Alok Behera

• Feb 12th, 2014

 

If the earthing transformer on the Delta Side is outsides the Zone of protection the Earth Fault(E/F)in the delta system outside Current Transformer(CT) locations would produce current distributions as shown which circulate within the differential CT secondaries and is kept out of operating coils.

Zig-Zag or inter connected star grounding transformer has normal magnetizing impedance of high value but for E/F, currents flow in windings of the same - core in such a manner that the ampere turn cancel and hence offer lower impedance.

In cases where the neutral point of three phase system is not accessible like the system connected to the delta connected side of a electrical power transformer, an artificial neutral point may be created with help of a zigzag connected earthing transformer.

zigzag star connected earthing transformer

This is a core type transformer with three limbs. Every phase winding in zigzag connection is divided into two equal halves. One half of which is wound on one limb and other half is wound on another limb of the core of transformer.

1st half of Red phase winding is wound on the 1st limb of the core and 2nd half of same Red phase is wound on 3rd limb.