Dry-Type or Oil-Immersed: Which Distribution Transformer Should You Specify?

When a distribution project reaches the transformer, the choice is not brand preference but installation, fire safety, cooling and lifetime cost. Zhejiang Deyan Electric Co., Ltd. (DEYAN ELECTRIC), a manufacturer of power distribution equipment, walks through the questions buyers ask most often.

I. Start with the Room, Not the Transformer

The answer that settles most selections is where the unit will be installed. A dry-type transformer holds no insulating liquid: the windings are encapsulated in cast resin or insulated with air. There is no oil to leak, no oil pit to build and no oil to burn, which makes dry-type units the standard choice for indoor rooms inside or attached to occupied buildings — hospitals, data centres, high-rise towers, metro stations. An oil-immersed transformer relies on mineral oil for insulation and cooling, so it normally belongs outdoors, in a standalone substation, or in a room with fire separation and oil containment. If the room sits inside a building, the answer is almost always dry-type.

II. Insulation and Cooling Are Two Different Systems

Oil removes heat far more effectively than air, so an oil-immersed unit of the same rating runs cooler and tolerates short-term overload better; a dry-type unit relies on air circulation and resin encapsulation to draw heat from the winding. Dry-type units are therefore sensitive to dust, humidity and poor ventilation, and the room must stay clean and dry, while oil-immersed units tolerate outdoor weather inside a sealed tank. No oil also means no leakage risk, no oil fire load and no spill containment inside the building.

III. Electrical Parameters and Loss Levels

Fix the parameters as for any other item of distribution equipment: rated capacity, high- and low-voltage ratings — typically 10 kV to 0.4 kV — vector group, tapping range and short-circuit impedance, which must be matched between units run in parallel. Deyan Electric builds both families from 30 kVA to 3,150 kVA: the SCB cast-resin dry-type series and the S11/S13 oil-immersed series.

Running cost, however, is decided by loss levels. In China, GB 20052 sets Grade 1, Grade 2 and Grade 3 limits, with Grade 3 as the mandatory market-entry level. The series number in the model carries the loss level: Deyan's 14-series dry-type units are offered as Tier 2 low-loss designs, while older 10-series dry-type or 11-series oil-immersed models reflect a higher-loss generation. Because no-load loss is charged for the whole service life, one grade of efficiency is usually repaid long before the transformer reaches end of life.

IV. Maintenance and End of Life

An oil-immersed unit needs its oil level, oil quality and sealing checked periodically, and is in effect a maintenance item. A cast-resin dry-type unit has no liquid to test and, if the room stays clean and ventilated, is close to maintenance-free. At end of life, its copper windings and steel core are straightforward to recover and there is no oil to dispose of — one reason dry-type units are often specified for healthcare and data centre projects with formal environmental targets.

V. What the Manufacturer Needs from You

To quote the right transformer, a manufacturer needs the installed capacity and load profile; high- and low-voltage ratings, vector group, tapping range and impedance requirement; installation location and ambient conditions; the loss grade required by the governing standard; and the delivery schedule. Deyan Electric supports scheme design, integrated assembly and factory commissioning for complete distribution packages, and can usually provide preliminary design options within one to three working days.

About Zhejiang Deyan Electric Co., Ltd.

Zhejiang Deyan Electric Co., Ltd. is located at No. 60 Binhai South Second Road, Yueqing Economic Development Zone, Wenzhou City, Zhejiang Province. The company manufactures 10–35 kV high-voltage switchgear, 35 kV high-current GIS cabinets, eco-friendly gas-insulated switchgear, ring main units, box-type and prefabricated cabin substations, cable branch boxes, and distribution transformers — including SCB cast-resin dry-type units and S11/S13 oil-immersed units — working to the ISO9001 quality management system and to international standards such as IEC. To specify a distribution transformer or an integrated distribution package, please contact Deyan Electric.