1. Core and Winding Assembly

1.1 Core: construction and materials

The core provides the magnetic path for flux and is typically built from stacked, cold-rolled grain-oriented (CRGO) silicon steel laminations. Laminations are insulated from each other to reduce eddy current losses. Cores for distribution transformers are usually of the single-phase or three-phase stacked type; core window dimensions determine how easily windings fit and how heat is dissipated. Core clamping and bolting ensure mechanical stability under fault and inrush conditions.

1.2 Windings: HV and LV details

Windings transform voltage between high-voltage (HV) and low-voltage (LV) sides. They are wound from copper or aluminium conductors: copper offers better conductivity and shorter life-cycle losses, aluminium reduces cost and weight. LV windings are usually placed closest to the core, with HV windings outside them and an insulating barrier in between. Typical constructions include layer, disk, or helical windings depending on rating and short-circuit strength requirements.

2. Insulation, Oil and Tank System

2.1 Insulating medium: mineral oil vs. alternatives

Distribution transformers commonly use mineral insulating oil for dielectric strength and cooling. Alternatives include ester-based fluids (biodegradable) or solid epoxy in dry transformers. Oil fills voids between windings and core, reduces partial discharge risk, and transfers heat to the tank. Oil quality (moisture content, acidity, dielectric strength) is critical for reliability.

2.2 Tank, conservator and breather

The tank houses the core and windings and provides mechanical protection and cooling surface. For larger units, a conservator (expansion tank) compensates oil volume changes with temperature. A silica-gel breather fitted to the conservator prevents moisture ingress; periodic replacement or regeneration of silica gel is a routine maintenance task.

3. Bushings, Tap Changer and External Connections

3.1 Bushings and terminals

Bushings provide insulated passage of conductors through the tank. They are made from porcelain, epoxy resin, or composite materials and must be rated for rated voltage class and mechanical force. External HV and LV terminals permit cable or overhead line connection; torque specifications and appropriate surge protection at these terminations are essential during installation.

3.2 Tap changer: on-load and off-load types

Tap changers adjust the transformer's turns ratio to regulate output voltage. Off-load (no-load) tap changers require de-energizing for position changes; on-load tap changers (OLTC) allow switching under load and include diverter switches and resistor/capacitor transition networks. OLTCs require periodic inspection, contact maintenance, and may have oil or vacuum interrupter mechanisms.

4. Cooling Equipment and Thermal Management

4.1 Cooling modes and radiators

Common cooling modes: ONAN (Oil Natural Air Natural), ONAF (Oil Natural Air Forced), and OFAF (Oil Forced Air Forced). Cooling is achieved via radiator fins or corrugated panels that increase surface area and convection. Fans (for ONAF) and pumps (in forced oil systems) activate based on temperature relays or thermostats.

4.2 Temperature sensors and gauges

Winding and oil temperature sensors (RTDs, thermocouples) feed alarms and automatic cooling controls. A temperature gauge and alarm thresholds help protect against overheating and guide load management to prolong transformer life.

5. Protection, Monitoring and Safety Devices

5.1 Buchholz relay, pressure relief and surge arresters

Buchholz relays (for oil-filled, conservator-equipped transformers) detect gas accumulation and slow faults between tank compartments and trigger alarms or trips. Pressure relief devices protect the tank from internal overpressure during severe faults. Surge arresters (external or mounted) protect bushings and windings from transient overvoltages.

5.2 Fuses, relays and neutral grounding

Distribution transformers often include LV fuses or HV fuse links for fault isolation. Protective relays (overcurrent, differential, thermal) coordinate with system protection. A neutral grounding link or grounding transformer ensures safe neutral reference and fault current paths—important for distribution network behavior and safety.

6. Accessories, Mounting and Installation Components

6.1 Mounting hardware, pads and lightning protection

Mounting brackets, skid rails or pole-mount kits secure the transformer. Pad-mounted transformers require secure concrete pads and locking enclosures for public safety. Lightning protection (grounding electrodes, reinforced surge protection) reduces risk from direct or nearby strikes.

6.2 Metering, terminal boxes and nameplate

A metering compartment (on some distribution transformers) houses CTs/VTs and metering terminals for billing or monitoring. Terminal boxes protect connections from weather. The nameplate provides rated data (kVA, vector group, impedances, cooling class) that technicians must verify before operation.

7. Component–Function Quick Reference Table

8. Practical maintenance pointers for components

• Check oil dielectric strength and water content annually and after repairs.
• Inspect bushings for cracks and clean terminals to ensure good connections.
• Test tap changer operation and contacts at scheduled intervals; keep spare contact kits for OLTCs.
• Monitor temperature trends and set alarm thresholds to avoid thermal overstress.
• Replace breather silica gel when color indicates moisture saturation.

9. Final checklist before commissioning

Before energizing a distribution transformer: verify nameplate data vs system voltage, confirm correct tap-changer position, perform insulation resistance (megger) tests on windings, ensure proper grounding, torque all terminals to specification, fill and degas oil (if required), and set protection relays and temperature alarms. Document all tests and observations in the commissioning log.