- Email: [email protected]
Depolymerization processes in the thermal degradation of cellulosic paper insulation in electrical transformers
PII:
s0141-3910(97)00249-8
Polymer Degradation and Stabrlity 61 (1998) 507-5 1I C] 1998 Elsevier Science Limited. All rights reserved Printed in Great Britain 0141-3910/98/%see front matter
ELSEVIER
Depolymerization processes in the thermal degradation of cellulosic paper insulation in electrical transformers Sergei Levchik,“” John Scheirq”t Giovanni Camino, 4 Wander Tumiat@ & Mauro Avidano” aDipartimento di Chimica Inorganica, Chimica Fisicae Chimicadeih4aterialidell’Universitridi Torino, Via P. Giuria 7,10125 Torino, Italy bSea Marconi Technologies, Via Crimea 4, 10093, Collegno, Italy
(Received 13 February
1997; accepted 20 October 1997)
A dynamic heating experiment was used to examine the influence of temperature on the degradation of the Kraft insulating paper in transformer oil. The degradation was monitored by measuring a number of key indicators on selected samples as a function of progressive ageing. These indicators included the degree of polymerization (DP), level of furanic compounds and level of gases such as carbon oxides and oxygen. The results show that the formation of furanic compounds and of gases begins significantly after a large reduction in DP, indicatinga two-stage process: firstly scission leading to a sharp drop in DP and secondly formation of low molecular weight degradation products. 0 1998 Elsevier Science Limited. All rights reserved
INTRODUCTION
paper migrate into the oil and can be used as probe compounds to assess indirectly the condition of the insulating paper. A detailed understanding of cellulose degradation mechanisms is required for meaningful interpretation of insulating ageing data that may enable lifetime predictions of remaining paper life and give early warning of premature fajlure. In particular, furanic compounds originate specifically from cellulose whereas H20, CO2 and CO may also be formed by thermal degradation of oil. Thus relationships are being sought to relate the concentration of furanic compounds in the oil to the degree of polymerization (DP) of the paper. Such relationships are difficult to formulate using actual data collected from in-service transformers since the large number of operational variables lead to considerable scatter in the results. In view of this, laboratory simulations have been proposed in order to provide strict control over experimental variables (such as temperature, water, oxygen and oil/paper ratio). 1,2A limitation of these isothermal experiments, however, is that they take many months to yield results whereas the present study uses an accelerated ageing method based on a dynamic heating to provide a rapid overview of chemical process associated with paper ageing. The
In nearly all industrial, oil-filled transformers, Kraft paper tapes (90% cellulose, 67% lignin, 34% pentosans) are the main solid insulator for the winding conductors and for inter-winding, intercore and winding-to-earth insulation. Oil-filled transformers may contain as much as 12 tons of paper and up to 40 tons of oil. Oil-filled electrical transformers typically have an operating temperature range of 60-90°C and operation lifetime of up to 40years. These service temperatures can cause degradation of the oil and of the paper winding insulation during the lifetime of the transformer. Whereas the status of the oil in an electrical transformer can be directly evaluated because it is accessible to sampling, unfortunately the insulating paper is not. However, certain degradation products (e.g. furanics, water, COZ, CO, etc.) from the *Permanent address: Research Institute for Physical Chemical Problems, Byelorussian University, Leningradskaya, 14220080 Minsk, Belarus. tpermanent address: ExcelPlas Australia, PO Box 102, Moorabbin, 3 139, Australia. tTo whom correspondence should be addressed. Fax: 0039 011 6707 557; e-mail: [email protected] 507
508
S. Levchik et al.
dynamic experiment uses a heating rate of l”C/h and the total test duration is approximately 1.5 week.
CHO
2-Furfural [2-FALI
EXPERIMENTAL CH*OH
Paper
The Kraft paper used was transformer-grade insulation paper obtained from Imatran Voima OY (Finland). Elemental analysis by inductively coupled plasma emission spectrophotometry gave the following assay: Ca, 1350 ppm; Cr, 188 ppm; V, 163 ppm; Na, 113 ppm; K, 113 ppm; Fe, 63 ppm. The paper was conditioned to 2% moisture content by the procedure described in detail elsewhere.3
2-Furfurol 12-FOL]
HocH2eHo
S-Hydroxy methyl2-furfural [5_HMF]
5Methyl-2-furfural [SMEF]
Oil Fig. 1. Furanic
The oil was a high-grade naphtenic transformer oil (Nynas 10GB) produced by the Nynas Oil Company. The oil was saturated (2.7v/v) with air by bubbling dried air through it for 2 h.
