Oil Sampling
8 mins read
Dissolved Gas Analysis (DGA) IEC60567
Electrical equipment, especially transformers, are critical for the generation and transmission of electrical power. The deterioration and identification of any incipient faults in the early stages of development is by far the most cost effective way for the avoidance of unplanned outages or possible catastrophic failure. The method used is both sensitive, reliable and an industry recognised technique known as Dissolved Gas Analysis (DGA).
Insulating oil/fluids which are put under abnormal thermal and electrical stress break down and release gaseous by-products which can be quantified and characterised to determine the types of incipient fault conditions such as overheating (pyrolosis), partial discharge (corona) and arcing.
Furan Analysis IEC61198
Transformer lifespan depends on its paper insulation. Using oil analysis to estimate the Degree of Polymerisation (DP), this method identifies insulation aging. Compounds like 2-furaldehyde and 5-methyl-2-furaldehyde signal paper degradation, helping determine replacement needs when DP ratings drop to critical levels.
Polychlorinated Biphenyls (PCBs) Analysis IEC61619
Polychlorinated biphenyls (PCBs) belong to a broad family of man-made organic compounds known as chlorinated hydrocarbons. PCBs were domestically manufactured from around 1930 until they were banned in 1979. Due to their non-flammability, chemical stability, high boiling and electrical insulting properties, PCBs were used in hundreds of industrial and commercial applications including transformers.
PCBs were manufactured and sold under many names, the most common being Aroclor. There are many different types of Aroclors and each have a distinguishing number, the most common found in electrical equipment such as transformers and capacitors are: 1260, 1254 and 1242.
Transformers with over 50mg/Kg of PCB should be registered with the Environmental Agency (EA).
Routine Analysis
Routine analysis consists of tests essential for evaluating the quality of insulating oil or fluid, identifying signs of aging, contamination, or degradation:
- Neutralisation (Acid) Number (IEC62021): This test measures the oil's acidity, expressed in mg/KOH per gram of oil. Acids, produced by oxidation or decomposition, can form sludge that clogs the transformer’s cooling system.
- Neutralisation (Acid) Number (IEC62021): Assesses the oil's resistance to electrical stress by measuring the voltage at which current arcs between electrodes. Contaminants like water or fibres reduce dielectric strength.
- Water Content (IEC60814): Water accelerates degradation of oil and cellulose insulation. Sources include atmospheric exposure (via breather leaks) and internal degradation. Water content varies with temperature.
- Colour (ASTM D1500): Oxidation darkens oil from pale yellow to dark amber, providing a visual indicator of aging. Silicone oils remain clear, while synthetic esters like Midel 7131 may discolor from dyes.
- Appearance (ASTM D1500): Inspected for clarity—hazy or cloudy oil suggests contamination by water or sediment.
- Interfacial Tension (IFT) (ASTM D971): Measures the tension at the oil-water interface to detect polar contaminants or degradation byproducts. New oil typically has an IFT of 40–45 dynes/cm.
These tests collectively provide a comprehensive understanding of the fluid's condition, guiding maintenance to ensure transformer reliability.
Dielectric Dissipation Factor (DDF) and Resistivity (IEC60247)
The Dielectric Dissipation Factor (DDF), also known as the Loss Factor or tan δ, measures the leakage current in the insulating oil under electrical stress. It is indicative of the oil's insulating properties and the presence of contaminants, aging products, or moisture. A high DDF suggests deterioration in the oil's ability to insulate and indicates a higher likelihood of electrical breakdown.
On the other hand, Resistivity measures the charge-carrying properties of the oil under DC conditions. Both DDF and resistivity are extremely sensitive to contamination, particularly by water or polar substances. High DDF and low resistivity often signal decreased insulation resistance, which may lead to transformer failure if not addressed.
This makes both DDF and resistivity critical indicators for the effective maintenance and longevity of transformers.
Oil Sampling
At EPMS Limited our HV engineers have over 30 years experience in taking and providing customers with independent. Laboratory reports covering a complete range of tests. They include water content, acidity, fibre content, electrical strength, along with DGA tests PCB levels fungals all to BS 2010 standards.
We provide new clean sampling bottles and all ways clean any debris from sampling point before running off several litres to ensure are engineers get a accurate sample of each transformer we also offer different sampling points to get more accurate report on larger transformers. Each sample is individually labelled with transformer ID number, voltage, sample temperature together with customer name and date etc. Once sample is taken it then goes in its own box to minimise damage in transit.
Each transformer then undergoes a comprehensive visual check reporting on oil levels condition of tank and cooling radiators as well as LV and HV connection boxes. Visual inspection of all LV and incoming cables are also reported.
Oil leaks are mainly due to gasket failure, our vast experience enables us to confirm most oil leak faults there and then giving our customers confidence in a reliable and long lasting repair to any oil loss.