A MODIFICATION OF THE RATIO RELAY TO MAKE A DIRECTIONAL RELAY

MODIFICATION

THE CHARACTERISTIC CIRCLE CENTRED AT THE ORIGIN CAN BE OFFSET BY AN AMOUNT Z^’

Z - Z^’  < |Z_r| TRIP 

Z - Z^’  > |Z_r| BLOCK

BY MAKING | Z^’| =  | Z_r| THE

CHARACTERISTIC CAN BE MADE TO PASS THROUGH THE ORIGIN.

THIS IS KNOWN AS THE MHO CHARACTERISTIC

A FAULT TO THE LEFT OF THE BUS 1 NO MATTER HOW CLOSE TO THE BUS WILL NOT RESULT IN A TRIP DECISION

BECAUSE

Z = V₁/I₁₂

AND WILL LIE IN THE THIRD QUARDRANT.

ZONE OF A RELAY INCLUDES ALL OF THE POWER SYSTEM THAT IS SITUATED IN ONLY ONE DIRECTION FROM THE RELAY LOCATION

R₂₁ OPERATES FOR FAULT LEFT OF ITS LOCATION.

REMEMBER THAT THE ELECTRICAL POWER LINE IS PREDOMINANTLY A REACTIVE LINE

FAULT LEFT OF R₂₁  FAULT CURRENT FROM 2 TO 1 TO LEAD VOLTAGE AT BUS 2 BY 90^o

THE OPERATION:

DEPENDS ON THE DIRECTION OF CURRENT WITH RESPECT TO THE VOLTAGE.

THE QUANTITY THAT PROVIDE THE REFERENCE PHASOR IS CALLED A POLARIZING QUANTITY.

IN THIS CASE A POLARIZING VOLTAGE.

SOMETIMES, CURRENT IS USED.

Θ_min > θ_op > θ_max                             TRIP

Θ_min < θ_op < θ_max                             BLOCK

θ_op  : PHASE ANGLE OF OPERATING QUANTITY MEASURED WITH THE POLARIZING PHASOR AS REFERENCE

Θ_min , θ_max :TWO ANGLES DEFINING THE BOUNDRY OF THE OPERATING CHARACTERISTICS.

DIRECTIONAL RELAY

 

RELAY PERFORMANCE CHARACTERISTICS IN THE COMPLEX PLANE OF THE PHASOR DIAGRAM IS DIVIDED.

THE SHADED PART WILL TRIP

BACKUP PROTECTION