  Functionally graded materials (FGM) are applied in HVDC gas insulated lines
(GIL) to control the electric field within the DC insulation system. In HVDC
GIL, FGM with a spatial distribution of the electric conductivity
(conductivity-FGM) is applied to control the electric field under DC steady
state condition. However, besides DC steady state, different DC conditions
occur, e.g. DC-on process, polarity reversal and lightning impulse. Under these
conditions conductivity-FGM is not sufficient to control the electric field,
since these conditions result in transient capacitive fields, where the
permittivity is decisive for the electric field. In this paper, we suggest
combining conductivity-FGM and a spatial distribution of permittivity
(permittivity-FGM) in the spacer material to control the electric field around
DC-GIL spacer for various DC-conditions, considering nonlinear material models
for the insulating gas and the epoxy spacer. A variation of the spatial
distribution of permittivity and conductivity in the spacer is investigated in
this paper for an effective field reduction. The results show a reduction of
the electric field intensity up to 65.8 %, when conductivity/permittivity-FGM
is applied.
