Grounding transformers are designed to protect a system against phase-earth fault current for a given fault time duration. There transformers are used to get a neutral point and provide earthing for non grounding systems. These transformers are commonly “inter-star” or even known as Zig-Zag connected transformers.

To specify a zig-zag transformer, the following information should be considered

Provide the line to line voltage of the system.
Provide the magnitude of the neutral current.
Provide how long the neutral current will flow.
Provide the system impedance per phase.