Breakdown voltage behavior of PET thermoplastics at DC and AC voltages

The demand on thermoplastic encapsulation of coils and electronic devices has been increased recently as an alternative to thermosets for electrical insulation, heat dissipation and physical protection of the encapsulated devices from harsh environmental operating conditions. Compared with thermosets, thermoplastics offer excellent electrical properties, superior surface hardness, excellent dimensional stability and good chemical and heat resistance. This paper presents a Weibull statistical analysis of breakdown voltages of PET thermoplastics polyester cups used for encapsulating high voltage coils. The behavior of the polyester cups at DC and AC voltages and under dry and wet conditions is demonstrated. It was found in this work that the DC breakdown voltage of the polyester cups are much higher than the AC breakdown voltage under dry conditions. On the other hand, under wet conditions the DC breakdown voltage significantly decreased in comparison with the AC breakdown voltage. Moreover, it was found that the DC breakdown voltages are widely scattered in comparison with the AC breakdown voltages. It was also found that the statistical behavior of the DC breakdown voltages under wet conditions follow a mixed Weibull distribution. It is believed that this behavior is attributed to the presence of technical defects in some cups such as microvoids.