What happens when a transformer burns out

Transformers bring the alternating current to any desired voltage

On the left is the diagram of a transformer that feeds the electricity generated in the power plant into the high-voltage network: Since the electricity has to be stepped up in this case, the secondary windings (right) have more turns than the primary windings. The windings for the three-phase phases are connected on both sides in a "star-point connection". On the secondary side, a "center point conductor" branches off from the star point, via which a compensating current flows when the three phases in the network are loaded to different degrees.

The transformer on the right brings high voltage from 110 kilovolts to medium voltage of 20 kilovolts. On the frame on the left you can see the incoming and outgoing three-phase conductors at the top.

The principle of the "transformer" is based on induction, which enables electricity to be generated in generators. In the simplest case, these are two coils that sit close together on a soft iron core, but are completely isolated from one another. If an alternating current is sent through one coil, this also causes an alternating current in the other coil. The induced alternating current oscillates in the same cycle or with the same "frequency" as the causative alternating current.

If a transformer were to be connected to direct current, an effect in the second coil would only be noticed when the current is switched on and off. The primary coil would probably also burn out quickly, as it does not oppose an "inductive resistance" to the direct current, but only an "ohmic resistance".

The number of turns determines the voltage ratio

If both coils of the transformer have the same number of turns of wire, the voltage of the induced current in the second coil is the same as that of the causative current in the first. Such "isolating transformers" are used for the inductive separation of electrical circuits, e.g. to make the grounding of the neutral conductor ineffective when connecting devices to the household power supply.

As a rule, however, the coils on the "primary" and "secondary" side of transformers have different numbers of turns. This makes it possible to change the voltage of the alternating current as required: If, for example, the second coil has ten times the number of turns of the first, the voltage induced in it is ten times higher than in the first coil. At the same time, the usable current strength is ten times lower, since the transmitted power, which results from the product of voltage and current strength, remains approximately the same (transformers work with very high efficiencies of up to 99 percent).

Transformers enable the long-distance transport of electricity

Transformers play an important role in the power supply. They offer the possibility of transmitting the same power with a reduced current intensity. However, this is a prerequisite for overcoming the electrical resistance of the copper and aluminum cables with as little loss as possible (see box). Otherwise neither a large-scale supply of electricity nor the interconnection of the power plants would be possible. As in the early days of power supply, the supply area of ​​a power plant would then be limited to a few kilometers around the "control center". Because both for safety reasons and for other practical considerations, the current may only reach the consumer with a relatively low voltage.

The effort for the transformation and the higher voltages are therefore accepted by the electricity suppliers. The highest voltage in the electricity transport network is currently 380,000 volts. On the way from the generator to the consumer, the current passes through up to five voltage levels, with each connection from one level to the other being made by transformers:

Generator voltage
10,000 to 60,000 volts
Extra high voltage network
380,000 or 220,000 volts
High voltage network
110,000 volts
Medium voltage network
20,000 or 10,000 volts

The first transformation takes place directly at the power plant, which feeds the electricity into the extra-high or high-voltage network. Smaller power plants such as hydropower plants or large wind converters feed the generated electricity into the medium-voltage network. Very weak power sources (e.g. solar generators) can be fed directly into the low-voltage network.

The gradual disconnection of the electricity is provided by substations and - on the last stage towards low voltage - network stations. In addition to transformers, the substations also contain switching and measuring devices. The enormous voltages and currents require special technical precautions. For example, despite their relatively low losses, the transformers become so warm that they have to be cooled and are usually located in an oil container. The contacts of the circuit breakers for interrupting the current flow must be driven apart with compressed air; the resulting arc is cooled and extinguished with the aid of a gas mixture. The currents and voltages cannot be measured directly either, but must first be brought to measurable low-voltage values ​​using converters.