| Modular Multilevel Converter(MMC)is the preferred topology in flexible DC transmission engineering due to its high waveform quality,scalability of system and high reliability.Nearest Level Modulation(NLM)is the most useful modulation strategy in MMC,with the problem of high occupancy in computing resources and high switching frequency.Therefore,the method of fast balancing reducing switching frequency which used in MMC based on NLM are studied in this paper.The fast sorting algorithm based on historical information,the balancing strategy considering capacitor aging and the balancing strategy with the third harmonic injection are proposed in this paper.In the proposed new methods,the computation burden of the controller can be greatly reduced,the operational reliability of the converter can be improved due to the balance of the capacitor aging degree,and the switching loss can be reduced with the reduction of the sub-modules' switching frequency.The research results will promote the rapid and large-scale application of MMC in the field of medium and high voltage levels,which has important research significance and broad application prospects.The specific research content and research results obtained are as follows:(1)A fast sorting algorithm considered about the historical information of sub-module and a strategy for reducing the submodules' switching frequency are proposed in this paper.The characteristics of the sub-module voltages sequence of the inserted group and bypassed group are constant within one control period are used in this algorithm.The historical sorting results of sub-module voltages,the bridge arm current direction and the numbers of sub-modules in inserted group and bypassed group are considered when sorting,which reducing the reduce the range of voltage comparisons and sequencing.The purposes of reducing the time complexity of the sorting algorithm and increasing the speed of calculation are achieved in the proposed sorting algorithm.After that,the maximum voltage deviation threshold is introduced in this sorting algorithm,which can reduce the switching frequency by setting a threshold and sacrificing the performance of equalizing voltage.(2)A voltage balancing strategy consider about the difference in the aging state of the submodule capacitors was proposed in this part.The online monitoring method of sub-module capacitor aging state is lacking in the practical application of MMC.The capacitance would be smaller than others when the corresponding capacitor's aging state are seriously than others,which may lead to an increase in voltage variation with the same bridge arm current and result in an increase.in switching frequency.The aging speed of capacitor will be increased and the operational reliability of MMC will be influenced.To solve this problem,the intrinsic relationship between the switching frequency of the submodule and the relative aging state of the capacitor are study in this paper.And the online monitoring method was proposed,where the charge-discharge conduction ratio is preferably used as a characteristic amount for measuring the relative aging state of the capacitors.Then the analytical formula between the charge-discharge conduction ratio and the relative aging state is obtained based on the BPAdaboost learning algorithm.By detecting the ratio of charge-discharge conduction times,the relative aging state of the corresponding capacitor can be calculated.After that,a relative aging state of the capacitors balancing strategy is proposed.The capacitor voltage addition amount is generated by using the capacitance relative aging state coefficient value and superimposed to the actual sub-module capacitor voltage.This new virtual capacitor voltage is used to voltage balancing algorithm in MMC.With this strategy,the capacitance relative aging states were balancing and the switching frequency of submodules are reduced,which result in an increase in reliability of converter.(3)A modulation strategy for injecting a third harmonic component into a voltage modulation wave are proposed.The feasibility of applying the third harmonic injection modulated wave to MMC is first analyzed in this part.Then the effect of third harmonic injection on bridge arm current,submodule capacitor voltage fluctuation and switching frequency are analyzed theoretically.It is found that the amplitude and effective value of the bridge arm current,the amplitude of the capacitor voltage fluctuation and the switching frequency can be reduced with the third harmonic injection modulation strategy.And finally improve the current and voltage safety margin of the switching device,reduce the on-state loss and switching loss of the converter valve.The paper focuses on the voltage balancing control and the method of reducing the switching frequency for MMC.From the above three specific engineering requirements,the paper draws a series of theoretical research conclusions.The MMC electromagnetic transient simulation model was built in Matlab/Simulink to verify the validity of the theoretical analysis.The research results of the thesis will provide theoretical support for the specific application of MMC voltage balancing and reduce switching frequency,and further promote the rapid and wide application of MMC in the medium and high voltage field. |
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