“The existing research work on multi-level converters is mainly aimed at the voltage source converter (Vohage Source Inverter, referred to as VSI), and the current source converter (Current Source Inverter, referred to as CSI) is still less research. This is not only because the usual electrical energy sources such as generators, grids, batteries, etc. are all voltage sources, but also because the energy storage element capacitor in VsI is compared with the energy storage element Inductor in CSI, the energy storage efficiency and the volume of the energy storage element , the price has obvious advantages. However, with the development of superconducting technology, the energy storage efficiency problem of inductors in current-mode converters will be solved.Compared with VSI, CSI also has

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introduction

The existing research work on multi-level converters is mainly aimed at the voltage source converter (Vohage Source Inverter, referred to as VSI), and the current source converter (Current Source Inverter, referred to as CSI) is still less research. This is not only because the usual electrical energy sources such as generators, grids, batteries, etc. are all voltage sources, but also because the energy storage element capacitor in VsI is compared with the energy storage element inductor in CSI, the energy storage efficiency and the volume of the energy storage element , the price has obvious advantages. However, with the development of superconducting technology, the energy storage efficiency problem of inductors in current-mode converters will be solved. Compared with VSI, CSI also has its own characteristics. It is convenient to realize four-quadrant operation, and the work is more stable, and the output current is easier to control. Therefore, in active filtering (APF), reactive power compensation (SVG) and power systems, the application will become more and more extensive. For multi-level inverters, PWM technology is undoubtedly a solution to obtain an ideal output. Generally speaking, the control strategy applicable to multi-level VSI is also applicable to multi-level CSI, but an appropriate control strategy should be adopted for different topologies. For combined multi-level CSI, each unit CSI is relatively independent in control, so it is convenient to use PWM technology. For direct multi-level CSI, the conventional modulation method cannot be simply followed. Generally, the following factors must be considered when selecting a control strategy for three-phase multi-level CSI:

1) To maintain the continuous conduction of the DC side current;

2) To ensure the current balance of the shunt inductor;

3) The mutual coupling effect of three-phase currents should be considered.

1. Circuit topology analysis

Figure 1 is the three-phase five-level CSI topology introduced in this paper (hereinafter referred to as direct three-phase five-level), each switching device is connected by a MOS tube and a fast recovery diode in series. In the steady state, considering the symmetry of the circuit topology and ignoring the ripple of the inductor, a current source with a value of 1/2 can be used to replace each current-sharing inductor when analyzing the circuit. As shown in Figure 2, its output level is one I, one I/2, 0, 1/2. I five level.

The CSI in this paper, in form, is two common inverters connected in parallel at both ends of the same current bus, but its working principle is to use the idea of multi-level combination, and the five-level topology uses four current-sharing inductors to control the inverter. The input current and output current of the device are divided into two equal parts, and then the corresponding switches are used to reasonably distribute and combine the three-phase currents to obtain the required five levels. Taking phase A as an example, the output level and switch conduction are shown in Table 1.

2. Research on modulation mode of three-phase direct five-level CSI

Commonly used PWM control methods for multilevel converters are: multilevel harmonic elimination PWM (SHPWM), carrier phase shift (CPS-SPWM), switching frequency optimization (SFO-PWM), and space vector modulation (SVPWM). In this paper, the modulation method of the three-phase new five-level CSI shown in Figure 1 is a combinational logic PWM technology. Three sine wave modulation signals with a difference of 120° are used to compare with a triangular carrier (the amplitude of the sine wave is 5V , the triangle wave amplitude is 2.5V), and then output the corresponding PWM wave through the combination of digital and analog circuits.

Fig. 3 is the digital scheme adopted to realize PWM technology. All comparator units (determining the switching logic) receive three-phase sine wave modulation signals, each sine wave corresponds to a piece of EPROM and D/A converter, each EPROM is addressed by a different counter, and through the phase-locked loop Synchronized with the corresponding phase power supply. The amplitude control signal is Vmod, which is multiplied with the output of the EPROM containing the sinusoidal modulation signal after D/A conversion, and then used as the modulation signal of each comparison module unit.

