The cast resin versus oil-immersed question gets answered by building layout more often than by electrical argument. In Vietnam the deciding factor is usually moisture: a factory room that condenses will punish an unprotected winding every morning of the wet season, and an encapsulated coil simply does not care.
Where Each Type Normally Lands
Cast resin dry type transformers up to 6000 kVA are the default recommendation for indoor rooms — no oil, no oil handling, no bund, and the epoxy encapsulation is indifferent to condensation. Oil-immersed units from 250 kVA to 16 MVA are the default outdoors, where the plot has space, the ambient is high but stable, and the rating climbs past what a dry type unit reaches economically. Most industrial-zone projects end up with both: a compact substation at the plot boundary and dry type units inside the building.
- Cast resin — indoor rooms, multi-storey buildings, any location where fire risk and oil containment are part of the approval, and anywhere condensation is expected.
- Oil-immersed — outdoor plots, utility feeders, higher ratings, and sites where the transformer sits inside a compact substation enclosure.
- Compact substation — when the factory wants one tested scope covering MV switchgear, transformer and LV board rather than three separate packages.
The Humidity Argument in Detail
A dry type winding that has absorbed moisture does not fail the day it gets damp — it fails months later, when the next energisation or a switching surge finds a weakened insulation system. That is the reason Vietnamese consultants lean towards cast resin even where the electrical case is neutral. Where a dry type unit has already absorbed moisture, the recovery is a controlled dry-out with insulation resistance tracked over time, not simply switching it on and hoping.
Network Context
EVN operates the grid at 50 Hz, with industrial connections commonly at 22 kV or 35 kV and low-voltage supply at 380/220 V. Equipment follows TCVN standards, which align with IEC 60076 on ratings and testing. Coastal industrial parks add salt to the humidity, which raises the anti-corrosion class on outdoor enclosures and on the hardware.
What We Supply
- Cast resin dry type transformers up to 6000 kVA for indoor factory rooms.
- Oil-immersed transformers from 250 kVA to 16 MVA for outdoor and utility duty.
- Compact substations from 100 kVA to 8000 kVA for industrial-zone plots.
- Prefabricated substations where factory capacity is added in phases.
More on the market is on our Vietnam page.
Testing and Delivery
Routine tests follow IEC 60076, with factory witness testing arranged where the consultant or lender requires it. Conformity is supported per project. Standard lead time is 30 days from drawing approval with a two-year warranty, quoted FOB, CFR or CIF against the shipment plan.
Frequently Asked Questions
Which type should a Vietnamese factory specify?
Cast resin for indoor rooms and multi-storey buildings because of humidity and fire containment; oil-immersed for outdoor plots and higher ratings, usually inside a compact substation.
Can the dry type unit handle sustained humidity?
The encapsulated winding is designed for it, which is why cast resin is the standard recommendation here. Room ventilation and drainage still need to be detailed.
What is the lead time?
Standard lead time is 30 days from drawing approval, with a two-year warranty.
Talk to Our Engineering Team
Send the single-line diagram, the load list and the site conditions. We will come back with a proposed arrangement, the applicable standard set and a quotation on FOB, CFR or CIF terms. Email info@xsdfftransformer.com, call +8615867897761, or message WhatsApp +852 5416 2620.