| VSC-HVDC is a new generation of HVDC transmission technology in the early 1990s based on the fully controlled semiconductor and pulse-width modulation technology.Compared with CSC-HVDC based on thyristor,VSC-HVDC has the advantages of no commutation failure,no requirements of reactive power compensation and AC filters.VSC has three main topologies:two-level converter,three-level converter and modular multilevel converter(MMC).Two-level or three-level converters are mainly used in early VSC-HVDC projects in which the series structure of IGBT devices has the problem of dynamic voltage balancing and much harmonics and loss due to low voltage level.Compared with two-level and three-level converter,MMC can increase the output voltage level simplily by incresing sub-modules(SMs)in arms,which has the obvious advantages such as lower manufacturing difficulty,lower converter loss and lower harmonics contents.Therefore,MMC-HVDC has become more important in the field of HVDC transmission.Many MMC-HVDC projects are under construction nowadays all over the world,especially in China.The modelling,control strategies and stability analysis have become the major technical problems in development of MMC-HVDC,which need to be studied and solved urgently.This paper makes in-depth studies and some innovations on the modelling,control strategies and small-signal dynamics and stability of MMC-HVDC.Firstly,the modelling of the MMC-HVDC mainly contains two parts:the modelling of single MMC and HVDC cable.For single MMC,two improved averaged-value model ignoring high frequency switching effect under balanced and unbalanced AC conditions are built respectively.The two averaged-value models consider the dynamics of SM capacitor voltage and arm circulating current.For HVDC cable,the frequency-dependent parameters and propogation characteristics are analyzed based on steady state cable model firstly.Then the frequency-dependent cascaded state space model of the HVDC cable is developed.For control strategy,this paper gives detailed control structure and tuning method of control parameters of traditional closed-loop PI control(TPIC)under both balanced and unbalanced AC conditions which includes phase lock loop(PLL).phase sequence separation,inner current controller,outer power or dc-voltage controller and circulating current suppression.The simulation in PSCAD validates the TPIC and the correctness of state space model of MMC.However,essentially MMC is a multi-input multi-output nonlinear system,the TPIC considers no dynamics of arm circulating current and SM capacitor voltage,which cannot decouple the active/reactive power and circulating current fully.In view of this,this paper proposes a nonlinear decoupling control(NDC)strategy for MMC based on feedback linearization.The nonlinear feedback law and overall control structure for power-controlled station and DC-voltage-controlled station are developed.Then the zero dynamics stability is analyzed and the control parameters of PI controller is tuned.The steady and tranient dynamics of a two-terminal MMC-HVDC under TPIC and NDC are compared,which shows the advatages of NDC.Finally,for small-signal dynamics and stability,a general procedure of building small signal model(SSM)is described at first,then the full-order small-signal models(FOM)of single MMC and dc grid are established respectively.The FOM of MMC-HVDC can be construted by integrating FOM of single MMC and dc grid.However,the FOM of MMC-HVDC reduce the simulating efficiency due to high system dimension.To reduce the complexity of FOM,this paper builds reduced-order small-signal model(ROM)of single MMC and MMC-HVDC based on balanced realization theory(BRT).The detailed order-reducing process is presented for both asymptotically stable and non-stable state space model of single MMC.Based on the ROM of single MMC,two schemes of order reduction for MMC-HVDC are developed,which reduce the model complexity and enhance the simulation efficiency.The simulation gives the general principle of retaining order for ROM and validates that the small-signal dynamics of FOM and ROM in Simulink are consistent with the detailed model in PSCAD.Also,the root locus of the ROM and FOM can predict the instabily precisely of MMC-HVDC. |
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