Application of new test procedure for verification of water drop corona on innovative insulation cross-arms
The use of composite insulators has grown rapidly over the last couple of years. They are especially popular solution for refurbishment and upgrading of high voltage overhead lines. In these cases the insulator set-up can be a rather complex insulated cross-arm combining composite insulators in different operating positions. This will in turn lead to an increased electric field around the insulator end fittings and hardware including arcing horns and grading rings if they are not dimensioned properly. It is agreed internationally that to be able to dimension a composite insulator from corona point of view two main criteria for E-field parameters should be taken into consideration. The first one is the limitation of E-field on the metallic hardware at 1,8 kV/mm and the second is the limitation of the average electric field on the composite insulator surface at about 0,4 kV/mm. Both limiting parameters are primarily checked by E-field calculations. The first parameter can be verified with a standard RIV (Radio Interference Voltage) test. However, there is no available verification test for the second criterion. At the same time this criterion is based on water drop corona, which may in long-term reduce the hydrophobicity of the surface of composite insulators close to the high voltage fitting, and this issue should also be investigated.
STRI has already started to create a test method to verify that no water induced corona occurs; this work was supported by the Swedish TSO Svenska Kraftnät, which was looking to use such test for their internal guidelines. This method is based on a standard RIV-test with some adjustments, such as applying water drops in the region of the insulator where the highest electric field occurs (close to the high voltage end fitting). The preliminary suggested Water Drop Corona Induced test (WDCI-test) already went through a few modifications.
The overall goal is to finalize the development of a test method to verify that no “dangerous” water drop induced corona occurs at required levels of E-field and by this method to verify the results of E-field calculations.
The earlier test results have showed that the WDCI-test method could verify suspension composite insulators of several different types. A good compliance between E-field calculations and the results of pre-tests has been found as well. A verification of more complex structures, i.e. two composite cross-arm configurations is presented and discussed in this paper as well as the influence of the humidity range when performing the test.