A gas chromatograph measures the gas concentration levels of fault gases contained in the gas within a headspace above the electrical insulating oil supply contained in a power transformer. An on-line fault detection system involves gas samples taken from transformers at a utility site in sequence with automatically controlled timer valves which open to draw a gas sample from each transformer headspace and then transfer the gas directly to a single gas chromatograph. The gas chromatograph includes an integrator which calculates the concentration level of each fault gas in the gas sample. The resulting fault gas data are fed to a computer programmed with a partition function based on Henry's Law to convert the headspace fault gas data to fault gas concentration levels within the transformer oil. This produces gas fault analysis data which are used to provide an early warning of specific transformer problems correlated to, indicative of and detected by excessive dissolved fault gas concentration levels in the oil identified with particular transformer faults. The invention avoids costly and time consuming prior art analysis techniques of extracting fault gases from the oil sample in an independent operation before the concentration levels can be measured.
