Combustible Gas Analysis of Insulating Oil
4.2.3.1 Introduction
An oil-fi lled transformer insulation system consists of insulating oil and
cellulose (paper) materials. Under normal use, transformer insulation deteriorates
and generates certain combustible and noncombustible gases. This
effect becomes more pronounced when the transformer insulation is exposed
to higher temperatures. When cellulose insulation (i.e., winding insulation)
is overheated to temperatures as low as 140°C, carbon monoxide (CO), carbon
dioxide (CO2), and some hydrogen (H) or methane (CH4) are liberated. The
rate at which these gases are liberated depends exponentially on the temperature
and directly on the volume of the insulation at that temperature.
When insulating oil is overheated to temperatures up to 500°C, ethylene
(C2H4), ethane (C2H6), and methane (CH4) are liberated. When oil is heated to
extreme temperatures, such as an electrical arc, hydrogen (H) and acetylene
(C2H4) are liberated in addition to the above mentioned gases.
The main cause of gas formation in a transformer is due to the heating of
paper and oil insulation and electrical problems inside the transformer tank.
The electrical problems can be classifi ed as low-energy phenomena, such as
corona, or high-energy phenomena, such as an electrical arc. The interpretation
of the test data in terms of the specifi c cause (or causes) is based on the
type of gas (or gases) and the quantity of that gas found in the transformer.
The detection, analysis, and identifi cation of these gases can be very helpful
in determining the condition of the transformer. Establishing a baseline data
as a reference point for new transformers and then comparing with future
routine maintenance test results is a key element in the application of this test
method. However, monitoring or assessing the condition of a transformer
using this method can start at anytime even if the reference data is not available.
There are two methods for detecting these gases: (1) total com