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 oilair 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