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transformer services
 
TECHNICAL BULLETIN
700

DISSOLVED GAS-IN-OIL ANALYSIS

Analysis of the levels and ratios of dissolved combustible gasses in electrical insulating fluids has developed into one of the most powerful tools available to diagnose potential problems in electrical equipment. TSI employs a composite of various systems to interpret the data generated by our dissolved gas analysis. Our primary evaluation methods for transformers (in order of increasing importance) are:

Ø The total combustible gas content

Ø The individual concentrations of combustible gasses, and the ratios of the "key gasses"

Ø The rate of generation or accumulation of combustible gasses versus time

TOTAL COMBUSTIBLE GAS CONTENT (TCG)

The TCG is the sum of the concentrations of the following gasses: hydrogen, methane, carbon monoxide, ethane, ethylene and acetylene. This is used predominately as a quick reference when recommending follow-up sampling. If the TCG in a transformer is below 700 ppm, annual testing is normally suggested. If the TCG is greater than 700 ppm, semi annual, quarterly or even monthly testing should be considered. (Depending on the "key gas" levels - see below). Generally, an increase in the TCG of 5% or greater per month is indicative of an impending problem.

CONCENTRATIONS OF INDIVIDUAL COMBUSTIBLE GASSES AND THE RATIOS BETWEEN THESE "KEY GAS" LEVELS

We have listed our maximum recommended concentrations for each of the dissolved combustible gas contents below. These limits were developed in accordance with data from various sources, including IEEE, Doble Engineering and several transformer manufacturers, and apply to mineral oil filled transformers only.

Dissolved Gas Max Rec. Concentration Potential Source of Gas Generation

Hydrogen 100 ppm Partial discharge (Corona) electrolysis of water

Methane 100 ppm Overheating in the oil

Carbon Monoxide 500 ppm Overheating in the cellulose (paper) insulation

Ethane 60 ppm Overheating in the oil

Ethylene 100 ppm Overheating in the oil

Acetylene 5 ppm Arcing, severe overheating

The above limits are used as a guideline only; some units may contain higher levels of dissolved gasses and still be operating normally (in particular, specialty transformers and accessory equipment - see below).

By comparing ratios of various "key gasses" found in the transformer oil, we can attempt to further qualify the location of the possible fault (or faults) in the unit. We use both Rogers Ratios and Binary Method to diagnose problems and determine service options.

The ratios of carbon monoxide to carbon dioxide and oxygen to nitrogen are also significant. The CO:CO2 ratio should be in the range of 1:10; if this ratio decreases to 1:3 ratio or less, overheating in the cellulose is most likely occurring. The O2:N2 ratio in sealed headspace transformers (nitrogen blanket or bladder-type conservator) should be maintained at less than 1:10, in order to prolong the useful life of the fluid.

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GENERATION RATES OF COMBUSTIBLE GASSES

Monitoring the rate at which dissolved combustible gasses are being produced in a transformer is by far the most useful tool in evaluating the condition of a transformer. We need to establish generation rates in order to determine which "key gasses" are indicative of an active fault, as opposed to those which may only be residual from a past, isolated incident. Two or three samples over a short period will greatly aid us in diagnosing the potential problems and setting up a service program for the transformer.

EVALUATION OF COMBUSTIBLE GASSES IN HIGH FIRE POINT FLUIDS AND IN LTC's, OCB's, SWITCHES AND STEP REGULATORS

Presently, the only available method for evaluating "key gas" levels in silicone and RTEmp is to monitor the rate of gas generation. Increases in one or more combustible gas contents, verified by follow-up sampling, can be used to determine the potential problem areas. Maximum acceptable levels/ratios have not yet been established for high fire point fluids; TSI is continuing research in this area and will update our clients as soon as additional information is available.

Comparisons of data from periodic dissolved gas analyses is also the best method for interpreting conditions within furnace transformers, load tap changers, oil circuit breakers, switches and regulators. Unlike standard power transformers, this type of equipment can generate combustible gasses in the oil during normal operation; we evaluate these units on a case-by-case basis, looking for significant gains in particular gas concentrations as an indicator of potential faults in the unit.

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