This time, we introduce a **12-phase diode rectifier** technology that uses transformers to inject harmonics, which can make 12-phase diode rectifiers become 24-phase diode rectifiers to improve the input power factor of mains power.

The harmonic injection circuit is composed of an injection transformer and a single-phase rectifier bridge. The circuit utilizes the characteristic that the output currents of the two main rectifiers are 30° out of phase to generate injected harmonics. Using this method, the injection circuit can generate a square wave voltage whose frequency is 6 times the frequency of the commercial power frequency. Due to the injection of this voltage, a three-level staircase wave is generated in the output voltage of the rectifier, and a 24-phase staircase wave voltage is generated in the right circuit of the input inductor LS.

- Composition and working principle of harmonic injection circuit

The harmonic injection circuit is composed of injection transformer Tr and injection rectifier VD1~VD4. A 12-phase diode rectifier circuit using transformers to inject harmonics is shown in Figure 1. The two main rectifiers in series in the figure are replaced by current sources. Since the instantaneous values of the output currents of the two main rectifiers are staggered by 30°, a current of 6 times the power frequency will flow through the primary winding of the injection transformer and induce a voltage Δu on the secondary side of the injection transformer. After △u is rectified by the rectifiers VD1~VD4, a three-level step wave voltage is generated by injecting the DC output voltage from the output end of the rectifier, thereby generating a 24-phase step wave voltage in the right circuit of the input inductor LS.

Because the main rectifiers 1 and 2 are connected in series and the phase shifting effect of the input transformer makes the output current RMS of the two current sources the same, but the current instantaneous value waveform has a phase difference of 30°, so the current instantaneous value in the circuit is different. are equal and cause the output currents of the two main rectifiers 1 and 2 to have a 30° phase deviation, which is the source of the injected harmonics. Figure 2 shows the simplified equivalent circuit of Figure 1.

As can be seen from FIG. 1 , the waveforms of the instantaneous values of the output currents of the two main rectifiers 1 and 2 are shown in the upper part of FIG. 3 . The waveform of the difference Δi=i2-i1 of the output currents i1 and i2 of the two main rectifiers 1 and 2 is shown in the second waveform in FIG. 3 . △i is the waveform of the harmonic injection current. Since the phases of the currents i1 and i2 are staggered by 30°, the injection circuit will generate a DC voltage of 0~Ud whose natural switching frequency is 6 times the power frequency at the output end of the rectifier. This voltage is the harmonic injection voltage. This voltage can be used to generate a 24-phase staircase wave

Mains input current.

In Figure 2, the current Δi flows through the primary winding of the injection transformer Tr, and the injection voltage Δu is generated at the secondary side of Tr, which changes the output voltages Ud1 and Ud2 of the main rectifiers 1 and 2, namely

When the output current of the main rectifier 1 is greater than the output current of the main rectifier 2 (i.e. i1>i2), the injection current is negative, the injection current -Δi=i2-i1, and Δi flows from point F to point G. According to the principle of transformer ampere-turn balance, a negative current will be induced in the secondary winding of Tr to make the diodes VD2 and VD3 conduct.

When the output current of the main rectifier 1 is less than the output current of the main rectifier 2 (i.e. i, <₂), the injection current is positive, the injection current △i=i2-i1, △i flows from point G to point F, at the secondary of Tr A positive current will be induced in the winding to turn on diodes VD1 and VD4.

Since the output currents of the main rectifiers 1 and 2 are out of phase by 30°, the injected current and voltage are in phase with them, but their frequency is 6 times the power frequency, as shown in Figure 3. In FIG. 3 , the third waveform is the injection voltage Δu, and the fourth waveform is the output voltage Ud1 of the main rectifier 1 . Compared with the load current, the effective value of the injected current is small, and the amplitude of the injected voltage is 1/k of the output voltage. Therefore, the power of the harmonic injection circuit is very small, only 2% of the power of the entire rectifier circuit.

- Generation of 24-phase rectifier staircase waveform

Figure 4 shows the waveform of the main rectifier, which is also a three-level waveform in which the output voltages Ud1 and Ud2 compensate and cancel each other, and a schematic diagram of a 24-phase staircase waveform generated on the right side of the input inductor L. This figure is based on the series output rectifier circuit shown in Figure 1.

The first three waveforms in Fig. 4 are the output voltage of the main rectifier 1 to the voltages UAIG, UBIG and UCIG of the neutral point G on the DC output side. When the input current iAI is negative, the bottom diode of the bridge arm of the main rectifier 1 is turned on, UAIG=-Ud/2; when the input current iAI is positive, UAIG=△U.

Similarly, the input voltage waveforms UA2G, UB2G and UA2O of the main rectifier 2 can also be obtained, as shown in the lower part of FIG. 4 .

From the above introduction, we can know that for a 12-phase rectifier, 24-phase rectification can be achieved by using the harmonic injection method. The power of the harmonic injection circuit is only 2% of the power of the entire rectifier.

- Experimental results

The parameters of the experiment are: the three-phase input inductance LS=1mH; the three-phase transformer with the △/Y△ connection mode whose transformation ratio of the input rectifier transformer is 1:1, so as to achieve the purpose of shifting the secondary phase of the transformer by 30°; the DC filter inductance Ld=2mH, DC filter Cd1=Cd2=6800μF/450V; the transformation ratio of the harmonic injection transformer is 1:4.617; the rectifier circuit adopts the circuit shown in Figure 1; the load resistance RL=8Ω.

The experimental results show that the harmonic injection type 12-phase rectifier can achieve the effect of the mains input current of the 24-phase rectifier and improve the power factor of the mains input.

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