Transformer Oil Analysis

Transformer Oil Analysis Transformer Oil Analysis

Transformer Oil Analysis Service

Regular oil analysis is useful in monitoring the condition of engines, hydraulics, turbines and other oil lubricated equipment. The same applies to transformer oils and other electrical distribution equipment. The analysis of insulating oils provides not only information about the oil, but also enables the detection of other possible problems, including contact arcing, aging insulating paper and other latent faults and is an indispensable part of a cost-efficient electrical maintenance program.

We offer a comprehensive service in transformer oil analysis at our accredited ISO9001:2008 oil analysis laboratory. We test all forms of transformer oils: - mineral, silicone and Midel. A full range of test suites are available for insulating and transformer oils:

  • Dissolved Gas in Oli Analysis (DGA) (Oil Gas Analysis) PCB Testing (Polychlorinated Biphenyls)
  • Electric Strength
  • FFA (Furfuraldehyde) Resistivity
  • Water Content
  • Acidity
  • Fibre Estimation
  • Colour
  • Metal in Oil Analysis

Transformer Reliability

Transformer maintenance has evolved over the past 20 years from a necessary item of expenditure to a strategic tool in the management of electrical transmission and distribution networks. Extreme reliability is demanded of electric power distribution, and even though the failure risk of a transformer and other oil-filled electrical equipment is small, when failures occur, they inevitably lead to high repair costs, long downtime and possible safety risks. Moreover, transformers are too expensive to replace regularly and must be properly maintained to maximize their life expectancy.

By accurately monitoring the condition of the oil, suddenly occurring faults can be discovered in time and outages can potentially be avoided. Furthermore, an efficient approach to maintenance can be adopted and the optimum intervals determined for replacement. Some of the checks are relatively simple: the operation of the gas relays, the operation of the on-load tap-changer, checks on oil leaks, etc. However, breakdown of one of the most crucial elements, the oil paper insulating system, can only reliably be detected by routine oil analysis.

By measuring the physical and chemical properties of oil, in addition to the concentrations of certain dissolved gases, a number of problem conditions associated with either the oil or the transformer can be determined. The following are some common tests performed on electrical insulating oils.

Moisture Content

One of the most important functions of transformer oil is to provide electrical insulation. Any increase in moisture content can reduce the insulating properties of the oil, which may result in dielectric breakdown. This is of particular importance with fluctuating temperatures because, as the transformer cools down, any dissolved water will become free, resulting in poor insulating power and fluid degradation. In addition, many transformers contain cellulose-based paper used as insulation in the windings. Again, excessive moisture content can result in the breakdown of this paper insulation with a resultant loss in performance.

Acid Number

As in industrial oils, transformer oils are oxidized under the influence of excessive temperature and oxygen, particularly in the presence of small metal particles which act as catalysts, resulting in an increase in Acid Number, due to the formation of carboxylic acids. Further reaction can result in sludge and varnish deposits. In the worst-case scenario, the oil canals become blocked and the transformer is not cooled well, which further exacerbates oil breakdown. Furthermore, an increase in the acidity has a damaging effect on the cellulose paper. Oil degradation also produces charged by-products, such as acids and hydro peroxides, which tend to reduce the insulating properties of the oil. An increase in Acid Number often goes hand-in-hand with a decrease in dielectric strength and increased moisture content.

PCB (Polychlorinated Biphenyls)

PCB’s are fairly non-biodegradable and can collect in food chains; legislation has been brought in to prevent widespread contamination. Liquids containing over 50mg/kg (or 50 ppm by weight) must be classed as injurious substances and disposal must be by high temperature incineration, which is expensive.

We use capillary column chromatography to determine the PCB concentration in oil. Our analysis identifies the three main types of PCB i.e. 1242, 1254 and 1260 and reports the total PCB content present.

Important information relating to PCB

PCBs were used as dielectric filler liquids in some types of electrical equipment such as transformers, switchgear, capacitors and in the starter units of fluorescent lights and fractional horsepower motors. Some equipment is labelled as containing PCBs but if you come across old equipment with no identifying label you should check with:

  • your employer; or
  • the manufacturer or owner of the equipment.

