Performance of Oil and Paper in Transformers Based on IEC and Dielectric Response Techniques*

Transcription

1 F E A T U R E A R T I C L E Performance of Oil and Paper in Transformers Based on IEC and Dielectric Response Techniques* Key words: oil paper insulation, transformer, polarization, conduction, aging Introduction Similar to all dielectric materials, oil and paper have two basic electrical properties the ability to be polarized and to support an electrostatic field. Polarization and conduction phenomena occur to some extent in every electrical insulating material, and problems in a dielectric are produced by one of these two mechanisms. Polarization occurs within the molecules of a dielectric and may cause chemical change or deterioration of the material (the term deterioration products will be used in this article). Conduction in a dielectric is often due to the presence of impurities or contaminants (the term contaminants will be used in this article), rather than its basic structure [1], [2]. In a transformer, the main contaminant is moisture in the form of dissolved or free water, but carbon dust, conducting particles, and deposits can also cause problems. Moisture at the molecular level in solid insulation causes polarization, rather than conduction. Deterioration products include polar molecules in oil, e.g., polar aromatics, acid and non-acid type oxidation by-products (in aged oil), by-products from discharges, and thermal aging by-products in paper. The lifetime of service-aged oil is mostly influenced by water and oxidation products. Water in oil is mainly in the dissolved state, and its solubility increases with temperature. When its concentration exceeds the saturation value, free water is precipitated from the oil. Free water in an area of high electric stress can easily cause the breakdown of insulation. States and solubility of water in oil-filled high voltage equipment are discussed in [3] and [4]. Oxidation products are generated when oxygen reacts with unstable hydrocarbon in oil. Contaminants are the catalyst, and electrical stress is the accelerator. Thus oxidation of oil may commence before a transformer is energized, provided oxygen *Editors note: This article was previously presented at the CIGRE SC-D1 Colloquium in Budapest, Hungary, September 20 25, 2009, paper 19D1-244, and is republished here for a wider audience. Supatra Bhumiwat KEA-Consultant, Auckland, New Zealand Steve Lowe Northpower, Whangarei, New Zealand Philippe Nething ENERCAL, Noumea, New Caledonia Jude Perera Integral Energy, Blacktown, Australia Prasanna Wickramasuriya Ergon Energy, Virginia, Australia Permsak Kuansatit Electricity Generating Authority of Thailand, Nonthaburi, Thailand Conduction and polarization mechanisms in the oil and paper of a power transformer are discussed in this article. is present and the oil is of low quality. For a sealed transformer in which the oxygen content is low, the quality of the oil is the most important factor in reducing oxidation. Oxidation products can be acid or non-acid type. New oil can sometimes contain undesired compounds or polar molecules, leftovers from the refinery process. These may also be considered as deterioration products /07/$25/ 2010IEEE IEEE Electrical Insulation Magazine

2 In the analysis of contaminants in oil, water content is mostly determined by the Karl Fischer method, although free water and some contaminants may be roughly estimated by dielectric breakdown voltage tests. Other special tests are particle counting and metal analysis. Deterioration products in oil are detected electrically by measurement of the Dielectric Dissipation Factor (DDF), which can be expressed as power factor (ASTM-D924 method), tan δ (IEC method), or conductivity (IEC method). Interfacial tension (IFT) is a physical test of oil which detects the formation of sludges or accumulation of oxidation products, although it is also influenced by certain contaminants. Neutralization number (or acidity) is a chemical test of oil which detects oxidation products, but only of the acid type. Deterioration products in the main oil duct between windings of a transformer are detected by dielectric response techniques, which will be discussed in this article. Heat is the main enemy of insulating paper. It breaks the glycosidic bonds of the cellulose and reduces the degree of polymerization (DP) of the paper, which is a measure of the paper lifetime. The products of overheating are CO, CO 2, and water. Water in insulating paper can bubble as a result of a sudden change in load, or development of a fault, and cause the breakdown of the transformer. Oxidation products in oil can accumulate at the surface of the pressboard and block the main ducts between windings. In that