Partial discharge inception voltage of power transformers sealed by gas cushions

Summary

Hermetically sealed power transformers offer many advantages over conventional distribution transformers. The space for an expansion tank can be saved and there is no additional cost for the maintenance of drying equipment. In combination with high temperature liquids, such as synthetic ester or silicone fluid, it is possible to reduce the cooling surface and overall tank size. Together with the increased fire safety, these transformers are ideal for installation in confined spaces, especially in wind turbines in remote locations or offshore. However, the transformers in wind turbines have highly volatile loads, which lead to severe changes in temperature and internal pressure, due to the sealed design. This investigation shows that these pressure changes can significantly reduce the partial discharge inception voltage (PDIV) inside these transformers and are probably the cause of many transformer failures in the field. To investigate the influence of pressure on the PDIV in the laboratory, a vacuum and pressure tight test vessel was constructed. The test vessel possesses a volume of 3 liters and is double walled to control the temperature. With this test vessel the PDIV of different insulating liquids was tested with two different electrode setups at varying ambient pressures from 600 to 1400 mbar. The measurements were performed under different gases. The first setup is a model of the transformer insulation with two electrodes and aramid paper in between. The other setup is the standard needle-sphere configuration according to IEC 61294:1993. The investigations with the insulation model have shown that PDIV decreases significantly at lower pressures for both gases. With dried air and synthetic ester at 20°C, the PDIV was reduced by over 60 % when lowering the pressure from 1013 mbar to 600 mbar. The decrease also depends on the type of gas. PDIV of the transformer model decreases with rising temperature. However, both temperature and pressure have little to no effect on the PDIV of the IEC standard configuration, questioning its ability to provide useful data for real liquid applications.

Additional informations

Keywords

partial discharge inception voltage, compact transformer, liquid, ester, silicone, pressure