If a 65°C average winding rise transformer is considered, the equation for FAA is as follows:
F e
AA
B B
H
=
−
+
383 273Θ
If a 55°C average winding rise transformer is considered, the equation for FAA is as follows:
F e
AA
B B
H
=
−
+
368 273Θ
where:
●
L(Q
H
) = life hour
s at winding hottest-spot temperature
●
Q
H,r
= hottest-spot temperature at rated load
● B = constant B from life expectancy curve. This parameter is set by the user. IEEE Std. C57.91-1995
recommends a B value of 15000.
The residual life hours at reference hottest-spot temperature is updated every hour as follows:
L L
F
res H r res p H r
AA i H
i
( ) ( )
( )
, , ,
,
Θ Θ
Θ
= −
=
∑
1
3600
3600
where:
●
L
r
es,p
(Q
H,r
) = residual life hours at reference temperature one hour ago
●
F
AA,i
(Q
H
) = mean aging acceleration factor, as calculated above (calculated every second)
The accumulated loss of life is updated in non-volatile memory once per hour. It is possible to reset this to a new
value if required; for example if the device is applied in a new location with a pre-aged resident transformer.
The rate of loss of life (ROLOL) in percentage per day is as follows. It is updated every day:
ROLOL
L
F
H r
AA m H
= ⋅ ⋅
24
100
( )
( ) %
,
,
Θ
Θ
The mean aging acceleration factor, F
AA
,m
, is as follows. It is updated every day:
F
F t
t
F
N
AA m
n
N
AAn n
n
N
n
n
N
AAn
,
=
∑ ⋅
∑
=
∑
=
=
=1
1
1
∆
∆
where:
● F
AA
,n
is calculated every power cycle
●
Dtn = 1 cycle
● F
AA,m
states the latest one-day statistics of F
AA
. When the device is energized for the first time, F
AA,m
default
value is 1.
The residual life in hours at F
AA,m
is as follows. It is updated every day:
L F
L
F
res AA m
res H r
AA m
( )
( )
,
,
,
=
Θ
P64x Chapter 7 - Transformer Condition Monitoring
P64x-TM-EN-1.3 151
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