| As penetration of renewable energy(wind power,photovoltaic,etc.)increases,the conventional grid can no longer accommodate the grid connection and consumption of distributed generation.To increase the energy utilization rate,future renewable electric energy delivery and Management(FREEDM)is the new trend.Based on utility grids,FREEDM combines power electronic technology and internet techniques to achieve a more reliable way for grid interconnection.By doing this,it maximizes the energy utilization rate of distributed generation.As its core equipment,energy router should be a multi-port converter with high power rating and be able to achieve bi-directional power flow.As a major trend of the FREEDM,hybrid AC/DC grid needs a high-power bidirectional multiport converter for the interconnection between the grids.Multilevel converter is a type of high power converter widely adopted for its high voltage rating,low output voltage distortion,and low switching loss.However,research on multiport multilevel converter is still in the start-up stage.Hence,with the port expansion of multivel converter as the research subject,the contents and achievements of the thesis is as follows:(1)Based on the two-port hexagonal MMC,a three-port nonagonal MMC is proposed in this thesis.Both the hexagonal MMC and the nonagonal MMC is classified as the polygonal MMC,which has the polygon structure and introduce the AC ports from its vertexes.The universal model of the polygonal MMC is then derived,with the frequency components of the bridge arm instantaneous power analyzed.(2)To analysis the generating mechanism of the power imbalance encountered in polygonal MMC,a comparison is conducted between the polygonal MMC and a power balanced modular multilevel matrix converter.Through the comparison,it is reached that the power imbalance in polygonal MMC is a result of its quasi-triangle connection and when port number is increased to greater than 2 not only the reactive power difference but also the active power of the two ports on either side of the bridge arm will pose an influence on the imbalance power.To evaluate the degree of the power imbalance and the imbalance power's influence on the power components' voltage/current stress,two indicators are chosen which are respectively the imbalance rate and the stress increase ratio.Then,through mathematical analysis,the rule is obtained of the system parameter and port number's influence on imbalance rate and stress increase ratio.The maximum stress increase ratio is also gotten when power imbalance is at its worst.(3)The three-port nonagonal is used as the research object.First,the power balancecondition is derived,and the average value model is used to derive the relationship between the circulating current and the offset voltages.Through this,the needed offset voltage values are obtained to offset imbalance power.Based on this,the power balance control is proposed for nonagonal MMC,which inject offset voltages to control circulating current value and hence ensure the power balance and stable operation of the converter.As for parameter design,the expression of the submodule capacitor voltage is gotten,with its fluctuation derived.Taken into account the maximum stress increase ratio gotten above,the submodule capacitance value is designed.(4)Polygonal MMC achieved multiport expansion through adding a DC port through the low voltage DC link of each submodule.Both low-voltage DC port and medium/high voltage DC port can be achieved.For the low-voltage DC port,the solid state fault interruption characteristic is analyzed when DC side is underfault.With the mathematical model derived,solid state fault protection circuit is designed.For the medium/high voltage DC port,a multi-port circuit breaker is proposed when the microgrid connected is a multi-terminal DC microgrid.It combines multiple hybrid circuit breaker and achieves multiport fault protection with lower cost and smaller size. |
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