200 kVA Liquid-Immersed Transformer for Commercial and Industrial Loads

Why Liquid Cooling Fits Commercial and Industrial Sites

Liquid immersion transfers heat from the windings to the tank wall and radiators far more efficiently than air alone, so an oil-immersed transformer carries more apparent power in a smaller volume than a dry-type unit of the same rating. That compactness matters in commercial basements and factory service rooms where floor area is scarce and clearances are tight. A sealed or conservator tank also keeps the active part away from dust, humidity and mildly corrosive atmospheres, which suits workshops and coastal sites. The trade-off is that the unit contains insulating liquid, so fire protection, bunding and liquid containment have to be designed from the start.

Load Profiles in Commercial Buildings

In offices, hotels, shopping centres and hospitals the load is dominated by lighting, heating and ventilation equipment, lifts, pumps and IT systems. The daily profile rises quickly in the morning, peaks during working hours and then falls to a small night load, so the transformer must carry the peak without exceeding its temperature rise limits while remaining efficient at part load. A 200 kVA unit commonly feeds a single floor, a wing or a small building through a low-voltage main distribution board. Because lighting and socket circuits are single phase, the low-voltage winding accepts a substantial neutral load, and a Dyn11 vector group keeps triplen harmonic currents out of the supply.

Load Profiles in Factories and Workshops

Factory loads are motor driven and include pumps, compressors, conveyors, fans and machine tools. Direct-on-line starting draws a current several times the rated value and produces a voltage dip that can disturb other equipment, while variable-speed drives and rectifiers add harmonic content that raises stray and eddy losses in the windings and the core. Sizing therefore has to consider the starting duty and the harmonic spectrum, not only the steady apparent power. Where harmonic content is high, the designer may reduce the design flux density, specify a lower-loss core grade or select a larger rating so that hot-spot temperature stays within the limits of the insulation system.

Sizing, Impedance and Voltage Regulation

Sizing starts from a load schedule that lists connected loads, demand factors and diversity between consumers. The calculated maximum demand is compared with the rated power and a margin is added for future growth. Impedance voltage, commonly around four percent for distribution ratings, sets the voltage regulation under load and the magnitude of the short-circuit current, so it has to be coordinated with the downstream protection. A tapping range of plus or minus five percent, or two steps of two and a half percent, allows the output to be corrected where the supply is persistently above or below nominal. Total cost of ownership should weigh no-load loss against load loss at the expected average loading.

Installation, Cooling and Maintenance

The unit is usually installed in a dedicated room or an outdoor enclosure. The room must be ventilated for the heat rejected at full load, and the floor should be able to contain the insulating liquid as required by local codes. Connections are made through cable boxes, bushings or busbars, and the tank and neutral are earthed. Commissioning covers checking of the vector group, phase rotation, insulation resistance and liquid level. Routine maintenance includes inspection of gaskets and connections, verification of protective devices, cleaning of cooling surfaces and, for conservator designs, periodic oil sampling in line with IEC 60422. Sealed units need far less attention over their service life.

Frequently Asked Questions

Q: How do I decide between 200 kVA and a larger rating?
Compare the calculated maximum demand with the rating after applying diversity, then add margin for growth and for motor starting duty before moving up to the next standard size.

Q: Does harmonic load change the sizing calculation?
Yes. Harmonic currents increase stray and eddy losses, so a harmonic-rich load calls for a lower design flux density or a derated rating to keep hot-spot temperature within limits.

Q: Is indoor installation acceptable for a liquid-immersed unit?
Yes, provided the room is ventilated for the heat load and the required clearances and liquid containment are provided by the installation design.

Q: How does impedance voltage affect motor starting?
A lower impedance voltage reduces the voltage dip during starting but increases short-circuit current, while a higher impedance limits fault current at the cost of poorer regulation.

Q: What maintenance does a sealed transformer need?
Periodic visual inspection, tightening and cleaning of connections, verification of protective devices and checks of the liquid level are generally sufficient.