Hi,
It looks like my problem and your problem are same.
In fact I was going to start a thread about "internal waveport & PEC cap in HFSS" but as I saw your post, I thought it would be good to discuss here.
To answer some of your questions:
1) Yes, in this case it is internal waveport since it lies inside radiation box and simulation will not run. It will generate error message with red cross bubble. Therefore, I always adopt a technique while I have waveport in my antenna. I always restrict radiation box's boundary in the same plane in which plane waveport face lies.
2) I do not know what is PEC cap.
In one of my designs of planar antenna, I thought to use "wave port" inside radiation box and HFSS gives error message
If an internal wave port is desired,
please add a PEC cap to one side of the port surface.
(12:57:12 PM Dec 20, 2016)
Could you please tell me what is PEC cap? cap means what in this context?
3) No, for lumped port you do not need PEC cap fortunately.
4) Irrespective of the excitation, lumped port does not need PEC cap.
I am attaching some snap shots of lumped port vs waveport. Take a look. and please tell me more about PEC cap. I know nothing about it