Dissolved Gas Analysis
NPM Services offer Dissolved Gas Analysis (DGA) testing of
transformer insulating fluids. We use a state of the art
direct injection method Shimadzu 2014 Togas system. The system
allows us to analyze data via a PC output chart that
automatically captures the data.
Dissolved gas analysis (DGA) is the most important tool in
determining the condition of a transformer. It is the first
indicator of a problem and can identify deteriorating
insulation and oil, overheating, hot spots, partial discharge,
and arcing. The health of the oil is reflective of the health
of the transformer itself. We offer on-site dissolved gas
collection services or we will provide syringes and sampling
procedures for customers to collect their own samples and mail
to our lab.
A four condition dissolved gas analysis (DGA) collection
guide to classify risks to transformers with no previous
problems has been published in the Standard IEEE C57-104. The
guide uses a combination of individual gases and total
combustible gas concentrations as indicators. The condition
levels are shown in the table below.
Table: Dissolved Gas Analysis
Concentration Limits in Parts Per Million (ppm). CO2 in not
included in adding the numbers for TDCG because it is not a
Condition 1: Total dissolved combustible gas (TDCG) below
this level indicates the transformer is operating
Condition 2: Total dissolved combustible gas (TDCG) within
this range indicates greater than normal combustible gas
Condition 3: Total dissolved combustible gas (TDCG) within
this range indicates a high level of decomposition of
cellulose insulation and/or oil.
Condition 4: Total dissolved combustible gas (TDCG) within
this range indicates excessive decomposition of cellulose
insulation and/or oil.
The condition based method is a starting point for
evaluating dissolved gas analysis results. In
the absence of previous data it is the default method used in
determining transformer health. If past data is available a
sudden increase in key gases and the rate of gas production is
more important in evaluating a transformer than the
accumulated amount of gas.
One very important gas is acetylene (C2H2). Generation of
any amounts of this gas above a few ppm indicates high-energy
arcing. Trace amounts can be generated by a hot thermal fault.
A one-time arc, caused by a nearby lightning strike or a high
voltage surge, can also generate small amounts of C2H2. If
C2H2 is found in the dissolved gas analysis, oil samples
should be taken weekly or even daily to determine if
additional C2H2 is being generated. If no additional acetylene
is found and the level is below IEEE condition 4, the
transformer may continue in service. However if acetylene
continues to increase, the transformer has an active
high-energy internal arc and should be taken out of service
immediately. Further operation is extremely hazardous and may
result in explosive catastrophic failure of the tank,
spreading flaming oil over a large area.
In summary NPM Services, Inc offers dissolved gas analysis
testing to help determine transformer condition for utilities,
manufacturers, universities, and government entities to help
our customers maintain healthy oil filled transformers.
Assessing transformer condition through diagnostic techniques
is also important for conducting asset management studies for
transformer replacement planning. Contact
us today to discuss your needs for