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With their increasing number, it is getting important that decentralized and renewable energy sources are supporting the stabilization of the system voltage after short voltage dropouts. Often, in the past, these generators were disconnected during network faults. This is not adequate anymore, as it would lead to a loss of a considerable amount of feed-in power. However, if the generators stay connected, it must be ensured that they are not receiving reactive power, as this would lead to a collapse of the grid. In Germany, a number of legal requirements and grid codes are regulating the connection of such generators. These documents also stipulate the use of Directional Reactive Power Undervoltage Protection (Q→& U<; protection) that would disconnect such energy sources if they received reactive power during faults on the network. This paper gives an overview about the legal documents and an introduction to the protection function mentioned above. The basic principle of the Q→& U<; protection is explained by means of its requirement specifications. Furthermore, the paper will show up standardized test methods evaluating the Q→& U<; protection. It will describe how to test the releasing functions, the reactive power direction determination and also all binary inputs and outputs that are necessary for the protection function.