Schweitzer Engineering SEL-387-0 - Page 87

3.37
Date Code 20170601 Instruction Manual SEL-387-0, -5, -6 Relay
Differential, Restricted Earth Fault, Thermal, and Overcurrent Elements
Thermal Element (SEL-387-6 Relay)
any of the relay outputs to perform alarm or tripping functions. Assertion of
the CSE bit indicates that the cooling system (fans and/or pumps) is operating
below expected efficiency and may require maintenance.
CSEPx = Cooling System Efficiency Pickup setting, °C, where x = 1 if
XTYPE = 3 and x = 1, 2, 3 if XTYPE = 1
Serial Communications Failure
The COMFLG Relay Word bit is asserted under the following conditions:
➤ Serial communications link fails and NTHM setting > 0.
➤ Any of the received temperatures is not valid and the NTHM
setting > 0. The range for valid measured temperatures is:
–40°C to 350°C.
The COMFLG bit will never assert if the user configures the relay not to
accept any temperatures by setting the NTHM setting to 0.
Insulation Loss of Life
Insulation Aging Acceleration Factor
The relay thermal element uses the hot-spot temperature to calculate an
insulation aging acceleration factor, F
AA
, which indicates how fast the
transformer insulation is aging.
The relay calculates the insulation aging acceleration factor, F
AA
, for each
time interval, t, as follows:
Equation 3.25
Daily Rate of Loss of Life
The relay calculates daily rate of loss of life (RLOL, percent loss of life per
day) for a 24-hour period as follows:
Equation 3.26
The relay stores the RLOL value at midnight each day to provide the user with
trend information on the loss of insulation life.
where:
F
AA
= insulation aging acceleration factor
BFFA = a design constant, typically 15000, °C (set from Ta ble 3.3 )

H,R
= winding hot-spot temperature at rated load (95°C if THwr
= 55°C, 110°C if THwr = 65°C)
THwr = average winding rise over ambient at rated load (setting)
F
AA
e
BFFA

HR
273+
----------------------------------
BFFA

H
273+
----------------------------–=
where:
RLOL = rate of loss of life in percent per day
ILIFE = expected normal insulation life in hours (set from
Table 3.3)
RLOL
F
EQA
24•
ILIFE
-------------------------- 100• =