Zero Sequence Polarizing - GE MiCOM P40 Technical Manual

P446SV
5.3.1

ZERO SEQUENCE POLARIZING

Residual voltage is generated during earth faults. This zero-sequence quantity can be used to polarize the
directional decision of Aided DEF protection. This device can derive residual voltage if connected to a suitable
voltage transformer (VT) arrangement.
For the Aided DEF to use a derived voltage signal, the three phase voltage inputs must be supplied from a 5-limb
VT, or from three single-phase VTs having the primary star-point earthed. These VT arrangements allow the
passage of residual flux and hence allow the device to derive the required residual voltage. A 3-limb VT has no
path for residual flux and is therefore unsuitable for zero-sequence Polarization.
Small levels of residual voltage may be present under normal system conditions due to system imbalances, VT
inaccuracies and device tolerances. You can set a threshold (DEF VNPol Set) to provide stability against Aided DEF
operation under these conditions. Residual voltage (Vres) is nominally 180° out of phase with residual current so
Aided DEF elements are polarized from the -Vres quantity. This 180° phase shift is automatically compensated in
this device.
For products designed to control dual circuit breaker applications you can choose to re-allocate the second check-
synchronism VT to provide a measured voltage input to the Aided DEF function. To do this you need to set VT2
Selection to Broken Delta in the CT AND VT RATIOS COLUMN. This then reveals the DEF Vnpol Input setting in
the AIDED DEF column which can be set to Measured or Derived.
5.3.1.1 VIRTUAL CURRENT POLARIZING
A technique called Virtual Current Polarizing can allow the Aided DEF protection to operate even if the Polarizing
voltage is below the DEF VNPol Set threshold. If the superimposed current phase selector associated with the
Distance protection has identified a faulted phase, for example phase A, it removes that phase voltage from the
residual calculation Va + Vb + Vc, leaving only Vb + Vc. The resultant Polarizing voltage has a large magnitude and
is in the same direction as –Vres. This allows the protection to be applied even where very solid earthing behind
the protection prevents residual voltage from being developed.
This technique of subtracting the faulted phase is described as Virtual Current Polarizing because it removes the
need to use current Polarizing from a current transformer (CT) in a transformer star (wye)-earth connection behind
the device.
If you don't want to use this feature you should set Virtual I Pol to Disabled so that removal of the faulted
phase voltage from the residual voltage calculation does not happen. The Aided DEF protection is then Polarized
by the residual voltage only.
The directional criteria with zero sequence (virtual current) Polarization are as follows:
Directional forward
90° < (angle(VNpol) +180°) - (angle(IN )- RCA) < 90°
Directional reverse
90° > (angle(VNpol) +180° - (angle(IN) - RCA) > 90°
This is represented in the following figure:
P446SV-TM-EN-1
Chapter 8 - Carrier Aided Schemes
185