| Popis: |
Inside the paper-oil insulation of condenser-type bushings, the conducting surfaces or capacitive shields have been in use for many years to control electric field distribution. In the traditional design, the maximum radial stress in condenser body should not exceed the partial discharge inception stress. However, real operation experience and laboratory tests indicate that long termed damages caused by partial discharges frequently occur at the capacitive shields edges. This fact shows that the traditional approach is to some extent inappropriate. For this reason, an improvement of this criterion is that the maximum stress at the shield's edge should not exceed the partial discharge inception stress, which is determined through numerous laboratory tests on samples that represent bushing insulation. The condenser core of a bushing is built up around a center tube that may or may not be in the current carrying path. It is wounded from paper and impregnated with transformer oil. Capacitive shields within paper-oil insulation take the form of coaxial cylinders and constitute a system of cylindrical capacitors, arranged in such a way that the electrical stress in both radial and axial direction does not exceed certain critical values. The capacitance between any adjacent pair of capacitive shields is known as a partial capacitance and the bushing insulation is made up of a large number of partial condensers in series. Optimal number of shields and each shield's physical dimensions lead to acceptable dielectric stresses and the most economical design. Laboratory tests were performed according to test procedure described in [1] on three samples in order to determine criterion for partial discharge inception stress at shield edges. This test procedure is not destructive and when sample was given a suitable recovery time after the test its insulation returned to the initial state. The two parameters Weibull distribution, well known in reliability engineering, was used to fit the test data. As a result, voltage levels with low, medium and high probability of partial discharge inception were obtained. These results were coupled with finite element method based calculation results to acquire the value of electric field at shield's edge with low probability of partial discharge inception. This represents a new criterion for design of bushing insulation. Implementation of the new criterion into the algorithm that calculates optimal parameters of capacitive shields gave as result significant reduction of insulation volume, without field distributions deterioration. |
