125 kVA 11 kV Oil-Immersed Distribution Transformer

Introduction to the 125 kVA 11 kV Oil-Immersed Distribution Transformer

The 125 kVA oil-immersed distribution transformer is a three-phase step-down unit that receives power at 11 kV and supplies it at 0.55 kV to local distribution networks. It is cooled by natural oil circulation and natural air convection, ONAN, which keeps the unit simple and maintenance-friendly. Typical application is feeding small industrial, commercial, and rural loads from an 11 kV medium-voltage line, including solar integration projects where the transformer steps up or down between the PV plant voltage and the grid.

Core Design and Loss Performance

The magnetic core is built from cold-rolled grain-oriented silicon steel laminations using a 45-degree miter joint and step-overlap construction. This arrangement reduces the no-load loss and the no-load current compared with conventional butt joints, because the overlap distributes the flux transition across several steps and lowers the local saturation at the joint. The core is clamped to control magnetostriction noise, and the complete magnetic circuit is annealed to restore the magnetic properties after cutting and stacking.

Technical Specification

Parameter	Value
Model	S11-125 kVA-11/0.55 kV
Type	Oil-immersed distribution transformer
Standard	IEC 60076
Rated power	125 kVA
Frequency	50 Hz
Phase	Three
Cooling type	ONAN
Primary voltage	11 kV
Secondary voltage	0.55 kV
Winding material	Copper
Vector group	Dyn11
Impedance	4 percent
Tap changer	NLTC, +/-2x2.5 percent
No-load loss	0.24 kW
Load loss	1.8 kW
Accessories	Standard configuration

The loss values correspond to the S11 loss class, which is the common efficiency level for distribution transformers in this rating; higher efficiency classes are available for projects with strict loss capitalization.

Applications

General power distribution for commercial, industrial, and residential areas supplied from 11 kV lines.

Small-scale industries and factories requiring a dedicated step-down transformer.

Renewable energy projects, where the unit interfaces solar or wind generation with the local grid.

Infrastructure services such as airports, tunnels, and bridge facilities.

Public and private facilities, including buildings, hospitals, and mine sites.

Functions of the Transformer Oil

The insulating oil performs three duties in an oil-immersed transformer:

Electrical insulation: the oil provides insulation between the energized windings and between windings and the tank, and it protects metal surfaces from oxidation.

Heat dissipation: oil absorbs heat from the core and windings by conduction and carries it to the tank surface, where it is radiated to the atmosphere.

Diagnostic medium: oil samples reveal the condition of the internal insulation through dissolved gas analysis and dielectric testing.

The oil quality is maintained by controlling viscosity, pour point, and flash point, which are checked during routine sampling.

Maintenance Practice

Monthly: visual inspection of bushings, gaskets, and penetrations for oil leaks, and verification of the oil level indicator.

Twice a year: check cooling fans and ventilation openings where fitted.

Annually: oil sampling and analysis, and functional testing of protective devices such as temperature relays and pressure switches.

Periodically: clean the tank top and the primary bushings to prevent tracking and corrosion.

A documented maintenance log with oil test trends gives early warning of internal faults and supports the transformer's full design life.

Selection Notes for the 125 kVA Rating

Confirm the primary voltage, secondary voltage, vector group, and impedance against the local grid and the load characteristic before ordering.

Check the ambient temperature and altitude of the site; derating applies above 40 C ambient and above 1000 m altitude per IEC 60076.

For 60 Hz networks, the design and the test report must be adjusted accordingly; confirm the frequency in the specification.

Choose the tap range to match the voltage variation of the feeding line, typically +/-2x2.5 percent for distribution duty.

Specify the oil type for the climate: standard mineral oil for moderate climates and low-temperature oil where winter temperatures drop below -25 C.

Frequently Asked Questions

Why is the vector group Dyn11 used for this transformer?

Dyn11 means the HV winding is delta-connected and the LV winding is wye-connected with the neutral brought out, with a 30-degree phase shift. This combination provides a stable neutral for line-to-neutral loads, limits triplen harmonics on the HV side, and is the standard for distribution transformers in most markets.

What is the difference between no-load loss and load loss?

No-load loss is the core loss that occurs whenever the transformer is energized, independent of load. Load loss is the copper loss in the windings, which rises with the square of the load current. Both are guaranteed values that should be verified by test report.

Can this transformer be used for solar power integration?

Yes. The 11 kV primary connects to the distribution grid, and the 0.55 kV secondary suits the output of a photovoltaic plant after the inverter stage. The unit must be specified with the correct protection and, where required, a vector group that suits the inverter grounding scheme.

How often should the oil be tested?

At least annually. The routine test covers dielectric strength, moisture content, and acidity; dissolved gas analysis is recommended every two to three years or after any suspected overload or fault event, because gas ratios indicate developing internal faults.

What does the 4 percent impedance mean for the installation?

It defines the voltage drop at full load and the available fault current at the secondary terminals. Protection devices and cables are sized using this value, and it must match the network's short-circuit coordination study.

Why is the 45-degree miter joint better than a simple butt joint?

The mitered and step-overlapped joint spreads the flux transition over a larger area and reduces the local flux crowding at the joint corners, which lowers no-load loss, no-load current, and audible noise compared with butt-jointed cores.