Study On Low Frequency Oscillation Of The Weak Grid Connected To VSC-HVDC

When the AC power transmission technology is used to supply power to weak grid,the active power transmitted is limited because the charging capacitor of the cable is too large,and it is difficult to deal with the problem of the high no-load voltage in the end of the cable.The receiving-end of conventional HVDC must have a large capacity to provide commutation current to its inverter station to ensure the normal function of the inverter,so conventional HVDC cannot be used to supply power to weak grid either.In recent years with the rapid development of power electronics and control technology,the development of VSC-HVDC is growing very fast.As a result of the adoption of full-controlled devices and pulse width modulation technology,VSC-HVDC is capable of running in four quadrants and controlling active and reactive power independently.Unlike conventional HVDC,VSC-HVDC can supply power to the weak grid and even passive power network,so the prospects of VSC-HVDC are very bright.Although this new type of transmission technology is developing rapidly,there are still a great number of problems which need further research and discussion because of the short developing time.This thesis studies the low frequency oscillation problem when the VSC-HVDC supplies power to weak grid using virtual synchronization generator control strategy.Firstly,the vector control mode used in the rectifier is introduced.Because the vector control mode cannot work properly when the VSC-HVDC connects to weak grid,a virtual synchronous generator control strategy is used in the inverter station of VSC-HVDC system in order to introduce the excellent characteristics of synchronous generator and enhance the stability of voltage and frequency of the weak grid.At the same time,reasonable design of virtual moment of inertia can reduce the transient frequency deviation.At this time a problem of low frequency oscillation may occur when the VSC-HVDC supplies power to the weak grid,and this not only affects the quality of frequency of the weak grid,but also influences the stability of the system.This thesis analyzes the low frequency oscillation theoretically,and then applies the method of increasing the damping coefficient of the virtual synchronous generator to suppress the low frequency oscillation.Finally,simulation model is established using PSCAD/EMTDC software and the results show that the adopted method can suppress the low frequency oscillation in the grid effectively.Compared with conventional HVDC,a big advantage of VSC-HVDC is the ability to supply power to weak grid without additional attachments,so the prospects are very bright.In this thesis,the anti-disturbance of the connected weak grid can be enhanced by the virtual synchronous generator control strategy which is applied in the inverter station of VSC-HVDC,and the low frequency oscillation can be suppressed effectively by increasing the damping coefficient of the virtual synchronous generator.