P54x/EN OP/La4 Operation
(OP) 5-80
MiCOM P543, P544, P545 & P546
OP
Curve description Standard S constant
Moderately Inverse IEEE 4.85
Very Inverse IEEE 21.6
Extremely Inverse IEEE 29.1
Inverse US 5.95
Short Time Inverse US 2.261
1.25.2 Directional overcurrent protection
The phase fault elements of the MiCOM P54x relays are internally polarized by the
quadrature phase-phase voltages, as shown in the table below:
Phase of protection Operate current Polarizing voltage
A Phase
A
VBC
B Phase
B
VCA
C Phase
C
VAB
Under system fault conditions, the fault current vector will lag its nominal phase voltage by
an angle dependent upon the system X/R ratio. It is therefore a requirement that the relay
operates with maximum sensitivity for currents lying in this region. This is achieved by
means of the relay characteristic angle (RCA) setting; this defines the angle by which the
current applied to the relay must be displaced from the voltage applied to the relay to obtain
maximum relay sensitivity. This is set in cell >Char Angle in the overcurrent menu. On the
MiCOM P54x relays, it is possible to set characteristic angles anywhere in the range –95° to
+95°.
The functional logic block diagram for directional overcurrent is shown overleaf.
The overcurrent block is a level detector that detects that the current magnitude is above the
threshold and together with the respective polarizing voltage, a directional check is
performed based on the following criteria:
Directional forward
-90° < (angle(I) - angle(V) - RCA) < 90°
Directional reverse
-90° > (angle(I) - angle(V) - RCA) > 90°
To Next Page
Loading...
