Distribution transformers work with full efficiency


Lexicon> Letter T> Transformer

Definition: an electrical device that can bring alternating current or three-phase current to a different level of voltage and current

More specific terms: autotransformer, isolating transformer, machine transformer, distribution network transformer, local network transformer, network transformer

English: transformer

Category: electrical energy

Author: Dr. Rüdiger Paschotta

How to quote; suggest additional literature

Original creation: 04/26/2010; last change: 02.10.2020

URL: https://www.energie-lexikon.info/transformator.html

Why does the current consumption of a transformer depend on the load on its secondary coil, even though it is not electrically connected to the primary coil?

A transformer (short transformer) is an electromagnetic component (or equipment) that can bring alternating current or three-phase current (but not direct current) to a different level of electrical voltage and current. The basic principle is that an electromagnet (usually a coil on an iron core) is used to generate a magnetic field that changes in direction and strength and which can induce other voltages and currents in a second coil (or in several coils). It is possible in this way to obtain a lower or higher voltage than the primary voltage; the tension (the translation) is essentially determined by the ratio of the number of turns of the coils involved. If the secondary winding has fewer turns than the primary winding, the secondary voltage is lower than the primary voltage, and the current output by the transformer can be correspondingly higher than the one consumed. In the ideal case, the power (the product of voltage and current) would be retained, but in practice certain energy losses occur (see below).

The basic function of a transformer is usually to change the electrical voltage to another level. When the tension is increased, it is called “high tensioning”, otherwise it is referred to as “down tensioning”.

Another often important function of a transformer is galvanic isolation; an electrically conductive connection between the various coils is generally not necessary. This is also important in power engineering, since this way you can have different types of networks on the primary and secondary side - for example a TN-C system as a four-wire system with a four-wire system with a grounded star point on the secondary side for a local network transformer and a TN-C system on the primary side Three-wire system with a different earthing concept.

In many cases the primary coil of a transformer is connected to the mains voltage (e.g. 230 V) and the secondary coil generates a significantly lower voltage for the supply of a device. Often a rectifier and capacitor follow to get a smoothed DC voltage.

Although the primary and secondary coils are mostly not electrically connected to each other (except for the seldom used Autotransformers), there is a reaction via the magnetic field: The primary coil draws more current, the higher the current drawn from the secondary coil. However, a certain small primary current remains even if the secondary coil is not loaded; it is largely one Reactive current (which does not transport any power in the time average) and partly by an active current, which means energy losses. This standby consumption can be considerable depending on the performance and quality of the device.

The primary and / or the secondary coil can contain taps. This makes it possible to adapt the number of turns of the primary coil used to the respective mains voltage, or to generate different secondary voltages. Several separate secondary coils are also possible.

Some old transformers turn into dangerous poison hoppers in a fire.

Large transformers have very high levels of efficiency - often well over 99%. Nevertheless, a loss z. B. from only 0.5% with a transmission capacity of 200 MW still leads to a power loss of 1 MW, which occurs as heat. (This corresponds roughly to the power required to heat 200 modern single-family houses on a cold winter day.) Inside a large transformer, the heat is often transported to the outer walls with the help of an oil, which also serves as an electrical insulator. In the past, transformer oils containing PCB were used. B. released extremely toxic dioxins and other substances in transformer fires. Because of the long service life of such systems, there are still many such transformers today.

Transformers with outputs of many 100 MW each cost several million euros. However, the costs per kilowatt are much lower than for small transformers. For this reason, the overall costs incurred for the consumers are determined more by the small transformers - for example in the transformer stations within residential areas, each of which only serves a relatively small number of consumers.

The short-term overload of a transformer by drawing an increased current from the secondary coil leads to increased heating and a certain drop in the secondary voltage, but is usually tolerated without any problems. An excessive primary voltage, on the other hand, can bring the iron core into magnetic saturation, which leads to a sharp increase in the primary current. Destruction can then take place relatively quickly.

