Application Notes; Lol Setting Guidelines; Example - GE P642 Technical Manual

Chapter 7 - Transformer Condition Monitoring
3.2

APPLICATION NOTES

3.2.1

LOL SETTING GUIDELINES

Set the life hours at the reference maximum Hotspot temperature. According to IEEE Std. C57.91-1995, the normal
insulation life at the reference temperature in hours or years must be user-defined. The following table extracted
from IEEE Std. C57.91-1995 gives values of normal insulation life for a well-dried, oxygen-free 65°C average
winding temperature rise insulation system at the reference temperature of 110°C.
50% retained tensile strength of insulation (former IEEE Std
C57.92-1981 criterion)
25% retained tensile strength of insulation
200 retained degree of polymerization in insulation
Interpretation of distribution transformer functional life test
data (former IEEE Std. C57.91-1981)
Note:
Tensile strength or degree of polymerization (D.P.) retention values were determined by sealed tube aging on well-dried
insulation samples in oxygen-free oil.
Note:
Refer to I.2 in annex I of the IEEE Std. C57.91-1995 for discussion of the effect of higher values of water and oxygen and also
for the discussion on the basis given above.
You should set the designed Hotspot temperature setting (Designed HS temp) to 110°C if the transformer is rated
65°C average winding rise. If the transformer is rated 55°C average winding rise, set it to 95°C.
As recommended by IEEE C57.91-1995, set the Constant B Set setting to 5000.
If the calculated aging acceleration factor is greater than the setting FAA> Set and the time delay tFAA> Set has
elapsed, the FAA alarm DDB signal is activated.
If the calculated loss of life is greater than the setting LOL>1 Set and the time delay tLOL> Set has elapsed, the
LOL alarm DDB signal is activated.
3.2.2

EXAMPLE

Consider a new 65°C average winding rise rated transformer whose life hours figure at designed maximum
Hotspot temperature is 180,000 hrs. As a result, you set the Life Hours at HS setting to 180,000, and the
Designed HS temp setting to 110.0°C. Set Constant B Set to 15,000 as recommended by IEEE. The aging
acceleration factor takes into consideration the constant B and the hottest spot temperature calculated by the
thermal function. For a distribution transformer, IEEE suggests 200°C as the limit for the maximum hot spot
temperature. The aging acceleration factor alarm will be asserted when 70% of the 200°C has been reached. The
aging acceleration factor is calculated as follows:
B

−

383 hottest spot tempt
FAA e

=
Therefore:
FAA>set should be set to 17.2. tFAA> Set may be set to 10.00 min. LOL>1 Set may be set to 115,000 hrs, if
it is considered that the transformer has 65,000 hrs left (Life Hours at HS – hours left = 180,000 – 65,000 = 115,000
hrs). tLOL> Set may be set to 10.00 min. Finally, the Reset Life Hours setting determines the value of the LOL
152
Basis
B
B

 
−


273
− − + 383 0 7 200 273 .
e
 
=
Normal Insulation life Hours
65000
135000
150000
180000
B


× +
= 17 2 .

P64x
Normal Insulation Life Years
7.42
15.41
17.12
20.55
P64x-TM-EN-1.3