Two-port MMC Electromagnetic Transient Efficient Modeling And Optimization Of Capacitor Voltage Sorting Algorithm

Modular multilevel converter(MMC)has been widely applied in the Voltage Source Converter based HVDC project all over the world for its prominent technical advantages.With the number of converter terminals and the number of MMC submodules increasing,When simulating a multiterminal DC power grid composed of a large number of ultra high level MMC,The efficiency of the existing MMC equivalent model is still low,and the simulation time is significantly increased,because the complexity of the MMC based sorting algorithm will increase greatly.Besides,For the problem that the half-bridge MMC can not clamp the DC fault and the computation complexity is high,some new two-port sub-module topologies with functions such as DC fault clamping and self-balancing are proposed,and is considered one of the future candidate topologies for MMC submodules.Unlike the previous equivalent method for single-port MMCs,the two-port MMC does not enjoy the feature that the same arm current flows through all the SMs.This paper proposes the accurate and high-speed electromagnetic transient(EMT)general equivalent modelling method for arbitrary novel two-port submodule MMC by using paralleled full bridge sub-module(SMs)based modular multilevel converter(MMC)topology.All the internal nodes both inside the SMs and between SMs are eliminated recursively,then the whole bridge arm is represented by 4 nodes in the main EMT solver.During this process,all the internal information are preserved though increased the bookkeeping effort.After solving the reduced order admittance matrix,the previously eliminated node information such as the capacitor voltages and interfacing node currents are updated.Therefore,the proposed algorithm is very time efficient yet satisfactorily accurate to reproduce identical external as well as internal dynamics.Besides,Previous research proposed the linear ranking algorithm(at most O(N)comparisons are required)for MMC equivalent models by representing the turn-off resistance of the IGBT switches using ideal zero conductance.This paper will theoretically prove that the linear ranking algorithm is also valid even the typical two-value switches are used in the MMC models.Therefore the internal switching failures can be accurately reproduced by the enhanced high speed models.In addition,this paper investigated the applicability of the linear ranking algorithm using both identical SM capacitance and Normal Distributing SM capacitance to show that the algorithm is even robust and converging in case of more practical scenarios.