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.
Negative Seq factor
R
R
R
R
=
2
1
GUID-EA5AD510-A3CA-47FB-91F0-75D7272B654E V1 EN
(Equation 19)
R
R2
Rotor negative sequence resistance
R
R1
Rotor positive sequence resistance
A conservative estimate for the setting can be calculated:
Negative Seq factor
I
LR
=
175
2
GUID-13CE37C5-295F-41D4-8159-400FA377C84C V1 EN
(Equation 20)
I
LR
Locked rotor current (multiple of set
Rated current). The same as the start-up current at the 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
r
,
Negative Seq factor
=
=
175
5 7
5 4
2
.
.
GUID-DF682702-E6B1-4814-8B2E-31C28F3A03DF V1 EN
(Equation 21)
Setting the thermal restart level
The restart disable level can be calculated as follows:
Section 4
1MAC059074-MB A
Protection functions
310
615 series ANSI
Technical Manual
Summary of Contents for Relion 615 series
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