| In order to better adapt to the increasing penetration of distributed generations such as wind and solar energy,and the access of generalized loads such as energy storage devices and electric vehicles,the medium-voltage interconnected power distribution system,whose core is flexible interconnection devices(FIDs),has attracted extensive attention from the academia and engineering.The basic idea is to realize the interconnection of distribution network(DN)feeders by back-to-back voltage source converters(VSCs),and further realize the closed-loop operation and optimized power flow.The main difficulties of FID realization are topology selection and control strategies.The traditional two-level and three-level VSCs are not applicable to mediumvoltage DNs due to their limited voltage and power capacity,while the modular multilevel converter(MMC)adopts the submodule cascading technology,and becomes the mainstream topology for FIDs due to its good scability and high output waveform quality.Generally,MMCs are connected to the grid via transformers for voltage adjustment and electrical isolation.However,the installation space in DNs is limited,requiring the FID to be more compact.It is expected to remove the grid-connected transformer,but the problems of voltage levels mismatch and the difficulty of zero-sequence current isolation may appear.In addition,power fluctuation and voltage asymmetry are common in DNs,which puts forward higher requirements for the control system of FID.Therefore,the converters at each terminal should realize good coordination control to maintain the stability of dc voltage and achieve the power balance.Besides,the control system of each terminal should give full consideration to the control requirements of MMC itself,and take into account multiple objectives inluding output current,circulating current and capacitor voltages under balanced or unbalanced grid conditions.Based on the above problems,this paper takes MMCs as research object,carries out the topology selection with and without connection transformer,and studies the control strategies of MMCs from the aspects of dc-side voltage coordination control,submodule capacitor voltage ripple suppression and comprehensive control under unbalanced conditions to ensure the reliable operation of MMC.The main research contents of this paper are as follows:Firstly,the topology requirements for applications with and without connection transformers are analyzed,and the power regulation capabilities of different topologies are evaluated,which provides a theoretical basis for topology selection.Combined with the requirements of multivoltage interconnection and zero-sequence current control,it is indicated that the converters in transformerless applications should have the capability of boosted ac-voltage operation,while the traditional MMCs are suitable for the applications with transformer.The potential topology sets are summarized respectively,and power regulation ranges of different topologies are calculated by analyzing their operation constraints.The comparison results show that the traditional backto-back MMC has the best comprehensive performance in applications with transformer,and in transformerless application,the hybrid MMC is the optimal choice.Secondly,a dc-side voltage coordination control method based on fuzzy logic is proposed aiming at the frequent power fluctuations in DNs,which can change the droop coefficient in real time,smooth the dc voltage fluctuation,and realize the power balance of FID.The general model of MMC outer loop control mode is established,and the influence of droop coefficient on the control mode is analyzed.Furthermore,the fixed dc voltage control terminal and the droop control terminal are set up,and the demand for droop coefficient under different conditions is discussed from the aspects of dc voltage steady-state deviation and stability time.Accordingly,the variable droop coefficient based on fuzzy logic is designed.The droop coefficient can be adjusted adaptively according to the dc voltage deviation and its variation trend,and the power vacancy can be reasonably distributed among multiple terminals,so as to reduce the dc voltage overshoot and speed up the system recovery process.Thirdly,to avoid the problem that large capacitor voltage ripples can deteriorate the MMC performance,a voltage ripple suppression method for hybrid MMC based on second-order circulating current injection(CCI)is proposed.The reason why the traditional CCI method is no longer applicable to hybrid MMCs is explained firstly.Then,the characteristics and influencing factors of capacitor voltage ripple in hybrid MMCs are discussed by analyzing the charging and discharging process of full-bridge and half-bridge submodule capacitors,and the optimal modulation ratio that minimizes the voltage ripple is derived.Besides,a new second-order CCI method is proposed.This CCI method analyzes the influence of injected circulating current on the maximum energy ripple of hybrid MMC capacitors,and calculates the optimal second-order circulating current.The proposed method can realize the voltage ripple amplitude minimization and is applicable to different modulation indexes and power factor angles.Finally,in view of the frequent occurrence of three-phase voltage asymmetry in DNs,an arm-current-based model predictive control(MPC)strategy is proposed,realizing the comprehensive control of ac current and circulating current of different phase sequences under balanced or unbalanced grid conditions.The control requirements and operation characteristics of MMC are analyzed under unbalanced conditions.An arm-current-based MPC method is proposed,and the generation method of arm current reference is provided.The proposed method takes the arm current as control object,avoiding the extraction of multiple current components and complicated design of various control loops.And the arm voltage reference is obtained directly according to the prediction model,which can significantly reduce the calculation amount.Moreover,the zero-sequence current suppression in transformerless applications is considered,and the suppression scheme is designed based on the proposed MPC method,which can effectively block the flow of zero-sequence current in the distribution system.Overall,in this paper,the medium-voltage DN is taken as the research background,and the topology selection and control strategies of key equipment are studied.For the application scenarios with and without transformer,the topology requirements are analyzed and a series of control strategies are proposed.The dc-side voltage coordination control of multi-terminal FIDs,the capacitor voltage ripple suppression of hybrid MMCs and the comprehensive control of various current components under unbalanced grid conditions are realized.Therefore,the MMC can better adapt to the complex DN circumstances such as frequent power fluctuations and unbalanced voltage conditions.The effectiveness of the proposed control strategies is verified on the MATLAB/Simulink simulation platform and the experimental prototype. |
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