CHAPTER 4 INVESTIGATIONS OF FURANIC DERIVATIVES AND PERFORMANCE EVALUATION OF TRANSFORMER OIL USING UV-VISIBLE SPECTROPHOTOMETER

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1 7 CHAPTER 4 INVESTIGATIONS OF FURANIC DERIVATIVES AND PERFORMANCE EVALUATION OF TRANSFORMER OIL USING UV-VISIBLE SPECTROPHOTOMETER 4. INTRODUCTION Life of a Power transformer is mainly determined by its insulation system. It provides electrical insulation and mechanical support to windings. Solid insulation paper degrades due to thermal ageing, oxidation, moisture, faults etc and produces furanic compounds which are dissolved along with transformer oil. Estimation of furanic compounds in oil provides an idea about the level of degradation of insulation system. Various furanic compounds produced are of 5-Hydroxy methyl furaldehyde (5-HMF), -Furaldehyde (-FAL), -Acetyl furanic compound (-AF) and 5-Methyl furaldehyde (5-MF). It is observed that -Furaldehyde is the most stable product of cellulosic ageing; hence it is mostly used as a key indicator for prediction of Degree of Polymerization (DP) of insulation paper and percentage life remaining of transformer. Masanori Kohtoh et al () presented characteristics analysis of transformer oil for various level of thermal ageing. Blue et al (998) estimated the presence of Furanic compounds in transformer oil using fluorescence property. Blue et al (997) estimated the presence of FFA compounds in transformer oil using solid state materials. Sin Pin Lai et al (8) determined the life of power transformer oil using UV-Vis spectral response by considering -FAL alone. Sin Pin Lai et al (8) correlated the spectral response of Ultra violet light with furanic measurement considering -FAL. Zhang et al (6) offered a review for importance of -FAL as main indicator of insulation paper ageing. Verma et al (5) analyzed the generation of furanic compounds in transformer oil for various ranges of thermal ageing. Shroff et al (985)

2 73 presented a review for ageing of paper insulation in power transformers. Emsley and Steven (994) has reported a review on chemical indicators of degradation of cellulose electrical paper. In this work, as an initiative all the furanic derivatives are individually mixed along with transformer oil for various levels of concentrations (ppm). Based on the spectral response like bandwidth and peak absorption, the type of furanic derivatives with their concentration is determined. DP and percentage values of tensile strength of paper remaining are estimated using -FAL concentrations. Regression analysis has been carried out to examine the linearity between concentrations of furanic derivatives with their absorbance. -FAL compound is always available with maximum concentrations, presence of other furanic derivatives is less than % and their prevalence also indicates particular type of problems in transformer. Hence other compounds also considered for investigation. Under operating conditions, the transformer oil undergoes severe mechanical, electrical and thermal stresses (Fofana et al ). When the transformer is subjected to overloaded operating conditions and elevated operating temperature, the possibility of chemical interactions between solid and liquid part of the insulation increases. These interactions deteriorate the quality of the insulating fluid and over certain period of time, the insulating fluid losses its ultimate reason of existence in the transformer. Therefore it is important to monitor the quality of the transformer oil and to make sure that the insulating fluid still holds on to its electrical properties. If the specified electrical properties are not met, then necessary action has to be taken regarding maintenance such as filtration of transformer oil or its replacement. Since transformer oil contains 7% of diagnostic information, by continuous monitoring, we can prevent any potential fault inside the transformers. Variation in different oil characteristics may be therefore be used to identify the type of problem in transformer.

