ANALYSIS OF TRANSFORMER OIL USING IRANALYZERS

M. P. Zakharich,a* I. I. Zaitsev,b V. P. Komar,a

F. N. Nikonovich,a M. P. Ryzhkov,b and I. V. SkornyakovaUDC 543.42

Spectral characteristics of samples of transformer oil that differ in technical grade, service life, andcontent of an antioxidant additive and dissolved water are investigated. It is suggested to determineservice deterioration of insulating oils from absorption in the region 1710 cm−1 using IR filter ana-lyzers.

Keywords: IR spectroscopy, transformer oil, analysis.

Introduction. Transformer oil is used for filling in transformers, oil-immersed switches, and otherhigh-voltage equipment as the main electroinsulating and cooling material. In the majority of cases, trans-former oil is a product of petroleum refining and is a complex mixture of paraffin, naphthene, and aromaticand naphthenoaromatic compounds. Small amounts of derivatives of hydrocarbons that include atoms of otherelements, such as nitrogen, sulfur, and oxygen, also enter into the composition of transformer oil. The oper-ating characteristics of transformer oil are governed by the conditions of refining and cleaning of low-viscos-ity oily distillates. In particular, the stability of the oil (the capacity for retaining its chemical compositionduring the service time) is caused by the amounts and composition of the aromatic hydrocarbons contained init [1−3]. To improve chemical stability and to retard autooxidation processes, synthetic antioxidant additivesare added to certain grades of transformer oil [2, 4]. At the present time, the most widespread antioxidantinhibitor is 2,6-ditertiary butylparacreosol (ionol).

The change in the composition of transformer oil in operation is related to the chemical processesthat occur in the dielectric medium under the action of temperature and high voltage. This leads to the oxi-dation and decomposition of chemical compounds that enter into the composition of the oil and to the appear-ance in it of new gaseous (CO, CO2, and volatile hydrocarbons), liquid (aldehydes, ketones, alcohols, acids,ethers, resins, and water), and solid (asphaltenes and carbenes) chemical products [5].

Because of the violation of storage, transportation, and operating rules, there can also be water, me-chanical impurities, and chemical contaminants (motor and vegetable oils, corrosion inhibitors, etc.) in the oil.The presence of impurities and of oxidation and decomposition products alters the operating characteristics ofthe transformer oil, which can lead to failure of electrical equipment. Therefore, transformer oil is subjectedto a number of control checks in operation. They include measurement of a breakdown voltage, determinationof the flash temperature and the loss factor, and also determination of the content of mechanical impurities,water-soluble acids, soluble sludge (potential sediment), and ionol in the oil and also of the moisture content.

aInstitute of Molecular and Atomic Physics, National Academy of Sciences of Belarus, 70 F. SkorinaAve., Minsk, 220072, Belarus; e-mail: lirp@imaph.bas-net.by; b"Bele′nergospetstekhnika" Design Office,Minsk, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 68, No. 1, pp. 47−50, January−Febru-ary, 2001. Original article submitted August 17, 2000.

Journal of Applied Spectroscopy, Vol. 68, No. 1, 2001

1021-9037/01/6801-0061$25.00 2001 Plenum Publishing Corporation 61

*To whom correspondence should be addressed.

The methods that are used to analyze the oil are expensive and laborious; they take a good deal of time,during which the transformer in question must be taken out of the operating regime. In this connection, it isof interest to develop continuous and rapid methods of spectral analysis of transformer oil using a simplifiedspectral analytical equipment, whose capabilities are described in [6].

Experimental Procedure. To investigate the spectral characteristics of transformer oil we selectedsamples of serviceable and waste oils of the grades TSp, TKp, T-750, T-1500, GK (with an antioxidant ad-ditive of ionol), and TK (without an antioxidant additive). The analyzed samples differed in service life (from3 to 26 years) and content of water and oxidation products. For transformers without special protection thecontent of dissolved water in transformer oil must be no higher than 20 mg/kg. In the samples of the oils inquestion, it was in the concentration range from 6.9 to 33.8 mg/kg. The content of volatile and nonvolatilewater-soluble acids in transformer oil is governed by the acid number, i.e., the amount of potassium hydrox-ide (KOH) necessary to neutralize the carboxylic acids contained in 1 g of oil. In the initial transformer oil,a content of nonvolatile and volatile water-soluble acids no higher than 0.05 mg of KOH per g of oil isconsidered to be permissible. In the operating period, the acid number for the oil with an additive must notbe higher than 0.2 mg of KOH per g of oil and 0.35 mg of KOH per g of transformer oil without an addi-tive. In the samples in question, the acid number was within 0.09 to 2.15 mg of KOH per g of oil.

