Research On Islanding Detection Technology Of Inverter Cluster

In China,relying on the government's financial fund subsidies and the encouragement of photovoltaic priority grid-connected policies,the number and capacity of PV inverters connected to the grid are increasing.In this context,the probability of occurrence of the islanding effect increases,and the test of the distributed generation system is more severe.At present,most of the islanding detection technology research is only for the single inverter.When these detection methods are applied to the inverter cluster,the detection performance is significantly degraded,and even the anti-islanding protection function is malfunctioned or rejected.Therefore,improving the existing islanding detection technology to improve the detection performance and reduce the negative impact on the power quality is of great significance in both theoretical development and practical engineering applications.In China,relying on government financial fund subsidies and photovoltaic priority grid-connected policies,the number and capacity of PV inverters are increasing.In this case,islanding operations will make the distributed power generation system face a more severe test.Most of the existing islanding detection researches only target a single inverter off-grid,but the detection performance in the inverter cluster environment is significantly degraded,and even lead to the detection failure phenomenon.Therefore,improving the detection performance of existing islanding detection technology under the condition about inverter clusters are having great significance in both theoretical development and practical engineering applications.In this paper,the islanding detection technology of grid-connected inverter clusters is studied.The research contents include:(1)Explain the causes and harms of the islanding effect,briefly introduce the existing local detection method and the principle of remote detection method and evaluate its advantages and disadvantages.The islanding detection idea of "centralized detection "for inverter clusters is proposed,and the difference between this idea and the existing island detection of multiple inverters is pointed out.(2)Inverter cluster simulation model construction.Investigate the inverter control strategy,adopt the constant power current type control strategy,and build the control loop of the current inner loop and the power outer loop.The inverter cluster is represented by three inverters to simulate the power balance environment of the inverter cluster and the local load in the grid-connected state.Using formula reasoning and simulation verification,the relationship between the power matching degree and the voltage amplitude and frequency of the common connection point after the islanding is obtained.(3)Study on the centralized islanding detection algorithm based on interharmonic impedance measurement.Briefly introduce the principle of impedance measurement and point out the shortcomings of the traditional method.Design the harmonic disturbance injection device,the relationship between the interharmonic frequency and the switching frequency of the disturbance device is derived by the formula,and the influence of the interharmonic disturbance on the power quality is analyzed.The feasibility and versatility of the method are verified by theoretical analysis and model simulation.The relationship between system impedance and local load capacity,power or load switching,measurement frequency and load quality factor is studied.It also solves the problems that may affect the detection performance under different working conditions.(4)Study the improved active frequency shifting method applicable to inverter clusters.Derived from the formula to explain the drawbacks of the traditional method about dilution effect and no detection zone.The S-function is used to adjust the output phase angle of the phase-locked loop of the three-phase inverter,so that the three-phase inverter generates a "dead zone" controllable distortion current at the same time as the constant power output.Improve the Sandia active frequency shift method,add the frequency shift input criterion that divides the non-islanding condition and the suspected islanding condition;predict the frequency shift direction after the islanding based on the reactive power exchange amount on the grid side,and improve the chopping coefficient formula.Using the active frequency shift phase angle and the load characteristic phase angle to formulate the equation,and obtain the positive feedback gain range of the dead zone without no detection zone.The simulation proves that the algorithm can make the common connection point frequency shift of the inverter cluster and detect islanding smoothly in a short time.(5)According to the specific situation of demonstration project,the anti-islanding protection device algorithm applicable to the site is proposed.Taking the realistic scene as an example,present the installation of distributed generation equipment and photovoltaic access of the distribution network.Explain the functional requirements of the anti-islanding protection device and comprehensive consideration of the site situation,and propose a new detection method based power exchange measurement of grid-connected points.Through the calculation of the no-load loss of the transformer,the detection criterion of the power exchange method is set,and finally the algorithm is verified by simulation.