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HVAC Breakdown of R12, R12 – Air Mixtures in Sphere gap Electrodes and Proposed Composite insulating Liquid to Reduce the Amount of Transformer Oil In Transformers

von Muhammad Junaid (Autor) Mehran Tahir (Autor) Muhammad Aamir (Autor)

Forschungsarbeit 2013 5 Seiten

Ingenieurwissenschaften - Energietechnik

Leseprobe

HVAC Breakdown of R12, R12 – Air Mixtures in Sphere gap Electrodes and

Proposed Composite insulating Liquid to Reduce the Amount of Transformer Oil

In Transformers

- 1 Engr. Muhammad Junaid, 2Engr. Mehran Tahir, 3Engr. Kamal Haider, * 4Engr. Muhammad Aamir
- Department of Electrical Engineering, Sarhad University of Science and Information Technology, Peshawar Pakistan.

Abstract -- Breakdown behavior of a 10 mm point-sphere gap is investigated for R12, R12 (Dichlorodifluoromethane, CCL 2 F 2 ), 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 SF6. And make a comparison of R12 and SF6 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 were investigated, with the aim of reducing the amount of oil and thus the cost. Tests were conducted on a sphere–sphere electrode in a horizontal arrangement and a sphere–plane electrode in a vertical arrangement. The Breakdown characteristics were also investigated in a modal in which Nomex paper was inserted between filler and the high- voltage electrode. Basedon 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 described and discussed in this paper.

Index Term -- High Voltage, R12 Gas, SF6 Gas, Insulation, Transformer, Nomex Paper, Glass Beads

1. INTRODUCTION

1.1 Gas

A number of high-voltage gas-insulated switchgears have been installed that utilize compressed 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 substations, 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 1.

SF6 decomposes at high temperatures (above about 500˚C) and stable 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 property2.

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 3.

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 4.

1.2 Composite liquid

Previously, we investigated the insulating property of silicone oil and the surface breakdown characteristics of combined insulation for application to electric power apparatus [5, 6] 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 was 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 use transformers.

2. EXPERIMENTAL ARRANGEMENTS AND

PROCEDURES

In the case of h/d=1.0, the glass beads touches the surface of the high-voltage electrode. And in the case the ratio of 0.84, the glass beads were 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 90psi. The electrodes were made of brass. The Point-sphere sphere electrode had diameter of 50mm. From the Test transformer (220v/100kv, 10kvA) RMS value of AC voltage is applied and the breakdown voltages were measured from a control desk digital display of voltage and current values.

A 10 mm gap was used for all these measurements. The maximum pressure of these investigations was restricted to 60 psi. Proper gas mixing procedures were adopted to form different mixtures with different ratios of R12 gas. The gas used was of commercial purity and were always filtered and dried prior to their admission into the pressure vessel. In addition, a small quantity of silica gel was placed inside the chamber to remove any moisture that may be present or produced in the vessel as a result of electrical discharges. Several measurements of the breakdown were made at different pressures. Before each series of measurements the electrodes were cleaned thoroughly with distilled water.

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Fig. 1. Test set up to investigate R12, R12 -Air breakdown voltage using Point-Sphere Electrode

2.2 Composite liquid

Tests were conducted for two types of assemblies, vertical (sphere–plane electrode with 12 mm diameter sphere and 30 mm diameter disc with a gap of 6.2 mm) and horizontal (50 mm diameter spheres with a gap of 2.5 mm). The filler used for experiments was made of glass spheres (relative permittivity 7.3) of 3 mm in diameter.

We dried the filler at 75 ºC for at least 12 h. Each test was conducted at atmospheric pressure. Ac voltage breakdown tests were 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.

(BDV) of different samples.

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 Nomexpaper on the BDV.

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Fig. 2. Horizontal arrangement

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Fig. 3. Vertical arrangement

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Fig. 4. Vertical arrangement, with Nomex act as a barrier

3. EXPERIM ENTAL RESULTS

3.1 Gas

As we can see that by mixing R12 in the air the BDV raises and drastically increases at higher pressures.

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Fig. 5. Breakdown curve of pure R12

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Fig. 6. Breakdown curves of Air & R12-Air

We also observed and identified spark discharge in R12 at high pressure. We investigated the spark 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.

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Fig. 7. Spark formation comparison

We also identified the by products formed on decomposition of R12 the by product contains carbon monoxide, carbon dioxide, hydrogen fluoride, hydrogen chloride. On several breakdowns it was seemed that carbon deposited on the electrodes.

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Fig. 8. Carbon deposits on electrodes

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 just before the glass 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/dؤ1.00), 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/dأ1.00), 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 was no significant change in BDV found.

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Fig. 9. Breakdown curve of composite liquid in vertical arrangement

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Fig. 10. Breakdown curve of composite liquid in horizontal arrangement

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Fig. 11. Breakdown curve of composite liquid with Nomex paper along with glass beads

CONCLUSION

- As relative breakdown strength of SF6 is 3.0 and R12 is 2.9, hence by using R12 the breakdown strength is 99.6% same.
- This alternative is 49% cheaper in cost as compared to the SF6.
- This alternative has 65% less GWP as compared to SF6.
- R12 tends to suppress the spark, 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 and an alternative to high cost gas SF6.
- The insulating property of transformer 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 upto 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 alternative to high cost gas filled transformer.

REFERENCES

1 Mauthe, G. and Pettersson, K. et al (1991)“ Handling of SF6 and its Decomposition Products in Gas Insulated Switchgear (GIS) ELECTRA” No.136, June 1991, pp 69---89 and No. 137, August 1991, pp 81---108.

2 IEC Publication 60376: 1971 Specification and acceptance of new sulphur Hexafluoride (SF6).

3 IEC Publication 60480: 1974 Guide to the checking of sulphur hexafluoride (SF6) taken from electrical equipment.

4 Hideyuki Miyahara, Akitoshi Nakajima, Tatsuya Ishikawa and Satoru Yanabu “ Insulating System to Reduce the Amount of Oil”, IEEE Transactions on Dielectrics and Electrical Insulation Vol. 15, No. 2; April 2008

5 H. Yoshida and S. Yanabu, “ Thebreakdown characteristics of the silicone oil for electric power apparatus”, ISH, Delft, The Netherlands, pp.389-392, 2003.

6 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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Details

Seiten
5
Jahr
2013
ISBN (eBook)
9783656457985
Dateigröße
920 KB
Sprache
Englisch
Katalognummer
v229763
Note
Schlagworte
hvac breakdown mixtures sphere electrodes proposed composite liquid reduce amount transformer transformers

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Titel: HVAC Breakdown of R12, R12 – Air Mixtures in Sphere gap Electrodes and Proposed Composite insulating Liquid to Reduce the Amount of Transformer Oil In Transformers