1 Overview Since the 1980s, all countries in the world have been accelerating the development of AC variable frequency speed controllers. Today, with the development of large-capacity GTOs, 4000kWG TO inverters have been put into practical use. Most of these inverters use the AC-DC-AC conversion method. The input rectifier circuit of the conversion method adopts large-capacitance filtering, and its DC-side voltage remains basically unchanged, and has a constant voltage source characteristic. Therefore, they essentially belong to the voltage-type harmonic source. During operation, they generate a large amount of harmonics into the grid, causing serious harmonic pollution. Series Active Power Filter (Active Power Fil-ter, abbreviated as APF) is a new type of power electronic device capable of dynamically suppressing harmonics, and is very suitable for Compensating the equal-voltage harmonic source of the inverter 4. This article describes the basic working principle of the series active power filter, and analyzes and studies its control method and control circuit. Based on this, it has developed a set. During operation, the voltage command signal uC and the compensation voltage uc actually generated by the series APF are sent to a comparator for comparison, and then the output of the comparator controls the on-off of the main circuit device so that Uc can track the command voltage U.sub.ly in real time. The purpose of the purpose of timing comparison is to allow the PWM signal to change at least once in one clock cycle.
In this way, the frequency of the clock signal limits the maximum frequency of device operation, thus avoiding the high switching frequency and eventual device damage. This control method requires only one comparator and one D flip-flop per phase, and the hardware circuit is very simple. According to the above analysis, the output harmonic voltage command of the series active power filter should be such that the control method needs to detect the load harmonic voltage.
The load harmonic voltage can be detected by using the proposed 5 harmonic voltage real-time detection method.
Because the energy loss of APF will cause its DC-side capacitor voltage Ud to decrease during operation, in order to ensure the normal operation of the PWM inverter, the DC-side voltage Ud must be maintained constant, so it is necessary to introduce a DC voltage feedback control link.
The deviation after comparison between the given value of the harmonic voltage command form Ud after feedback voltage control is introduced and the feedback value is introduced, and the harmonic voltage value UL obtained by subtracting -p calculated by the PI regulator and the detection circuit is obtained. h, ULvh, ULwh added the additional fundamental voltage components AuLu'AuLvi and Aulw, so that the command voltage uC also contains additional fundamental components, and ultimately make the PWM inverter to generate the required harmonic voltage, while providing A certain fundamental voltage, which is the fundamental voltage, interacts with the fundamental current of the power supply to control the energy flow of the PWM inverter to maintain the constant voltage of the DC-side capacitor voltage Ui as shown in the figure, where LPF is a low-pass filter. The PLL is a phase-locked loop circuit whose purpose is to generate standard sine and cosine signals at the same frequency as the supply voltage. The circuit conversion constants are the current 5 series active power filter inputs. After compensation, the current waveform of the power supply and its amplitude-frequency characteristics can be seen from the figure. After the compensation, the current waveform of the power supply is basically close to a sine wave and its harmonics. The content is greatly reduced. By analyzing and comparing the frequency spectrum, the THD of the power supply current after compensation can be calculated to be 3.9%, and the power factor can be increased to Q 998. Thus, it can be seen that the series active power filter has an effect on the frequency converter. Ideal compensation effect.
The compensated power supply current and its frequency spectrum (a) Current (b) Spectrum 6 Conclusion This article introduced the system structure of the series active power filter, analyzed its basic working principle, and studied its control circuit and control method. On this basis, using IGBT devices, an experimental study of a series APF experimental prototype was developed and showed that the series APF has a good harmonic compensation effect on the frequency converter. Lays the foundation for the development of a practical series active power filter
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