6.3.2.4 FIRST EARTH FAULT DETECTION
VN> Voltage Set
VN> Start
VAB
VBC
VCA
&
IN
1P Mode
Mode
Selector
1P Time Delay
IS/Comp EF
IAB
IBC
IN> Current Set
V02778
342
IN> Bias
1335
ICA
Voltage equilibrium
analysis
Biased Neutral
Level Detector
( LDBN )
V
N
I2
IN> Bias I2 check
Figure 90: First earth fault detection
Detection of the first fault primes the detection of a second earth fault and blocks tripping of phase to phase
elements for zones 1,2,3,4 (zones P and Q do not get blocked). It also creates a DDB signal indicating a single-
phase earth fault (IS/Comp EF). Typically, with zones P and Q, the external starting zones will be used to detect if a
second earth loop converges into the impedance characteristic.
The VN> Start signal is established by comparing the neutral voltage VN with a threshold set by the setting VN>
Voltage Set. This signal is gated with another signal indicating whether the phase voltage triangle is balanced or
not and then fed into the mode selector. If the voltage triangle is balanced and there is an overvoltage condition,
then a single-phase fault is indicated.
The mode selector decides whether the single phase-to-earth fault is determined by VN> only, IN> only, VN> or
IN>, or VN> and IN>. The mode is set by the setting 1P Mode in the DISTANCE SETUP column. The single phase-to-
earth fault signal (IS/Comp EF) is then produced after a time delay set by 1P Time Delay.
6.3.3
FAULT DETECTION LOGIC
It is unlikely that a second fault would occur on the same phase, because the phase to neutral voltage on this
phase has collapsed to zero as a result of the first fault. However, a second fault is highly likely in one of the other
phases as the voltage has increased by a factor of Ö3 and the extra voltage will put more stress on the insulation,
increasing the likelihood of a fault further down the line.
A second earth fault (on a different phase) will cause an imbalance in the voltage triangle. This is used as part of
the second fault detection process. The criteria for the detection of a second fault is as follows:
A second earth fault is confirmed if a first earth fault has been detected AND the phase to phase voltage triangle is
unbalanced AND the neutral voltage has exceeded the set threshold AND the neutral current has exceeded the
level set by the threshold together with the bias neutral level detector (LDBN).
Chapter 7 - Distance Protection P543i/P545i
186 P54x1i-TM-EN-1
To Next Page
Loading...
Need help?
General
