Grounding Transformer

The grounding transformer is used to provide a path to an ungrounded system or when the system neutral is not available for some reason, for example, when a system is delta connected. It provides a low impedance path to the neutral and also limits the transient overvoltage when the ground faults occur in the system. The grounding of the system can be done in the following way

• By using a delta-star grounding transformer.
• By using a zig-zag grounding transformer

Delta-Star Grounding Transformer

In a case of delta-star grounding transformer, the delta side is closed to provide a path for zero-sequence current. The star winding must be of the same voltage rating as the circuit that is to be grounded, whereas the delta voltage rating can be chosen to be any standard voltage level.

The selection of the type of grounding depends on the type of the system and its voltage levels. The following considerations are made for the selection of the grounding

• Transient overvoltage developed.
• The Magnitude of ground-fault current as a percentage of 3-phase fault current.
• Dip in line voltage due to fault conditions.

Generally, solid grounding is used for a low-voltage system up to 600V. For voltages up to 11KV resistance grounding is used.