Transformers

The transformer is the reason AC power systems exist. It changes voltage up or down with efficiencies above 98%, has no moving parts, and can run for forty years. Electricians install and connect transformers constantly: pad-mounted utility units, dry-type transformers that step a building's distribution voltage down to 120/208 V for receptacles, control transformers in motor starters, buck-boost transformers that correct a supply voltage, and current transformers that feed meters and relays. This lesson explains how transformers work, the ideal transformer relationships, ratings, losses and efficiency, percent impedance and fault current, taps and polarity, single-phase and three-phase connections, autotransformers and buck-boost transformers, instrument transformers, and the rules for protecting and installing them safely.

How a transformer works

A transformer consists of two or more coils wound on a common iron core. An alternating voltage applied to the primary winding drives a small magnetizing current that produces an alternating magnetic flux in the core. The core guides almost all of that flux through the secondary winding, where the changing flux induces a voltage (mutual induction). Because the same flux links both windings, the voltage per turn is the same in each, so the voltages are in the ratio of the turns. A transformer works only with changing flux, so it does not transfer DC; a DC voltage applied to a primary would drive a large current limited only by the winding resistance and burn the winding out.

Cores are made of thin laminations of silicon steel, insulated from each other, to limit eddy currents induced in the core itself. Windings are copper or aluminium, insulated with enamel, paper or resin. Small and medium transformers in buildings are usually dry-type, cooled by air; large ones are immersed in mineral oil or less-flammable fluids for insulation and cooling.

The ideal transformer

Neglecting losses and leakage, the relationships are:

where \(a\) is the turns ratio. A step-down transformer (\(a > 1\)) lowers voltage and raises current; a step-up transformer does the opposite. Power in equals power out: the transformer trades voltage for current. The load current determines the primary current; with no load on the secondary, the primary draws only a small magnetizing current, typically 1 to 5% of rated current.

Reflected impedance

An impedance \(Z_S\) connected to the secondary appears from the primary side as \(Z_P = a^2Z_S\). A 6 Ω load on the secondary of a 5:1 transformer looks like 150 Ω from the primary. This impedance transformation is used for matching in electronics and explains why impedances on the two sides of a transformer must be referred to a common side (or put in per unit, as in ELE-380) when calculating fault currents.

Ratings

Transformers are rated in volt-amperes (VA or kVA), not watts, because their heating depends on current, regardless of the load's power factor. The nameplate also gives the primary and secondary voltages, frequency, phases, percent impedance, insulation class and temperature rise, connection diagram, taps and, for three-phase units, the vector group or phase displacement. Rated currents follow from the kVA:

Losses, efficiency and heating

Real transformers have two kinds of losses:

• Core (no-load or iron) losses, from hysteresis (energy to reverse the magnetization of the steel each half cycle) and eddy currents in the laminations. They are present whenever the transformer is energized, regardless of load, and are nearly constant.
• Copper (load) losses, \(I^2R\) in the windings, which grow with the square of the load current. Harmonic currents add extra losses through eddy currents in the windings, which is why transformers serving nonlinear loads are derated or K-factor rated.

Efficiency is output power divided by input power. Because core losses are constant and copper losses rise with the square of load, efficiency peaks where the two are equal, typically at 35 to 50% of rated load for distribution transformers. Energy efficiency regulations in both Canada and the US set minimum efficiencies for distribution transformers at a specified load.

Insulation class and temperature rise limit loading. A dry-type transformer with 220 °C insulation might be rated for a 150 °C average winding rise over a 40 °C ambient; lower-rise designs (115 °C or 80 °C) run cooler, last longer and tolerate overloads better. Overloading or blocking ventilation shortens insulation life rapidly.

Percent impedance, regulation and fault current

A real transformer's windings have resistance and leakage reactance, together called its impedance, stated on the nameplate as a percentage. The percent impedance is the percentage of rated primary voltage that would drive rated current through a short-circuited secondary. It determines two important things:

• Voltage regulation: the drop in secondary voltage from no load to full load, roughly proportional to the impedance and to the load's power factor. Lower impedance gives better regulation.
• Fault current: with an infinite (stiff) source, the maximum fault current at the secondary is rated current divided by the per-unit impedance, \(I_{sc} = I_{rated}/Z_{pu}\). Lower impedance gives higher fault current, which equipment downstream must be rated to withstand and interrupt.

