Alstom MiCOM P546 - 1.29 Directional earth fault protection; 1.30 Residual voltage polarization

Operation P54x/EN OP/La4
MiCOM P543, P544, P545 & P546 (OP) 5-85
OP
0
1
2
3
4
5
6
7
8
9
10
1 10 100
I/IN>
Operating time (seconds)
IDG Is Setting Range
IDG Time Setting Range
P2242ENa
Figure 56 IDG characteristic
1.29 Directional earth fault protection
As state
d in the previous sections, each of the four stages of earth fault protection may be
set to directional if required. Consequently, as with the application of directional overcurrent
protection, a suitable voltage supply is required by the relay to provide the necessary
polarization. Two options are available for polarization; Residual Voltage or Negative
Sequence.
1.30 Residual voltage polarization
With e
arth fault protection, the polarizing signal requires to be representative of the earth
fault condition. As residual voltage is generated during earth fault conditions, this quantity is
commonly used to polarize DEF elements. The relay internally derives this voltage from the
3 phase voltage input which must be supplied from either a 5-limb or three single phase VTs.
These types of VT design allow the passage of residual flux and consequently permit the
relay to derive the required residual voltage. In addition, the primary star point of the VT
must be earthed. A three-limb VT has no path for residual flux and is therefore unsuitable to
supply the relay.
Note: Residual voltage is nominally 180 out of phase with residual current.
Consequently, the DEF relays are polarized from the '-Vres' quantity.
This 180 phase shift is automatically introduced within the relay.
The directional criteria with zero sequence (residual voltage) polarization are given below:
Directional forward
-90
o
< (angle(IN) - angle(VN+180
o
) - RCA) < 90
o
Directional reverse
-90
o
> (angle(IN) - angle(VN+180
o
) - RCA) > 90
o
The virtual current polarizing feature is not available for use with the backup earth fault
elements - that is used exclusively in DEF aided schemes only.
The logic diagram for directional earth fault overcurrent with neutral voltage polarization is
shown overleaf.