E) machine cooling, Where: tc, Thermal capacity used tc – GE 489 User Manual
Page 116: Value caused by overload condition tc
4-66
489 Generator Management Relay
GE Multilin
4.10 S9 THERMAL MODEL
4 SETPOINTS
4
e) 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:
(EQ 4.31)
(EQ 4.32)
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%
×
=
808705A1.CDR
0
25
50
75
100
0
30
60
90
120
150
180
Time in Minutes
T
h
er
m
a
l
Ca
p
a
ci
ty
U
s
ed
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
T
h
er
m
a
l
Ca
p
a
ci
ty
U
s
ed
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
T
h
er
m
a
l
Ca
p
a
ci
ty
U
s
ed
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
T
h
er
m
a
l
Ca
p
a
ci
ty
U
s
ed
Cool Time Constant= 30 min
TCused_start= 100%
Hot/Cold Ratio= 80%
Motor Stopped after Overload Trip
TCused_end= 0%
MOTOR TRIPPED
100% LOAD
MOTOR STOPPED
80% LOAD