A novel DC current source TPWM cascade multi-level inverter

introduction

I have published an article titled "Technical Reform of DC Power Supply PWM Cascade and Multi-level Inverter", which was written for voltage-type inverters. In fact, based on the duality principle of voltage-type and current-type inverters, this reform idea can also be applied to power-type inverters.

The so-called cascade of current-type TPWM (Trapezoida-PWM) inverters is to parallel and superimpose N three-phase inverters with the same output current of TPWM waveform. The common feature of the currently commonly used three-phase current-type TPWM inverters is that the parallel superposition and TPWM control are both performed on the inverter. This cascade method has the disadvantages of using more devices, large switching losses, and high manufacturing costs. If the parallel superposition and TPWM control are moved to the DC current source, the number of components used can be reduced, especially the number of TPWM switching devices used, and the inverter switch can naturally work in the ZCS state. In this way, not only the inverter efficiency is improved, but also the manufacturing cost of the inverter is greatly reduced.

l Working principle of basic three-phase TPWM DC current source inverter

The principle circuit of the basic three-phase TPWM DC current source inverter is shown in Figure 1. This is a new type of three-phase TPWM DC current source inverter. It is very different from the general three-phase current type TPWM inverter, that is, the TPWM control of the output current is not performed on the inverter switch, but on the DC current source. That is, between the DC current source and each phase output 2H bridge inverter 2HA, 2HB, 2HC, a switch tube VTA, VTB, VTC are connected in series, and these three switch tubes are used to perform TPWM control on the DC current source, so that the DC current of each phase is IdA, IdB, IdC. A TPWM DC current source waveform like the output voltage of a single-phase full-wave rectifier is obtained, and then this waveform is converted into a three-phase TPWM AC current output by the synchronous ZCS inversion of the GTO2HA, GTI2HB, GTO2Hc inverter bridge.

However, TPWM control of the DC current source of the current type inverter is different from SPWM control of the DC power supply of the voltage type inverter, that is, it cannot perform TPWM control on the DC current source independently for each phase. It is necessary to perform TPWM control on the DC current sources of the three phases simultaneously according to the characteristics of TPWM control of the current three-phase inverter, so that the output current of the three-phase DC current source IdA+IdB+IdC=Id, so as to ensure that the output current of the DC current source Id remains stable during the TPWM modulation operation.

The working principle of the three-phase basic TPWM DC current source inverter is introduced below.

The three-phase basic TPWM DC current source inverter shown in Figure 1 adopts variable carrier triangular wave TPWM control, in which the waveform of the trapezoidal modulation wave and the waveform of the two groups of carrier triangular waves with a phase difference of 180o are shown in Figure 2. In the TPWM control process, the two groups of carrier triangular waves uc and uc' must be switched alternately at intervals of the period of each phase modulation wave uT, and compared with the trapezoidal modulation wave uT of each phase. A positive pulse is generated in the part where the trapezoidal wave is larger than the triangular wave, and a zero pulse is generated in the part where it is smaller than the triangular wave. With such a TPWM control method, the DC current source of the three-phase current source inverter is controlled by TPWM on switches TVA, TVB, and TVc respectively, which can ensure that the commutation is automatically and accurately performed between adjacent phases, and ensure that the output current Id=IdA+IdB+IdC at both ends of the DC current source Id is stable and unchanged.

The corresponding relationship between the switching positions of the three-phase trapezoidal modulation waves uTA, uTB and uTC and the two sets of carrier triangle waves uC and uC' is shown in Figure 3. Each phase must be switched alternately according to the period of the trapezoidal wave.

For the current source Id in the three-phase basic TPWM DC current source inverter shown in Figure 1, the working waveforms of the DC currents IdA, IdB, IdC and Id of each phase obtained by the above-mentioned TPWM control are shown in Figure 4. From this working waveform diagram, it can be seen that the inverter commutation is carried out between adjacent phases. For example, during the period t1 to t5 of interval A in Figure 4, the current is converted between phases A and C; during the period t6 to t10 of interval B, the current is converted between phases B and C; in interval C, the current is converted between phases A and B; in interval D, the current returns to converting between phases A and C...

The direction of current transfer is shown by the arrow in Figure 4. In this way, the DC current waveforms of the three-phase GT02H bridge DC current source are shown as IdA, IdB, and Idc in Figure 4, and are all TPWM modulated. The TPWM DC current waveform of each phase is like the TPWM current waveform of the output voltage of the single-phase full-wave rectifier. The fundamental zero crossing point of this waveform is zero potential. Therefore, after the ZCS synchronous inversion of the GT02H bridge behind it, the output of the three-phase AC current iA, iB, and iC of the three-phase basic TPWM DC current source inverter can be obtained. Since the GT02H bridge inverter works in the ZCS state, cheap low-frequency switching devices such as GTO or SCR can be selected.

The schematic diagram of the control circuit of the three-phase basic TPWM DC current source inverter is shown in Figure 5. There are four core parts in the figure, namely, a three-phase trapezoidal wave generator, two sets of carrier triangular wave generators, two sets of carrier triangular wave switching circuits, and a comparator for comparing the trapezoidal modulation wave with the carrier triangular wave to generate a driving pulse, wherein the two sets of carrier triangular wave switching circuits are unique to the current type TPWM inverter. In Figure 5, since the inverter output current is adjusted by controlling the rectified voltage of the current source, it is not drawn in the figure.

One point that should be pointed out here is that there are two parameters that affect the TPWM modulation mode: one is the modulation index M=UT/Uc, and the other is the carrier ratio F=ωC/ωT, which is a multiple of 3. Generally, M=1 and F≥9. The larger the F, the better the improvement effect on the output current waveform, but at the same time, the switching speed of the switch tube is required to be faster and faster, and the cost is also getting higher and higher.

