Section 4
1MAC050144-MB C
Protection functions
376
615 series ANSI
Technical Manual
Second harmonic blocking
This module compares the ratio of the current second harmonic (IG_2H) and IG to the set
value
Pickup value 2.H
. If the ratio (IG_2H / IG) 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 “2nd harmonic”.
Timer
Once activated, the timer activates the
PICKUP
output. The time characteristic is
according to DT. When the operation timer has reached the value set by
Minimum trip
time
, the
TRIP
output is activated. If the fault disappears before the module trips, the reset
timer is activated. If the reset timer reaches the value set by
Reset delay time
, the reset
timer resets and the
PICKUP
output is deactivated.
The timer calculates the pickup duration value PICKUP_DUR which indicates the
percentage ratio of the pickup situation and the set trip time. The value is available through
the Monitored data view.
Blocking logic
There are three operation modes in the blocking functionality. The operation modes are
controlled by the
BLOCK
input and the global setting
"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 IED 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. In the "Block all" mode, the whole function
is blocked and the timers are reset. In the "Block TRIP output" mode, the function operates
normally but the TRIP output is not activated.
The activation of the output of the second harmonic blocking signal
BLK2H
deactivates
the
TRIP
output.
4.6.3.5
Application
A ground-fault protection using an overcurrent element does not adequately protect the
transformer winding in general and the 87LOZREF winding in particular.
The restricted ground-fault protection is mainly used as a unit protection for the
transformer windings. 80LOZREF is a sensitive protection applied to protect the
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