EARTHING Transformers

The Earthing Transformers is a special electrical equipment that physically creates the star center of the MV system to connect it to ground directly or through an appropriate impedance (Petersen coil). The structure of the Earthing Transformers is similar to a three-phase transformer in which two identical and concentric windings, instead of being galvanically separated to form primary and secondary, are connected to each other in the typical zig-zag connection, i.e. two windings, a zig and a zag , of cyclically successive columns are connected in opposite series forming a phase; the three phases are star connected. If required, the Earthing Transformers can be equipped with an auxiliary low voltage winding to power any loads present in the substations.

Earthing in electrical power systems plays a very important role as reliability, the ability to withstand fault currents and overvoltages depend on the characteristics of the neutral earthing. The main applications of Earthing Transformers are:

• Earthing the electrical system
• Limit the magnitudes of transient overvoltages when restriking earth faults occur
• Limit the current line to earth faults
• In phase-to-ground faults, limit the current value
• Allow the circulation of unbalanced load current in the neutral
• Allow the connection of phase to neutral loads when desired