Answer / raghavendran.r

In percentage differential relays, the current from each
current transformer flows through a restraint coil. The
purpose of the restraint coil is to prevent undesired relay
operation due to current transformer errors. The operating
coil current | i1 - i2 | required for tripping is a
percentage of the average current through the restraint
coils.

i1-i2 >=k(i1+i2)/2= k*iavg

where k is the proportion of the operating coil current to
the restraint coil current. For example if k = 0.1, the
operating coil current must be more than 10% of the average
restraint coil current in order for the relay to operate.

When we change the tap position of 10MVA transformer it suddenly tripped with Out of step indication and also with Static differential relay indication, Transformer Bucholz relay indn and Low oil level indication. Before 1 week the same problem occured by changing the tap position. but at that time only oltc bucholz relay activated. After testing the TxR Relay team & PET team cant find any solid reason. The values are ok also. So plz answer wat's the reason for this incident? anybody plz help.

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