Just thought I'd mention this for interest. I was using a cement mixer fed through a large 240 to 110 volt transformer. First thing each morning when the transformer is plugged in (with nothing connected to the 110v side of the transformer) the RCD trips. Reset it; it trips again. Third time lucky, it is OK. Then all works OK all day. Exactly the same at the start of each day. Three attempts required. It is as if the transformer has to "charge up" or "warm up" initially, otherwise the RCD trips. I can't really think of a mechanism to cause this. (except maybe a tiny bit of damp in the transformer, or similar)??
Is it an RCD or an RCBO If it's an RCBO it's probably tripping on the magnetic trip part of the curve due to the transformer inrush. If you put them on and off a few times the inrush does drop, due to the core remaining magnatised for a while after power down