Modelling of the DC Flow in AC Systems Considering the Mitigating Measures

The earth return currents (ERCs) of highvoltage direct current transmissions can lead to DC bias of neutral-grounded transformers. This paper provides a coherent framework for modelling and mitigating of the ERC flow in bulk AC systems. Based on the in-situ earth resistivities, the proposed model has been verified by the measured neutral currents of a bulk AC system. Results show that the ERC flow is closely relative to the Green's function representing the earth surface potential.