ABB 615 Series - Page 651

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
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 Timer.
Timer
Once activated, 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 operation 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.
For the DT characteristics, the deactivation of the TRIP output activates the cooling
timer. The timer is set to the value entered in the Cooling time setting. The
BLK_RESTART and COOL_ACTIVE outputs are kept active until the cooling timer is
reset. If the excitation increases above the set value during this period, the PICKUP output
is activated immediately. For IDMT, the deactivation of BLK_RESTART and
COOL_ACTIVE depends on the curve type selected.
The T_ENARESTART output indicates in seconds the duration for which the
BLK_RESTART output still remains active. The value is available in the Monitored data
view.
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.
1MAC059074-MB A Section 4
Protection functions
615 series ANSI 645
Technical Manual