Dielectric Fluid Sampling & Analysis

Burlington Electrical Testing procures oil samples to perform a Dissolved Gas Analysis (DGA) and an Oil Content evaluation of switchgear insulating fluids in accordance with NETA and IEEE/ANSI (C57.104-1991) specifications. Insulation fluids analyzed include Mineral Oils, PCBs, Perc, R-Temp, and Silicone based.

Hydrocarbon, mineral-based, oils are commonly used in transformers as insulating fluids because of their high dielectric strength and chemical stability. Under normal operating conditions little decomposition occurs in these oils. However, under a fault condition, the oil will undergo chemical degradation. This breakdown releases various gases and chemicals into the oil. Only from a qualitative and quantitative analysis of the gases present, is it possible to distinguish such fault processes as Corona, Sparking, Overheating and Arcing occurring in the Transformer.

A Dissolved Gas Analysis can provide:

1. Indication of an ongoing problem
2. Prevention of a serious occurrence such as explosion or sudden outtage
3. Advanced warning of developing faults
4. A determination of the improper use of the apparatus tested
5. Status checks on new and repaired units
6. A means to trend the levels in order to conveniently schedule repairs

The interpretation of data from a DGA can be complex due to the many different characteristics and operating conditions that affect gas formation. Therefore establishing a baseline and performing regular testing and monitoring the content is the best way to determine and control the fault process.

A summary of mineral oil analysis and interpretations can be found below:

Dissolved Gas Analysis	Interpretation	Acceptable Limits
Hydrogen* H 2	Thermal decomposition of the cellulose insulation material - overheating condition.	500 ppm (0.05%) (Max.)
Methane* CH 4	Overheating or other fault conditions have occurred or exist inside the transformer.	100 ppm (0.010%) (Max.)
Ethane* C 2H 6	Low energy arcing has occurred or exists inside the transformer.	50 ppm (0.005%) (Max.)
Ethylene* C 2H 4	Transformer is operating hot or localized overheating has occurred or exists.	50 ppm (0.005%) (Max.)
Acetylene* C 2H 2	High energy arcing has occurred or exists inside the transformer.	0 ppm (0.00%) (Max.)
Oxygen O 2	Thermal decomposition of the cellulose insulation material - overheating condition.	20,000 ppm (2.00%) (Max.)
CarbonMonoxide* CO	Thermal decomposition of the cellulose insulation material - overheating condition.	1,000 ppm (0.10%) (Max.)
Carbon Dioxide CO 2	Thermal decomposition of the cellulose insulation material - overheating condition.	10,000 ppm (1.00%) (Max.)
* Combustible Gas
Oil Screen Analysis	Interpretation	Acceptable Limits
Moisture Content	Reveals if transformer tank is sealed or cellulosic breakdown via the level of water present.	25 ppm ↓ 69KV XFRM (Max.) 35 ppm ↑ 69KV XFRM (Max.)
Interfacial Tension	Reveals the presence of sludge. Precursor to the insulation’s failure to properly cool the unit.	24 dyne/cm (Min.)
Acid Number	The amount of acidic or alkaline material present.	0.20 mg KOH/g-oil (Max.)
Color Number	Elevated number indicates oil contamination or deterioration of the oil.	0.5 (New Oil) – 8.0 (Worse case )
Dielectric Breakdown	Reveals conductive contaminants in the oil such as metallic cuttings, fibers or free water.	24 KVAC (Min.)
Specific Gravity	Helps reveal if oil has been mixed with other fluids.	Approx 0.875 @15°C
Power Factor	Reveals leakage current through oil. Elevated power factor reveals moisture, resins or fuel oils.	0.5% ↓ 69KV XFRM @ 25°C 1.0% ↑ 69KV XFRM @ 25°C (Max.)

Furanic Compound testing is used to determine the approximate age of transformer insulation and to estimate the remaining life of the transformer.

This process is performed by first procuring an insulating fluid sample from a transformer (which can be performed on line), and then analyzing the sample with a Liquid Chromatograph. The chromatograph detects decomposed cellulose compounds in the oil.

The most significant compound is 2-Furfuraldehyde; concentrations of this compound correlate the Degree of Polymerization (DP), which is a number that relates to the average number of D-Glucopyranose monomers present in cellulose chain.

The higher the numbers of D-Glucopyranose monomers present the better the condition of the insulation in the transformer.

The following chart gives a quick reference of the DP # to the age of the transformer.

New Kraft Paper    1000 – 1300    Middle aged Paper       approx. 500    Old paper    less than 250       Dust    less than 150