Research On Islanding Detection Method For Grid-Connected Inverter Based Photovoltaic Power System

The photovoltaic power generations has been developing rapidly, due to effects of energy deregulation and environmentalist influence. Although the connection of distributed generations brings much advantage to power system, but the effect to the stability of system can’t be ignored, especially the unintentional islanding phenomenon that may lead the system to a dangerous situation. So it is necessary to study the islanding detection techniques. This paper studies the passive and active islanding detection methods, analyzes the efforts of parameter changes to detection performance. And this paper is focused on the research of hybrid method based on different detection principles. Details are as follows:Anti-islanding and the detection methods are studied. This paper analyzes the influence of the short of power to voltage and frequency and also the generation of islanding after the grid disconnection. The protection schemes based on different principles are studied and the non-detection zones (NDZ) to evaluate the performance of anti-islanding methods are chosen for the followed analysis. Then passive detection methods based on grid-connected inverters are studied, mainly including over/under voltage protection(O/UVP)、over/under frequency protection(O/UFP)、and rate of change of frequency(ROCOF) detection methods, and detection performance is discussed.The active detection methods and the effect of parameter changes to detection performance. Active methods introduce the perturbation to the system for increasing the change amplitude of electric parameters, and the parameter settings of perturbation affects the performance. This paper discussed the principles and non-detection zones of the following active methods:active frequency shift (AFD), Sandia frequency shift (SFS) and slip-mode frequency shift (SMS). Based on it, the comparative research on the different parameters is continued to set the parameters for various methods and optimized the performance. The voltage-shift method is also studied. The detection speed are accelerated by the improvement of given form of currentThe hybrid anti-islanding methods that can eliminates the drawbacks of passive and active ones are studied. A new hybrid islanding detection method based on discrimination of non-detection zone is proposed by merging O/UVP、O/UFP and SFS. By analyzing and deriving the NDZ with passive methods, the NDZ index is derived in the hybrid method. Sandia frequency shift method is introduced based on the detection index by calculation of data from the operating system within one frequency cycle. The hybrid method has the advantage of realizing detection with non NDZ and less effect on power quality. Simulations with MATLAB has verified the effectiveness of the proposed method.A hybrid islanding detection method based on improved phase disturbance is proposed. Only one islanding detection method cannot give consideration to both NDZ and power quality. To solve this problem, a robust hybrid islanding detection method, based on SMS method as the active method, and ROCOF method as the passive method is proposed. In the proposed hybrid technique, SMS method gets activated only when islanded condition is suspected by ROCOF relay, which means the parameter of rate change of frequency decides wether the disturbance of phase is imposed. The simulation results indicate that the islanding detection performance and the robustness of system have been improved.