output" mode, the function operates normally but the OPERATE output is not
activated.
4.2.3.5 Application
INTRPTEF is an earth-fault function dedicated to operate in intermittent and
permanent earth faults occurring in distribution and sub-transmission networks.
Fault detection is done from the residual current and residual voltage signals by
monitoring the transients with predefined criteria. As the function has a dedicated
purpose for the fault types, fast detection and clearance of the faults can be
achieved.
Intermittent earth fault
Intermittent earth fault is a special type of fault that is encountered especially in
compensated networks with underground cables. A typical reason for this type of
fault is the deterioration of cable insulation either due to mechanical stress or due
to insulation material aging process where water or moisture gradually penetrates
the cable insulation. This eventually reduces the voltage withstand of the insulation,
leading to a series of cable insulation breakdowns. The fault is initiated as the
phaseto- earth voltage exceeds the reduced insulation level of the fault point and
mostly extinguishes itself as the fault current drops to zero for the first time, as
shown in
Figure 210
. As a result, very short transients, that is, rapid changes in the
form of spikes in residual current (Io) and in residual voltage (Uo), can be repeatedly
measured. Typically, the fault resistance in case of an intermittent earth fault is only
a few ohms.
Residual current Io and residual voltage Uo
FEEDER
FEEDER MEAS
Iov
Fault
Point
K
R
e
-0.3
-0.2
-0.1
Residual Current (kA)
Residual Voltage x 10
2
(kV)
INCOMER
COMP. COIL
Ioj
Uo
K
R
e
0
0.1
Ioj
(Faulty
Feeder)
Iov
(Healthy
Feeder)
Uo
Pulse width
400 - 800
s
Pulse interval
5 - 300 ms
Peak value
~0.1 ... 5 kA
U
tres
U
tres
I
ctot
R
f
Figure 210: Typical intermittent earth-fault characteristics
Earth-fault transients
In general, earth faults generate transients in currents and voltages. There are
several factors that affect the magnitude and frequency of these transients, such
1MRS757644 H
Protection functions
620 series
Technical Manual
425
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