Auxiliary Transformer — Types, Working Principle & Full Specifications

Auxiliary Transformers in electrified sections are required for colour light signal equipment (signal lights), switching stations, and traction sub-stations. The Auxiliary Transformer provides 25kV/240V, 50Hz single-phase supply for cabins, sub-stations, feeding and switching posts, stations, block-huts, RRIs, and level crossings with gate signals. It is a static device working as a step-down transformer, transforming power from one AC circuit to another at the SAME frequency — the circuits are conductively disjointed but magnetically coupled by a common time-varying magnetic field.

Working Principle

Based on electromagnetic induction: (a) when a conductor cuts magnetic flux (or flux cuts the conductor), an EMF is induced; (b) the magnitude of this EMF is proportional to the rate of change of flux. When the primary winding connects to AC mains, current flows and — since this winding links with an iron core — produces alternating flux that also links the secondary winding, inducing an EMF there at the SAME frequency as the supply. This induced secondary EMF delivers current to an external load. The core's flux also links the primary winding itself, producing a self-induced EMF that OPPOSES the applied voltage — known as 'back EMF' — which limits primary current, analogous to how back EMF in a DC motor limits armature current.

Technical Data — All Four Capacity Ratings

All four ratings SHARE the following common specifications: Type — double-wound, single-phase, oil-immersed, natural-air-cooled (ONAN), step-down, for outdoor installation. Secondary voltage: 240V. Rated frequency: 50Hz ±3%. Insulation level: primary lightning impulse withstand 190kV peak, primary short-duration power-frequency withstand 70kV rms (secondary 3kV rms), primary induced over-voltage 80kV rms. Tapings: off-circuit tap changer, +5% to -10% in steps of 5%, all taps carry full load current continuously. Polarity: Subtractive. Temperature rise limits: winding 50°C (resistance method), oil 40°C (thermometer), current-carrying parts in air 40°C, hot spot NOT to exceed 115°C — all over a 45°C ambient. Type of cooling: ONAN. Insulation class: 'A' for all windings. Current density (max): 2.5 A/mm² for both windings. Minimum creepage distance: 900mm for 36kV class, 90mm for 3.6kV class.

Construction — The 11 Parts & Components

The Auxiliary Transformer consists of: HV winding and LV winding (electrically separated but magnetically coupled), wound on a common magnetic core. One end of the primary connects to the 25kV system through a bushing; the other end connects to the tank (earth) through a bushing and link. Both secondary terminals mount on the tank through bushings — one used as PHASE, the other EARTHED and used as NEUTRAL. Windings and core are immersed in insulating oil in the tank.

• 1. Transformer Tank: constructed from good commercial-grade low-carbon steel, single-tier construction (minimizes welding), strengthened with lifting lugs for hoisting the oil-filled transformer.
• 2. Core: high-permeability cold-rolled grain-oriented/non-ageing electrical silicon steel laminations per IS:3024, free from storage/atmospheric defects. Both sides insulation-coated to withstand stress-relief annealing. Air gaps avoided during assembly. Electrically connected to the tank to drain electrostatic potential.
• 3. Windings: spiral/helical type, continuous electrolytic copper conductor, paper-insulated to Class 'A'. NORMALLY no joints; if unavoidable, brazed with high-silver-alloy grade Ba,Cu Ag6 per IS:2927, or electrically butt-welded. Width-to-thickness ratio of copper conductor kept small, NOT EXCEEDING 5:1, to avoid conductor tilting under axial/radial short-circuit forces.
• 4. HV & LV Bushings: comply with IS:2099 and IS:3347. Primary line terminal brought out through a 36kV-class bushing; neutral terminal through a 3.6kV-class bushing, connected externally to the tank via a tin-coated copper link (covered by ≥2mm sheet steel, welded and bolted). Porcelain housing per IS:5621, single-piece construction (no joint), lip at extremities but no underside ribs (avoids dust/pollutant accumulation, eases cleaning), corona-free.
• Bushing terminal stems: 20mm diameter (36kV class), 12mm diameter (3.6kV class), threaded to metric size per IS:1367.
• 5. Conservator: a drum containing transformer oil, mounted at the top, connected to the main tank by a pipe — maintains oil level in the tank and provides space for oil expansion as heat production causes the oil volume to rise and fall.
• 6. Breather: attached to the conservator, contains silica gel — prevents moist air entering the tank during oil contraction. When oil is hot, it expands and gas passes to atmosphere through the breather; when cooled, it contracts and air enters — the silica gel prevents moisture contamination during this cycle.
• 7. Tap Changer: on primary or secondary side, used to MANUALLY regulate output voltage according to line voltage — taps can be changed manually as OHE voltage varies.
• 8. Radiator: tubes or fins that increase the tank's surface area to atmosphere, providing better/effective transformer-oil cooling.
• 9. Outlet (Drain) Valve: provides a passage to drain oil during overhauling, or when an oil sample is needed for testing.
• 10. Inlet Valve: provides a passage to pour transformer oil into the tank during purification, or if a shortage is found in the tank.
• 11. Oil Level Indicator: indicates the level of insulating oil in the transformer tank, with markings on a transparent sheet for maximum and minimum levels.