Research On Key Technology Of Shunt Active Power Filter Based On Three-level Topology

With the development of power electronics technology,more and more nonlinear loads with power electronics are used in industrial and civil applications.But the new problem are coming,the nonlinear load causes the current distortion,and injects a large amount of harmonic current into the power grid,which will affect the power quality.Therefore,the power quality problems have also been more and more extensive attention.And the active power filter(APF)has a strong ability to compensate the harmonic current of the nonlinear current,which is considered as the main solution to improve the power quality.In addition,compared with two-level topology,three-level topology has the advantages,such as the voltage of each power device is half of DC link voltage,and the output current has a lower harmonic content and lower switching loss of power device.So,three-level topology applied to the active power filter will achieve better harmonic characteristics,and can improve the harmonic current compensation performance.Firstly,some common modulation methods used in NPC three-level converter are introduced in this paper.In case of the neutral point potential offset,an ASPDPWM method is introduced to eliminate the low frequency even harmonic in the output current of converter.In order to avoid overmodulation when the neutral point potential offset occures,a modulation method is proposed to expand the DC bus voltage utilization.To the problem of the neutral point potential offset,the neutral point potential is balanced by the method of injecting common mode voltage(CMV).And CMV is calculated by the upper and lower capacitance values,and the upper and lower capacitor voltage and the output current of the converter.And then,the effect of the dead time on the output voltage of converter is analyzed in the different commutation modes.Secondly,the mathematical model of NPC three-level APF is introduced,and then the discrete time mathematical model in ?? frame is obtained.On this basis,a deadbeat(DB)current control method is introduced.And two-step prediction is used to reduce the influence of control delay.In order to obtain the phase angle information of the grid,a phase locked method based on the positive sequence voltage component is introduced.The positive sequence voltage component can be obtained by the forgotten integral filtering algorithm.Thirdly,the error analysis of the conventional DBC method is carried out.On this basis,an improved DBC method is proposed to suppress the influence of the current sampling error on the current control accuracy.Furthermore,the stability and dynamic performance of the conventional DBC method and the improved DBC method are analyzed by using the transfer function of DBC model in z domain.When inductance variation exists in the filter inductor,the robustness of the conventional DBC method and the improved DBC method is compared.And then,the effectiveness of the improved DBC method is verified by experiments.Finally,two kinds harmonic current compensation methods are introduced,namely,load current open-loop detection compensation method and grid side current closed-loop detection compensation method.In open-loop detection compensation method,due to the defects of open-loop,the compensation capacity of the harmonic current is limited in the regional where the harmonic current changes rapidly,which will result in the upper convex and lower concave phenomena in grid side current.So,a current tracking error compensation method is proposed,which can improve the compensation ability of harmonic current to a certain extent.At last,the harmonic current compensation capacity of two harmonic current detection compensation methods is verified on the experimental platform of NPC three-level APF.Compared with the conventional DBC method,the improved DBC method has a better current control performance,and the current sampling error is suppressed effectively.Compared the closed loop detection compensation with the open loop detection compensation,the former has a better harmonic current compensation capacity.