V02784
Ground
VA=Ground
VA
VC VC
VB VB
All phases healthy
Phase A ground fault
Figure 88: Voltage distribution in an isolated system for a Phase-A-to-Earth fault
Therefore, we can establish an earth fault by seeing if the neutral voltage V
N
exceeds a settable threshold VN>
Voltage Set, and checking whether the phase-phase voltages are still balanced.
0.8(max. V
ph-ph
) < (min. V
ph-ph
)
Neutral displacement is established by comparing the neutral voltage VN with a threshold set by the setting, VN>
Voltage Set
6.3.2.2 NEUTRAL CURRENT CRITERIA
The bias neutral level detector (LDBN), detailed in the Biased Neutral Current Detector section, is used to improve
stability and is complemented with an additional minimum threshold controlled by setting IN> Current set . A
negative sequence current check is also performed for long and heavily loaded lines.
 
ACCBBABIAS
I IIIIII ï€ï€ï€ï€½ ,,max
Bias current
N
e
u
t
r
a
l
c
u
r
r
e
n
t
Figure 89: Biased Neutral Current Detector
6.3.2.3 RELEVANT SETTINGS FOR FIRST EARTH FAULT DETECTION
The following settings are visible if the setting Dist.Earth Mode is set to Is/Comp Earthing.
IN> Current Set: Sets the threshold for the first fault earth fault overcurrent (default – 0.1In)
VN> Voltage Set: Sets the threshold for the first fault earth fault overvoltage (default – 12V)
1P Mode: Sets the first fault detection method (default – VN>)
1P Time Delay: Sets the first fault pickup delay (default – 50 ms)
P543i/P545i Chapter 7 - Distance Protection
P54x1i-TM-EN-1 185
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