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Composite insulators in valve tower of UHVDC converter station withstand combined ac-dc voltage. The flashover performance of silicone rubber under combined voltage is investigated in this paper. The voltages on the post insulator of valve tower under different operating conditions are simulated in PSCAD/EMTDC, and the harmonic components of the voltages are analyzed. The simulation results indicate that the ac frequencies of the voltage on the post insulator are mainly 50 Hz, 3n and 6n±1 times harmonics. The flashover voltages of silicone rubber under different combinations of ac and dc voltages are tested, including different single frequency ac voltages superimposed on different dc voltages, and different combined harmonic voltages superimposed on different dc voltages. The test results indicate that different ratios of dc voltage to ac voltage and different percentages of harmonics have various influences on the flashover voltage of silicone rubber. Moreover, argon atmosphere pressure plasma jet (Ar APPJ) is applied to modify the surface of the silicone rubber under different modification times and voltages. The flashover voltages and the surface morphologies of the silicone rubber before and after surface modification are compared. Surface modification by Ar APPJ can improve the surface resistivity of the silicone rubber significantly, and the flashover voltage also increases. [ABSTRACT FROM AUTHOR]

Thyristors of valve tower in converter station are controlled through fiber. Valve fiber withstands strong electric field and harmonic voltage. Partial discharge (PD) and flashover faults sometimes happen in valve tower. In this paper, flashover test is conducted for different fiber sheath materials under different voltage types (DC voltage and harmonic voltage superimposed on DC voltage). PD characteristics of fiber under combined voltage of AC voltage and DC voltage are measured. And influence of sheath materials on the PD inception voltage is discussed. Test results indicate that the flashover voltages of fiber sheath materials are different under different voltage types. Sheath material with higher surface resistivity has higher flashover voltage under DC voltage. Moreover, PD of fiber under vertical electric field concentrates on the falling edge of the positive half-circle of the AC voltage, and the PD strengthens with the increase of DC voltage. And the sheath material whose ratio of volume resistivity to relative dielectric constant is approximate to that of the fiber body (SiO2), has higher PD inception voltage. [ABSTRACT FROM AUTHOR]