AUTOTRANSFORMER 220-750 kV
Autotransformers are frequently used in power applications to interconnect systems operating at different voltage classes, for example 220/750 kV for transmission. They are also often used for providing conversions between the two common domestic main voltage bands in the world (400, 200, 66 kV). The links between the UK 400 kV and 275 kV networks are normally three-phase autotransformers with taps at the common neutral end. Autotransformers are built with common main winding and a separate low voltage winding. Autotransformers are often used to step up or step down voltages in the 110-115-120 V range and voltages in the 220-230-240 V range - for example, providing 110 V or 120 V (with taps) from 230 V input, allowing equipment designed for 100 or 120 V to be used with a 230 V supply. This allows US electrical equipment to be fed from the higher European voltage. Autotransformers can also be used to supply European 230 V appliances from a 100 or 120 V supply in countries outside Europe. In all cases the supply and the autotransformer must be correctly rated to supply the required power.
Autotransformers are tailor-made products and the design varies with the impedance levels (constant Ohmic and constant percentage), transmission voltage and cooling requirements. These are normally provided with on load tap changer for voltage variation. For long distance rural power distribution lines, special autotransformers with automatic tap-changing equipment are inserted as voltage regulators, so that customers at the far end of the line receive the same average voltage as those closer to the source. The variable ratio of the autotransformer compensates for the voltage drop along the line.