Effect of Humidity on High Voltage Testing of VPI Insulation Systems before Impregnation

Quality assessment of insulation systems at the white stage - i.e., after taping and installing coils/bars in slots but prior to vacuum pressure impregnation (VPI) - is an important and critical step. At the white stage, various factors can affect the quality and integrity of insulation systems. These factors include mechanical stresses related to winding manufacturing process and storage condition of mica tapes and un-impregnated coils/stators. Consequently, dielectric testing is essential to evaluate the condition of insulation systems. However, the moisture content of mica tapes, un-impregnated insulated coils and assembled windings is influenced by storage conditions, possibly leading to failure during high voltage testing. Because proper drying may enhance performance sufficiently to pass high voltage testing, work was undertaken to investigate the relationship among humidity, temperature and breakdown voltage. The samples consist of mica tapes were stored under various conditions of temperature and humidity. AC and DC dielectric testing was performed. In addition, a variety of insulation parameters were measured to determine appropriate voltage levels for dielectric testing.