| With the depletion of the traditional fossil energy resources and the increasing demand of power energy,the distributed generation system based on renewable energy is paid more and more attention.In particular,when these concepts,including global energy internet and energy router,are put forward,the research on distributed generation systems with interconnected type has a significant meaning.In the distributed generation system,grid-connected converter has become an important common unit.Control performance of the common unit and power quality of grid current have very strict code.In order to suppress the PWM ripple harmonic,LCL filter has obvious advantage over L filter due to higher PWM ripple suppression.Thus,LCL filter is regarded as one of important candidates in industry application.However,there is weak damping ability of LCL filter,stability issue may easily occur.Thus,to enhance the stability of grid-connected converter with LCL filter,the damping performance improvement is especially important.But,damping performance is also affected by different factors.There will be a big challenge to improve damping performance.To realize the active and reactive power individual control for grid-connected converter with LCL filter,the rotating reference frame is often adopted.However,this control frame will bring the multiple dq-axis coupling issue.As a result,it will be very difficult to realize full decoupling function.Meanwhile,resonance issue needs to be taken into account.Both issues will also bring a big challenge.At remote PV generation or offshore wind generation base,there is long transmission cable or boost transformer to connect main grid.These auxiliary devices will lead to grid impedance changed widely.Meanwhile,grid impedance is also difficult to obtain directly.As a result,the stability issue for grid-connected converter with LCL filter will be occurred in practical application because of largely changed grid impedance.Under the condition of unknown grid impedance,research on a new control method to operate safely and reliably is also a big challenge.Therefore,taking grid-connected converter with LCL filter as the research object,this thesis focuses on active damping control under different factors,the decoupling and resonance damping under the rotating reference frame,and stability performance improvement under grid impedance.For the weak damping issue of grid-connected converter with LCL filter,capacitor current proportional feedback active damping method is usually adopted.This method can obtain good damping performance.But,there needs an extra current sensor and it leads to increase bulk and cost.From the perspective of cost,active damping method under grid-side current feedback is employed in this thesis.In existing references,although the grid-side current proportional feedback active damping method is simple,the analysis result illustrates that the damping performance is not acceptable when the ratio γ of resonance frequency to sampling frequency is among large range.Meanwhile,the design of current controller is also complex.To improve damping performance and extend γ range,a proportional derivative-based grid-side current feedback active damping method is presented by large comparisons and analysis.However,in practical application,damping performance of the proposed active damping method is affected by digital control time,the ratio of resonance frequency to sampling frequency and feedback algorithm.it is imperative to research the damping characteristic simultaneously under these factors and find the maximal possible damping ratio between 0 and 1.According to the characteristic of active damping loop transfer function,a damping ratio synthesis design and optimization method is presented.By the proposed design method,the maximal possible damping ratio curve can be obtained under different conditions.Furthermore,the envelop curve of maximal possible damping ratio is gained among γ∈[0.025,0.45].It is convenient to obtain optimal control parameters directly and improve damping performance as much as possible.To verify the correctness and effectiveness of the proposed active damping method,simulation and experimental platforms are built.Simulation and experimental results illustrate that the proposed active damping method can improve damping performance.For the dq-axis coupling and resonance issue of grid-connected converter with LCL filter,the control strategy including the grid-side current proportional decoupling feedback and capacitor current proportional feedback are mostly employed in existing references.Based on the complex-vector analysis method,the self-axis and cross-axis closed-loop transfer functions of the conventional control are derived.The results illustrate that the conventional control can not realize full decoupling.Meanwhile,there is interaction effect between active damping control and decoupling control.As a result,the control parameters need to design by trail and error method repeatedly.It will be difficult to design the control parameters accurately in terms of control performance.To solve this issue,a novel multi-cell network control structure is proposed.By the cooperation of different control cells,the proposed structure can realize decoupling fully and improving damping performance under the condition of the least sensors.At the same time,the steady-state and dynamic performances have also been improved simultaneously on the premise of stability.Considering the multiple unknown parameters,the corresponding design will be very difficult.In particular,the cooperation control is considered to design.Thus,a zero-pole full placement multiple parameters design method in this thesis is proposed.The concrete theory expressions of all control cells are derived by the proposed design method.The main merits of the proposed design method are that the whole parameters of control cells can be designed accurately and simply according to the expected response performance.There is no any trail and error process.Particularly,only one step is needed for the design process.It is very convenient for engineers in practical application.The proposed control is verified by a built simulation and experimental platform,the simulation and experimental results are both in line with theory analysis.Stability issue of grid-connected converter with LCL filter is affected by grid impedance.The conventional control method can realize a limited stability frequency range or only consider inductive impedance.As a result,it will be difficult for grid-connected converter with LCL filter to promise safe and stable operation under the condition of different types or wide range variation grid impedance.To address this issue,a delay-based output impedance magnitude phase shaping method at full frequency range is proposed.By designing and optimizing the parameters of the proposed shaping method,the real part of output impedance can be larger than zero at full frequency range.That is to say,the phase of output impedance is between-90° and 90°.Meanwhile,the magnitude of output impedance is also met the design demand.To design the control parameters of the proposed shaping method,a multiple constraint conditions with graphical solution design and optimization method is proposed.It can give control parameters directly.In addition,a concrete design flow chart of the proposed shaping method is also given,it is very convenient to apply in practice.The significant merit of the proposed shaping method is that the phase of output impedance can be between-90°and 90° without any extra sensor.The stability issue for grid-connected converter with LCL filter under different types or wide range variation grid impedance can be addressed.Moreover,there is no need to redesign the control parameters as well as measure grid impedance on line,so that the realization difficulty and computation burden will be both decreased.Meanwhile,the magnitude at fundamental and low-order harmonic frequencies are all realized the expected values,so that the power quality of grid-side current is also improved.Through simulation and experiment verification,test results illustrate that the proposed control can realize safe and stable operation under different types or wide range variation grid impedance. |
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