Short Communication Obtaining a high Accurate Fault Classification of Power Transformer based on Dissolved Gas Analysis using ANFIS Patil Pallavi and Ingle Vikal PG Department of Electronics Engg., Bapurao Deshmukh College of Engg, Sewagram, MS, INDIA Available online at: www.isca.in (Received 18th December 2011, revised 28th December 2011, accepted 24th January 2012) Abstract Power Transformers are a vital link in a power system. Well-being of power transformer is very much important to the reliable operation of the power system. Dissolved Gas Analysis (DGA) is one for the effective tool for monitoring the condition of the transformer. To interpret the DGA result multiple techniques are available.IEC codes are developed to diagnose transformer faults. But there are cases of errors and misleading judgment due to borderline and multiple faults. Methods were developed to solve this problem by using fuzzy membership functions to map the IEC codes and heuristic experience to adjust the fuzzy rule. This paper proposes a neuro-fuzzy method to perform self learning and auto rule adjustment for producing best rules. Keywords: Dissolved gas analysis, fault diagnosis, fuzzy inference system, gas concentration. Introduction Power transformer plays an important role in power system. Hence for the reliable operation of power supply system the health of transformers must be properly maintained. Power transformer is subject to various thermal, electrical and mechanical stresses. These stresses can cause incipient faults, deterioration or even failure of power transformer. To prevent this timely fault detection via a suitable standard technique is must. Dissolved Gas Analysis (DGA) is an important tool for online monitoring of transformers of transformers1-3. Different standards are available such as the IEC 599 ratio codes, Rogers's ratio and Triangle ratio1-3. These ratio methods are efficient and simple to use. There are certain limitations of the ratio methods. In case of more than one fault present, only the dominating faults are represented in these rules. Due this some of the faults are left unrecognized. Another shortcoming is due to the structure of the IEC codes used. Known as the gas ratios these codes are quantized to define the crisp boundaries. Practically mostly these figures are fuzzy. Thus these codes could lead to errors. To find out the solution of this difficulty some of artificial neural network (ANN) methods were employed1-4. In these ANN based methods the key gas methods were not used which is very important for the correct fault diagnosis. Finally a fuzzy DGA method was developed which uses both IEC 60599 and the key Gas concentration5. Material and Methods IEC 60599 DGA Codes: During faults in the transformer due to electrical and thermal stresses, oil and paper decomposition occurs evolving gases that will decrease the heat dissipation capability and the dielectric strength of the oil. These released gases get dissolved in the oil, which are known as dissolved gases. Different fault are associated and are reflected by the different concentration of the gases in oil. IEC 50699 uses 3 gas ratios C2H 2/C 2H4 (acetylene upon ethylene) , CH 4/H2 (methane upon hydrogen) and C2H4/C2H6 (ethylene upon ethane). Each ratio is quantized to a classification code 0, 1 or 2. Thus there must be total 27 combinations but IEC 50699 defines only 11 combinations leading to non-decision diagnosis. Table-1 IEC gas ratios C2H 2/C2H4CH 4/H2C2H4/C2H6<0.10100.1-0.251100.25-11001-3121>3222 The IEC codes are extended into the expert rules using experiences in the field by filling in the gaps created by IEC. The new knowledge base is given in the tables 1 and whose main advantage is elimination of the non-decision problem with all the 27 combinations included. It shows the IEC codes for different gas concentration ratios. Fuzzy Diagnosis System: The fuzzy logic analysis involves three successive process namely: fuzzification, fuzzy inference and deuzzification. Fuzzification converts a crisp gas ratio into a fuzzy input membership. A chosen fuzzy inference system (FIS) is responsible for obtaining conclusions from the knowledge based fuzzy rules set of if - then linguistic statements. Defuzzification then converts the output values back into the crisp values. Input of the System: The input for our fuzzy diagnosis system are three gas ratios C 2H 2/C 2H4 (acetylene upon ethylene), CH4/H2 (methane upon hydrogen) and C2H4/C2H6 (ethylene upon ethane). The values of these ratios i.e Code 0, Code 1 or code 2 are each represented by a trapezoidal membership function. These inputs are given to a sugeno model for obtaining the output. Figure-1 Fuzzy (sugeno) analysis of the fuzzy rules Fuzzy Rules Base: The three gas ratios constitutes the fuzzy inputs. The if-then statements is used to make decision on the fuzzy inputs derieved from the three gas ratios. The fault s are described in the given table 2. For example if C2H2/C2H4 is 0, CH4/H2 is 2 and C2H4/C2H6 is also 0 then the fault type corresponding to this combination of the ratios is OH_T2 i.e thermal fault (overheating)150