7.4.1.1 DIRECTIONAL EARTH FAULT LOGIC WITH RESIDUAL VOLTAGE POLARISATION
V00744
VTS Slow BlockVTS Slow Block
IN1> BlockingIN1> Blocking
VTS Blocks IN>1VTS Blocks IN>1
IN1> VNpol SetIN1> VNpol Set
Directional
check
IN1> DIRECTIONALIN1> DIRECTIONAL
Low Curren t Th re sholdLow Curren t Th re shold
&
To EF logicTo EF logic
IN1>1 Char AngleIN1>1 Char Angle
IN1>1 Trip Ang leIN1>1 Trip Ang le
Note: This diagram shows the logic for IN1 (measured earth fault). The logic for
IN2 (derived earth fault) follows simila r p rinciples.
This diagram does not show all st ages. Ot her stages follow similar principles.
Note: This diagram shows the logic for IN1 (measured earth fault). The logic for
IN2 (derived earth fault) follows simila r p rinciples.
This diagram does not show all st ages. Ot her stages follow similar principles.
VNVN
IN1IN1
Figure 38: Directional EF logic with neutral voltage polarization (single stage)
V
oltage Transformer Supervision (VTS) selectively blocks the directional protection or causes it to revert to non-
directional operation. When selected to block the directional protection, VTS blocking is applied to the directional
checking which effectively blocks the Start outputs as well.
7.4.2 NEGATIVE SEQUENCE POLARISATION
In some applications, the use of residual voltage polarisation may be not possible to achieve, or at the very least,
pr
oblematic. 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
(Ð
V2 + 180°) + RCA - 90° + (180° - tripping angle)/2 <
Ð
I2 < (
Ð
V2 +180°) + RCA +90° - (180° -
tripping angle)/2
Directional reverse
(Ð
V2 + 180°) + RCA - 90° - (180° - tripping angle)/2 >
Ð
I2 > (
Ð
V2 +180°) + RCA +90° + (180° -
tripping angle)/2
This can be best visualised with reference to the following diagram:
P24xM Chapter 6 - Current Protection Functions
P24xM-TM-EN-2.1 113
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