Research On Control Strategy Of DFIG Grid-connected Inverter Under Grid Voltage Distortion

Energy transformation plan is put forward in China's 13 th Five-Year plan,which requires wind power and other clean energy to change gradually from the role of auxiliary energy to substitute energy.Therefore,more stringent safety and reliability of wind power must be taken seriously.Wind power resources are abundant in China and mostly are located in remote areas far from the economic burden,such environment is equivalent to a "weak grid" environment for wind power systems.Thus,harmonic distortion and other faults of grid voltage often occur.Under the severe circumstances,the wind power may not be satisfied with ?Technical rule for connecting wind farm to power network?.In order to maintain the operation stability of the wind power system and the output power quality of the wind power system,the operation control of wind power system under grid-voltage faults must be studied.At present,the doubly fed induction generator(DFIG)is most popular,in this paper,the control of grid side inverter in DFIG wind power system under harmonic distortion is studied.The effect of grid voltage harmonic distortion on the inverter of grid side,performance evaluation of traditional vector control,improved control based on resonance and application scheme of sliding mode variable structure in grid side inverter are studied.When the grid voltage contains low harmonics,the accurate mathematical model of the grid side is the basis of its control analysis.Firstly,the accurate mathematical model of the grid side converter is established when the grid voltage contains 5th and 7th harmonics,according to the mathematical model,the influence of the grid voltage harmonic distortion on the output voltage,current,active power,reactive power and the DC bus voltage of the grid side inverter is analyzed,then to verify the correctness of the established mathematical model,the mathematical model is set up in the related simulation software.The performance of traditional vector control of GSC under grid voltage harmonic distortion is analyzed and improved.The traditional vector control based on two phase rotating coordinate system is the most widely used strategy of DFIG wind power system.However,when grid voltage harmonic distortion occurs,the traditional PI control has limited harmonic suppression capability.Besides,the method of restraining harmonics by increasing the filter inductance at the grid side and increasing the bandwidth of the current inner loop is not advisable in practical application.To enhance the harmonic suppression of traditional vector control,resonant control is introduced.Firstly,the advantages of resonance controller for AC volume regulation without static difference are analyzed,and then the improved Proportional-Integral-Resonance(PIR)control is designed on the basis of traditional vector control.It is applied to the control of the DFIG grid side inverter under the harmonic distortion of the grid voltage,which is compared with the traditional PI control.Finally,in order to enhance the robustness of the DFIG grid side inverter under model parameter variations,sliding mode variable structure control is introduced.Firstly,a full order sliding mode control strategy is proposed in two-phase static coordinate system,and the feasibility of the proposed scheme is proved.Then,based on high robustness of full order sliding mode control,when the grid voltage contains low harmonics,the calculation of current reference value is improved.Thus,the ability of full order sliding mode control for harmonic suppression is improved without losing its robustness.Finally,based on the proposed control strategy,the related experiment is carried out on a grid side inverter platform of a small wind power system.