Grounding Transformer Impedance

[newbie01]

Could someone please assist me with the following questions (providing references if possible).

(1) What are the limitations for specifying the impedance of a zig-zag grounding transformer. Can it be as much as 100%?

(2) What are the pros/cons of purchasing a transformer with a high impedance(approx 100%) vs using a transformer with a low impedance along with a ground resistor or reactor to obtain the same ground fault current?

 

[kingpb]

A zigzag is designed to only allow zero-sequence current to flow; positive and negative cannot. Since the resistance of the transformer is very low, the ground path is essentially reactance. If the zigzag overall impedance is too low for the desired fault level, additional resistance or reactance can be added to the ground, this will limit the current to the desired level.

Transformer impedance is typically limited to variations above or below a nominal design value based on the KVA or MVA of the transformer. You can select another impedance, but that will affect the voltage regulation and performance of the transformer. The greater the variation, the higher the cost. If you are wanting to limit ground fault current, then use the proper rated transformer, and simply add a ground resistor or reactor. Much, much cheaper, and would probably be the more accepted approach.