Page 92 - Electrical Equipment Handbook _ Troubleshooting and Maintenance

TRANSFORMER COMPONENTS AND MAINTENANCE

TRANSFORMER COMPONENTS AND MAINTENANCE 4.21

the oil and solid insulation. Gases are also generated from the decomposition of oil and
insulation exposed to arc temperatures. The following paragraph was extracted from the
IEEE Guide for the interpretation of gases generated in oil-immersed transformers:

The detection of certain gases generated in an oil filled transformer is frequently the first avail-
able indication of a malfunction that may eventually lead to failure if not corrected. Arcing,
corona discharge, low-energy sparking, severe overloading, pump motor failure, and overheat-
ing in the insulation system are some of the possible mechanisms. These conditions occurring
singly, or as several simultaneous events, can result in decomposition of the insulating materi-
als and the formation of some gases. In fact, it is possible for some transformers to operate
throughout their useful life with substantial quantities of combustible gas present. Operating a
transformer with large quantities of combustible gas present is not a normal occurrence but it
does happen, usually after some degree of investigation and an evaluation of possible risk.

The generated gases in the transformer can be found dissolved in the oil, in the gas blan-
ket above the oil, or in gas-collecting devices. If an abnormal condition is detected, an eval-
uation is required to determine the amount of generated gases and their continuing rate of
generation. When the composition of the generated gases is determined, some indication of
the source of the gases and the kind of insulation involved will be gained.
There are many techniques for detecting and measuring gases. However, the interpreta-
tion of the significance of these gases is not a science at present. It is an art subjected to vari-
ability. It is difficult to establish a consensus due to the variability of acceptable gas limits.
The main reason for not developing an exact science for fault interpretation is the lack of cor-
relation between the fault-identifying gases with faults found in actual transformers. Table 4.5
provides a general description of the various fault types with associated developing gases.

TABLE 4.5 A General Description of the Various Fault Types with Associated Developing Gases
Fault type Description

Arcing Arcing is the most severe of all fault processes. Large amounts of hydro-
gen and acetylene are produced, with minor quantities of methane and
ethylene. Arcing occurs through high-current and high-temperature
conditions. Carbon dioxide and carbon monoxide may also be formed if
the fault involved cellulose. In some instances, the oil may become car-
bonized.
Corona Corona is a low-energy electric fault. Low-energy electric discharges pro-
duce hydrogen and methane, with small quantities of ethane and ethyl-
ene. Comparable amounts of carbon monoxide and carbon dioxide may
result from discharges in cellulose.
Sparking Sparking occurs as an intermittent high-voltage flashover without high
current. Increased levels of methane and ethane are detected without
concurrent increases in acetylene, ethylene, or hydrogen.
Overheating Decomposition products include ethylene and methane, together with
smaller quantities of hydrogen and ethane. Traces of acetylene may be
formed if the fault is severe or involves electrical contacts.
Overheated cellulose Large quantities of carbon dioxide and carbon monoxide are evolved from
overheated cellulose. Hydrocarbon gases, such as methane and ethyl-
ene, will be formed if the fault involved an oil-impregnated structure. A
furanic compound and/or degree of polymerization analysis may be per-
formed to further assess the condition of the insulating paper.

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