238
1MRS751802-MUM
Protection Relay
Technical Reference Manual, Standard Configurations
REX 521
11.7.3.4.
Short-circuit (high-set overcurrent) protection
The high-set overcurrent protection (3I>> or 3I>>>) constitutes an interwinding
short-circuit protection for the motor and a phase-to-phase short circuit protection
for the feeder cable.
It is recommended to use doubling function which automatically doubles the high-
set overcurrent setting during start-up. Thus, the current setting can be given a value
lower than the motor starting current. Normally, the setting can be chosen to
0.70...0.90 times the motor starting current. With a suitable operating time set, this
feature enables the high-set overcurrent unit to operate in case the motor is jammed
while the motor is running. Generally, a setting value as low as 75% gives good
results but if the inrush current causes trippings during start-up, choose a higher
setting.
The doubling is controlled by the Is2t n< function block. The checksum for the
switchgroup in
Main menu\Configuration\Input SWGRP\Double
should be 1 to double the 3I>> settings, and 2 to double the 3I>>> settings.
11.7.3.5.
Unbalance protection
The negative phase sequence protection (I2> and I2>>) monitors the current
asymmetry of the network and protects the motor against heavy network unbalance
or single-phasing. Typically, the operating time of the protection follows an inverse
time characteristic.
Selecting Start value -setting
The
Start value
-setting is the negative phase sequence (NPS) current, which
the machine is able to continuously withstand without damage. The setting is
selected as instructed by the machine manufacturer.
In case the maximum permitted negative phase sequence voltage is known (instead
of NPS current), the NPS current is approximately equal to the product of the NPS
voltage and the ratio of starting current to full load current.
Example:
Motor start-up current is 6
x
FLC, the highest permitted NPS voltage is 4% and
thereby the estimated NPS current is 6
x
4% = 24%.
Selecting K-setting
The machine constant K is equal to the I
2
2
x
t capability of the machine, as stated by
the machine manufacturer. It defines the rotor capability of accepting additional
heating caused by negative sequence current.
A conservative estimate is calculated as:
where
I
LR
=locked-rotor current (times the Full Load Current of the motor
K
2
175
I
LR
(
)
2
---------------
=
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