250 KVA Oil-immersed Power Transformer: Design Features and Application Scope

250 kVA Oil-immersed Power Transformer: Design Features and Application Scope

The 250 kVA oil-immersed power transformer is a medium-capacity distribution transformer widely used where a three-phase supply at 10 kV, 11 kV or 22 kV must be stepped down to 400/230 V. This article explains its design features and the applications it serves, following the rating and test requirements of IEC 60076 and GB/T 6451.

Core Design

The magnetic core is built from cold-rolled grain-oriented silicon steel laminations, typically grades such as 30Q120 or 27QG110. Step-lap joints and precise lamination reduce no-load loss and noise. The core is clamped with low-loss insulation to keep the magnetizing current stable over the transformer's service life.

Winding and Insulation System

Copper windings are standard for 250 kVA units because copper gives lower resistance loss and better heat transfer than aluminum at the same rating. The LV winding is usually wound in layers or as helical coils, and the HV winding uses continuous disk or layer windings with kraft paper and pressboard insulation. All insulation materials are class A (105 degC) or higher for oil-immersed duty, and the oil itself provides both insulation and cooling.

Cooling and Tank Structure

The standard cooling mode is ONAN: oil circulates naturally and air cools the radiator surface. The corrugated tank or bolted radiator increases the heat dissipation area. A conservator with a silica gel breather, or a fully sealed tank with nitrogen cushion, keeps the oil dry and slows aging. The tank is pressure-tested and painted for outdoor service.

Protection and Accessories

Buchholz relay for gas and oil surge detection (for units with conservator).

Pressure relief valve and oil level indicator.

Winding temperature indicator and oil temperature indicator with alarm contacts.

Off-circuit tap changer, usually +/-2 x 2.5% or +/-5%, to compensate for local voltage variation.

Nameplate with all ratings per IEC 60076-1.

Application Scope

A 250 kVA transformer fits commercial buildings, small factories, workshops, agricultural processing plants, pumping stations and rural distribution networks. It can also serve as a step-down unit in combined heat and power or renewable generation sites where the grid voltage is 10 kV or 11 kV. For indoor installations, check clearances and oil containment; for outdoor poles or plinths, specify the enclosure class and lightning protection.

Selection and Acceptance Notes

Confirm the actual load profile: a transformer loaded above 80% of rating on a continuous basis shortens insulation life.

Verify no-load loss and load loss from the routine test report; efficiency directly affects operating cost.

Check vector group (normally Dyn11 for distribution), impedance voltage and tapping range against the local grid.

Specify altitude and ambient temperature; above 1000 m or 40 degC derating may apply.

Frequently Asked Questions

Q: What voltage does a 250 kVA oil-immersed transformer typically work with?
Common combinations are 11 kV/0.4 kV, 10 kV/0.4 kV and 22 kV/0.4 kV, three-phase, 50 Hz or 60 Hz, with other primary voltages available on request.

Q: What does ONAN mean?
ONAN means oil natural air natural: the insulating oil circulates by natural convection and heat is rejected to ambient air by the tank or radiators, without fans or pumps.

Q: How long does a 250 kVA oil transformer last?
With proper oil care and loading, a typical design life is 20-30 years; periodic oil tests and preventive maintenance extend it further.

Q: Can it be used indoors?
Yes, but indoor installations need ventilation, oil containment and fire protection in line with local electrical codes.

Q: What is the difference between oil-immersed and dry-type at this rating?
Oil-immersed units have higher overload capacity and lower cost per kVA, while dry-type transformers avoid fire and oil-containment concerns and suit indoor or high-rise sites.

Q: What protection and monitoring accessories can be fitted to a 250 kVA unit?
Typical accessories are winding and oil temperature indicators with alarm contacts, an off-circuit tap changer of about +/-2 x 2.5% or +/-5% to compensate local voltage variation, a pressure relief device and an oil level gauge. Conservator-type designs can additionally carry a Buchholz relay, and options such as a silica-gel breather or an oil thermometer with trip contacts are selected according to the operating conditions.