Scaling Mechanism And Descaling Method Of External Cooling System For HVDC Converter Valve

In order to ensure the normal and efficient operation of converter stations in HVDC projects,deionized cooling water is usually used to take away the heat generated by the core equipment of converter stations,i.e.the valve group,while the deionized water re-cooling is mainly carried out in the external cooling system,and its effect directly affects the heat transfer process.This paper analyzes the influence of the fouling of the spray pipe in the external cooling system of the converter valve on the water-cooling system of the converter valve.It is found that the fouling will significantly reduce the heat transfer performance,generate a lot of waste of resources,and also cause the performance of the converter valve block electronic device,unpredictable losses such as reduced,shortened life,etc.In this paper,EDS spectrum analysis is carried out on the components of the fouling collected from the spray water pipeline.It is found that the fouling consists of three main elements: Ca,O and C,and five minor elements such as Fe,Mg,Al,Si and S,which are come from spray water.This paper also simulates the fouling of the external cooling system of the converter valve.The effects of concentration ratio,displacement,temperature and spray water flow rate are analyzed through experiments.The formula of the fouling rate is preliminarily estimated.The results show that large discharge capacity can significantly control fouling,and the fouling rate will decrease when the temperature drops and the concentration multiple decreases.When the spray water flow rate decreases,the fouling rate does not decrease all the time,and there is an optimal flow rate for controlling fouling.The converter valve external cooling system can estimate the fouling rate by continuously detecting the water hardness and the pipe wall thickness.In order to solve the problem of scaling of spray water,several conventional pretreatment techniques and several core scale inhibition techniques such as ion exchange,dosing in water treatment,membrane treatment and electromagnetic scale inhibition are compared and analyzed.The conductivity and hardness of the process effluent are analyzed,and the combined scale inhibition process combining conventional mechanical filtration,softening and membrane treatment is selected to achieve the scale inhibition of the spray water.Then,a set of double coil closed cooling towers is used for comparative tests.One set of coils is sprayed with water that is not treated by the scale inhibition process at the converter station,and the other set of coils is sprayed with water treated by the combined scale inhibition process.The comparison test results show that the selected composite scale inhibition process can effectively inhibit scale,and the nanofiltration technology in the membrane treatment technology is slightly better than RO in removing hardness,but relatively weak in removing the remaining ions.Finally,the ultimate evaporation test shows that when the core water treatment process is nanofiltration,maintain no fouling for 17 days,when the core water treatment process is RO,maintain no fouling for 34 days.