Transformer Liquids and Testing

Oil Types and Oil Tests

Polychlorinated Biphenyls

Man made substance with high dielectric strength. Is not flammable but extremely toxic and not biodegradable. PCB’s are very hard to dispose of and create more toxic products when burned. Equipment must be labeled if they contain any PCB’s

Mineral Oil

Most common insulating fluid found in transformers. Byproduct of petroleum distillation. Has a low fire point, and is non-renewable.

Silicone Oil

Liquid silicone polymers that are fully synthetic. Possess high ignition temperatures, and have the best overall thermal stability. They are hardly biodegradable, and have high viscosity at high temperatures. Have very poor lubricant properties

Synthetic esters

Derived from chemicals. At room temperature, the viscosity is around 4 times higher than that of mineral oil. Is oxygen stable, so more suitable for free breathing transformers. Absorbs water from insulating paper which slows degradation.

Natural Esters (Vegetable Oil)

High water saturation point. Very biodegradable. High fire point. Lower clean up costs. Absorb moisture from the insulating paper, which slows the rate of degradation.

Oil Sampling

Removing a set amount of oil to be tested to determine quality of oil, rate of degradation, and observe possible issues.

Oil Sampling Process

Flush 2-4 liters of oil through valve (half to a whole gallon). If taking syringe AND bottle, take the syringe last. Rinse bottle with about 1/3 volume unless particle free bottle. Fill without causing aeration.

Sampling Fill Levels

• Glass bottles

  • Fill to the shoulder or roughly an inch from the top to ensure room for expansion

• Syringe

  • Initially fill syringe a little past 50mL, then slowly push out until 50mL is achieved. This removes air bubbles

  • Do not remove bubbles after sample has cooled

Containers for Sampling

• General Sampling

  • Glass bottles (amber or clear), aluminum, steel or tin cans, HD polyethylene bottles

• DGA Sampling

  • Ground glass syringes or SS Cylinders

• Water Content

  • Ground glass syringes, glass or metal bottles, no plastic containers

Specific Fill Levels

• Bottles

  • Fill to shoulder and seal

• Cans, Steel Cylinders, Plastic Bottles

  • Fill to overflowing and seal

• Syringes

  • Flush 2-3 times

  • Remove gas bubbles immediately

  • Do not remove once sample has cooled

Oil Processing

Filtering Oil Steps

• Pump out transformer

• Pull vacuum on transformer

• Filter oil

• Pump oil into transformer after being filtered

• Test transformer oil

American Society for Testing Materials

ASTM

Formed in 1898

Establishes a global standard for determining what tests should be and what acceptable results are

Dielectric Breakdown

Minimum voltage at which flash over occurs in an oil

Measures the ability of the oil to withstand electrical stress at power frequencies without failure

Low value = contamination

Results for Dielectric Breakdown

Doble

• Doble

  • 20 kV minimum (1 mm electrode gap)

  • 40 kV minimum (2 mm electrode gap)

Water Content

Measuring amount of water found in transformer

Acceptable results for Mineral Oil

• Doble

  • 30 ppm max on new oil

• NETA MTS 2019 (Used Oil)

  • Up to 69kV = 35 ppm at 60 degrees Celsius

  • 69kV to 230kV = 25 ppm at 60 degrees Celsius

  • 230kV and above = 20 ppm at 60 degrees Celsius

• NETA ATS 2017 (New)

  • Up to 69kV = 20 ppm max

  • 69kV and up = 10 ppm max

Color (ATSM D1500)

Determination of level of degradation of oil based on visual color

Higher color numbers indicate contamination, degradation, or both

Acceptable Color Results

• Doble

  • 0.5 max on new oil

• NETA MTS 2019 (Used Oil)

  • All voltages = 3.5 max

• NETA ATS 2017 (New Oil)

  • Up to 345kV = 1.0 max

  • 345kV and up = 0.5 max

Viscosity (ASTM D445)

Thickness of oil

High viscosity = more thick

Low viscosity = more thin

Acceptable Results

• Doble

  • 100 degrees Celsius = 3.0

  • 40 degrees Celsius = 11.0

  • 0 degrees Celsiuis = 76.0

Neutralization Number (ASTM D974)

Number of acidic or alkaline materials present

As oil ages in service acidity/alkalinity increases

High number means contamination/oxidized

Acceptable Results

• Doble

  • Maximum 0.015 mg KOH/g

• NETA MTS 2019 (Used)

  • Up to 69kV = maximum 0.20 mg KOH/g

  • 69 kV to 230kV = maximum 0.15 mg KOH/g

  • 230kV and above = 0.10 mg KOH/g

• NETA ATS 2017 (New)

  • All voltages = 0.015 mg KOH/g

Interfacial Tension (ATSM D971)

Amount of force needed to break the oil film existing in an oil-water interface

Soaps, paints, oxidation, and varnishes lower film strength

Higher number = better

Acceptable Results

• Doble

  • Minimum 40mN/m at 25 degrees Celsius

• NETA MTS 2019 (Used)

  • Up to 69 kV = minimum 25mN/m

  • 69kV to 230kV = minimum 30mN/m

  • 230kV and above = minimum 32mN/m

• NETA ATS 2017 (New)

  • All voltages = minimum 38mN/m

Power Factor (ASTM D923)

Dielectric Loss of Insulation

High power factor = contamination and deterioration

Acceptabl Results

• Doble

  • 25 degrees Celsius = maximum 0.5%

  • 100 degrees Celsius = maximum 0.30%

• NETA MTS 2019 (Used)

  • 25 degrees Celsius = max 0.5%

  • 100 degrees Celsius = max 5.0

• NETA ATS 2017 (New)

  • 25 degrees Celsius = maximum 0.05%

  • 100 degrees Celsius for up to 230kV = max 0.40%

  • 100 degrees Celsius for 230kV and up = max 0.30%

Specific Gravity (ASTM D1298 or D4052)

Ratio of Equal Weights of oil to water (relative density)

Used to determine if ice is present in extremely cold environments

Acceptable Results

• Doble

  • 60/60 = maximum 0.910

• NETA MTS 2019 (Used)

  • 15 degrees Celsius = max 0.910

Visual Examination

Visual inspection of oil to determine the clarity and if particles are floating in the oil

Acceptable results

• Doble

  • Bright and clear

• NETA MTS 2019 (Used)

  • Bright, clear, and free of particles

• NETA ATS 2017 (New)

  • Bright and clear

PCB’s in Fluids (ASTM D4059)

Testing oil to determine levels of PCB’s present

Regulated and prohibited from commercial distribution

Products should not contain large amounts of PCB’s

Less than 50 ppm

Furanic Compounds (ASTM 5837)

Serves as insulation degradation indicator

Byproducts of cellulosic material like paper, pressboard, and wood

Low baseline = no significant furanic compounds in oil

Acceptable Results

• Doble

  • Max 25 ug/L

Oxidation Inhibitor Content (ASTM D2688 or D4760)

Determines the quantity of oxidation inhibitors in oil

Control of inhibitor content = longer service life of oil

Acceptable Results

• Doble

  • Type 1 = maximum 0.08% by weight

  • Type 2 = maximum 0.3% by weight

Parts Per Million

Weight of value/ weight of oil

Dissolved Gas Analysis

DGA

Measurments of gas levels inside transformers for deterimining types of pending or occurring faults within transformer

Great for predicting faults or failures

Sensitive and accurate measurements