Transformer Glossaries (A–Z)

A

  • Alternating Current (AC): A current that periodically reverses direction, unlike direct current (DC) which flows in one direction.

  • Alternator: A rotating machine that generates alternating current (AC).

  • Ambient Temperature: The surrounding environmental temperature.

  • Ammeter: A device for measuring electric current in amperes.

  • Autotransformer: A transformer with one winding acting as both primary and secondary.

  • Air-Core Inductor: An inductor using air (or vacuum) as its magnetic core.

  • Ampacity: The maximum amount of electric current a conductor or device can carry before sustaining immediate or progressive deterioration.

  • Arc Suppression Coil: A device used to suppress electrical arcs.

B

  • Bushing: An insulating component allowing a conductor to pass through a grounded enclosure.

  • Buck-Boost Transformer: Used for small voltage adjustments in lighting and control systems.

  • Breakdown Voltage: The voltage at which insulation fails and becomes conductive.

  • BIL (Basic Insulation Level): The maximum voltage that electrical equipment can withstand without breakdown.

  • Banking: Connecting two or more transformers to function as a single unit.

  • Buchholz Relay: A gas-actuated relay used for the protection of oil-immersed transformers.

C

  • Circuit Diagram: Schematic representing electrical connections using standard symbols.

  • Conduction: The transfer of energy (electric or heat) through a conductor.

  • Contactor: A large relay used to control power circuits.

  • Cooling: The process of dissipating heat from a transformer.

  • Copper Losses (I²R): Heat losses due to current flowing through winding resistance.

  • Coupling: Transfer of energy between circuits, often via magnetic fields.

  • Core Saturation: A condition where an increase in current does not result in an increase in magnetic flux.

  • Capacitance: The ability of a system to store an electric charge.

D

  • Direct Current (DC): Electric current flowing in a single direction.

  • Dielectric: Insulating material between conductors in a capacitor or transformer.

  • Distribution Transformer: Steps down voltage for residential or commercial use.

  • Delta Connection: A method of connecting three transformer windings in a triangle-like configuration.

  • Detuning: Adjusting a circuit to prevent resonance at a particular frequency.

  • Duty Cycle: The ratio of operating time to total time in a repeating cycle.

E

  • Efficiency: Ratio of output power to input power, expressed as a percentage.

  • Electrical Insulation: Material preventing unwanted current flow.

  • Electromagnetic Induction: Generating voltage through magnetic field change.

  • Exciting Current: The current required to energize the transformer's core.

  • Electrostatic Shield: A grounded conductor placed between windings to reduce capacitive coupling.

  • Eddy Currents: Circulating currents induced within conductors by a changing magnetic field.

F

  • Flux (Magnetic): Magnetic field lines generated in a transformer core.

  • Frequency: Number of AC cycles per second, measured in hertz (Hz).

  • Fuse: A safety device that melts under excessive current to break the circuit.

  • Faraday's Law: The principle that a change in magnetic field can induce an electromotive force (EMF).

  • Ferrite Core: A type of magnetic core made of ferrite, used in high-frequency transformers.

  • Flux Leakage: Magnetic flux that does not follow the intended path in a magnetic circuit.

G

  • Grounding (Earthing): Connecting components to the earth for safety.

  • Grading Ring: Metal ring used on bushings to manage electric fields.

  • Gauss: A unit of magnetic flux density.

  • Gap Loss: Losses due to air gaps in the magnetic path.

  • Gassing: The release of gas within transformer oil due to decomposition.

  • Ground Fault: An unintentional connection between an electric circuit and the ground.

H

  • Harmonics: Voltage/current distortions at multiples of the base frequency.

  • HV (High Voltage): Voltage levels typically above 1 kV.

  • Heat Run Test: A test measuring transformer temperature rise under load.

  • Hysteresis Loss: Energy loss due to the lag between magnetic field strength and magnetization.

  • Hot Spot Temperature: The highest temperature within the transformer winding.

  • Hermetically Sealed Transformer: A transformer sealed to prevent moisture ingress.

I

  • Impedance: Resistance to AC, including both resistance and reactance.

  • Inrush Current: Surge of current when a transformer is first energized.

  • Insulation Resistance: Resistance offered by insulating material to current flow.

  • Instrument Transformer: Used to measure voltage or current without directly connecting to high-voltage lines.

  • Isolating Transformer: Provides electrical isolation between its primary and secondary windings.

  • Inductance: The property of a conductor by which a change in current induces an electromotive force.

J

  • Joule Effect: Heat produced by electric current in a conductor.

  • Jumper Cable: A temporary cable used for electrical connections.

  • Junction Temperature: The temperature at the point where different materials meet in a device.

  • Jacket: The outer protective covering of a cable or transformer.

