ABB RELION REX640 Technical Manual page 405

1MRS759142 C
REX640
Technical Manual
Setting the overload factor
The value of Overload factor defines the highest permissible continuous load. The
recommended value is 1.05.
Setting the negative sequence factor
During the unbalance condition, the symmetry of the stator currents is disturbed and
a counter-rotating negative sequence component current is set up. An increased stator
current causes additional heating in the stator and the negative sequence component
current excessive heating in the rotor. Also mechanical problems like rotor vibration
can occur.
The most common cause of unbalance for three-phase motors is the loss of phase
resulting in an open fuse, connector or conductor. Often mechanical problems can be
more severe than the heating effects and therefore a separate unbalance protection is
used.
Unbalances in other connected loads in the same busbar can also affect the motor. A
voltage unbalance typically produces 5 to 7 times higher current unbalance. Because
the thermal overload protection is based on the highest TRMS value of the phase
current, the additional heating in stator winding is automatically taken into account.
For more accurate thermal modeling, the Negative Seq factor setting is used for taking
account of the rotor heating effect.
R
NegativeSeq factor
=
R
GUID-1C0889EC-48C3-4E14-A21F-B8383D8F9A46 V1 EN
R rotor negative sequence resistance
R2
R rotor positive sequence resistance
R1
A conservative estimate for the setting can be calculated:
175
NegativeSeq factor
=
I
GUID-D2F93566-E55F-4FEF-935F-4079FED3B54F V1 EN
I locked rotor current (multiple of set
LR
beginning of the motor start-up.
For example, if the rated current of a motor is 230 A, start-up current is 5.7 x I
175
NegativeSeq factor =
5 7
GUID-045D0689-AB6C-4891-89E0-4B7FC4CB98C1 V1 EN
R
2
R 1
2
LR
Rated current ). The same as the start-up current at the
5 4 .
=
2
.
Section 4
Protection functions
(Equation 40)
(Equation 41)
,
r
(Equation 42)
399