| Modular multilevel converter(MMC)adopts the topology of cascaded sub-modules(SMs)and has the merits such as low switching frequency,good output waveform,and low switching consistency.In recent years,voltage source converter-high voltage direct current(VSC-HVDC)based on MMC has been developing rapidly in the fields of wind power integration and AC grids asynchronous interconnection.Before the actual project is put into operation,it is necessary to test the control and protection system using hardware-in-the-loop experiments in the real-time simulation environment,in order to simulate various operating conditions without damage,and ensures the safe and stable operation after the project is put into operation.A single MMC bridge arm usually contains hundreds of SMs in real system to obtain higher voltage level and transmission capacity.On the one hand,a large number of electrical nodes will bring great challenges to the real-time simulation of the MMC primary system.On the other hand,the MMC control system needs to deal with massive data acquisition,transmission,complex control computation,resulting in long control link delay,deterioration of the system dynamics,and even the risk of oscillation when connected to the power grid.In recent years,a number of high frequency resonance(HFR)problems have been reported in the VSC-HVDC projects during commissioning or operation stage.In this thesis,the HFR in grid-connected VSC-HVDC system is studied,the mechanism of HFR is analyzed,the active damping control strategy of HFR is proposed,and the corresponding hardware-in-the-loop experimental platform is developed to verify the theoretical analysis and the active damping control strategy.The specific research contents in this thesis are summarized as follows:A generic AC/DC-side impedance calculation method of MMC considering grid impedance coupling is proposed,which can accurately calculate the MMC DC-side impedance,AC-side positive-sequence impedance and AC-side negative-sequence impedance.The similarities and differences of MMC internal harmonic variables under small-signal excitation at AC-and DC-side are analyzed.The impedance model is established by harmonic linearization in the frequency domain.When modeling the primary system,the current path under each coupling frequency is analyzed in detail,and the impact of AC-and DC-side network impedances connected with MMC is considered.When modeling the control system,all the control blocks including the positive and negative sequence separation and the negative-sequence inner loop control are considered.The impedance of MMC is calculated by solving linear equations,and the frequency scan of a two-terminal back-to-back MMC-HVDC system in 1Hz-5kHz under various working conditions is carried out in offline and real-time simulation environment to verify the accuracy of this method.The influence of network impedance coupling,control link delay,power level,negative sequence inner loop control,and outer loop control on MMC impedance is analyzed in detail.This thesis proposes an active damping control strategy of HFR in the grid-connected MMC-HVDC system based on the feedforward voltage low-pass filter and the current cascaded adaptive notch filter.Firstly,based on the established MMC impedance model,the simplified AC-side impedance expression of MMC in the high-frequency range is derived to locate the control blocks related to HFR.Then,considering the complex topology and changeable operation mode of the AC network,which makes it difficult to predict the resonance frequency,the inner loop control is modified,and the HFR damping control strategy of the feedforward voltage low-pass filter and the current cascaded adaptive band-stop filter is proposed to eliminate the negative resistance effect caused by the control link delay.Finally,the proposed strategy is verified by hardware-in-the-loop experiments.The proposed strategy can reshape the MMC impedance partially at the resonance frequency so that it does not meet the resonance conditions and achieves the purpose of dissipating the oscillation.It also has good compatibility with different operation modes of the AC system and ensures the safe and stable operation of the system.A real-time MMC simulation model based on field programmable gate array(FPGA)is developed.The parallel characteristic of FPGA is used to realize the real-time calculation of the MMC valve.And the calculated result is equivalent to the controlled Thevenin branch in RTDS small time step environment.The SMs in each bridge arm are divided into several groups,and each group completes the equivalent calculation through pipeline architecture to improve the computation efficiency,while the groups are computed in parallel.For the applications of pure real-time simulation and the hardware-in-the-loop experiment with the external controller,the real-time simulation model with sorting network integrated and with high-speed serial Aurora communication interface integrated are designed,respectively.The accuracy of the developed model is verified by comparing the simulation results under power flow reversal,AC/DC-side fault,block start-up charging with PSCAD/EMTDC.An MMC integrated controller based on FPGA is developed,which can implement all the control strategies of MMC in a single FPGA board.The controller makes full use of the parallel characteristics of FPGA,adopts the parallel design in each control block,the parallel processing between blocks,and the overall timing optimization,which greatly improves the calculation speed of MMC control system.Based on the RTDS and the developed MMC real-time simulation model,the hardware-in-the-loop experimental platform is constructed.The performance of the controller is verified through power flow reversal,switching frequency optimization,and circulating current suppression on/off experiments.Furthermore,based on the experimental platform,the accuracy of the proposed impedance model and the effectiveness of the HFR suppression strategy are verified,which has a certain guiding significance for practical engineering. |
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