There are two ways to shift the phase of the mains input polyphase rectifier transformer to obtain the polyphase output: one is to use the star and delta connection (Y/△ or △/Y) between the primary and secondary to shift the phase by 30°; One is to use the series connection (zigzag connection) between the secondary windings of different phases to shift the phase. Using these two phase shifting methods, three-phase input, multi-phase rectification output of 6-phase, 12-phase, 18-phase, 24-phase and 30-phase output can be realized on a three-phase transformer. Phase rectifier transformer.
Polyphase rectifier transformers are different from conventional transformers in structure. There are multiple windings in each phase coil, one of which is the primary winding and the other is the secondary winding, while the conventional transformer has only two windings in each coil, one is the primary winding and the other is the secondary winding . The connection method of the secondary winding of the multi-phase rectifier transformer is more complicated. First, the two windings in different phases should be connected in series in a certain direction, and then connected to the SCR three-phase bridge rectifier.
After this connection, the voltage of each winding in each phase coil of the transformer and the corresponding output phase voltage will produce a phase shift. It should be pointed out here that there are two directions when the windings in different phase coils are connected in series, and two results of “phase forward shift” and “phase backward shift” will be produced accordingly. By changing the ratio of turns of the two series windings, the magnitude of the phase shift angle can be changed. If several secondary windings are sequentially moved out of a same phase angle, such as 30°, 20°, 15° and 12°, etc., input currents of different phases will be generated on the primary side. After these input currents are superimposed multiple times, a staircase wave current close to a sine wave will be generated, and this staircase wave current is basically in phase with the input mains voltage waveform (there is only one phase shift depending on the phase control angle), then by The SCR polyphase phase-controlled power frequency rectifier composed of the polyphase rectifier transformer can obtain a high mains input power factor, and the waveform distortion rate THDi of the mains input current is also very small.
The phasor synthesis diagram of the 12-phase, 18-phase, 24-phase and 30-phase mains input multi-phase rectifier transformers obtained according to the above-mentioned phase-shift synthesis method is shown in Figure 1.
Next, the phasor composition relationship and the sine theorem shown in Figure 2(a)~(f) are used to calculate the value of each synthetic phasor.
According to the 18-phase composite phasor diagram in Figure 1(b), the structure of the 18-phase polyphase rectifier transformer connected by Y/YYY can be obtained, as shown in Figure 2(e). The structure diagram of the 18-phase polyphase rectifier transformer connected by Y/△△△ is shown in Figure 2(f).
The advantage of the multi-phase rectifier transformer is that it can reduce the distortion rate THDi of the input current, improve the input power factor PF of the mains and reduce the ripple of the rectified output DC voltage. However, since the two-phase secondary windings are connected in series and phase-shifted, the utilization rate of the windings is not high, or the power utilization rate of the secondary windings is lower than that of the normal transformer, so its volume and mass are larger than those of the normal transformer.
With the multi-phase rectifier transformer, the multiple superposition method can be used to realize the idea of reducing the distortion rate of the mains input current and improving the power factor of the mains input.
Before introducing the multiple superposition method, first introduce the shape of the input mains current waveform when the phase is shifted by 30° using the star and delta connections. Under ideal conditions, assuming that the capacity of the mains grid is infinite, and the filter inductance of the DC side is large enough, the saturation and nonlinear effects of the transformer core are ignored, then when the primary and secondary connections of the transformer are Y/Y, Y /△, △/△ and △/Y, the waveform of the input current ia of the three-phase bridge rectifier is shown in Figure 3. When the initial and secondary connections are the same (Y/Y and △/△), the waveform of the primary input current of the transformer is a square wave with a pulse width of 120°; when the initial and secondary connections are different (Y/△ and △/Y), the transformer The primary input current is a step wave with equal steps.
It can be seen from the above two formulas that the mains input current. Contains a large number of 5.7, 11.13…. harmonics, and their amplitudes are respectively 1/5, 1/17, 1/11, 1/13….
Read more: Definition and characteristics of SCR polyphase phase-controlled power frequency rectifier