ABB RELION 620 Series Technical Manual page 407

1MAC504801-IB E
620 series ANSI
Technical Manual
Figure 183: The influence of Weighting factor p at prior load 1xFLC, timeconstant =
640 sec, and Overload factor = 1.05
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
R 2
Negative Seq factor
=
R
R 1
GUID-EA5AD510-A3CA-47FB-91F0-75D7272B654E V1 EN
R rotor negative sequence resistance
R2
R rotor positive sequence resistance
R1
A conservative estimate for the setting can be calculated:
175
Negative Seq factor
=
2
I
LR
GUID-13CE37C5-295F-41D4-8159-400FA377C84C V1 EN
I locked rotor current (multiple of set
LR
the motor startup.
For example, if the rated current of a motor is 230 A, startup current is 5.7 x I
Rated current ). The same as the startup current at the beginning of
Section 4
Protection functions
(Equation 30)
,
r
401