case, heat generated from the paper cannot be removed by the oil, and the lifetime of the paper is shortened by overheating. Thermal aging products of insulating paper are detected by furans analysis, especially 2-Furfural (2-FAL). Dielectric response measurements [5] [7] in the time or frequency domain are popular for the determination of moisture in pressboard [8] [11]. A technique based on time-domain measurement of polarization and depolarization current (PDC) [12], [13] can distinguish between conduction and polarization in dielectrics. All the transformers in the studies presented in this article were free of electrical and thermal faults. Thus the article is concerned with normal aging of oil-paper insulation. Performance of Oil Based on the IEC Method CIGRE TF introduced IEC 61620, which prescribes an indirect method of determining DDF by the measurement of the conductivity and permittivity of an insulating liquid under very low voltage stress for a short time. Data from this method and from direct tan δ measurements using ac bridges are in good agreement. Thermodynamic equilibrium can be assumed, and values of the conductivity σ obtained by following IEC can be considered characteristic of the insulating liquid, because electron or hole conduction are unlikely to occur under the prescribed conditions. Further details can be found in Electra 185, 1999; it was concluded that the dc resistivity measured by the IEC method is not characteristic of the liquid because of the high applied voltage applied for the relatively long time of 1 minute. In this section, the performance of oil in transformers is described using the results of measurements on more than 1,000 oil samples taken from transformers in four countries in the Asia- Pacific region. Of these samples, 90% were taken from freely breathing type transformers; 40.9% had σ >5 ps/m, 36.3% in the range σ 1 5 ps/m, 7.9% in the range σ ps/m, 13.6% in the range σ ps/m, and 1.2% had σ <0.1 ps/m. The conductivity was measured at room temperature and corrected to 20 C. Independence of Oil Conductivity Measured Using IEC on Moisture Content Whereas both moisture and oxidation products influence the oil resistivity measured using IEC 60247, the conductivity of oil measured using IEC is independent of moisture content, as shown in Table 1 (also in [11]). Table 1 also shows the relative efficiency of each oil treatment method. Moisture (measured by the Karl Fischer method) is most efficiently removed by vacuum dehydration, which also removes some volatile acids. Clay removes acid type oxidation products. Oil conductivity and acidity (or neutralization number) data allow non-acid type polar molecules or oxidation products to be detected (see Tx-002 after the second treatment and Tx-003 after the first treatment). Many in-service transformers develop problems due to moisture and deterioration products, as shown in Figure 1. Most of the data lie in the σ range ps/m and in the moisture range ppm. The moisture content for most of the transformers Table 1. Oil test results before and after various treatments, showing the independence of oil conductivity measured using IEC on moisture content. ID Oil condition 1 Oil temperature ( C) when sample is taken Moisture in oil (ppm) (Karl Fischer method) Conductivity at 20 C (ps/m) (oil test: IEC 61620) Acidity (mg of KOH/g) Tx-001 A C B + C Tx-002 A C B + C Tx-003 A B B + C Tx-004 A D E A: before treatment; B: after vacuum dehydration; C: after clay treatment; D: after on-line drying; E: after oil replacement. May/June Vol. 26, No. 3 17

3 is not high because most of them were operated at about 50% of their rating. Thus more moisture was stored in the insulating paper than in the oil. DDF (IEC 60247) at 90 C Verses Conductivity (IEC 61620) at 20 C The high voltage stress prescribed in the IEC method can sometimes cause the generation of new ionic charges. An increase in temperature increases the mobility of ionic charges and may also increase the density of ionic charges due to dissolved substances. It follows that high temperature and high stress may modify the nature of the oil. There is no simple way to describe the variation of DDF with temperature. However, the data of Figure 2 for 1,066 oil samples tested by both test methods suggest a logarithmic relationship between DDF at 90 C measured using the IEC test method, and conductivity at 20 C measured using the IEC test method. Deterioration Products in Oil Figure 3 shows neutralization number (acidity) versus conductivity at 20 C. It suggests that acids are not the only cause of high σ. When the acid value was 0.01 mg of KOH/g, the oil conductivity varied between 0.05 and 73.6 ps/m. High σ can be caused by non-acid type oxidation products or other polar