For the voltage IDMT mode of operation, the used IDMT curve equations contain
discontinuity characteristics. The
Curve Sat relative
setting is used for preventing
unwanted operation.
For more detailed description on IDMT curves and usage of
Curve
Sat Relative
setting, see
Chapter 11.3.2 IDMT curves for undervoltage
protection
in this manual.
The level detector contains a low-level blocking functionality for cases where one
of the measured voltages is below the desired level. This feature is useful when
unnecessary starts and operates are wanted to avoid during, for example, an
autoreclose sequence. The low-level blocking is activated by default (
Enable block
value
is set to "True") and the blocking level can be set with the
Voltage block value
setting.
Phase selection logic
If the fault criteria are fulfilled in the level detector, the phase selection logic detects
the phase or phases in which the fault level is detected. If the number of faulty
phases match with the set
Num of start phases
, the phase selection logic activates
the Timer.
Timer
Once activated, the Timer activates the START output. Depending on the value of
the set
Operating curve type
, the time characteristics are selected according to DT
or IDMT.
For a detailed description of the voltage IDMT curves, see
Chapter 11.3.2
IDMT curves for undervoltage protection
in this manual.
When the operation timer has reached the value set by
Operate delay time
in the DT
mode or the maximum value defined by the IDMT, the OPERATE output is activated.
When the user-programmable IDMT curve is selected, the operate time
characteristics are defined by the parameters
Curve parameter A
,
Curve parameter
B
,
Curve parameter C
,
Curve parameter D
and
Curve parameter E
.
If a drop-off situation occurs, that is, a fault suddenly disappears before the
operate delay is exceeded, the reset state is activated. The behavior in the drop-
off situation depends on the selected operate time characteristics. If the DT
characteristics are selected, the reset timer runs until the set
Reset delay time
value
is exceeded. If the drop-off situation exceeds the set
Reset delay time
, the Timer is
reset and the START output is deactivated.
When the IDMT operate time curve is selected, the functionality of the Timer in the
drop-off state depends on the combination of the
Type of reset curve
,
Type of time
reset
and
Reset delay time
settings.
1MRS757644 H
Protection functions
620 series
Technical Manual
647
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