Grounding transformers are used to obtain a neutral when an existing delta-connected or ungrounded systems are to be grounded. These transformers provide a path for the flow of fault currents during unbalanced ground faults. They can be found in transformer substations where neutral might not be available as the transformer windings are delta connected.
Types of grounding transformers
Grounding transformers are of two types: zig-zag type and wye-delta (star-delta) type. The neutral obtained from either type can be grounded directly or through a current limiting impedance.
Zig-zag transformer
A zig-zag transformer is a three-phase grounding transformer with no secondary winding. They are also known as interconnected star transformers. The transformer neutral may be earthed through an impedance. It does not have a secondary winding. Should there be a secondary winding, it shall be treated as an ordinary transformer. The schematic diagram of a zig-zag transformer is shown below:
Each phase winding is split into halves and interconnected such that the winding connection produces a phase shift of 300. Hence an equal and opposite current flows through the winding halves wound on the same core. Therefore the vector sum of MMF produced by the windings will be zero under normal conditions.
The impedance of the transformer to the three phase voltage is high such that only the magnetizing current flows through it under normal conditions (when there is no fault in the system). However the impedance of the transformer to the ground fault currents is negligible.
During ground faults, the transformer acts in the similar way to any neutral grounded system. Under normal conditions, the flowing through the windings of each leg cancel the flux produced by the other winding on the same leg since thery are equal and opposite. Hence, on the event of ground faults, this zero net flux forces the zero sequence current to divide itself into equally through the three phase windings and accounts for the low impedance of the transformer-to-ground currents.
A resistance or impedance can be inserted between the neutral point and the ground in order to attain desired method of grounding. The short time kVA rating of a zig-zag transformer is product of the line-to-ground voltage and the rated neutral current. They are designed to carry the fault current for upto 60 seconds. A low resistance grounding using a zig-zag transformer with 51G (overcurrent relay) to sense the ground fault current is shown below:
Wye-delta (star-delta) grounding transformer
Another method to obtain neutral for grounding is using a star-delta transformer. The star winding terminals are connected to the three phases of the network and the delta winding is left unconnected. Also, the delta connection must be closed to provide a path for zero-sequence current on the event of ground fault.
The voltage rating of the star winding is need not exceed the three phase system voltage. An impedance can be inserted betweent the neutral and the ground to limit the fault currents. Schematic of a low resistance grounding using a star-delta transformer with 51G (overcurrent relay) to sense the ground fault current is shown below:
For high resistance grounding, it is economical economical to connect impedance as shown below in order to reduce the cost. Three single phase transformers (transformer bank) are used in this case, with their primaries connected in wye and secondary in delta through a high resistor to limit the fault current within the desired low levels.
Wye-delta or star-delta grounding transformer provides a low impedance path to the ground fault currents due the circulating path offered by the delta winding. In a normal system it behaves like an open circuited tranformer drawing a magnetizing current from the network.
View technical specifications for grounding transformers at Swedish neutral.
The notes are elaborate and simplified.