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discharges propagating over pressboard of different thicknesses immersed in jatropha curcas methyl ester oil (JMEO) and mineral oil (MO) under positive and ...
Creeping Discharges over Pressboard immersed in Jatropha Curcas Methyl Ester and Mineral oils Henry B.H. Sitorus1,2,3, Abderrahmane Beroual1, Rudy Setiabudy2, Setijo Bismo2, 1

Ecole Centrale de Lyon, University of Lyon, Ampere CNRS UMR 5005, 36 avenue Guy Collongue, 69134 Ecully, France

[email protected]

2

Engineering Faculty, Universitas Indonesia (UI), Depok, Indonesia

[email protected]

3

Electrical Engineering Department, Engineering Faculty, Universitas Lampung (Unila), Bandar Lampung, Indonesia

Abstract—Due to their high biodegradability, non-toxicity and higher fire safety guarantee, vegetable-based oils are considered today as a potential substitute for mineral and synthetic liquids for electrical insulation and especially in high voltage power transformers. However most of known vegetable oils are derived from food materials (rape-seeds, sunflower, palm, olive). And one has to be vigilant to the fact that the replacement of mineral oil which is a petroleum-based product by natural ester fluids based on “renewably sourced” vegetable oils, does not create new problems as this could cause global food crisis due to diversion of food. An interesting product can be jatropha curcas oil extracted from jatropha curcas plants (“miracle tree”) which is non-food crops. This paper presents an experimental study of creeping discharges propagating over pressboard of different thicknesses immersed in jatropha curcas methyl ester oil (JMEO) and mineral oil (MO) under positive and negative lightning impulse voltages (1.2/50 µs), using two divergent electrode configurations (electrode point perpendicular and tangential to pressboard). It is shown that the thickness of pressboard significantly influences the characteristics of creeping discharge and especially the stopping (final) length Lf and the density of branches. For a given thickness, Lf increases with the voltage and decreases when the thickness increases. Lf is longer when the point is positive than with a negative point. For a given voltage and thickness of pressboard, the values of Lf in mineral oil and JMEO are very close Keywords—jatropha curcas methyl ester oil (JMEO); mineral oil; pressboard; creeping discharge; stopping length; discharge current)

I. INTRODUCTION Due to the limitation of resources of mineral oils which are non-renewable and the fact that they are poorly biodegradable (the biodegradability of typical mineral oils is just less than 30%) [1], researchers still continue to look for other alternative materials for replacing mineral oils. Currently, vegetable oils are considered as one of the potential replacements for mineral and synthetic liquids for high voltage electrical equipment, especially high voltage transformers. Even some brands of vegetable oils have been applied in distribution transformers, such as FR3 and BIOTEMP. In general, vegetable oils are derived from food products (rape-seeds, sunflower, olive ...). 978-1-4799-8903-4/15/$31.00 ©2015 IEEE

Massively conversion of food into insulating material oils or others (fuel) can lead to a global food crisis. Therefore, it is necessary to look for alternative materials derived from nonfood crops. Previously we have introduced vegetable oil derived from jatropha curcas (non-food crops) that we called Jatropha curcas methyl ester (JMEO) [2]. We observed that the breakdown voltage of JMEO and MO are very close, even under AC, the breakdown voltage of JMEO is higher than that of MO. We have also analyzed the pre-breakdown phenomena in JMEO and MO [3]. We observed that the streamers characteristics namely the streamers shape, their stopping length (final length), the associated current and electrical charge in JMEO and MO are too close. This paper is in the continuity of our previous work about the ability of JMEO as a substitute for mineral oil in power transformers. It is aimed at the creeping discharges propagating over pressboard immersed in JMEO and MO with two divergent electrode configurations. The knowledge of the characteristics of these discharges is of a great interest to make the good choice of the kind and dimensions of the constituents of insulating system. II. EXPERIMENTAL SETUP The experimental setup consists of a Marx generator (200 kV - 2 kJ) that provides negative and positive standard lightning impulse voltage (1.2/50 μs) and its control desk, a test cell containing the electrodes arrangement and solid/liquid insulating structure, optical and electrical discharge measurement systems (Figure 1). We used two types of test cells to arrange point-plane and point-bar electrodes as illustrated in Figure 2. The first test cell for perpendicular position consists of cylindrical core of 90 mm high and 110 mm inner diameter made of Teflon; the upper cover was of Plexiglas and the lower one which constitutes also the electrode plane, is of brass. The second test cell having 112 mm of inner diameter and 80 mm of height was made also of Teflon. Its upper and lower covers were made of Plexiglas (transparent material) top permits the visualisation of creeping discharge on pressboard. The point electrode was of tungsten; its radius of curvature being 15 μm. A CCD camera – HR58 type SONY XC SVGA resolution (767x580 pixels) connected to PC

