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| Study on the Influence of the Dynamic Film Resistance of Scale Layer on the Dynamic Deposition Characteristics of Grading Electrodes in Inner Cooling System of Converter Calve |
| Li Tong1, Liu Penglong2, Li Ningrui1, Kang Chao1, Lu Binxian2 |
1. State Grid Gansu Electric Power Company Extra High Voltage Company Lanzhou 730000 China;
2. School of Electrical and Electronic Engineering North China Electric Power University Beijing 102206 China |
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Abstract The failures of the cooling system in the converter valve is directly or indirectly caused by the problem of thegrading electrode deposition which accounts for more than 70% and has become an important factor that affects the safe and stable operation of the HVDC transmission system. The cooling medium boundary and the electrode reaction kineticsareaffectedby dynamic deposition of the scale layer on the surface of the grading electrode. The deposition reaction involves ohmic polarization, concentration polarization and electrochemical polarization process. The deposition reaction is complex one. However, most of the existing studies on the grading electrode deposition ignore the influence of turbulent field, mass transfer field and dynamic deposition of scaling ion in scale layer in the internal cooling system. A three-dimensional simulation model of the cooling system in a single valve segment is established, in which electric field, turbulence fluid field and mass transfer field are coupled. Not only is the dynamic growth mechanism of the grading electrode scale layer analyzed and predicted, but also are the corresponding fitting formulas obtained. They provide the basis for the maintenance and scale removal of the grading electrode in the converter valves.
Firstly, the potential difference of scale layer is introduced into the electrode kinetic equation, and the traditional Butler-Volmer equation is corrected. A dynamic film resistance model of scale layer is constructed to study the influence of dynamic scale layer deposition on the deposition reaction. The results show that the growth of scale layer can inhibit the deposition reaction. Secondly, the equivalent conductivity of the scale layer is determined by weighting conductivities of Al(OH)3 and deionized water. The maximum thickness of the grading electrode scale layer with the running time of 3 and a half years is 0.716 mm, which is 0.85% deviation from the statistical data of the scale layer in the actual converter station. It is 11.35% lower compared with the existing research. The simulation model is verified. Thirdly, not only are the growth rules of scale layer within 5 years predicted, including the change curves of scale layer thickness, deposition quality and deposition rate, but also are these curves fitted with a formula. The results show that the variation law of the scale layer average thickness and deposition mass are approximated with the zero-state response of the first-order RC circuit. Both forms are linear, and have the same time constant, their time constantsare 1 111 days. When the time reaches 3~5 τ, the deposition mass deposited on the grading electrode and the average thickness of the scale layer approximately reach the steady state. The deposition reaction on the grading electrode surface almost stops. The ultimate deposition mass is 377 mg, and the ultimate average thickness of the scale layer is 1.03 mm. However, the deposition rate decreased in a piecewise single-exponential pattern, with the decay time constant τ=36 d between 0~150 days and τ =406 d after 150 days. Finally, the electrolyte potential of the scale layer surface isonly decreased by 2.5%. The total leakage current in the branchpipe is only increased by 1.53% with the running time of 5 years,indicating that the increase of the scale layer thickness on the surface of the grading electrode hasa weak impact on the voltage equalization effect of the grading electrode.
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Received: 22 November 2021
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2023, Vol. 38 
