IOSR Journal of Electrical and Electronics Engineering (IOSR-JEEE) e-ISSN: 2278-1676,p-ISSN: 2320-3331, Volume 9, Issue 2 Ver. V (Mar – Apr. 2014), PP 36-41 www.iosrjournals.org
Reduce the Amount of Transformer Oil using Air Mixtures in Sphere Gap Electrodes and Proposed Composite Insulating Liquid Muhammad Abid Saeed1, Niaz Ali2, Zeeshan Akbar1, Muhammad Waqas1 1
Department of Electrical Engineering, Sarhad University of Science and Information Technology, Peshawar, Pakistan Department of Electrical Engineering, Sarhad University of Science and Information Technology, Peshawar, Pakistan 1 Department of Electrical Engineering, Sarhad University of Science and Information Technology, Peshawar, Pakistan 2 National Power Control Center, NPCC, Pakistan 1
Abstract : Breakdown behavior of a 10 mm point-sphere gap is investigated for R12, R12 (Dichlorodifluoromethane, CCL2F2), air mixtures for pressures of up to 60psi. It is shown that at high gas pressures, mixtures containing high R12 content can have breakdown voltages higher than the corresponding values in air. Furthermore, these voltages are compared with the S F6. And make a comparison of R12 and S F6 in different scenarios i.e. GWP, cost, dielectric strength. Then Ac voltage breakdown characteristics of various combinations of insulating filler in transformer oil for use in transformers are investigated, with the aim of reducing the amount of oil and thus the cost. Tests are conducted on a sphere–sphere electrode in a horizontal arrangement and a sphere–plane electrode in a vertical arrangement. The Breakdown characteristics are also investigated in a modal in which Nomex paper is inserted between filler and the high - voltage electrode. Based on the experiments, optimum conditions are proposed for the application of transformer oil to transformers. Detailed results of investigations of both gas and composite insulating oil are describe d and discussed in this paper. Index Term: High Voltage, R12 Gas, S F6 Gas, Insulation, Transformer, Nomex Paper, Glass Beads
1.1 Gas A number of high –voltage gas insulated switchgears have been installed that utilize c o m p r e s s e d SF6, which has excellent insulating and arc-quenching properties. In Electric power Transformers circuit breakers, nonflammable SF6 gas – insulated transformers are increasingly being applied to prevent fires at indoor and underground facilities. However, as the global warming potential (GWP) of SF6 gas is very high and the Kyoto Protocol designates SF6 as a restricted. Greenhouse gas, SF6 is heavier than air, hence in case of a leakage, SF6 gas will collect in the lowest regions of the sub-stations, which are normally the cable trenches, and replace all the air. This could present a danger of asphyxiation due to oxygen deficiency if personnel are working . SF6 decomposes at high temperatures (above about 500˚C) and s table gaseous and solid decomposition products will be formed when arcing occurs during circuit breaker switching. These products can cause irritation of the skin, eyes and mucous membranes, such as in the respiratory tract in concentration they have toxic property . SF6 decomposition products (and moisture) inside equipment in service can damage the equipment, and material such as alumina, molecular sieves is used for safe removal and absorption purposes . It is important to consider the global environment when designing electric power system. The investigation is carried out to develop a gas mixture which can act as a replacement of SF6. There are two basic reasons for carrying out such investigations. Firstly, the aims are to develop an insulating medium which is technically as well as economically attractive. The other reason is to obtain a better understanding of the breakdown mechanism, operating in refrigerant gases, other compressed gases, and gas mixtures with air . 1.2 Composite liquid Previously, we investigated the insula ting property of silicone oil and the surface breakdown characteristics of combined insulation for application to electric power apparatus ,  to reduce the cost of transformer oil used in transformer. We investigated the breakdown characteristics in a model in which a filler of small insulating spheres is mixed into the transformer oil and in a modal in which Nomex paper is used between the filler and oil. Based on the experimental results we propose a composite insulating liquid that reduces the amount of transformer oil required for us ed transformers. www.iosrjournals.org
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Reduce the Amount of Transformer Oil using Air Mixtures in Sphere Gap Electrodes and Proposed II.
Experimental Arrangements and Procedures
In the case of h/d=1.0, the glass beads touches the surface of the high-voltage electrode a nd in the case the ratio of 0.84, the glass beads are not touches with the high voltage electrode. For h/d>1.0, the high -voltage electrode is covered with the glass beads. We measured the breakdown voltage. 2.1 Gas Pressure vessel used for the experiments have maximum operating pressure of 90ps i. The electrodes are made of bras s. The Point-sphere electrode had diameter of 50mm. From the test transformer (220v/100kv, 10 kvA) RMS value of AC voltage is applied and the breakdown voltages are measured from a control des k digital dis play of voltage and current values. A 10mm gap is used for all these m e a s u r e m e n t s . The maximum pressure of these investigations is restricted to 60ps i. Proper gas mixing procedures are adopted to form different mixtures with different ratios of R12 gas. The gas used is of commercial purity and are always filtered and dried prior to their admission into the pressu r e vessel. In addition, a small quantity of silica gel is placed ins ide the chamber to remove any moister that may be present or produced in the vessel as a result of electrical dis charges . Several measurements of the breakdown are made at different pressures. Before each series of measurements the electrodes are cleaned thoroughly with dis tilled water.
