Unit
- Electromagnetic induction: A transformer operates on Faraday's law of induction, where an alternating current in the primary winding creates a time-varying magnetic flux in the core, which induces a voltage in the secondary winding. The voltage ratio is determined by the turns ratio of the windings.
- Core construction: The core is typically made from laminated grain-oriented silicon steel to minimize eddy current losses and hysteresis. Each lamination is coated with an insulating layer to prevent inter-lamination current flow.
- Winding types: Power transformers use concentric windings (low-voltage winding placed closer to the core) while distribution transformers often use sandwich or disc windings for better voltage regulation under load.
- Insulation system: A combination of solid insulation (paper, pressboard, Nomex) and liquid insulation (mineral oil, ester fluids) provides dielectric separation between turns, layers, and windings, as well as heat dissipation.
- Tap changers: Off-circuit or on-load tap changers allow adjustment of the turns ratio to compensate for voltage variations in the power system, typically providing plus or minus 5-10 percent range in steps.
- Cooling methods: ONAN (Oil Natural Air Natural) is the most common for distribution transformers, while large power transformers use OFAF (Oil Forced Air Forced) or ODAF (Oil Directed Air Forced) systems to remove heat generated by core and winding losses.
- Bushings and tank: High-voltage bushings provide insulated electrical connections from the windings to the external circuit, while the tank houses the core-coil assembly and contains the insulating oil, with expansion conservator or nitrogen blanket for oil volume variation.
- Production phases: Transformer manufacturing consists of core preparation, winding manufacturing, core-coil assembly, active part drying, tanking, oil filling, final testing, and packaging for dispatch.
- Testing procedures: Transformer testing includes routine tests such as ratio measurement, polarization index, insulation resistance, winding resistance, short-circuit impedance, no-load loss and current, partial discharge, and induced overvoltage, as well as type tests like impulse, temperature rise, sound level, and short-circuit withstand to verify design integrity and production quality.
- Losses and efficiency: Transformer losses are divided into no-load losses (core losses from hysteresis and eddy currents, constant for a given voltage) and load losses (I²R losses in windings plus stray losses in structural parts). Efficiency peaks around 70-80% of rated load due to the balance between fixed core losses and variable copper losses.
- Transformer types: Classification includes power transformers (above 500 MVA, used in transmission grids), distribution transformers (up to 500 kVA, for local voltage step-down), instrument transformers (current and voltage transformers for metering and protection), and special-purpose types (rectifier, furnace, earthing, and dry-type transformers) each with unique design criteria and production considerations.
Process Flow
- Stage 1 - Core building: Laminations are cut, stacked, and clamped into the desired core shape (core-type or shell-type) with precise air gap control to minimize magnetizing current.
- Stage 2 - Winding fabrication: Conductors (copper or aluminum) are wound on cylindrical or disc formers with interlayer insulation and cooling ducts, then lead connections are brazed or welded.
- Stage 3 - Active part assembly: Windings are assembled onto the core legs, with phase barriers and clamping structures installed to withstand short-circuit forces.
- Stage 4 - Drying and impregnation: The active part is placed in a vacuum autoclave to remove moisture, then impregnated with insulating oil under controlled temperature and vacuum cycles.
- Stage 5 - Tanking and final assembly: The dried active part is lowered into the tank, bushings, tap changer, and cooling equipment are mounted, and the tank is sealed and filled with oil.
Quick Revision
- A transformer uses electromagnetic induction to transfer electrical energy between circuits with voltage transformation based on turns ratio.
- Core laminations are made from grain-oriented silicon steel with insulating coatings to reduce losses.
- Winding types include concentric and disc designs, each suited to different voltage and power ratings.
- Insulation systems combine solid materials and liquid dielectrics to ensure electrical isolation and cooling.
- Production follows a linear sequence from core building through winding, assembly, drying, tanking, and testing.