A fuzzy logic based relay for power transformer protection

rotection schemes employing the differential relaying principle exhibit certain limitations in applications with power transformers. This is because the detection of a differential current does not clearly distinquish between internal faults and other possible conditions. Among the phenomena most likely to upset the current balance and cause the relay to malfunction are inrush magnetizing currents, stationary overexcitation of a core, external faults in the presence of current transformer (CT) saturation, and/or CT and power transformer ratio mismatch. To mitigate these problems, a number of more or less reliable protection criteria have been developed which support the traditional biased differential characteristic in combination with 2nd and 5th harmonic restraints [1,2]. They include the ∆-differential principle, direct waveshape identification, protective algorithms based on electromagnetic equations of the protected transformer, and adaptive approaches, to name just some of the recognition techniques introduced in recent decades. The measuring units of contemporary relays are also improved by the use of Fourier methods, Kalman filtering techniques and optimal state observers. Research work in the above areas naturally focuses on a multi-criteria approach to power transformer relaying. One result of this work has been the development of a general fuzzy logic based platform for a multi-criteria transformer relay that introduces several new artificial intelligence (AI) related concepts [3–5]. A protective device with AI features offers enormous potential for optimization. It has a number of internal coefficients, functions and thresholds [3, 4] that can be adjusted in order to tune a relay to a protected element and improve the quality of the protection. However, no recommendations exist for an approach of this kind; neither is any practical experience available that could be used to set the internal relay parameters mentioned. Research was therefore undertaken to resolve this problem. In the following, a look is taken at a 12-criteria Fuzzy Logic protective Relay (FLR) for power transformers and at the unique algorithms used for its automatic, off-line self-adjustment prior to installation.