Consequences of Transformer DC Bias in AC Earthed Low Voltage DC Grids

This paper investigates the behaviour of DC earth faults and their consequences on the AC side transformer(s) for AC earthed TN-S and IT grids. In hybrid AC/DC grids, a LVDC designer can choose to earth the system on the AC side, by connecting the neutral transformer secondary with the PE. This is typically done in industrial AC grids and referred to as a TN-S system. In this case, earthing the DC side is not allowed anymore: the DC grid needs to remain floating. However, a DC earth fault will give rise to a DC current in the PE and in the transformer secondary coils, creating a DC magnetization of the core. This paper will investigate the consequences of the transformer DC bias and the impact on the protection strategy.