ABB 615 Series ANSI, Техническое руководство

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1MAC050144-MB C
Section 4
Protection functions
615 series ANSI 251
Technical Manual
If the input frequency (f
m
) is less than 20 percent of the nominal frequency (f
n
), the
calculation of the excitation level is disabled and forced to zero value. This means that the
function is blocked from picking up and tripping during a low-frequency condition.
The calculated excitation level (V/f ratio or volts/hertz) VOLTPERHZ is available in the
monitored data view.
Level detector
The level detector compares the calculated excitation level to the Pickup value setting. If
the excitation level exceeds the set limit, the module sends an enabling signal to start the
timer.
Timer
Once activated, the timer activates the PICKUP output. Depending on the value of the
Operating curve type setting, the time characteristics are according to DT or IDMT. When
the trip timer has reached the value set by Trip delay time in the DT mode or the value
defined by the inverse time curve, the TRIP output is activated.
In a drop-off situation, that is, when the excitation level drops below Pickup value before
the function trips, the reset timer is activated and the PICKUP output resets after the time
delay of
Reset delay time for the DT characteristics. For the IDMT curves, the reset
operation is as described in the Timer characteristics chapter.
For the IDMT curves, it is possible to define the maximum and minimum trip times via the
Minimum trip time and Maximum trip time settings. The Maximum trip time setting is used
to prevent infinite pickup situations at low degrees of overexcitation. The Time multiplier
setting is used for scaling the IDMT trip times.
The activation of the TRIP output activates the BLK_RESTART output.
The beginning of the cooling process deactivates the TRIP output and activates the
COOL_ACTIVE output. COOL_ACTIVE is kept active during the cooling process. If a
new overexcitation pickup ceases cooling, COOL_ACTIVE is deactivated during that
pickup time. BLK_RESTART is kept active until the set total cooling time has elapsed. It
means that even during the new overexcitation pickup ceases cooling, BLK_RESTART is
kept active. Due to the updated cooling time, the BLK_RESTART activation time is
prolonged with these new starts during cooling. A new overexcitation pickup during
cooling does not immediately reactivate TRIP, but PICKUP is first activated and a new
TRIP activation depends on the already run cooling time. If, for example, 60 percent of the
set cooling time has run before a new pickup, 40 percent of the operating time is needed
to reactivate TRIP.
The T_ENARESTART output indicates the duration in seconds for which the
BLK_RESTART output still remains active. The value is available in the monitored data
view.
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 in the
monitored data view.