Research On Control Of Permanent Magnet Synchronous Wind Power System Converter Under Unbalanced Grid Voltage

Wind energy has the advantages of rich resources,wide distribution,green and environmental protection.In recent years,countries all over the world attach great importance to the research and application of wind power generation.Although the achievements are very remarkable,there are still some problems to be further studied.Considering that the actual power grid is easy to be affected by unbalanced loads,faults and other factors,resulting in the imbalance of power grid voltage.Therefore,the influence of unbalanced power grid on the system should be considered in the design of converter control strategy of wind power system to ensure the reliable and safe operation of permanent magnet synchronous wind power system.In the case of unbalanced grid voltage,due to the introduction of negative sequence voltage component,it will cause fluctuations on dc bus voltage and system output power,output current distortion and overcurrent.The uncertainty of circuit parameters of grid side converter may also be caused by the variation of external ambient temperature and measurement error.Besides,if the power grid frequency fluctuates,how to design the control strategy of grid side converter to improve the frequency stability of power grid should be considered.These are urgent problems to be solved in the control of permanent magnet wind power system under the condition of non ideal power grid.This dissertation mainly focuses on the above problems,and the specific contents include:(1)In the case of unbalanced grid voltage,aiming at the problems of system output power fluctuations and current distortion,the dual current loop(Proportional Integral,PI)control is used to suppress active power fluctuation,reactive power fluctuation and negative sequence current.However,the control scheme needs to separate the positive and negative sequence of current.And there are many PI controllers,it is difficult to set parameters.Therefore,a integrated power control scheme in positive sequence dq+coordinate system is proposed.A resonant integrator and a resonant filter are introduced into the outer loop of the grid side control to suppress the double frequency fluctuations,and reduce the impact of unbalanced operation of the system on the output current.The inner loop uses a(Quasi Proportional Resonance,QPR)controller to control the current and avoid the separation of positive and negative sequence currents.Based on(Radial Basis Function,RBF)neural network,the PI controllers parameters of dc bus voltage and reactive power loop are adjusted online to improve the anti-interference ability of the system.The control scheme not only suppresses the fluctuations on output power,but also reduces the low harmonic contents of output current.(2)Because the instantaneous power control strategy and current balance control strategy can only realize the constant power or balanced current independently,a power/current coordinate control scheme is proposed to solve this problem.The control scheme is based on(Proportional Complex Integral,PCI)controller.In the harmonic control branch,the current harmonic contents are adjusted by introducing the weight coefficient to achieve the purpose of coordinate control of grid connected power and current.The control strategy does not need harmonic detection module,and the structure of the control system is simple and easy to implement.(3)Because the traditional control scheme has limitations in taking into account the output active power,reactive power and grid connected current quality at the same time.To solve this problem,a optimal control scheme of power and current is proposed.The regulation coefficients α and β are introduced into the calculation of current command.The analytical expressions between active power fluctuation amplitude,reactive power fluctuation amplitude,current total harmonic distortion rate and regulation coefficients are derived.In addition,the influences of regulation coefficients on control performance are analyzed.Taking the current harmonic distortion rate limit as the constraint condition and the minimum overall fluctuations on active and reactive power as the control objective,the optimization model of regulation coefficients is established.The optimal control of power and current is realized by using(Quantum Particle Swarm Optimization,QPSO)based on feasibility law.(4)Unbalanced grid voltage will not only lead to fluctuations on the dc bus voltage and output power,but also increase the peak current,which will bring hidden danger to the reliable operation of the wind power system.To solve these problems,the causes of excessive grid connected current and the calculation method of current are analyzed,a simple current limiting control scheme without auxiliary equipment is proposed.The electromagnetic power of the generator is adjusted according to the grid connected power of the grid side converter.At the same time,the unbalanced power between the machine side and grid side is converted into rotor kinetic energy to avoid dc bus overvoltage.The grid side controller can not only ensure that the grid connected current is within the maximum safety range that the converter can bear,but also provide reactive power support for the power grid.In addition,the fluctuations on dc bus voltage and output power can be effectively eliminated when using this scheme.(5)In the actual permanent magnet synchronous wind power control system,the uncertainty of circuit parameters of grid side converter may be caused due to ambient temperature and measurement accuracy.To solve this problem,a(Model Free Adaptive Control,MFAC)scheme based on grey prediction is proposed,which takes the inductance as the uncertain parameter to control the grid side converter of permanent magnet wind power system.It can be seen from the simulation diagrams that the control scheme can further reduce the fluctuations on power,improve the current quality,and has good static and dynamic performance.Furthermore,this control strategy improves the robustness of the system.(6)When the grid frequency fluctuates,(Virtual Synchronous Generator,VSG)technology is introduced into the control of grid side converter to support the grid frequency.Due to the large transient overshoot of VSG control with fixed inertia damping,the frequency is easy to exceed the frequency fluctuation threshold.To solve this problem,a comprehensive adaptive control scheme of inertia damping considering damping ratio is proposed,which improves the transient performance of the system and further improves the frequency stability under the condition of power grid frequency fluctuation.