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European Journal of Scientific Research - EuroJournals

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926 Samuel N. Ndubisi and Marcel .U. Agu<br />

STEP 1: Determination <strong>of</strong> the degree <strong>of</strong> firing (DOF) <strong>of</strong> the rules<br />

The (DOF) <strong>of</strong> the ith rule consequent is a scalar value which equals the minimum <strong>of</strong> the two antecedent<br />

membership degrees.<br />

Table 1: Decision table <strong>of</strong> 121 rules<br />

Integral <strong>of</strong> voltage error VI<br />

Voltage error Ve NV NL NB NM NS ZR PS PM PB PL PV<br />

NV NV NV NV NV NV NV NL NB NM NS ZR<br />

NL NV NV NV NV NV NL NB NM NS ZR PS<br />

NB NV NV NV NV NL NB NM NS ZR PS PM<br />

NM NV NV NV NL NB NM NS ZR PS PM PB<br />

NS NV NV NL NB NM NS ZR PS PM PB PL<br />

ZR NV NL NB NM NS ZR PS PM PB PL PV<br />

PS NL NB NM NS ZR PS PM PB PL PV PV<br />

PM NB NM NS ZR PS PM PB PL PV PV PV<br />

PB NM NS ZR PS PM PB PL PV PV PV PV<br />

PL NS ZR PS PM PB PL PV PV PV PV PV<br />

PV ZR PS PM PB PL PV PV PV PV PV PV<br />

For instance, if Ve is PM with a membership degree <strong>of</strong> 0.4 and VI is PS with a membership<br />

degree <strong>of</strong> 0.2 then the degree <strong>of</strong> firing <strong>of</strong> this rule is 0.2.<br />

STEP 2: Inference mechanism<br />

The inference mechanism consists <strong>of</strong> two processes called fuzzy implication and rule aggregation. The<br />

degree <strong>of</strong> firing <strong>of</strong> rule interacts with its consequent to provide the output <strong>of</strong> the rule, which is a fuzzy<br />

subset. The formulation used to determine how the DOF and the consequent fuzzy set interact to form<br />

the rule output is called a fuzzy implication. In fuzzy logic control the most commonly used method for<br />

inferring the rule output is mamdani method [9, 10, 11].<br />

STEP 3: Defuzzification<br />

To obtain a crisp output value from the fuzzy set obtained in the previous step, a mechanism called<br />

defuzzification is used. In this work output u is defuzzified according to the membership functions<br />

shown in fig 2. Centre <strong>of</strong> area method [7] is used for defuzzification.<br />

Figure 2: Membership functions for output u<br />

Tuning: To achieve a fast response from the controller during faults, the FLC can be tunned with the<br />

parameters g0, g1 and g2 as shown in fig 4.<br />

3. A Power System Dynamic Model<br />

In this paper, simplified dynamic model <strong>of</strong> a single-machine infinite bus power system is considered<br />

[12]. This model consists <strong>of</strong> a single synchronous machine connected through a parallel transmission

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