molecules, which are leftovers from the refinery process in the case of new oil. In our population, non-acid polar molecules that caused very high σ in service-aged oil originated in on-load tap changers, with oil from the changers leaking directly into the main tank. Table 2 shows results for new and short-service-time transformers. New oil in NT-001 through NT-005 contained significant concentrations of polar molecules, with σ as high as ps/m. This was not true for NT-006 through NT FAL in Oil Verses Conductivity (IEC 61620) at 20 C Figure 4 shows 2-FAL concentration (due to thermal aging of paper) versus oil conductivity. The transformers with σ <1 ps/m Figure 2. Dielectric dissipation factor (DDF) (IEC 60247) at 90 C verses conductivity (IEC 61620) at 20 C. are in good condition, regardless of their age. Those with σ in the range 1 5 ps/m and 2-FAL concentration >200 ppb are in acceptable condition in their middle age. However, those with σ in the range 1 5 ps/m and 2-FAL concentration <200 ppb will probably fail in later years. Transformers with oil leakage from on-load tap changers, as mentioned above, have σ >5 ps/m. When by-products of discharge or oxidation accumulate in oil ducts or at oil-paper interfaces, heat transfer is greatly reduced, resulting in paper overheating and very high 2-FAL concentrations. Suggested Oil Conductivity Levels (IEC 61620) at 20 C When a new transformer is filled with new oil, the conductivity of a sample taken from the transformer can be more than 3 times that of the same oil measured before filling, depending on how the transformer was prepared after the factory tests. Reasonable upper limits for conductivity are as follows: New oil before filling into a new transformer: 0.05 ps/m Oil in a new transformer after filling: 0.1 ps/m Figure 1. Moisture in oil verses conductivity (IEC 61620) at 20 C. Figure 3. Neutralization number (acid number) verses conductivity (IEC 61620) at 20 C. 18 IEEE Electrical Insulation Magazine

4 Table 2. Oil test results for new and short-service-time transformers with poor quality oil (NT-001 through NT-005) and good quality oil (NT-006 through NT-009). Oil from transformer no. Rating of transformer (MVA) Age in service Water content (ppm) ASTM-D1533 [35 max] σ at 20 C (ps/m) IEC Acidity (mg of KOH/g) ASTM-D974 [0.03 max] Interfacial tension at 25 C (dynes/ cm) ASTM-D971 [40 min] DDF at 90 C IEC NT months NT month NT months NT years NT years NT NT month NT year NT year Service-aged oil after treatment, or reclamation before refilling: 0.5 ps/m (sample taken from transformer, not from the treatment plant) Dielectric Response of Power Transformers The main purpose of dielectric response measurements is to assess the condition of the insulation system between the windings, through evaluation of the pressboard moisture content and the conductivity of the oil in the main ducts. In many in-service transformers, the σ of the oil in the main ducts is the same as that in the main transformer tank, when measured following IEC However, in some situations they may differ. The first is a new transformer before it is energized but after on-site oil filling, as reported in [14]; new oil in the transformer had σ = 2.03 ps/m, but σ measured in the main duct was 0.79 ps/m (at 20 C). Another is when oxidation products block the oil duct [9]; σ of oil in the main duct is then higher than in most of the oil in the main tank. The dielectric response data presented in this article were obtained using a PDC-analyzer-1 MOD [12]. The analyzer makes time-domain PDC measurements and provides recovery voltage polarization spectra and insulation resistance values. It also makes frequency-domain measurements that yield values for capacitance and DDF (tan δ). Conduction and Polarization Time-Domain PDC Measurements During PDC measurement, the current I pol during polarization (or charging) consists of conduction and absorption components, the former due to contaminants and the latter to deterioration products. The current I depol during depolarization (or discharging) has zero or very little conduction component, and is due almost exclusively to deterioration products. If the insulation is dry, I pol and I depol will be nearly equal at about one-tenth of the charging/discharging time. Thus problems due to contaminants and to deterioration products may be easily separated. The pressboard moisture content and the σ of the oil in the main duct are determined by the PDC software. PDC magnitude depends on capacitance, geometry, and the insulation condition. Figure 5 shows PDC results for transformers with different insulation conditions. Charts 5.1 to 5.6 are for transformers T1 to T6 filled with oil, and charts 5.7 to 5.9 are for transformers T4 to T5 and T7 to T8 without oil. Further details are given in Table 3. PDC Measurements on Oil-Filled Transformers For oil-filled transformers, the initial current is determined largely by the condition of the oil, and the current at longer time by the pressboard condition. The oil-paper interface dominates around the time of the main slope change, which in many cases Figure 4. 2-Furfural (2-FAL) verses conductivity (IEC 61620) at 20 C. May/June Vol. 26, No. 3 19