compounds produced by the degradation Kraft paper and detected in oil.
of
The paper (1 g) was introduced into 150ml glass vials that were flame sealed after addition of 100 ml oil and heated in an oven at l”C/h from room temperature to 200°C. Samples were removed for analysis at intervals of approx. 10°C beginning from 70°C.
ing DP of approximately 1250 corresponds to the initial DP of paper used in transformers. A sigmoidal decrease in the DP occurs with increasing temperature beginning at about 70°C (first sampling). The maximum rate of change occurs at about 100°C and a limiting DP value of approximately 200 is reached above 150°C. A DP of 200 is generally recognized as the failure criterion for insulating paper since at this point the paper becomes brittle and loses its mechanical integrity.
Analytical tests
Furanics
All measurements were conducted on duplicate test samples with the exception of the blank samples containing only oil, with methods reported in detail elsewhere.3 Briefly, the DP of the paper was measured by viscometry of cupric ethylene diamine solution (Method IEC 450, 1974), furanic compounds (Fig. 1) were analysed by HPLC.3 Gases dissolved in oil were measured by gas chromatography (ASTM D3612) and water in oil by the Karl Fisher method after cooling to room temperature.
Figure 3 shows the increase in the concentration of furanic compounds in oil as a function of
Ageing
RESULTS
AND DISCUSSION
Degree of polymerization (DP)
0
50
100
150
2dO
Temperature, OC
Figure 2 shows the reduction of DP of the insulating paper as a function of temperature. The start-
Fig. 2. Degree of polymerization air) as a function
(DP) of Kraft paper in oil (in of temperature.
Depoiymerization processes in the thermal degradation of cellulosic paper insulation in electrical transformers
509
400 15w
-
k 2 300 a ?? Y * .g 200
d
2-FAL
d
5-HMF
+
2-FOL
E 5 t 0 u
100.
0 50
100
150
200
0
Temperature, OC
50
100
150
200
Temperature, OC
Fig. 3. Furanic compounds
Fig. 4. Furanic
temperature. Furanic compounds are formed in the paper in ppm levels and diffuse into the oil. The furanic derivatives found in the highest concentrations were 2-furfuraldehyde (ZFAL), 5hydroxymethylfurfuraldehyde (5-HMF) and furfurylalcohol (2-FOL). This is consistent with the situation in operating transformers, where 2-FAL is generally found in the highest concentration in oil relative to the other furanics.4 The analysis of 2-FAL in the oil has recently received much attention as a specific oil-soluble, chemical indicator of paper insulation degradation since it cannot be produced by degradation of other organic materials in the transformer.4 Furthermore, Burton et ~1.~ have found that the ratio of 2-FAL to 5-HMF in the oil decreases by a factor of 5 between 200 and 350°C. Thus, the 2-FAL to 5HMF ratio in the oil might be used to predict the temperature of localized over-heating in a transformer. Figure 4 shows that the concentrations of the furanic compounds in paper increased markedly above 150°C. This temperature also corresponds to the point at which the DP of the paper dropped to the limiting value of about 200 (see Fig. 2) when the mechanical properties of the paper are significantly affected. In paper, the furanics found in highest amounts is 5-HMF (due to its higher polarity and hence affinity with the paper phase) and this is followed by 2-FAL and 2-FOL (Fig. 4). Again the trend is the same as that which occurs in the industrial transformer, thus demonstrating that this accelerated test is closely simulating the paper degradation processes that are occurring in actual field transformer environments. When the total furanics
are considered (i.e. the sum of the concentration in the paper and the oil) then the order of decreasing concentration is 2-FAL > 5-HMF > 2-FOL. The majority of the literature suggests that furanit compounds originate from the intermediate levoglucosan (LG), which is formed when heating causes cleavage of the glucosidic bonds. No reports in the literature have examined the use of LG as a probe compound in oil to assess cellulose degradation, probably because of its low solubility in hydrocarbon oil. To investigate its potential as a probe compound the solubility of LG in transformer oil (both mineral and naphtenic oil) was examined by adding 0.5 g of LG to lOOm1 of oil. Mixtures were left to stand at room temperature for 1 week, heated for 1 week at 130°C and ultrasonicated for 5 h, respectively. In all cases, no soluble LG could be detected in the oil by FTIR (using the diagnostic band at 1047cm-*) or by the HPLC technique. Hence, the use of LG as a probe compound in the diagnosis of paper insulation in oil-filled transformers is not possible.
detected in oil containing paper as a function of temperature.
compounds detected in Kraft paper as a function of temperature.
