Compared with the common double winding transformer, autotransformer has the following advantages:
1) Less material consumption 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 reduced, and the cost is also low.
2) Less loss, higher efficiency. Due to the reduction of copper wire and silicon steel sheet, the copper loss and iron loss of autotransformer are less than those of double winding transformer at the same current density and magnetic flux density, so the benefit is higher.
3) Easy to transport and install. Because it is lighter than double winding transformer with the same capacity, smaller size and smaller floor area.
4) The ultimate manufacturing capacity of the transformer is increased. The limited manufacturing capacity of transformer is generally limited by transportation conditions. Under the same transportation conditions, the capacity of autotransformer can be larger than that of double winding transformer.
The use of autotransformer in power system will also have adverse effects. Its disadvantages are as follows:
1) The short-circuit current of power system is increased. Due to the electrical connection between the high and medium voltage windings of the autotransformer, its short-circuit impedance is only (1-k / 1) times the square of the common double winding transformer with the same capacity. Therefore, the three-phase short-circuit current will increase significantly after the autotransformer is used in the power system. Because the neutral point of the autotransformer must be directly grounded, the single-phase short-circuit current of the system will increase greatly, sometimes even exceed the three-phase short-circuit current.
2) It causes some difficulties in voltage regulation. It is mainly caused by the electrical connection between the high and medium voltage windings. At present, there are three possible voltage regulation modes of the autotransformer. The first is to install the voltage regulation device with load to change the split position inside the autotransformer winding; the second is to install the additional transformer on the high voltage and medium voltage lines. These three methods are not only difficult to manufacture and uneconomic, but also have shortcomings in operation (such as affecting the voltage of the third winding), so they are not ideal.
3) The over-voltage protection of winding is complicated. Due to the self coupling of high and medium voltage windings, when one side falls into a lightning impulse wave with amplitude corresponding to the insulation level of the winding, the amplitude of overvoltage impulse on the other side may exceed the insulation level. In order to avoid this phenomenon, a group of valve type arresters must be installed at both high and medium voltage outgoing terminals.
4) It makes relay protection complex.
Although the autotransformer has some shortcomings, many countries still attach great importance to the application of autotransformer, which is inseparable from the development of power system to large capacity and high voltage. With the increase of capacity and voltage, the advantages of autotransformer become more prominent.
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