| HVDC plays an important role in the long-distance transmission project,which has developed rapidly in China in recent years.When the valve system is running normally,a lot of heat will be generated,so it must be brought out by the internal cooling water system.However,after several years of actual operation,there are varying degrees of deposition phenomenon on equalizing electrode in converter stations,some of which even cause serious faults,threatening the normal operation of the whole HVDC project.So far,the problems of corrosion and deposition have not been solved.Therefore,it is of great significance to study the corrosion principle of HVDC converter valve cooling system to solve the hidden trouble of internal cooling water safety.This paper has shown the whole process from corrosion of radiator to deposition of aluminum on the surface of equalizing electrode.By using the potential-pH diagram of Al and the existing form diagram of Al element in aqueous solution with pH change,the main existing form of Al ion in inner cooling water was determined.Based on the solubility product constants of the related reactions,the ratios of two forms of Al ions at different pH were calculated.The step-by-step hydrolysis process of Al3+ in aqueous solution was analyzed,and the stable pH value of inner cooling water without other metal corrosion and side reaction was determined.A simplified three-dimensional simulation model of the internal cooling water system of the converter valve was established.The potential difference of the branch pipe in the parallel water cooling system considering the electrode dynamics was calculated,and the difference of the electric field intensity distribution in the internal cooling water with or without the equalizing electrode was compared.The influence factors of potential difference in branch pipe section were studied,and the influence of installing equalizing electrodes at different positions on radiator corrosion was calculated.The distribution of electrolyte potential in a single radiator was studied,and the effect of extending the length of branch pipe on the corrosion of radiator was considered.The corrosion simulation model of the radiator with stainless steel electrode at the inlet and outlet was established.The corrosion of stainless steel ring and Al was calculated by using the secondary current field,and verified by the test results.The corrosion rate before and after the installation of stainless steel ring in different positions of radiator were compared,and the relevant suggestions for the installation of stainless steel ring were put forward. |
