Answer / vinod g

In Restricted earth fault protection Three phase CT is
parralleling (shorting all S1 andshorting all S2 together)
and paralleel conection with Neutral Relay and CT. IN
Healthy conditon current in all the three phase will be
balanced and summation will be zero. During earth fault out
side CT area summation of phase CT currents and Neutral CT
zero, so relay will not pick up. When earth fault inside
the transformer the current in the CT will differ and Relay
will pick and trip the tranformer breaker,so it is clear
that REF is for Transformer Winding protection and Bushing
CT is normally using for phase CT

Answer / pintu

THIS IS CALLED 5 CT CONFIGURATION (3 PH. CT + 1 X'MER NCT +
1 BB N CT)OF REF PROTECTION. ALL THESE FIVE CTS ARE TO BE
CONNECTED IN PARALLEL. TRANSFORMER NCT SHOULD BE LOCATED
AFTER NEUTRAL BIFERCATION (I.E. PATH TOWARDS EARTH) AS THIS
IS DEDICTAED FOR DETECTING THE RESIDUAL CURRENT IN CASE OF
GROUND FAULT. BUT NEUTRAL BUSBAR CT IS NOT FOR DETECTING
THE FAULT CUURENT BUT FOR THE UNBALANCE CURRENT IN THE
SYSTEM. IT SHOULD NOT BE TRIPPED DURING THE EARTH FAULT.

IN 4 CTS CONFIGURATION OF REF PROTECTION, NEUTRAL BUSBAR CT
CONNECTION WILL NOT BE THERE. YOU SHOULD DECIDE THE 5 OR 4
CT CONFIGURATION AS PER YOUR SYSTEM DESIGN.

Answer / anup mahajan

The purpoae of Restricted earth fault (REF) is to provide
the protection of transformer neutral in case of residual
current. As the fault which lies near phases are known as
external faults and is to be detected by differential
protection but the residual current which is not going to
ground and circulates between neutral and phases are
detedted by REF protection. Please keep in mind that CT
characteristics of this REF must be matched with phase side
CT characteristics.

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