Application - ABB RELION 620 Series Technical Manual

Section 4
Protection functions
4.2.7.5
316
input, a horizontal communication input or an internal signal of the protection relay
program. The influence of the BLOCK input signal activation is preselected with the global
Blocking mode setting.
The Blocking mode setting has three blocking methods. In the "Freeze timers" mode, the
trip timer is frozen to the prevailing value. In the "Block all" mode, the whole function is
blocked and the timers are reset. In the "Block TRIP output" mode, the function operates
normally but the TRIP output is not activated.

Application

Systems with critical motor applications provide backup power sources to those motors
that can be switched in when an undervoltage on the original power source is detected. The
three-phase undervoltage protection 27R is used to monitor for an undervoltage condition
over a decaying frequency after original power is lost, before allowing re-application of
backup power. An independent undervoltage function, operating at nominal frequency,
can be used to detect the original loss of primary power and initiate the transfer to backup
power. 27R provides a permissive signal indicating when restoration of backup power can
be completed safely.
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 remnant voltage decays as the motor slows to a stop at a
rate depending on the motor and load. Applying the backup power source while the
remnant voltage is still present can result in damage to the motor shaft and windings.
27R operates over the range of 10...70 Hz so it can monitor the voltage while the motor is
slowing and prevent transfer of the backup power until the motor voltage has decayed to
a safe level. The undervoltage setting is typically set 20...30 percent of rated voltage.
There is also a time delay setting to ensure that the remnant voltage remains below the
undervoltage setting for the set time. 27R includes a settable value for the detection of
undervoltage in either a single phase, two phases, or in all three phases. This setting value
should always be set for three phases in the remnant undervoltage application but can be
changed if 27R is used as a standard undervoltage function.
The function contains a self-blocking functionality. It is possible to block function
outputs, timer or the function itself, if desired.
1MAC504801-IB E
620 series ANSI
Technical Manual