In order to addresses the shortage of misdiagnosis with only the accuracy rate index

the concept of fault diagnosis sensitivity rate is introduced

together with the prevailing accuracy rate

a new criterion to evaluate the reliability of the gas-in-oil fault diagnosis methodologies for power transformers is formed by weighting. Considering the differences of occurrence probability among the fault types

a composite reliability evaluation index is proposed by introducing the reliability weightiness of each fault

which makes the assessment of the gas-in-oil diagnosis effectiveness more comprehensive and accurate. Seven typical gas-in-oil diagnostic methods are used to diagnose the 628 fault cases from power transformer operating nationwide in terms of the accuracy and sensitivity rates for each type of faults

further

both the advantages and disadvantages of each method as well as a comprehensive reliability are obtained. The new outcomes present the basis for appropriate selection of the most suitable gas-in-oil fault diagnosis method in practical applications

and lay a foundation for amelioration of the prevailing gas-in-oil diagnosis schemes.