Elements of Fuzzy Logic in Protective Relays - Digital Signal Processing in Power System Protection and Control - page 238

Fig. 11.20 Equivalent
diagram of the line P-Q with
phase-to-ground fault

Z SP Z PK Z KQ Z SQ

P I P Q

K

3 I 0

R F

E P E Q

• simple description of systems, for which a detailed mathematical model is too
complex or is not known.

Among shortcomings of the fuzzy approach the following can be mentioned:

• problems with choice of the effective defuzzification method for given task and
its practical implementation in on-line mode,
• problems with obtaining enough data for fuzzy system design, especially for the
systems already in operation.
The following (not exhaustive) list of possible applications of fuzzy systems in
power systems [ 1 , 3 , 4 , 7 , 11 , 13 - 20 ] is intended as exemplary only:

• identification of fault type in power system transmission lines,
• fault location in transmission and distribution lines,
• multi-criteria differential protection of power transformers,
• as well as, being examples of non-relaying problems,
• voltage and speed control of synchronous generators,
• diagnostics of power transformers,
• development planning and quality assessment of power networks,
• reactive power flow control,
• load forecasting, etc.

Below an example of fuzzy scheme is described that was intended for
improvement of distribution/transmission line distance protection [ 14 , 16 ]. The
scheme incorporates the ideas of signal fuzzification, fuzzy settings and fuzzy
decision-making that is based on Mamdani approach and compositional reasoning.

11.4.1 Example of Fuzzy Logic Application for Distance
Protection

The distance relaying technique is used as primary protection mainly in trans-
mission lines but in some countries it is also applied for distribution lines, mostly
in meshed urban networks. Although the idea of distance protection was intro-
duced a few decades ago, its present-day digital implementations are not free from
problems evoked by impedance measurement errors due to both inaccuracy of
used algorithms and such system factors as influence of fault resistance, etc. The
commonly recognized problem concerns so called reactance effect. Figure 11.20
shows equivalent circuit that can be used to determine the impedance seen by the
distance relay installed at the beginning of the line.