3 74 UV spectrophotometer diagnostic system is based on the absorbance of light by transformer oil and has the immense potential to be a diagnostic tool to detect abnormalities in transformer. Karmakar et al () has examined the change in optical quality of the transformer oil due to three parameters specifically arcing, thermal degradation and partial discharge in transformer oil. Abu-Siada et al (9) showed the accurate interpretation of UV-spectrophotometry and informed about correlation between oil spectral response and its furan contents. John A. Palmer et al () examined the change in absorbance property of transformer oil degraded in the laboratory and in service transformers. Ota et al (9) has analyzed the effect of acetic acid in transformer oil. On considering the above literature review, UV-visible spectrophotometer has been considered as a tool for condition monitoring of transformer. But to make the purpose more effective, the faults can t be limited within arcing, thermal and partial discharge. Increased acidic content in transformer oil over period of service, presence of free sulphur, degradation of Kraft paper with elapsed heating and the real time faults occurring in the transformers such as Line-Line fault, Lightning, Overload, Short circuited faults should also be considered. Therefore the ultimate aim of this research work is to make the UV spectrophotometer technique as an effective monitoring tool to monitor the performance of transformer oil and transformer. 4. INVESTIGATIONS OF FURANIC DERIVATIVES Furanic concentration in transformer oil is correlated with UV-visible spectral property. -FAL is the major furanic derivative and the most stable compound of deterioration of insulation paper. High concentration of Furanic derivatives may indicate in some cases that transformer has reached end of its life operation. Therefore continuous monitoring of furanic compounds in transformer oil leads to avoid incipient faults. Factors that

4 75 accelerate the ageing of solid insulation are of moisture, temperature, oxygen, electrical stress etc. ASTM D5837 recommends HPLC method for estimation of furanic compounds. The method is time consuming and expensive. Further it requires expensive devices and trained persons. In order to overcome the above difficulties an initiative has been made to determine furanic compounds using UV-visible spectrophotometry. Even though furanic compounds absorb light from nm to 45 nm, the absorbance values are much higher than the instrument limit up to 35 nm, where as the absorbance values are with in instrument limit from 35 nm to 45 nm and there are well defined maxima in this range. Hence the absorbance values in the range of 35 nm to 45 nm only are considered for further studies. Various concentrations of test samples ( ppm,.5ppm, 3 ppm, 5 ppm, 7 ppm, ppm, 5 ppm, 5 ppm, 7 ppm, ppm, 5 ppm) is prepared by mixing the furanic derivatives like 5-HMF, - AF, -FAL, 5-MF individually with pure transformer oil. The concentrations used are on a preliminary base and it should be lower a lot (as low as 5 ppb) which will be useful for the needs of electrical industry. 4.. Estimation of 5 Methyl Furaldehyde 5 Methyl Furaldehyde is one of the furanic compounds which are produced by thermal degradation of solid insulation mainly caused due to overheating. Solutions of 5-MF in transformer oil are prepared keeping the concentration to 5 ppm. The absorption spectra of the resulting solutions are recorded between 35 nm to 45 nm. From the absorption spectra the absorption maxima and the absorbance at absorption maxima are noted and illustrated in Table 4.. From the results it is inferred that whenever the concentration increases the absorbance also increases.

5 76 Table 4. UV response of 5-MF Concentration PPM Absorption Maximum nm Absorbance Representative spectra of 5-MF at.5, 5, ppm concentrations are given in Figure 4., 4. and 4.3. Figure 4. Spectral response of 5-MF (.5 ppm) to Peak absorption

6 77 Figure 4. Spectral response of 5-MF (5 ppm) to Peak absorption Figure 4.3 Spectral response of 5-MF ( ppm) to Peak absorption Regression analysis of 5-MF concentrations with light absorbance at absorption maximum indicates moderate linearity (R =.88) as shown in the Figure 4.4.

7 y =.73x R = MF Concentrations(ppm) Figure 4.4 Variations of 5-MF to Peak absorption 4.. Estimation of 5 Hydroxy Methyl Furaldehyde 5-Hydroxy Methyl Furaldehyde is another furanic derivative produced by oxidative degradation of solid insulation. UV spectra of 5-HMF solutions in transformer oil are prepared keeping the concentrations - ppm. The absorption spectra of the resulting solutions are recorded between 35 nm to 45 nm. From the absorption spectra the absorption maxima and the absorbance at absorption maxima are noted which is shown in Table 4.. From the test result it is identified that wavelength of 5 HMF falls mostly in region of 356 nm. Table 4. UV response of 5-HMF Concentration PPM Bandwidth value (Absorption Maximum) Nm Absorption Peak Value

