Grounding Transformers

Grounding transformers are used to derive a neutral for three phase four wire operation when the neutral of the system is not otherwise available.
With balanced three phase loads, the grounding transformer carries only magnetizing current, but with unbalanced 3 phase or single phase loads, the unbalanced current in the neutral will divide equally in the three phases of the transformer, one third flowing in each leg.

Traditionally, Wye-Delta connected transformers have been used for grounding transformer applications, however Zig-Zag connected transformers have become more common, and are generally better suited for grounding applications.

Grounding Transformers are generally used for the following purposes:

• Permitting the connection of L-N connected single phase loads
• Providing a low impedance path for triplen harmonics (3rd, 6th, 9th, etc..) when non-linear loads are present to reduce THD on supply side.
• Providing a means for detecting and limiting L-G faults in an ungrounded system
• Provide a relatively low-impedance path to ground, thereby maintaining the system neutral at or near ground potential