Is dry transformer suitable for high humidity environment?

Cast resin dry-type transformers can adapt to high-humidity surroundings properly with standardized structural design and protective treatment, while open-structured VPI dry transformers are generally not recommended for continuous high-humidity working conditions. Humidity has always been a key restrictive factor affecting the insulation stability of dry transformers, as excessive ambient moisture easily penetrates loose insulating materials, reduces insulation resistance, raises partial discharge risk and accelerates insulation aging. Different manufacturing technologies lead to entirely different anti-humidity performances between two mainstream dry transformer categories, directly determining their applicable damp installation scenarios such as coastal projects, underground pump rooms, river-side substations and tropical wet industrial plants.

Epoxy cast-resin dry transformers are the preferred choice for long-term high-humidity environments. Their high-voltage and low-voltage coils are entirely wrapped in solid epoxy resin through integral vacuum casting, forming a fully closed waterproof and moisture-proof protective shell with no exposed insulating paper or winding gaps. Dense epoxy effectively blocks water vapor from contacting internal copper conductors and core insulation. Most qualified manufacturers add anti-moisture additives inside epoxy raw materials and configure IP20 or higher metal protection enclosures with dehumidification space reserved inside the cabinet. Meanwhile, Class F or Class H high-grade insulation matching the casting structure further strengthens environmental adaptability. Even under relative humidity between 85% and 90% without condensation, encapsulated windings maintain stable insulation parameters. For extra severe damp coastal areas with salt fog besides high humidity, manufacturers can provide customized anti-corrosion surface coating and sealed cabinet with built-in space heaters to eliminate condensed dew inside the transformer chamber during temperature alternation.

On the contrary, conventional open-type VPI dry transformers feature exposed Nomex insulated windings after vacuum varnish impregnation rather than full encapsulation. Numerous gaps between coil layers leave channels for moist air infiltration. In persistent high humidity, water vapor adheres to insulating fiber surfaces, gradually lowering insulation resistance value and triggering creeping discharge or local breakdown in serious cases. VPI units can only run normally under relative humidity below 70% with constant ventilation and dry air supply. If users have to install VPI transformers in humid sites, extra independent air dehumidification systems and constant-temperature heating devices are compulsory, which largely increases overall project investment and later operation maintenance cost.

Several auxiliary design details further improve dry transformer humidity resistance. Most indoor damp installation projects equip cast resin dry-type units with automatic space heating strips. When ambient humidity rises to a preset threshold, heating equipment automatically starts to evaporate residual moisture inside the cabinet and prevent dew formation on coil surfaces. In addition, AN natural cooling plus AF forced fan dual configuration helps discharge internal damp hot air timely during daily operation.

In summary, epoxy cast dry transformers are fully suitable for high humidity environments after standard anti-damp configuration, whereas ordinary VPI open dry transformers avoid long-term placement in continuously humid locations unless supporting full set of dehumidification auxiliary facilities. Project buyers need to distinguish transformer types based on local annual average humidity before ordering equipment to prevent hidden insulation faults caused by improper model selection.

1. APA 7th

Ferreira, L., & Costa, M. (2020). Humidity influence on insulation performance of cast-resin and VPI dry-type transformers. IEEE Electrical Insulation Magazine, 36(5), 34–41. 

2. MLA 9th

Ferreira, Luis, and Marcos Costa. "Humidity Influence on Insulation Performance of Cast-Resin and VPI Dry-Type Transformers." IEEE Electrical Insulation Magazine, vol.36, no.5, 2020, pp.34-41,

3. GB/T 7714-2015

FERREIRA L, COSTA M. Humidity influence on insulation performance of cast-resin and VPI dry-type transformers[J]. IEEE Electrical Insulation Magazine, 2020,36(5):34-41.