Fig.1. Test setup to investigate R12, R12 -Air breakdown volt age using Point -Sphere Elect rode 2.2 Composite liquid Tests are conducted for two types of assemblies, vertical (sphereâ€“plane electrode with 12mm diameter sphere and 30mm diameter disc with a gap of 6.2mm) and horizontal (50mm diameter spheres with a gap of 2.5 mm). The filler used for experiments is made of glass spheres (relative permittivity 7.3) of 3mm in diameter. We dried the filler at 75 ÂşC for at least 12 h. Each test is conducted a t atmo s pheric p r e s s u r e . Ac voltage breakdown tests are performed by varying the level of glass beads in the oil testing cup by varying the ratio of the height (h) of the glass beads to the distance (d) from the high -voltage electrode. 2.3 Different samples of BDV We also used Nomex paper (relative permittivity 3.3) in some samples, inserted between the glass beads and high voltage electrodes and see the effect of inserting the Nomex paper on the BDV.
Fig.2. Horizontal Arrangement
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Reduce the Amount of Transformer Oil using Air Mixtures in Sphere Gap Electrodes and Proposed
Fig.3. Vertical Arrangement
Fig.4. Vertical Arrangement, with Nomex act as a Barrier
3.1 Gas As we can see that by mixing R12 in the air the BDV raises and drastically increases at higher pressures.
Fig.5. Breakdown curve of pure R12
Fig.6. Breakdown curves of Air & R1 2-Air www.iosrjournals.org
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Reduce the Amount of Transformer Oil using Air Mixtures in Sphere Gap Electrodes and Proposed We also observed and identified spark discharge in R12 at high pressure. We investigated the s park formation in R12-Air and air, at different pressures and voltages. We investigated that with the addition of R12 in air, the spark tends to suppress and turns into a bluish glow, which can be clearly seen from the following pictures captured during HV testing.
Air: 20psi 20psi
Air -R12: 30psi
Air: 50 psi
Air-R12: 50 psi Fig.7. Spark formation Comparison
We also identified the byproducts formed on decomposition of R12 the by product contains carbon monoxide, carbon dioxide, hydrogen fluoride, hydrogen chloride. On several breakdowns it is seemed that carbon deposited on the electrode.
Fig.8. Carbon deposits on electrodes www.iosrjournals.org
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Reduce the Amount of Transformer Oil using Air Mixtures in Sphere Gap Electrodes and Proposed 3.2 Composite liquid The BDV characteristics of the vertical arrangement are as follows: The BDV decreases from the grounding electrode (h/d=0.0) until jus t before the g lass beads touches the high -voltage electrode (h/d<1.00). When the glass beads approaches the high-voltage electrode (h/d=0.84), the BDV decreases by only about 20% from the normal value when h/d=0. If the glass beads touches the upper spherical electrode of the high electric field, i.e. the high voltage electrode (h/d =1.00) or (h/d>1.00), the BDV decreases significantly to about 46% from the normal value. The BDV did not changes until the glass beads touches the lower side of the high voltage and grounded electrodes (h/d< 1.00) or (h/d=1.00), and decrease by only 10% compared to the normal value when h/d=0. When the glass beads. If h/d=1.52, the high-voltage electrode is covered by the glass beads , BDV falls to 50% of the normal value when h/d=0. By inserting a Nomex paper between the high voltage electrode and glass beads, the BDV increases by 60% of the normal value. By further increasing the no of Nomex papers there a r e n o significant change in BDV found.
Fig.9. Breakdown Curve of Composite Liquid in Vertical Arrangement
Fig.10. Breakdown curve of composite liquid in horizontal arrangement
Fig.11. Breakdown curve of composite liquid with Nomex paper along with glass beads www.iosrjournals.org
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Reduce the Amount of Transformer Oil using Air Mixtures in Sphere Gap Electrodes and Proposed IV.
As relative breakdown strength of SF6 is 3.0 and R1 is 2.9, hence BVD using R112 the breakdown strength is 99.6%. This alternative is 49% cheaper in cost as compared to the SF6. This alternative has 65% les s GWP as compared to SF6. R12 tends to suppress the s park, produces during HV switching, quenching etc. Our research will be useful for the development of pollution-free incombustible and low cost insulating gas for HV applications a n d an alternative to high cost gas SF6. The insulating property of t ransformer oil does not significantly decrease as long as the glass bead does not touch a high-voltage electrode. When the glass beads touch a high -voltage electrode, the insulating property is influenced, because volume of oil under stress decreases,and non-homogeneity of material increases. For the sake of reducing cost and increasing BDV Nomex paper introduced which raises the BDV up to 60%. Our research will be useful for the development of pollution-free incombustible and low cost power apparatus for transformer oil and silicone oil substations and a l t e r n a t i v e t o hi g h cost gas filled transformer.
References      
Maut he, G. and Pettersson, Ketal “ Handling of SF6 and its decomposition products in Gas insulated switchgear (GIS) Electra” No.136, pp. 69-89 and No. 137, pp.87-108, Aug.1991. IEC Publication 60376: 1971 “Specification and acceptance of new sulphur Hexafluoride (SF6), 1971. IEC Publication 60480: 1974 Guide to the checking of sulphur hexafluoride (SF6) taken from electrical equipment, 1971. Hideyuki Miyahara, Akitoshi Nakajima, Tatsuya Ishhikawa and Satoru Yanabu, “Insulating System to reduce the amount of oil,” IEEE transactions on Dielectric and Electrical Insulation, Vol. 15, No.2: April 2008. H. Yoshida and S.Yanabu, “The breakdown characteristics of the silicone oil for electric power apparatus,” ISH, Delft, Netherlands, pp.389-392, 2003 H. Yoshida and S. Yanabu, “ The breakdown characteristics of silicone oil for electric power apparatus,” IEEJ Trans. Power Energy, Vol. 124, pp.582-587, 2004
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