Electric Field Grading and Discharge Inception Voltage Improvement on HVDC GIS/GIL Spacer With Permittivity and Conductivity Graded Materials (ε/σ-FGM)

Functionally graded materials (FGM) application with graded permittivity and conductivity is promising as an effective technique for electric field relaxation in SF6 gas around HVDC gas insulated switchgear (GIS)/gas insulated transmission line (GIL) spacers. To approach the practical application of FGM to HVDC GIS/GIL spacers, this article investigates the electric field reduction effect given by permittivity and conductivity graded materials ( $\varepsilon /\sigma $ -FGM) based on actual measured permittivity ( $\varepsilon $ ) characteristics of SrTiO3-filled epoxy composites and conductivity ( $\sigma $ ) characteristics of SiC-filled epoxy composites. In addition, theoretical discharge inception voltage (TDIV) of $\varepsilon /\sigma $ -FGM spacer is calculated under standard lightning impulse (LI) voltage based on the volume–time theory. The results show that the $\varepsilon /\sigma $ -FGM spacer with grading to lower permittivity (GLP) ( $\varepsilon _{r}$ from 12.7 to 4) containing 0–26.9-vol% SrTiO3-filled epoxy composite and U-shaped graded conductivity containing 5–10-vol% SiC-filled epoxy composite is effective for electric field relaxation under DC steady state (DC-SS) and LI voltage where both resistive and capacitive fields present. It is attributed to the higher $\varepsilon $ and $\sigma $ of FGM spacer near the HV conductor/spacer interface. TDIV under LI voltage is also estimated to be 26% higher at 0.5 MPa-abs, compared with the conventional spacer without $\varepsilon /\sigma $ grading.