The Mechanism Research On The Corrosion Of The Aluminum Heat Sink And The Deposition Of The Grading Electrode

The corrosion of aluminum heat sinks(AHSs)and the deposition of grading electrodes(GEs)in converter valve cooling system have been threatening the safe and stable operation of HVDC transmission system.Although many domestic and foreign scholars have conducted much research on this problem and put forward several suppression measures,the problems of corrosion and deposition have not been exhaustively solved,and the existing corrosion mechanism of AHSs and deposition mechanism of GEs are not completely clear.Therefore,based on the parallel cooling system of a single valve segment in the±500 kV HVDC transmission system,it is of great significance to study the specific mechanism of AHSs corrosion and GEs deposition,and then propose effective suppression measures,which is to effectively solve the safety risks of cooling system and improve the reliability and safety of the operation of the HVDC transmission system.Firstly,according to the substitution theorem,the equivalent circuit of the parallel cooling system of a single valve segment is established.And combined with the principle of REDOX reaction,it is found that the leakage current in the branch pipes is the root cause of the corrosion of AHSs.By establishing the Thevenin equivalent circuit,the distribution law of leakage current flowing into(or out)AHSs through the branch pipes is visually analyzed.And the reaction principles of AHSs corrosion and GEs deposition are analyzed in detail by combining the electrochemical theory.Next,the 3-D finite element quasi-static electric field simulation model and the multi-electrode conductance system analysis model of the single valve segment parallel cooling system are established,the conductance matrix of the cooling system is solved,and the accuracy of the calculation method and results is verified by using the theory of improved nodal method and comparing the simulation calculation results with the experimental results in literatures.The electric field distribution and leakage current distribution of the cooling system are calculated and analyzed accurately by the electric field simulation model.Combined with the conductance matrix of the cooling system,it is concluded that the concentrated distribution of the electric field near the GE is the root cause of the leakage current in the branch pipes,which in turn causes the corrosion of AHSs.By changing the parameters of the electric field simulation model,not only the factors and laws affecting the leakage current flowing through the AHS are analyzed,but also a new multi-electrode installation scheme is proposed,which effectively inhibit the corrosion of the AHSs from the root and the inhibition efficiency reached 98.46%.Finally,the 3-D finite element simulation model of coupled electric field,fluid field and mass transfer field is established.By modifying the traditional electrode dynamic equation,a dynamic film resistance model of scale layer is introduced.The equivalent conductivity of scale layer is determined by weighting method.The calculation error of scale layer is only 0.85%compared with the statistical data of scale layer measured at the converter station.The dynamic growth laws of the scale layer on the surface of the GE are analyzed comprehensively.And the dynamic deposition index analysis model of scale layer is proposed.The deposition time constant is 1111 days,the ultimate deposition mass is 377 mg,and the ultimate average thickness of the scale layer is 1.03 mm.The influence of the growth of scale layer on the equalizing voltage effect of the GEs and the corrosion of AHSs is quantitatively analyzed,which provides a reliable basis for the growth prediction of the scale layer and the maintenance and descaling work.