621.319 amplitude of pulses of partial discharges cumulative sum control chart dielectric barrier dielectric loss tangent dielectric spectroscopy distribution of electric potential electric field electric field distribution electrical capacitance electrical insulation systems external partial discharges fiberglass substrate frontal part of the rod glass-mica paper tape high-voltage composite insulation induction traction motor insulation resistance interfacial polarization location of the defect long-term electric and thermal agin long-term electrical strength manufacturing process stability partial discharges regulation of the electric field replacement circuit resonance frequency semiconductive coating stability of nonlinear properties surface resistivity