A member of the Set Point group OIL ANALYSIS

ISSUE 36 A basic introduction to an essential part of a cost-efficient maintenance programme

It is well known networks. Extreme reliability is that regular oil demanded of electric power analysis is distribution and, even though the extremely failure risk of a transformer and useful in other oil-filled electrical equipment monitoring the is small, when failures occur they condition of inevitably lead to high repair engines, costs, long downtime and very drivetrains, real safety risks. Moreover, hydraulics, are too expensive turbines and to replace regularly and must be many other properly maintained to maximise Neil Robinson types of oil their life expectancy. lubricated equipment. The same can be said for transformer oils By accurately monitoring the which are used to insulate many condition of the oil, many types transformers and other electrical of faults can be discovered distribution equipment. The before they become serious analysis of transformer oils not failures and outages can only provides information about potentially be avoided. the oil, but also enables the Furthermore, an efficient detection of other potential approach to maintenance can problems, including contact be adopted and the optimum arcing, ageing insulating paper intervals determined for and other latent faults and is an replacement. Some of the checks indispensable part of a cost- are relatively simple: on the efficient electrical maintenance operation of the gas relays, programme. on the operation of the on-load tap-changer, on oil leaks, etc.

ENSURING However, breakdown of one of the most crucial elements, the / paper insulating system, can RELIABILITY only reliably be detected by routine oil analysis. By measuring certain physical and chemical Transformer maintenance has properties of the oil, in addition evolved over the past 20 years to the concentrations of certain from a necessary item of dissolved gases, a number of expenditure to a strategic tool problem conditions associated in the management of electrical with either the oil or the transmission and can be determined.

WEARCHECK AFRICA IS AN ISO9001 AND ISO14001 REGISTERED COMPANY The following are some common tests the oil. An increase in acid number often performed on electrical transformer oils: goes hand in hand with a decrease in dielectric strength and increased moisture content as shown in Figure 1. Again, just like their MOISTURE CONTENT industrial cousins, the acid content of One of the most important functions of transformer oils is determined by transformer oil is to provide electrical potentiometric titration with potassium insulation. Any increase in moisture content hydroxide. can reduce the insulating properties of the oil, which may result in dielectric breakdown. Sludge and Water and oil, due to their differing chemical varnish properties are not mutually soluble. However, up to a certain limit, a small amount of water Oil oxidation Paper degradation will dissolve in the oil. This limit is a function Temperature Temperature of the temperature of the system and the

solubility increases exponentially with Oil Paper increasing temperature. This is of particular importance with fluctuating temperatures OxygenMetal Water because, as the transformer cools down, catalysts any dissolved water will become free, Acids Paper chain and scissors resulting in poor insulating power and oil hydroperoxides + water degradation. A point to note is that as the oil ages in service, a certain amount of Figure 1 oxidation occurs which changes the chemical make up of the oil. This, in turn, allows more DIELECTRIC STRENGTH water to dissolve. In addition, many The dielectric strength of a transformer oil transformers contain cellulose-based paper, is a measure of the oil’s ability to withstand which is used as insulation in the windings. electrical stress without failure. Because Again, excessive moisture content can result transformer oils are designed to provide in the breakdown of this paper insulation electrical insulation under high electrical with a resultant loss in performance. The potentials, any significant reduction in the moisture content of the oil is determined dielectric strength will indicate that the oil is using a coulometric Karl Fischer instrument. no longer able to perform this vital function. This is an extremely sensitive test and can Some of the things that can cause a reduction detect water at levels down to a few parts in dielectric strength include contaminants per million. such as water, sediment, conducting particles, ACID NUMBER oil degradation by-products and cellulose Just like lubricating oils, transformer oils paper breakdown. The test method for are oxidised under the influence of excessive determining dielectric strength is relatively temperature and , particularly in the simple and involves applying an AC voltage presence of small metal particles which can at a controlled increasing rate to two act as catalysts. Oxidation products are electrodes immersed in the transformer oil. usually acidic in nature and result in an The gap is a specified distance and when the increase in acid number. Further reaction current arcs across this gap the voltage of these acids with the bulk oil can result in recorded is used to determine the dielectric sludge and varnish deposits. In the worst strength. case scenario, the oil canals become blocked and the transformer is not cooled adequately, POWER OR DISSIPATION FACTOR which further exacerbates oil breakdown. The power factor of a transformer oil is the Furthermore, an increase in the acidity has ratio of true power to apparent power and a damaging effect on the cellulose paper. is a measure of the current leakage through Oil degradation by-products such as acids the oil, which in turn is a measure of the and hydroperoxides also generally have the contamination or deterioration of the oil. In ability to conduct an electrical charge, which a transformer, a high power factor is an in turn reduces the insulating properties of indication of significant power loss in the

