Prevention of Recovery Overvoltages on High Voltage Testing Transformers

During flashover tests with ac voltages, both the transformer and the test object may be subjected to excessively high overvoltages. If the recovery overvoltages occur during the negative half cycle, as is generally the case, external flashovers may not necessarily take place and the resultant high stress can damage the internal insulation of the transformer or the test object. The danger of the recovery overvoltage is even more pronounced in extra high voltage tests where several transformer units are cascaded to provide the required test voltage levels.