| 1 | Conservator-Type Oil-Immersed Transformer | Mineral-oil insulation with a conservator tank, radiator surfaces and ONAN cooling Oil-immersed ONAN | Typically 50 kVA to 2,500 kVA for distribution networks | Uses an expansion space to accommodate changes in oil volume caused by temperature variation. A breather is normally fitted to control moisture entering the conservator. | Outdoor substations, utility distribution networks, industrial facilities and commercial developments with adequate space. | Established design, good efficiency, strong overload capability and generally lower initial cost than dry-type alternatives. | Requires oil-quality testing, leak inspection and periodic checks of bushings, breathers and protection devices. Combustible liquid requires suitable fire and environmental controls. | Choose this type when outdoor installation, cost efficiency and conventional maintenance access are more important than minimizing fire risk. |
| 2 | Hermetically Sealed Oil-Immersed Transformer | Sealed tank containing insulating oil, with no free communication between the oil and the atmosphere; commonly cooled by ONAN Sealed tank ONAN | Typically 50 kVA to 2,500 kVA | The tank may use flexible corrugations or a gas cushion to accommodate oil expansion. Atmospheric moisture and oxygen are largely excluded. | Outdoor substations, locations with high humidity or dust, and installations where low routine maintenance is preferred. | Reduced moisture ingress, no silica-gel breather maintenance and lower exposure of oil to oxidation. | Internal faults can be more difficult to inspect directly. Tank condition, pressure-relief equipment and oil leaks still require regular inspection. | Consider this design where a sealed enclosure is beneficial and the transformer will be installed outdoors with appropriate oil containment. |
| 3 | Oil-Immersed Transformer with Off-Circuit Tap Changer | Oil-immersed transformer fitted with a de-energized tap changer, normally offering several voltage adjustment positions DETC Oil-immersed | Commonly available across small and medium distribution ratings | Allows the 11 kV primary winding ratio to be adjusted only when the transformer is isolated and de-energized. Typical adjustment is often around ±2.5% or ±5%, depending on the design. | Networks where the incoming medium-voltage level varies seasonally or where commissioning voltage adjustment is required. | Simple, robust and less costly than an on-load tap changer. It can improve the 400 V output voltage under known supply conditions. | Voltage cannot be changed while energized. Incorrect tap selection can produce an unsuitable secondary voltage or excessive flux. | Select the tap range from the actual 11 kV operating voltage, not only the nominal voltage. Confirm the required secondary voltage at normal load. |
| 4 | Natural-Ester-Immersed Transformer | Transformer filled with biodegradable natural ester fluid; available in sealed or conservator configurations and normally ONAN cooled Natural ester ONAN | Commonly used from small distribution ratings to large plant transformers, subject to manufacturer design | Natural ester fluids generally have a higher fire point than conventional mineral oil and can provide good moisture tolerance for cellulose insulation. | Indoor or environmentally sensitive sites, renewable-energy projects, urban substations and areas requiring improved fire performance or biodegradability. | High fire point, good biodegradability and potential to extend paper insulation life when the system is correctly designed. | Usually higher purchase cost. Fluid compatibility, temperature limits, testing procedures and local fire regulations must be verified. | Use when environmental impact, fire safety and long insulation life justify the additional cost and specialized fluid management. |
| 5 | Synthetic-Ester-Immersed Transformer | Transformer filled with synthetic ester fluid, typically in a sealed tank and cooled by natural air circulation through the tank and radiators Synthetic ester Sealed or conservator | Commonly selected for distribution and special-purpose installations where high fire performance is required | Synthetic ester fluids have a high fire point and are generally less biodegradable than natural esters but can offer strong thermal and oxidation stability. | Indoor substations, tunnels, high-rise buildings, transport infrastructure and sites with demanding fire-safety requirements. | Improved fire safety compared with conventional mineral oil and suitability for installations with limited fire separation. | Higher fluid and equipment cost. The specification must address fluid compatibility, enclosure ventilation and disposal requirements. | Choose this type when fire performance and thermal stability are critical and the project budget supports a specialty insulating fluid. |
| 6 | Cast-Resin Dry-Type Transformer, AN | Medium-voltage windings encapsulated in solid resin; no liquid insulation; cooled by natural air circulation Dry type AN | Typically 100 kVA to 3,150 kVA, subject to design and installation conditions | Uses air as the cooling medium and cast resin as the principal insulation around the windings. It is commonly installed inside a ventilated enclosure or room. | Indoor commercial buildings, hospitals, data facilities, factories and locations where liquid leakage is unacceptable. | No liquid leakage, low fire risk, reduced environmental contamination risk and generally straightforward visual inspection. | Usually larger or more expensive than an equivalent oil-filled unit. Ventilation, dust control, humidity management and acoustic design are important. | Select AN cooling when the continuous load can be served without forced fans and maximum simplicity and low auxiliary power are desired. |
