Investigation of Fracture Mechanism and Performance Optimization of Grading Electrode Applied in HVDC System

Grading electrode is widely used to the converter valve in the high voltage direct current (HVDC) system for balancing the voltage between the components and protecting components from electrolytic corrosion. The grading electrode is made of pure Pt in a needle shape. Recently, some Pt electrode needles fractured during service which should be paid more attention. Multiple fracture causes have been analyzed utilizing a combination methods of chemical composition analysis, mechanical property test, fracture surface observation and microstructure observation. It is demonstrated that the failure mechanism of the Pt needle is fatigue fracture. This mainly due to the low shear strength of the Pt needle, which is around 140MPa. In order to increase the safety and reliability of grading electrode during service, the processing technology of the Pt needles has been optimized. The microstructure of the Pt needle has been improved and the shear strength has been increased to 297MPa.