The autotransformers can be single-phase, two-phases and three-phases, they can be used in power systems up to 15.000 kVA and 36 kV. Autotransformers use simple connections of two winders keeping the same number of losses as when they were connected as two windings transformer. They dispose of imposed limitations to high short circuits and pulse voltages. When the changing reason gets near to the unite, low impedance allows a high and possibly harmful high short circuit current, this makes that principal limitations are produced when the benefits raise. A good way to solve these limitations is to connect a lightning rod through a serial winding.
If you choose autotransformers instead of transformers it can be more useful and cheaper in low voltages, in addition the price of medium voltage and potential is reduced, 100kVA more or less, and these are used to adapt the available voltage to the double or a half, according to the needs.
If the difference between primary voltage and secondary voltage is reduced, the transformer price will be lower.
When the objective is to reduce or amplify similar magnitudes electric voltages, you can use a conventional single-phase or three phase autotransformer, yet usually, when the reason between primary and secondary voltages is the same or less than two, it is economically convenient to use these electric systems.
Three phase autotransformers are usually constructed with star connected, with accessible neuter, whether in the primary or the secondary. In middle potential systems, a delta windings is incorporated, called stabilization tertiary, and it can be used to detect eventual high-short circuit currents in distribution lines.
However, it is important to take into account that these systems have a reduced impedance, what makes them weaker to withstand mechanical efforts if there is a high short circuit.
Autotransformers can dispose of he same accessories as a conventional transformer, for example temperature indicator, refrigerant liquid level, protection elements as pressure relief valve for a secure operation, etc. Three phase autotransformers have three bushings insulators in medium voltage and three more in low voltage, one or two bushings corresponding to common neuter and eventually, two insulators of stabilization tertiary. Autotransformers are designed and built refrigerated in mineral oil or biodegradable vegetal oil (FR3), to voltage up to 33 [kv] in medium voltage. They are also built dry-type, impregnated with insulating barns, to voltages up to 15 [kV]