| 7 | Cast-Resin Dry-Type Transformer, AF | Cast-resin dry-type transformer fitted with fans for forced-air cooling Dry type AF | Commonly used where additional capacity is needed from a dry-type platform | Fans increase heat removal and may allow a higher permissible rating than the natural-air rating, provided the manufacturer specifies the rating and duty. | Indoor substations with intermittent peaks, restricted footprint, variable loads or limited space for a larger naturally cooled transformer. | Higher output capability from a given enclosure size and improved response to short-duration load peaks. | Fans add noise, control complexity and maintenance requirements. The transformer must be able to operate safely at its natural-air rating if fans fail. | Use AF only after confirming fan redundancy, alarms, controls, acoustic limits and the required continuous rating without forced cooling. |
| 8 | Open-Wound VPI Dry-Type Transformer | Windings impregnated with resin under vacuum and pressure, normally air-cooled; the core and coils are not fully cast in solid resin VPI Dry type | Commonly used for low- and medium-voltage industrial distribution, subject to insulation and environmental class | Vacuum pressure impregnation improves mechanical strength and moisture resistance without completely encapsulating the windings in cast resin. | Industrial plants, motor-control systems, indoor substations and installations with controlled, relatively clean environments. | Good thermal performance, serviceability and flexibility for industrial applications; no liquid insulation is required. | More sensitive to conductive dust, salt, chemicals and persistent condensation than a fully cast design. Enclosure and environmental protection are essential. | Choose VPI when the room is clean and dry, service access is available and industrial thermal performance is more important than maximum encapsulation. |
| 9 | Amorphous-Core Oil-Immersed Transformer | Oil-immersed transformer using an amorphous-metal magnetic core to reduce no-load losses; commonly ONAN cooled Low no-load loss Oil-immersed | Most commonly applied to distribution ratings where the transformer remains energized for long periods | The core material has lower hysteresis losses than conventional grain-oriented electrical steel, particularly when the transformer operates at light load. | Utility distribution, commercial networks and renewable-energy sites with long energization periods and relatively low average loading. | Reduced no-load energy consumption can lower lifetime operating losses, especially for continuously energized transformers. | Higher initial cost and possible constraints on core size, mechanical design and availability. Load losses still depend on winding design and loading. | Evaluate total cost of ownership using local electricity tariffs, annual load profile, capitalization of losses and expected operating life. |
| 10 | Compact Packaged Substation Transformer | Integrated 11 kV/400 V transformer package combining the transformer with medium-voltage switching, low-voltage distribution and a protective enclosure; the transformer may be oil-filled or dry-type Packaged unit Outdoor or indoor | Commonly supplied from approximately 100 kVA to several MVA, depending on the package design | Factory-assembled solution with coordinated protection, metering, cable compartments and an enclosure. The transformer technology is selected separately within the package. | Construction sites, solar and wind facilities, remote loads, industrial compounds and projects requiring rapid installation. | Reduced site assembly, compact footprint, coordinated equipment and potentially shorter installation time. | Limited customization after manufacture. Access, ventilation, segregation, arc-fault performance, transport weight and civil works must be checked carefully. | Select this option when speed, compactness and integrated protection are priorities. Specify the transformer medium, cooling, enclosure rating and maintenance access clearly. |
| Common Technical Basis for an 11 kV to 400 V Transformer: The primary voltage is normally 11 kV line-to-line and the secondary voltage is 400 V line-to-line at 50 Hz in many distribution systems. A 400/230 V, three-phase, four-wire secondary is commonly used, with 230 V available between phase and neutral. Dyn11 is a frequently used vector group, but the correct vector group, neutral arrangement, impedance, tap range, insulation level, short-circuit withstand, losses and protection requirements must be confirmed against the local network standard and project load-flow study. |
| Important Selection Checks: Confirm maximum demand, diversity factor, motor starting current, harmonic content, expected ambient temperature, altitude, indoor or outdoor location, fire and environmental requirements, fault level, earthing arrangement, voltage regulation, noise limits, enclosure protection, future expansion and applicable standards such as the IEC 60076 series. |
