New DGA sensor for power transformers sealed by gas cushion

Summary

Many transformer faults such as electrical discharges, thermal overheating, etc. produce specific gases. Depending on the nature of the damage, the insulating oil or the cellulose paper is decomposed. In this process, gas is released in dissolved or undissolved form. Through Dissolved Gas Analysis (DGA) by means of gas chromatography, the gas produced by the failure can be broken down to its constituent gas elements. The concentration, as well as the time course, is of interest in the evaluation of these key gases. In the case of severe faults in power transformers, generation of considerable amount of undissolved gases is expected, which escape from the insulation oil and diffuse into the gas cushion. As a cost-effective and compact alternative to gas chromatography, the development of a semiconductor-based monitoring system of the undissolved fault gases caused by malfunctions is proposed. For this purpose, gas sensors are examined with regard to their suitability and accuracy when exposed to harsh environments similar to those inside a transformer. Hydrogen is one of the main indicators of most common faults in a transformer. Therefore, the focus of the investigation lies on detecting and tracking the concentration of undissolved hydrogen, since hydrogen is a key gas with a very poor solubility in insulating liquids. The proposed monitoring system is based on a set of integrated sensors, tracking the concentration of hydrogen continuously, through which abnormalities in the insulating system can be detected. In order to determine a suitable sensor that can withstand the harsh environment inside a transformer, the behavior of various sensors was investigated in different pressure and temperature ranges, as well as their long term application in inert oxygen-free surroundings. Their sensitivity in comparison to application in atmospheric air was also investigated. The monitoring system is further equipped with environment sensors, making it possible to monitor and compensate the acquired data from the gas sensors.

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Keywords

DGA Sensor, Power Transformer, Online Monitoring, Hermetically Sealed Transformer