Research On Control Strategies Of Large-scale Wind Farm Integration By VSC-LCC Hybrid HVDC Transmission

It is inevitable for energy development that the supply and consumption of renewable energy such as wind power will increase.However,the development and utilization of wind power in China has the problem of imbalance between the geographical locations of energy-rich areas and load centers.So wind power in the northwest needs to be transmitted to load centers in the east.DC transmission is effective for long-distance transmission of large scale wind power.For areas where both wind power and thermal power need to be exported,in order to realize the reliable integration and transmission of large scale wind power,a VSC-LCC hybrid HVDC system with appropriate topology and control strategies that combines advantages of line commutated converter high voltage direct current(LCC-HVDC)and voltage source converter high voltage direct current(VSC-HVDC)can be devised.Firstly,a suitable VSC-LCC hybrid HVDC system for wind power integration is proposed.LCC converter is used to transmit more power for the system with lower cost,and VSC converter is used to provide with stable voltage support for the wind farm.Basic control strategies are designed for bundled wind-thermal operation mode and isolated wind farm operation mode.Mathematical model of the system under steady state operation is built.Start-up control strategy of the hybrid HVDC system is designed,which includes charging preparation stage and power rising stage.The simulation model verifies that the system can achieve stable operation in about 1.5 seconds.Fluctuating output characteristics of the wind farm is analyzed.Relationship between output power of the wind farm and wind speed is established,and t location-scale distribution model is used to fit the variation rate of fluctuating output wind power.Considering capacity limitation of VSC converters,basic coordinated control strategy of the system is designed.When output power of wind farm is low,the system will not be able to transmit rated power.In order to make full use of transmission capacity of the system,an improved coordinated control strategy that use the complementary characteristics of wind power and thermal power is designed.Under this strategy,the system can transmit rated power in a larger range of output power of wind farm,which increases stability and economy of the system.Operation characteristics are compared under different short circuit ratio(SCR)of AC system at the sending end.When the SCR is low and power transmitted on the AC line is large,voltage of the AC bus at the sending end is low,which will make the system unable to achieve steady-state operation states based on designs of the improved coordinated control strategy.When AC system at the sending end is weak,it is necessary to adjust control strategy of VSC converter connected to AC system at the rectifier.And reactive power compensation is also equipped to realize the coordinated and stable operation of the system.