Research On Islanding Detection For Three-phase Grid-connected Inverters

When the power failure came down which resulted in the suspension of electricitysupply, distributed generation system will operate independently with the local load. Thisphenomenon is called islanding. The existence of islanding will cause serious damage tomaintenance personnel and electric equipment. Hence, as the interface between distributedgeneration units and grid, the grid-connected inverter must have the ability of detectingislanding. By taking three-phase grid-connected inverter as a target, a further study ofislanding detection on the basis of the predecessors is discussed in this paper.Slip-mode frequency shift (SMS) method is widely applied in islanding detectionbecause of the little impact on power quality and the inexistence of non-detection zoneunder the standard quality factor. However, with the load quality factor increasing, theislanding detection speed reduces rapidly which resulted in the detection time exceeding2s. To avoid this shortage of the traditional method, an improved SMS algorithm is putforward, in which the sine curve used by traditional SMS algorithm is replaced byparabola curve. Since the slope of the selected parabola is infinite at peak, the frequencyof the voltage at the point of common coupling rapidly deviates from the grid frequencyafter islanding and the islanding detection time is substantially shortened. The proposedalgorithm is conducted with analysis of the non-detection zone and the parameter of thecurve is determined. Also, the impact on power quality of the improved algorithm isdiscussed and the two slip-mode algorithms are verified by MATLAB/Simulink.Negative-sequence current perturbation method is a new islanding detection which isput forward in recent years. It is widely used because the algorithm has no impact onpower quality and no non-detection zone. The negative-sequence current perturbationmethod with positive feedback of unbalance factor is described in detailed in this paperfirstly. Considering the linear feedback method will affect the symmetry of grid currentbecause of the fluctuation of grid voltage’s unbalance factor, a novel islanding detectionwith nonlinear positive feedback which injects less negative-sequence perturbation currentis put forward. Due to the feedback function is greater than the positive feedback threshold and is less than the restrictive value under the maximum unbalance factor of grid voltage,not only the proposed algorithm can detect islanding successfully, but also makes theunbalance factor of grid current getting a quantitative restriction. Simulations andanalysises of both mentioned negative-sequence current perturbation methods are verifiedby MATLAB/Simulink.Finally, a experiment platform of three-phase grid-connected inverter which iscontrolled by DSP is completed. And the main mentioned islanding algorithms in thispaper have been carried on the experimental verification using the platform. Theexperimental results show that the parabolic slip-mode frequency shift method comparedwith the traditional algorithm can greatly improve the detection speed of islandingdetection; the negative-sequence current perturbation method with nonlinear positivefeedback of unbalance factor compared with linear feedback method can significantlyreduce the impact on the symmetry of grid current.