In order to further improve the mains input power factor of the SCR polyphase phase-controlled power frequency rectifier, reduce the waveform distortion rate THDi of the mains input current to less than 10%, and make the mains input current and the mains input voltage in the same phase, there is a A simple and effective method is to use the harmonic injection method. Through the harmonic injection method, the 12-phase rectifier can reach the harmonic elimination level of the 24-phase rectifier. This method is currently the rectification method that uses the fewest components and has the best effect on improving the input current waveform of the mains.
- SCR12-phase rectifier with balanced inductor secondary injection harmonics
In the previous sections, the method of using multiple superposition methods and increasing the number of phases of the rectifier circuit with the input rectifier transformer was introduced to reduce the waveform distortion rate of the mains input current and increase the mains input power factor. The following introduces the method of injecting harmonics into the secondary winding of the balanced inductor without increasing the number of phases of the rectifier circuit to reduce the harmonic content of the mains input current and improve the mains input power factor. The circuit is shown in Figure 1. In this circuit, a secondary winding is set on the balanced inductance of the three-phase SCR fully-controlled bridge rectifier circuit, which is connected in parallel and superimposed, and the output of the secondary winding is used for harmonic injection.
1.1. Harmonic injection circuit and its working principle
In Figure 1, the part inside the dotted box is the harmonic injection circuit. It is formed by adding a secondary winding to the balanced inductor connected in parallel between two three-phase SCR full-controlled bridge main rectifiers, and a single-phase half-controlled rectifier composed of Sm1, Sm2, VDm1, and VDm2. Its working principle is: Assuming that the two SCR three-phase fully controlled main rectifiers id1 and id2 are replaced by current sources, since the phase difference between the two main rectifiers is 30°, the phase angles of their output currents id1 and id2 are also different. 30°. Although the effective values of id1 and id2 are equal, the instantaneous values are not equal. When id1 and id2 flow through the primary winding of the balanced inductor, a current of id1-id2=Δi will be generated in the secondary. Therefore, a voltage udβ with a natural switching frequency of 6 times the power frequency will be induced on the secondary side of the balanced inductor. After udβ passes through the half-controlled rectifier composed of Sm1, Sm2, VDm1, and VDm2, a natural switching frequency of 6 times will be obtained. The DC voltage of 0~udβ of the power frequency is the harmonic injection voltage. This voltage is injected from the output terminal of the 12-phase rectifier into the DC output voltage, and a three-level step wave voltage will be generated, so that the mains power 24-phase mains input current is generated on the input power supply.
- Generation of 24-phase mains input current waveform
The portion within the dotted box in Figure 1 is the harmonic injection circuit portion of the circuit. After the voltage in the secondary winding of the balanced inductor is rectified, the DC voltage udβ is obtained and added to the original DC voltage.
The balanced inductor secondary winding is connected to a thyristor SCR switch Sm1, Sm2. On the single-phase half-controlled bridge rectifier circuit composed of diodes VDm1 and VDm2 (referred to as half-controlled bridge), its control angle is A; the input voltage phase difference between rectifiers 1 and 2 is 360°/2×6=30° , and their control angle is α. The operation of this circuit is divided into regions according to the control angle a of rectifier 1 and rectifier 2 and the control angle β of the half-controlled bridge. The area 1 (15°≤α≤90°, 0≤B≤30°) as the main work area is introduced here.
Figure 2 shows the voltage and current waveforms of each part of the region 1. Among them, Figure (a) is the voltage and current waveforms, and Figure (b) is the AC input current i. (Region 1) of the waveform.
In Fig. 2(a), Fig. (al) shows the output voltages of the rectifiers 1 and 2, and the control angle a is a common method. Figure (a2) shows the voltage u on the balanced inductor. The waveform of , the half-controlled bridge controls the angle β to illustrate u. The zero point is the reference. Figure (a3) shows the DC output voltage, and the interval A is the state where the two diodes VDm1 and VDm2 of the half-controlled bridge are conducting. Sections B and C are states in which one thyristor SCR switch Sm1 or Sm2 and one diode VDm1 or VDm2 are conducting. Figures (a4) to (a6) show the input current waveform of rectifier 1.
AC input current i. The waveform of is shown in Figure 2(b). In the case of no balanced inductor secondary winding, each current subscript is attached with the word “0”, and its ia10, ic10, and ia20 are shown in Figures (b2) to (b4). The synthetic AC input current iao at this time is shown in Figure (b5). In the absence of a balanced inductor secondary winding, in order to cancel the current component of the secondary magnetomotive force input to rectifier 1 and rectifier 2, the current im shown in Figure (b6) can flow from the AC input side, so that the AC input The current is the waveform of the superposition of iao and im, as shown by im in Figure (b7).
The AC input current of this circuit is greatly affected by the turns ratio am=W2/W1 between the balanced inductor and its secondary winding and the control angle β of the half-controlled bridge.
Therefore, in the case where am and β are optimally selected (am=0.4, β=150°), the harmonics of the AC input current and the ripple of the DC output voltage are almost the same as those of the usual 24-phase rectifier, which can eliminate the 23rd order All harmonics below. The power output by the harmonic injection circuit (ie, the balanced inductor secondary winding) is only 2% of the entire rectifier capacity.