400 kVA 11 kV Oil Immersed Transformer: Technical Guide
Nov 10, 2025
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Overview
The 400 kVA 11 kV oil immersed transformer is a three-phase liquid dielectric distribution transformer that steps 11 kV primary voltage down to 0.415 kV for secondary distribution. Mineral oil serves as both dielectric and coolant, giving the unit high insulation strength and efficient heat transfer. Typical models follow the S11 series and are designed to IEC 60076.
Technical Specification
| Parameter | Value |
|---|---|
| Model | S11-400 kVA-11/0.415 kV |
| Type | Oil immersed transformer |
| Standard | IEC 60076 |
| Rated power | 400 kVA |
| Frequency | 50 Hz |
| Phase | Three |
| Cooling type | ONAN |
| Primary voltage | 11 kV |
| Secondary voltage | 0.415 kV |
| Winding material | Copper |
| Vector group | Dyn11 |
| Impedance | 5% |
| Tap changer | NLTC, ±2x2.5% |
| No-load loss | 0.57 kW (570 W) |
| On-load loss | 4.3 kW |
The typical no-load loss for the S11-400 rating is 0.57 kW (570 W).
Core and Winding Design
The core is built from cold-rolled grain-oriented silicon steel laminations with step-lap joints to reduce no-load loss and noise. Copper windings are used for both HV and LV sides to lower resistance losses and improve overload capacity. The Dyn11 vector group provides a neutral for single-phase loads and is standard for distribution networks.
Applications
Building infrastructure: hospitals, shopping centres, residential and commercial towers.
Industrial plants: factories and manufacturing facilities.
Utility distribution: step-down substations feeding local networks.
Infrastructure: airports, subway stations and bridges.
Oil and gas, water and wastewater facilities.
Tank Types and Protection
Small and medium transformers commonly use barrel type tanks, while large units use bell type tanks for easier core and winding access. A pressure relief valve mounted on the tank cover releases internal pressure in fault conditions, protecting the tank. Gas relays and oil temperature indicators can be added for alarm and tripping functions.
Maintenance Points
Routine checks include oil level and dielectric strength, breather condition, bushing integrity and thermometry. Oil samples should be tested periodically per IEC 60296 and the tank kept sealed to prevent moisture ingress.
FAQ
What does ONAN cooling mean?
ONAN means oil natural, air natural: the oil circulates by natural convection and dissipates heat through the tank and radiators to ambient air without pumps or fans.
Why choose copper windings?
Copper has higher conductivity than aluminium, reducing I2R losses and allowing a more compact winding with better short-circuit withstand.
What is the Dyn11 vector group used for?
Dyn11 means the HV winding is delta connected and the LV winding is wye connected with the neutral brought out, giving a 330 degree phase shift and suitability for mixed three-phase and single-phase loads.
What is NLTC tapping?
NLTC, or no-load tap changing, allows voltage adjustment only when the transformer is de-energized, typically at plus/minus 2x2.5% around nominal.
Can this transformer operate at 60 Hz?
The design in the table is for 50 Hz. Operation at 60 Hz would require verification of the design, since flux density and loss behaviour change with frequency.