Power Transformers

Vocabulary practice flashcards covering operational principles, equivalent circuits, ratings, losses, construction details, and performance metrics of electrical transformers.

Mutual Inductance

The electromagnetic phenomenon where an electromotive force is induced in one coil due to a changing current in an adjacent coil.

Henry ($\text{H}$)

The unit of mutual inductance defined as the induction of an electromotive force of $1\,\text{V}$ in a secondary coil when current changes at a rate of $1\,\text{A/s}$ in the primary coil.

Primary Winding

The transformer coil directly connected to the electrical power supply that receives alternating input energy.

Secondary Winding

The transformer coil in which voltage is induced through magnetic flux and which delivers output power to an electrical load.

Step-Up Transformer

A transformer whose secondary output voltage ($V_2$) is higher than its primary input voltage ($V_1$).

Step-Down Transformer

A transformer whose secondary output voltage ($V_2$) is lower than its primary input voltage ($V_1$).

Transformer Turns and Voltage Ratio Formula

The relationship defining transformer action in an ideal unit, expressed as $\frac{V_1}{V_2} = \frac{N_1}{N_2} = \frac{I_2}{I_1}$.

Transformer Nameplate Rating

The capacity specified in volt-amperes ($\text{VA}$) or kilovolt-amperes ($\text{kVA}$) indicating the maximum product of rated voltage and current a unit can handle continuously without overheating.

Transformer EMF Equation

The mathematical formula calculating root-mean-square induced voltage: $E = 4.44\,\Phi_{\text{max}}\,f\,N$, where $\Phi_{\text{max}}$ is maximum flux in Webers, $f$ is frequency in Hertz, and $N$ is turn count.

Form Factor of a Sine Wave

The ratio of root-mean-square value to average value, which equals $1.11$ for a sinusoidal waveform.

No-Load Current ($I_0$)

The small current taken by the primary winding when the secondary is on open circuit, normally constituting less than $5\%$ of full-load current.

Magnetizing Current Component ($I_{0m}$)

The reactive component of the no-load current that establishes the magnetic flux in the ferromagnetic core and is in phase with that flux.

Core-Loss Current Component ($I_{01}$)

The active power component of the no-load current that is in phase with applied primary voltage and supplies hysteresis and eddy-current losses.

Stalloy

A high-grade silicon steel alloy with high electrical resistivity and low hysteresis loss, manufactured in sheets up to $3.5\,\text{mm}$ thick for transformer cores.

Core Lamination

The assembly technique of stacking thin steel stampings separated by insulating varnish to restrict the paths of circulating eddy currents.

Concentric (Cylindrical) Windings

A coil arrangement used primarily in core-type transformers where the low-voltage winding is placed closest to the core limb and encircled by the high-voltage winding.

Sandwich (Disc) Windings

A coil arrangement used mainly in shell-type transformers where the primary and secondary turns are split into interleaved flat sections along the core.

Leakage Flux

The magnetic lines of force set up by winding currents that link only one winding instead of traveling through the main ferromagnetic core, causing an inductive reactance voltage drop.

Methods for Reducing Leakage Flux

Design techniques including utilizing long and narrow core windows, arranging coils concentrically or interleaved, and adopting a shell-type core construction.

Conservator Tank

An auxiliary oil drum situated above the main transformer tank that allows cooling oil to freely expand and contract with temperature variations.

Dehydrating Breather

An air-filtering device attached to the conservator that contains a desiccant and an oil bath to extract atmospheric moisture and particulate impurities during thermal breathing cycles.

Silica Gel

A desiccant chemical that is blue when dry and turns pink when saturated with water, regenerated by spreading out and heating on a metal sheet.

Buchholz Relay

A gas- and oil-actuated safety relay mounted in the pipe connecting the main tank and conservator that uses floats and mercury switches to sense internal faults.

Buchholz Relay Alarm Operating Conditions

Incipient or minor transformer defects including slow accumulation of gas from localized heating, short-circuited core bolts, insulation breakdown between laminations, or a minor drop in oil level.

Buchholz Relay Trip Operating Conditions

Severe faults including violent gas and oil surges from phase-to-phase short circuits, internal earth faults, bushing breakdowns, or complete loss of insulating liquid.

Arcing Horns

Spark-gap electrodes fitted across transformer bushings that provide a lower-breakdown path to earth, diverting lightning strikes and switching surges away from insulator surfaces.

ONAN Cooling

A cooling method designation meaning Oil Natural Air Natural, where heat transfer occurs purely through natural convection currents in both the insulating oil and surrounding ambient air.

ONAF Cooling

A cooling method designation meaning Oil Natural Air Forced, where insulating oil circulates naturally while external blowers force air over radiator fins.

OFAF Cooling

A cooling method designation meaning Oil Forced Air Forced, where mechanical pumps circulate the oil and electric fans drive air across external cooling radiators.

Differential Protection

A protective relaying scheme that monitors and compares incoming and outgoing currents, instantly tripping circuit breakers if a discrepancy indicates an internal fault.

Iron (Core) Losses

Constant power losses occurring within the ferromagnetic core comprising hysteresis and eddy-current dissipations, independent of load current under rated operating voltage.

Eddy-Current Losses

Power dissipated as heat within the conductive core structure due to closed circulating currents induced by the alternating magnetic flux.

Hysteresis Losses

Energy consumed as heat during every cycle of magnetic reversal, proportional to the area enclosed by the ferromagnetic material's $B$-$H$ loop.

Copper ($I^2R$) Losses

Variable power losses generated by current passing through the internal electrical resistance of primary and secondary windings, changing with the square of the load current.

Open-Circuit (No-Load) Test

A transformer factory test conducted at rated voltage and frequency with the secondary open; the wattmeter reading indicates constant core iron losses.

Short-Circuit Test

A transformer factory test performed by shorting the secondary through an ammeter and applying a reduced voltage (approximately $4\%$ of rated voltage) to circulate full-load current, measuring full-load copper losses via a wattmeter.

Condition for Maximum Efficiency

The transformer loading level where variable copper losses ($I^2R$) equal the constant core iron losses.

Voltage Regulation

The difference between no-load secondary terminal voltage and full-load secondary terminal voltage expressed as a percentage of no-load voltage: $\frac{V_{\text{no-load}} - V_{\text{full-load}}}{V_{\text{no-load}}} \times 100\%$.

Current Transformer ($\text{CT}$)

An instrument transformer whose single-turn primary is connected in series with a power line to step down high operating currents to a standard small secondary current (such as $5\,\text{A}$) for monitoring.

Current Transformer Secondary Open-Circuit Hazard

The dangerous situation where eliminating opposing secondary ampere-turns causes core flux to rise dramatically, inducing lethal high voltages and triggering severe insulation-destroying overheating.

Autotransformer

A transformer design utilizing a single continuous tapped electrical winding that functions simultaneously as both primary and secondary circuits.

Autotransformer Operational Advantages

Lower manufacturing cost, reduced copper and iron material requirements, and increased electrical efficiency due to lower overall core and winding losses.

Autotransformer Operational Disadvantage

The absence of electrical isolation between the primary and secondary sides, which risks exposing the low-voltage output to full supply voltage if an open circuit develops in the common winding.

On-Load Tap Changer

A mechanism typically fitted to the high-voltage winding that adjusts the active turns ratio during operation without interrupting load current, compensating for supply voltage fluctuations.

Three-Phase Delta-Star Connection

A transformer configuration where primary windings are interconnected in delta and secondaries in star, providing an accessible neutral terminal essential for commercial and residential consumer supplies.