Numerical simulations of sediment deposition on grading electrodes in cooling water dielectric of HVDC converter

The platinum pin-type electrodes are installed in inner water cooling circuits of HVDC converter valves to grade the electrical potential distribution and to protect metal components from electrolytic corrosion. In practice, the inorganic particles such as aluminum compounds in cooling water dielectric would be transported to the surfaces of pin-type grading electrodes and aggregate the inorganic sediment. The previous researches showed the thickness of deposited sediment depended on the electric current density through the surface of grading electrode. In this paper, a mechanism of electrophoretic deposition was proposed to clarify the inorganic sediment deposition on the pin-type grading electrode surfaces. The transport and aggregate processes of charged inorganic particles were simulated by COMSOL software. Based on the calculations of the electric and fluid fields along the grading electrodes in a ±500 kV converter valve, the effects of the electrode current and electrode size on the electrophoretic depositions were analyzed. The calculated and analyzed results indicate that the maximum electric field near the surface of a single-pin electrode tip is much higher than that of a combined triple-pin electrode. Under the electric field, the charged particles in water dielectric near the plastic tube wall with a lower fluid velocity will be expediently transported to the grading electrodes, and aggregate on the electrode surface. The thickness of accumulated or deposited sediment during first 50 days is half of that in duration of 180 days. The transport trajectories of charged particles around the grading electrodes were also obtained. The rate of sediment deposition on the electrodes is dependent mainly on the local current density and the sediment properties, and rarely on the charges of the inorganic particles under a critical value. The rate of sediment deposition would decrease with increasing of sediment thickness due to the high insulation resistance of sediment.