225 kVA 11 kV Oil-Immersed Distribution Transformer
Nov 10, 2025
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Product Overview
The 225 kVA oil-immersed distribution transformer is a three-phase step-down unit that converts an 11 kV primary supply into a 400 V three-phase low-voltage network. It is a compact, field-proven design used by utilities, industrial plants and commercial facilities that need a reliable boundary between the medium-voltage grid and the utilization voltage. With a rated power of 225 kVA, the unit suits small residential districts, workshops, agricultural installations and auxiliary supplies of larger plants.
Representative Technical Specification
| Parameter | Value |
|---|---|
| Model | S11-225 kVA-11/0.4 kV |
| Type | Oil-immersed distribution transformer |
| Design standard | IEC 60076 |
| Rated power | 225 kVA |
| Frequency | 50 Hz |
| Phase | Three |
| Cooling type | ONAN |
| Primary voltage | 11 kV |
| Secondary voltage | 0.4 kV |
| Winding material | Copper |
| Vector group | Dyn11 |
| Impedance voltage | 4% |
| Tap changer | NLTC, tapping range ±2 x 2.5% |
| No-load loss | 0.4 kW |
| On-load loss | 3.05 kW |
| Accessories | Standard configuration |
The S11 designation indicates a loss level defined in the GB/T 6451 series for oil-immersed distribution transformers; the same unit can be manufactured to the equivalent loss classes of IEC 60076 where required by the local tender specification.
Typical Applications
Urban and rural power distribution: stepping 11 kV down to the low voltage used by residential and commercial customers
Industrial and mining enterprises: supplying workshops, processing lines and auxiliary power in factories and mines
Compact substations: the small footprint of the 225 kVA class fits into pole-mounted, pad-mounted and package substations where installation space is limited
Commercial buildings and facilities: hospitals, airports, hotels and shopping centers that need a dedicated medium-voltage intake with low-voltage distribution
Key Components and Their Functions
Transformer Bushings
Bushings guide the high-voltage and low-voltage leads through the tank wall. They insulate the live conductors from the grounded tank, fix the leads in position and seal the tank against moisture and air. Porcelain or epoxy-impregnated paper bushings are selected according to the voltage class and the pollution level of the site.
Core and Winding
The magnetic core is built from cold-rolled grain-oriented silicon steel laminations to keep no-load loss low, while the copper windings carry the load current with minimum copper loss. The Dyn11 vector group provides a neutral point on the low-voltage side, which is required for single-phase loads and for earth-fault protection.
Installation and Operating Notes
The unit is designed for outdoor or indoor installation with ONAN cooling, meaning natural oil circulation and natural air cooling. Keep the ventilation openings clear, maintain the minimum clearance to walls specified in the installation drawing, and provide an oil containment pit where local regulations require it. Before energizing, verify the tap position against the actual service voltage and test the insulating oil for dielectric strength.
Frequently Asked Questions
What does ONAN cooling mean?
ONAN stands for oil natural, air natural: the oil circulates by thermal convection inside the tank and the heat is rejected to the ambient air through the radiator surface. No pumps or fans are used, which keeps the unit simple and quiet.
Why is the Dyn11 vector group used on this transformer?
Dyn11 offers a low-voltage neutral for single-phase loads, reduces third-harmonic voltages in the secondary system and is the standard vector group for parallel operation of distribution transformers in most networks.
What is the purpose of the tap changer?
The off-circuit tap changer adjusts the turns ratio to compensate for variations in the primary voltage, typically ±2 x 2.5%. The unit must be de-energized before the tap position is changed.
What is the difference between no-load loss and on-load loss?
No-load loss is the core loss that occurs whenever the transformer is energized, independent of load. On-load loss is the winding loss that increases with the square of the load current. Both values determine the efficiency class of the unit.
Can the 225 kVA transformer be used with a 60 Hz network?
The standard unit is designed for 50 Hz. For 60 Hz networks the core flux density and the loss values must be re-verified, and the design is normally re-optimized to IEC 60076 or the applicable ANSI/IEEE standard.
What routine maintenance does an oil-immersed transformer require?
Periodic inspection of oil level and color, dielectric strength testing of the oil, checking of the Buchholz relay and pressure relief device, and verification of the earthing and lightning protection. Dissolved gas analysis is recommended annually for units in critical service.