ABB RELION REX640 Technical Manual page 782

Section 4
Protection functions
776
U_A_AB
U_B_BC
U3P
U_C_CA
BLOCK
GUID-4B98F2BD-8A2A-4668-9121-F0A6F0BC8B99 V1 EN
Figure 425: 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 the
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.
Phase
Level
selection
detector
logic
Blocking
logic
IDMT curves for undervoltage protection
1MRS759142 C
OPERATE
Timer
OPR_A
OPR_B
t
OPR_C
START
ST_A
t
ST_B
ST_C
REX640
Technical Manual