Transformer oils of early fault characteristics of the gas is carbon monoxide and hydrogen, which is mainly the organic insulation materials and insulating oil inside the transformer, along with the increase of the transformer running time, and under the long-term effects of heat and electricity produced by aging, decomposition, and together they produce gas and carbon dioxide and hydrocarbon gases, are most of these gases dissolved in transformer oil.

The work principle of transformer on-line monitoring device is characteristic of transformer oil gas through attached directly to the transformer body sensor inside the osmosis membrane selectively into the gas detector, and with the sensor inside the fuel cell and oxygen in the air react to produce a reaction rate is proportional to the output signal, After the rectifier amplifies the output signal and temperature compensation signal, the device display in the form of volume fraction value μL/L to the technical personnel.

The sensor is a passive component, no rotating part, and does not need any reagents or other materials. The fuel it needs is characteristic gas. The sensitivity of on-line partial discharge monitoring of transformer is generally above 1000 pC, which is higher than the value specified in off-line transformer factory test. Therefore, the existing system mainly analyzes the transformer insulation status according to the long-term development trend of PD, rather than completely according to a single measurement value. In addition, partial discharge ellipse, two-dimensional spectrum and three-dimensional spectrum can be used to analyze the types of partial discharge.

In addition, the transformer core grounding current can be monitored online to judge whether the transformer core has multi-point grounding fault. The status of on-load voltage regulating switch of transformer and the dielectric loss of bushing are monitored on line, but the moisture in transformer oil, the temperature distribution of winding (determining the temperature of winding hot spot) and the aging parameters of paper insulation (such as furfural) cannot be monitored on line.