Schweitzer Engineering Laboratories SEL-749M - Page 89

4.23
Date Code 20080918 Instruction Manual SEL-749M Relay
Protection and Logic Functions
PTC/RTD-Based Protection
RTD Biasing
When you have connected an ambient temperature sensing RTD and set trip
temperatures for one or more winding RTDs, the relay gives you the option to
enable RTD trip temperature biasing by setting ERTDBIAS equal to Y. The
thermal model can also be biased by setting ETHMBIAS equal to Y (see
Table 4.6) When you enable either biasing, the relay does the following:
➤ Calculates RTD % Thermal Capacity and adds the value to the
Thermal Meter values.
➤ Automatically reduces the winding RTD trip temperatures if
ambient temperature rises above 40°C and ERTDBIAS is set
equal to Y.
➤ Automatically reduces the thermal model element trip
threshold if ambient temperature rises above 40°C and
ETHMBIAS is set equal to Y.
➤ Provides an RTD bias alarm if the winding temperature exceeds
60°C rise over ambient and the RTD % Thermal Capacity
exceeds the thermal element % Thermal Capacity by more than
10 percent.
The relay uses Equation 4.7 to calculate RTD % Thermal Capacity.
Equation 4.7
As ambient temperature rises, the ability of a motor to shed heat to the
surroundings is reduced and internal temperatures rise. To preserve insulation
life, NEMA standards suggest a 1°C reduction in RTD trip temperature for
each 1°C rise in ambient temperature over 40°C.
When you enable RTD biasing, the SEL-749M automatically reduces the RTD
trip temperatures for all winding RTDs when ambient temperature is above
40°C. The relay reduces the trip temperatures by 1°C for each degree rise in
ambient temperature over 40°C.
Finally, when you enable thermal model biasing, the relay provides an RTD
bias alarm when the RTD % Thermal Capacity exceeds the thermal element
% Thermal Capacity by more than 10 percentage points while the winding
temperature rise is higher than 60°C over ambient. This alarm can be a useful
indicator that the motor has lost coolant flow or that the winding RTD trip
temperature is conservatively low.
For all the RTD thermal capacity and bias calculations described above, the
relay uses the winding RTD with the measured temperature closest to the
winding trip value.
RTD%
Thermal Capacity
Winding RTD
Temperature
⎝⎠
⎛⎞
Ambient
Temperature
⎝⎠
⎛⎞
–
Winding RTD Trip
Temperature
⎝⎠
⎛⎞
Ambient
Temperature
⎝⎠
⎛⎞
–
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Courtesy of NationalSwitchgear.com