Power transformer

Power transformers are electrical transformers that are designed for high outputs in the range from 10,000 kVA to 1000 MVA. This type of transformer has a very high output and is usually connected to high-voltage devices in electrical energy networks, i.e. in the power grid, and used for voltage regulation. Power transformers are often used as three-phase transformers in the power grid or as single-phase transformers for traction power supply. Various standards and general regulations apply to power transformers. In Germany/Europe, power transformers are generally designed in accordance with EN or IEC 60076. Further standards and regulations exist, e.g. for converter transformers.

A transformer consists of a magnetic circuit, which is referred to as the core. The core of a power transformer consists of laminated electrical steel. The core has at least two current-carrying windings, the so-called primary winding and the secondary winding. If an alternating electrical voltage is applied to the primary winding, an alternating current begins to flow through the primary winding. This alternating current generates a magnetic field in the winding, which changes its strength and frequency with the alternating current source. The magnetic flux of the transformer changes with the frequency of the AC voltage.

The manufacturing technology for the core and the quality of the transformer core used have an effect on the magnetic circuit. Ideally, the magnetic circuit of a transformer (magnetic field) should generate low eddy current losses and have low remagnetization losses (hysteresis losses). Another aspect is the resistances in the winding of a transformer. Winding losses can only be reduced with layered and ordered windings on the primary and secondary coils and the best winding metal. The voltage is regulated by the number of turns on the coil. The current strength determines the diameter of the winding metal.

The power rating of a transformer is expressed in VA, kVA or MVA (VA stands for volt-ampère and is the unit of measurement for apparent electrical power, kVA for kilovolt-ampère and MVA for megavolt-ampère).

With the exception of silver, copper has the best conductivity value with γ = 56, while aluminum only has γ = 36. Aluminum follows with a gap of around 35 percent. Copper is therefore the best metal and aluminum “only” the second best of the technically and economically usable conductor materials for electrical energy. All other metals cannot be considered as conductors, and alloys generally have a considerably lower conductivity than pure metals. Silver or gold are ruled out completely due to their high price.

Power transformers are manufactured either as oil-filled or dry-type transformers (cast resin transformers). An oil-filled transformer consists of at least one active part. An active part is the combination of the winding (this is arranged concentrically as a cylindrical winding or disk winding made of copper or aluminium), the core (layered electrical sheets with low eddy current losses) and pressed parts. Depending on the design, other active parts can be, for example, chokes for short-circuit current limitation or current limiting chokes for tap changers used. A dry-type transformer (cast resin transformer) is used when oil transformers can only be used unsafely due to the fire load and water hazard. Dry-type transformers have similar components and consist of the same technology as an oil transformer. The core of a dry-type transformer consists of a laminated core made of electrical sheets, just like an oil transformer. The windings of the transformer are designed as wire or strip windings. The windings are often cast in cast resin, which is where the name cast resin transformer comes from.

Power transformers are used in a wide variety of areas. The area of application varies depending on the design of the transformer. Power transformers can therefore be used for various solutions and applications. Power transformers are used in the field of power generation and transmission as generator step-up transformers and grid access transformers between the power plant and the grid, as system connection transformers in high-voltage substations and as system supply transformers for industrial applications. As dry-type power transformers are generally not so good at dissipating heat losses, they are limited to outputs of up to 40 MVA. For this reason, these transformers are mostly used in medium-voltage grids. Here, they are mainly used as distribution transformers to supply low-voltage networks in large systems or building complexes.