5 is in the range seconds but can be much longer for a large transformer such as T2 (chart 5.2). In cases of very high insulation moisture content, e.g., T4 (chart 5.4), no change of slope occurs. PDC Measurements on Transformers Without Oil Measurements on transformers without oil allow assessment of the condition of the insulating paper. A new transformer can be tested before oil filling, and an in-service transformer can be tested during major refurbishment after the oil is drained. However, the test voltage applied to a transformer without oil should not be higher than 100 V, in order to avoid partial discharge. Moisture in the pressboard of a transformer without oil (homogeneous) is moisture in the molecular state, distributed inside the material by diffusion [3]. It causes polarization and increases both I pol and I depol. Surface humidity and free water influence the conduction component of I pol. Transformers T4 and T5 were twin transformers that had been operated under different conditions before refurbishment (chart 5.4 for T4 and chart 5.5 for T5). Paper drying reduced the moisture content of the pressboard in both units to 3%. However, the tank of T4 was wet, and so its I pol was higher than that of T5 (chart 5.7). Overheating in the LV ground insulation of T7 caused the bending in the I depol plot around 100 seconds shown in chart 5.8. The influence of surface humidity on I pol (no change in slope) is also seen in the same chart. In T8 (chart 5.9), the high initial current with slight bending is due to oxidation products remaining on the surface of the paper after the oil was drained. In conclusion, the shape of the I depol plot characterizes the type of aging in the insulating paper, provided the applied voltage stress is very low. Evaluation of DDF and Capacitance (C) Ratio by PDC Measurement In figure 6, the top row presents DDF results and the bottom row presents C ratios (the ratio of capacitance at a given frequency to capacitance at 50 Hz). Both were evaluated from the PDC results of figure 5. Further details are given in Table 3. Oxidation products at the oil-paper interface influence the maximum DDF values; the higher the frequency at which the maximum DDF appears, the worse the condition of the oil-paper interface. Thus chart 6.1 indicates that the condition of the oilpaper interface is much worse in T5 than in T4. Transformer T6 Figure 5. Polarization and depolarization current (PDC) measurement results of transformers in different aging conditions. Charts are from the measurement of transformer filled with oil (multilayer oil paper). Charts are from the measurement of transformers without oil (homogeneous). 20 IEEE Electrical Insulation Magazine

6 Figure 6. Dielectric dissipation factor (DDF; top row) and capacitance (C) ratio (bottom row), evaluated from the measurement results of polarization and depolarization currents. (chart 6.2) is one of the worst. Both polarization and conduction increase the magnitude of DDF. Polarization causes an increase in the C ratio, but conduction does not. Moisture at the molecular level in insulating paper causes polarization, but free or dissolved water, and surface humidity, cause conduction. Chart 6.4 shows that T4 had extremely high pressboard moisture content before refurbishment, and T5 had extremely high oxidation product content, especially of the acid type. Both had very high C ratios despite the difference in the shape of the two plots. Although T6 before refurbishment had much higher oil conductivity than T5, the oxidation products were mostly of the non-acid type. The C ratio shape of T6 (chart 6.5) also differs from that of T5 (chart 6.4). Recovery Voltage Polarization Spectra Recovery voltage polarization spectra were obtained from the PDC data. Charts 7.1 and 7.2 in Figure 7 indicate that the first peak of the spectra is influenced more by pressboard moisture Table 3. Details of the transformers, charts, and dielectric response results.1 Transformer description PDC chart no. DDF chart no. C ratio chart no. Recovery voltage chart no. Insulation system analyzed Moisture in pressboard (% wt.) σ of oil in main duct at 20 C (ps/m) T1: 20 MVA, new, not yet in service 5.1 HV-LV T2: 180 MVA, age HV-LV T3: 20 MVA, new, 1 day after oil filling 5.3 HV-LV T4: 20 MVA, age HV-LV >> T5: 20 MVA, age 28 (a twin of T4) HV-LV T6: 100 MVA, age HV-LV The twins T4 and T5 after refurbishment but before oil refilling. Case 2 of [14]. T7: 25 MVA, age unknown, tested after the oil was drained. A case of paper overheating. T8: 25 MVA, age 36, tested after the oil was drained. A case of high σ of oil before draining HV-LV LV-G LV-G PDC = polarization and depolarization current; DDF = dielectric dissipation factor; C = capacitance. May/June Vol. 26, No. 3 21