Water Figure 5 shows that in the time scale of our experiment, diffusion of water initially present in the paper (2%) to the oil is negligible because its concentration in oil is constant for the first 50 h of treatment, as expected from larger affinity of polar water for paper as compared to hydrophobic oil. Above 100°C water tends to diffuse in oil, probably because its concentration in paper increases owing to elimination reactions in cellulose6 when a significant drop of DP has occurred at a somewhat lower temperature than that at which furanics
S. Levchik et al.
510 50
30000
1
./
Oi)
1
0
50
Fig. 5. Formation
100
150
200
Temperature, “C
Temperature, OC
of carbon oxides in oil containing a function of temperature.
paper as
become detectable (150°C). It has been found recently by Burton et al5 that 5-HMF is formed more rapidly in IQ-aft paper insulation when water is present. The present results would also suggest that the production of 5-HMF during paper ageing may be influenced by the presence of water. Oxygen consumption Figure 6 shows that there is a sharp decrease in the oxygen concentration at about 100°C. The consumption of oxygen is due to either the degradation of the cellulose and/or the oxidation of the oil. An explanation for the levelling off of the oxygen consumption above 150°C may be due to the available oxygen in the closed test system being
Fig. 7. Water
formation
as a function paper in oil.
of temperature
for
completely utilized. There was no measurable increase in the acid number of the oil, suggesting that the oil had not been appreciably oxidized. Carbon oxides Figure 7 shows the concentration of carbon monoxide and carbon dioxide in the oil. The carbon oxides steadily increase in concentration above about lOO”C, as in the case of water. It is significant to note that the level of CO2 detected was one order of magnitude higher than the level of CO. This may reflect the higher solubility of CO2 in the oil. Although oxidation of the oil also produces some CO2, the predominant source is attributed to decomposing cellulose, particularly when moisture is present. The carbon oxides together with the furanic compounds comprise the three key paper ageing indicators currently in use to monitor transformer performance.7
25000 i 3.
CONCLUSIONS
8
50
100
1.50
260
Temperature, OC Fig. 6. Concentration
of oxygen detected in oil containing paper as a function of temperature.
The accelerated heating experiment used to examine the influence of temperature on the degradation of Kraft insulating paper in transformer oil gave a good simulation to the degradation processes that occur in actual power transformers. This seems to occur in two steps: firstly, scission leading to a sharp drop in DP and secondly decomposition with formation of low molecular weight species such as furanics, water and carbon oxides, which are somewhat soluble in oil.
Depolymerization
processes in the thermal degradation of cellulosic paper insulation in electrical transformers
REFERENCES 1. De Pablo, A. and Pahlavanpour, B., International Conference on Large High Voltage Electrical Systems, Working Group 15-02, Task Force 03 (CIGRE, WG1501, TF 03) Paper Ageing Experiment, 1993 (Available from Mr Alfonso De Pablo, ASINEL, Apartado 233, 28930 Mostoles, Madrid, Spain). 2. De Pablo, A. and Pahlavanpour, B., International Conference on Large High Voltage Electrical Systems, Working Group 15-02, Task Force 03 (CIGRE, WGl5-01, TF 03) Paper Ageing Experiment (II), February, 1994 (Available from Mr Alfonso De Pablo, ASINEL, Apartado 233, 28930 Mostoles, Madrid, Spain).
5 11
3. Scheirs, J., Camino, G. and Tumiatti, W., Study of the origin of furanic compounds in the thermal degradation of cellulosic insulating paper. J. Appl. Polym. Sci., accepted. 4. Emsley, A.M., Polym. Degrad. Stab., 1994, 44, 343. 5. Burton, P. J., Carballeira, M., Duval, M., Fuller, C. W., Graham, J., de Pablo, A., Samat, J. and Spicar, E., in CZGRE 1988 Session, paper 15-08. CIGRE, Paris, France, 1988, p. 357. 6. Burton, P. J., Graham, J., Hall, A. C., Laver, J. A. and Oliver, A. J., CZGRE 1984 Session, paper 12-09. CIGRE, Paris, France, 1984, p. 1. 7. Scheirs, J., Camino, G., Tumiatti, W., Serena, E., Allan, D., Jones, C., Emsley, A. and Avidano, M., Review of some developments in cellulose insulation condition monitoring (to be submitted).