Special designs and applications

Three-phase transformers

Three-phase transformers are obviously transformers that can work with three-phase current. A simple design is the Three-limb transformer, an improved type of Five-limb transformer. However, it is also possible to use three separate single-phase transformers, which are combined as one Transformer bank are designated.

On the primary and secondary side, the windings can be connected in delta or star connection. The star connection tends to be preferred for very high voltages (because of the lower insulation requirements), whereas the delta connection is preferred for high currents (e.g. with low voltage transformers). Even more complicated types of shutdown are sometimes used, such as zigzag switching to reduce the influence of unbalanced loads.


Normally the primary and secondary coil (s) of a transformer are galvanically isolated from one another, i.e. H. there is no electrical connection between them. In the case of an autotransformer, however, this is different; it usually only has a single winding with different taps. For example, if there is a coil with a tap in the middle and the primary voltage is applied to the entire winding, half the primary voltage is obtained between one end of the winding and the tap in the middle. With this approach, you first save an additional secondary winding, which would have to have half as many turns as the primary winding. In addition, the transformer core can also be made smaller than with a conventional transformer with the same gear ratio. This is because some of the power is not transmitted magnetically, but directly electrically. This saving effect is particularly pronounced when the secondary voltage is as high as the primary voltage. This is why autotransformers are mainly used when the transformation ratio is between 2: 1 and 1: 2.

Some application examples for autotransformers:

  • They are often used for power adapters (travel adapters) for the use of devices in other countries with different mains voltages. For example, a mains voltage of 110 V in the USA can be transformed to 230 V for devices built in Europe.
  • Ignition coils for gasoline engines are usually designed as autotransformers. Here the transmission ratio is very high and therefore hardly any savings effect is given. On the other hand, nothing speaks against this concept here.
  • Auto-transformers are also sometimes used in high-voltage networks.

Because of the lack of galvanic isolation, autotransformers cannot of course be used for many other purposes. For example, they are usually not suitable for power packs for devices that operate with low voltage, since depending on the orientation of the plug in the socket, the device could work with a high voltage to earth, which would make it dangerous if a line was touched.

Isolating transformers

Isolation transformers are transformers whose main purpose is galvanic isolation. Often the secondary voltage is roughly identical to the primary voltage; H. there is no translation of the voltage. Such transformers are often not used for the transmission of energy, but for analog audio signals, for example, in order to avoid ground loops.

Machine transformers

In a large power plant, one or more so-called ones are usually used Machine transformers (or Block transformers) to increase the voltage of the generators (typically a few kilovolts up to a few tens of kilovolts) for feeding into the high voltage.

Network coupling transformers

When two medium-voltage or high-voltage networks are to be coupled with one another in order to exchange energy, this is often used Network coupling transformers.

Distribution transformers

Distribution transformers are transformers that are used to distribute energy. This includes those that are built into low-voltage transformer stations, but also those that step down from high voltage to medium voltage.

Energy efficiency of transformers

There are essentially two types of energy loss in transformers:

The power loss in a transformer has a component that is independent of the load and a further component that increases roughly with the square of the power drawn.
  • On the one hand, as soon as a transformer is in operation, there is a fixed contribution to the losses, which mainly has to do with the periodic magnetization reversal of the transformer core and with eddy current losses. The contribution of the ohmic losses in the primary winding is less important, since the current strength occurring there is low in no-load operation. One speaks here of idling losses, since these occur already when idling (but of course not only then).
  • In addition, there is a load-dependent power loss, which is mainly caused by the ohmic resistance of the windings. This contribution to the power loss increases with the square of the current drawn, i. H. If the withdrawn power is halved, this contribution drops to a quarter.

The highest degree of efficiency often occurs in the middle partial load range, where the load-dependent losses are not yet too high, but the idling losses are no longer a major factor. In the lower part-load range, on the other hand, idling losses are a problem, since they are quite high relative to the low output.