You should assume that any capacitor or transformer manufactured before 1976 may contains PCBs unless you have information to the contrary. It is also possible that there may be PCBs present in capacitors and transformers manufactured between 1976-1986. Even if the PCBs have been replaced by another liquid, significant amounts of PCBs may still be present. PCBs may occur as contaminants in the oil used in oil-filled electrical equipment. Always check with your employer if you are in any doubt. Well into the 1970s, polychlorinated biphenyls (PCB)s were often used as a dielectric fluid since they are not flammable. However, they are toxic, and, under incomplete combustion, can form highly toxic products such as furan. Starting in the early 1970s, concerns about the toxicity of PCBs have led to their being banned in many countries.Today, nontoxic, stable silicone-based or fluorinated hydrocarbons are used, where the added expense of a fire-resistant liquid offsets additional building cost for a transformer vault. Natural or synthetic esters are also becoming increasingly common as alternatives to naphthenic mineral oil. Esters are non toxic, readily biodegradable, and have higher flash points than mineral oil.

For information on the handling and disposal of PCBs see Waste Management Paper No 6 - Polychlorinated biphenyls (DoE 1994, available from HMSO) or contact your local Waste Regulation Authority (listed in your local telephone directory) or the Department of the Environment Enquiry Unit, 2 Marsham Street, London SWIP 3EB (Tel 0171 276 3000).

The National Association of Waste Disposal Contractors (NAWDC) will be able to give you a list of contractors who can handle PCBs and PCB waste. Their address is Mountbarrow House, 6-20 Elizabeth Street, London, SWIW 9RB.

Dielectric Strength

The dielectric strength (ASTM D300-00) of a transformer oil is defined as the maximum voltage that can be applied across the fluid without electrical breakdown. Because transformer oils are designed to provide electrical insulation under high electrical fields, any significant reduction in the dielectric strength may indicate that the oil is no longer capable of performing this vital function. Some of the things that can result in a reduction in dielectric strength include polar contaminants, such as water, oil degradation by-products and cellulose paper breakdown.

Dissolved Gas Analysis (DGA)

Dissolved gas analysis (often referred to as DGA), is used to determine the concentrations of certain gases in the oil such as nitrogen, oxygen, carbon monoxide, carbon dioxide, hydrogen, methane, ethane, ethylene and acetylene. The concentrations and relative ratios of these gases can be used to diagnose certain operational problems with the transformer, which may or may not be associated with a change in a physical or chemical property of the insulating oil.

For example, high levels of carbon monoxide relative to the other gases may indicate thermal breakdown of cellulose paper, while high hydrogen, in conjunction with methane may indicate a corona discharge within the transformer.

The following is a brief summary of the significance of the gases that can be found through analysis:

  • Oxygen/Nitrogen - Present due to air dissolved in the oil, normally through the breather and headspace above the oil. These constitute the majority of gases.
  • Hydrogen - Largely associated with partial discharges which is an electrical phenomenon within the unit. Hydrocarbon Gases (Methane, ethylene, ethane, acetylene) - associated with thermal problems within the unit and in some cases arching and sparking
  • Methane and ethane - mainly associated with thermal problem indicators.
  • Ethylene - can be thermal or arching indicator.
  • Acetylene - Indicators of high temperature thermal faults and arching

Dielectric Furans

Furan derivatives are a measure of the degradation of cellulose paper. When the paper ages, its degree of polymerization reduces and so its mechanical strength decreases. The degree of polymerization can only be determined directly by taking a sample of paper, a very complex operation and almost never performed in practice.

However, the degree of polymerization of the paper can be directly related to the concentration of furan derivatives in the oil. Furan derivatives are formed as a direct result of the breakdown of the polymeric structure of cellulose paper.

The content of furan derivatives is relatively easy to measure in the oil, using HPLC and is thus a way of measuring the aging of the paper.

Just like machinery oil analysis, electrical insulating oil analysis can play a vital role in preventing unscheduled outages in electrical transmission and distribution equipment by determining the condition of the equipment itself, and other vital components including the condition of the oil and the cellulose paper insulation. For all critical oil-filled electrical equipment, including transformers, circuit breakers and voltage regulators, regular, routine oil analysis should be the cornerstone of any PM program.

Featured product

Complete (UK) Lube Oil Analysis & Sampling Kit (LUBEKIT1)

Kit contents

60ml PETG sample bottle, sample tubing, equipment ID labels, re-sealable plastic bag and a pre-paid, addressed postal Jiffy bag to return your sample.

All-inclusive oil analysis service

Price is inclusive of the laboratory oil analysis, recommendations and comprehensive report, which will be issued to you in pdf format via e-mail.

Fast Service Turnaround

Turnaround time from receipt of the sample in our laboratory to return of your report and recommendations is 2 working days.