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IB02602004E
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Instructional Literature
Effective: May 2008
Page
171
Instructions for the FP-6000 Protective Relay
If the direct stator temperature measurements are available, the
thermal replica model will be modified to include the heat loss
between stator and rotor. As a result, motor will be able to run
longer under overload conditions. The heat loss serves as a
cooling. At some point, the cooling effect will cancel the heat
increment so that the thermal capacity used will reach some
steady-state level that may be below the trip or alarm limit. This
equivalently raises the service factor and shifts the trip curve
right. If the thermal capacity used is held at a level that is below
the trip threshold, the thermal model will not trip. To prevent the
protected equipment from overheating, the direct temperature
trip function must be enabled. Keep in mind that in order for the
stator temperature to be effective in the thermal replica model,
the following conditions must be met:
•
Some RTD channels must be configured to measure the
winding temperatures; and
•
These RTD channels must be enabled for trip for 49MT or
alarm for 49MA.
In addition, at least one of these winding temperatures must be
valid.
Knowing the maximum steady stator temperature
θ
S
(C
°
), the
thermal capacity used can be estimated by the following
formula.
Take an example, I
LR
= 6FLA, T
LR
=15, and thermal trip level of
100%. The relationship between the effective current threshold
and the stator temperature can be seen in Figure 68. From the
figure, it is seen that the lower the stator temperature, the higher
the effective current threshold. Without stator temperature,
given the current threshold of 1.0FLA and 2.0FLA of the stator
phase current, the thermal model will use the full thermal
capacity in 139.54 seconds. However, if the stator temperature is
known of 100
°
C (212
°
F), the effective current threshold is raised
to 2.55FLA and the thermal capacity used will reach a steady
state of 77.5%. As a result, the thermal model will never trip
under this condition. From this example, it can be seen that
stator RTD could keep the motor running under overload
condition. In this case, the appropriate direct stator temperature
trip function must be enabled.
Figures 69 and 70 show the thermal model trip curves with and
without RTD based on the above example. The unmarked lines
in the figures are the thermal limit curves and the marked lines
are the trip curves. From Figure 69, it can be seen that one can
play the thermal current threshold to shift the upper portion of
the trip curve right to allow the motor to run at a higher overload
condition than it is specified with the service factor. From Figure
70, it can be seen that the stator RTD pushes the effective
thermal current threshold to 2.55FLA on the thermal limit curve
(unmarked line). The marked line is the trip curve with 80%
thermal capacity trip threshold, so actual effective thermal
current threshold for the trip curve is about 2.05FLA. Although in
this case, the thermal current threshold is set to 1.50FLA, it is
effectively raised to a higher level with the stator RTD. Keep in
mind that thermal limit and trip curves shown are based on the
example above. They will vary with other sets of the settings.
Figure 68. Stator Temperature Effect on Current Threshold.
FLA
Ith
I
when
T
I
I
TC
ef
LR
LR
ef
S
Used
!
¸
¸
¹
·
¨
¨
©
§
,
50
2
40
%
2
2
Q
Effective Current Threshold vs. Maximum Stator Temperature
0
0.5
1
1.5
2
2
.5
3
3.5
0
50
100
150
2
00
2
50
Stator Temperature (C)
Current
T
hreshold
M
ultiple
o
f
F
LA