Research On Technologies Of Stability Control And Anti-islanding Protection For Grid Integration Of Photovoltaic Inverters

With applications of photovoltaic power generation in large scale,the number of inverters connected into the grid increases significantly.However,the presence of grid impedance leads to interactions among inverters and between inverters and the grid.These interactions could cause the increasing of harmonics in the point of common coupling,the disconnection of inverters from the grid,unexpected islanding operations etc.,and consequently make great threats to the safety and reliability of the power grid operation.To ensure the integration of photovoltaic inverters into the grid safe,reliable,flexible,and efficient,this dissertation,sponsored by the National High Technology Research and Development Program,systematically analyzes and studies the following problems,such as the off-grid of inverters caused by resonance,the poor performance of traditional phase locked-loop under abnormal grid voltages,and multi-inverter anti-islanding protection failure,and so on.Some innovative results have been obtained.In this dissertation,the mechanism of the influence of the digital control time delay on the stability of inverters is revealed,and the constraint conditions of the inverter stability are concluded.First of all,mathematical models of the inverter with single loop control of the inverter-side current feedback and with single loop control of the grid-side current feedback are established respectively.Then,the constraint relationship between the control frequency and the resonant frequency of the filter is quantitatively analyzed under with different digital control time delays.At the same time,the constraint relation of the current controller proportional gain is given when the inverter is stable.The mechanism of resonance between inverter(s)and the power grid is revealed,and a new method is proposed to damp the resonance by reshaping the output admittance of the inverter.First of all,based on the model of single/multi inverter and grid,it is found that the frequency characteristics of ratio of the inverter's equivalent output admittance to the grid admittance does not satisfy the Nyquist stability criterion,which is the cause of inverter-grid resonance.Then the frequency ranges of the negative real part of the inverter output admittance under single-loop control of inverter-side current feedback and single-loop control of grid-side current feedback are found by means of mathematical analyses.It is found that changing the inverter control frequency and changing the current controller control parameters cannot completely eliminate the negative output admittance of the inverter.Finally,the method of the output admittance reshaping is put forward for resonance damping.This method is realized by associating passive damping with digital control to make the ratio of the inverter output admittance to the grid admittance satisfy the Nyquist stability criterion.Simulation results show that the proposed method can effectively damp resonance caused by the mutual interactions between inverter(s)and the grid,and improve the robustness of the inverter,and the adaptability of the inverter to the grid.A novel islanding detection method for multi-inverter grid-connected system is also proposed.First,the analyses and comparisons of the existing islanding detection methods show that the frequency positive feedback-based islanding detection methods are suitable for the multi-inverter systems.Then this dissertation proposes the power line carrier communication based Sandia frequency shift islanding detection method.With the rational design of the positive feedback gain,the performance of the proposed islanding detection method is decoupled from the inverter output power,and the islanding detection capability of the multi-inverter grid-connected system is improved.Moreover,our theoretical analysis shows that the impact of the frequency measurement error and the line impedance on the performance of this novel method can be ignored.Our experimental results shows that the proposed method with the optimized design parameters has a shorter anti-islanding protection time(<200ms)and good reliability,and there are no non-detection zones.In addition,this novel method also introduces little degradation on the power quality of the inverter output current.A new phase locking method with high adaptability for grid voltage is proposed to improve the stability of grid-connected inverter control.Based on the SRF-PLL small-signal model,the mechanism of influence of the negative sequence component and voltage amplitude in the grid voltage on the performance of the traditional phase-locked loop is analyzed.In order to obtain the positive sequence voltage component,the decoupled double synchronous reference frame is adopted to obtain the positive and negative sequence component separation.The positive sequence component is used for phase-locked loop,which avoids the influence of negative sequence components on the phase-locked phase.The decoupled double synchronous reference frame PLL is modified by the normalized method for the elimination of influence of the voltage amplitude fluctuation.Simulations and experiments shows that the enhanced decoupled double synchronous reference frame PLL has good adaptability to the grid voltage,strong robustness,fast dynamic response,and does not subject to abnormal power grid voltages.Three prototypes of 10kW three-phase photovoltaic grid-connected inverters have been developed.An experimental platform has also been built to verify the correctness of the theoretical analyses and the effectiveness of the proposed methods.