Before designing DC gas insulated transmission lines (GIL)

the insulation characteristics of spacers under a lightning impulse (LI) voltage with a superimposed DC voltage must be clarified. Many experiments have been performed to guide the development of the DC spacer. However

the impacts of thermal stresses on their insulation performance under combined voltage are always neglected

and little attention has been given to the insulation characteristics of spacers in an eco-friendly gas filled DC-GIL. To solve these problems

an experiment setup with the abilities to load the thermal field and the DC voltage with LI voltage superimposed on the cone-type spacer are built. After that

the insulation characteristics of spacers under the combined voltage of DC with lightning impulse at 0.1 MPa SF

6

/N

2

gas mixtures are investigated

under different thermal stress conditions. The experiment's results indicate that: 1) The flashover voltage of spacers under the same polarity superimposed voltages is slightly higher than that under LI alone. Conversely

the flashover voltage under reverse polarity superimposed voltages is lower than that under LI alone. 2) The longe

r the duration of the foregoing DC voltage

the differences between the flashover voltage under superposed voltage and that under LI alone are larger. 3) The combined flashover voltages are reduced under thermal stress. To ensure the insulation performance of newly designed DC spacers

the insulation test is suggested to be applied under thermal stress and combined voltage of the DC with lightning impulse

especially for superimposed reverse polarity.