4.4 EARTH FAULT PROTECTION LOGIC
&
IN>1 Timer Blk
IN>1 Start
IN>1 Trip
&
V00737
IN
Inhibit IN >1
CTS Block
IN>1 Current Set
VTS Slow Block
IN> Blocking
VTS Blocks IN>1
VN
IN> VNpol Set
&
IDMT/ DT
Directional
check
IN>1 Directional
IN
Low Current
&
Residual voltage polarisation
V2
IN> V2pol Set
I2
Negative Sequence Polarisation
IN> I2pol Set
Note: For the purpose of clarity, this diagram shows the first
relevant stage number for each signal and setting name .
928
467
833
405
777
671
Figure 101: Earth Fault Protection logic diagram
4.5 APPLICATION NOTES
4.5.1 RESIDUAL VOLTAGE POLARISATION SETTING GUIDELINES
It is possible that small levels of residual voltage will be present under normal system conditions due to system
imbalances, V
T inaccuracies, IED tolerances etc. Hence, the IED includes a user settable threshold (IN> VNPol Set)
which must be exceeded in order for the DEF function to be operational. In practice, the typical zero sequence
voltage on a healthy system can be as high as 1% (i.e. 3% residual), and the VT error could be 1% per phase. A
setting between 1% and 4% is therefore typical. The residual voltage measurement may assist in determining the
required threshold setting during commissioning, as this will indicate the level of standing residual voltage present.
4.5.2 SETTING GUIDELINES (DIRECTIONAL ELEMENT)
With directional earth faults, the residual current under fault conditions lies at an angle lagging the polarising
v
oltage. Hence, negative RCA settings are required for DEF applications. This is set in the cell I> Char Angle in the
relevant earth fault menu.
P54A/B/C/E Chapter 9 - Current Protection Functions
P54xMED-TM-EN-1 205
To Next Page
Loading...
Need help?
General
