| Scaling in the cooling system of the converter valve has always been a severe problem endangering the safety of the UHVDC transmission system.The three hazards of scaling are as follows: Scaling attached to the electrodes will weaken the pressure sharing capacity of the converter valve and increase the leakage current of the valve cooling system;Scaling and peeling will clog the aluminium radiator in the cooling system and affect its cooling capacity.Scale removal needs to be done during the annual overhaul,which takes a long time.In order to solve the scaling problem of the cooling system,the following studies have been carried out:(1)Hazard assessment of electrodes scaling.On the one hand,according to the electrical connection of the valve layer,a laboratory simulated waterway experiment is carried out.The experimental analysis shows that the contact resistance between electrodes and waterways can not be ignored.The contact resistance between electrodes and waterways under different scaling conditions is solved by numerical simulation.Furthermore,the model for solving the leakage current and the voltage of the valve segment was improved,and a calculation index was proposed to evaluate the harm of scale on the valve cooling system by attaching it to the equalizing electrodes.On the other hand,the convection heat transfer model of the valve group is established,and the simulation of different blockages is carried out,and the overall situation of the valve group overheating caused by the blockage of the radiator is analyzed.This paper introduces the reliability theory,establishes the risk assessment model of valve group overheating,and puts forward the index to evaluate the risk of valve group overheating.(2)For the detection of scale on electrodes.Based on the thermal and electrical methods,the non-disassembly detection methods of scale thickness and electrode-water contact resistance are given.For online detection of radiator blockage,the correlation between radiator cooling capacity and valve group temperature distribution is deduced by thermoelectric analogy and Newton’s heat transfer law.According to the corresponding relationship between the blockage of the radiator and the cooling capacity,the blockage status of the radiator of the valve cooling system in operation is judged.(3)Scaling removal for the equalizing electrodes.An ultrasonic descaling platform for the equalizing electrodes of the converter valve was set up,and the descaling effect of ultrasound on the equalizing electrodes was explored,and the threshold value for the effect of ultrasound on the electrodes descaling was determined.Further,an online non-disassembly descaling method was designed,and the embracing scheme and ultrasonic cavitation were simulated to determine the availability of a non-disassembly ultrasonic descaling scheme. |
