GE P642, Technical Manual

Page: 176 / 592

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.
Basis Normal Insulation life Hours Normal Insulation Life Years
50% retained tensile strength of insulation (former IEEE Std
C57.92-1981 criterion)
65000 7.42
25% retained tensile strength of insulation 135000 15.41
200 retained degree of polymerization in insulation 150000 17.12
Interpretation of distribution transformer functional life test
data (former IEEE Std. C57.91-1981)
180000 20.55
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:
FAA e e
B B
hottest spot tempt
B B
= =
−
− − +






−
× +
383 273
383 0 7 200 273 .






=
17 2 .
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
Chapter 7 - Transformer Condition Monitoring P64x
152 P64x-TM-EN-1.3