Updated: Feb 16, 2021
Why Grounding/ Earthing transformer required?
In a delta connected alternator and transformer neutral point is not available. To create neutral point grounding or earthing transformer is used. The grounding or earthing transformer provides a source for zero sequence current, also the grounding transformer prevents the rise of line voltage of healthy phase during phase to earth fault. The zero sequence current flows through the grounding transformer in case of earth fault. Therefore, the grounding transformer must be capable to handle the earth fault current of the system. The grounding of the system can be done by two methods.
By using of delta-star grounding transformer
By using Zig-Zag grounding transformer
Delta - Star Grounding Transformer
In delta star transformer, the delta winding must be closed to provide the path of zero sequence current. The star winding voltage must be equal to the system voltage. The grounding transformer must capable to handle the earth fault current and the unbalance current in the load side.
Zig - Zag Grounding Transformer
The Zig-Zag transformer is used to provide the neutral point.
The VA rating of the Zig-zag transformer is very less as compared to VA rating of the delta star grounding transformer. The Zig-Zag transformer have only primary winding, and it do not have secondary winding. The primary winding is divided into two paths with the same number of turns, and both the winding in each phase is connected in such a way that current flowing in one part is in opposite direction of other part. Thus the resultant MMF produced in any of the core limb is zero under normal condition.
During line to ground fault, heavy fault flow through the grounding transformer and the fault current is supplied by the power source. A high resistance must be connected to neutral point of the grounding transformer to ground in order to limit the fault current.
The neutral point voltage can be monitored and connected to protection relay to timely trip the circuit under earth fault or unbalance load condition to avoid heavy stress on the grounding transformer.