7/22/2019 Eddy Current & circulating current

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SHEATH/ARMOUR LOSSES

Sheath/armour losses are current-dependent losses and are generated by the inducedcurrents when load current flows in single core cable conductors. In the existing setup,

there are 12 300sq mm single core incomer cables whose sheath/armour has beenbonded and grounded both on the incomer side and panel side thereby resulting in aclosed loop for leakage current.

Due to the high current flowing in the main conductor there is a magnetic field createdwhich has induced voltages in the sheath/armour and hence there is a current flow inthese sheaths due to the transformer effect ( i.e.by the magnetic field of alternatingcurrent flowing in cable conductor which induces voltages in cable sheath or otherparallel conductors.)

This sheath/armour induced electromotive forces (emf) has generated two types of

losses i.e circulating current losses (leakage current) and eddy current losses whichhas caused the 450A leakage current and gland plate to heat up (72oC).

However to reduce circulating current in sheath/armour the following measures have to

be taken

1. Single core cables to be laid in trefoil formation. (Due to heating effect on oneanother the heat dissipation will be poor)

2. Sheath/armour special bonding Involves earthing the single core cablesheath/armour at one end only and insulating all other points of the

sheath/armour from the earth.

Under the current installed circumstances only option 2 was viable and practical. Eddy

currents can also be reduced by splitting the gland plate to cut the eddy current flow or

to use an alloy plate for entry of single core cables.