Cast Resin Dry-Type Transformer

Oil-free power distribution inside the furnace building

Overview

A cast resin dry-type transformer is the unit that distributes power inside a melt shop building without oil. Its windings are encapsulated in epoxy resin under vacuum, the core and coils are cooled by air, and there is no tank, no conservator and no insulating liquid to burn. That last point is the reason the type exists in a steel plant: the electrical room at the furnace platform, the continuous casting cellar and the rooms under the crane runway are spaces where an oil-filled transformer would require fire separation, drainage and a suppression system that the building may not be able to provide.

The family runs from 100 kVA to 2500 kVA at 10 kV, 20 kV or 35 kV, with F class insulation, 100 K temperature rise and AN or AF cooling. Where a conventional distribution transformer would be installed outdoors in a yard, this one goes where the load is: beside the drives of a casting line, next to a dedusting fan house, in a basement room below the melt deck. HANI Metallurgy supplies these units for the electrical rooms that feed a billet continuous casting machine and for auxiliary distribution around the furnace. The engineering that matters is not the absence of oil; it is what that absence does to overload capability, to protection and to the way the room is ventilated.

Technical Parameters

Typical values for reference. Final ratings, impedance and cooling scheme are fixed on the nameplate and the approved project datasheet.

How It Works Under Furnace Duty

The coil of a cast resin transformer is a winding embedded in epoxy. The resin is poured under vacuum so that no voids remain between conductor and insulation, because a void is where partial discharge starts and where the insulation eventually fails. Everything else in the design follows from that: the winding has to be dried and cast in one operation, the resin has to be formulated for the thermal expansion of copper, and the finished coil cannot be repaired the way an oil-immersed winding can. Partial discharge testing at the works is therefore not a formality; it is the only evidence that the casting is sound.

Heat leaves a dry-type unit by conduction through the resin to the surface and then by convection into the room air. There is no oil circulation to carry heat from the middle of the winding to a radiator, which is why the temperature rise is limited to 100 K and why the winding hot-spot is monitored rather than guessed. F class insulation is rated for a 155 class temperature, so a 100 K rise leaves margin above the rated ambient; that margin is what absorbs a hot afternoon in a room beside a furnace, a blocked air filter and a short overload. It is not a licence for continuous operation above the nameplate.

The practical consequence is that dry-type units are less forgiving than oil units under overload and under dust. Forced air cooling can raise the rating for a limited period, but only while the fans run and the air path is clear. In a melt shop the enemy is metallic dust from charging and tapping, which is conductive and which settles on the coil surfaces and in the air ducts. An enclosure of IP20 or higher is the minimum indoors, plus a scheduled clean-down; a unit installed open at IP00 in a dusty room will track across the coil surface long before it reaches its thermal limit.

Key Advantages

• No fire load: there is no insulating liquid to ignite or to spread, so the unit can sit in a furnace-side electrical room, a cellar or a room under the casting floor without a dedicated fire compartment.
• No oil maintenance: no oil sampling, no leak repair, no conservator and no gas relay; the maintenance list is inspection, cleaning and a check of the fan and temperature devices.
• F class margin at 100 K rise: the difference between the insulation class and the specified rise is the reserve that covers high ambient, dust and short overload.
• Short-time boost with forced air: fans raise the usable capacity for the peak hours of a casting sequence, without changing the continuous rating.
• Enclosure from IP20 to IP54: the same core and coil can be fitted for a clean electrical room or for a dusty platform, with the air path designed for the actual dirt load rather than assumed.

Selection and Commissioning

1. Fix the ambient before the rating. A dry-type unit is rated at a stated ambient; a room beside a furnace is not the same as a clean substation, and the rating has to be adjusted for the temperature the room actually reaches in summer.
2. Check the air path. Louvres, inlet area, outlet height and the position of the unit relative to the wall decide whether convection works; a unit pushed into a corner will run hot at half load.
3. Decide the enclosure with the dust load. IP20 indoors as a minimum, IP54 for a yard or a very dusty platform, and a filter regime that can actually be followed by the maintenance team.
4. Set the temperature devices. Winding temperature alarms and trips come from the embedded sensors; they should act on the upstream breaker and, where relevant, on the process interlock.
5. Plan the earthing and the screens. Core, enclosure, screens and the neutral arrangement are agreed on the drawing, because none of them can be added after the coils are cast.
6. Commission with a load run and a thermal image. After the first loaded hours, scan the coil surfaces and the connections; a hot joint at a cable lug looks exactly like a winding problem until it is photographed.

Frequently Asked Questions

Choose an Oil-Free Unit for the Melt Shop

Send the load list, the room ambient and the dust conditions, and HANI Metallurgy will return a rating, a cooling scheme and an enclosure class for your installation.