Dissolved Gas Analysis

NPM Services offer Dissolved Gas Analysis (DGA) testing of transformer insulating fluids. We use a state of the art direct injection method Shimadzu 2014 Togas system. The system allows us to analyze data via a PC output chart that automatically captures the data.

Dissolved gas analysis (DGA) is the most important tool in determining the condition of a transformer. It is the first indicator of a problem and can identify deteriorating insulation and oil, overheating, hot spots, partial discharge, and arcing. The health of the oil is reflective of the health of the transformer itself. We offer on-site dissolved gas collection services or we will provide syringes and sampling procedures for customers to collect their own samples and mail to our lab.

A four condition dissolved gas analysis (DGA) collection guide to classify risks to transformers with no previous problems has been published in the Standard IEEE C57-104. The guide uses a combination of individual gases and total combustible gas concentrations as indicators. The condition levels are shown in the table below.

Status Hydrogen    (H2) Methane (CH4) Acetylene (C2H2) Ethylene
(C2H4)
Ethane
(C2H6)
Carbon Monoxide
(CO)
Carbon Dioxide
(CO2)
TDCG
Condition 1 100 120 35 50 65 350 2,500 720
Condition 2 101-700 121-400 36-50 51-100 66-100 351-570 2,500-4,000 721-1,920
Condition 3 701-1,800 401-1,000 51-80 101-200 101-150 571-1,400 4,001-10,000 1,921-4,630
Condition 4 >1,800 >1,000 >80 >200 >150 >1,400 >10,000 >4630

Table: Dissolved Gas Analysis Concentration Limits in Parts Per Million (ppm). CO2 in not included in adding the numbers for TDCG because it is not a combustible gas.

Condition 1: Total dissolved combustible gas (TDCG) below this level indicates the transformer is operating satisfactorily.

Condition 2: Total dissolved combustible gas (TDCG) within this range indicates greater than normal combustible gas level.

Condition 3: Total dissolved combustible gas (TDCG) within this range indicates a high level of decomposition of cellulose insulation and/or oil.

Condition 4: Total dissolved combustible gas (TDCG) within this range indicates excessive decomposition of cellulose insulation and/or oil.

The condition based method is a starting point for evaluating dissolved gas analysis results. In the absence of previous data it is the default method used in determining transformer health. If past data is available a sudden increase in key gases and the rate of gas production is more important in evaluating a transformer than the accumulated amount of gas.

One very important gas is acetylene (C2H2). Generation of any amounts of this gas above a few ppm indicates high-energy arcing. Trace amounts can be generated by a hot thermal fault. A one-time arc, caused by a nearby lightning strike or a high voltage surge, can also generate small amounts of C2H2. If C2H2 is found in the dissolved gas analysis, oil samples should be taken weekly or even daily to determine if additional C2H2 is being generated. If no additional acetylene is found and the level is below IEEE condition 4, the transformer may continue in service. However if acetylene continues to increase, the transformer has an active high-energy internal arc and should be taken out of service immediately. Further operation is extremely hazardous and may result in explosive catastrophic failure of the tank, spreading flaming oil over a large area.

In summary NPM Services, Inc offers dissolved gas analysis testing to help determine transformer condition for utilities, manufacturers, universities, and government entities to help our customers maintain healthy oil filled transformers. Assessing transformer condition through diagnostic techniques is also important for conducting asset management studies for transformer replacement planning.  Contact us today to discuss your needs for dissolved gas analysis.

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