Short-Circuit Protection - ABB REM 610 Technical Reference Manual

REM 610
4.1.4.4.
30
Motor Protection Relay
Technical Reference Manual
The operate time is calculated as below. However, the shortest possible operate time
2
of stage I x t is ~300 ms.
s s
2
I ( > ) ×
t
s s
t s [ ] -------------------------------------------
=
2
I
where
t = operate time
I > = set start-up current for motor
s
t > = set start-up time for motor
s
I = phase current value
An advantage of start-up supervision based on thermal stress calculation is that the
operate time will be automatically extended during low-voltage conditions as it
depends on the start-up current of the motor.
Note!
2
Stage I x t cannot be used concurrently with stage I
s s
Start-up supervision with speed switch
In case the safe stall time is shorter than the start-up time of the motor stated by the
motor manufacturer, as with motors of ExE-type, for instance, a speed switch on the
motor shaft is required to give information on whether the motor is accelerating
during motor start up. The speed switch should be open at standstill and closed
during acceleration.
2
Stages I > and I x t will be blocked on activation of the speed switch input.
s s s

Short-circuit protection

The non-directional short-circuit protection detects overcurrent caused by
interwinding, phase-to-phase and phase-to-earth short circuits.
When one or several phase currents exceed the set start value of stage I>>, the stage
will generate a start signal after a ~ 50 ms' start time. When the set operate time at
definite-time characteristic elapses, the stage will generate a trip signal. The
high-set overcurrent stage can be given an instantaneous characteristic by setting the
operate time to the minimum, i.e. 0.05 s. The stage will be reset in 50 ms after all
three phase currents have fallen below the set start value of the stage.
The set start value of stage I>> can be set to be automatically doubled in a motor
start-up situation, i.e. when the object to be protected is being connected to a
network. Consequently, a set start value below the connection inrush current level
can be selected for the stage. In this case, the short-circuit protection will still detect
overcurrent caused by a locked rotor when the motor is running, which in turn may
be caused by bearing failure, for instance. The selection is made in SGF3.
Note!
When automatic doubling is in use and the PU scale has been set to be very low, it
must be assured that the doubled set start value of stage I>> does not exceed the
maximum measured current.
>
1MRS 752263-MUM
>.
s