| As an effective complement of centralized power generation,distributed generation systems(DGs)are the important way to develop and utilize renewable energy,which are used to ease the global energy crisis and improve energy sustainable development ability.Meanwhile,as a key interface between DGs and utility grid,the inverter is the core area of "green energy transformation".Actually,the inverter can feed power to the utility grid in grid-connected mode,and can also provide power to the local loads with operating independently in island mode.In this paper,the LCL-type photovoltaic inverter is discussed as the research object to study the key technologies of grid-connected control and multi-inverters parallel control under different conditions.So,these technologies can provide the important theoretical and practical guidance for DGs to improve the reliability,security,stability,high quality and efficient operation,which have a great significance to improve the development of the renewable energy DGs and active utility grid.This paper is finished by financial support of the key project of National Natural Science Fund(51237003)"multi-inverter parallel and power quality control method of microgrid" and the China Europe International cooperation project "power quality analysis and control of micro-grid and micro-network distribution system"(2010DFA61640).The research contents are composed with the grid power and damping control for the LCL inverter,the robust active damping control in the weak grid,the delayed compensation under the digital control,and the accurate power distribution and circulation suppression method for multiple LCL-type inverters in the microgrid island operation to study.The main work and innovation are reflected in:(1)For single-phase LCL-type grid-connected DGs installed at the end of the grid,a new two-degrees freedom single-current grid control(N2DOF-SCGC)method with power regulating and active damping is proposed to provide high-quality active energy for the grid,compensate the reactive power for local loads in time,suppress the resonance peak,and improve the robustness against the grid frequency fluctuation.Since the proposed method only needs to sample the grid current and the grid voltage without additional voltage/current sensors,it will reduce the hardware costs and increase the system reliability.Aiming at the robustness problem that the grid-frequency fluctuation affects distribution accuracy of active and reactive powers,the synchronous-reference-frame-quasi-proportional-integral(SRFQPI)control is proposed to improve the system adaptability and robustness against the grid frequency fluctuations.Aiming at the instability problem easily caused by the LCL-resonance peak,the grid-current-feedback-active-damping(GCFAD)control method is proposed to damp well the system resonance.And its essence is analyzed in depth form the perspectives of the negative feedback and the virtual impedance mode.Aiming at the deficiencies that parameters of the proposed method require repeated trial and error,a optimized parameters design method is proposed to improve the system steady-state and dynamic performance through the analysis of the grid frequency disturbance range,the grid current steady-state error and stability margin in detains.The value areas of parameters meeting the design requirements are deduced and the appropriate closed-loop parameters are selected.Finally,the experimental platform of the 2k W single-phase LCL-type photovoltaic power grid-connected generation built verifies the validity of the proposed method.(2)Aiming at the robustness problem that the wide range variation of grid impedance under the weak grid condition can cause easily LCL-type grid-connected inverter unstable,the virtual impedance model of the grid current feedback active damping is discussed.Then,the real reason of the weak robustness is revealed that the positive and negative cutoff frequency of the virtual resistor is located at the range from 1/6 to 1/3 times switching frequency.For this,the robust grid current feedback active damping(RGCFAD)control method is proposed to make the virtual resistance present a positive resistance characteristic all the time in the actual LCL-resonant frequency.Hence,the stability problems that the actual LCL-resonant frequency of LCL-type grid-connected inverter gets through the cutoff frequency in the weak grid is solved,and the system stability and robustness against the grid-impedance is enhanced.Finally,the experimental platform of the 50 k W three-phase LCL type photovoltaic power grid-connected generation built verifies the validity of the proposed method.(3)Aiming at the system stability and control performance problems introducing by the control delay in the digital control,the mechanism generated by the control delay is analyzed.And the influences that the different control delays impact on the system loop gain and control performance is deeply revealed.Then,the tri-mode two load real-time PWM control strategy is proposed to improve the system control performance.Obviously,the maximum duty cycle limited situations in the proposed PWM control is relieved,and the superior positive and negative half-cycle transition performance is obtained.Meanwhile,a shot-lag calculated delay of the grid current loop and the active damping are completely eliminated to greatly improve the grid current waveform power quality and enhance the stability and dynamic performance of the LCL-type grid-connected inverter.In addition,a half of the switching cycle time for sampling and duty cycle calculating is allowed,and an excellent positive and negative half-cycle transition performance is existed in this mode.The proposed PWM method is suitable for single-phase/three-phase power generation system that has the higher switching frequencies.Obviously,compared with the other real-time PWM control method,the proposed PWM method is also easier to apply in engineering.(4)In order to improve the power quality of the multiple LCL-type inverter parallel system in the island operation,and achieve high precision power distribution and circulation suppression,the low-delay and robustness power droop multi-loop control method are proposed based on the analysis of parallel multi-inverters' power distribution mechanism.The proposed method is composed of the output-inductance voltage and current double-closed loop control,and low-delay robust power droop control with the virtual impedance.Through redesigning the equivalent output impedance of the inverters as the pure resistance,it can decouple the active/reactive powers in the droop controller,and weaken the negative impact on the voltage that the network side filter inductance has.Then,by constructing integrator 1/s in active power control,the Q-f/P-U? droop controller and U? recovery mechanism is proposed to improve the robust of the output power against the line impedance and equivalent output impedance.Thus the parallel multi-inverters can share the load power with regard to the ratio of their rated capacities accurately.In additional,considering the phase characteristic of equivalent output impedance is unchanged in this method,it does not need to design the equivalent output impedance again and reduces the design difficulty.Moreover,the system dynamic can be improved by the least-mean-square adaptive algorithm,which reduces the control hysteresis of the power calculation. |
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