Dissolved Gas Analysis Method

Topics: Carbon dioxide, Transformer, Hydrocarbon Pages: 8 (1947 words) Published: November 30, 2012

AUTHOR’S NAME:Ruban s/o Paramasivam
SUPERVISOR’S:Mr. John Steven
PROJECT TITLE:Dissolved Gas Analysis in determining
Transformer Faults SUBMISSION DATE:16TH JULY 2012

1.0 Background of Studies
Oil sampling analysis is a useful, predictive, maintenance tool for determining transformer health. DGA is identified as one of the sufficient method of oil sampling in evaluating transformer health. The breakdown of electrical insulating material inside the transformer generates gases within the transformer. The identity of gases being generated is useful in any preventive method maintenance program. DGA method involves oil sampling method and testing the sample to measure the concentration of the dissolved gases. The two typical principal cause of gas formation within an operating transformer are electrical disturbance and thermal decomposition. All transformers generate gasses to some extent at normal operating temperature. Insulating mineral oils for transformer are mixtures of many different hydrocarbons and the decomposition process for these hydrocarbons are complex. During this process, active hydrogen atoms and hydrocarbons fragments are formed. These fragments can combine with each other to form gasses such as Hydrogen (H2), Methane (CH4), Acetylene (C2H2), Ethylene (C2H4), Ethane (C2H6) and many more. The gasses listed are considered combustible. The rate at which each gas are produced depends on the temperature. Therefore, the concentration of the individual dissolved gasses found in transformer insulating oil may be used directly to evaluate the transformer and suggest any faults within the transformer. After samples have been taken and analysed, the first step in evaluating DGA result is to consider the concentration level of each gasses. Basically, any sharp increase of the key gasses stated above indicates potential problem within the transformer. The type of faults which the key gasses can produce will be further discussed in the study.

Literature Review
2.0 Dissolved Gas Analysis

Power Transformers are filled with oil which acts as a dielectric medium and also as a heat transfer agent besides being an insulator to the transformer. The insulated oil is made up of saturated hydrocarbons. These molecules are connected together to form a chain liked manner by carbon and hydrogen.[1]

Table 1: Chemical structure of insulating oil and fault gases

During normal use, there is a slow degradation of mineral oil which produces gases that dissolve in the oil, but when there is a electrical fault, the oil starts to degrade and temperature rises.Different patterns of gases are generated due to different intensities of energy dissipated according to the type of faults. This happens because of the broken chain of the chemical structure of the insulating oil. Therefore, the broken chain will form its own chemical structure which is known as hydrocarbon gases or also known as fault gases. It can be divided into 3 categories which is Thermal heating, Corona and Arching, The most severe intensity of energy dissipation occurs with arching, followed by thermal heating and the least severe is Corona. Figure 1 illustrates the process of breaking chain within the insulating oil chemical structure of the fault arcing, thermal heating, and corona.

Figure 1: Breaking chain process of fault arcing, corona, thermal heating and pyrolysis of cellulose

Gases which are produced by the degradation of oil because of the increase of temperature may be caused by several factors: [2]

* severe overloading
* lighting
* switching transients
* mechanical flaws
* chemical decomposition of oil or insulation
* overheated areas of the windings
* bad connections which have a high contact resistance

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