7 Figure 7. Recovery voltage polarization spectrum at charging voltage 500 V, evaluated from the measurement results of polarization and depolarization currents. than by the conductivity of the oil in the main duct. The oil in T6 (chart 7.2) had a high concentration of oxidation products (σ = ps/m), and the first peak of the spectrum appeared at a charging time of 2.8 seconds, whereas T4 had a high pressboard moisture content and the first peak appeared at a charging time less than 1 second (chart 7.1). Chart 7.3 (T4 and T5 without oil) shows that surface humidity slightly decreases the maximum recovery voltage. Conclusions Some of the electrical characteristics of an insulating oil sample can be determined accurately at room temperature using the IEC test procedure. High voltage stress and high temperature may modify these characteristics. Deterioration products, both acid and non-acid types, influence the measured conductivity, but moisture or contaminants do not do so. Reasonable upper limits have been suggested for new and in-service oil. Pressboard moisture content and conductivity of the oil in the main ducts of a transformer may be measured using dielectric response techniques, which also give information on the state of the insulation at interfaces. Measurement of polarization and depolarization currents in a transformer without oil allows conduction and polarization phenomena occurring in the insulating paper to be identified. The depolarization current characterizes the type of aging in the paper, provided the applied voltage stress is very low. References [1] T. W. Dakin, Conduction and polarization mechanisms and trends in dielectrics, IEEE Electr. Insul. Mag., vol. 22, no. 5, pp , [2] B. Tareev, Physics of Dielectric Materials. Moscow: Mir Publishers, [3] V. G. Arakelian and I. Fofana, Water in oil-filled, high voltage equipment, Part I: States, solubility and equilibrium in insulating materials, IEEE Electr. Insul. Mag., vol. 23, no. 4, pp , [4] V. G. Arakelian and I. Fofana, Water in oil-filled, high voltage equipment, Part II: Water content as physiochemical tools for insulation condition diagnosis, IEEE Electr. Insul. Mag., vol. 23, no. 5, pp , [5] W. S. Zaengl, Dielectric spectroscopy in time and frequency domain for HV power equipment, Part I: Theoretical considerations, IEEE Electr. Insul. Mag., vol. 19, no. 5, pp. 5 19, [6] W. S. Zaengl, Applications of dielectric spectroscopy in time and frequency domain for HV power equipment, IEEE Electr. Insul. Mag., vol. 19, no. 6, pp. 9 22, [7] CIGRE Task Force , Dielectric response methods for diagnostics of power transformers, IEEE Electr. Insul. Mag., vol. 19, no. 3, pp , [8] S. Bhumiwat and P. Stattmann, Quality assurance after transformer refurbishment by means of polarisation depolarisation currents analysis, in IEEE Bologna Power Tech Conf., Bologna, Italy, June 23 26, 2003, p. 90. [9] S. A. Bhumiwat and P. Phillips, Verification of on-site oil reclamation process by means of polarisation depolarisation currents analysis, in IEEE Int. Symp. Electr. Insul., Indianapolis, IN, Sept , 2004, pp [10] S. A. Bhumiwat, Insulation condition assessment of transformer bushings by means of polarisation/depolarisation current analysis, in IEEE Int. Symp. Electr. Insul., Indianapolis, IN, Sept , IEC Fluids for electrotechnical applications Unused mineral insulating oils for transformers and switchgear. [11] S. Bhumiwat, The latest on-site non-destructive technique for insulation analysis of electrical power apparatus, Weidmann-ACTI Annual Technical Conf., Sacramento, CA, Nov. 8 10, bizland.com/papers/technical/weidmann.pdf [12] J. Alff, V. Der Houhanessian, W. S. Zaengl, and A. J. Kachler, A novel, compact instrument for the measurement and evaluation of relaxation currents conceived for on-site diagnosis of electrical power apparatus, in IEEE Int. Symp. Electr. Insul., Anaheim, CA, April 2 5, 2000, pp [13] A. Bouaicha, I. Fofana, M. Farzaneh, A. Setayeshmehr, H. Borsi, E. Gockenbach, A. Beroual, and T. Aka-Ngnui, Dielectric spectroscopy techniques as quality control tool: A feasibility study, IEEE Electr. Insul. Mag., vol. 25, no. 1, pp. 6 14, [14] S. A. Bhumiwat, Advanced applications of polarisation/depolarisation current analysis on power transformers, IEEE