Design of 400 Hz Inverter Power System Based on Pole Placement

2 Simulation and Analysis

According to the characteristics of the 400 Hz inverter power supply, its switching frequency is selected as f sw = 20 kHz, and the damped oscillation angular frequency of the system is determined to be r = 3 500 rad/sec. Since the inverter power supply corresponding to this system is designed under no-load conditions, a certain margin is left for loading during operation, and r = 0.6 is taken. The comprehensive equivalent resistance of the system is r = 0.1.

There are many methods for designing filters. This paper adopts a relatively simple LC filter design method to determine the parameters of LC. Since the switching frequency of the system is much larger than the fundamental frequency, the cutoff frequency of the filter is generally selected as 1/10~1/15 times the switching frequency. This paper selects the cutoff frequency as fc=1600 Hz, that is, the 4th harmonic, and then determines the filter parameters according to the method used in the literature [7]: L=530!H, C=11.9!F. Substituting the above determined parameters into formula (7), the feedback gain of the system can be calculated. After calculation, it can be obtained that the parameter selection of the 400 Hz inverter power supply is not easy to determine compared with the 50 Hz system. This paper takes k1i=231.9, k1p=0.0776, k2i=235909, k2p=24.4. Substituting the above parameters into equation (5) and performing MATLAB simulation, the no-load system characteristics are shown in Figures 2 to 7.

Figure 2 is the Nyquist curve of the system. It can be seen that the curve does not surround the point (-1, j0). According to the Nyquist stability criterion, the system is stable. Figure 3 is the root locus diagram of the system. It can be seen that a pair of conjugate dominant poles are located in the left half of the S domain close to the imaginary axis, which has a major impact on the system characteristics. The non-dominant poles are also located in the left half of the S domain and have a smaller effect on the system characteristics. It can also be seen from the figure that the four poles are all located in the left half of the S domain, indicating that the system is stable. Figure 4 is the frequency characteristic curve of the system. From the amplitude-frequency characteristic and the phase-frequency characteristic, it can be seen that the system has a stable low-frequency characteristic, and the high-frequency amplitude decays rapidly. It has a wide bandwidth and a large stability margin within the operating frequency range. Figure 5 is the unit step response of the system. It can be seen from the figure that the system has a faster dynamic characteristic.

Figure 6 and Figure 7 are the output waveform of the inverter power system at no-load and the corresponding waveform distortion. As can be seen from the figure, the use of PI control based on pole configuration greatly enhances the output performance of the system and reduces the waveform distortion of the system. The THD value of the system at no-load is only 2.98%, and the sinusoidal nature of the waveform is good. Next, the system is subjected to a loading experiment. Since the 400 Hz power supply is mainly used in precision occasions and is not used as an AC input for re-rectification, this paper only applies resistive load and resistive-inductive load to the system. The simulation results are shown in Table 1.

Table 1 System loading simulation results

From the analysis of Table 1, it can be seen that when a resistive load is added, the system distortion rate THD is smaller than the THD value when no-loaded, and the voltage change is also within the allowable range (5%); when a resistive-inductive load is added, due to the presence of inductance, the voltage distortion rate is larger than that of a pure resistive load, but the effective value of the output voltage changes within a small range and remains basically stable.

It can be seen that this system has a strong load-carrying capacity.

3 Conclusion

This paper analyzes and calculates a 400 Hz inverter power supply by using the pole configuration method. It is found that the design of 400 Hz inverter power supply parameters is more difficult than that of power frequency inverter power supply. Factors such as calculation error and phase angle offset have a greater impact on system performance. This paper adopts a method combining theoretical calculation with simulation verification, and finally determines the relevant parameters of the system. Through the MATLAB IMULINK module simulation, it is verified that the system has fast dynamic response, small steady-state error and large stable range, small output waveform distortion of the inverter power supply, strong load capacity, and achieves the expected effect.