8 79 Spectrum of 5-HMF at 3ppm concentration is given in Figure 4.5. Regression analysis of 5-HMF concentrations with Peak absorption implies moderate linearity (R =.8) as shown in the Figure 4.6. Figure 4.5 Spectral response of 5HMF (3 ppm) to Peak absorption y =.7x +.56 R = HMF Concentrations (PPM) Figure 4.6 Variations of 5HMF to Peak absorption

9 Estimation of Acetyl Furanic Derivatives -Acetyl Furanic compound is one of the furanic compounds which are produced mainly due to solid insulation degradation by means of moisture. The absorption spectra of -AF solutions in transformer oil are recorded between 35 nm to 45 nm. From the absorption spectra the absorption maxima and the absorbance at absorption maxima are noted in Table 4.3. Table 4.3 UV response of -AF Concentration PPM Bandwidth value (Absorption Maximum) Nm Absorption Peak Value Spectra of -AF at,, 7 ppm concentrations are given in Figures 4.7, 4.8 and 4.9. Regression analysis of -AF concentrations with Peak absorption indicates strong linearity (R =.98) as shown in the Figure 4.. From the test results it is observed that absorbance value increases on increase of concentration.

10 8 Figure 4.7 Spectral response of -AF ( ppm) to Peak absorption Figure 4.8 Spectral response of - AF ( ppm) to Peak absorption

11 8 Figure 4.9 Spectral response of - AF (7 ppm) to Peak absorption y =.x R = AF Concentrations (ppm) Figure 4. Variations of -AF to Peak absorption

12 Estimation of Furaldehyde -Furaldehyde is one of the furanic compounds which are produced mainly due to the solid insulation degradation by overheating and faults. The absorption spectra of -FAL solutions in transformer oil are recorded between 35 nm to 45 nm. From the absorption spectra the absorption maxima and the absorbance at absorption maxima are noted in Table 4.4. Table 4.4 UV response of -FAL Concentration PPM Bandwidth value (Absorption Maximum) Nm Absorption Peak Value Spectra of -FAL at,, ppm concentrations and overline spectra response are given in Figure 4., 4., 4.3 and 4.4. From the test results is observed that bandwidth value falls between range of nm.

13 84 Figure 4. Spectral response of - FAL ( ppm) to Peak absorption Figure 4. Spectral response of - FAL ( ppm) to Peak absorption

14 85 Figure 4.3 Spectral response of -FAL ( ppm) to Peak absorption Figure 4.4 Over line spectra response of - FAL to Peak absorption

15 86 Regression analysis of -FAL concentrations with Peak absorption implies strong linearity (R =.9) as shown in the Figure y =.575x R = FAL Concentrations(ppm) Figure 4.5 Variations of -FAL to Peak absorption 4.3 DEGREE OF POLYMERIZATION Paper insulation is mainly composed of cellulose. It is a polymeric material linked by D-Glucospyranose monomers. DP is the number of monomeric units in a macromolecule or chain. Furanic compounds are generated due to decomposition reaction of cellulose chains. It is a key indicator for determining the mechanical strength of insulation paper. DP of paper can be estimated by Viscometric degree method by ASTMD 443 standard, the method requires samples of insulation paper which is invasive and needs the entire unit to be in off line. Hence by alternative methods DP can be identified by presence of -FAL compounds in oil, which is a stable one and it is directly related to determination of DP. De Pablo (997) equation relates the concentration of -FAL to DP of paper. Whenever DP value is it indicates the insulation level as healthy one, when the value reaches 5, paper has lost 6 % of its initial tensile strength. DP can be used to

16 87 investigate the tensile strength of paper insulation based on furanic derivatives dissolved in transformer oil. Estimation of DP using -FAL doesn t give exact location of weekend insulation system. It provides information for complete mechanical strength of insulation system based on -FAL concentrations. Variations of furanic concentration to DP and Percentage tensile strength of paper remaining are given in Table 4.5. From the result it is illustrated that whenever DP decreases % tensile strength of paper decreases. Table 4.5 Variations of - FAL with DP Concentration PPM Degree of Polymerization Percentage values of tensile strength of paper remaining % % % % % % 34 4.% % % % 5.% Regression analysis of Degree of Polymerization with -FAL concentrations implies strong linearity (R =.9) as shown in the Figure 4.6.

