GE Multilin 489 Instruction Manual page 198

Note
5–88
Courtesy of NationalSwitchgear.com
where: RTD_Bias_TC = TC used due to hottest stator RTD
used
Temp = current temperature of the hottest stator RTD
actual
Temp = RTD Bias minimum setpoint
min
Temp = RTD Bias center setpoint
center
Temp = RTD Bias maximum setpoint
max
TC = TC used defined by the
used at RBC
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.
It should be noted that the RTD bias feature alone cannot create a trip. If the RTD bias
feature forces the thermal capacity used to 100%, the machine current must be above the
over-load pickup before an overload trip occurs. Presumably, the machine would trip on
stator RTD temperature at that time.
No biasing occurs if the hottest stator RTD is open or short.
100
80
60
40
20
0
–50
HOT/COLD SAFE STALL RATIO
RTD Bias Maximum
Hot/Cold = 0.8
Rated Temperature=130°C
Insulation Rating=1 °C
RTD Bias Center Point
RTD Bias Minimum
0 50 100 150
Ma imum Stator RTD Temperature
FIGURE 5–24: RTD Bias Curve
489 GENERATOR MANAGEMENT RELAY – INSTRUCTION MANUAL
CHAPTER 5: SETPOINTS
setpoint
200 250
808721A1.CDR