s0141-3910(97)00249-8
Polymer Degradation and Stabrlity 61 (1998) 507-5 1I C] 1998 Elsevier Science Limited. All rights reserved Printed in Great Britain 0141-3910/98/%see front matter
ELSEVIER
Depolymerization processes in the thermal degradation of cellulosic paper insulation in electrical transformers Sergei Levchik,“” John Scheirq”t Giovanni Camino, 4 Wander Tumiat@ & Mauro Avidano” aDipartimento di Chimica Inorganica, Chimica Fisicae Chimicadeih4aterialidell’Universitridi Torino, Via P. Giuria 7,10125 Torino, Italy bSea Marconi Technologies, Via Crimea 4, 10093, Collegno, Italy
(Received 13 February
1997; accepted 20 October 1997)
A dynamic heating experiment was used to examine the influence of temperature on the degradation of the Kraft insulating paper in transformer oil. The degradation was monitored by measuring a number of key indicators on selected samples as a function of progressive ageing. These indicators included the degree of polymerization (DP), level of furanic compounds and level of gases such as carbon oxides and oxygen. The results show that the formation of furanic compounds and of gases begins significantly after a large reduction in DP, indicatinga two-stage process: firstly scission leading to a sharp drop in DP and secondly formation of low molecular weight degradation products. 0 1998 Elsevier Science Limited. All rights reserved
INTRODUCTION
paper migrate into the oil and can be used as probe compounds to assess indirectly the condition of the insulating paper. A detailed understanding of cellulose degradation mechanisms is required for meaningful interpretation of insulating ageing data that may enable lifetime predictions of remaining paper life and give early warning of premature fajlure. In particular, furanic compounds originate specifically from cellulose whereas H20, CO2 and CO may also be formed by thermal degradation of oil. Thus relationships are being sought to relate the concentration of furanic compounds in the oil to the degree of polymerization (DP) of the paper. Such relationships are difficult to formulate using actual data collected from in-service transformers since the large number of operational variables lead to considerable scatter in the results. In view of this, laboratory simulations have been proposed in order to provide strict control over experimental variables (such as temperature, water, oxygen and oil/paper ratio). 1,2A limitation of these isothermal experiments, however, is that they take many months to yield results whereas the present study uses an accelerated ageing method based on a dynamic heating to provide a rapid overview of chemical process associated with paper ageing. The
In nearly all industrial, oil-filled transformers, Kraft paper tapes (90% cellulose, 67% lignin, 34% pentosans) are the main solid insulator for the winding conductors and for inter-winding, intercore and winding-to-earth insulation. Oil-filled transformers may contain as much as 12 tons of paper and up to 40 tons of oil. Oil-filled electrical transformers typically have an operating temperature range of 60-90°C and operation lifetime of up to 40years. These service temperatures can cause degradation of the oil and of the paper winding insulation during the lifetime of the transformer. Whereas the status of the oil in an electrical transformer can be directly evaluated because it is accessible to sampling, unfortunately the insulating paper is not. However, certain degradation products (e.g. furanics, water, COZ, CO, etc.) from the *Permanent address: Research Institute for Physical Chemical Problems, Byelorussian University, Leningradskaya, 14220080 Minsk, Belarus. tpermanent address: ExcelPlas Australia, PO Box 102, Moorabbin, 3 139, Australia. tTo whom correspondence should be addressed. Fax: 0039 011 6707 557; e-mail: [email protected] 507
508
S. Levchik et al.
dynamic experiment uses a heating rate of l”C/h and the total test duration is approximately 1.5 week.
CHO
2-Furfural [2-FALI
EXPERIMENTAL CH*OH
Paper
The Kraft paper used was transformer-grade insulation paper obtained from Imatran Voima OY (Finland). Elemental analysis by inductively coupled plasma emission spectrophotometry gave the following assay: Ca, 1350 ppm; Cr, 188 ppm; V, 163 ppm; Na, 113 ppm; K, 113 ppm; Fe, 63 ppm. The paper was conditioned to 2% moisture content by the procedure described in detail elsewhere.3
2-Furfurol 12-FOL]
HocH2eHo
S-Hydroxy methyl2-furfural [5_HMF]
5Methyl-2-furfural [SMEF]
Oil Fig. 1. Furanic
The oil was a high-grade naphtenic transformer oil (Nynas 10GB) produced by the Nynas Oil Company. The oil was saturated (2.7v/v) with air by bubbling dried air through it for 2 h.
