Risk of MV distribution systems uncontrolled islanding supplied by LV embedded generators

The increasing penetration of distributed generation has lead international authorities to integrate the rules for the connection of active end-users to distribution networks. Thus, power regulating functions aiming at supporting the voltage and frequency stability of the main grid have been recently introduced in European grid codes. In this paper, the consequences of adopting such regulating functions have been examined in detail. In particular, the risk of uncontrolled islanding on a Medium Voltage network with increasing amount of embedded generation connected to Low Voltage subsystems has been studied. Both existing regulating requirements, such as active and reactive power modulation according to locally measured frequency and voltage, and additional stabilizing functions, such as Inertia Emulation and Fast Voltage Support, have been investigated. Results of dynamic simulations, carried out making use of DIgSILENT PowerFactory 2016 software, demonstrate the interactions between stabilizing functions, generators protection systems and network automatic reclosing procedures.