how to connect a single-phase single-bushing power transformer ( 220 volt secondary) to 13.8 kv power line?

Answer 1

Overhead distribution transformers with only one HV bushing use the tank ground as the H2 conection. These transformers can only be used on a multi-grounded circuit.

You connect the H1 bushing to one of the phase conductors -- generally through a cutout fuse assembly. The case ground gets connected to the system neutral and to a driven ground at the pole location.