2 transformer oil, usually as a result of contaminants such as water, oxidised oil and cellulose paper degradation. It may also be any substance in the oil that either resists or conducts electricity differently to that of the oil itself, which may include diesel fuel, lubricating oil and kerosene. The test is not specific in what it detects and is usually carried out at elevated temperatures because contaminants that affect the test may remain undetected at 25ºC and only Degree of polymerisation reveal themselves at >90ºC. Furanic concentration

INTERFACIAL TENSION (IFT) The interfacial tension of transformer oil is Figure 2 related to its deterioration. Transformer oil is generally a hydrocarbon and thus The cellulose materials are the weakest link hydrophobic. However, when the sample in the insulation system. Since the life of the undergoes oxidative degradation, oxygenated transformer is actually the life of the cellulose species such as carboxylic acids are formed, insulation, and degradation of the cellulose which are hydrophilic in nature. Interfacial is irreversible, the decay products should be tension is the surface tension of a sample removed before they can do any further of the oil carefully floated on top of a layer damage to the cellulose. With proper of water. The more hydrophilic the oil maintenance the cellulose can virtually have becomes, the lower the value of the surface an indefinite life. To test for furanics, a tension between the two liquids. Studies sample of the oil is obtained and certain have shown that there is a definite chemical techniques are used to extract the relationship between acid number and IFT. furans from the oil. The extract is then An increase in acid number generally shows analysed using a process called high a decrease in IFT. However, when there is performance liquid chromatography (HPLC). a loss in IFT without the corresponding The results are usually reported in terms of increase in acid number, it is generally parts per billion (ppb). because of contamination with another hydrophilic substance not derived from (DGA) oxidation of the oil. The analysis of gases from petroleum products has been performed for decades FURANICS OR DEGREE using . However, this OF POLYMERISATION (DP) technique was not applied specifically to The solid insulation (cellulose-based products) transformer mineral oils until the late 1960s in transformers degrades with time at or early 1970s and is now commonly called rates which depend on the temperature, dissolved gas-in-oil analysis (DGA). DGA has moisture content, oxygen and acids in the become a standard in the electrical insulation system. Heat and moisture are maintenance industry throughout the world the main enemies of the solid paper insulation and is considered to be the most important with oxidation as the primary culprit. oil test for transformer oils in electrical When degradation occurs, the cellulose apparatus. More importantly, an oil sample molecular chains (polymers) get shorter. can be taken at any time from most Chemical products such as furanic derivatives equipment without having to take it out of are produced and dissolve in the service, allowing a “window” inside the electrical transformer oil. Of the furanic compounds, apparatus that helps with diagnosing and 2-furaldehyde is the most abundant. Its troubleshooting potential problems. concentration in oil has been related to the degree of polymerisation (DP) and As the insulating materials of a transformer consequently to the physical strength of the break down from excessive thermal or solid insulation (see Figure 2). electrical stress, gaseous by-products form.

