Research On Output Performance Optimization Control Strategy Of Cascaded Multilevel Dynamic Voltage Restorer

In recent years,electricity has been highly valued by the Chinese government as the cleanest,most effective and most convenient form of energy supply.With the rapid development of the economy,various sensitive loads are increasingly used in industry,and these sensitive loads have higher and higher requirements for power quality.At the same time,power quality problems such as voltage dips,uplifts,fluctuations and flicker in the power grid have also occurred from time to time.These power quality problems have caused serious economic losses to the national economy,seriously affecting the safe,stable and efficient operation of electricity.Among various power quality problems in power systems,voltage sag is the power quality problem with the highest probability of occurrence.The Dynamic Voltage Restorer(DVR)is a series-type power quality controller that can solve the voltage drop problem economically and effectively.In this paper,the cascaded H-bridge dynamic voltage restorer(CHB-DVR)is taken as the research object.By summarizing the research contents at home and abroad,and investigating the research progress of the existing related technologies,some key technical problems existing in CHB-DVR are summarized and the output performance is optimized.The main innovations of this paper are as follows:(1)A hybrid pulse width modulation(HPWM)technique for CHB-DVR and a DC-side voltage equalization control method for the inverter are proposed.For the cascaded multilevel converter system,the modulation mode used plays an important role in the realization of the converter power conversion,and directly affects the harmonic content of the output voltage and the operating efficiency of the entire system.Based on the loss analysis of H-bridge inverters with different modulation modes for CHB-DVR,a hybrid pulse width modulation(HPWM)technique and DC-side voltage balance control method for inverters are introduced.The modulation technology can reduce the switching frequency of the power device while ensuring a higher quality multi-level output waveform,and is beneficial for reducing s witching loss.(2)An output voltage harmonic optimization control technology of CHB-DVR inverter based on HPWM modulation is introduced to ensure that the inverter can achieve maximum level output under different grid voltage drop levels.Firstly,The relationship between the modulation ratio and the number of levels of Hybrid Pulse Width Modulation(HPWM)is analyzed,and the range of the modulation ratio satisfying the maximum level output of the cascaded H-bridge inverter is given.To solve the problem that CHB-DVR can not guarantee the maximum level output when the voltage drop of the power grid is low,by analyzing the range of the modulation ratio when the maximum level output of the inverter is satisfied,a minimum energy based on the fixed voltage of the DC side is proposed.Compensation uses a control strategy that automatically adjusts the phase angle of the load reference voltage.The control strategy can maintain the maximum level output of the inverter under different levels of voltage drop of the power grid,improve the utilization ratio of the voltage on the DC side,and give full play to the advantage of cascading multi-level.(3)A capacity optimization design method for CHB-DVR inverter based on minimum energy compensation is proposed.Based on the analysis of CHB-DVR compensation strategy,a capacity optimization design method for CHB-DVR inverter based on minimum energy compensation is proposed.By increasing the series coupling capacitance on the output filtering branch of the conventional LC low-pass filter CHB-DVR system,the design method can reduce the output compensation voltage of the device and output the active power,thereby reducing the series inverse of the CHB-DVR system.The capacity of the transformer and improve the operating efficiency.The capacity optimization design principle of CHB-DVR inverter based on series capacitive coupling is introduced.Then,the design principle of CHB-DVR parameters based on series capacitive coupling and the adjustment strategy of the system under load fluctuation are given.