compounds produced by the degradation Kraft paper and detected in oil.
of
The paper (1 g) was introduced into 150ml glass vials that were flame sealed after addition of 100 ml oil and heated in an oven at l”C/h from room temperature to 200°C. Samples were removed for analysis at intervals of approx. 10°C beginning from 70°C.
ing DP of approximately 1250 corresponds to the initial DP of paper used in transformers. A sigmoidal decrease in the DP occurs with increasing temperature beginning at about 70°C (first sampling). The maximum rate of change occurs at about 100°C and a limiting DP value of approximately 200 is reached above 150°C. A DP of 200 is generally recognized as the failure criterion for insulating paper since at this point the paper becomes brittle and loses its mechanical integrity.
Analytical tests
Furanics
All measurements were conducted on duplicate test samples with the exception of the blank samples containing only oil, with methods reported in detail elsewhere.3 Briefly, the DP of the paper was measured by viscometry of cupric ethylene diamine solution (Method IEC 450, 1974), furanic compounds (Fig. 1) were analysed by HPLC.3 Gases dissolved in oil were measured by gas chromatography (ASTM D3612) and water in oil by the Karl Fisher method after cooling to room temperature.
Figure 3 shows the increase in the concentration of furanic compounds in oil as a function of
Ageing
RESULTS
AND DISCUSSION
Degree of polymerization (DP)
0
50
100
150
2dO
Temperature, OC
Figure 2 shows the reduction of DP of the insulating paper as a function of temperature. The start-
Fig. 2. Degree of polymerization air) as a function
(DP) of Kraft paper in oil (in of temperature.
Depoiymerization processes in the thermal degradation of cellulosic paper insulation in electrical transformers
509
400 15w
-
k 2 300 a ?? Y * .g 200
d
2-FAL
d
5-HMF
+
2-FOL
E 5 t 0 u
100.
0 50
100
150
200
0
Temperature, OC
50
100
150
200
Temperature, OC
Fig. 3. Furanic compounds
Fig. 4. Furanic
temperature. Furanic compounds are formed in the paper in ppm levels and diffuse into the oil. The furanic derivatives found in the highest concentrations were 2-furfuraldehyde (ZFAL), 5hydroxymethylfurfuraldehyde (5-HMF) and furfurylalcohol (2-FOL). This is consistent with the situation in operating transformers, where 2-FAL is generally found in the highest concentration in oil relative to the other furanics.4 The analysis of 2-FAL in the oil has recently received much attention as a specific oil-soluble, chemical indicator of paper insulation degradation since it cannot be produced by degradation of other organic materials in the transformer.4 Furthermore, Burton et ~1.~ have found that the ratio of 2-FAL to 5-HMF in the oil decreases by a factor of 5 between 200 and 350°C. Thus, the 2-FAL to 5HMF ratio in the oil might be used to predict the temperature of localized over-heating in a transformer. Figure 4 shows that the concentrations of the furanic compounds in paper increased markedly above 150°C. This temperature also corresponds to the point at which the DP of the paper dropped to the limiting value of about 200 (see Fig. 2) when the mechanical properties of the paper are significantly affected. In paper, the furanics found in highest amounts is 5-HMF (due to its higher polarity and hence affinity with the paper phase) and this is followed by 2-FAL and 2-FOL (Fig. 4). Again the trend is the same as that which occurs in the industrial transformer, thus demonstrating that this accelerated test is closely simulating the paper degradation processes that are occurring in actual field transformer environments. When the total furanics
are considered (i.e. the sum of the concentration in the paper and the oil) then the order of decreasing concentration is 2-FAL > 5-HMF > 2-FOL. The majority of the literature suggests that furanit compounds originate from the intermediate levoglucosan (LG), which is formed when heating causes cleavage of the glucosidic bonds. No reports in the literature have examined the use of LG as a probe compound in oil to assess cellulose degradation, probably because of its low solubility in hydrocarbon oil. To investigate its potential as a probe compound the solubility of LG in transformer oil (both mineral and naphtenic oil) was examined by adding 0.5 g of LG to lOOm1 of oil. Mixtures were left to stand at room temperature for 1 week, heated for 1 week at 130°C and ultrasonicated for 5 h, respectively. In all cases, no soluble LG could be detected in the oil by FTIR (using the diagnostic band at 1047cm-*) or by the HPLC technique. Hence, the use of LG as a probe compound in the diagnosis of paper insulation in oil-filled transformers is not possible.
detected in oil containing paper as a function of temperature.
compounds detected in Kraft paper as a function of temperature.
