RATIO RELAY

CONSIDER RELAY R12 WHICH OPERATES WITHIN A GIVEN DISTANCE FOR ANY FAULT FROM BUS 1

THE DISTANCE ALONG THE LINE OR THE IMPEDANCE BETWEEN BUS 1 AND THE FAULT DESCRIBED THE VICINITY

A ZONE OF PROTECTION IN THIS CASE IS DESCRIBED BY THE LENGTH OF LINE FROM THE BUS1 SUCH THAT THE IMPEDANCE IS LESS THAN A SPECIFIED SETTING |Z_r|

THIS HAS DIMENSION OF IMPEDANCE

Z = V₁/I₁₂ 

PERFORMANCE SPECIFIED BY

|Z|  <  |Z_r| TRIP

|Z|  >   |z_r| BLOCK

THIS IS REFERRED TO AS AN IMPEDANCE OR DISTANCE RELAY.

IMPEDANCE RELAY CHARACTERISTICS SHOWING ZONE OF PROTECTION FOR R₁₂

DURING NORMAL OPERATION, THE RATIO IS A COMPLEX NUMBER WITH ARBITRARY PHASE ANGLE DETERMINED BY THE POWER FACTOR.

OPERATION:

WHEN NO FAULT, I (CURRENT IN THE SYSTEM) IS SMALL HENCE Z IS LARGE; WITH AN ARBITRARY ANGLE.

WHEN A FAULT OCCURS, Z APPEARS AS A LOAD WHOSE IMPEDANCE IS THAT OF THE LINE BETWEEN THE RELAY LOCATION AND THE FAULT. IN THIS CASE THE CURRENT I IS A FAULT CURRENT.

THE ANGLE OF Z IS θ OR π+θ DEPENDING UPON IF THE FAULT IS TO THE RIGHT OR LEFT OF BUS 1.

IMPEDANCE RELAY CHARACTERISTICS SHOWING ZONE OF PROTECTION FOR R12

 

Z = V₁/I₁₂

COMPLEX PLAIN IN WHICH MEASURED IMPEDANCE IS PLOTTED FOR A NON – DIRECTIONAL RELAY.