The secondary winding of an autotransformer is an integral part of the primary winding. Such a transformer seems to have only one winding, so it is also called a "single winding transformer". The primary and secondary coupled inductors can be decoupled according to the three-terminal coupled inductors connected to the opposite ends in the circuit theory. Applying voltage on the primary side and the secondary side is short-circuited or applying voltage on the secondary side and the primary side is short-circuited, the equivalent leakage reactance reduced to the primary or secondary side of the autotransformer can be obtained. The autotransformer is a special transformer that does not need to be insulated between the primary and secondary, that is, a special transformer that shares a set of coils for output and input. In other words, a transformer with primary and secondary windings on the same winding.

I. Working Principle of Auto Transformer

Two or more coils are wound on a closed iron core. When one coil is connected to an AC power supply (the primary coil), an alternating current flows through the coil. This alternating current generates an alternating magnetic field in the iron core. The variable main magnetic flux generates its own induced electromotive force in the primary coil, while the other coil (that is, the secondary coil) induces a mutual induction electromotive force. By changing the relationship between the primary and secondary coil turns ratio to change the primary and secondary coil terminal voltage to achieve voltage conversion, the general turns ratio is 1.5:1~2:1. Because the primary and secondary coils are directly connected, there is a risk of inter-level leakage. Therefore, it cannot be used as a running light transformer.

Points to take:

1. The autotransformer is a special transformer that shares a set of coils for output and input. The step-up and step-down are realized by different taps. The voltage of the taps that are less than the common coil will decrease, and the voltage of the taps that are more than the common coil will increase. high.

2. In fact, the principle is the same as that of an ordinary transformer, except that its primary coil is its secondary coil. In general transformers, the primary coil on the left uses electromagnetic induction to make the secondary coil on the right generate voltage, and the autotransformer influences itself.

3. The autotransformer is a transformer with only one winding. When used as a step-down transformer, a part of the turns are drawn from the winding as the secondary winding; when used as a step-up transformer, the applied voltage is only applied to part of the winding Turn up. Usually the part of the winding that belongs to both the primary and the secondary is called the common winding, and the rest of the autotransformer is called the series winding. Compared with the ordinary transformer, the autotransformer of the same capacity is not only small in size, but also more efficient, and the transformer The larger the capacity, the higher the voltage. This advantage becomes more prominent. Therefore, with the development of power systems, the improvement of voltage levels and the increase of transmission capacity, autotransformers have been widely used due to their large capacity, low loss, and low cost.

II. Features of Auto Transformer

Because the calculated capacity of the autotransformer is less than the rated capacity. Therefore, under the same rated capacity, the main size of the autotransformer is smaller, and the effective materials (silicon steel sheets and wires) and structural materials (steel) are reduced accordingly, thereby reducing the cost. The reduction of effective materials reduces copper loss and iron loss correspondingly, so the efficiency of the autotransformer is higher. At the same time, due to the reduction of the main size and quality, a single transformer with a larger capacity can be manufactured under allowable transportation conditions. But usually only when k≤2 in an autotransformer, the above advantages are obvious.

- Since the short-circuit impedance per unit value of the autotransformer is smaller than that of the double-winding transformer, the voltage change rate is smaller, but the short-circuit current is larger.

- Due to the direct electrical connection between the primary and secondary autotransformers, when the high-voltage side is over-voltage, it will cause serious over-voltage on the low-voltage side. In order to avoid this danger, lightning arresters must be installed on both the primary and secondary windings. Don't think that the primary and secondary windings are connected in series. Once the primary and secondary windings are installed, the secondary can be omitted.

- In the general transformer. The on-load voltage regulating device is often connected to the neutral point of the ground, so that the voltage level of the voltage regulating device can be lower than that of the line end voltage regulation. The neutral point voltage regulation side of the autotransformer will bring so-called related voltage regulation problems. Therefore, when the autotransformer is required to adjust the voltage under load, only the end-of-line voltage adjustment method can be used.

1. Advantages of Auto Transformer

The transformation ratio of the autotransformer in the step-down starter is fixed, while the transformation ratio of the contact voltage regulator is variable. Compared with general transformers of the same capacity, autotransformers have the advantages of simple structure, less materials, and small size. Especially when the transformation ratio is close to 1, it is particularly economical, so it is used more in high-power transmission transformers with similar voltages. In addition, it is widely used in step-down starters for asynchronous motors above 10 kilowatts. However, because the primary and secondary windings share a single winding and are electrically connected, they are not suitable for use on some occasions, especially not used as a running light transformer. Compared with ordinary dual-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, it is related to the capacity of the winding, the autotransformer winding capacity is reduced, the material consumed is also reduced, and the cost is also low.

(2) Low 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 reduced compared with the double winding transformer, so the benefit is higher.

(3) Easy to transport and install. Because it is lighter than the double-winding transformer of the same capacity, the size is small, and the floor space is small.

(4) Improved the ultimate manufacturing capacity of the transformer. The ultimate manufacturing capacity of transformers is generally restricted by transportation conditions. Under the same transportation conditions, the capacity of autotransformers can be larger than that of dual-winding transformers.

2. Disadvantages of Auto Transformer

(1) Increase the short-circuit current of the power system.

Since there is an electrical connection between the high and medium voltage windings of the autotransformer, its short-circuit impedance is only (1-1/K) square times that of the ordinary double-winding transformer of the same capacity. Therefore, after the autotransformer is used in the power system, the Make three-phase short-circuit current increase significantly. In addition, since the neutral point of the 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) Cause some difficulties in voltage regulation.

There are three possible voltage regulation methods for the autotransformer mainly due to the electrical connection of the high and medium voltage windings. The first is to install a voltage regulation device with load to change the position of the splitter in the autotransformer winding; One is to install additional transformers on high-voltage and medium-voltage lines. These three methods are not only difficult to manufacture, uneconomical, but also have shortcomings in operation (such as affecting the voltage of the third winding), and the solutions are not ideal.

(3) Make the overvoltage protection of the winding complicated.

Due to the auto-coupling of the high and medium voltage windings, when either side falls into a lightning shock wave with an amplitude that matches the insulation level of the winding, the amplitude of the overvoltage shock that appears 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 voltages.

(4) Make the relay protection complicated.

Although autotransformers have certain shortcomings, all countries still attach great importance to the application of autotransformers, which are mainly inseparable from the development of power systems to large-capacity and high-voltage. As the capacity increases, the voltage rises, and the advantages of the coupling transformer are even more significant.

III. Applications

Autotransformers are used in occasions where primary and secondary isolation are not required and have the advantages of small size, fewer consumables, and high efficiency. Common AC (manual rotating) voltage regulators, transformers in small household AC voltage regulators, transformers in three-phase motor auto-decompression starter boxes, etc., are all application examples of auto-transformers.

With the rapid development of power systems in the direction of large capacity and high voltage, autotransformers have been widely used in high-voltage power networks due to their low cost and high efficiency, becoming voltage conversion equipment that transfers important electrical energy. As one of the most important equipment in the high-voltage power grid, the autotransformer is of great significance to ensure the safe and reliable operation of the power grid and the flexible distribution of electrical energy.

With the rapid development of high-speed railways, the reliability of autotransformers is critical to the safe operation of high-speed railways. Direct lightning strikes and foreign objects in the catenary cause frequent high-speed rail short-circuit trip accidents. The short-circuit impulse current generated can easily cause autotransformer winding failure, greatly reducing the reliability of transformer operation and seriously affecting the safe operation of high-speed rail.