The IR spectra of the oil samples in question were obtained using UR-20 and Perkin−Elmer 180 spec-trometers in the middle spectrum and a Beckman 5270 spectrometer in the near IR spectrum. The opticaldensity of the samples at analytical wavelengths was also recorded by an ANB-315 filter IR analyzer, whosespecifications are given in [7]. The thickness of the layer in question was selected in the interval from thethickness of a layer of an oil droplet crushed between two KCl windows to a 1-cm layer in a quartz cell inrecording the spectra in the near IR region. In order to obtain the lowest error in measuring the transmissioncoefficients and for subsequent computation of the optical density, recording conditions for the spectra wereselected according to the recommendations given in [8]. The values of the optical density presented here areobtained for an oil-layer thickness of 1 mm. The transmission of the background was measured in the region1820 cm−1.

The 1.2, 1.4, and 1.72 µm absorption bands observed in the spectra of the transformer oil in the nearIR region are overtones of the stretching vibrations of CH groups and also component frequencies and com-binations of the frequencies of these groups with other vibrations. The complete coincidence of the spectra ofthe serviceable and waste oils shows the purposelessness of using the near IR region for an analysis of im-purities and oxidation products in transformer oil.

Based on a possible composition of the chemical substances formed in the service of transformer oil,the most efficient portions for analyzing in the middle IR region (400−4000 cm−1) are the spectral portions inthe absorption regions of the OH and C=O groups. In the remaining portions of the spectrum, the absorptionbands of the initial components of the oil are superimposed on the absorption bands of the impurities andsubstances formed as a result of oxidation and decomposition of transformer oil.

Figure 1 gives the spectra of the samples of transformer oil in the region of absorption of the OH andC=O groups. The samples of transformer oil differed in serviceability, in-service time, and amount of dis-solved water, ionol, and carboxylic acids. The spectra of the remaining samples in question are identical inspectral characteristics to the spectra of the figure. In the region 3200−4000 cm−1, in all the transformer-oilspectra we observe the intensity-constant 3340 cm−1 absorption band and the 3650 cm−1 absorption band. Themaximum intensities of the 3650 cm−1 absorption band are obtained for the samples of TSp transformer oilwith a service life of 4 years (curve 1). The minimum values are recorded for the samples of TKp (curve 4)and TK oils characterized by the absence of a synthetic additive of ionol (curve 3). These samples of trans-former oil have been in operation for more than 20 years. In the spectra of the transformer oils of othergrades, for the 3650 cm−1 absorption band we observe a tendency for a decrease in the absorption intensitywith increase in the service life. In addition to these absorption bands, we observe the 3550 cm−1 band in the

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spectra of the waste oils and samples of transformer oils of different grades characterized by a 14-year andover service life of the oil under working conditions (curves 2−4).

In the region 1700−2000 cm−1, the spectra of the transformer oils are characterized by the set of1800−1950 cm−1 absorption bands. In the spectra of the transformer oils, there is also the 1730 cm−1 absorp-tion band, while for the samples of waste oils and transformer oils with a service life of over 11 years the1710 cm−1 absorption band is observed. The intensity of the 1730 and 1710 cm−1 absorption bands increaseswith the service life of the oil. Therefore, it can be assumed that these absorption bands are related to theprocesses of oxidation and formation of C=O bonds.

The spectral region 1800−2000 cm−1 is characterized by the presence of the absorption bands of over-tones and component frequencies of the CH groups of aromatic compounds [9]. From the observed numberof absorption bands and their relative intensity, in this spectral region the type of substitution in the benzolring is identified regardless of the type of the added cluster and its chemical functionality [9, 10]. Since thegrade of transformer oil is related to the chemical composition of the aromatic compounds in it, the spectralcharacteristics in this range can be used for determining the grade of the oil (see curves 1 and 2).

