Solutions for blinding of protection in today's and future German LV grids with high inverter penetration – simulative and experimental analysis

Abstract

The behaviour of today's installed interfaced distributed generators (IIDGs) during grid faults in German low-voltage grids is regulated in the valid guideline VDE-AR-N 4105. Presently, a new revision of this guideline is discussed, defining a higher voltage threshold of 0.85·VN and requiring the IIDG to remain synchronised for a given period of time during low-voltage dips. An analysis of IIDG using a CIGRE LV benchmark grid shows the possibility of an increase in minimum short-circuit currents, following the new guideline. Nevertheless, there are still cases in which the behaviour of NH fuses as feeder protection is tempered up to an uncertain tripping behaviour, leading to possible threats for persons and equipment. Furthermore, the used CIGRE LV benchmark grid does not cover all worst-case scenarios for the analysis of blinding of protection in German grids. It is shown that the usage of digital (directional) overcurrent detection can be a proper solution to cope with the lower short-circuit currents. The combination of a fuse link allowing the triggered disconnection in the overcurrent range can be a promising solution, especially regarding the retrofit of existing feeders. Laboratory tests with a prototype show a reliable tripping with tripping times of <300 ms.