Lubezine Volume 8 * NOVEMBER 2013 - JANUARY 2014 | Page 22

RECYCLING

FEATURE

insulation. (See figure 2).

Indefinite life
The cellulose materials are the weakest link
in the insulation system. Since the life of the
transformer is actually the life of the cellulose
insulation and degradation of the cellulose
is irreversible, the decay products should
be removed before they can do any further
damage to the cellulose.
With proper maintenance, the cellulose
can virtually have an indefinite life. To test
for furanics, a sample of the oil is obtained
and certain chemical techniques are used to
extract the furans from the oil. The extract
is then analysed using a process called high
performance liquid chromatography (HPLC).
The results are usually reported in terms of
parts per billion (ppb).

Dissolved Gas Analysis (DGA)
The analysis of gases from petroleum products
has been performed for decades using gas
chromatography. However, this technique
was not applied specifically to transformer
mineral oils until the late 1960s/early 1970,s
and is now commonly called dissolved gas-inoil analysis (DGA).
DGA has become a standard in the electrical
maintenance industry throughout the world
and is considered to be the most important oil
test for transformer oils in electrical apparatus. More importantly, an oil sample can be
taken at any time from most equipment without having to take it out of service, allowing a
“window” inside the electrical apparatus that
helps with diagnosing and trouble-shooting
potential problems.
As the insulating materials of a transformer
break down from excessive thermal or electrical stress, gaseous by-products form. The
by-products are characteristic of the type
of incipient-fault condition, the materials
involved and the severity of the condition.
Indeed, it is the ability to detect such a

variety of problems that makes this test such
a powerful tool for detecting incipient-fault
conditions and for root-cause investigations
after failures have occurred.
Dissolved gases are detectable in low
concentrations (ppm level), which usually
permit early intervention before failure of
the electrical apparatus occurs, and allow for
planned maintenance. The DGA technique
involves extracting or stripping the gases from
the oil and injecting them into a gas chromatograph (GC). Typical gases generated from
mineral oil / cellulose (paper and pressboard)
-insulated transformers include: Hydrogen,
H2; Methane, CH4;Ethane, C2H6; Ethylene,
C2H4;Acetylene, C2H2;Carbon Monoxide,
CO and Carbon Dioxide, CO2.
Additionally, oxygen and nitrogen are
always present, their concentrations vary
with the type of preservation system used on
the transformer. Also, gases such as propane,
butane, butene and others can be formed as
well, but their use for diagnostic purposes is
not widespread.
The concentration of the different gases provides information about the type of incipientfault condition present as well as the severity.
For example, four broad categories of fault
conditions have been described and characterized in Table 1.
The severity of an incipient-fault condition
is ascertained by the total amount of combustible gases present (CO, H2, C2H2, C2H4,
C2H6, CH4) and their rate of generation.
Generally, transformers will retain a large
portion of the gases generated and therefore
produce a cumulative history of the insulating
materials’ degradation.
This is an important tool for detecting and
trending incipient problems. However, it also
means that care is needed in interpreting
values for a first-time analysis on service-aged
transformers (more than several years old),
which could contain residual gases from
previous events.

Abnormal behaviour
Some gas generation is expected from normal
ageing of the transformer insulation and it is
therefore important to differentiate between
normal and excessive gassing rates. Normal
ageing or gas generation varies with transformer design, loading and type of insulating
materials. Routinely, general gassing rates for
all transformers are used to define abnormal
behaviour. Specific information for a family
of transformers can be used when sufficient
dissolved gas-in-oil data are available.
Acetylene is considered to be the most significant gas generated. An enormous amount
of energy is required to produce acetylene,
which is formed from the breakdown of oil at
temperatures in excess of 700°C.
Excessively high overheating of the oil
will produce the gas in low concentrations.
However, higher concentrations are typically
symptomatic of sustained arcing, a more serious operational issue that can cause a transformer failure if left unchecked.
DGA is used not only as a diagnostic tool but
also to stem apparatus failure. Failure of a large
power transformer not only results in the loss
of very expensive equipment but it can cause
significant collateral damage as well. Revenue
losses due to operational outages may be the
least worrisome consequence of a failure.
Replacement of that transformer can take up
to a year if the failure is not catastrophic and
can result in tremendous revenue losses.
If the failure is catastrophic, then additional losses could be realised, such as adjacent
transformers, environmental problems from
the release of oil, (which could be as much as
20,000 litres), and the resulting fire that must
be contained and smothered.
In order to avoid such a failure H