Second harmonic blocking
This module compares the ratio of the current second harmonic (I
0
_2H) and I
0
to
the set value
Start value 2.H
. If the ratio (I
0
_2H / I
0
) value exceeds the set value, the
BLK2H output is activated.
The blocking also prevents unwanted operation at the recovery and sympathetic
magnetizing inrushes. At the recovery inrush, the magnetizing current of the
transformer to be protected increases momentarily when the voltage returns to
normal after the clearance of a fault outside the protected area. The sympathetic
inrush is caused by the energization of a transformer running in parallel with the
protected transformer connected to the network.
The second harmonic blocking is disabled when
Restraint mode
is set to "None" and
enabled when set to "Harmonic2".
Timer
Once activated, the Timer activates the START output. The time characteristic is
according to DT. When the operation timer has reached the value set by
Minimum
operate time
, the OPERATE output is activated. If the fault disappears before the
module operates, the reset timer is activated. If the reset timer reaches the value
set by
Reset delay time
, the reset timer resets and the START output is deactivated.
The Timer calculates the start duration value START_DUR which indicates the
percentage ratio of the start situation and the set operate time. The value is
available through the Monitored data view.
Blocking logic
There are three operation modes in the blocking function. The operation modes
are controlled by the BLOCK input and the global setting in Configuration >
System > Blocking mode which selects the blocking mode. The BLOCK input can be
controlled by a binary input, a horizontal communication input or an internal signal
of the protection relay's program. The influence of the BLOCK signal activation is
preselected with the global setting
Blocking mode
.
The
Blocking mode
setting has three blocking methods. In the "Freeze timers"
mode, the operate timer is frozen to the prevailing value, but the OPERATE output
is not deactivated when blocking is activated. In the "Block all" mode, the whole
function is blocked and the timers are reset. In the "Block OPERATE output" mode,
the function operates normally but the OPERATE output is not activated. The
activation of the output of the second harmonic blocking signal BLK2H deactivates
the OPERATE output.
4.3.3.5 Application
An earth-fault protection using an overcurrent element does not adequately protect
the transformer winding in general and the star-connected winding in particular.
The restricted earth-fault protection is mainly used as a unit protection for the
transformer windings. LREFPNDF is a sensitive protection applied to protect the
star-connected winding of a transformer. This protection system remains stable for
all the faults outside the protected zone.
LREFPNDF provides higher sensitivity for the detection of earth faults than the
overall transformer differential protection. This is a high-speed unit protection
scheme applied to the star-connected winding of the transformer. LREFPNDF is
Protection functions
1MRS757644 H
562 620 series
Technical Manual
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