Dissolved gas analysis (DGA) has been widely used for fault diagnosis in a transformer. Artificial neural
networks (ANN) have high accuracy but are regarded as black boxes that are difficult to interpret. For
many problems it is desired to extract knowledge from trained ANN so that the user can gain a better
understanding of the solution arrived by the NN. This paper applies a pedagogical approach for rule
extraction from function approximating ANN with application to incipient fault diagnosis using the concentrations
of the dissolved gases within the transformer oil, as the inputs. The proposed method derives
linear equations by approximation the hidden unit activation function and splitting the input space into
subregion. For each subregion there is a linear equation. The experiments on real data indicate that the
approach used can extract simple and useful rules. Transformer incipient fault diagnosis can be made that
matches the actual fault present and at times the predictions better than those of the IEC/IEEE method.
The rule sets generated have been successfully checked for accuracy of predictions by applying them to
case studies.