Water Figure 5 shows that in the time scale of our experiment, diffusion of water initially present in the paper (2%) to the oil is negligible because its concentration in oil is constant for the first 50 h of treatment, as expected from larger affinity of polar water for paper as compared to hydrophobic oil. Above 100°C water tends to diffuse in oil, probably because its concentration in paper increases owing to elimination reactions in cellulose6 when a significant drop of DP has occurred at a somewhat lower temperature than that at which furanics
S. Levchik et al.
510 50
30000
1
./
Oi)
1
0
50
Fig. 5. Formation
100
150
200
Temperature, “C
Temperature, OC
of carbon oxides in oil containing a function of temperature.
paper as
become detectable (150°C). It has been found recently by Burton et al5 that 5-HMF is formed more rapidly in IQ-aft paper insulation when water is present. The present results would also suggest that the production of 5-HMF during paper ageing may be influenced by the presence of water. Oxygen consumption Figure 6 shows that there is a sharp decrease in the oxygen concentration at about 100°C. The consumption of oxygen is due to either the degradation of the cellulose and/or the oxidation of the oil. An explanation for the levelling off of the oxygen consumption above 150°C may be due to the available oxygen in the closed test system being
Fig. 7. Water
formation
as a function paper in oil.
of temperature
for
completely utilized. There was no measurable increase in the acid number of the oil, suggesting that the oil had not been appreciably oxidized. Carbon oxides Figure 7 shows the concentration of carbon monoxide and carbon dioxide in the oil. The carbon oxides steadily increase in concentration above about lOO”C, as in the case of water. It is significant to note that the level of CO2 detected was one order of magnitude higher than the level of CO. This may reflect the higher solubility of CO2 in the oil. Although oxidation of the oil also produces some CO2, the predominant source is attributed to decomposing cellulose, particularly when moisture is present. The carbon oxides together with the furanic compounds comprise the three key paper ageing indicators currently in use to monitor transformer performance.7
25000 i 3.
CONCLUSIONS
8
50
100
1.50
260
Temperature, OC Fig. 6. Concentration
of oxygen detected in oil containing paper as a function of temperature.
The accelerated heating experiment used to examine the influence of temperature on the degradation of Kraft insulating paper in transformer oil gave a good simulation to the degradation processes that occur in actual power transformers. This seems to occur in two steps: firstly, scission leading to a sharp drop in DP and secondly decomposition with formation of low molecular weight species such as furanics, water and carbon oxides, which are somewhat soluble in oil.
Depolymerization
processes in the thermal degradation of cellulosic paper insulation in electrical transformers
REFERENCES 1. De Pablo, A. and Pahlavanpour, B., International Conference on Large High Voltage Electrical Systems, Working Group 15-02, Task Force 03 (CIGRE, WG1501, TF 03) Paper Ageing Experiment, 1993 (Available from Mr Alfonso De Pablo, ASINEL, Apartado 233, 28930 Mostoles, Madrid, Spain). 2. De Pablo, A. and Pahlavanpour, B., International Conference on Large High Voltage Electrical Systems, Working Group 15-02, Task Force 03 (CIGRE, WGl5-01, TF 03) Paper Ageing Experiment (II), February, 1994 (Available from Mr Alfonso De Pablo, ASINEL, Apartado 233, 28930 Mostoles, Madrid, Spain).
5 11
3. Scheirs, J., Camino, G. and Tumiatti, W., Study of the origin of furanic compounds in the thermal degradation of cellulosic insulating paper. J. Appl. Polym. Sci., accepted. 4. Emsley, A.M., Polym. Degrad. Stab., 1994, 44, 343. 5. Burton, P. J., Carballeira, M., Duval, M., Fuller, C. W., Graham, J., de Pablo, A., Samat, J. and Spicar, E., in CZGRE 1988 Session, paper 15-08. CIGRE, Paris, France, 1988, p. 357. 6. Burton, P. J., Graham, J., Hall, A. C., Laver, J. A. and Oliver, A. J., CZGRE 1984 Session, paper 12-09. CIGRE, Paris, France, 1984, p. 1. 7. Scheirs, J., Camino, G., Tumiatti, W., Serena, E., Allan, D., Jones, C., Emsley, A. and Avidano, M., Review of some developments in cellulose insulation condition monitoring (to be submitted).