Active Power Filter For Single-Phase Rectifier Based On The Traction Converter

Single phase grid is widely used in the electrical railway traction system. The gridvoltage is to be converted to a three phase AC voltage source with variable voltage andvariable frequency (VVVF), to drive the motors. While operating, the single phase rectifiercircuit will generate instantaneous pulsating power and hence cause the DC bus voltage tofluctuate. This fluctuating component of DC bus voltage would increase the maximumvoltage of the DC bus, and introduce harmonic component into both the grid and the loadcurrent, due to the cross-coupling in modulation calculation. The traditional way to get ridof this is to install passive filters. There are two mainstream types of filters, and one ofthem is to increase the DC bus capacitance. Because of the huge capacitance needed towithstand the pulsating power, electrolytic capacitors become the only choice, which areinferior to film capacitors in reliability, therefore causing an increased burden onmaintenance, and a decrease in system life. The extremely immense energy stored in thecapacitors also complicates the short-circuit protection. Another way is to install an LCresonant circuit to absorb the pulsating power, whose performance is highly sensible to thedeviation of the system parameters and the grid frequency, and this solution is also very lowin power density.Based on the H-bridge Pulse Width Modulation (PWM) rectifier, this paper analyzesthe formation and the effect of the pulsating voltage of the DC bus. The operating principlesand the way to determine the system parameters are also discussed of the two passive filters.To eliminate the pulsating power and meanwhile avoid all the drawbacks of the passivefilters, this paper mainly focuses on the Active Power Filter (APF) solution. Throughclassifying, analyzing and comparing all the given topologies and control methods, somespecific rules are given to guide the APF design process. Multi-Watt traction convertershave a high power level and are switching at a significant low frequency, consequentlysome control methods with high bandwidth and high dynamic responses requirement arenot practical for the high power converter application. With this concern, this paperdiscusses the detailed characters of the waveforms in all the given basic topologies, thepossibilities to apply these topologies and control methods to the multi-watt application,and relationship between the major system parameters and the system performance, including the stability margin, robustness, current and voltage stress, and the systemvolume. Some improvements, including repetitive controller and closed-loop ofinstantaneous power, are also introduced to obtain better performance in low switchingfrequency.