In the region 3400−3800 cm−1, there can appear stretching vibrations of the OH group of water, alco-hols, carboxylic acids, phenols, and other products of oxidation of transformer oil. Ionol is also absorbing inthis spectral region [11]. It is known that the in-service time of an oil is always accompanied by a decreasein the content of an inhibitor in it. The antioxidant, being an active substance in itself, enters into interactionwith the radicals of the base oxidizing material, transferring them to an inactive state. The formed radicals ofthe antioxidant itself are dimerized into stable compounds of quinoid structure [2]. The decrease in the inten-sity of the 3650 cm−1 absorption band with increase in the service life of transformer oil and the minimumintensity of this absorption band in the grades of oil without an antioxidant additive make it possible to as-

Fig. 1. IR spectra of transformer oils with different service lives, mois-ture contents, and acid numbers: 1) TSp, 4 years, 11.9 mg/kg, and 0.09mg of KOH; 2) TKp, 14 years, 12.6 mg/kg, and 0.19 mg of KOH; 3)TK, 26 years, 12.9 mg/kg, and 1.1 mg of KOH; 4) TKp, 21 years, 38.2mg/kg, and 2.15 mg of KOH.

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sume that the 3650 cm−1 absorption band is due to the absorption of the ionol hydroxyls. The presence of the3650 cm−1 absorption band in the spectrum of TK oil (without an antioxidant additive) may well be causedby the absorption of the oxidation products formed (phenols and alcohols) [12] or by the presence of residualnatural antioxidants.

It should be noted that in the spectra of TK and TKp oils that have a service life of over 20 yearsand are characterized by a minimum value of the intensity of the 3650 cm−1 band, there appears a neighbor-ing 3620 cm−1 absorption band insignificant in intensity. It can be due to the absorption of hydroxyl groupsof dissolved water or secondary or tertiary alcohols formed as a result of oxidation [10, 12]. In the spectra ofthe other samples of transformer oil, the 3620 cm−1 absorption band is masked by the absorption of the in-hibitor.

The appearance of the 3550 cm−1 absorption band seems to be related to the oxidation reactions oc-curring in the transformer oil and to the formation in it of chemical compounds that contain hydroxyl groups.According to [2, 4, 5], the basic final product of oxidation of branched hydrocarbons are acids. The stretchingvibrations of the free OH group of carboxyl are located near 3550 cm−1; there is a wide region of absorptionbetween 3000 and 2500 cm−1 for the dimers of carboxylic acids [10]. The appearance of the 3550 cm−1 ab-sorption band is also accompanied by increase in the intensity of the absorption band in the region 1730cm−1 and by the appearance of a new 1710 cm−1 absorption band, where in addition to the absorption ofaldehydes and ketones, there is also absorption due to the stretching vibrations of the C=O groups of carbox-ylic acids [10]. As a consequence, we can assume the presence of carboxylic acids, predominantly in mono-mer form, in the samples of waste oils and transformer oils with a prolonged service life. The stabilitycriterion of a transformer oil under operating conditions is the absence of a tendency for the formation ofwater-soluble acids in the beginning of aging [2]. Therefore, from the intensities of the 3550 and 1710 cm−1

absorption bands that are due to the presence of carboxylic acids in transformer oil we can determine theserviceability of the oil and monitor the processes occurring in electrical equipment.

In the region 1710 cm−1, the analyzed samples of transformer oil had optical density (OD1710) in theinterval from 0.16 (for the samples with a service life of 3−4 years) to 0.7−1.3 (waste oil). The given valuesof OD1710 corresponded to values of the acid number of 0.09 mg of KOH per g of oil for OD1710 of 0.16and 2.15 mg of KOH for OD1710 of 1.31. To determine the serviceability criterion we prepared extra samplesof oil with additions of oleic acid at concentration equal to the maximum permissible content of carboxylicacids in transformer oil. Measurement of the optical density of the oil with oleic-acid additions shows that themaximum permissible content of carboxylic acids in transformer oil corresponds to OD1710 = 0.22 ± 0.01. Inthe region 3550 cm−1, we observe no absorption band. For a tenfold excess over the maximum possible normof carboxylic acids the spectrum is characterized by the appearance of absorption in the region 3350 cm−1

and by OD1710 = 1.22. The maximum value of OD1710 was observed for TKp oil after 21 years of operation;it was 1.31. This value is close to the magnitudes obtained for model compounds where the content of car-boxylic acids exceeds the maximum permissible one by a factor of 10.