through a video card Meteor-II/Multichannel and installed on the test cell was utilized for capturing the integral light image of creeping discharges over the pressboard. Figure 2 shows the three different positions of camera on pressboard used in this study. In the arrangement of the needle electrode perpendicular to the pressboard, the camera was located on the top of the test cell with the same direction with the electrode point, as shown in Figure 2a. While in parallel position between electrode point and pressboard, the position of camera sited on two different positions: (1) the camera was perpendicular to the pressboard that permits to observe the morphology and the ramification of discharge on pressboard as depicted in Figure 2b; and (2) the camera was parallel to the pressboard that allows us to know whether the discharge propagates along the pressboard surface as shown in Figure 2c. The electrode gap distance is set at 37.5 mm for obtaining a same gap distance between point electrode and grounding in perpendicular and parallel positions.

Rseries

CCD Camera PC

Test Cell

Impulse HV Generator

Capacitive Divider Discharge Current Detection TEK CT 2

Oscilloscope

Digital Impulse Analysing System Haefely DIAS 730

Control Desk

water content of oils was measured using a Karl Fischer titration method based on ASTM standard D-1533. The solid insulating samples are square pressboard of 75 mm side and 2 mm thickness. The test was performed with 3 types of thickness of samples namely 2, 6 and 10 mm. For obtaining the 6 mm and 10 mm of thickness of samples, we superimposed 3 sheets and 5 sheets of pressboard having 2 mm of thickness, respectively. The pressboard samples were dried at 80°C of temperature for 10 hours so we obtained the moisture level of pressboards less than 0.2% by weight. The pressboards were then immersed into the insulating liquid (JMEO or MO) for 48 hours. III. EXPERIMENTAL RESULTS A. Morphology of Creeping Discharge Figure 3 and 4 show some examples of creeping discharge forms on JMEO/pressboard and MO/pressboard interfaces for different thicknesses of pressboard under negative and positive lightning impulse voltage with needle electrode perpendicular to the pressboard. It is observed that the thickness (t) of pressboard significantly influences the morphology and the stopping length (Lf) of creeping discharge, as well as the density and number of branches/ramification. This evidences the implication of the capacitive effects and the electric field in the propagation mechanism as reported by others [4]. We observe that Lf decreases when the thickness of pressboard is increased. We also note that the voltage required to obtain the same Lf is greater when the point is positive than when it is negative; in other words, for the same voltage level, Lf is longer with a positive point than a negative one. t = 2 mm

t = 2 mm

Fig. 1. Experimetal setup Camera

Point electrode Pressboard

Point electrode

t = 6 mm

t = 10 mm

t = 10 mm

Point electrode

15o Pressboard

t = 6 mm

(a)

Camera

15o

Plane electrode

Pressboard

Bar electrode

Bar electrode Camera

(b)

(c)

Fig. 2. Sketch of electrodes arrangement, pressboard and camera; (a) point electrode perpendicular to the pressboard, (b) and (c) point electrode parallel to the pressboard with camera in different position

The investigated oils are jatropha curcas methyl ester oil (JMEO) and mineral oil (MO). Before the tests, the water content of JMEO was controlled by using molecular sieve 3A so it is less than 100 ppm. While the water content of mineral oil is less than 10 ppm that is in the range required of standard. The

(a)

(b)

Fig. 3. Evolution of creeping discharge over (a) JMEO/pressboard and (b) MO/pressboard interfaces for different pressboard thickness t, U = -78 kV, point electrode and camera are perpendicular to presboard.

t = 2 mm

t = 2 mm

t = 6 mm

t = 6 mm

surface of pressboard, while in positive polarity; besides propagating along the surface, sometimes some branches propagate laterally without touching the pressboard in both liquids. Similar observations have been reported by Q. Liu et al [5]. 86 kV kV -- 86

- 86 kV

JMEO (a)

MO (b)

+ 66 kV

+ 66 kV

Pressboard

t = 10 mm

MO (d)

JMEO (c)

(b)

Fig. 4. Evolution of creeping discharge over (a) JMEO/pressboard and (b) MO/pressboard interfaces for different pressboard thickness t, U = +66 kV, point electrode and camera are perpendicular to presboard.

