Comparative Study Of Various Dissolved Gas Analysis Methods To Diagnose Transformer Faults

Dissolved gas analysis (DGA) is a reliable technique for detecting the presence of incipient fault conditions in oil immersed transformers. In this method the presence of certain key gases is monitored. The various analysis methods are : Rogers ratio, IEC ratio, Doernenburg, Duval triangle, key gas, artificial neural network (ANN) method. In this paper the various DGA methods are evaluated and compared. The comparative study is carried out from DGA data obtained from published papers. The key gases considered are hydrogen, methane, ethane, ethylene, acetylene. INTRODUCTION: Mineral oils is mixture of saturated hydro carbon paraffin whose general molecular formula is CnH2n+2 with ‘n ‘ in the range of 20-40. This oil acts as di electric medium and this heat transfer agent when used in transformers. During the occurrence of fault in the transformer, these gases are released within the unit. The rate of gas generation and its distribution indicates the severity of fault. Fault may occur due to overheating, arcing, partial discharge, over heating in cellulose, etc. The fault gases are methane(CH4),ethane (C2H6), ethylene (C2H4),acetylene(C2H2), hydrogen(H2),carbon monoxide(CO),carbon di oxide(CO2).non fault gasses are nitrogen(N2),Oxygen(O2). Depending up on the fault gas there are several technique to analyse the type if transformer fault. METHODOLOGY: The insulating oils breakdown to release small quantity of gases up on occurrence of fault. It is possible to distinguish fault such as partial discharge (corona), overheating, arcing, by means of DGA 1. Roger ratio method: In this method four ratio CH4/H2, C2H6/CH4, C2H4/C2H6 and C2H2/C2H4 are utilised. The code number that is generated can be related to a diagnostic interpretation as shown in Table 1,2 & 3. Table(1): 2. IEC method: This method similar to Roger’s ratio method except that the ratios C2H6/CH4 is excluded as it indicates only a limited range of decomposition. A detailed description of IEC method shown in table(4).