For the current-mode inverter circuit, the switching action must meet three conditions, namely, maintaining the continuous conduction of the DC side; considering the influence of the mutual coupling of the three-phase currents; and the average voltage on the current-sharing inductor should be zero. Based on the above considerations, this paper adopts the following PWM control strategy.

1) Taking phase A as an example, as shown in Figure 4(a), a two-level pulse signal is obtained by comparing SINA, a SINA and the same triangular carrier, and the two signals are superimposed to obtain a three-level pulse signal PA. (Similarly, signals PB, Pc can be obtained). The three-level pulse signals PA and PB are superimposed to obtain a group of five-level modulation signals PA-PB, as shown in Figure 4(b). Similarly, PB-PC and PC-PA can be obtained.

2) Considering the above current level combination conditions, take PA (PA1, PA2, PA3, PA4 represent the control signals on switches SA1, SA2, SA3, SA4 respectively) as the level signal whose positive half cycle is greater than zero, and PA2 is positive The level signal whose half cycle is greater than 1/2, similarly, PA3 is a level signal whose negative half cycle is less than zero, and PA4 is a signal whose negative half cycle is less than 1/2. Through the analog switch, PA1, PA2, PA3, and PA4 are evenly output alternately. In this way, the driving signal on each switch tube is alternately wide and narrow pulse width, which ensures that the conduction time of the left and right bridge arms is the same in several cycles, so that it can be It is better to realize that the average voltage of the current sharing inductor is zero, as shown in Figure 5(b), which is the driving signal of the three switches of the same bridge arm.

3) In this experiment, each bridge arm of the main circuit cannot have no DC path. In order to eliminate the state of no DC path, take a bridge arm as an example, using the logic control as shown in Figure 5(a), after “OR” the three switch signals on one bridge arm, the bridge arm is non-conductive. After the signal is “ANDed” with the phase signal it is in, it is then “ORed” with the signal of this channel to obtain the control signal on each switch tube. In this way, it can be well ensured that each bridge arm of the main circuit does not have a DC no-path situation.

3. Direct three-phase five-level CSI simulation and experiment

3.1 Simulation parameters and results

In order to verify the correctness of this control strategy, a simulation study of the three-phase five-level CSI composed of the new topology is carried out. The simulation parameters are as follows: the current source current is 20A, each current sharing inductance is 100mH, the output operating frequency of the inverter is 50Hz, the load resistance is 8Ω, the load is star-connected without neutral, and the output filter is composed of LC. into (L=5mH, C=60μF).

Figure 6(a) shows the output PWM current waveform of the three-phase five-level CSI. Figure 6(b) shows the filtered three-phase load current waveform. It can be seen that the filtered current waveform is very close to a sine wave.

3.2 Results of experimental parameters

Based on the previous analysis and discussion, the three-phase direct five-level CSI topology is experimentally verified. The main circuit of the experiment is shown in Figure 1. The parameters are as follows: each current sharing inductor is 100mH, the frequency of the output current is 50Hz, the load resistance is 8Ω, the output filter capacitor is 60μF, the filter inductor is 8mH, and the carrier ratio is 32. The experimentally measured input current is about 4A. Figure 7(a) is the three-phase load current waveform before filtering, and Figure 7(b) is the filtered waveform. It can be seen that the output waveform is very close to a sine wave.

4. Conclusion

The direct CSI multilevel inverter is a very distinctive topology. With the breakthrough development and practical application of high temperature superconducting technology, superconducting technology will solve the energy storage inductance energy storage in current-mode converters. At the same time, the energy storage coil in the electric superconducting energy storage system has the characteristics of a current source. Therefore, the current mode converter will have a wide range of application prospects, and the research on the topology and control of the three-phase direct CSS will be of great significance. .

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