ABB Switzerland Ltd REG 316*4 1MRB520049-Uen / Rev. F
3-108
E. Setting instructions
Settings:
Negative-sequence component of stator
current
I2-Setting
Delay Delay
An NPS current is usually caused by asymmetrical loading of the
three phases, but may also be the result of an open-circuit phase
(single-phasing).
An asymmetrical load on a generator produces a magnetic field,
which rotates in the opposite direction to the positive-sequence
field. The negative-phase sequence flux induces currents in the
rotor and these result in additional rotor losses and increased
rotor temperature. The latter can represent a hazard for the rotor
and this is the reason for applying NPS protection.
The asymmetry of the load on a generator is defined in terms of
the negative-sequence stator current I
2
which is therefore the
quantity monitored.
The definite time NPS function is intended for systems where
asymmetries are of longer duration and do not change fre-
quently. This generally applies in the case of small to medium
generators. Two NPS stages are used, one for alarm and one for
tripping.
The maximum continuous NPS current rating I
2¥
is stated by the
generator manufacturer, usually as a percentage of the genera-
tor rated current I
GN
.
The alarm stage is normally set to I
2¥
or somewhat lower, e.g.
for I
2¥
= 10 % I
GN
, “I2-Setting” is set to 8 % I
GN
.
The tripping stage is set 50 to 100 % higher than the alarm
stage, e.g.
I2-Setting = 15 % I
GN
The NPS protection is always delayed to avoid false tripping
during transient phenomena and especially during phase-to-
phase and earth faults on the power system. The delay may be
relatively long, because the rate at which the temperature of the
endangered parts of the rotor rises is relatively low.
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