3 The by-products are characteristic of the KEY GASES GENERAL FAULT type of incipient fault condition, the materials CONDITION involved and the severity of the condition. Indeed, it is the ability to detect such a , , Thermal condition variety of problems that makes this test and small involving the oil amounts of such a powerful tool for detecting incipient fault conditions and for root cause , Methane Partial discharge and small amounts of investigations after failures have occurred. Acetylene and Ethane Dissolved gases are detectable in low Hydrogen, Acetylene Sustained arcing concentrations (ppm level), which usually and Ethylene permits early intervention before failure of and Thermal condition the electrical apparatus occurs, and allows involving the paper for planned maintenance. The DGA technique involves extracting or stripping the gases Table 1: Categories of key gases and general fault condition from the oil and injecting them into a gas chromatograph (GC). time analysis on service-aged transformers (several years old), which could contain Typical gases generated from mineral oil / residual gases from previous events. cellulose (paper and pressboard) insulated transformers include: Some gas generation is expected from normal ageing of the transformer insulation and it

Hydrogen H2 is therefore important to differentiate

Methane CH4 between normal and excessive gassing rates.

Ethane C2H6 Normal ageing or gas generation varies with

Ethylene C2H4 transformer design, loading and type of

Acetylene C2H2 insulating materials. Routinely, general gassing Carbon Monoxide CO rates for all transformers are used to define

Carbon Dioxide CO2 abnormal behaviour. Specific information for a family of transformers can be used when Additionally, oxygen and are always sufficient dissolved gas-in-oil data is available. present. Their concentrations vary with the type of preservation system used on the Acetylene is considered to be the most transformer. Also, gases such as propane, significant gas generated. An enormous butane, butene and others can be formed amount of energy is required to produce as well, but their use for diagnostic purposes acetylene, which is formed from the is not widespread. The concentration of breakdown of oil at temperatures in excess the different gases provides information of 700ºC. Excessively high overheating of about the type of incipient fault condition the oil will produce the gas in low present as well as the severity. For example, concentrations. However, higher four broad categories of fault conditions concentrations are typically symptomatic of have been described and characterised in sustained arcing, a more serious operational Table 1. issue that can cause a transformer failure if left unchecked. The severity of an incipient fault condition is ascertained by the total amount of DGA is used not only as a diagnostic tool combustible gases present (CO, H2, C2H2, but also to stem apparatus failure. Failure

C2H4, C2H6, CH4), their rate of generation of a large power transformer not only results and their ratios with one another. Generally, in the loss of very expensive equipment but transformers will retain a large portion of it can cause significant collateral damage as the gases generated and therefore produce well. Revenue losses due to operational a cumulative history of the insulating outages may be the least worrisome materials’ degradation. This is an important consequence of a failure. Replacement of tool for detecting and trending incipient that transformer can take up to a year if problems. However, it also means that care the failure is not catastrophic and can result is needed in interpreting values for a first in tremendous revenue losses. If the failure

