Impacts and Analysis of an Unbalanced Grid on the DC-Link of a Grid Connected Three-Phase Inverter and a Hybrid Storage System

The technical connection guidelines for power generation systems on the medium voltage grid, which were newly defined by VDE in 2017, make it clear through adapted fault ride through curves that converter systems should remain connected to the grid in the event of voltage dips or other grid faults. This requires longer operation on an unbalanced grid and leads to new advantages for inverter and storage systems. On this basis, this paper analyses the effects of an unbalanced grid on a grid connected three-phase inverter, a DC/DC-converter and storage system and the DC-link itself, using a simulation model created in Matlab-Simulink. The focus is on the negative effects on the DC-link, including oscillations in the active and reactive power, the DC-link voltage and currents. Furthermore, this paper investigates the impact of several factors, which have a huge impact on these oscillations and the DC-link voltage. Finally, this paper gives a recommendation on how these oscillations can be reduced.