Taps

Most distribution transformers have taps on the primary winding to adjust the turns ratio when the supply voltage is consistently high or low. A typical arrangement provides two taps of 2.5% above nominal and four below (written 2 × 2.5% FCAN and 4 × 2.5% FCBN, "full capacity above/below nominal"). If the supply is consistently 3% low, connecting to the −2.5% tap uses fewer primary turns, raising the secondary voltage. Taps are changed only with the transformer de-energized; large utility transformers have on-load tap changers that adjust automatically.

Polarity

The relative direction of the windings determines whether the secondary voltage is in phase with the primary at given terminals. Transformer terminals are marked H1, H2 (high-voltage side) and X1, X2 (low-voltage side), so that at any instant H1 and X1 have the same polarity. In schematics this is shown with dots. Polarity matters when paralleling transformers, connecting single-phase units into three-phase banks, and connecting dual windings in series or parallel. Connecting windings with the wrong polarity can short-circuit a winding or produce voltages that cancel instead of adding.

Dual-voltage windings are common: a primary rated 240 × 480 V has two 240 V coils connected in parallel for 240 V or in series for 480 V, and a secondary rated 120/240 V has two 120 V coils in series with the centre tap as neutral. The nameplate diagram shows which terminals to link.

Three-phase transformers

Three-phase transformation uses either three single-phase transformers or one three-phase unit with three windings on a common core. The windings on each side can be connected in delta or wye:

The secondary of a delta-wye transformer is a separately derived system: its neutral must be grounded and bonded at the transformer or the first disconnect, as covered in ELEC-120.

Open delta

Two single-phase transformers connected in an open delta (V-V) can supply three-phase power, either as a planned economy or when one unit of a delta-delta bank fails. An open delta can carry only 57.7% (\(1/\sqrt3\)) of the capacity of the full three-transformer bank, or 86.6% of the combined rating of the two transformers used.

Autotransformers and buck-boost transformers

An autotransformer has a single winding with a tap, so part of the winding is common to primary and secondary. It is smaller and cheaper than a two-winding transformer for small voltage changes, but it provides no isolation between input and output. Variable autotransformers are used in test benches; reduced-voltage autotransformer starters reduce motor starting current (ELEC-310).

Buck-boost transformers are small two-winding transformers with low-voltage secondaries (typically 12, 16, 24 or 32 V) that are connected as autotransformers to raise (boost) or lower (buck) a supply by a few percent, for example to run 230 V equipment from a 208 V supply. Because the transformer carries only the boost voltage, a small unit handles a large load: the kVA required is the load current times the secondary (buck or boost) voltage.

Instrument transformers

Current transformers (CTs) and potential (voltage) transformers (PTs or VTs) reduce high currents and voltages to standard levels, typically 5 A or 1 A and 120 V, for meters, relays and controllers. A CT's primary is the power conductor passing through its window, and its secondary current is the primary current divided by its ratio: a 400:5 CT produces 5 A when 400 A flows.

A CT secondary must never be open-circuited while primary current flows. The CT is designed to drive current into a low-impedance burden; with the secondary open, the primary current becomes all magnetizing current, saturating the core and inducing a dangerously high voltage, possibly thousands of volts, across the open terminals, which can injure workers and destroy the CT. CT circuits are therefore fitted with shorting terminal blocks that short the secondary before a meter or relay is removed.

Installation and protection

The Canadian Electrical Code (Section 26) sets requirements for transformers, including maximum overcurrent protection on the primary, and on the secondary where required, as percentages of the transformer's rated currents; working space and ventilation; separation from combustible materials; vault requirements for large oil-filled units indoors; and grounding of the secondary system. Secondary conductors between the transformer and their overcurrent device must be protected and kept short.

Energizing a transformer draws a magnetizing inrush current of 8 to 12 times rated current (sometimes more for small units) for a few cycles, as the core is driven into saturation depending on the point on the voltage wave at switching. Primary breakers and fuses must ride through inrush without tripping, which is why their instantaneous settings and time-current characteristics are chosen with inrush in mind.

Lesson 9 continues with three-phase connections, banks, paralleling and testing, and Lesson 11 extends the fault current calculation from the transformer terminals to the rest of the system.