Control Strategy Of Suppresing The Effection Of DC Ripple In Single-phase UPQC

With the increasing complexity of load components in a distribution system,several kinds of power quality problems may occur in the distribution system.The custom power technology for distribution system is an important way to solve power quality problems.As a part of custom power technology,unified power quality conditioner(UPQC)with comprehensive compensation capability can deal with several problems.Reducing the requirement of DC-link capacitance helps to reduce the cost of UPQC.In this thesis,the main research of DC ripple in single-phase two-level full-bridge UPQC is as follows:Firstly,the cause of DC ripple of parallel side converter in single-phase UPQC is studied.The influence of DC ripple on single-phase UPQC and its path are analyzed.It is found that the DC ripple will affect the output compensation current of the parallel side converter from the control segment and the modulation segment respectively,and the output compensation voltage of the series side converter from the modulation segment only.According to the influence path,the parallel side converter adopts instantaneous current control.A filter is added in the voltage outer loop to filter the DC ripple which enters the controller,and a harmonic feedback in the current inner loop to suppress the influence from the modulation segment.A method of voltage signal detection is proposed.Based on this method,double closed-loop control of voltage and current is adopted,and DC ripple is introduced as a correction parameter to restrain the influence of DC ripple on series side converter.The validity of analysis about DC ripple and the feasibility and validity of the proposed control method for DC ripple suppression are verified by simulation with matlab.Secondly,the power flow inside single-phase UPQC is analyzed and the capacity of single-phase UPQC is designed.The cause of DC ripple in single-phase UPQC is analyzed.It is concluded that the main cause of DC ripple is reactive current and harmonic current compensated by parallel-side converter,which is independent of the output compensation voltage of series-side converter.The power quality of compensation voltage and current of UPQC under the proposed control method and the control method without suppressing effection of DC ripple is compared by simulation,which shows the correctness,feasibility and dynamic and static performance of the proposed control for the effection of DC ripple.Finally,a low-power experimental platform based on single-phase two-level full-bridge topology is built.Hardware programming language named Verilog HDL is used to realize the above control method.The experimental verifications of the series and parallel side converter are completed and compared with the control method without suppressing the influence.The result of experiments shows the correctness and feasibility of the control method of series-parallel converter for DC-side voltage ripple,and analyze the dynamic and static performance of the system.