Function Block; Functionality; Operation Principle - ABB RELION 620 Series Technical Manual

Section 4
Protection functions
4.2.7.2
4.2.7.3
4.2.7.4
314

Function block

27R
V_A_AB
V_B_BC
V_C_CA
BLOCK
GUID-5676E44B-85C1-4F11-84D5-AB775D9E2582 V1 EN
Figure 142: Function block

Functionality

Systems with critical motor applications may provide backup power sources to those
motors that can be switched in when an undervoltage on the original power source is
detected. The 27R function is used, after primary power has been removed, to monitor for
an undervoltage condition over a decaying frequency before allowing re-application of
backup power.
When the power to a motor is suddenly lost, the motor terminal voltage does not
immediately fall to zero but remains as the rotating motor now acts as a generator
producing its own voltage. This remanent voltage decays as the motor slows to a stop at
a rate depending on the motor and load. Re-applying a power source while the remanent
voltage is still present can result in damage to the motor shaft and windings.
The three-phase undervoltage protection 27R operates over the range of 10...70 Hz so it
can monitor the voltage while the motor is slowing and prevent application of the backup
power until the motor voltage has decayed to a safe level. 27R includes a settable value for
the detection of undervoltage either in a single phase, two phases, or all three phases.
The function contains a blocking functionality. It is possible to block function outputs,
timer or the function itself, if desired.

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 functionality can be described
by using a module diagram. All the modules in the diagram are explained in the next
sections.
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1MAC504801-IB E
620 series ANSI
Technical Manual