Research On Transient Characteristics And Primary Regulation Method Of Doubly-fed Induction Wind Turbine

As the proportion of wind power continues to increase,the large-scale offgrid problem caused by external faults of wind turbines has become a major hidden danger of power grid security and stability.At the same time,the share of traditional power generation is constantly crowded out,which leads to the decrease of inertia of the system,the reduction of thermal reserve capacity,and the reduction of the safety and stability margin of the power grid.Doubly-fed induction generator(DFIG),as the main wind power model,is very important to the power grid,and its weak immunity and grid-connection stability problems need to be solved urgently.Therefore,the purpose of this paper is to improve the disturbance resistance and stability of wind turbines and improve the friendliness of wind turbines connected to the grid.Firstly,based on the measured data of low voltage crossing,the full electromagnetic transient simulation model of doublyfed induction wind turbine is established accurately.On this basis,the transient stability of wind power system is improved,and the DFIG continuous fault crossing scheme is improved.The active transient stability mechanism of doublyfed induction wind turbine is studied,and the transient operation strategy of variable power point tracking and the comprehensive control method of oscillatory voltage crossing are put forward,which can realize the continuous high and low voltage fault crossing of wind turbine and solve the problem of weak disturbance resistance of wind turbine.In chapter 2 and 3 of this paper,it has been elaborated in detail.Secondly,the new idea of DC side bus with a small amount of energy storage is adopted to increase the control freedom.Considering the random fluctuation of source load power,the inertia support and primary frequency modulation method of DFIG based on ultracapacitor control and the primary frequency smoothing method of DFIG based on variable power point tracking and coordinated control of ultracapacitors are proposed.In chapter 4,5 and 6 of this paper,specific elaboration has been made.Finally,the dynamic reactive power coordination control of DFIG stator-side and grid-side converter is studied to improve the reactive power limit capacity,which is elaborated in Chapter 7 of this paper.The main contents are as follows.1)The simulation model and parameters of wind turbines are an important basis for the safety simulation operation after a high proportion of new energy is connected to the large power grid.It is particularly important to establish the full electromagnetic transient model of DFIG accurately according to the actual wind turbine operation.To solve this problem,this paper determines the action characteristics and control strategy of DFIG based on the measured data and actual operation curve of field low voltage crossing experiment.The improved genetic algorithm is used to identify the parameters of doubler wind turbine,fan power control parameters and Crowbar resistance value of low voltage crossing protection device.Combined with the electrical parameters provided by the wind turbine manufacturer,the accurate electromagnetic transient simulation model of the actual doubly-fed wind turbine in a wind farm in Mengxi,China was established.Within the range of full wind speed,the wind turbine can quickly enter stable operation state within ls after starting.Finally,through data comparison,it is verified that the parameter identification method proposed in this paper can reflect the key characteristics of actual wind power,and the established electromagnetic transient simulation model is accurate and has good practical application value.2)Due to the continuous expansion of the grid-connected scale of wind power generation,the problem of grid security and stability of large-scale offgrid due to external faults of wind turbines has become more and more serious.In response to this problem,this paper proposes a comprehensive control method based on variable power point tracking for transient stable operation and oscillating voltage ride-through.It analyzes the "flywheel effect" of the DFIG rotor and the speed change characteristics of the input mechanical power.A positive sequence voltage component compensation method based on autodisturbance rejection control is proposed to stabilize the grid voltage and effectively enhance the auto-disturbance immunity of doubly-fed wind turbines.Combined with the Great Britain’s "8·9" blackout in 2019,the incident was reproduced by constructing a model of the British Horn wind farm system.Finally,the simulation results show that the application of the above control strategy to the regional power grid can effectively avoid the occurrence of the accident,and realize the continuous high and low voltage fault ride-through of wind turbines.It can provide a reference for studying similar problems in other regional power grids.3)Under the maximum power point tracking control of DFIG,the generator power output is difficult to respond to grid frequency fluctuations.Although the conventional overspeed and load reduction control can retain part of the active power reserve to participate in the system frequency modulation,there are problems such as reduced power generation efficiency of wind turbines,reduced speed adjustment range,and frequent start of pitch angle control.In response to this