The difference of characteristics of creeping discharges over pressboard immersed in JMEO and MO can be observed for a pressboard thickness of 2 mm. It cursorily appears that the characteristic of the creeping discharge namely the shape and stopping length over pressboard immersed in JMEO and MO are very similar. However, when we examine more carefully, we observe that the number of branching / ramification or density of discharges are higher in MO than JMEO, for both polarities. Figure 5 shows the morphology of creeping discharge on JMEO / pressboard and MO / pressboard interfaces for point electrode and camera, respectively perpendicular and parallel to pressboard of 2 mm thickness. There is no obviously visible difference between the discharge shape in JMEO and in MO. However, we note that the negative streamers are more filamentary than the positive ones in both liquids. - 86 kV

+ 66 kV

+ 66 kV

Fig.6. Creeping discharge shape over JMEO/pressboard and MO/pressboard interfaces with thickness t = 2mm, electrodes gap distance d = 37.5 mm, under voltage level U = -86 kV ((a) and (b)) and (c) and (d) U = +66kV, point electrode and camera are parallel to pressboard.

B. Stopping Length of Discharge The stopping length (Lf) of creeping discharge in both liquids increases when the voltage is increased and/or the thickness of pressboard is decreased whatever the electrode configurations whatever the polarity of voltage (Figures 7 and 8). Such a result has been reported elsewhere for various types of oil [6-9]. This increasing is first quasi-linear (at low voltages) and then accentuates. Negative point

60 MO - 10 mm JMEO - 10 mm JMEO - 6 mm MO - 6 mm JMEO - 2 mm MO - 2 mm

50

Final length (mm)

(a)

- 86 kV

Propagating in oil

t = 10 mm

40 30 20 10

JMEO (a)

MO (b)

JMEO (c)

MO (d)

Fig.5. Creeping discharge shape over JMEO/pressboard and MO/pressboard interfaces with thickness t = 2mm, electrodes gap distance 37.5 mm under voltage level U = -86 kV ((a) and (b)) and U = +66kV ((c) and (d)), point electrode and camera are perpendicular and parallel respectively to pressboard.

For determining whether streamers propagate on the surface of pressboard, the camera also has been positioned parallel to the pressboard, as illustrated in Figure 2c. Figure 6 shows examples of streamer propagation under U = -86 kV and U = +66 kV. We observe that in negative polarity, the streamers propagate on the

0 40

60

80 100 Voltage (kV)

120

140

Fig.7. Stopping length Lf of negative creeping discharge propagating on pressboard surface for different thickness in JMEO and MO. The point electrode is perpendicular to pressboard.

However, when the point is negative, Lf in JMEO is longer than in MO for voltage levels up to 50% of flashover voltage level. While it is the inverse for higher voltage levels that is above 50% of flashover voltage (Figure 6). Meanwhile, the

stopping length of positive creeping discharge in JMEO is generally longer than in MO. This result is similar to that reported by Beroual et al. [8]. Positive Point

Final Length (mm)

50

MO - 10 mm JMEO - 10 mm JMEO - 6 mm MO - 6 mm JMEO - 2 mm

40 30

The influence of point electrode direction (with respect to pressboard) on the stopping length of creeping discharge is shown in Figures 9 and 10. It is seen clearly that the stopping length of negative creeping discharges on pependicular direction (between the point electrode and the pressboard) is longer than in the parallel direction in both liquids. This indicates that the configuration in which the point electrode is perpendicular to pressboard is the most dangerous. Meanwhile, when the point is positive, the stopping length is similar for both electrode directions (perpendicular and parallel).

