What is a transformer? A transformer is a device that transfers energy from one AC system to another. A transformer can accept energy at one voltage and deliver it at another voltage. This permits electrical energy to be generated at relatively low voltages and transmitted at high voltages and low currents, thus reducing line losses, and to be used at safe voltages.

[McPherson G. , Laramore R. D.] Transformer in electric network

Three-phase transformer Three-phase power may be transformed by using either two or three single-phase transformers, or by a single three-phase transformer. When a set of single-phase transformers is employed to transform three phases, it is called a three-phase bank of transformers.

Symmetrical three-phase transformer

[Sähkötekniikan käsikirja]

Figure below presents a three-phase core transformer and a three-phase shell transformer. Every phase of the core transformer has its own bar with two windings belonging to phase. In the shell transformer the one-phase magnetic flux has a return path, independent from the other phases.

[Sähkötekniikan käsikirja]

Types of distribution transformers

A two-winding transformer The transformer may be defined as a device in which two or more stationary electric circuits are coupled magnetically, the windings linked by a common timevarying magnetic flux. One of these windings, known as the primary, receives power at a given voltage and frequency from the source; and the other winding known as the secondary, delivers power, usually at a different voltage but at the same frequency, to the load.

[McPherson G. , Laramore R. D.]

Figure below presents a one-phase core transformer and a one-phase shell transformer. To the iron core in a one-phase core transformer belong two bars with cylindrical windings around them. Bars are connected to each others with yokes. Bars and yokes build together a closed magnetic circuit. There are two windings. They are called high-voltage- and low-voltage windings or, according to the power direction, primary or secondary windings. As material are used copper or aluminum. A one-phase shell transformer´s iron core is constructed of three bars and two yokes. The windings are placed on the middle bar.

[Sähkötekniikan käsikirja]

Three-phase transformer connections

[Sähkötekniikan käsikirja]

Yy0, Dy11, Yd11 and Yz11 connections are used in Finland. In the case of one three-phase transformer or of three single-phase transformers, several

three-phase arrangements can be used. The following are quite common: the delta-delta, the wye-wye, wye-zigzag and the wye-delta or delta-wye connections.

Exciting current The exciting current I e, is considered as having two components, the core-loss current I fe and the magnetizing current I m. The core-loss current is a real-power component and is due to the core losses. The magnetizing current is, in effect, the component of current that furnishes the mmf to overcome the magnetic reluctance of the core.

The waveform of the exciting current is not sinusoidal. However, it is symmetrical, the exciting current can therefore be represented by a series of odd harmonics.

Current inrush of a transformer Frequently upon energizing a power transformer, there is an inrush of exciting current which may initially be as high as eight times the rated current of the excited winding even with all other windings open. The inrush is most severe when the transformer is energized at the instant the voltage goes through zero immediately following which the polarity of the voltage is such that the flux increases in the direction of the residual flux.

Figure. Inrush current for a transformer energized at zero instantaneous voltage

Voltage Regulation The voltage regulation is an important measure of transformer performance and is expressed by the formula:

The percent regulation is of 100 times the per-unit regulation. In the above expression, V 2 Io is the secondary voltage under load, and V 2 nI is the secondary no-load voltage (I 2 = 0), with primary voltage held constant at the value it had under load. The quantities used in the formula are magnitudes, not phasors.

[McPherson G. , Laramore R. D.]

Losses and efficiency The losses in a transformer are the core losses, which for a given voltage and frequency are practically independent of the load; the copper losses due to the resistance of the windings; and the stray losses, largely due to eddy currents induced by the leakage fluxes in the tank and other parts of the structure. The sum of copper losses and the stray losses is called the load losses. The efficiency of a transformer at rated load is quite high. A value of 90 percent is not uncommon for transformers as small as 1 kVA, with greater values of efficiency as the rating increase. The efficiency is expressed by

Kinds of core alloys

There are five main groups of magnetically soft alloys classified primarily by the chief constituents of the metal.

low-carbon steel

silicon steel

nickel-iron

cobalt-nickel-iron

cobalt-iron

Also look at:

Hysteresis curve of a hard and soft alloy

Look at trade names

Core types The configuration of a three-phase transformer depends on the core type of the transformer. There are three choices:

The "shell" type

The "core" type

3x single-phase

Core configuration Core type shape is mostly used in three-phase distribution transformers. The window height H a depends on a coil height and the core area A r depends on the rated power S n.

