P54x/EN OP/La4 Operation
(OP) 5-54
MiCOM P543, P544, P545 & P546
OP
1.22.1 DEF zero sequence polarization with “virtual current polarizing” (Distance option only)
With earth fault protection, the polarizing (directional reference) 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 the directional decision of 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.
It is possible that small levels of residual voltage will be present under normal system
conditions due to system imbalances, VT inaccuracies, relay tolerances etc. Hence, the
relay includes a user settable threshold (DEF VNPol Set) which must be exceeded in order
for the DEF function to be operational. Note that 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.
A distinct advantage of the MiCOM P54x is that the relay can trip by this method of
polarizing, even if VNpol is less than the set threshold. Provided that the superimposed
current phase selector has identified the faulted phase (suppose phase A), it will remove that
phase from the residual calculation Va + Vb + Vc, leaving only Vb + Vc. The resultant
polarizing voltage will have a large magnitude, and will be in the same direction as –Vres.
This allows the relay to be applied even where very solid earthing behind the relay prevents
residual voltage from being developed.
This technique of subtracting the faulted phase is given the description “virtual current
polarizing” as it removes the need to use current polarizing from a CT in a transformer star
(wye)-ground connection behind the relay. This would have been necessary with traditional
relays.
The directional criteria with zero sequence (virtual current) polarization are given below:
Directional forward
-90 < (angle(IN) - angle(VNpol+180) - RCA) < 90
Directional reverse
-90 > (angle(IN) - angle(VNpol+180) - RCA) > 90
Where VNpol is as per the table below:
Phase selector pickup VNpol
A Phase Fault VB + VC
B Phase Fault VA + VC
C Phase Fault VA + VB
No Selection VN = VA + VB + VC
1.22.2 DEF negative sequence polarization (Distance option only)
In certain applications, the use of residual voltage polarization of DEF may either be not
possible to achieve, or problematic. An example of the former case would be where a
suitable type of VT was unavailable, for example if only a three-limb VT was fitted. An
example of the latter case would be an HV/EHV parallel line application where problems with
zero sequence mutual coupling may exist.
In either of these situations, the problem may be solved by the use of negative phase
sequence (nps) quantities for polarization. This method determines the fault direction by
comparison of nps voltage with nps current. The operate quantity, however, is still residual
current. It requires a suitable voltage and current threshold to be set in cells DEF V2pol Set
and DEF 2pol Set, respectively.
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