20

IV. CONCLUSSION

10 0 45

55

65

75

85

95

105

Voltage (kV) Fig.8. Stopping length Lf of positive creeping discharge propagating on pressboard surface for different thickness in JMEO and MO. The point electrode is perpendicular to pressboard.

ACKNOWLEDGMENT

Negative point ; t = 2 mm

40

Final length (mm)

It appears from this study that the characteristics (shape and final length) of creeping discharge propagating over pressboard immersed in JMEO and MO are very similar whatever the electrodes configuration (i.e., the electrode point perpendicular or parallel to pressboard). Basing on the results of this study and those related to breakdown and steamers phenomena, one can conclude that JMEO could be a potential replacement for mineral oil for electrical insulation, especially in high voltage power transformers The authors gracefully acknowledge the financial support of DIKTI (Direktorat Jenderal Pendidikan Tinggi) Ministry of Education and Culture Indonesia under the International Joint Research (IRC) Grants No: 2238/H2.R12/HKP.05.00/2014.

JMEO - Parallel M O - Parallel JMEO - Perpendicular MO - Perpendicular

30 20

REFERENCES [1]

10 [2] 0 40

70 80 90 100 Voltage (kV) Fig.9. Stopping length Lf of negative creeping discharge propagating on pressboard surface for 2 mm of thickness in JMEO and MO versus the crest value of applied voltage. The point electrode is perpendicular and parallel to pressboard.

60

[3]

[4]

Positive point ; t = 2 mm

40

Final length (mm)

50

JMEO - Parallel MO - Parallel JMEO - Perpendicular MO - Perpendicular

30

[5]

20

[6] [7]

10 0 40

50

60 Voltage (kV)

70

80

Fig.10. Stopping length Lf of positive creeping discharge propagating on pressboard surface for 2 mm of thickness in JMEO and MO versus the crest value of applied voltage. The point electrode is perpendicular and parallel to pressboard.

[8]

T. V. Oommen, C. C. Claiborne, and J. T. Mullen, “Biodegradable electrical insulation fluids,” Proceeding of Electrical Insulation and Electrical Manufacturing Coil Winding., pp. 465 – 468, Sep-1997. H.B.H. Sitorus, R. Setiabudy, S. Bismo, and A. Beroual, “Physicochemical and Electrical Properties of Jatropha Curcas Methyl Ester Oil as a Substitute for Mineral Oil,” in The 18th IEEE International Conference on Dielectric Liquids, Bled, Slovenia, 2014. H.B.H. Sitorus, A. Beroual, R. Setiabudy, and S. Bismo, “Comparison of Streamers Characteristics in Jatropha Curcas Methyl Ester Oil and Mineral oil under Lightning Impulse Voltage,” in The 18th IEEE International Conference on Dielectric Liquids, Bled, Slovenia, 2014. A. Beroual and L. Kebbabi, “Influence of Capacitive Effects on the Characteristics of Creeping Discharges Propagating over Solid/Liquid Interfaces under Impulse Voltages,” in Annual Report Conference on Electrical Insulation and Dielectric Phenomena, 2008. CEIDP 2008, 2008, pp. 357–360. Q. Liu and Z. D. Wang, “Streamer characteristic and breakdown in synthetic and natural ester transformer liquids with pressboard interface under lightning impulse voltage,” IEEE Trans. Dielectr. Electr. Insul., vol. 18, no. 6, pp. 1908 –1917, Dec. 2011. P. Atten and A. Saker, “Streamer propagation over a liquid/solid interface,” IEEE Trans. Electr. Insul., vol. 28, no. 2, pp. 230–242, 1993. R. Hanaoka, T. Kohrin, T. Miyagawa, and T. Nishi, “Creepage discharge characteristics over solid-liquid interfaces with grounded side electrode,” IEEE Trans. Dielectr. Electr. Insul., vol. 9, no. 2, pp. 308 –315, Apr. 2002. A. Beroual and V.-H. Dang, “Fractal analysis of lightning impulse surface discharges propagating over pressboard immersed in mineral and vegetable oils,” IEEE Trans. Dielectr. Electr. Insul., vol. 20, no. 4, pp. 1402–1408, 2013.