Performance Improvement And Weight Reduction Technology Of MMC For HVDC Transmission

Recently,China is in the critical period of energy structure transformation and upgrading.In order to realize the large-scale rational allocation and efficient utilization of renewable energy,the modular multilevel converters based high voltage direct current(MMC-HVDC)system has attracted widespread attention from industry and academia circles.At present,the further development and large-scale engineering application of MMC-HVDC are mainly restricted by the following key technical problems:(1)Lightweight of converter.(2)DC fault clearing.(3)Power coupling between AC system and DC system.In view of the above technical problems,this paper makes an in-depth study on the performance improvement and lightweight technology of MMC for HVDC system.The main contents of each chapter are as follows:(1)To realize the lightweight of converter,a novel circulating current injection controller for capacitor voltage fluctuation suppression of modular multilevel converters is proposed.Firstly,the mathematical model of MMC based on average switching function is studied,and the relationship between capacitor voltage and circulating current is derived.The proposed controller consists of two PI controllers,namely the outer controller and the inner controller.The outer controller determines the reference value of the circulating current,and the inner controller generates the circulating current in each arm.The outer controller obtains the circulating current reference value through the PI controller.Compared with the existing research using instantaneous value to calculate the reference value,it avoids the influence of measurement error on the control effect,and the second harmonic fluctuation in the capacitor voltage can be accurately suppressed to 0.Finally,a comparison between the conventional circulating current suppressing controller and the proposed circulating current injection controller is made in PSCAD.The simulation results show that the proposed circulating current controller can effectively suppress the capacitor voltage fluctuation,so as to reduce the required capacitance of submodules and realize the lightweight of converter.(3)For the problem of DC fault clearing,the hybrid HVDC system based on the LCC and the hybrid MMC are studied.Firstly,the operation principle and mathematical model of the hybrid MMC is studied,and the relationship between DC operating range and submodule proportion is analysed.Then,two kinds of steady-state control strategies of hybrid HVDC system are designed,and two feasible DC fault clearing strategies are introduced,which are converter blocking strategy and DC fault ride through strategy.Finally,a bipolar hybrid HVDC test system is built in PSCAD,the response characteristics of two control strategies and DC fault clearing strategies are compared and analysed.The feasibility and effectiveness of proposed control strategies are proved by simulation results.(4)For the problem of DC fault clearing,a modified hybrid MMC with lower proportion of FBSMs is proposed.Firstly,three main conditions for successful DC fault clearing of the hybrid MMC are analysed,which are:(a)blocking AC fault current;(b)Suppression of DC fault current;(c)capacitor voltage under threshold value.Then the topology and the DC fault clearing strategy of the proposed modified hybrid MMC are introduced.Since the installed AC current blocking switches block the AC current feed during the fault,the number of FBSMs is only limited by the third condition.Furthermore,the maximum capacitor voltage after the DC fault is analszed to obtain the minimum required number of FBSMs.Finally,the DC fault responses of the modified hybrid MMC are compared with the conventional hybrid MMC in a three-terminal MMC-HVDC test system in PSCAD.Simulation results demonstrate the effectiveness and feasibility of the proposed modified hybrid MMC.(5)For the problem of power coupling between AC system and DC system,a modified MMC named active MMC with bidirectional fault isolation capability is proposed.Firstly,the topologies of the active MMC and SMs with energy storage are introduced.Then the operation principle of the active MMC under different faults is analysed in an MMC based HVDC grid.Thirdly,the complete control scheme and fault ride-through strategy of the active MMC is proposed.Finally,for two typical application scenarios of active MMC,the corresponding test systems are built in PSCAD,and the fault response of the active MMC and conventional MMC are compared by simulation.Simulation results show that the active MMC can effectively protect the AC system and DC system from each other's faults.