Figure 484: Functional module diagram
Level detector
The fundamental frequency component of the measured three phase voltages are
compared phase-wise to the set
Start value
. If the measured value is lower than
the set value of the
Start value
setting, the level detector enables the phase
selection logic module. The
Relative hysteresis
setting can be used for preventing
unnecessary oscillations if the input signal slightly varies above or below the
Start
value
setting. After leaving the hysteresis area, the start condition has to be fulfilled
again and it is not sufficient for the signal to only return back to the hysteresis area.
The
Voltage selection
setting is used for selecting the phase-to-earth or phase-to-
phase voltages for protection.
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.
Protection functions 1MRS759142 F
860 REX640
Technical Manual
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