250 kVA Three-Phase Oil-Immersed Transformer: Technical Parameters and ISO 9001 Certification
Nov 03, 2025
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What a 250 kVA Three-Phase Oil-Immersed Transformer Is Used For
A 250 kVA three-phase oil-immersed distribution transformer is a standard building block of medium-voltage distribution. It steps a 6 kV, 6.3 kV, 10 kV, 10.5 kV or 11 kV primary down to a 0.4 kV secondary for commercial buildings, factories, water works, mines and remote settlements. Because the windings are immersed in insulating oil, the unit combines compact dimensions with effective cooling and insulation, and it is normally supplied as a sealed tank without an oil conservator, with the oil volume accommodated by the elasticity of the tank and the radiator fins as load and temperature change.
Why Sealed Tank Construction Is Used
A fully sealed transformer removes the conservator and the oil-to-air interface, so the oil does not absorb atmospheric moisture and the oil does not need periodic topping up. The tank must be designed and tested for the pressure variation produced by thermal cycling, which is covered by the pressure and vacuum tests required in the power transformer standards. The practical benefits for the operator are lower maintenance and a longer insulation life, because water content and oxygen content in the oil stay low.
Technical Parameters of a 250 kVA Unit
The table lists the parameters of a 250 kVA, 6 kV to 11 kV / 0.4 kV sealed oil-immersed distribution transformer rated for 50 Hz operation.
| Parameter | Value | Basis |
|---|---|---|
| Rated capacity | 250 kVA | GB/T 6451, IEC 60076-1 |
| High voltage / tapping range | 6, 6.3, 10, 10.5, 11 kV, plus or minus 5 % or plus or minus 2 x 2.5 % | Off-circuit tap changer |
| Low voltage | 0.4 kV | Standard distribution secondary |
| Vector group | Dyn11, Yzn11 or Yyn0 | GB/T 1094.1 |
| No-load loss | 400 W | Measured at rated voltage, rated frequency |
| Load loss | 3,200 W at 75 degrees C reference | Measured at rated current |
| Impedance voltage | 4 % | Short-circuit impedance |
| No-load current | 1.7 % | Percentage of rated current |
| Overall dimensions | 1,310 x 940 x 1,605 mm | Length x width x height |
| Gauge / track | 550 x 550 mm | Wheel centre spacing |
| Weight | Active part 695 kg, oil 210 kg, total 1,135 kg | Dry weight reference |
Loss Levels and Energy Cost
No-load loss is the part of the loss bill that runs 8,760 hours a year regardless of load, so it dominates the operating cost of a lightly loaded distribution transformer. Each successive national design series has cut no-load loss at the same rating, and a 250 kVA unit in current production shows a no-load loss of about 400 W against the noticeably higher figure of older S7-class designs. Load loss, at 3,200 W for this rating, appears only in proportion to the square of the load factor, so the two loss figures must be evaluated separately rather than added together as a single efficiency number.
Applicable Standards and Routine Tests
The governing standards are the GB/T 1094 series for power transformers, read together with GB/T 6451 for the technical parameters and requirements of three-phase oil-immersed power transformers, and the IEC 60076 series for international projects. IEC 60076-2 and GB/T 1094.2 set the temperature rise limits used in design, including a top-oil temperature rise of 60 K and an average winding temperature rise of 65 K under continuous rated load. Routine tests carried out on every unit cover winding resistance measurement, voltage ratio and phase displacement check, short-circuit impedance and load loss measurement, no-load loss and no-load current measurement, and the dielectric routine tests. Insulating oil is normally supplied to IEC 60296 class requirements, with breakdown voltage and water content verified by certificate.
Installation Conditions and Site Requirements
The standard environmental envelope for this class of unit is a maximum ambient temperature of plus 40 degrees C, a minimum ambient temperature of minus 25 degrees C, an installation altitude below 1,000 m and a monthly average relative humidity of up to 90 % at 20 degrees C. Where the destination is above 1,000 m, the dielectric withstand and the temperature rise both have to be corrected. Units are used in chemical plants, factories, remote sites and other harsh locations, so the degree of protection of bushings, cable boxes and the tank finish should be agreed as part of the specification.
What an ISO 9001 Quality System Covers
An ISO 9001 certification on a transformer works covers the design and production system rather than the product itself, and it is the process controls that matter to a buyer. A compliant system documents incoming inspection of core and winding materials, oil testing before filling, control of the vacuum drying and oil impregnation process, calibration of the loss measuring bridge and the high-voltage test set, and traceability of routine test records to the serial number of each unit. Buyers should request the test certificate per serial number rather than relying on the certificate of the management system alone.
Frequently Asked Questions
Q: What is the no-load loss of a 250 kVA oil-immersed transformer?
A: About 400 W for a current sealed design at 10 kV class, measured at rated voltage and rated frequency, with a load loss of about 3,200 W.
Q: What does the vector group Dyn11 mean?
A: The high-voltage winding is delta connected and the low-voltage winding is star connected with a 30 degree phase shift and a brought-out neutral, which is the usual arrangement for a 0.4 kV distribution secondary.
Q: Which standards apply to this transformer?
A: The GB/T 1094 series and GB/T 6451 for parameters, or the IEC 60076 series for international projects, with insulating oil to IEC 60296.
Q: Why use a sealed tank instead of one with a conservator?
A: A sealed tank eliminates the oil-to-air interface, so moisture and oxygen ingress are greatly reduced and the oil does not need routine topping up.
Q: What temperature rise is allowed?
A: The design limits are a top-oil temperature rise of 60 K and an average winding temperature rise of 65 K under continuous rated load, as set out in the power transformer temperature rise standards.
Q: Does ISO 9001 certification guarantee the transformer parameters?
A: No. It certifies the quality management system. The parameters are confirmed by the routine and type test certificates issued for the individual serial number.