Electronic instrument transformers and merging units are important components of a digital substation. Compared with conventional instrument transformers， electronic instrument transformers have many advantages such as simplified insulation structure， wide dynamic range， excellent transient characteristic， reductions in size and weight， and digital output etc. The structures and characteristics of an active electronic transformer and a passive electronic transformer， and the function and application of a merging unit are introduced. A new opinion of how to dispose electronic transformers and merging units in a digital substation is put forward.