The efficiency of very large transformers, such as those used in large power plants or in substations, can be well above 99% at full load and especially at medium partial load; so there are only relatively small energy losses, which lead to corresponding amounts of waste heat. Smaller transformers, for example with a power of the order of one megawatt, can also be very efficient. On the other hand, miniature transformers for household appliances, for example, are often quite inefficient (with efficiencies sometimes even below 50%) because there are few incentives for manufacturers to optimize efficiency while accepting higher production costs.

Even larger transformers can become very inefficient in the lower partial load range because of the no-load losses, and depending on the application, such load cases can occur more or less frequently. For example, the average load of transformers in the distribution grids is typically only around 20% of the maximum load, since the power drawn by the consumers fluctuates greatly and the capacity of the transformers must be sufficient in any case. This problem can hardly occur in a power plant, since most power plants are never operated in the lower partial load range.

In principle, the energy efficiency of a transformer is one of several factors that should be optimized during development. How much energy efficiency is weighted here depends on the respective priorities. For example, the energetic optimization of distribution network transformers would be very desirable, as they remain in operation for many decades, i.e. they can waste an enormous amount of energy over the course of their service life. However, the acquisition costs for better transformers are significantly higher, and their use can also be hindered by other aspects - for example, by a slightly higher space requirement (i.e. with retrofitting possibly the need to create more space) and the need for some designs ( e.g. with amorphous core) stronger noise development. Certain minimum standards for the energy efficiency of distribution network transformers are also specified by standards, which, however, have so far hardly developed any steering effect due to their low requirements.

At EU level, the permitted energy losses from many types of transformers are now limited by a regulation (at least for new systems) [1]. For example, the no-load losses for a transformer with a rated power of 1 MVA may be a maximum of 1550 W since July 1, 2015, and only 1395 W from July 1, 2021. This then corresponds to only approx. 0.14% of the maximum power - im Compared to the actual load in practice, of course, accordingly more. Large power transformers must adhere to certain minimum values ​​for maximum efficiency. For outputs of 100 MVA or more, this value has been 99.737% since July 1, 2015, and even 99.770% is required from July 1, 2012. These values ​​apply to a certain optimal performance, which is considerably below the maximum performance. For example, if a transformer achieves 99.7% efficiency with a converted output of 50 MW, this corresponds to a power loss of 150 kW - which can still correspond to that of a boiler for an apartment building.

Use of transformers

Transformers are very important in power engineering:

  • In a high-performance power plant, a machine transformer (block transformer) translates the voltage of the generator (up to 27 kV) to a high level of hundreds of kilovolts, while the current is reduced accordingly. This enables the low-loss transmission of power with extra-high voltage lines.
  • The maximum voltage from the transmission network is in Substations translated to the high voltage level of 110 kV. The very large transformers used here can transmit outputs of several hundred megawatts. Systems for high-voltage direct current transmission also contain one or more transformers on the alternating current side (see Figure 2).
  • Medium-voltage networks (in Germany with 10 kV, 20 kV or 30 kV) are supplied from the high voltage level via transformer stations with transformers for several tens of megawatts.
  • The low-voltage networks (three-phase current with 400 V) are fed with a few hundred kilowatts from the medium-voltage level via compact transformer stations (network stations), each of which contains a distribution transformer. B. is sufficient for households on a street. Most of the consumers are connected to the low-voltage networks.

So-called adjustable local network transformers are used, the transmission ratio of which can be automatically adjusted within certain limits in order to be able to regulate the voltage level in the event of fluctuations in the load and infeeds (→Voltage maintenance). Basically, a type of step switch is used here, implemented either in a purely mechanical form or with electronics.

In the household, various low-voltage transformers in power supplies generate even lower voltages of e.g. B. 12 V or less for the operation of various devices. For example there is Plug-in power supplies with an output of just a few watts for the operation of small electronic devices. Such plug-in power supplies often contain a small transformer; however, these are increasingly being replaced by even more compact, powerful and low-loss systems Switching power supplies replaced.

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See also: transformer station, power supply unit, electrical energy, alternating current, three-phase current, power grid, standby consumption
as well as other items in the electrical energy category