Research On The Harmonic Detection And Control Strategy For The Three-phase Four-wire APFs

With the rapid development of modern power electronics technology,varieties of nonlinear and time-varying electronic devices have been widely used,though the negative effects of these applications become even more obvious.,At the users side of the three-phase four-wire power system,large amount of harmonic current is introduced into the system by tremendous office appliances and household appliances,which causes serious distortion in voltage and current of the power system.The contaminated power system will,in turn,seriously affects the functional operation of electric equipment of power system and power users.Therefore,controlling harmonics at user side of the three-phase four-wire power system is important and urgent for the development of modern electric power production.Firstly,the measuring principle of harmonic current in three-phase four-wire power system APF is introduced briefly in the thesis,then,the measuring method of harmonic current in three-phase four-wire system,based on instantaneous reactive power theory is discussed separately,including the analysis of harmonics with conventional p-q method and positive sequence component p-q method of PI phase locked loop.When the three-phase four-wire power system carries non-linear and asymmetrical voltage,the conventional p-q method and the positive sequence component p-q method of PI phase locked loop will be unable to analyze the harmonics.To measure the harmonic current accurately in this system,an improved measurement method of harmonic current by extracting the positive sequence component of main voltage with a no phase-locked loop structure of two dimensional adaptive is proposed in the thesis.The MATLAB simulation of positive sequence component p-q method of PI phase locked loop and improved p-q method is carried out respectively,Simulation and experimental results show that the new method has higher detection accuracy and better dynamic performance.Secondly,a control strategy of three-level four-leg inverter based on traditional reference voltage decomposition is analyzed.because of some problems in the contol strategy,such as complicated calculation process,complex control and unbalanced voltage of medium voltage,a new control strategy for reducing dimension of three-level four-leg inverter is proposed in this paper.The control strategy of three-level four-leg inverter is decomposed into a control strategy of three-level three-leg inverter and a four-leg relation,where the 3D-SVPWM modulation method is used to control the control strategy of three-level three-leg inverter,and the fourth leg switch vector is controlled by the bridges relation.The new control strategy not only reduces the complexity of the mathematical model of the three-level four-leg inverter,but also the midpoint voltage balance of DC side can be effectively controlled.The simulation and experimental results verify the new control strategy and the stability of the neutral point voltage.Finally,the experimental platform of a three-level three-phase four-leg active power filter based on the TMS320F2815 is designed and the relevant experiments is carried out,the experimental results verify the dynamic performance of the three-phase four-wire harmonic current measure method based on instantaneous reactive power and the effectiveness of the new control strategy of the three-level three-phase four-leg inverter.