Operation P54x/EN OP/La4
MiCOM P543, P544, P545 & P546
(OP) 5-13
OP
1. OPERATION OF INDIVIDUAL PROTECTION FUNCTIONS
The MiCOM P54x is a line protection relay that includes phase differential protection on a
per phase basis and optionally comprehensive full scheme distance protection. Each one of
these protection functions can be selected to work separately or simultaneously.
The distance protection can also be set to operate upon failure of the relay protection
communications. With the inclusion of Aided Directional Earth fault (DEF) the MiCOM P54x
is a fully comprehensive and versatile line protection relay.
The following sections detail the individual protection functions.
1.1 Phase differential characteristics
MiCOM P54
x calculates the difference between the currents entering and leaving a
protected zone. The protection operates when this difference exceeds a set threshold.
Differential currents can also be generated during external fault conditions due to CT
saturation. To provide stability for through fault conditions, the relay adopts a biasing
technique. This method effectively raises the setting of the relay in proportion to the value of
through fault current to prevent relay maloperation. Figure 1 shows the operating
cha
racteristics of the P54x phase differential element.
The differential current is calculated as the vector summation of the currents entering the
protected zone. The bias current is the average of the measured current at each line end.
It is found by the scalar sum of the current at each terminal, divided by two.
Each of these calculations is done on a phase by phase basis. The level of bias used for
each element is the highest of the three calculated for optimum stability.
P1001ENa
=
Operate
I
1
I
2
I
3
Restrain
Percentage
bias k2
Percentage
bias k1
I
s1
I
s2
I
bias
I
1
+
I
2
+
I
3
2
I
diff
I
1
+
I
2
+
I
3
=
Figure 1 Relay bias characteristic
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