Had a message from a friend today who lives in Edgbaston and walked along Broad Street. He wrote
The sub-station to provide power to the tram was installed on the
foundations within Five Ways roundabout last weekend. As there are no
overhead wires at that point, I presume it must feed the short section in
Broad Street that is so-equipped via an underground cable, and my guess is
that it's really there to provide power to any future extension down Hagley
Road.