Compared with ordinary two-winding transformers, autotransformers have the following advantages:

1) Less consumption of materials and low cost. Because the amount of silicon steel sheet and copper wire used in the transformer is related to the rated induced potential and rated current of the winding, that is, related to the capacity of the winding, the winding capacity of the autotransformer is reduced, the material consumption is also reduced, and the cost is also low.

2) Less loss and high benefit. Due to the reduction in the amount of copper wire and silicon steel sheet, at the same current density and magnetic flux density, the copper loss and iron loss of the autotransformer are lower than those of the double-winding transformer, so the benefit is higher.

3) Easy to transport and install. Because it is lighter in weight, smaller in size and smaller in footprint than a two-winding transformer of the same capacity.

4) The limit manufacturing capacity of the transformer is increased. The ultimate manufacturing capacity of the transformer is generally limited by the transportation conditions. Under the same transportation conditions, the capacity of the autotransformer can be larger than that of the double-winding transformer.

The use of autotransformers in power systems can also have adverse effects. Its disadvantages are as follows:

1) Increase the short-circuit current of the power system. Due to the electrical connection between the high and medium voltage windings of the autotransformer, its short-circuit impedance is only (1-k/1) square times that of an ordinary two-winding transformer of the same capacity. Therefore, after the autotransformer is used in the power system, the The three-phase short-circuit current is significantly increased. And because the neutral point of the autotransformer must be directly grounded, it will greatly increase the single-phase short-circuit current of the system, and sometimes even exceed the three-phase short-circuit current.

2) Cause some difficulties in voltage regulation. Mainly due to the electrical connection between the high and medium voltage windings, there are three possible voltage regulation methods for the autotransformer at present. The second is to install additional transformers on high-voltage and medium-voltage lines. These three methods are not only difficult to manufacture and uneconomical, but also have shortcomings in operation (such as affecting the voltage of the third winding), and the solutions are not ideal.

3) The overvoltage protection of the winding is complicated. Due to the self-coupling connection of the high and medium voltage windings, when either side falls into a lightning shock wave whose amplitude is compatible with the insulation level of the winding, the amplitude of the overvoltage shock on the other side may exceed the insulation level. In order to avoid this phenomenon, a set of valve-type arresters must be installed at the outlet ends of both high and medium pressure.

4) The relay protection is complicated. Although the autotransformer has certain shortcomings, countries still attach great importance to the application of the autotransformer, which is mainly inseparable from the development of the power system to large-capacity and high-voltage. The advantages of the coupling transformer are even more prominent.