17 y = -9.9x R = FAL Concentrations (ppm) Figure 4.6 Variations of -FAL to Degree of Polymerization Increase in -FAL concentrations in transformer oil shows the reduction in Percentage tensile strength of paper remaining, as shown in Figure % 45.% 4.% 35.% 3.% 5.%.% 5.%.% 5.%.% FAL concentrations (ppm) Figure 4.7 Variations of -FAL to % tensile strength of paper remaining Percentage values of tensile strength of paper remaining is calculated using the formula, % Life= x (DP-)/ (-) (4.)

18 PERFORMANCE EVALUATION OF TRANSFORMER OIL In this work samples of transformer oil with various levels of fault and composition of materials were prepared. The samples include both laboratory prepared sample and those obtained from in-service transformer. The test samples analysed are listed in the Table 4.6. Sample S- stands for new transformer oil which acts as a reference for both wavelength and absorbance for the rest of the samples. Photographs of test samples are given in Figure 4.8. Table 4.6 Description of transformer oil samples Name of the Sample S- Pure Transformer Oil S- Years Oil S-3 Oil + CU S (6 ppm) S-4 Oil +Sulphur (5 ppm) S-5 Oil +Acetic acid ( ppm) Samples Description S-6 Oil + Kraft paper (heated at ºC, hrs) S-7 Oil (heated at ºC, hrs) S-8 Oil (arcing 5 times) S-9 Transformer oil from Line-Line faulted transformer S- Highly burned oil S- Transformer oil from Overloaded transformer S- Transformer oil from Lightning affected transformer

19 9 Figure 4.8 Snap shots of Test samples Most of the transformers are intended to operate for a maximum of -3 years, if properly designed, installed and maintained. The cause of the most of the transformer failure is their breakdown of the insulation system. So anything that adversely affects the insulating properties inside the transformer reduces the transformer life. S- is years worked out transformer oil obtained from power transformer rated kv/66kv. As per ASTM D 864, elemental sulphur and thermally unstable sulphur compounds in electrical insulating oil can cause corrosion of certain transformer metals such as copper. Deposition of sulphur and its compounds are more pronounced in sealed transformer and often lead to the deposition of the copper-sulphur compounds in the insulation paper. This in turn increases the conductivity of the insulation paper and results in the total deterioration of the property of the insulation. In addition presence of these compounds in transformer oil results in corrosion of the windings in transformer (Claes Bengtsson et al 6). In order to prevent the copper corrosion in transformer the level of sulphur contents should be maintained below.%. Sulphur level in the depleted transformer oil (Oommen et al 993) ranges from 48ppm to 9 ppm. It becomes important to detect the presence of sulphur and its

20 9 compounds in transformer oil to prevent the corrosion of windings. Therefore to monitor the presence of sulphur, sample S-3 is prepared for 6 ppm concentration of cuprous sulphide (CU S) and sample S-4 is prepared is prepared for 5 ppm concentration of sulphur. For the complete miscibility of CU S and sulphur with the transformer oil, the samples S-3 and S-4 were stirred by magnetic stirrer for hours separately in the laboratory. Mechanical property of insulation paper reduces as it ages, although electrical properties may not show significant change. In transformers it is mainly the oil impregnated cellulose system of windings that governs larger risks. Acetic acid is formed along with formic and levulinic acids due to ageing of cellulose in transformer Lundgaard et al (5). Therefore in order to depict the presence of acidity in transformer oil and also to avoid further corrosion of transformer tank, sample S-5 is prepared for ppm concentration of acetic acid. For the complete miscibility of acetic acid with the transformer oil, the sample S-5 was stirred by magnetic stirrer for hours in the laboratory. When transformer is subjected to overload operating conditions, the increase in temperature within the windings causes the mechanical strength of the insulation paper to get reduced. This in turn may adversely impact the insulation s performance, which may lead to transformer failure. Further cause of most transformer failures is the breakdown of the insulation system. Therefore in order to analyse the change in quality of transformer oil due to elapsed heating, samples S-6 and S-7 were prepared. The sample S-6 is prepared by heating transformer oil along with Kraft paper for the time period of hours at the temperature of C, the sample is prepared mainly to illustrate the initial performance of oil along with the insulation system and sample S-7 is prepared by heating transformer oil for the time period of hours at the temperature of C.

