Power transformers transform electrical voltage from one level or phase to another, usually "stepping down" voltage ratio from a higher to a lower level. This is accomplished through the principle of magnetic induction between coils to convert voltage and/or current levels. Power transformers include a broad range of electrical transformers, such as autotransformer, control transformer, current transformer, distribution transformer, general-purpose transformer, instrument transformer, isolation transformer, and potential (voltage) transformer.
There is a difference between power transformers and distribution transformers operating at normal levels. Usually, power transformers are utilitized in electric utility transmission networks (that use higher voltages) for either step up or step down applications (400 kV, 200 kV, 110 kV, 66 kV, 33kV) and are normally rated higher than 200MVA. Utility distribution transformers are put to work in distribution networks, as a way of transforming electricity to a level (11kV, 6.6 kV, 3.3 kV, 440V, 230V) that can be utilized by an end user and are normally rated lower than 200MVA.
PTs can be configured as either a single-phase primary configuration or a three-phase configuration. Other important aspects to consider when specifying may include:
PTs provide a system with a number of AC supplies of various primary voltage and secondary voltage levels and suitable values of current, from the primary winding and secondary windings (depending on the number of turns ratio) and magnetic fields. In addition it may be required to provide electrical isolation between the electronic circuitry and the external public power supply.
PTs operating properly can be manufactured as either a toroidal or laminated transformer. The differnce is that toroidal transformers typically have copper wire wrapped around a cylindrical laminated core type so the magnetic flux (which occurs within the coil) doesn't escape. The coil efficiency is superior, while the magnetic flux has hardly any influence on other transformer components. On the other hand, laminated PTs contain laminated-steel cores. This means that steel laminations are insulated with a nonconducting material, such as varnish, and then formed into a core that reduces electrical losses.
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