GE 469 Instruction Manual page 88

4.6 S5 THERMAL MODEL
The following two figures illustrate the resultant overload protection curves for 80% and 100% line voltage, respectively. For
voltages in between, the 469 will shift the acceleration curve linearly and constantly based on measured line voltage during
a motor start.
HIGH INERTIA LOAD OVERLOAD CURVES
8800 HP, 13.2 kV, REACTOR COOLANT PUMP
1000
900
800
700
600
500
400
300
200
100
90
80
70
60
4
50
40
30
20
10
9
8
7
6
5
4
3
2
1
1 2 3
Figure 4–11: VOLTAGE DEPENDENT OVERLOAD PROTECTION AT 80% AND 100% VOLTAGE
d) UNBALANCE BIAS
Unbalanced phase currents also cause additional rotor heating not accounted for by electromechanical relays and also not
accounted for in some electronic protective relays. When the motor is running, the rotor rotates in the direction of the posi-
tive-sequence current at near synchronous speed. Negative-sequence current, with a phase rotation opposite to positive-
sequence current (and hence, opposite to the rotor rotation), generates a rotor voltage that produces a substantial rotor
current. This induced current has a frequency approximately 2 times the line frequency: 100 Hz for a 50 Hz system or
120 Hz for a 60 Hz system. The skin effect in the rotor bars at this frequency causes a significant increase in rotor resis-
tance and therefore a significant increase in rotor heating. This extra heating is not accounted for in the thermal limit curves
supplied by the motor manufacturer, as these curves assume only positive-sequence currents from a perfectly balanced
supply and motor design.
The 469 measures the ratio of negative to positive-sequence current. The thermal model may be biased to reflect the addi-
tional heating that is caused by negative sequence current when the motor is running. This biasing is accomplished by cre-
ating an equivalent motor heating current rather than simply using average current ( I
calculated using the equation shown below.
4-38
Courtesy of NationalSwitchgear.com
GE Multilin
4 5 6 7
MULTIPLES OF FULL LOAD AMPS
469 Motor Management Relay
HIGH INERTIA LOAD OVERLOAD CURVES
8800 HP, 13.2 kV, REACTOR COOLANT PUMP
1000
900
800
700
600
500
400
300
200
100
90
80
70
60
50
40
30
20
10
9
8
7
6
5
4
3
2
1
8 1 2
806825A4.CDR
4 SETPOINTS
GE Multilin
3 4 5 6
MULTIPLES OF FULL LOAD AMPS
). This equivalent current is
per_unit
GE Multilin
7 8
806826A4.CDR