Research On Current Distribution During Monopolar Operation Of HVDC Systems With Ground Return

During monopolar operation of HVDC systems with ground return, there is a large scale DC route between soil and AC power net and currents returned from earth may enter into neutrals of transformers, which cause DC bias of transformers and influence transformers running. Considering the problems of DC bias of nuclear power stations, the main works of this dissertation consist of the distribution of DC currents in AC system, the distribution of potential of vertical Grounding Electrode and evaluation of tolerated DC current of transformers.By mean of deriving the Green's function in horizontal-layer and vertical-layer soil structures, the formula of in complex soil layer for calculating potential is derived using the concept of image. The calculation results of DC currents through transformer neutrals with the influence of sea considered are evidently different from those calculated by terrestrial soil model. The resistivity of sea influences the distribution of ground potential, so the ground potential closed to sea will drop to zero. According to actual model of 500kV power grid, simulation results show that larger DC currents will flow through transformer neutral, which are near the seaside and far from the ground of HVDC system. The reason of this phenomenon lies in that influence of sea makes the substation potential decreased, so more DC currents entered into AC power network will flow outwards from AC network via substations with low ground potential. The topology of AC power net caused the different DC currents flowing through neutrals of transformers which are in the same site. The calculations of dc currents through the neutrals of transformers are match to the measures, taking into account complex soil model and AC power grid. Simulation results well explain the phenomena that larger DC currents flow through transformer neutrals in Ling'ao and Daya Bay nuclear power stations. The distribution of ground potential of vertical electrode is analyzed in this paper. The calculation of potential brought by vertical electrode across multi-layers soil is proposed. The results show that the potential near electrode is more influenced and less far from electrode when electrode deep buried. Therefore the deep buried electrode makes less change on the current distribution. Near-sea ground electrode can decrease potential of distant place and increase potential of terrestrial soil near sea for low resistivity of sea. It can be used to alleviate the problems of DC currents entered into AC grid.The reason of DC bias of transformers and the factor of tolerated DC bias are analyzed in this paper. According to simulation of a single phase two-winding transformers model the exciting currents of transformers under AC over exciting and DC bias analyzed. Exciting current can be as the indicating depicting the change of magnetic field within transformer. A method of evaluation of tolerated DC currents based over-exciting is presented.