Research On Optimization Of The Power Decoupling Technology For Single Phase Converter In Mirco-grid

Tradiationlly,electrolytic capacitors can be installed in the dc-link of single phase converter as power decoupling elements to buffer the second order ripple power.However,the well-known bukly,short lifetime and unreliable issue of electrolytic capacitor may threaten the reliability of the single phase converter.Alternative active power decoupling techniques have thus been developed,which in most cases,require additional energy storage and switches for diverging the ripple power away from the converter.These additional components will,no doubt,raise the cost and complexity of the inverter,which can definitely be reduced by developing topologies that use less components.Starting from those issues,this dissertation focuses on the split-capacitor based active power decoupling technology to enhance converter reliability with reduced cost.Start from the cost of the converter,the topology of AC-split-capacitor based differential inverter is employed to explore the active power decoupling function,which only need one auxiliary decoupling capacitor when compare with convertional H-bridge inverter.The differential inverter is treated as two bidirectional DC-DC converters working in the differential mode(DM)and common mode(CM).The differential mode is to transfer energy between the DC port and AC port,and the common mode is to realize power decoupling.Based on the average switching model of CM and DM,the resonance of power decoupling control is identified and an autonomous reference generation technique is thus proposed to provide accurate ripple power compensation,and also,the stability of power decoupling control.Furthermore,the inverter suffering the harmonics from non-linear load,and rise the higher order even harmonic power in the DC-link.Harmonic mitigation criterions is then identified and realized by using additional resonant compensate both in DM and CM.The LCL filter is combined with AC-split-capacitor power decouping,to improve the switching harmonic attenuation.A generalized small-signal model of the inverter is built first with the averaged switching model,which shown that the LCL filter resonance merely occur in the differential mode,while an LC filter resonance exists in the common mode.A unified active damping control approach is then proposed for stabilizing the inverter control,which realized by independent feedback the capacitor current,and introduce the virtual impedance on both of capacitro.Existing studies related to power decoupling using a differential inverter have however focused on developing control schemes with equal storage capacitances assumed for split-capacitor.This is unquestionably not realistic since the two capacitances will vary in practice due to thermal,humidity and time effect.In chapter 4,according to the mathematical model,the quantitative ac and dc imperfections experienced by the differential inverter when storage mismatch occurs can be calculated.Based on analisis,mitigation criterions under capacitance mismatch is then identified,and A simple improved scheme is then proposed for raising performance of the differential inverter in case of capacitance mismatched,based on the control scheme used for grid connected mode.The improvement tolerates 20% of capacitance mismatch,and higher reliability of inverter.According to the investigation form AC-split-capacitor,capacitance mismatch,also threatens the performance of DC-split-capacitor.Chapter 5 and 6 contribute to design a robust control scheme for DC-split-capacitor based power decoupling.Chapter 5 addresses to quantify the ac and dc degeneration during capacitance mismatch,for both standalone installed and integrated with H-bridge case.Based on analysis,a mitigation criterion is proposed to suppress the harmonic,which is realized by an improved control scheme based on the existing one.To simplify the control scheme,a more robust compensation scheme is thus proposed in Chapter 6 for the dc-split-capacitor circuit,by controling the capacitor voltage to be dc with double line frequency.The proposed scheme is less sensitive to capacitance mismatch,and will not introduce any other harmonics.But unfortunately,the proposed method is unable to be appliedto the integrated topology,due to the unrealistic high dc-link voltage requirement.At last,a dSPACE 1006 based experimental platform is designed to verify the control effectiveness and theoretical quantification of the AC-split-capacitor circuit with linear and nonlinear load,and also,the DC-split-capacitor active power decoupling.The grid-connected mode is tested based on a dSPACE 1103 platform.The experimental result shows the proposed control schemes can mitigate the dc side second order ripple power effectively,also make the grid current meet the requirement of standard IEEE 1547 in case of capacitance mismatch.