

10

1

0.8

8

0.6

0.4

6

0.2

t [s]

real fault
impedance value

0

4

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1

(c)

1

2

0.8

0

0.6

0.4

-2

0.2

t [s]

0

-3 -2 -1 0 1 2 3 4 5 6 7 8

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1

R [ Ω ]

Fig. 11.25 Distance relay operation for a single-phase in-zone fault (close to first-zone reach):
a trajectory of measured impedance b response of classical relay c response of FL adaptive relay

reactance component DX 1 is obtained that is further used for adaptation of the
relay. The new relay 1st zone reach in X direction is defined and then fuzzified
taking also into account the estimated information amount in the signal X (block
7). Finally, the fuzzified reactance signal is compared with the new fuzzy setting to
generate the final relay decision (block 8). More details about the presented
scheme can be found in [ 14 ].
The performance of the developed fuzzy adaptive distance relay has been
verified with various signals obtained from simulation of in-zone and out-of-zone
faults on EMTP-ATP model of a test power system (two sub-systems with a tie
line, to be protected). The operation of the new scheme has been compared with
distance protection with polygon characteristic as proposed in [ 2 ]. Below the relay
operation for a selected case of single-phase-to-ground fault is shown (Fig. 11.25 ).
The short-circuit occurred in-zone, at 80% of the line length while the relay reach
was set at 85% of the line impedance and thus proper response of the relay was
line tripping in shortest possible time (first-zone trip). As seen in Fig. 11.25 a the
trajectory of measured impedance did not enter the relay first-zone area and the
relay without adaptation failed (no response, Fig. 11.25 b). The cause for that was
the reactance effect that was not compensated for in the classical relay. On the
contrary, the developed FL adaptive relay responded properly in the considered
case, i.e. classifying the event as first-zone fault (Fig. 11.25 c).
The fuzzy distance protection developed has also been tested with the signals from
out-of-zone faults. In Fig. 11.26 the case of phase-to-ground fault at 90% of the line
length is shown. No malfunctions have been noticed in this case (Fig. 11.26 c),
contrary to classical non-adaptive relay, where improper operation was observed for
some faults due to underestimating of the measured fault-loop impedance
(Fig. 11.26 b). The FL protection responded also correctly to all unambiguous in-
zone faults with the decision time of the same range as for the classical non-fuzzy
protection.
One can say thanks to introduction of signal fuzzification and adaptive settings
the new relay has gained improved sensitivity and selectivity, especially for faults