Modern oil-immersed transformers are built with relatively tight insulation tolerances compared to older equipment and as a result are subjected to higher level of stress in service. It is therefore very important to closely monitor their in-service behaviours so as to avoid catastrophic failures, costly outages and consequential losses of production.

The Dissolved Gas Analysis (DGA) is a very efficient tool that helps to assess and monitor the condition of transformers by analyzing dissolved gases found in the mineral or silicone oil.

Through DGA, any abnormal condition in transformers can be detected. It can give warning about an impending problem, facilitate an early diagnosis and increase the chance of finding an appropriate remedy. Other benefits of DGA include the prevention of major failures of transformers, monitoring of overloaded transformers and convenient scheduling for transformer repairs.

The operating principle of DGA is centered on the deterioration of insulation that accompanies incipient faults. These faults can come in various forms as follow:

These stresses will result in the chemical breakdown of oil and cellulose molecules that constitute the dielectric insulation. The main degradation products are combustible gases, which are entirely or partially dissolved in the oil. These gases are very soluble in oil and may not be released as free-gases until the oil is fully gas-saturated. Thus by the time the combustible gases are released from the oil as free-gases where they can be detected by protective devices like Buchholz gas and oil actuated relays, the transformer insulation may already have sustained serious damage beyond repair. The dissolved gases can now be detected by DGA analysis.

A precise DGA is a laboratory test of oil samplings taken directly from the transformer. The analytical procedures involve removing dissolved gases from the samplings and then injecting the gases into a dedicated gas chromatograph (GC) via a carrier gas. The vacuum degassing technique is employed, which sprays the sample oil into a cylinder which then chamber-vacuum it effectively. By evaluating the respective quantities of dissolved gases in the transformer oil, transformer problems such as partial discharge, arcing and thermal fault can be identified. The interpretation of DGA results is based on the IEC 60599 standard.

One word of caution regarding DGA is that to avoid drawing any conclusions on the basis of a single oil sample (i.e. a sample taken at a particular point in time). The other important aspect to note is that certain amounts of combustible gases are normally present in the insulating oil of most healthy power transformers. Thus it is not generally advisable to obtain an indication of the fault condition of a particular transformer, or of its life expectancy, simply by carrying out a single DGA. The most reliable indications are those obtained when a DGA history has been maintained and a discernable change in an established pattern is observed. Therefore it is important to perform DGA on a regular basis (please refer to the transformer operation and inspection manual for the recommended frequency of DGA).

Our dissolved gas analyzer can process 12 mineral and silicone oil samples sequentially. In total 9 dissolved gases are monitored :-

			Detection Limit
	Non-Combustible Gases
1.	Nitrogen		10 ppm
2.	Oxygen		5 ppm
3.	Carbon Dioxide		5 ppm
	Combustible Gases
4.	Carbon Monoxide		5 ppm
5.	Hydrogen		1 ppm
6.	Methane		0.2 ppm
7.	Ethane		0.2 ppm
8.	Ethylene		0.2 ppm
9.	Acetylene		0.1 ppm

Besides the DGA, we also perform oil characteristics tests which complement the DGA in evaluating the internal conditions of a power transformer.

1. Water Content   This gives an indication of the water content in the paper insulation material. Overly high water content in the paper will adversely affect the ageing process of the paper, leading to irreversible damage that may cause an electrical breakdown in the transformer. Water is produced through oxidation of the oil as well as the paper.
2. Volume Resistivity   Volume resistivity is very sensitive to various contaminants and ageing materials.
3. Dielectric Strength   This is a measure of the ability of the insulating oil to withstand electrical stress. The level of breakdown voltage is dependent on both the water and particle contents in the oil. Water and particles are created when oil and paper deteriorate.
4. Total Acid Value   This value indicates the acidity of the oil. An elevated or increasingly elevated value indicates that the oil has started to oxidize. Acids also promote the degradation of the paper insulation (catalytic action).
		Back to Top

The oil characteristics tests complement the DGA in evaluating the internal conditions of a power transformer. It is highly recommended to perform these oil tests together with DGA.