| With the rapid development of modern power system,the demand for power by users is increasing significantly,however,the energy and load are reversely distributed,so longdistance and large-capacity transmission methods are required to satisfy power needs.In addition,problems such as uneven distribution of power flow in the backbone network and lack of reactive power support capacity are emerging,which bring new challenges to the operation control of the power grid.Unified Power Flow Controller(UPFC),which has important research value and broad application prospects in flexible AC transmission systems,can fully exploit the potential of existing power grid and promote the construction of power grid to an environment-friendly development.Modular Multilevel Converter(MMC)is considered to be the most suitable topology for high-voltage and high-capacity applications,which is a topology suitable for highcapacity UPFC applications due to its modular cascading characteristics and flexible scalability as well as common DC bus structure.This thesis mainly studies the control strategies of UPFC based on MMC.The main research contents are as follows:Firstly,a detailed mathematical model is established for the topology and working mechanism of MMC.On this basis,the MMC's various modulation strategies,submodule capacitor voltage equalization control strategies and circulating current suppression strategies are studied respectively,and the MMC modulation and control strategies suitable for UPFC applications are analyzed and determined.Secondly,the model of the MMC-UPFC topology is established under the single circuit wiring,and the reactive power compensation,DC voltage control strategy on the shunt side,and the four power flow control modes on the series side are studied separately,and the corresponding control parameters are calculated according to the mathematical model.A highly reliable DC voltage coordinated control strategy is proposed for the optimal topology of double converters on the shunt side under double circuit wiring,which combining constant DC voltage control,droop control and margin control to achieve reliable and stable operation and fast power flow control of UPFC system.Thirdly,under the unbalanced power grid due to asymmetric three-phase line parameters,remote faults in AC systems and lightning strikes,the operating characteristics of the MMCUPFC are analyzed.Based on the current source characteristics of MMC-UPFC shunt side,a positive-negative double sequences control strategy is proposed,the positive-sequence control is used to stabilize DC bus voltage and compensate reactive power,and the negative-sequence control is used to compensate the negative sequence current of the grid.Based on the voltage source characteristics of the series side,the positive and negative sequences control strategy of the series side is proposed.The positive sequence control is used to regulate the power of transmission line,and the negative sequence control is used to compensate the negative sequence voltage of the grid.This control strategy realizes the safe and stable operation of the MMC-UPFC under unbalanced conditions and the grid negative sequence voltage and current compensation.Finally,based on the control strategies mentioned above,the simulation models are established on Matlab/Simulink platform.The results of the simulation verify the correctness and effectiveness of the above control strategies. |
PDF Full Text Request