Earth currents in HVDC grids: An example based on 5 terminal bipolar configurations

Abstract

With the rapid development of Voltage Source Converters and the recent progresses for DC fault management through DC breakers or inherent fault limiting converters, HVDC has become more and more attractive to grid operators and the possibility of a DC grid layer superimposed to the AC grid is gaining ground. Still, a lot of questions remain concerning the appropriate operation of such a grid. One question that has only attracted limited interest is the possibility of ground currents circulating between stations of the grid. The current levels and the criticity of this question are highly dependent on the topology of these hypothetical DC grids and theirs modes of operation. Still, the environmental and legal issues associated with ground currents as well justify the need to study this aspect as well as the mission profile information required for the proper HVDC station electrode design. The aim of this document is to provide illustration of ground current circulation in the case of bipolar HVDC grids with 5-terminal meshed and radial examples. The influence of the mode of operation of the grid is studied, in the transient case with a DC-fault and in the permanent case of operation with a faulted pole. The influence of the impedance connection to ground is illustrated with both high and low resistance cases. Finally, the possibility of using a pole voltage balancing control unit is investigated and its limits are demonstrated, showing the need for a supervision unit or an enhanced local ground current control.