  • Jitter: Small, rapid variations in a waveform resulting from fluctuations in the signal.

  • Junction Box: An enclosure housing electrical connections.

K

  • kVA (Kilovolt-Amperes): Unit of apparent power in a transformer.

  • K-Factor: Indicates a transformer's ability to handle non-linear loads.

  • Knee Point Voltage: The voltage at which a transformer's core begins to saturate.

  • Kilowatt (kW): Unit of real power.

  • Knee Point: The point on the magnetization curve where the core begins to saturate.

  • KVA Rating: The maximum apparent power a transformer can handle.

L

  • Load Loss: Power loss when supplying load due to resistance in windings.

  • Lightning Arrester: Protects transformer from lightning-induced surges.

  • Leakage Reactance: Reactance due to leakage flux not linking both primary and secondary windings.

  • Line Voltage: Voltage measured between two lines in a three-phase system.

  • Load Tap Changer: A device that adjusts transformer voltage under load conditions.

  • Laminations: Thin sheets of electrical steel used to construct the transformer core.

M

  • Magnetizing Current: Current needed to energize the magnetic core.

  • Mineral Oil: Oil used in transformers for insulation and cooling.

  • MVA (Megavolt-Amperes): Unit of apparent power for large transformers.

  • Mutual Inductance: The ability of one inductor to induce voltage in another nearby inductor.

  • Magnetic Flux Density: The amount of magnetic flux in a given area.

  • Moisture Content: The amount of water present in transformer insulation or oil.

N

  • No-Load Loss: Power consumed by a transformer energized without load.

  • Neutral Point: Common return path for electrical current in three-phase systems.

  • Nameplate Rating: The manufacturer's specified operating limits for a transformer.

  • Noise Level: The sound emitted by a transformer during operation.

  • Nominal Voltage: The standard operating voltage of a system.

  • NEMA Rating: Standards set by the National Electrical Manufacturers Association for equipment.

O

  • Oil-Immersed Transformer:A type of power transformer in which the windings and magnetic core are fully submerged in insulating mineral oil. The oil provides both electrical insulation and efficient heat dissipation.

  • Overload Capacity:The ability of a transformer to operate temporarily above its rated kVA without damage, often defined for specific durations and ambient conditions.

  • Open Circuit Test:A test typically performed on the low-voltage side of a transformer to measure core (iron) losses and magnetizing current under no-load conditions.

  • OLTC (On-Load Tap Changer):A tap changer that enables voltage regulation while the transformer is energized and under load, used in grid and industrial applications requiring continuous voltage control.

  • Overvoltage:A condition where the applied voltage exceeds the transformer's design limit, potentially leading to insulation breakdown or other failures.

  • Overtemperature Alarm:A protective feature that alerts operators when internal temperature exceeds predefined thresholds, helping to prevent overheating and failure.

  • Off-Circuit Tap Changer (OCTC):A manual or motor-driven device that adjusts winding taps for voltage regulation, but only when the transformer is de-energized.

  • Oil Temperature Indicator (OTI):A device that monitors and displays the top oil temperature inside an oil-immersed transformer, often linked with alarms or trip circuits.

  • Oil Level Gauge:An instrument that displays the current level of insulating oil inside the transformer tank, essential for routine maintenance and leak detection.

  • Oil Conservator:An expansion tank mounted above the main transformer tank, allowing for thermal expansion and contraction of the insulating oil while minimizing exposure to air and moisture.

  • Oil Filtration:The process of purifying transformer oil by removing moisture, gas, and particulates to restore its insulating and cooling properties.

  • Oil Leakage Detector:A sensor or visual indicator system that identifies unintended loss of oil from the tank, signaling potential seal or weld failures.

  • Operational Life:The expected lifespan of a transformer under standard conditions, typically ranging from 25 to 40 years depending on design, loading, and maintenance.

  • Overcurrent Protection:A safety mechanism (like fuses or circuit breakers) designed to disconnect the transformer from the source in the event of current levels exceeding the safe limit.

  • Oil Sampling Valve:A valve installed on the transformer tank to extract small quantities of oil for periodic analysis of dielectric strength, moisture content, and dissolved gases.

  • ONAN (Oil Natural Air Natural):
    A passive transformer cooling system where heat is dissipated through natural convection of insulating oil and ambient air, ideal for standard load and ambient conditions.
  • Optional Upgrade – ONAF (Oil Natural Air Forced):
    An enhanced version using forced-air fans to increase cooling capacity, suitable for high-load or high-temperature environments.

P

  • Primary Winding: The input winding connected to the power source.

  • Power Factor: The ratio of real power to apparent power in a circuit.

  • Partial Discharge: Localized dielectric breakdown of insulation under high voltage.

  • Power Transformer: Used in transmission networks to transfer electrical energy at high voltages.

