Modern methods performed in 400/200 kV substation-Iron Gates 1-for 400 MVA, 400/220/22 kV autotransformers rehabilitation in order to increase their installed life and their service reliability

The paper shows an interesting case of a power station, one of the greatest power station of the Romanian Power System called Iron Gates I. The power station is near the Danube River at the border with Yugoslavia, where, between 1965 and 1970, was built a big hydroelectric power plant. The paper shows the major problems that occurred and some of the methods for solving these problems. All of these problems are related to the reliability of the autotransformers of the substation, in order to increase their installed life. One of these problems is the ageing of the insulating oil, which leads to the decrease of its dielectric strength. Another problem is the ageing of the winding insulation, which leads to a constant increase, towards and over the normal limits, of the dielectric dissipation factor of the autotransformer windings. The third problem is the ageing of the bushings, which leads also, to a constant increase, towards and over the normal limits, of the tangent of the loss angle of the bushings.