GE P642, Technical Manual, Page: 174 / 592

3 LOSS OF LIFE STATISTICS
Deterioration of transformer insulation is a time dependent function of temperature, moisture and oxygen content.
The effects of moisture and oxygen can be minimized through designing in preservation systems for most modern
transformers, therefore it is temperature that is the main reason for transformer aging. Frequent overloading will
shorten the life expectancy of a transformer due to the elevated winding temperatures.
Insulation deterioration is not uniform and will be more pronounced at Hotspots within the transformer tank.
Therefore any asset management system intending to model the rate of deterioration is based on simulated real-
time Hotspot temperature algorithms. These models may take ambient temperature, Top Oil temperature, load
current, oil pump status (pumping or not), and radiator fan status (blowing or not).
Alstom transformer protection devices provide such a loss of life monitoring facility in accordance with the thermal
model defined in IEEE C57.91. The protection algorithm determines the current rate of life-loss, and uses that
information to indicate the likely remaining service time. The asset owner can be alerted in advance, so that he
can plan an outage in which to carry out the required maintenance such as reconditioning or rewinding.
3.1 LOSS OF LIFE IMPLEMENTATION
Loss of life settings are found in the THERMAL OVERLOAD column of the required setting group.
The device provides two single-stage definite time delay alarms based on aging acceleration factor (FAA) or loss of
life (LOL).
A reset command is provided to allow you to reset the calculated parameters displayed in the MEASUREMENTS 3
column: Loss of Life status (LOL status), Loss of Life aging factor (LOL Aging Factor), mean aging factor (FAA,m),
rate of loss of life (Rate of LOL), residual life at mean aging factor (Lres at FAA,m), and residual life at designed
(Lres at designed).
The loss of life model is executed once every power cycle.
3.1.1 LOSS OF LIFE CALCULATIONS
IEEE C57.91-1995 defines the aging acceleration factor as the rate at which transformer insulation aging for a
given maximum Hotspot temperature is accelerated compared with the aging rate at a reference maximum
Hotspot temperature. For transformers with average winding temperature rise of 65°C, the reference maximum
Hotspot temperature is 110°C. For transformers with average winding temperature rise of 55°C, the reference
maximum Hotspot temperature is 95°C. For maximum Hotspot temperatures in excess of the reference maximum
Hotspot temperature, the aging acceleration factor is greater than 1. For maximum Hotspot temperatures lower
than the reference maximum Hotspot temperature, the aging acceleration factor is less than 1.
The model used for loss of life statistics is given by the equations for LOL and FAA. LOL is calculated every hour
according to the following formula:
LOL = L(
Q
H,r
) - L
res
(
Q
H,r
)
where:
●
L(
Q
H,r
) = life hours at reference winding hottest-spot temperature (Life Hours at HS setting)
●
L
res
(
Q
H,r
) = residual life hours at reference winding hottest-spot temperature
The aging acceleration factor FAA is calculated once every cycle as follows:
F
L
L
e
e
e
AA
H r
H
A
B
A
B
B
H r
H
H
=
( )
( )
= =
+
+








+
+






Θ
Θ
Θ
Θ
Θ
,
,
273
273
,, r H
B
+
−
+








273 273
Θ
Chapter 7 - Transformer Condition Monitoring P64x
150 P64x-TM-EN-1.3