ABB RELION Series Technical Manual page 390

Section 4
Protection functions
384
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 unearthed (as in
this case this setting is compared to the amplitude of
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 earth-fault current values produced by the background
network in case of a solid earth fault with a security margin.
The main task of the current magnitude supervision module is to
secure the correct directional determination of an earth 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 earth fault in the network.
Transient detector affects the operation of MFADPSDE (START and OPERATE
outputs) when operation mode is "Intermittent EF". For other operation modes,
("General EF", "Alarming EF"), PEAK_IND and INTR_EF outputs can be used for
monitoring purposes. The operation of the Transient detector is illustrated in
200.
Several factors affect the magnitude and frequency of fault transients,
such as the fault inception angle on the voltage wave, fault location,
fault resistance and the parameters of the feeders and the supplying
transformers. If the fault is permanent (non-transient) in nature, the
initial fault transient in current and voltage can be measured, whereas
the intermittent fault creates repetitive transients. The practical
sensitivity of transient detection is limited to approximately few
1MRS758755 A
1
I
). The selected setting value
o stab
REC615 and RER615
Technical Manual
Figure