Dielectric properties of stator coil insulation of medium voltage motor

One of the non-destructive methods commonly used to characterize the electrical properties of power devices is polarization and depolarization current (PDC) method. The objective of this paper is first to compare the dielectric properties of the stator coil without overhang insulation, the stator coil after hot-press process, and the stator coil with overhang insulation. The second objective is to compare dielectric properties of newly wound stator coil, the stator coil heated at {\mathrm {60^{\circ}C}}, and the stator coil heated at {\mathrm {90^{\circ}C}} for a specific time. The stator coils in this research were designed for a 6.6 kV 260 kW synchronous motor. For the stator coil without overhang insulation, the slot portion stator coil was wound by Nomex paper, Kremica tape, and conductive tape. For the overhang insulation, the end portion was wound by Filosam tape and polyester tape. Then, the PDC measurement was performed. The parameters from PDC analysis were used to assess dielectric properties. These parameters were composed of the PDC shapes, capacitance ratio, polarization loss and conduction loss. Test results showed that different insulation types applied for each part of the stator coil manufacturing provided different dielectric properties. This point should be considered when PDC measurement is performed with the whole insultion system of the high voltage motors.