GE 489

4-62 489 Generator Management Relay
GE Power Management
4.10 S9 THERMAL MODEL 4 SETPOINT PROGRAMMING
4
4.10.4 MACHINE COOLING
The 489 thermal capacity used value is reduced exponentially when the motor current is below the
OVERLOAD PICKUP
set-
point. This reduction simulates machine cooling. The cooling time constants should be entered for both stopped and run-
ning cases (the generator is assumed to be running if current is measured or the generator is offline). A machine with a
stopped rotor normally cools significantly slower than one with a turning rotor. Machine cooling is calculated using the fol-
lowing formulae:
where: TC
used
= thermal capacity used
TC
used_start
= TC
used
value caused by overload condition
TC
used_end
= TC
used
value dictated by the hot/cold curve ratio when the machine is running
(= 0 when the machine is stopped)
t = time in minutes
τ = Cool Time Constant (running or stopped)
I
eq
= equivalent heating current
overload_pickup = overload pickup setpoint as a multiple of FLA
hot / cold = hot/cold curve ratio
Figure 4–17: THERMAL MODEL COOLING
TC
used
TC
used_start
TC
used_end
–() e
t
τ⁄
–
()TC
used_end
+=
TC
used_end
I
eq
overload_pickup
-------------------------------------------


1
hot
cold
-----------–


100%×=
0
25
50
75
100
0 30 60 90 120 150 180
Time in Minutes
Thermal Capacity Used
Cool Time Constant= 15 min
TCused_start= 85%
Hot/Cold Ratio= 80%
Ieq/Overload Pickup= 80%
0
25
50
75
100
0 30 60 90 120 150 180
Time in Minutes
Thermal Capacity Used
Cool Time Constant= 15 min
TCused_start= 85%
Hot/Cold Ratio= 80%
Ieq/Overload Pickup= 100%
0
25
50
75
100
0 30 60 90 120 150 180
Time in Minutes
Thermal Capacity Used
Cool Time Constant= 30 min
TCused_start= 85%
Hot/Cold Ratio= 80%
Motor Stopped after running Rated Load
TCused_end= 0%
0
25
50
75
100
0 30 60 90 120 150 180
Time in Minutes
Thermal Capacity Used
Cool Time Constant= 30 min
TCused_start= 100%
Hot/Cold Ratio= 80%
Motor Stopped after Overload Trip
TCused_end= 0%
Thermal Model Cooling, 80% Load
Thermal Model Cooling, Motor Stopped
Thermal Model Cooling, 100% Load
Thermal Model Cooling, Motor Tripped