Dear czping,
Thanks for your additional inputs.
a) reply to your question:
Some maintenance practices call for comparison of DGA results to that of gas accumulated in the Buchholz. So a tank top sample is considered to be more representative. You see how 'poor circulation' is correlated!
b) I have reproduced an extract of available literature with me. Hope this will be of some use. Please note the "longer than..."
...................
Dissolved Gas Analysis on
transformer oils from the main
tank of a transformer
...................
Sampling has to be done in accordance with
IEC 60567 or ASTM 3613
Analysis has to be performed in accordance
with IEC 60567/599(??) or ASTM 3612
DGA should be performed as frequently as
possible, but at least once a year as long as
the values remain below the limits of Table 1.
When values exceed the limits of Table 1, a
more frequent analysis is necessary (contact
Manufacturer).
Table 1 Limits for normal values
THRESHOLD VALUES
Rates of Concentrations
production(*,**) ppm
ml/day
GAS
H2 5 150
CH 5 110
CO2 200 13000
CO 50 900
C2H6 2 90
C2H 2 280
C2H2 0.1 50 (***)
#) If the measurement (one of the gasses)
exceeds one of the threshold values
(concentrations OR rates of production) in
Table 1 (and measurement of the second
sample confirms these results), a new
sample should be taken and analyzed.
##) If the values of the new sample still
exceed the threshold values in table 20,
Manufacturer should be contacted as soon as
possible.
Remarks:
1)If C2H2/H2 ratio is higher than 2, a
contamination from the OL TC
compartment is probable. If this is the
case, DGA of the main tank can be
unreliable.
2)If CO2/CO ratio is lower than 3, paper
deterioration is possible. This has to be
checked with a FAL analysis (specific
analysis for paper deterioration.
*) Rates of production ml/day = (dc x V)/(M x 1000)
where,
dc = difference in concentration over the
observed period in ppm
V = volume of the transformer litres
M = expired period between measurements in days
**) To have a reliable rate, intervals should be
longer than 2 weeks
***)If a communicating OL TC conservator is in
use, this value will be 270 ppm
MOST IMPORTANT:
1) Were you associated with all the earlier measurements on this particular transformer, to the same capacity/extent as in the recent ones?
2) Have attempted to analyse the Buchholz sample and compare with your results?
best regards,