Hot/Cold Curve Ratio; Rtd Bias - GE 489 Instruction Manual

4 SETPOINT PROGRAMMING
When thermal limit information is available for both a hot and cold machine, the 489 thermal model will adapt for the condi-
tions if the

HOT/COLD CURVE RATIO

used that the relay will settle at for levels of current that are below the
ning at a level below the
OVERLOAD PICKUP LEVEL
age phase current and the entered
minute or fall as dictated by the running cool time constant.

×
TC I 1
= – ---------- -

used_end eq
cold
where: TC = Thermal Capacity Used if I
used_end
I
= equivalent generator heating current
eq
hot/cold =
HOT/COLD CURVE RATIO
The hot/cold curve ratio may be determined from the thermal limit curves, if provided, or the hot and cold safe stall times.
Simply divide the hot safe stall time by the cold safe stall time. If hot and cold times are not provided, there can be no differ-
entiation and the
HOT/COLD CURVE RATIO
The thermal replica created by the features described in the sections above operates as a complete and independent
model. However, the thermal overload curves are based solely on measured current, assuming a normal 40°C ambient and
normal machine cooling. If there is an unusually high ambient temperature, or if machine cooling is blocked, generator tem-
perature will increase. If the stator has embedded RTDs, the 489 RTD bias feature should be used to correct the thermal
model.
The RTD bias feature is a two part curve, constructed using 3 points. If the maximum stator RTD temperature is below the
setpoint (typically 40°C), no biasing occurs. If the maximum stator RTD temperature is above the
RTD BIAS MINIMUM
setpoint (typically at the stator insulation rating or slightly higher), then the thermal memory is fully biased
BIAS MAXIMUM
and thermal capacity is forced to 100% used. At values in between, the present thermal capacity used created by the over-
load curve and other elements of the thermal model, is compared to the RTD Bias thermal capacity used from the RTD Bias
curve. If the RTD Bias thermal capacity used value is higher, then that value is used from that point onward. The
should be set at the rated running temperature of the machine. The 489 automatically determines the ther-
CENTER POINT
mal capacity used value for the center point using the
TC @ RTD_Bias_Center
used
At temperatures less that the RTD_Bias_Center temperature,
Temp
RTD_Bias_TC = --------------------------------------------------------
used
Temp
At temperatures greater than the RTD_Bias_Center temperature,
Temp
RTD_Bias_TC = ------------------------------------------------------------ -
used
Temp
where: RTD_Bias_TCused = TC used due to hottest stator RTD
Temp = current temperature of the hottest stator RTD
acutal
Temp = RTD Bias minimum setpoint
min
Temp = RTD Bias center setpoint
center
Temp = RTD Bias maximum setpoint
max
TCused @ RTD_Bias_Center = TC used defined by the
In simple terms, the RTD bias feature is real feedback of measured stator temperature. This feedback acts as correction of
the thermal model for unforeseen situations. Since RTDs are relatively slow to respond, RTD biasing is good for correction
and slow generator heating. The rest of the thermal model is required during high phase current conditions when machine
heating is relatively fast.
GE Power Management
is programmed. The value entered for this setpoint dictates the level of thermal capacity
, the thermal capacity used will rise or fall to a value based on the aver-
HOT/COLD CURVE RATIO

hot
×
100%

remains steady state
per_unit
setpoint
should be entered as "1.00".
HOT/COLD SAFE STALL RATIO
 
hot
×
= 1 – ---------- - 100%
 
cold
Temp
–
actual min × (
100 TC –
used
– Temp
center min
Temp
–
actual center × (
100 TC –
used
– Temp
max center
489 Generator Management Relay
OVERLOAD PICKUP LEVEL
. Thermal capacity used will either rise at a fixed rate of 5% per
setpoint.
)
@ RTD_Bias_Center + TC @ RTD_Bias_Center
used
)
@ RTD_Bias_Center + TC
used
HOT/COLD SAFE STALL RATIO
4.10 S9 THERMAL MODEL
4.10.5 HOT/COLD CURVE RATIO
. When the generator is run-

4.10.6 RTD BIAS

RTD BIAS
@ RTD_Bias_Center
setpoint
4
RTD
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