100 kVA Copper Oil-Immersed Industrial Power Transformer
Oct 29, 2025
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Typical Industrial Duty
A 100 kVA oil-immersed transformer is often the interface between a medium-voltage supply and the low-voltage distribution board of a factory, workshop, water treatment plant or commercial building. It can be installed outdoors on a plinth or inside a ventilated substation room. The duty is usually continuous with a moderate daily load cycle, so loss performance over the whole day matters as much as the peak capacity.
Voltage Combinations and Connections
| Parameter | Typical Value |
|---|---|
| Rated capacity | 100 kVA |
| High-voltage winding | 6 kV, 6.3 kV, 10 kV, 10.5 kV or 11 kV class |
| Low-voltage winding | 400 V or 415 V, three-phase, four-wire |
| Vector group | Dyn11 or Yyn0 depending on the earthing and harmonic requirement |
| Tapping | Plus and minus 5 %, off-circuit, in 2.5 % steps |
| Cooling designation | ONAN, oil natural air natural, self-cooled |
| Insulating liquid | Mineral insulating oil to IEC 60296 |
| Enclosure arrangement | Sealed tank with corrugated walls, or tank with radiators |
The vector group fixes the phase displacement between primary and secondary and determines how zero-sequence current behaves on the low-voltage side. Dyn11 is widely used because it provides a low-impedance path for triplen harmonics in the delta winding and a stable neutral for unbalanced single-phase loads.
Tank, Cooling and Sealed Design
Sealed-tank (corrugated-wall) designs allow the oil volume to breathe through the elastic deformation of the tank wall rather than through contact with outside air, which greatly reduces moisture ingress and oil oxidation. Units with radiators or finned panels give a larger cooling surface for the same footprint and are used where ambient temperature is high or where the load cycle is severe.
Steelwork is grit-blasted and painted with a system rated for outdoor exposure. Cooling performance should be checked against the highest expected ambient temperature at the installation site, because the temperature-rise limits specified in IEC 60076-2 assume the reference ambient conditions defined in that standard.
Protection and Ancillary Fittings
Typical fittings on an industrial unit include an oil level indicator, a filler and drain valve, a pressure relief device or conservator with a breather, a grounding terminal, a nameplate and lifting lugs. Where a conservator is fitted, gas and oil actuated relays can be provided for internal fault detection; protective fittings for liquid-immersed transformers are covered by the IEC 60076-22 series of accessory standards. Enclosure ingress protection, where a cubicle-mounted arrangement is used, is classified according to IEC 60529.
Losses, Temperature Rise and Tolerances
Declared no-load loss and load loss values are nameplate data. Under IEC 60076-1 the measured total loss may not exceed the declared total loss by more than 15 %, and short-circuit impedance carries a tolerance of plus or minus 10 % of the declared value. Temperature-rise verification follows IEC 60076-2, which sets a top-liquid rise of 60 K and an average winding rise of 65 K for the standard temperature-rise classes.
Because capital cost is dominated by the loss evaluation over the life of the unit, purchasers should evaluate designs on total owning cost, weighting no-load loss for lightly loaded industrial sites and load loss for plant with sustained high loading. Efficiency levels required by regional eco-design regulations should be checked against the installed voltage and rating.
Stricter parameters for three-phase oil-immersed distribution transformers, including loss levels by rating, are given in GB/T 6451, while the general, temperature-rise, insulation-level and short-circuit requirements are set out in the GB/T 1094 series and the corresponding parts of IEC 60076.
Testing Before Delivery
Routine tests cover winding resistance, ratio and connection check, no-load loss and current, load loss and impedance, applied potential, induced potential and insulation resistance. Special or type tests may include temperature rise, lightning impulse withstand to IEC 60076-3, sound level determination to IEC 60076-10 and short-circuit withstand verification to IEC 60076-5. Insulating oil is tested for dielectric breakdown according to IEC 60156 before and after filling.
Frequently Asked Questions
Q: Which vector group should be specified?
A: Dyn11 is the common choice where unbalanced single-phase loads and triplen harmonics are present; Yyn0 is used where the application permits it and the distribution practice requires it.
Q: Can the transformer be installed indoors?
A: Yes, provided the room is ventilated so that the ambient temperature stays within the assumptions behind the IEC 60076-2 temperature-rise limits and the fire regulations for the insulating liquid are met.
Q: What is the benefit of a sealed corrugated tank?
A: The oil does not exchange air with the atmosphere, so moisture and oxygen ingress are minimised and the oil retains its dielectric strength for longer.
Q: How often should the oil be tested?
A: Dielectric strength is normally tested at commissioning and then at intervals agreed with the operator, using the breakdown test method of IEC 60156, with shorter intervals where loading is heavy.
Q: What loss tolerance applies?
A: Under IEC 60076-1 the total loss measured in test may exceed the declared total loss by no more than 15 %.
Q: Is a 100 kVA unit suitable for a motor load?
A: It can supply motor loads, but the starting current of large motors must be checked against the transformer's impedance to keep voltage dip within acceptable limits.