problem,this article combines the control characteristics of the DFIG gridside converter,and proposes a DFIG inertia support and primary frequency modulation strategy based on supercapacitor energy storage control.Among them,both the virtual inertia adjustment and the primary frequency adjustment are realized by the super capacitor control,and there is no need to change or increase the additional control of the wind turbine,which improves the stability and immunity of a single wind turbine.The capacity of the energy storage unit is optimally configured according to the actual cost and charging and discharging efficiency of the supercapacitor module.By comparing and evaluating the economics of the reserved primary frequency modulation scheme,it shows that the proposed scheme has strong economic advantages.Finally,simulations and experiments show that its inertia support and primary frequency adjustment capabilities and power generation efficiency are significantly improved compared to conventional primary frequency control.4)When DFIG based on super capacitor control participates in system inertia support and primary frequency modulation,if only virtual inertia control is used,the steady-state frequency deviation cannot be reduced.If only the virtual droop control is used,it is impossible to hinder the frequency drop speed and reduce the maximum frequency deviation variation.Therefore,the two used alone cannot reasonably coordinate the contradiction between the steady-state frequency deviation and the transient frequency drop speed and the maximum frequency deviation change rate.In response to this problem,this paper proposes a DFIG inertia support and primary frequency modulation adaptive control strategy that takes into account the supercapacitor energy storage state of charge(SOC)control.On the basis of maintaining the energy storage SOC and combining the advantages of virtual inertia control and virtual droop control,a proportional coefficient model that can automatically adjust two frequency modulation controls to participate in frequency modulation with changes in frequency deviation value and frequency deviation change rate is proposed.It realizes the smooth switching of the two frequency modulation modes and improves the frequency adjustment effect.Finally,simulation experiments show that this control can smooth the output of supercapacitors while improving the frequency adjustment capability.It reduces the depth of charge and discharge,increases the service life,and provides new ideas and new applications for the transformation and upgrading of DFIG.5)At present,the frequency fluctuation range of the low-voltage side of the transformer in the wind farm or the grid connection point is relatively large,and the situation that exceeds the threshold of a frequency modulation operation(0.033Hz)frequently occurs.In addition,the parallel configuration of energy storage devices on the DC bus of wind turbines for charging and discharging requires a large capacity for the primary frequency modulation of the participating system.It requires more frequency adjustments for charging and discharging,which affects the life of the energy storage system.This paper comprehensively considers the influence of random fluctuation of source-load power on system frequency,and proposes a smooth adjustment method of primary frequency of DFIG variable power point tracking and coordinated control of supercapacitors.Based on the historical data of wind power fluctuations in a strong wind weather cycle in a wind farm,the influence of wind power fluctuations on the system frequency of a single wind turbine unit under different time scales is studied.This paper determines the capacity of the energy storage device required for the next frequency smoothing on the best time scale,and optimizes the configuration of the energy storage device.Finally,simulations and experiments show that the control strategy proposed in this paper can greatly improve the frequency quality of the system and improve the stability of the system while reducing the capacity of the configured energy storage device.6)As the scale of wind farms continues to expand,improving the reactive voltage regulation capability and response speed of DFIG and improving the safety and stability characteristics of large-scale wind power integration into the grid are key issues that need to be resolved urgently.In response to this problem,this paper quantitatively analyzes the reactive power and voltage regulation capability and limiting factors of a single DFIG in a wind farm in West Mengxi Power Grid,and formulates a dynamic reactive power coordinated control strategy based on the reactive power regulation mechanism of a single unit.If the system reactive power demand exceeds the DFIG reactive power output limit,on the basis of ensuring the maximum power generation benefit of the unit,a DFIG active power additional control based on the reactive power difference is proposed.And through the improved genetic control algorithm identification to obtain additional controller parameters,this control reduces the minimum output of the unit while ensuring that the reactive power output limit of the unit is increased,thereby meeting the reactive power demand of the system.Finally,the experiment verifies the feasibility and accuracy of the DFIG dynamic reactive power coordinated control proposed in this paper,and enhances the unit’s voltage stability capability.