| The development and utilization of wind power is an important way to promote the energy structure transition,and its utilization methods are divided into centralized and decentralized.China's northern wind power mainly uses centralized development model,the most typical is the "three north" wind power base.Wind power through the UHV transmission channel to the east and central region of long-distance power transmission,its ability to reduce the capacity is vulnerable to the constraints of the delivery channel.Mountain wind farm is the mainstream way to develop and utilize wind power in southern and central and eastern regions of China,which is mostly in areas with complex terrain,weak network frame structure and abundant hydropower,and is faced with technical bottlenecks such as multi-type harmonic overlay problem,high/low voltage ride-through and reactive voltage coordination and dispatch difficulty.In order to solve the above problems,the integrated control filtering and dynamic reactive power compensation(ICFDRC)of dispersed wind farms,three-phase voltage imbalance coordinated treatment technology,optimal hierarchical control of the internal voltage of wind farms,and coordinated control of reactive power voltages when wind farms are connected to the grid are carried out.The basic research work,and the innovative research results obtained are mainly reflected in the following aspects:(1)This chapter studies the engineering application of ICFDRC in wind power grid-connected systems.First,it introduces the main electrical wiring method of ICFDRC.Then,a theory of transformer reactance integration is proposed,which realizes the integrated design of cascaded SVG connecting reactance and grid-connected transformer.The equivalent circuit of ICFDRC is established,and the transfer characteristics of harmonics in this type of transformer is studied,finaly,the requirements for the realization of ICFDRC can be obtained,that is,the equivalent impedance of the third winding is 0 and the current flowing into the third winding side must meet a specific relationship to make the filter branch equivalent to the harmonic short-circuit loop,so that the harmonics of the wind farm can be shielded from the transformer.On this basis,the design scheme of the control system and the harmonic impedance matching control method were completed;Finally,the actual operation of the wind farm based on ICFDRC was simulated and field tested.The results show that the ICFDRC can achieve high-efficiency filtering of harmonics and fast dynamic reactive power compensation.(2)Firstly,an equivalent mathematical model of negative sequence admittance of cascaded SVG and PMSG is established,which theoretically proves the feasibility of suppressing the negative sequence voltage of wind farm grid-connected points by changing the output negative sequence admittance of cascaded SVG and PMSG.Next,the mathematical relationship between the negative sequence current and the maximum output current and maximum voltage of the cascaded SVG is established,and the optimal compensation domain for the negative sequence current of the cascaded SVG is obtained.At the same time,in order to avoid the negative sequence current required by the system from exceeding the negative sequence For the instability problem caused by the current compensation domain,a fast calculation process of the negative sequence reference current is proposed to ensure that the negative sequence current is limited within the compensation domain.In addition,in order to solve the problem of "on-demand" voltage imbalance management for PMSGs with different remaining capacities,a negative sequence admittance adaptive adjustment method based on participation coefficients is proposed,in which participation coefficients and voltage imbalances,and their own remaining capacity are positive.Related,the cascaded SVG and PMSG can change their own output negative sequence admittance by adjusting their respective participation coefficients,so as to realize the reasonable distribution of the task of controlling the voltage imbalance at the grid connection point of the wind farm,avoiding the remaining capacity caused by the traditional proportional distribution Fewer fans are prone to overcurrent problems.This method is also applicable to photovoltaic power plants that encounter the problem of three-phase voltage imbalance.(3)In this chapter,we have conducted an in-depth analysis of the voltage rise of the PCC of dispersed wind farm.It is pointed out that the reverse power flow of the wind farm is the main reason for the voltage rise of PCC.At the same time,the interaction between the charging capacitor of the collector line and the active output is the main factor that causes the terminal voltage of the wind turbine to rise.Aiming at the problem of internal voltage rise in wind farms,a wind farm voltage layered coordinated optimization control method that takes into account active power curtailment is proposed,and an approximate voltage sensitivity calculation method is introduced,which overcomes the calculation complexity caused by frequent calculation of voltage sensitivity in traditional methods.Subsequently,through the approximate voltage sensitivity,the voltage-reactive power relationship between the wind turbine and all nodes in the wind farm is established,and each wind turbine has a unique droop gain that changes with the change of operating conditions.The minimum deviation between the terminal voltage and the grid connection point voltage is the goal,the droop gain is the decision variable,and the reactive power constraints and voltage constraints of the wind turbines are used as the limiting conditions to make all wind turbines work at the optimal droop gain.When the wind farm is at full power or close to full power and the PCC voltage exceeds the limit,the active power of some wind turbines is reduced to some value,which can meet the reactive power demand of the system.The simulation results verify the superiority of the proposed control method to the internal voltage control of the wind farm.Compared with the traditional droop control method,the reactive power distribution in the wind farm is more reasonable,which solves the problem of PCC voltage and terminal voltage fluctuation and rise.(4)Aiming at the problem of voltage coordinated control in large-scale dispersed wind power integrated into the grid,a distributed voltage coordinated control method based on reactive power and voltage division is proposed.Firstly,an electrical distance control space is defined to solve the problem that PV nodes cannot be characterized by the reactive power-voltage relationship,and then wind farms with voltage regulation capabilities in the power grid are used as reactive power sources to analyze the reactive power output of this type of wind farms The strength of the correlation with the voltages of important nodes in the power grid,the quantitative evaluation of the strength of the wind farm's participation in the grid voltage control,the establishment of an adaptive zoning criterion for the participation of wind farms in voltage control,and the clustering tree tracking method to classify the distributed wind power reactive voltage strong control area.Secondly,a power grid voltage stability control model was established,and the optimal power flow model was simplified to a standard quadratic programming problem using linear decision principles.Finally,a real power grid in Hunan Province was taken as the research object,combined with the typical daily output of wind farms.The ADMM algorithm is called for distributed solution based on the optimal reactive voltage partition.The simulation results show that the proposed control strategy can control the system voltage to the maximum near the target value,effectively suppress the voltage fluctuation problem,and also reduce the network loss.Compared with the centralized method,the proposed method is more suitable for the voltage control problem of large-scale distributed wind farms integrated into the grid. |
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