Abstract: Characterized by big volume and large o amount of oil in ultra high-voltage(UHV) transformers, the concentrations of the characteristic dissolved gases relating to malfunctions may become too low to be monitored normally by certain equipment if the detection limit of the testing method is not low enough. Thus a new testing method adopting gas chromatography (GC) with helium ionization detector (PDHID) is proposed, using the technology of helium ionization detector supplemented with ten-port valves, heart cutting and back flush to carry out the separation and analysis for 7 kinds of dissolved gases (H₂, CH₄, C₂H₆, C₂H₄, C₂H₂, CO and CO₂) in single injection within 7 minutes. The detection limit can reach 10-9 grade. Especially for C₂H₂ and H₂, the detection limit can reach 5×10^-9and 11×10^-9, respectively. The repeatability of the method is about 1% with the linearity correlation R² of all the components higher than 0.99. Compared with traditional GC, the new method of GC with PDHID is easier to operate, and the detector has much stronger peaking signal with the detection limit increased by 5-80 times. Moverover, hydrogen is no longer needed as the auxiliary gas such that the potential risk is reduced. Using the method to analyze the dissolved gases in the transformer oil is of great significance to discover the potential internal malfunctions of the UHV transformers in a timely way.

Key words: ultra high-voltage, transformer oil, dissolved gas, gas analysis, gas chromatography with helium ionization detector