Power transformers are electrical transformers designed for high power in the range of 10,000 kVA to 1000 MVA. This type of transformers have a very high power and are usually connected in the high voltage area to 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 railroad power supply. Various standards and general regulations apply to power transformers. In Germany/Europe, power transformers are generally constructed according to EN or IEC 60076. Further standards and regulations exist, e.g. for converter transformers.
A transformer consists of a magnetic circuit, this is called the core. The core of a power transformer is made of laminated electrical sheet. The core has at least two current-carrying windings, called the primary winding and the secondary winding. When 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 technique for the core and the quality of transformer core used affects the magnetic circuit. The magnetic circuit of a transformer (magnetic field) should ideally produce low eddy current losses and have low remagnetization losses (hysteresis losses). Another aspect is the resistances in the winding of a transformer. Only with layered and ordered windings on the primary coil and the secondary coil and the best winding metal can the winding losses be reduced. The voltage is controlled with the number of turns on the coil. The current determines the diameter of the winding metal.
The construction power of a transformer is expressed in VA, kVA or MVA (VA is the name for voltampère and stands for the unit of measurement of apparent electrical power, kVA for kilovoltampère and MVA for megavoltampère).
Except for silver, copper has the best conductance with γ = 56. Aluminum, on the other hand, has only γ = 36. Aluminum thus follows with a gap of about 35 percent. Thus, copper is 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 of electricity, and alloys generally have considerably lower conductivity than pure metals. Silver or gold are ruled out altogether because of their high price.
Power transformers are manufactured either as oil-filled transformers or as dry-type transformers (cast resin transformers). An oil-filled transformer consists of at least one active part. An active part is the composite of the winding (this is arranged concentrically as a cylindrical winding or disc winding on top of each other made of copper or aluminum), the core (laminated electrical sheets with low eddy current losses) and pressed parts. Depending on the construction, further active parts can include, chokes for short-circuit current limitation or current-limiting chokes for on-load tap-changers. A dry-type transformer (cast resin transformer) is used when oil transformers can only be used unsafely because of the fire load and water hazard. Dry-type transformers have similar components and consist of the same technology as an oil transformer. Just like an oil transformer, the core of a dry-type transformer consists of a laminated core made of electrical sheets. The windings of the transformer are wire or tape winding. The windings are often cast in cast resin, which is where the name cast resin transformer comes from.
Power transformers are used in a very wide range of applications. Depending on the construction of the transformer, the area of application varies. Thus, power transformers can 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 plant supply transformers for industrial applications. Since power transformers as dry-type transformers usually cannot dissipate heat very well, they are limited to power ratings of up to 40 MVA. For this reason, these transformers are mostly used in medium-voltage networks. Here, they are mainly used as distribution transformers to supply low-voltage networks in large plants or building complexes.