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 pickup 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 the IDMT curves for under voltage
protection section 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 the
IDMT curves for under voltage protection section 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.
1YHT530004D05 D Section 4
Protection functions
615 series 475
Technical Manual
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