151 – Insulation Evaluation Analysis of a Power Transformer

Application Note / Model Data

Overview

A transformer is an electrical device that uses electromagnetic induction to convert the voltage level of alternating current power. Among transformers, power transformers that are used in power conversion have extremely high electric fields applied to their coils, so insulation technology like the winding structure, insulating materials, and insulation structure are vital in attaining miniaturization and higher capacity. In addition to normal power conversion, a transformer’s insulation structure has to be designed to be able to withstand lightning strikes and excess voltage during short circuits.
A transformer’s dielectric strength is dependent on the electric field intensity applied to the insulating material, so the electric field intensity needs to be examined before the safety factor for the insulation breakdown can be estimated.
This Application Note shows how to obtain the electric field intensity distribution when a lightning strike or short circuit occurs and the assumed maximum voltage is applied to the winding.

Electric Filed Distribution

Fig.1 shows the electric field distribution when a voltage difference is applied between the primary coil’s Fig.1 shows the electric field distribution when a voltage difference is applied between the primary coil’s insulating material and the secondary coil’s insulating material.
The maximum electric field intensity is about 8 kV/mm, which does not exceed the dielectric strength. From the distribution, it is also possible to confirm which parts large electric fields are generated in.

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