Research On The Control Strategy Of HVDC System Based On Modular Multilevel Converter

Modularized multilevel converter(MMC)has been an important and popular research area in the field of power electronics.Comparing with traditional diode clamping,flying capacitance and cascaded H bridge multilevel converter,the high modulation structure of cascade of MMC can realize multilevel output and easily realize redundant design,and content less output harmonic.The advantages of four-quadrant operation and adjustable power transmission have become one of the main types of voltage source converters and been widely used in flexible direct current transmission(HVDC)systems.High voltage direct current transmission system which is based on overhead line has a broad application prospect and development potential in China.However,there are still a series of key problems that have not been solved well.Therefore,it is of practical engineering significance to study the HVDC which is suitable for overhead lines.In this paper,it is which the operating mechanism,working characteristics and system control strategy of MMC are studied.The main contents are as follows:The advantages of MMC,and the development of voltage source converter in HVDC transmission system,as well as the research hot spot of MMC at home and abroad are introduced in the paper.The circuit model based on capacitor voltage of sub-module and simplified the model based on current flow direction are established.The topology and working principle of MMC and its sub-module are analyzed in detail,and the driving principle of MMC is analyzed in detail.The principle and output waveform of mmc output multilevel are explained.According to the simplified mathematical model and the principle of power balance,the range of capacitance and inductance of mmc system is calculated.The three control levels of MMC-HVDC system are introduced.According to the three different control levels,the power synchronization control outer loop,the voltage synchronous control outer loop and the inner control strategy are designed.The capacitance voltage balance of MMC includes two aspects: capacitor voltage balance in the sub-module of the bridge arm and voltage balance between the two arms.The double closed loop controller solves the problem of voltage balance between the bridge arms only.The capacitance voltage balance control of the sub-module is discussed in this paper.The improved carrier modulation and cyclic permutation mapping are adopted.Because of the topological structure and working principle of MMC inevitably cause the internal circulation of the bridge arm,the circulation increases the loss of the system and components,and interferes with the operation of the system,thus causing instability,so the suppression of the circulation is very important.In this paper,the mechanism and mathematical model of circulation are analyzed.Aiming at the lowest energy loss,the optimal reference value expression of bridge arm circulation suppression is derived,and the circulation suppressor is designed.The proposed control scheme is simulated in PSCAD/EMTDC software to verify the feasibility and effectiveness of the proposed control strategy and suppression strategy.The application status of MMC-STATCOM,the circuit model and compensation principle of STATCOM based on half-H bridge MMC structure,and the problem of phase imbalance are analyzed.In this paper,the internal and external control strategies of MMC-STATCOM are proposed.The internal control module includes capacitive voltage balance control,capacitor voltage balance control between the bridge arms,interphase capacitor voltage balance control and circulation control.The external control module includes three phase total capacitance voltage control,output voltage and current control.By constructing the PSCAD/EMTDC simulation model,it is verified that the STATCOM system based on MMC can effectively compensate reactive power,restrain harmonics and compensate negative sequence current under the condition of unbalanced power network and unbalanced load.