ABB RELION 620 Series Technical Manual page 282

Section 4
Protection functions
276
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Figure 126: Functional module diagram
Level detector
The fundamental frequency component of the measured three phase voltages are
compared phase-wise to the set Pickup value. If the measured value is lower than the set
value of the Pickup 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 Pickup 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-ground 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
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
pickups and trips 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 pickup phases, the phase selection logic activates the Timer.
IDMT curves for under voltage protection
1MAC504801-IB E
section
620 series ANSI
Technical Manual