Schweitzer Engineering SEL-387-0 - Page 86

3.36
SEL-387-0, -5, -6 Relay Instruction Manual Date Code 20170601
Differential, Restricted Earth Fault, Thermal, and Overcurrent Elements
Thermal Element (SEL-387-6 Relay)
Equation 3.20
For any EXPn (oil exponent) value and any load value, the relay calculates the
thermal top-oil time constant according to the following expression:
Equation 3.21
Thermal Element With No Measured Temperature Inputs
In this case, where your relay receives no measured ambient temperature or
top-oil inputs, the relay uses a default ambient temperature value (DTMP
setting) that you select and calculates a hot-spot temperature and top-oil
temperature.
The relay calculates hot-spot temperature according to the following:
Equation 3.22
and the top-oil temperature according to the following:
Equation 3.23
The relay has no measured ambient temperature input, so you must select an
ambient temperature setting (DTMP) for the thermal element calculation of
top-oil temperature as follows:
Equation 3.24
Top-Oil Temperature Comparison to Indicate Cooling System Efficiency
With measured top-oil temperatures available, top-oil temperatures are also
calculated using measured currents and the two results compared. If the
measured top-oil temperature exceeds the calculated top-oil temperature by a
margin specified by the user (via the CSEPx setting), a Cooling System
Efficiency (CSE) Relay Word bit is asserted. This bit can be programmed to
where:
RATL = ratio of load loss at rated load to no-load loss (set from
Table 3.3)
EXPn = oil exponent (set from Table 3.3)
TH
or
= top-oil rise over ambient temperature at rated load, °C (set
from Ta ble 3.3)
where:
OTR = thermal time constant in hours at rated load with initial
top-oil temperature equal to ambient temperature (set
from Ta ble 3.3)

TO U
K
2
RATL 1+• 
RATL 1+
--------------------------------------------


EXPn
TH
or
• =
T
o
OTR

TO U
TH
or
---------------------

TO i
TH
or
-------------------–

TO U
TH
or
---------------------


1
EXPn
---------------

TO i
TH
or
-------------------


1
EXPn
---------------
–
------------------------------------------------------------------------------------








• =
where:
DTMP = user-selectable default ambient temperature

H

TO

H
+=

TO

A

H
+=

TO
DTMP 
TO
+=