4 is catastrophic, then additional losses could costly decontamination work. occur, such as adjacent transformers, environmental problems from the release PCDDs and PCDFs cause damage and of oil (which could be as much as 20 000 death in doses as low as 1ppb to litres), and the resulting fire that would have 5000ppb. They are some of the most to be contained and smothered. In order to potent cancer promoters known and can avoid such a failure, the sample frequency damage organs such as the liver, kidney of most large power transformers is between and digestive tract as well as cause one and three years. However, sampling miscarriage and sterility. frequencies will increase as an incipient fault is detected and monitored. Often sampling Methods of PCB analysis frequencies are dictated by insurance Current methods of analysis are divided requirements, which often stipulate that into two major groups: PCB specific and annual transformer oil analysis must be PCB non-specific. Non-specific methods conducted to ensure continued coverage. test for PCBs indirectly by detecting one of the components of the PCB PCB ANALYSIS compound, usually chlorine. In general, PCBs (polychlorinated biphenyls) are a group non-specific methods are quicker and of synthetic oil-like chemicals of the less expensive than the specific methods. organochlorine family. Until their toxic nature However, these tests are susceptible to was recognised and their use was banned false positive results, since the test does in the early 1980s, they were widely used not detect PCB itself. as insulation in electrical equipment, particularly transformers. Three types of Specific methods utilise some type PCBs are normally used in electrical of chromatography to separate transformers: Aroclor 1242, 1254 and PCB molecules from each other and 1260, commonly known by various brand interfering compounds. It is not a case names which include Askarel, Chlorectol, of simply finding an easily quantifiable Elemex, Inerteen and Pyranol. compound, but of quantifying a complex mixture of compounds. Of the three One of the most important problems with major chromatography types, gas PCBs is that they concentrate in the fatty chromatography (GC), thin layer parts of microorganisms. This concentration chromatography (TLC) and liquid factor between the organism and the water chromatography, GC is the preferred can be as much as a million times. and most extensively used method. Concentrations are further amplified as the microorganisms become food for animals Terminology associated with PCBs is further up the food chain, ultimately ending defined below up in humans. PCBs are very stable and their degradation process is slow, making Non PCB for yet greater amplification in organisms. Any fluid, including that in electrical Although not overly toxic in themselves, equipment and any item that has a PCBs are poisons, which have been shown measurable PCB concentration of less to cause damage to the reproductive, than 50ppm of PCB, is considered a neurological and immune systems of wildlife non-PCB item. and humans. PCB contaminated Far more serious are the risks of a fire or Any fluid, including that in electrical an explosion. At temperatures around equipment, and any item which has a 500ºC, extremely toxic compounds - measurable PCB concentration of 50ppm polychlorinated dibenzofuranes (PCDF) and or greater but less than 500ppm is polychlorinated dibenzodioxins (PCDD) - are regarded as being PCB contaminated. formed. Small amounts of these compounds Transformer oil that has not been tested have been found at accidents where must be classified as PCB contaminated transformers and capacitors have been until shown to be otherwise. exposed to fire or have exploded. Even if the amounts have been extremely small and PCB item have caused no personal injuries, it has been Any fluid, including that in electrical necessary to perform very extensive and equipment and in any item which has a

5 measurable PCB concentration equal to or dependent on the quality of the oil sample greater than 500ppm, is regarded as a PCB that is sent to the lab. A sampling point on item. Once the PCB status is determined, any equipment should be identified and clearly a sticker is issued and fixed to the item in labelled for the technician. Also, as with question. This allows for quick reference and sampling locations in other types of ensures that potential cross contamination equipment, the same location should be used is avoided during future sampling, each time a sample is collected to ensure maintenance and decommissioning if representative conditions are tested. necessary. This point should be located in a place where a live oil sample can be collected rather Blending PCB contaminated oil with virgin than in an area where the oil is static. or other oil to meet the legal requirements is obviously an illegal practice that has been Just like machinery oil analysis, electrical shown to happen from time to time. transformer oil analysis can play a vital role This practice simply has the effect of in preventing unscheduled outages in electrical contaminating virgin oil supplies and ensures transmission and distribution equipment by that the PCBs persist in the environment, determining the condition of the equipment leading to further contamination. itself, as well as other vital components, including the condition of the oil and the Greater detail on PCBs, their management, cellulose paper insulation. Regular routine oil disposal and applicable legislative issues analysis should be the cornerstone of any surrounding them, can be viewed on the PM programme for all critical oil-filled electrical Wearcheck web site (www.wearcheck.co.za) equipment, including transformers, circuit in an article entitled “Guide for PCB breakers and voltage regulators. management of insulating oils in South Africa” by I.A.R. Gray under the Additional Info REFERENCES section. Anne Reygaerts – Laborelect, courtesy of Noria Corporation. PROPER NTT WorldWide Technical Bulletins TRANSFORMER Lance R. Lewand, Doble Engineering SAMPLING Company, courtesy of Noria Corporation

I.A.R. Gray - Transformer Chemistry Services As with machinery oil analysis, the ability of transformer oil analysis to provide an early Neil Robinson is managing director of warning sign of a problem condition is Wearcheck Africa.

Copies of previous Technical Bulletins can be accessed on Wearcheck’s web site: www.wearcheck.co.za

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6 Felicity Howden Public Relations 6/2006