Просмотр полной версии : High voltage transformer bushings

13.07.2012, 16:27
Bushings are insulation devices used where live conductor passes through earthed metal case of the transformer, reactor etc, or through walls and roof of a structure. Bushings ensure physical fixture of the conductor and the required degree of electric strength.

The simplest bushing design for a transformer lid is shown in the picture below. This bushing consists of the following elements:

A live conductor rod (or tube) position 1;
Insulation position 2, which mechanically secures the conductor rod and its electric insulation from the lid grounding;
Metal bushing position 3 with flange, which secures the bushing to the lid.
Top screen position 4, which evens the electric field near the top contact point for increased electric strength to eliminate stream corona in the air at operating voltage;
Bottom screen position 5, which evens the electric field to increase electric strength of the insulation oil gap between the bushing and transformer case.

The actual design of bushings, especially those for extreme voltages, is much more complex than this. They contain certain assemblies to ensure required technical and economic parameters, including high operational performance. However, this simplification is required to demonstrate the important features of all bushings.

The first thing to consider is that the bushing, just as many other (but not all) insulation systems, has outer and inner insulation. The processes occurring in the outer and inner insulation which define their respective electric strength, are significantly different and depend on a variety of factors. Therefore, the required electric strength of the outer and inner insulation is reached by different methods.

The second consideration is that the electric field in the simple, basic assembly is highly irregular in radial and axial directions. As a result, creation of bushings for voltages of 110 kV and above is impossible without taking special measures to regulate the electric field. The field in the insulation material between the conductor rod and the bushing away from the bushing’s edges is a field between coaxial cylinders, where radial intensity drop is inversely proportionate to the radius. Therefore, for this section, the ration of maximum and minimum intensities Emax and Emin is equal to the ratio R/r, where R is the inner radius of the bushing and r is he radius of the conductor rod.

The Emax / Emin ratio, defined by R/r of 2.5 to 4.0, as is typical for real bushings (if electric field regulation is facilitated), will, in the simplest bushing design as shown in the figure above, have the same values. This is why the use of dielectric strength of the insulation material (position 2 in the figure) will be quite uneven in thickness, which will lead to unreasonable increase of the bushing's radius.

The electric field near the edges of the grounded bushing according to the figure is sharply uneven. Field intensity immediately at the edges of the bushing (in the air and in the insulation material) may exceed the average intensity between the rod and the bushing in 5 to 10 times and above (depending on the curvature of the bushing’s edges). The intensity here has a significant component normal to the surface of the insulation material. The intensity drops significantly when moving away form the edges of the bushing in axial direction.

The electric strength of the insulation material in the area next to the edges of the bushing is the lowest due to the highly irregular electric field. It is here that corona discharge forms in the air at relatively low voltage, which is unacceptable at normal operating voltage, as it is a source of powerful radio interference and has a destructive effect on the insulation material.