ABB RELION REF615R, Technical Manual

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4.2.2.2 Function block
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Figure 110: Function block
4.2.2.3 Functionality
The three-phase undervoltage protection function 27 is used to disconnect from the
network devices, for example electric motors, which are damaged when subjected to
service under low voltage conditions. 27 includes a settable value for the detection of
undervoltage either in a single phase, two phases or three phases.
The function contains a blocking functionality. It is possible to block function outputs,
timer or the function itself, if desired.
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 27 can be described using a module diagram. All the modules in the
diagram are explained in the next sections.
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Figure 111: 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.
Section 4
1MRS240050-IB C
Protection functions
230 REF615R
Technical Manual