2 Parallel superposition of N three-phase basic TPWM DC current source inverters

The parallel superposition of the three-phase basic TPWM DC current source inverter is to directly parallel superpose the TPWM DC current sources of each phase in the three-phase basic TPWM DC current source inverter shown in Figure 1 according to the three phases A, B, and C, and then perform synchronous inversion through the GT02H bridge of each phase to obtain a three-phase AC current output. A circuit of N TPWM DC current sources in phase A directly parallel superposed is shown in Figure 6. The characteristic of this inverter is that the cascade superposition and TPWM control are moved from the switch of the inverter to the DC current source TPWM control switch VTAl, VTA2...VTAN. This reform brings many advantages to the cascade multilevel inverter: reducing the number of switch tubes; reducing the total number of equivalent TPWM switches; making the inverter switch work in the zCS state; using cheap low-speed switching devices such as GTO or scR; improving the inverter efficiency; reducing the manufacturing cost; very suitable for 7~15 level high-power current inverter. This is a new type of current-type multi-level inverter that we have recently developed. The output current of this multi-level inverter is m=2N+l. To obtain such an AC current waveform output, the TPWM control of each DC current source control switch VTA1, VTA2, and VTAN in Figure 6 must use the same A-phase trapezoidal wave μTA as the modulation wave, and the two groups of carrier triangle waves μc1~μcN; μcl'—μcN must lag 2丌/N phase angles in sequence, so that IdA1, IdA2...IdAN have the same fundamental current, which is convenient for cascade superposition.

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The principle circuit of a three-phase 9-level inverter with N=4 three-phase basic TPWM DC current source inverters in parallel cascade superposition is shown in Figure 7. The currents of the four DC current sources Id1=Id2=Id3=Id4 are controlled by TPWM through their respective TPWM control switches VTAl, VTA2, VTA3, and VTA4. Their carrier triangle waves uC1, uc2, uc3, uc4 and uCl', uc2', uc3', and uc4' lag 2/4 phase angles in turn, and use the same A-phase trapezoidal wave uTA as the modulation wave for modulation, and obtain the TPWM output currents IdA1, IdA2, IdA3, and IdA4 of each current source. IdA=IdA1+IdA2+IdA3+IdA4 is the DC input current source current output from phase A to the GT02HA inverter. This current is a DC TPWM waveform similar to the single-phase full-wave rectified voltage waveform as shown in the waveform in Figure 7. Then, after synchronous inversion of this DC current source current through the GT02HA bridge behind, the 9-level TPWM A-phase AC current iA output can be obtained. The working principle and output current waveform of phase B and phase C in the three-phase AC output current are the same as those of phase A.

The control circuit of the multi-level inverter is shown in Figure 8. This is a principle schematic diagram, which is mainly composed of the four control circuits shown in Figure 5. The most important one is the switching circuit of the two groups of carrier triangle waves uC and uC', which is the key to ensure accurate commutation of phase-to-phase TPWM control.

Characteristics and development prospects of parallel superposition of N three-phase basic TPWM DC current source inverters

The original 120. square wave current inverter has a poor output current waveform and contains many odd harmonics, which will have a very adverse effect on the motor. However, the parallel superposition of N three-phase basic TPWM DC current source inverters can greatly improve the output current waveform and greatly reduce the harmonic content.

N three-phase basic TPWM DC current source inverters are connected in parallel and superimposed. It is developed based on the DC power supply PWM cascade multilevel inverter introduced in the literature through the duality of voltage-type and current-type inverters. Therefore, the two have the same circuit structure, the output current (voltage) is close to sine, and both do not need output AC filters. They both have fast dynamic response speeds, use the least number of components, have the least switching losses, have the highest inverter efficiency, and are the lowest cost multilevel inverters. It's just that one is a voltage-type multilevel inverter and the other is a current-type multilevel inverter. It is precisely because of this difference that the following different characteristics appear between the two: the voltage-type multilevel inverter uses superposition diodes and feedback diodes, and adopts SPWM control; while the current-type multilevel inverter does not use superposition diodes and feedback diodes, making the circuit simpler, but must use TPWM control instead of SPWM control to ensure accurate phase commutation and stable output current of the current source.

The three-phase basic TPWM DC current source parallel superposition multi-level inverter is a new type of current-type multi-level inverter that we have recently developed after a long period of research. Its characteristics are that the cascade superposition and TPWM control are separated from the inverter and moved to the DC current source TPWM control switch to achieve the purpose of reducing the number of switching devices and reducing switching losses. The guiding ideology of this research and development is actually a technical reform to simplify the circuit of the multi-level inverter. Due to this reform, the performance of the current-type multi-level inverter has been significantly improved, which is a new and important development direction of the current multi-level inverter research.

4 Conclusion

Through the introduction of this article, the new DC current source TPWM cascade multilevel inverter is a newly developed multilevel inverter with superior performance. Compared with the current clamped or cascaded voltage type multilevel inverter and the typical current type TPWM superposition multilevel inverter, it has the following advantages:

①The minimum number of components used and the lowest cost;

②The equivalent total switching times are minimal and the inverter efficiency is the highest;

③ The inverter switch works in ZCS state, and GTO or SCR can be used as a switch;

④TPWM control method is simple and has fast dynamic response;

⑤The output current is close to sinusoidal, so no AC filter is required;

⑥ Most suitable for large and medium power inverters with level number m=7~15;

⑦Small size and light weight.