| With the continuous development of new power systems,the trend of the high proportion of power electronic equipment and large-scale new energy access will become more prominent.The VSC-HVDC(voltage source converter-high voltage direct current,VSC-HVDC)technology based on the modular multilevel converter(MMC)has been widely used in various DC transmission systems due to its advantages of low switching frequency,low harmonic ratio,and high modularization degree.At the same time,MMC also has broad application prospects and some engineering applications in the field of new energy consumption.As the core equipment of the VSC-HVDC system,MMC still has many problems.Compared with the traditional HVDC converter valve,MMC with the same capacity and voltage level has a large volume/weight,poor seismic performance,and high cost.With the continuous improvement of transmission capacity and voltage level,the continuous increase of the series number of sub-modules,the technical problems caused by excessive volume/weight will be more prominent,and it is urgent to improve its power density to reduce its cost.Therefore,the lightweight research of MMC is imperative.In addition,the impact of low capacitance values on MMC DC fault conditions also needs to be studied.The following contents were studied in this paper:(1)Analysis of MMC ripple reduction mechanism and classification of existing lightweight schemesIn response to the problem of large volume and weight caused by the capacitance value of MMC large submodules(SMs),based on the average switching function model,the internal energy flow model and capacitor voltage fluctuation models of MMC are established.By exploring the internal energy flow law of MMC,the key factors affecting the capacitor voltage ripple of the SMs are sought.After analysis,the thesis concludes that the current reconstruction scheme of the MMC bridge arm is one of the mainstream measures to reduce the capacitor voltage ripple of the SMs.The topology optimization scheme or additional control scheme is used to reconstruct different components of the bridge arm current,thereby reducing the capacitor voltage ripple of the SMs,and achieving the goal of improving the power density of the MMC.This paper summarizes and integrates the current lightweight schemes for MMC,and divides them into three categories:topology optimization schemes,additional control scheme,and the combination schemes between topology scheme and control schemes.For each category,corresponding lightweight strategies are proposed.(2)Additional control MMC lightweight scheme for coupled injection of harmonic current and harmonic voltageIn response to the problem that existing MMC lightweight schemes of harmonic current injection will increase the root mean square(RMS)of MMC bridge arm current,a reverse injection method is proposed to determine the amplitude and initial phase angle of the second harmonic current injection amount.This method can greatly reduce the capacitance of the SMs in MMC based on ensuring that the RMS value of the bridge arm current after harmonic current injection is the same as the RMS value of the inherent bridge arm current of the MMC.At the same time,the additional impact of harmonic current injection on the entire MMC and corresponding improvement measures are explored and analyzed.The scheme of improving the modulation ratio through the third harmonic voltage injection can effectively alleviate the additional effects caused by harmonic current injection.Based on the PSCAD/EMTDC simulation environment,the Xiamen project is built to verify the additional control strategy.Simulation results show that the proposed strategy can effectively reduce the capacitance of SMs by more than 30%under bidirectional power flow conditions of the converter.(3)Two kinds of lightweight MMC schemes based on topology optimizationTwo types of MMC optimization topology models that can effectively reduce the capacitance requirements of SMs are proposed to address the problem of low power density caused by large capacitance values of SMs in conventional half-bridge MMC topologies,which can effectively reduce the demand for capacitance values of SMs.Firstly,a bridge arm inductance variable MMC topology which can effectively reduce the capacitance of the SMs of the MMC is proposed.The inductance value of the bridge arm is dynamically adjusted based on the time-varying characteristics of the equivalent capacitance of the bridge arm,and the corresponding auxiliary controller is also designed.At the same time,the lightweight scheme based on the sub-module hybrid MMC is analyzed and discussed,and the two over-modulation operation modes and the lightweight mechanism under the steady-state operation conditions are specifically discussed.A method of adding half-bridge SMs is proposed to optimize the lightweight capability of sub-module hybrid MMC.Simulation and experiment show that two optimized MMC can both effectively reduce the capacitor voltage ripple of the SMs in MMC,and then reduce the capacitance value of the SMs by more than 20%.The bridge arm inductance variable MMC topology is verified by the experimental platform.(4)Lightweight combination scheme for harmonic injection MMC under optimized topologyBy analyzing the lightweighting mechanism of different topology optimization schemes and additional control schemes,this paper points out that the additional control scheme based on sub-module hybrid MMC has better lightweighting effect.The proportion of the full-bridge sub-modules(FBSMs)is optimized based on the submodule hybrid MMC topology.At the same time,a method for determining the amount of the second harmonic current injection is invented.The amplitude of the injected harmonic current is determined from the perspective of the RMS value of bridge arm current,and the initial phase angle of harmonic current injection is determined from the multi-objective optimization of the peak value of bridge arm current,converter valve loss,etc.,and the parameters are designed.Simulation results show that the proposed combination scheme can effectively reduce the capacitance of SMs by more than 40%under bidirectional power flow conditions.(5)DC fault clearing strategy for low capacity MMC based on the equal chargeAiming at the system low voltage blocked problem caused by the fast discharge speed of DC fault SMs of low capacitance MMC compared to conventional MMC.This paper explores the DC fault characteristics of low capacitance half-bridge MMC and the influence of low capacitance on the typical parameters of MMC,mainly including the DC short-circuit fault current,the equivalent capacitance of MMC,and the discharge process of capacitance voltage in SMs.At the same time,it analyzes the influence of different capacity reduction conditions on the fault characteristics of halfbridge MMC.Based on the above conclusions,a low capacitance MMC fault-clearing strategy based on equal charge is proposed,which can effectively alleviate the additional problems caused by low capacitance,and the corresponding additional controller is designed.Simulation and experiment results show that the proposed auxiliary strategy can effectively reduce the discharge speed of the sub-module capacitor voltage under fault conditions,and avoid the locking of MMC during fault clearing. |
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