The cross section of the core

The bulk factor of the leg can be defined when the cross sectional area of the leg is known:

k = a coefficient that depends on the number of steps

Core sheet

Coil material The coil material can be copper or aluminum. The term copper loss is still used to indicate resistance losses of winding materials whether copper or aluminum is used.

Copper ρ k = 8.93 kg/dm3

Aluminum ρ k = 2.69 kg/dm3

ρ k = The weight by volume

Coil configuration

Foil coil Show the primary!!

Tank The tank and the cover are manufactured of hot-rolled, unalloyed steel sheet and profile balks. Fine granular steel is used in transformers for low ambient temperatures.

Areas where strong eddy currents can be generated due to high currents, and where ordinary hot-rolled steel can become too warm, are made of non-magnetic (austenite) steel. Such areas are for example the surroundings if high current bushings and bushbars. The tanks are welded and manufactured in accordance with modern welding methods.

The tank has lifting lugs for lifting the transformer (fully-equipped transformer including oil) and at least four jacking points an the lower part if the tank for lifting by hydraulic

jacks. For transport wheels there are fixing points at the bottom of the tank. The tank is provided with at least two earthing lugs made of stainless steel.

The connecting flanges of the coolers and the flanges for filling, draining, filtering and sampling valves are welded to the tank. Also the fixing brackets of cooler fans are welded to the tank. Usually the support of the oil conservator is fastened to the tank too. The transformer cover is fixed to the tank usually by means of a bolt joint using oil resistant rubber cork as the gasket. A gasket made of special rubber can also be used. The cover can also be fixed to the tank by welding. The welded seams are tighter and more reliable than bolted ones. They can be quite easily opened and rewelded.

The transformer cover is constructed so that no water pockets or other water collector points are formed. An air pipe is connected to the gas relay from all the turrets, flanges etc. where it is possible for gas pockets to develop.

[ABB Catalogue. Power Transformers Construction.]

Coolers Transformers are usually provided with radiators for cooling (cooling method ONAN or ONAN/ONAF). The radiators are manufactured of welded elements and they are vacuum-proof.

The radiators are connected to the transformer tank by means of shut-valves. This method allows individual radiators to be removed without draining oil from the transformer. The shut-off valve is provided with a position indicating handle and with a locking spring. The lower part of the radiators has a plug for oil outlet and the upper part a plug for air release.

In the transformers which have ONAN/ONAF cooling, the fans forcing the air circulation are under or at a low noise level and they are equipped with steel sheet guard and the necessary protective mesh. The fan motors can normally be connected to the 380/220 V supply, but if required, motors with other voltage ratings can also be used.

The transformer can also be provided with water cooling, cooling method OFWF or oil-air cooler, cooling method OFAF. In each case, the oil circulation through coolers is handled by means of an oil pump. The coolers can be installed so as to rest upon the transformer tank or on a frame separated from the transformer. The pipe work is provided with the necessary valves for removing the cooler and the pump for inspection and maintenance.

[ABB Catalogue. Power Transformers Construction.]

Heating of the windings The heating of the windings depend on the current density and dimensions of coil wires. The smaller the selected current density is the more copper or aluminum is needed for the coil. As the heating is smaller, the load losses become lower. The heating is squarely proportional to the current density. Standards set some restrictions to the heating of the windings. Therefore, the designer should always calculate the heating when designing the coils. An average heating of the windings compared to the outer air according to the IEC 76 standards can be:

V < 65 0 C

Losses of a transformer