Research On The Method Of Secondary Ripple Suppression By Differential Frequency Reactive Active Capacitor

The rapid development of the world economy has brought environmental pollution,energy consumption and other problems,however,the total amount of fossil fuel energy is limited,but the extraction reserves are increasing day by day.Therefore,countries around the world have begun to pay more and more attention to the field of new energy,and green renewable energy has become the key to solving environmental and energy problems.Photovoltaic energy is nonpolluting,easy to obtain and widely distributed,and has become the focus of social attention.Single-phase inverters are widely used as key devices in the field of new energy generation.Due to the common Second harmonic power problem on the DC side of single-phase inverters,it directly reduces the power generation efficiency of new photovoltaic energy.For the Second harmonic power problem,an electrolytic capacitor is usually connected in parallel to the DC side of the inverter to absorb the Second harmonic power.Electrolytic capacitor has the advantages of large capacity and low price.However,its short life span and poor environmental adaptability become its fatal shortcomings.In order to overcome the short life span and poor environmental adaptability of electrolytic capacitors.To find an active capacitor solution with high versatility,low coupling to the main circuit,and can simulate the external characteristics of electrolytic capacitors,so as to realize the modular replacement of electrolytic capacitors,the research of de-electrolytic capacitance in the field of new energy generation is of great significance.By analyzing the working principle of active capacitors and the current research status of domestic and foreign scholars.According to the technical characteristics,the active capacitors are divided into three categories: parallel active capacitors,series active capacitors and active capacitor control strategies.Shunt active capacitors,connected in parallel on the bus side to absorb current fluctuation components,are subject to higher bus voltage due to their circuit structure and working principle,which makes the system efficiency decrease significantly.In series type active capacitor,the switching tube is connected in series with the main absorption capacitor,and the control purpose is achieved by changing the series voltage value.Since the switching tube only needs to bear part of the bus voltage drop,it has a higher system efficiency compared with the parallel active capacitor.The control strategy of active capacitor is to make the active capacitor have the same external characteristics as the electrolytic capacitor through the control strategy.Active capacitors have better filtering effect,but they are currently used only in small and medium power applications.In this paper,a differential frequency reactive power theory is proposed to achieve heterodyne power conversion by increasing the system frequency ω and using this high frequency power to synthesize low frequency power.Applying this mechanism to active capacitors can increase the equivalent capacitance of active capacitors to a larger extent,thus expanding the application range of active capacitors.First,the differential frequency reactive power theory is introduced in detail,and then,the differential frequency reactive power theory is combined with the new active capacitor topology to form a differential frequency reactive active capacitor,and its working principle is analyzed.Then,the control strategy is designed by establishing the mathematical model of the differential frequency reactive active capacitor system.The differential-frequency reactive power theory is obtained as a complex fluctuation function of two high-frequency rotating vectors,and the DC-side fluctuations are suppressed by designing a control system based on a high-frequency rotating coordinate system that transforms high frequencies into low frequencies for control.Finally,according to the characteristics of the differential frequency reactive active capacitor,the hardware design and software writing of the differential frequency reactive active capacitor are carried out.The simulation model of the differential frequency reactive active capacitor is built by MATLAB/Simulink simulation software,and the simulation results are consistent with the expected ones,which verifies the feasibility of the differential frequency reactive power theory.The simulation results show that the differential frequency reactive capacitor has the ability to replace the electrolytic capacitor at higher power.