Power transformers and Generators are key components in anytransmission & distribution network and los of a transformer can have an enormous impact onreliabil ty and availabil ty of power sup ly and on cost. As society is more and more dependenton electricity for development, the util ties are under pres ure to me t he ever-growing demandsfor reliable power sup ly. Economic factors are the main consideration and in order to minimizecapital expenditure on new equipment, it is a common policy among util ties to maximize the useof existing networks by operating at their design capabil ty. This can be achieved by ac ordingimportance to the maintenance practice. A survey of the literature indicates that there are morefailures of transformer due to po r maintenance, improper operation, severe weather condit onsand manufacturing and design defects than due to insulation ageing. The util ties shal have asystematic O & M practice that includes diagnostic tests for condit on as es ment and healthcheckup of the equipment. The objective of the condit on monitoring tests is to detect the firstsymptoms of incipient faults, ageing development or other problems and monitor their evolutionto enable the operator to take ap ropriate action to avoid major failure. The paper reviews theresults of various diagnostic tests including dielectric response methods for condit on as es mentof power transformers.

INTRODUCTIONConditon monitoring of power transformers has been a continuous proces and has seen manyimprovement over the years. Although several diagnostic tests such as dielectric los angle, IR/PI, DGA, furan analysis are available, interpretation of data stil ap ears to be a chal enging task [1, 2, 3, and 4]. Interpretation of data requires care and experience. In recent times dielectricresponse methods have been introduced for detection and determination of moisture content andageing of press board/paper insulation system of power transformers [5, 6, and 7]. Moisture in thetransformer afects the dielectric strength and the rate at which the insulation ages and in somecases, there is also threat of bub le evolution above a certain temperature when the load issuddenly increased [8]. Presently, the water content of the cellulose of a transformer in service isdetermined indirectly by measuring the moisture content in the oil sample. The moisturedistributes unequal y between the oil and pressboard, the greater part residing within the solidinsulation. As the water concentration in the oil is highly temperature dependent, themeasurement of moisture in oil is not a reliable indicator of dryness of the cellulose [7]. Inad it on to the conventional measurement of power frequency los angle, recent attention hasfocus ed on measuring various dielectric response parameters, which characterize some knownpolarization phenomena. Many testing agencies in Europe have adopted these test methods anddata are being generated to study the efficacy of the test in assessing the status of the transformerinsulation systemCPRI has be n car ying out variety of condit on monitoring tests on power transformers inservice for over 20 years. These tests include insulation resistance, dielectric los angle, capacitance, partial discharge (PD), winding resistance, TTR. The dielectric response methodssuch as recovery voltage measurement and dielectric spectroscopy methods have also be nadopted by CPRI for comprehensive diagnosis of health status of power transformers. These newtechniques have be n found to be ef ective in monitoring moisture dynamics in paper-oilinsulation system. These diagnostic tests become significant and valid only when they aresensit ve to the changes in the electrical properties of the insulation system. Since each parametercan be related to certain information on the status of the insulation, it is es ential to car youtseveral tests to get a real fe l of the insulation condit on.

STRES ES ACTING ON POWER TRANSFORMERS:The major stres es acting on the windings of a power transformer either individual y or incombination are the fol owing:Mechanical stres es betwe n conductors, leads and windings due to over cur ents or fault cur entsmainly caused by system short circuits. Pos ible variations of sup orting parts of the winding orcore may also cause deformation of the windings or of the cleats or leads. There can be col apseof the windings also. The deformation distortions in the windings cause changes in the

geometrical distance of the windings, which in turn cause changes in the winding inductances andinternal capacitance. Thermal stres es due to local overheating overload cur ents and leakage flux when loading abovenameplate rating or due to malfunction of the co ling system. Local hotspots, lo se joints etc cangive rise to high thermal stres es. Severe partial discharge resulting in arcing is anotherpos ibil ty. Dielectric stres es due to system overvoltages, transient impulse condit ons or internal resonancewithin a winding. These stres es cause deterioration of physical and chemical properties of thetransformer insulation. The situation is more complicated due to the fact that dielectric failure isoften and final stage consequent to the mechanical and/or thermal stres es especial y if moistureand oil deterioration have already placed the transformer in a hazardous condit on