Research and design of an improved wireless power transmission system based on the traditional dual LCL WPT system

0 Preface

WPT technology has received extensive attention from international and domestic scholars in recent years. It has clear practical value and broad application prospects in the future, and can bring significant economic and social benefits. In order to adapt to different requirements, some new compensation topologies have been proposed at home and abroad. Among them, the research on dual LCL WPT systems is the most prominent. Because of its excellent characteristics such as stable system resonance operating frequency, constant primary transmitting coil current, and constant system output current, it can be used in large-scale applications such as electric vehicle charging and desktop multi-load power supply. However, when the system is lightly loaded, the inverter output current is severely distorted. In response to this problem, this article proposes an improved method of series LC filter, and the improved method is verified by simulation and experiment to reduce the harmonic distortion of the inverter output current. rate.

1 Working principle analysis of double LCL WPT system

Figure 1 shows the equivalent circuit model of the dual-LCL WPT system, in which Ud is a DC voltage source; S1~S4 are IGBTs, and D1~D4 are anti-parallel diodes respectively; Lp and LS are the free coils of the primary and secondary coils respectively. Inductance, M is the mutual inductance; L1 and C1 are the primary side compensation inductance and primary side compensation capacitor respectively; L2 and C2 are the secondary side compensation inductance and secondary side compensation capacitor respectively; Uin and Iin are the inverter output voltage and output current respectively ; Ip and Is are the primary coil current and the secondary coil current respectively; R is the system equivalent resistance load; Io is the system output current. The operating frequency of the system is f, where f0 is the resonant operating frequency of the system.

According to the theoretical analysis in literature [5], when the system parameters in Fig. 1 satisfy the relational formula (1)

From equations (2) and (3), it can be seen that the size of the primary coil current Ip and the size of the system output current Io are independent of the size of the system equivalent resistance load R, which can indicate that the system has the constant current characteristics of the primary coil And the characteristic of constant output current of the system. At the same time, it can be seen from equation (4) that the total impedance Zin of the inverter output system is always purely resistive, that is, the change of the equivalent resistance load R of the system will not affect the natural resonant operating frequency of the system, and the system has frequency stabilization characteristics.

The previous analysis is based on the fundamental component of the inverter output voltage Uin, without considering the harmonic components of the inverter output voltage. The traditional double LCL type WPT system uses a voltage-type full-bridge inverter circuit. When the drive circuit is controlled by 180° conduction mode, the expression of the inverter output voltage uin is:

Similarly, the third and fifth harmonic components of the inverter output current iin can be obtained as

`

It can be seen from Figure 2 that when the load R is small, the harmonic distortion rate of the inverter output current is relatively large, which will result in large inverter losses, reduced system efficiency, and aggravated system instability. Aiming at this problem of dual LCL WPT system under light load, this paper proposes an improvement method, which will be specifically analyzed as follows.

2 Theoretical analysis of improved double LCL WPT system

An LC filter is added to the branch where the original system C1 is located, and the improved system circuit is shown in Figure 3.

Among them, Lf1 and Cf1 satisfy the following relationship:

At this time, when only the uin fundamental wave component is considered, the total impedance Zinf11 of the downstream system at the output of the inverter is:

Among them, Zr3 and Zr5 are as shown in formula (12) and formula (13) respectively.

It can be seen from Figure 4 that when the load R is small, that is, when the system is lightly loaded, the harmonic distortion rate of the inverter output current of the improved system is significantly lower than that of the system before the improvement, indicating that the improved system has achieved better results. effect.

3 System simulation and experiment

In order to verify the correctness of the above theoretical analysis, this paper determines the simulation and experimental parameters shown in Table 1.

3.1 Simulation analysis

Build MATLAB/SIMULINK simulation models according to the simulation parameters in Table 1 and Figure 1 and Figure 3.

When the load resistance R is selected as 10 Ω, the inverter output currents of the original system and the improved system are shown in Figure 5 (a) and 5 (b) respectively.

It can be seen from Figure 5 that the inverter output current waveform of the improved system is smoother than that of the original system. It can be seen intuitively that the improved system has achieved better results.

Perform Fourier analysis on all the waveforms in Fig. 5 and get the histogram of harmonics as shown in Fig. 6.

It can be seen from Figure 6 that compared with the original system, the total harmonic distortion rate of the improved system has dropped by 70.27%. From the data, it can be seen that the improved system can achieve better results.

3.2 Experimental analysis

According to the experimental parameters in Table 1 and Figure 1 and Figure 3, the experimental platform was built.

When the load resistance is selected as 10 Ω, the inverter output current waveforms of the original system and the improved system are shown in Figure 7 (a) and 7 (b) respectively.

It can be seen from Fig. 7 that the experimental waveform of the inverter output current of the improved system is smoother than the experimental waveform of the inverter output current of the original system, which can achieve better results, which is consistent with the previous theoretical analysis and simulation analysis.

Use Fluke Norma 4000 high-precision power analyzer to test the efficiency of the experimental platform, and draw the result as a broken line as shown in Figure 8.

It can be seen from Figure 8 that the improved WPT system has higher transmission efficiency than the original system, and it is especially obvious when the system is lightly loaded, indicating that the improved system can improve the overall efficiency of the system and has obvious beneficial effects.

4 Conclusion

In this paper, the traditional dual-LCL WPT system has the problem of serious distortion of the inverter output current when the system is lightly loaded. The root cause of this problem is analyzed, and an improved method is proposed based on this. The effectiveness of the two improved methods is proved from three aspects: theoretical analysis, simulation and experimental verification. The improved method can better solve the problems of the traditional dual-LCL WPT system, and can achieve better results in improving the output current waveform of the inverter, reducing the switching loss of the inverter and improving the system efficiency.

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