1MAC050144-MB C
Section 4
Protection functions
615 series ANSI
231
Technical Manual
4.2.2.4
Operation principle
The function can be enabled and disabled with the
Operation
setting. The corresponding
parameter values are Enable and Disable.
The operation of the three-phase undervoltage protection can be described by using a
module diagram. All the modules in the diagram are explained in the next sections.
Figure 109:
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.
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.
For more detailed description on IDMT curves and usage of
Curve Sat
Relative
setting, see the
General function block features
section in this
manual.
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