Transformer FAQ
Cast resin vs VPI dry-type transformer: what is the difference?
Cast resin embeds the winding in a solid resin body, while VPI impregnates the winding insulation with varnish under vacuum and pressure.
Direct answer
Cast resin construction provides a solid encapsulated winding with strong resistance to moisture and contamination. VPI construction uses vacuum-pressure impregnation to fill winding voids with insulating varnish while retaining a more open winding structure. Neither system is universally superior; the selection depends on environment, thermal duty, mechanical requirements, repair strategy, standards and cost.
Cast resin characteristics
Cast resin windings can provide strong environmental protection and low partial-discharge performance when correctly manufactured. Thermal expansion, resin formulation and quality control are important.
VPI characteristics
VPI windings generally offer good heat transfer and avoid a thick rigid casting. Environmental protection may rely more heavily on insulation system quality and enclosure design.
How to choose
Compare humidity and contamination, thermal cycling, overload duty, partial-discharge requirement, enclosure, repairability, certification and local market practice.
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