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**Nov 23, 2020

Research on Repetitive Control Strategy of High Frequency Link Inverter

Abstract: The topology of the high-frequency chain is introduced, the simulation model of the high-frequency chain inverter and its repetitive control strategy is established, and the system is simulated and analyzed. It is verified by simulation that the repetitive control strategy can make the high-frequency chain inverter obtain high-quality output waveforms.

Keywords: high frequency chain; inverter; repetitive control; harmonic

l Introduction

Bidirectional voltage source high-frequency link inverters are currently a research hotspot in the photovoltaic field because of their high conversion efficiency, high power density, and ease of use in high-power applications. On the basis of the voltage source type high frequency chain inverter model, the mathematical model of the inverter in the continuous time domain and the discrete time domain is established. The discrete repetitive control technology based on voltage feedback is studied, and the principle of repetitive control to eliminate periodic waveform distortion of the output voltage is analyzed. Finally, the PSIM simulation software is used to conduct a system test, and the key test waveforms are analyzed.

2 Establishment of the mathematical model of the inverter main circuit

It is based on Forward as the basic unit. The DC input voltage is inverted by a high-frequency inverter, and high-frequency positive and negative pulses are obtained on the primary side, and the transformation ratio is adjusted and isolated by a high-frequency transformer. The secondary side of the transformer gets high-frequency positive and negative pulse waves with the same phase as the primary side. , The cycle converter performs low-frequency demodulation on the high-frequency pulse, and obtains the low-frequency AC pulse voltage at both ends of the output filter circuit, and then the filter circuit filters out higher harmonics.

In the main circuit mathematical model of the high frequency chain inverter, the switching frequency of the inverter is much higher than the oscillation frequency of the LC filter. Therefore, the dynamic characteristics of the inverter are mainly determined by the LC filter, which can be equivalent to a second-order system composed of output LC filter links. Suppose the filter inductance is Lf, the filter capacitor is Cf, and the equivalent resistance of the capacitor, inductance, and Rz:.

Using a zero-order holder and choosing a suitable sampling period T, the equivalent transfer function of the inverter can be discretized.

In the main circuit of the experiment, the carrier frequency of the inverter is 10kHz. Considering that the cut-off frequency of the output filter is 1/10 to 1/5 of this frequency, choose Lf=2mH, Cf=6μF, Rz=1Ω.

The no-load damping of the inverter is very small, and there is a large resonance peak at the natural frequency. The output voltage error of the PWM inverter is mainly caused by factors such as load disturbance, DC side voltage fluctuation, and dead zone effect. Most of the error frequency components are located in the middle and low frequency bands. As long as the system has a strong ability to suppress these middle and low frequency error components. It can greatly improve steady-state indicators such as harmonic distortion (THD%) and voltage steady-state error. therefore. The amplitude and phase compensation should be mainly concentrated in the middle and low frequency bands. That is, the amplitude at the natural frequency is attenuated to below -3dB.

3. Duplicate control strategy

Repetitive control is a control strategy based on the principle of internal model. Its function is that when the input signal repeats in the fundamental period, the output accumulates the input signal cycle by cycle. Even if the input attenuates to zero, the internal model will continue The cycle repetitively outputs the same signal as the previous cycle waveform. The periodic signal holder is introduced into the feedback control system, and the system is stabilized through the compensation link, which can track the given and eliminate disturbances in one cycle.

3.1 Selection of cycle delay coefficient

The sampling times of the output voltage per fundamental wave period, N=fc/f.

Among them fc is the reference input fundamental frequency, f is the carrier frequency.

3.2 Design of compensator

From the controlled system P(z) amplitude-frequency characteristic curve, it can be seen that there is a resonance peak at ω=4564rad/s. The compensator S(z) is used to eliminate the resonance peak. The second-order oscillation link is selected to cancel P(z) in the middle and low frequency bands, and attenuate sharply in the high frequency band, where ζ=l, ωn=4600. In order to improve the compensation performance, the Notch function is introduced into the design to trap the resonant peak. Then the complete compensator form is: S(z)=S1(z)F(z).

3. The design of 3zKKrQ(z)

The controlled object P(z) and the compensator S(z) both have a large phase lag, so the phase compensation link Zk is used for compensation. It is determined by comparing the phase-frequency curve that when k=10, S(z)P(z) and z-10 are consistent with the mid-low frequency band, and the phase is compensated. The value range of Kr is 0～1, which is similar to the function of a proportional controller. In this example, Kr is 0.21 to get a satisfactory adjustment effect.

Q(z) should be a constant close to 1. When Q(z) is a constant less than 1, the unit circle whose origin is the end of Q(z) is shifted to the left as a whole to ensure the stability of the system in the full frequency band. In this example Take the experience value of 0.95.

3.4 Simulation experiment

Under resistive and resistive inductive load conditions, simulate the system and compare the output voltage with the reference input waveform.

After the introduction of the repetitive controller, the system has less harmonic content and good sine of the output voltage waveform under resistive load conditions; the phase is also consistent with the reference voltage. The waveform quality is better. The effect of repeated control on the adjustment of waveform amplitude is not as good as that of resistive load. The sine of the waveform is obviously deteriorated, and the harmonic content is large. This shows that repetitive control has certain difficulties in suppressing random interference and has the disadvantage of poor dynamic performance. However, it can also be seen from the results that after the introduction of repetitive control, the phase adjustment effect is good, and the output waveform phase is basically consistent with the input reference. This provides an advantageous condition for the inverter waveform control and the parallel connection of the inverters.

4 Conclusion

The structure and principle of the two-way voltage source high frequency chain inverter commonly used in the inverter field are analyzed, and the discrete repetitive control technology based on voltage feedback is studied. It can be seen through experiments that after the introduction of the repeat controller, the harmonic content of the system output waveform is significantly reduced, and the quality of the output voltage waveform is greatly improved; the effect of phase adjustment is more obvious, so that the output waveform can be consistent with the input reference in phase , Provides favorable conditions for the parallel connection of inverters.