21 9 Electrical discharge is often possible in high voltage Power apparatus. Discharge is caused in the insulating fluid due to failure of withstand capability of dielectric liquids. Discharges may seriously damage other parts of the insulating materials and if these discharges persists continuously it will leads to total failure of transformer. Sample S-8 is prepared (5 times of arcing) by means of test set up which comprises of HV AC test transformer and test cell. HVAC is fed to Test cell which consist of two sphere electrodes separated by.5 mm, the test cell is filled with transformer oil, by means of varying the test voltage using HV transformers the arcing is produced. Test set up is shown in Figure HVAC Transformer, -Voltage divider, 3-Test cell Figure 4.9 Experimental setup under which the transformer oil is arced 5 times As the insulating oil deteriorate, the probability of occurrence of transformer failure increases. It is therefore important to regularly monitor the transformer s insulation. If the data s obtained through continuous monitoring allows one to determine the condition of transformer oil, then the fault can be predicted and rectified. In this aspect sample S-9 (Transformer oil from Line- Line faulted transformer), S- (Highly burned oil), S-(Transformer oil from Overloaded transformer) and S- (Transformer oil from Lightning

22 93 affected transformer) where obtained from transformers rated kv/433v, kv/3v, kv/433v and kv/433v respectively. 4.5 RESULTS AND INFERENCE Transformer oil is of highly absorbing material, the value of absorbance generally exceeds the spectrophotometer saturation range (max. peak absorbance range is 4). Moreover during measurement of furan contents it is found that oil spectral response reaches its saturation level. In order to overcome this complexity, drop of Test samples is diluted by ml of isooctane solution. During measurement of spectral response using two beam spectrophotometer, iso-octane is used as reference solvent in one quartz curvette and another curvette consists of drops of samples diluted by ml of iso-octane. Spectral responses for various test samples obtained are listed in Table 4.7. From the test results it is illustrated that each sample has its own spectral characteristics, thereby helps to find out the performance of transformer using their own spectral characteristics. Each samples prepared resemble its own fault and characteristics, the above samples listed shows their own bandwidth and absorbance value, therefore a study on correlation on spectral response and characteristics will be taken us as a separate study in future

23 94 Table 4.7 Spectral response of the sample Test samples Absorption Maximum (nm) Absorbance S S S S S S S S S S S S Sample S- of pure transformer oil serves as the reference for remaining samples. Its wave length is found to be in the region of 65 nm, absorbance of.3. Spectral response of S is given Figure 4..

24 nm Figure 4. Spectral response of the pure transformer oil (S-) Twenty years worked out sample S- immediately shows the drastic reduction in both wavelength and absorption peak which is shown in Figure nm Figure 4. Spectral response of years worked out transformer oil (S-)

25 96 Samples S-3 (presence of cupric sulphide) and S-4 (presence of sulphur) shows similar wavelength, but their absorbance peak slightly varies which is shown in Figure 4. and 4.3. Sample S5 (presence of acetic acid) shows a unique type of wavelength (66 nm) and absorbance which is shown in Figure 4.4. Elapsed heating of transformer oil deteriorates the quality of the transformer oil. The transformer oil heated along with Kraft paper Sample S-6 doesn t show any increased bandwidth than that of S- but shows increased absorbance peak (.83) which is shown in Figure 4.5. Sample S-7 (transformer oil heated at C) shows abrupt change in the values of both bandwidth and absorption peak. The sample S-7 has a unique type of wavelength (8 nm) lower than that of S- and absorbance peak (.864) is much higher than S-, which is shown in Figure nm Figure 4. Spectral response of transformer oil with CU S (S-3)

26 nm Figure 4.3 Spectral response of transformer oil with sulphur (S-4) nm Figure 4.4 Spectral response of transformer oil with acetic acid (S-5)

27 nm Figure 4.5 Spectral response of overheated transformer oil heated with Kraft Paper (S-6) nm Figure 4.6 Spectral response of overheated transformer oil (S-7) Spectral response of sample S-8 (5 times arcing) shows a wave length of 68.4nm with absorbance peak.5. Maximum absorbance wavelength of S-8 is more or less same as that of S-6 and S-, but S-8 absorbance peak value entirely differs. Spectral response is shown in Figure 4.7.

