Impact of the correct modeling of low voltage grid with high DG share on the medium voltage grid calculations

Abstract

The penetration of distributed generators are effecting the operation of power grids. They, especially photovoltaics, are commonly connected via an inverter to low voltage grids and equipped with a local Volt/var control schema. Usually, when calculating medium voltage grids, low voltage grids are modelled using lumped loads. In the case of Q(U) controlled PV systems, this modelling approach does not correctly reflect the reactive power behaviour of low voltage grids. This work analyses the impact of the correct modelling of low voltage grids with high PV share on the voltage and reactive power behaviour of the superordinate voltage levels. Results show that a more realistic modelling of the there do exist discrepancies between the lumped load modelling approach and the real reactive power behaviour of low voltage grids. The reactive power consumption of the whole low voltage grid increases with the rising of low voltage bus bar voltage on the secondary side of the distribution transformer. It is shown that this changed reactive power behaviour of low voltage grids has significant impacts on the voltage levels in the high and medium voltage grids.