Transformer terms, in plain English

Tests performed before energizing a new unit to confirm it meets spec and shipped undamaged.

A transformer with one shared winding instead of separate primary and secondary. Smaller and cheaper for small voltage changes, but with no electrical isolation.

Basic Impulse Level — the insulation's ability to withstand a voltage surge (like a lightning strike), in kV.

The insulated terminal that carries a conductor through the tank wall without shorting to it — where cables land on the unit.

A dry-type transformer with windings cast in epoxy resin — rugged, moisture- and contamination-resistant, and suited to harsh indoor or industrial sites.

An expansion tank mounted above an oil-filled transformer that gives the oil room to expand and contract with temperature.

Construction where all energized parts are fully enclosed and insulated, so the public and crews can't contact them.

A three-phase winding wired in a triangle, with no neutral brought out. Common on the primary (high-voltage) side of a distribution transformer.

De-Energized Tap Changer — taps adjusted only when the transformer is de-energized; for infrequent voltage setting.

Dissolved Gas Analysis — testing oil for gases that reveal internal faults like arcing or overheating.

A transformer that uses air and solid insulation instead of oil — common indoors and in public buildings.

The very high current that flows during a short circuit. Equipment must be rated to withstand and interrupt the fault current available at its location.

A biodegradable, high-fire-point vegetable-oil dielectric fluid, often used in fire-sensitive or eco-sensitive sites.

Group-Operated Air-Break switch — a gang-operated overhead switch for isolating a line section.

A transformer that steps a generator's output voltage up to transmission level so power can leave the plant.

Distortion in the current waveform created by electronic loads like drives, servers, and LEDs. Left unaddressed, it heats windings and eats into capacity.

The transformer's internal opposition to current, set in design. It governs voltage regulation and fault current.

The brief, large current surge when a transformer is first energized — often many times full-load current, which matters for breaker and fuse sizing.

The thermal rating of a unit's insulation system (e.g. 220°C for dry-type). It caps how hot the windings can safely get.

A rating (K-4, K-13, K-20…) for how much harmonic-rich, nonlinear load a unit can carry without overheating. Higher K = more tolerance for data-center and drive loads.

Kilovolt-ampere — the apparent power rating of a transformer. It sizes the unit to the load it can carry.

kW is the real power that does work; kVA is the total apparent power the transformer must supply. They're equal only at a power factor of 1.0.

Older construction where energized parts sit exposed behind a door, rather than fully insulated. Dead-front has largely replaced it for safety.

Energy lost as heat in the windings, rising with the load carried. With core loss, it defines a unit's efficiency.

A distribution arrangement where a unit can be fed from two directions, improving reliability.

Load Tap Changer — a device that adjusts the transformer's turns ratio under load to regulate output voltage.

The traditional petroleum-based dielectric fluid that insulates and cools a liquid-filled transformer. Effective and cheap, but flammable — hence ester alternatives like FR3.

Megavolt-ampere — 1,000 kVA. Used for larger power and substation transformers.

The metal rating plate on every unit — kVA, voltages, impedance, cooling class, weight, and fluid. The single source of truth for what you have.

Neutral Grounding Resistor — limits how much current can flow in a ground fault, protecting equipment and people while the fault is cleared.

Energy the transformer burns just being energized to magnetize its core — 24/7, load or no load. A key driver of lifetime operating cost.

Any load that draws current in pulses rather than a smooth sine wave — VFDs, rectifiers, servers. These generate the harmonics that drive K-factor sizing.

Cooling classes: Oil-Natural Air-Natural and Oil-Natural Air-Forced (fans). Higher cooling = higher rating.

A ground-level, pad-mounted distribution transformer in a locked steel enclosure — common for commercial and underground service.

An overhead distribution transformer mounted on a utility pole — the classic gray can on residential lines.

The ratio of real power (kW) doing work to apparent power (kVA) supplied. A low power factor means the system carries current that isn't doing useful work.

A spring-loaded valve on a liquid-filled tank that vents safely if internal pressure spikes during a fault.

The primary is the input winding fed from the source; the secondary is the output winding that feeds the load. A step-down unit has the higher voltage on the primary.

A distribution layout fed from a single direction. Simpler and cheaper than a loop feed, but a fault upstream drops everything downstream.

The fins or panels on a liquid-filled tank that shed heat. Adding fans (ONAF) boosts the unit's rating.

An automatic switch that opens on a fault and re-closes to restore power if the fault was temporary.

Sweep Frequency Response Analysis — detects mechanical winding movement or damage by its frequency signature.

A larger power transformer that steps transmission or sub-transmission voltage down to distribution level at a substation.

The assembly of switches, breakers, fuses, and metering that protects and isolates a transformer and the circuits around it.

Adjustable connection points on a winding that fine-tune the output voltage — typically ±2 steps of 2.5% — to match actual site conditions.

How much hotter the windings run than the surrounding air at full load — typically 55°C or 65°C. Lower rise generally means longer insulation life.

Transformer Turns Ratio test — verifies the ratio of primary to secondary windings is correct.

The ratio of primary to secondary winding turns — it sets how far the voltage is stepped up or down.

Shorthand like 'Dyn11' that describes how primary and secondary are connected (delta/wye) and the phase shift between them. It must match for units to be paralleled.

How far the output voltage sags from no-load to full-load, as a percentage. Tighter regulation means steadier voltage at the load.

Vacuum Pressure Impregnation — a dry-type coil sealed with varnish under vacuum and pressure for moisture resistance. A step below cast coil in ruggedness and cost.

A three-phase winding wired in a star with a common neutral point that can be grounded and used for single-phase loads. Common on the secondary side.