Comparison of the computed values of OD1710 with the in-service time of transformer oil made itpossible to establish that in optimum operation of transformers the stability of the operating characteristics oftransformer oil can be retained for up to 14 years. For this service time, regardless of the grade of the ana-lyzed transformer oil, the content of carboxylic acids does not exceed the maximum permissible values. Afterthis time, oxidation processes, whose rate is also governed by the operating conditions of transformers, beginin transformer oil. Deviation from these standard characteristics indicates the presence of additional malajust-ments in the transformer. Thus, for a sample of TSp oil after three years of operation, the spectrum displaysthe 1710 cm−1 absorption band (OD1710 = 0.22), which characterizes the supernormal formation of oxidationproducts in the transformer oil.

The performed investigations showed that a transformer oil can be checked for correspondence to thecriterion obtained (OD1710) on a simplified spectral equipment (ANB-315 filter analyzer) as well. In the

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working channel, a filter with a maximum of the transmission band of 1710 cm−1 is installed. To eliminatethe influence of the absorption of other components of the transformer oil, there was a filter with a transmis-sion maximum of 1820 cm−1 in the comparison channel. The thickness of the analyzed layer was 1 mm. Eachtransformer-oil sample was measured up to five times. We observed the coincidence of the values of OD1710measured on an analyzer with the data obtained on mass-produced spectrometers. The relative error of meas-uring OD1710 on an ANB-315 filter analyzer was no higher than 2.5% for the maximum permissible contentof carboxylic acids.

Conclusions. By the spectra in the middle IR region it is possible to identify the grade of transformeroil and to determine the content of ionol and of substances formed in the operation of transformer oil. Thetotal level of the content of decomposition and aging products is related to the magnitude of the absorptionin the region 1730−1710 cm−1 that is due to the vibrations of C=O groups occurring in oxidation. The higherthe optical density of the oil at a frequency of 1710 cm−1, the larger the degree of service deterioration of thetransformer oil. For OD1710 higher than 0.22 the level of oxidation products begins to exceed the maximumpermissible content in transformer oil. This analysis can be performed continuously, from the instant of fillingin the oil, without removing the transformer from the operating regime, using small-size IR spectral equip-ment with specifications analogous to the specifications of an ANB-315 analyzer.

REFERENCES

1. L. P. Kazakova and S. E′ . Krein, Physicochemical Principles of the Production of Petroleum Oils [inRussian], Moscow (1978).

2. S. E′ . Krein and R. V. Kulakova, Petroleum Insulating Oils [in Russian], Moscow−Leningrad (1959).3. N. I. Chernozhukov, S. E′ . Krein, and B. V. Losikov, Chemistry of Mineral Oils [in Russian], Moscow

(1959).4. S. E′ . Krein, Stabilization of Turbine and Transformer Oils [in Russian], Moscow−Leningrad (1948).5. N. I. Chernozhukov and S. E′ . Krein, Oxidizability of Mineral Oils [in Russian], Moscow (1955).6. A. D. Zamkovets, M. P. Zakharich, V. P. Komar, and I. V. Skornyakov, Zh. Prikl. Spektrosk., 65,

734−744 (1998).7. N. A. Borisevich, M. P. Zakharich, V. P. Komar, F. N. Nikonovich, and I. V. Skornyakov, Zh. Prikl.

Spektrosk., 64, 679−685 (1997).8. G. G. Petrash, Tr. Fiz. Inst. Akad. Nauk, 27, 3−62 (1963).9. C. W. Young, R. B. DuVall, and N. Wright, Anal. Chem., 23, 707−714 (1951).

10. L. Bellamy, Infrared Spectra of Complex Molecules [Russian translation], Moscow (1963).11. A. Smith, Applied IR Spectroscopy [Russian translation], Moscow (1982).12. K. Nakanishi, Infrared Spectra and Structure of Organic Compounds [Russian translation], Moscow

(1965).

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