Research On Frequency Stability Control Strategy Of Sending-end Hybrid AC/DC Power Grid With High Penetration Of Wind Power

Under the strategic goal of "carbon peak,carbon neutral",wind energy becomes one of the most promising new energy sources due to its renewability,no pollution and good economy.Due to the characteristics of reverse distribution of wind power resources and electric power center in our country,large-scale wind power through traditional line commutated converter based high voltage direct current(LCC-HVDC)has become the main way of new energy long-distance reliable consumption.However,the sending-end power grid of large-scale wind power sent through LCC-HVDC has the characteristics of low inertia and weak frequency modulation ability.If DC locking fault occurs,it will cause serious high-frequency problems.Only the traditional DC frequency limit controller(FLC)involved in surplus power modulation can not meet the demand for frequency modulation of the sending-end power grid.And excessive modulation may induce the DC power overload limit,which poses a serious threat to the frequency stability of the sendingend power grid.To this end,this paper fully exploits the controllable rotor kinetic energy and fast power support potential of wind power,and designs a frequency stability control strategy for largescale wind power sent out to the grid via LCC-HVDC.The details are as follows:(1)The mathematical model of LCC-HVDC,DC FLC and the doubly fed induction generator(DFIG)are established,and the control principle and control characteristics of DC FLC and wind power in large-scale wind power transmission grid via LCC-HVDC are analyzed.Then the frequency response analysis model of the sending-end power grid is established.The frequency response characteristics of the sending-end power grid under different wind power penetration rates and FLC control are analyzed,and the serious high-frequency cut-off risk faced by the sending grid under the increasing trend of wind power penetration is analyzed.Finally,the necessity of wind power participation in the frequency control of the sending grid is explained.(2)In order to fully exploit the potential frequency regulation capability of wind power,an integrated inertia control strategy for DFIG to actively support the frequency stability of the sending-end power grid is proposed.Firstly,the maximum power tracking characteristics of DFIG are analyzed,additional virtual inertia control is designed,and the mathematical model of doublyfed wind turbine based on virtual synchronous control is established.Then,the rotor speed and pitch angle control principle of DFIG is analyzed,and the sag control combining variable speed and variable pitch angle with DFIG participation in primary frequency control is designed by combining the primary frequency control demand of the grid.Secondly,the two control methods are cooperated to design a comprehensive inertia control strategy combining virtual inertia and primary frequency control.Finally,the simulation analysis of the modified 4-machine 11-bus test system containing large-scale wind power delivery shows that the wind power comprehensive inertia control strategy can significantly improve the frequency stability of the sending grid.(3)In order to take into account the frequency regulation characteristics of wind power and DC FLC,a cooperative control strategy is proposed for wind power and FLC to participate in the frequency regulation of the sending grid.First of all,the response characteristics of DC FLC to participate in the frequency regulation of the sending grid are analyzed,the sag relationship between the frequency of the sending grid and the power of the wind turbine is drawn,and the primary frequency regulation control method combining variable speed and variable pitch angle of the wind turbine is designed.Then,a comprehensive frequency response model including traditional primary frequency modulation,doubly-fed fan droop control and DC FLC control is established.Combined with the frequency requirements of the power grid,the wind power and DC FLC are used the sensitivity method to adjust the frequency regulation parameters,and the wind power-DC FLC cooperative frequency regulation control strategy is designed.Finally,simulation analysis is carried out through the modified 4-machine 11-bus system and an actual power transmission network in China,to verify the effectiveness and feasibility of the proposed strategy.The results show that the proposed frequency cooperative control strategy can effectively reduce the risk of high-frequency cutter and DC overload operation and improve the frequency stability of the sending-end power grid.