  • Protection Relay: A device that trips a circuit breaker in the event of a fault.

  • Phase Shift: A change in phase angle between input and output voltages in transformers.

Q

  • Quality Factor (Q): A measure of the efficiency of inductors and capacitors in an AC circuit.

  • Quenching: The suppression or extinguishing of electrical arcs.

  • Quiescent Current: The current drawn by a transformer or circuit when not delivering load.

  • Quick Disconnect: Connectors allowing fast disconnection of power circuits.

  • Quasi-Resonant Converter: A type of power converter using resonance to reduce switching losses.

R

  • Reactance: Opposition to AC caused by inductance and capacitance.

  • Regulation (Voltage): The change in secondary voltage from no-load to full-load.

  • Rated Capacity: Maximum power output a transformer can deliver under standard conditions.

  • Radiator: Component used to dissipate heat from transformer oil.

  • Resistance: Opposition to current flow, causing voltage drop and power loss.

  • Relay Protection: System used to detect transformer faults and activate protective devices.

S

  • Secondary Winding: The winding from which power is delivered to the load.

  • Step-Up Transformer: Increases voltage from primary to secondary winding.

  • Step-Down Transformer: Decreases voltage from primary to secondary winding.

  • Stray Loss: Losses due to leakage flux in transformer structure or tank.

  • Surge Arrester: Device protecting against overvoltages or lightning surges.

  • Silicon Steel: Magnetic material used in transformer cores to reduce losses.

T

  • Tap Changer: Device to adjust transformer output voltage by altering winding turns.

  • Temperature Rise: Increase in temperature during transformer operation above ambient level.

  • Three-Phase Transformer: Transformer for 3-phase power systems with three sets of windings.

  • Thermal Overload: Condition when temperature exceeds safe limits due to overloading.

  • Turns Ratio: Ratio of primary to secondary winding turns.

  • Toroidal Transformer: Donut-shaped transformer with reduced magnetic flux leakage.

U

  • UL Certification: Certification indicating compliance with Underwriters Laboratories safety standards.

  • Undervoltage: Operating below rated voltage, which may cause inefficiency or instability.

  • Uniform Field: An evenly distributed magnetic or electric field.

  • Utility Transformer: Transformer used by power utilities to distribute electrical energy.

  • Unbalanced Load: When the loads on the three phases of a transformer are not equal.

  • U-Shape Core: A transformer core shape designed for specific space or performance requirements.

V

  • Vector Group: Symbolic representation showing winding configuration and phase displacement.

  • Voltage Regulation: The ability to maintain constant output voltage despite varying load.

  • Vacuum Drying: Process to remove moisture from transformer insulation in manufacturing.

  • Volt-Ampere (VA): Unit of apparent power used in transformer ratings.

  • VPI (Vacuum Pressure Impregnation): Method to improve insulation of dry-type transformers.

  • Voltage Tap: Connection point on a winding to select voltage output.

W

  • Winding: Conductive coil in a transformer where EMF is induced.

  • Watt Loss: Total power lost as heat due to resistance and core losses.

  • Wye (Star) Connection: Three-phase connection where each winding joins at a common neutral point.

  • Wattmeter: Instrument used to measure real power in a circuit.

  • Winding Resistance: Resistance of the transformer windings affecting voltage drop and losses.

  • Weatherproof Transformer: Designed for outdoor use and protected from environmental elements.

X

  • X/R Ratio: Ratio of reactance to resistance, used in fault analysis.

  • X-Ray Transformer: High-voltage transformer used in medical X-ray machines.

  • X-Phase Shift: Adjustable phase shift to control power flow in networks.

  • X-Connected Winding: A unique configuration in high-voltage transmission.

  • X-Class Insulation: Temperature rating class for insulation materials.

  • Xenon Flash Supply: Transformer used in xenon light systems.

Y

  • Y-Connection: Another term for Wye connection.

  • Yoke: The top and bottom horizontal part of a transformer core.

  • Yttrium-Based Insulators: High-performance insulators used in advanced transformer designs.

  • Yield Strength: The maximum stress a material can withstand without permanent deformation.

  • Y-Δ Transformation: Mathematical conversion between wye and delta configurations.

  • Yaw Control Transformer: Used in wind turbines to control nacelle rotation.

Z

  • Zero Sequence Current: A current component in unbalanced systems affecting neutral and ground.

  • Zigzag Transformer: Provides grounding and reduces harmonics in three-phase systems.

  • Zinc-Oxide Arrester: Surge arrester using zinc oxide as its nonlinear resistor material.

  • Zonal Heating: Uneven heating in different transformer regions.

  • Z-Core Design: A core configuration for compact and efficient transformer layouts.

  • Zener Protection: Overvoltage protection using Zener diodes in control transformers.