Early Diagnosis of Transformer Troubles
Automated Dissolved-Gas Analysis Of Transformer Oils
by Jocelyn Jalbert and Roland Gilbert, Institut de Recherche en
Electricité HydroQuébec (Canada) and Susan Brillante,
HewlettPackard Co.
The malfunctions of highvoltage power transformers can be diagnosed by
analyzing the dissolved gases that typically form in the insulating
materials of the devices. A method for automated transformeroil gas
analysis (TOGA) has been developed by combining an HP 7694 static
headspace sampler with an HP 5890 Series II capillary gas chromatograph and an HP 3365 ChemStation.
Background of TOGA
Highvoltage power transformers are susceptible to problems such as
arcing, overheating and partial discharges. These malfunctions always
result in the chemical decomposition of the insulating materials,
mineral oil and cellulose. Decomposition, in turn, produces several
gases that are totally or partially dissolved in the oil, including H2,
CO, CO2, CH4, C2H2, C2H4, C2H6 and C3H8. The contact of ambient air
with the oil may also cause gases such as O2, N2 and CO2 to be present.
Because the relationship of dissolved gases to specific transformer
problems are known, it is possible to diagnose the presence of faults
at an early stage and take preventive action before failures (which can
include explosion) occur. Early diagnosis of faults by performing TOGA
on a routine basis will result in cost savings and improvements in
service reliability.
TCD and FID Detectors
An HP 7694 headspace sampler (HS) equipped with a sixport valve and a
1mL sample loop was used to reproducibly sample the dissolved gases in
transformer oils. The method uses two detectors in series: a thermal
conductivity detector (TCD) for the quantitation of H2, O2 and N2 and a
flame ionization detector (FID) with a methanizer for the quantitation
of the other gases.
The gases were separated by using a molecularsieve PLOT column
connected to a Carboxen(TM) PLOT column through a 6port valve. The TCD
and FID signals were recorded simultaneously by the ChemStation. The
figures show the FID signal and the TCD signal, respectively, for a
primary oil sample.
Standard Method Allows No Automation
Most electrical utilities perform TOGA according to Method A of the
D3612 standard procedure approved by the American Society for Testing
and Materials (ASTM). This method uses vacuum extraction of the gases of
interest, followed by packedcolumn GC chromatography. The method cannot
be automated and, to achieve reliable results, requires a dedicated and
welltrained analyst.
The authors have analyzed transformer oil samples using both the
ASTM D3612 Method A procedure and the HS/capillary GC technique (Jalbert & Gilbert, 1996).
The HS results demonstrated excellent correlation with the ASTM method
and lower detection limits.
Why Headspace?
Compared with the ASTM method, HS/capillary GC offers
significant benefits:
- equivalent precision
- lower detection limits
- shorter analysis times
- automation
These benefits result in higher sample throughput in the analytical laboratory. The analysis takes 55 minutes for the first sample, and 25 minutes for each additional one. It is at least five times faster than the ASTM D3612 method and potentially yields higherquality results with less skilled operators.
Reference
Jalbert, J., and Gilbert, R. Decomposition of Transformer Oils: A New
Approach for the Determination of Dissolved Gases. Accepted for
presentation to the IEEE Power Engineering Society, Denver, CO (1996).
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