Int. Symp. Electr. Insul., Vancouver, Canada, June 2008, p Supatra A. Bhumiwat (M 81) was born in Bangkok, Thailand, in She received her bachelor s degree in electrical engineering from Chulalongkorn University, Bangkok, in March In January 1980, she joined Electricity Generating Authority of Thailand (EGAT) and has been a test engineer since the end of In 1983, while working for EGAT, she received practical training on high voltage tests from BBC in Switzerland and Germany in addition to ETH-Zurich and ITR-Rapperswil in Switzerland. She worked at EGAT Extra-High Voltage Laboratory from 1984 until February Her main interest was dielectric diagnoses. Her research works included insulation aging of instrument transformers and power transformers. She was a member of CIGRE WG12.15 and CIGRE WG In March 1997, she migrated 22 IEEE Electrical Insulation Magazine

8 to New Zealand and has been an independent high voltage diagnostics consultant. Her services at present include transformer oil analysis and PDC analysis of power transformers, bushings, rotating machines, and power cables. Steve Lowe was born in Tanzania, Africa, in 1950 but at an early age shifted to New Zealand. He received a New Zealand Certificate in Engineering (Electrical) from the Central Institute of Technology, Wellington, in His employment experience was with New Zealand Electricity, and he still remains at Northpower as development engineer. During this period he helped run trials using PDC analysis to determine life expectancy on aging zone transformers and sub transmission cables. His special fields of interest include equipment life extension and testing. Philippe Nething was born in Nouméa, New Caledonia, in He graduated from Université des Sciences et Techniques du Languedoc, Montpellier, France. He joined Enercal in 1982 and has been working in various domains hydro plant construction ( ) and overhead line and power station construction ( ). Since 1993 he has been the head of Hydro Power and Transmission grid. With his role as a transmission asset manager, life extension of high voltage equipment, especially power transformers, is always his main interest. Jude Perera was born in Sri Lanka in He graduated with a BEng degree from the University of Moratuwa, Sri Lanka, in He also received a master s degree in engineering science from the University of Newcastle, Australia, in 1995 and a master s degree in business administration from Macquarie University, MGSM, Sydney, in He started his career with the Ceylon Electricity Board, Sri Lanka, where he gained experience in maintenance of HV and LV electrical assets. When he migrated to Australia in 1990, he joined the Electricity Commission of NSW (subsequently known as Pacific Power and TransGrid). Since 2003 he has worked for Integral Energy, Sydney, as the transmission asset engineering manager. He is currently a member of CIGRE AP B3 (Substations) panel. Throughout his career he has had extensive experience in maintenance and condition monitoring of power transformers. His main interest is in diagnostics of faults in power transformers. Prasanna Bhashitha Wickramasuriya was born in Sri Lanka in He graduated with a BEng (Hon) degree from the University of Moratuwa, Sri Lanka, in He also received a master s in business and technology from the University of New South Wales, Australia, in He started his career with the Ceylon Electricity Board, Sri Lanka, where he gained experience in maintenance of HV and LV underground electricity system assets. He migrated to Australia in 1991 and joined the Snowy Mountain Hydro Electric Authority (subsequently known as Snowy Hydro). During this time, he was mainly involved in condition monitoring and maintenance of 330-kV transformers and cables. Since 2002 he has worked for Ergon Energy, Brisbane, as the high voltage test engineer. His main responsibilities include testing and condition monitoring of a HV plant and failure investigations. He is currently a member of CIGRE AP D1 (Materials and Emerging Test Technologies) panel. Permsak Kuansatit was born in Bangkok, Thailand, in He received his bachelor s degree in electrical engineering from King Mongkut Institute of Technology, North Bangkok campus, Thailand. His employment has been with Electricity Generating Authority of Thailand (EGAT). His experience with dissolved gas analysis and transformer oil analysis started in He was the head of the EGAT oil laboratory for 10 years, serving oil tests in the whole country. Currently he is engineer level 10, a transformer-oil expert, and a researcher, providing technical support to EGAT Transmission System Maintenance Division. He has been a member of the Doble Oil Committee for more than 10 years and is an observer, at present, of CIGRE SC-A2 May/June Vol. 26, No. 3 23