1) Less materials and low cost. Because the amount of silicon steel sheet and copper wire in the transformer is related to the rated induction potential and rated current of the winding, that is, to the capacity of the winding, the capacity of the autotransformer winding is reduced, the consumed materials are also reduced, and the cost is also low.
2) Less loss and high benefit. Due to the reduction of copper wire and silicon steel sheet, under the same current density and magnetic flux density, the copper loss and iron loss of autotransformer are smaller than those of double-winding transformer, so the benefit is higher.
3) Convenient transportation and installation. Because it is lighter, smaller and occupies less area than the double-winding transformer with the same capacity.
4) The ultimate manufacturing capability of the transformer is improved. The final manufacturing capacity of transformers is generally limited by transportation conditions. Under the same transportation conditions, the capacity of autotransformer can be larger than that of double-winding transformer.
Secondly, the use of autotransformer in power system will also have adverse effects. Its disadvantages are as follows:
1) increases the short-circuit current of power system. Because of the electrical connection between high and medium voltage windings of autotransformer, its short-circuit impedance is only (1-k/ 1) times that of ordinary double-winding transformer with the same capacity, so the three-phase short-circuit current will increase significantly after autotransformer is used in power system. Because the neutral point of autotransformer must be directly grounded, the single-phase short-circuit current of the system will greatly increase, and sometimes even exceed the three-phase short-circuit current.
2) Some difficulties caused by voltage regulation. At present, there are three possible voltage regulation methods for autotransformer, mainly because of the electrical connection between its high and medium voltage windings. The first is to install a voltage regulator with load to change the split position inside the autotransformer winding; The second is to install additional transformers on high voltage and medium voltage lines. These three methods are not only difficult and uneconomical to manufacture, but also have shortcomings in operation (such as affecting the voltage of the third winding), and they are not ideal.
3) The winding overvoltage protection is complex. Due to the self-coupling relationship between high-voltage and medium-voltage windings, when either side is caught in lightning shock wave with amplitude suitable for winding insulation level, the overvoltage impact amplitude on the other side may exceed insulation level. In order to avoid this phenomenon, a set of valve arresters must be installed at the outlet terminals of high and medium voltage sides.
4) The relay protection is complex.
Although autotransformer has some shortcomings, countries still attach great importance to the application of autotransformer, which is closely related to the development of power system to large capacity and high voltage. With the increase of capacity and voltage, the advantages of autotransformer are more prominent.