Under normal circumstances, as long as a rectifier device has two rectifier transformers, an equivalent 12-phase system is adopted, because this system does not require special phase shifting and can be achieved by changing the connection mode of the windings. When the DC capacity is large, A rectification system with an equivalent of more than 18 phases is used. IGBT is equivalent to the thyristor. It is used in the rectification system to control on-off but not phase.
1. Working principle of phase-shifting transformer
To put it bluntly, it is to toss the secondary winding of the transformer and achieve phase shifting through different connections of the secondary winding. You can find professional information to learn more. Take the more complex phase-shifting transformer with delta connection in Yanbian as an example. The secondary winding is a connection method between delta connection and star connection.
The figure below is the working principle diagram of a phase-shifting transformer. In Figure (a), if two double-coil transformers of the same specification are connected to the same phase, the secondary voltages are added to obtain a new voltage with doubled voltage amplitude and the same phase; if they are as shown in Figure (b) or (c) In this way, when connected to two different phases, the secondary voltages are added, that is, the vectors are added, and a new voltage with the same voltage amplitude and a phase difference of 60° is obtained; or the secondary voltages are subtracted, which is the vector phase. Subtract, and you get a new voltage with a voltage amplitude that is twice as large and a phase difference of 30°.
Obviously, by selecting different phases for connection on the primary side of the transformer, and using different wiring methods for the secondary coil, the phase of the output voltage can be changed as needed.
2. Classification of phase-shifting transformers
There are many types of phase-shifting transformers. From the perspective of use, there are three-phase three-wire, three-phase four-wire or single-phase lines. If divided by phase shift angle, there is a range of 0~360° continuous or stepwise. If divided according to the value of the power exponent m of the regular 6x2 polygon connected in the circle, when m=0, it is a regular hexagon; m=1, dodecagon; m=2, twenty-four-sided... Regular hexagonal phase shifters can be divided into circumscribed triangular hexagons, inscribed triangular hexagons, triangular star hexagons and double star hexagons. The external triangular hexagonal phase shifter can be evolved into a variety of circuits depending on the input and output requirements. In addition, there are triangular extended side phase shifters and single-phase phase shifters.
3. Where are phase-shifting transformers mostly used?
Phase-shifting transformers are mostly used on the input side of high-power frequency converters and high-power DC power supplies. The purpose of using a phase-shifting transformer is to increase the pulse number of the rectifier. Three-phase rectification is called 6-pulse rectification. The three-phase transformer generates another set of three phases through phase shifting. The rectifier input is 6 phases, which is called 12-pulse rectification. The three-phase transformer generates another three sets of three phases through phase shifting, and the rectifier input is 12 phases, which is called 24-pulse rectification. The greater the rectifier pulse number, the smaller the input current harmonics and the smaller the harmonic pollution to the power grid.
1. Limited to the withstand voltage problem of IGBT. In order to obtain high voltage output, it must be obtained by superimposing multiple power units.
2. Avoid the impact of harmonics on the power grid.
4. Whether the 12-pulse inverter has a built-in phase-shifting transformer?
The 12-pulse inverter does not need a built-in phase-shifting transformer. The 12-pulse inverter is a 6-phase full-wave rectifier. It only needs to use a three-phase to six-phase wiring method, so it does not need a phase-shifting coil. For example, if a three-phase transformer has a delta connection on the primary side and two inverted y connections on the secondary side, it can use axial double splitting to form a 6-phase (12-pulse) full-wave output wiring method. Of course, it can also be Y/d-d or other internal wiring methods of the transformer. As long as 6 phases can come out of the secondary side and one coil is connected in delta, it will be fine.
The purpose of using a phase-shifting transformer is to increase the pulse number of the rectifier. Three-phase rectification is called 6-pulse rectification. The three-phase transformer generates another set of three phases through phase shifting. The rectifier input is 6 phases, which is called 12-pulse rectification. The three-phase transformer generates another three sets of three phases through phase shifting, and the rectifier input is 12 phases, which is called 24-pulse rectification. The greater the rectifier pulse number, the smaller the input current harmonics and the smaller the harmonic pollution to the power grid.
