| It is of great significance for distributed generation system to guarantee the stability and safety of the LCL type grid-connected inverter,which serves as the energy converting unit from the new clear energy generator to the grid.However,because the photovoltaic,wind power and other new energy power generation equipment in China is generally installed in remote areas,the corresponding grid-connected inverter system will be far from the main grid,which makes that the power grid in the interface of the distributed power generation systems is generally weak.When the non-ideal conditions like harmonic and unbalanced grid happen,the quality of the power which the inverter injects to the grid will be influenced directly and the safe operation of the equipment will be threated indeed.Besides,the LCL type filter which is used to filter harmonic waves for the grid-connected inverter can achieve the same filtering effect as the L type filter and can reduce the volume and costs at the same time.However,the resonance problem of it will also affect the stability of system.Based on the above,the control strategy of the LCL type grid-connected inverter for the condition of harmonic and unbalanced grid as well as the damping method of LCL filter is instituted in this thesis.Firstly,the mathematical model of the LCL type grid-connected inverter is built in three phase abc stationary frame,two phase αβ stationary frame and two phase dq rotating frame respectively.Based on the instant power theory,the instant power models are separately built in two phase αβ stationary frame and two phase dq rotating frame considering the two conditions of unbalanced grid as well as the harmonic and unbalanced grid.Then,to realize the stable operation of the grid-connected inverter under harmonic and unbalanced grid,the corresponding control strategy is instituted in this thesis.Firstly,three kinds of positive-sequence,negative-sequence and harmonic components extracting methods separately based on the traditional second-order generalized integrator,the second-order generalized integrator with prefilter and the multi-second-order generalized integrator are compared and analyzed.The multi-second-order generalized integrator can obviously improve the gain of the voltage component extracted and has a notch character for other voltage components,of which the positive-sequence,negative-sequence and harmonic components extracting property is the best.Based on the above,three kinds of control target on inhibition of negative sequence current,active power fluctuation and reactive power fluctuation is set on the premise of realizing the grid current sinusoidal.The corresponding current commands are derivated in the condition of distorted and unbalanced grid and the current inner loop is controlled by the proportional multi-resonant controller.After then,for the LCL filter resonance problem has an influence on the system stability,the stability of the LCL type grid-connected inverter using the digital control without any damping method or with capacitor current proportional feedback active damping is instituted in the discrete domain.Then,a virtual RLC active damping method is proposed which can make the effective resonance region of system expend to(0,fs/3)∪(fs/3,fs/2)from(0,fs/6)and the robustness of the system against the grid impedance variation is improved.The optimal damping parameter is acquired based on the analysis of the stability of the system by the Nyquist theorem.Finally,Simulation and experiment is realized on the MATLAB/Simulink and 10 kW three phase grid-connected inverter experiment platform.The result of simulation and experiment verified the correctness and effectiveness of theory analisys in this thesis. |
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