assigning excitation to non planar surfaces

I am trying to assign voltage difference between two plates of a spherical capacitor using HFSS 13.inner metal is a sphere from copper and outer one is a shell from copper and between the plates is filled with teflon. Since my geometry is not planar I can not assign excitation to it, so I defined a circular plane between metals and assigned voltage to this plane and defined the vector in x direction. The problem with assigning voltage is that after simulation, EF is not radial and it is in x direction everywhere on the plane. I also tried lumped port on that plane and it gives me radial EF but just on the plane. EF pattern on other planes or in the volume is wrong. How can I tell HFSS that this excitation is for the whole sphere not just this plane?
Is there any way to assign voltage to a non planar surface?
Thanks for your help.