-Apart from the positive technical consequences like reducing losses, the increasing penetration of Distributed Generation (DG) units connected to the distribution network, has lead to the increase of the short-circuit (sc) currents and the fault level. Therefore it is important to know the contribution of each unit. This paper presents the available analytical equations to calculate the short-circuit current, and makes a comparison between the IEC 60909 and the results obtained by the simulations in a test network that incorporates these units.

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Abstract. In this paper the possibility to use Decentralized Generation (DG) units for voltage support in Distribution Networks during and after a Short Circuit (S/C) event is discussed. Two types of DG units will be examined, Combined Heat-Power (CHP) plants and Doubly-Fed Induction Generators (DFIG). Earlier approaches would oblige the disconnection of these units during grid disturbances, in order to avoid unwanted conditions such as interference with the protection system and islanding [1]. Newer grid codes for Transmission Networks like [2], taking into consideration the increased share of these units in energy production have set specific Fault Ride-Through (FRT) criteria which oblige them to stay connected during SC events when they are combined to large plants directly connected to the transmission grid, such as (off shore) wind farms.. If ever more DG is connected directly to distribution grids, the same reasoning leading to applying rault ride through criteria to DG connected in large groups to transmission grids, may be applied to DG connected to distribution grids, as a loss of a a large amount of DG connected to distribution grids has the same adverse effect as using DG connected directly to the transmission grid.In order to examine the effects of applying FRT criteria to DG connected to the distribution grid, simulations with a detailed modeled network for different types of outages were carried out. In the simulations, the application of FRT criteria currently only used for DG connected to the transmission grid is discussed. From the specific test case, which corresponds to a strong network with extended cable connections and locally high penetration level of DG in distribution system it was shown that application of strict fault ride-through criteria combined with active voltage support is not necessary in case of DG connected to distribution networks.

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