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What is the difference between a rectifier and a standard transformer in Oil Distribution Transformers?

There is nothing special on the mechanical part (the tank) in Oil Distribution Transformers. However the electrical design, the active part differs compared to a standard transformer. Power electronic circuits can convert alternating current (ac) to direct currents (dc). These are called rectifier circuits. The vice versa is also applicable in a power circuit which is called inverter circuits. A transformer that has one of its windings connected to one of these circuits is considered to be a converter or a rectifier transformer. Electronic circuits provide many types of control today, and they are getting more and more in practice. These circuits are generally more efficient than previous types of control. Rectifier circuits are used to provide high-current dc for electromechanical process like chlorine production as well as copper and aluminium production. They are also used in variable-speed-drive motor controls, transit-traction applications, mining applications, electric furnace applications, higher-voltage laboratory-type experiments, high-voltage direct-current power transmission (HVDC) static precipitators, and others. The operation of the semiconductor rectifier produces harmonic voltages and currents. The harmonic currents are at higher frequencies than the fundamental frequency of the transformer. The problem is coming from the harmonic current. These higher-frequency currents cause high levels of eddy-current losses and other stray losses in other parts of the transformer. This can create potentially high temperatures, which degrade the insulation of the transformer and can cause earlier failure of the transformer. So the idea is to do such a design that will withstand and overcome those affects that are generated by the harmonic currents. We usually limit the flux density and the current density at a certain level and integrate an electrostatic shield between the primary and the secondary windings. A rectifier transformer has a bigger footprint, is heavier and is more expensive compared to a standard transformer.
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