Alstom MiCOM P54 Series - 1.28 Earth fault (ground overcurrent) and sensitive earth fault (SEF) protection

Operation P54x/EN OP/La4
MiCOM P543, P544, P545 & P546 (OP) 5-83
OP
Overall, the dual time constant characteristic provided within the relay serves to protect the
winding insulation from ageing, and to minimize gas production by overheated oil.
Note: The thermal model does not compensate for the effects of ambient
temperature change.
The thermal curve is defined as:
0.4e
(-t/)
+ 0.6e
(-t/)
=

2
- (k.FLC)
2

2
- p
2
Where:
1 = Heating and cooling time constant of the transformer windings;
2 = Heating and cooling time constant for the insulating oil.
In practice, it is difficult to solve this equation to give the operating time (t), therefore a
graphical solution, using a spreadsheet package, is recommended. The spreadsheet can be
arranged to calculate the current that will give a chosen operating time. The equation to
calculate the current is defined as:
 =
0.4p
2
.e
(-t/1)
+ 0.6p
2
.e
(-t/2)
-k
2
.
FLC
2
0.4 e
(-t/1)
+ 0.6 e
(-t/2)
-1
…….. Equation 1
Current IA
Current IB
Current IC
Reset
Thermal State
Measurerment
1
Thermal
Trip
Thermal
Characteristic
Thermal State
Measurement
Thermal Alarm
Thermal Trip
Start
P1629ENa
Figure 55 Thermal overload protection logic diagram
The functional block diagram for the thermal overload protection is shown in Figure 55.
The m
agnitudes of the three phase input currents are compared and the largest magnitude
taken as the input to the thermal overload function. If this current exceeds the thermal trip
threshold setting a start condition is asserted.
1.28 Earth fault (ground overcurrent) and sensitive earth fault (SEF) protection
The P54x re
lays include backup earth fault protection. Two elements are available; a
derived earth fault element (where the residual current to operate the element is derived
from the addition of the three line CT currents) and a sensitive earth fault element where low
current settings are required. The sensitive earth fault element has a separate CT input and
would normally be connected to a core balance CT. The derived and sensitive earth fault
elements both have four stages of protection. The first two stages can be set either inverse
time or definite time only. The third and fourth stages have a DT characteristic only. Each
stage can be configured to be directional forward, directional reverse or non-directional.