p
security factor = 0.5…0.7
This setting should be set based on the total resistive ground-fault current of the network
including the parallel resistor of the coil and the network losses. It must be set to a value
which is lower than total resistive ground-fault current in order to enable dependable
operation.
For example, if the resistive current of the parallel resistor is 10 A (at primary voltage
level), then a value of 0.5 · 10 A = 5 A could be used. The same setting is also applicable
in case the coil is disconnected and the network becomes ungrounded (as in this case this
setting is compared to the amplitude of
I
o stab
1
). The selected setting value must never
exceed the ampere value of the parallel resistor in order to allow operation in the faulty
feeder. In case of smaller ampere value of the parallel resistor, for example 5 A, the
recommended security factor should be larger, for example 0.7, so that sufficient margin
for CT and VT errors can be achieved.
In case the “Amplitude” operating quantity is selected, the setting should be selected
based on the capacitive ground-fault current values produced by the background network
in case of a solid ground fault with a security margin.
The main task of the current magnitude supervision module is to secure
the correct directional determination of a ground fault, so that only the
faulty feeder is disconnected or alarmed. Therefore, the threshold values
should be selected carefully and not set too high as this can inhibit the
disconnection of the faulty feeder.
The residual current should be measured with accurate core balance
current transformer to minimize the measurement errors, especially phase
displacement.
Transient detector
The Transient detector module is used for detecting transients in the residual current and
zero-sequence voltage signals. Whenever transient is detected, this is indicated with the
PEAK_IND
output. When the number of detected transients equals or exceeds the
Peak
counter limit
setting (without the function being reset, depending on the drop-off time set
with the
Reset delay time
setting),
INTR_EF
output is activated. This indicates detection
of restriking or intermittent ground fault in the network. Transient detector affects the
operation of 67YN (
PICKUP
and
TRIP
outputs) when operation mode is “Intermittent
EF”. For other operation modes, (“General EF”, “Alarming EF”),
PEAK_IND
and
Section 4
1MAC059074-MB A
Protection functions
448
615 series ANSI
Technical Manual
Summary of Contents for Relion 615 series
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