Negative Sequence Polarisation - GE P4A Technical Manual

Chapter 9 - Current Protection Functions
Small levels of residual voltage could be present under normal system conditions due to system imbalances, VT
inaccuracies, device tolerances etc. For this reason, the device includes a user settable threshold (IN> VNPol set),
which must be exceeded in order for the DEF function to become operational. The residual voltage measurement
provided in the MEASUREMENTS 1 column of the menu may assist in determining the required threshold setting
during the commissioning stage, as this will indicate the level of standing residual voltage present.
Note:
Residual voltage is nominally 180° out of phase with residual current. Consequently, the DEF elements are polarised from the
"-Vres" quantity. This 180° phase shift is automatically introduced within the device.
The directional criteria with residual voltage polarisation is given below:
● Directional forward: -90° < (angle(IN) - angle(VN + 180°) - RCA) < 90°
Directional reverse : -90° > (angle(IN) - angle(VN + 180°) - RCA) > 90°
●
4.3.2

NEGATIVE SEQUENCE POLARISATION

In some applications, the use of residual voltage polarisation may be not possible to achieve, or at the very least,
problematic. For example, a suitable type of VT may be unavailable, or an HV/EHV parallel line application may
present problems with zero sequence mutual coupling.
In such situations, the problem may be solved by using Negative Phase Sequence (NPS) quantities for polarisation.
This method determines the fault direction by comparing the NPS voltage with the NPS current. The operating
quantity, however, is still residual current.
This can be used for both the derived and measured standard earth fault elements. It requires a suitable voltage
and current threshold to be set in cells IN> V2pol set and IN> I2pol set respectively.
Negative phase sequence polarising is not recommended for impedance earthed systems regardless of the type
of VT feeding the relay. This is due to the reduced earth fault current limiting the voltage drop across the negative
sequence source impedance to negligible levels. If this voltage is less than 0.5 volts the device will stop providing
directionalisation.
The directional criteria with negative sequence polarisation is given below:
● Directional forward: -90° < (angle(I2) - angle(V2 + 180°) - RCA) < 90°
Directional reverse : -90° > (angle(I2) - angle(V2 + 180°) - RCA) > 90°
●
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P54A/B/C/E
P54xMED-TM-EN-1