Compact current relay and
protection assemblies
RXHL 401 and
RAHL 401
1MRK 509 062-BEN
Page 3
often required to clear the earth-faults with
residual currents of magnitudes which are as
low as 50-100A.
In high-impedance grounded networks a sen-
sitive non-directional earth-fault overcurrent
function can be used as a protection for cross-
country faults. This is due to the fact that
there is a risk that cross-country faults will
not activate directional earth-fault overcur-
rent relays.
In some systems a medium impedance resis-
tive system grounding is used. The neutral
point resistor will give an earth-fault current,
larger than the capacitive earth-fault current
of the lines and cables in the system. If the
system is operated radially the non-direc-
tional earth-fault overcurrent protection can
be used as earth-fault line protection.
In many applications a directional function of
the residual overcurrent protection is desir-
able. In such cases the measuring relay can be
used in combination with the directional relay
RXPDK 23H, that will provide enable criteria
in case of earth-faults in the forward direc-
tion.
Both inverse time characteristics protection
as well as three step definite time characteris-
tics are used. If inverse time characteristics
are used with equal currents and time settings
for all residual current protections in the sys-
tem. Selectivity is usually achieved as long as
there are more than two bays carrying fault
current to each substation.
It is also possible to use the protection as a
multi-stage earth-fault current line protection
where the first stage has instantaneous func-
tion and covers most of the protected line.
The second stage has a short delay (about 0.4
s) and covers the rest of the line. The third
stage has a longer delay and will give rela-
tively rapid and selective fault clearance of
high resistive phase to earth-faults.
Design
The earth-fault protection has a low set stage
with inverse or definite time delayed func-
tion. The inverse time characteristics are pro-
vided with minimum operate time for
improved selectivity in certain applications.
The low set stage also has a reset time logic
for detection of intermittent faults. If the pro-
tection starts and the fault current drops the
reset of the function will be made gradually
so that the integrated fault current time area
will be remembered for some time. In case of
an intermittent fault every re-strike of the
fault will increase the integrated current-time
area so that the fault can be tripped. The
earth-fault protection has two high set stages
with definite time delayed function. The
earth-fault protection is designed for low
transient-overreach which allows extended
reach and smaller setting margins.
The following characteristics are selectable
for the low set stage (diagrams are shown in
the chapter “Design description”):
1 Definite time delayed
2 Inverse time delayed:
- Normal inverse (NI)
- Very inverse (VI)
- Extremely inverse (EI)
- Long time inverse (LI)
- RI inverse (RI)
- Logarithmic inverse (LOG)
NI, VI, EI and LI according to IEC 60255-3.
RI-curve according to old electromechanical
relays manufactured by ASEA.
LOG-curve according to RXIDG relay manu-
factured by ABB.