28 nm Figure 4.7 Spectral response of transformer oil arced 5 times (S-8) Spectral response of sample S-9 (Oil from L-L faulted transformer) shows a very high wave length (8nm) with increased absorbance peak (3.493). The absorbance peak has reached more or less the saturation level. Spectral response is shown in Figure nm Figure 4.8 Spectral response of transformer oil obtained from L-L faulted transformer (S-9)

29 Spectral response of sample S- (highly burned oil) shows wavelength (78 nm) which is a unique type. Here also the absorbance peak has reached saturation level (3.643) of the instrument. Spectral response is shown in Figure nm Figure 4.9 Spectral response of highly burned transformer oil (S-) Spectral response of S- (Oil from over loaded transformer) shows a similar pattern of wavelength regarding S-6 and S-8 but the absorbance peak is much higher (3.64) when compared with S-6 and S-8. Spectral response is shown in Figure 4.3. Spectral response of S- (Oil obtained from Lightning affected transformer) shows a unique type of wavelength (3 nm) and absorbance peak (3.8) is very high, which is more or less reached instrument saturation level. Spectral response is shown in Figure 4.3

30 nm Figure 4.3 Spectral response of transformer oil obtained from overloaded transformer (S-) nm Figure 4.3 Spectral response of transformer oil obtained from lightning affected transformer (S-)

31 4.6 DISCUSSION It is customary to record absorbance spectrum in extended spectral range as in present case -45 nm and the useful range for analysis is selected based on the shape of spectrum. In the present study nm was selected, since the spectrum in this range was smooth with well defined characteristics absorbance band. Investigations of spectral response for mixed combinations of furanic derivatives at different proportions shall be carried out as a future work. Sulphur content in the transformer ranges from 48ppm to 9 ppm. It was decided to keep the concentration at lower limit. Therefore 6 ppm concentration for Cu S sample and 5ppm concentration for Sulphur content with transformer oil were chosen. (T.V.Oommen and Claes bengstsson, CIGRE 6) Acetic acid is formed along with formic and levulinic acids due to ageing of cellulose in transformer. Since lower molecular weight acids are most easily absorbed by insulation paper, sample S-5 is prepared to level of ppm of acetic acid. (Le Lundgard 5) Test results reveal that samples with corrosive sulphur, acidity, lightning affected oil etc. shows their own spectral response. Whenever more number of test samples prepared, an index value shall be prepared for samples which supports the industrial community to identify the fault in transformer based on the spectral response of transformer oil.

32 3 4.7 CONCLUSION This chapter has demonstrated the exploration of furanic derivatives in transformer oil using its Ultra-Violet absorption property. Regression analysis has been carried out to examine the linearity between furanic derivatives and peak absorption. Degree of polymerization for solid insulation system and percentage tensile strength of paper remaining has been determined. In this work, due to the limitation of available resources, the response of ppm to maximum of 5 ppm of furanic derivatives has been considered. However in future, assessment of furanic derivatives for lowest value less than 5 ppb shall be projected on considering the needs of electrical industry. Furthermore, the same method shall be utilized to investigate the furanic derivatives for highly oxidized oil, high acid content oil, natural ester oil, synthetic ester oil etc. It is observed that UV spectrophotometer acts as an effective tool for transformer s insulation assessment. An index value of test samples for various faults which occur in transformer has been formed. Performance evolution system of transformer oil can be optimized when more number of test samples is prepared for variety of faults. Based on results we shall establish a general index for variety of faults, thermal degradation and composition of materials. The index value of samples helps us to determine condition of insulation system. Correlation between the spectral response results obtained through UV-visible spectrophotometer for unknown test samples and the index statistics previously formed, enable us to determine type of fault and performance level of transformer oil. Moreover the analysis of transformer oil is done with in short time and at reduced cost. The next chapter deals with deterioration of solid insulation for thermal degradation of transformer oil.