Impulse Breakdown of Extruded Cable Insulation Materials

Rongsheng Liu, Gustavo Dominguez

ABB AB, Corporate Research 721 78 Västerås, Sweden

Andreas Farkas ABB Power Systems, High Voltage Cables

371 23 Karlskrona, Sweden

Abstract- A new test cell is designed for making breakdown test on solid insulation materials for DC cables. Different from a conventional method, the new system uses a specially designed test chamber instead of insulation oil as ambient medium in order to prevent experiment failure from external flashover. The measure eliminates the need of the liquid and has ability to create breakdown data closer to the nature of solid insulation. Lightning impulse breakdown tests were performed on selected insulation materials. Breakdown strength of 400 kV/mm has been obtained for films about 100 µm in thickness. The data was compared with those of model cables tested with a larger thickness of insulation. Breakdown volume theory and up-scale effect are discussed. Power curve fit was made between breakdown strength and the thickness of insulation. A good correlation has been found, which makes it possible to predict the low probability value of breakdown of extruded DC cables at any thickness of insulation. A hypothesis is proposed which indicates that the propagation velocity of breakdown made contributions to the lightning impulse breakdown strength of an insulation material. Higher breakdown strength was found with a lower propagation velocity of a breakdown. The breakdown results show good agreement with data reported by others for the similar materials, and for polyethylene with inorganic fillers the breakdown strength might be slightly decreased.

I. INTRODUCTION

Extruded DC cable system has been studied for over 40 years in the world in using as alternative of overhead lines for electrical power transmission [1, 2]. For industry application, the world’s first extruded HVDC cable system was commercially inaugurated on November 19, 1999 in Visby (Gotland) in Sweden with a transmission capacity of 50 MW, rated voltages of ±80 kV DC and a transmission distance over 72 km [3]. By now, such a system is commercially available by up to ±320 kV [4, 5, 6]. For research development, extruded DC cable systems up to ±500 kV have been developed in Japan [7].

Conventional DC power cables have been oil-filled (OF) or mass impregnated non-draining (MIND) cables. However, OF cables are fundamentally unsuited to long-distance power transmission due to their sophisticated oil refilling systems. MIND cables are thus today the major player on the market for long distance submarine DC power transmission and even for land cables. The MIND cables hold today mostly of the world-records for the highest DC voltage, the highest power transfer capability, the longest length and the deepest sea crossing [8, 9]. On the other hand, a trend has been seen in

recent years that extruded cable systems will play an important role in the world market on DC power transmission (Fig.1).

In this paper, we focus our attention on the fundamental study of dielectric properties of solid insulation for extruded DC cables. Several types of materials were selected and tested. Breakdown mechanism was investigated. The advantages of the new materials show technical potential in manufacturing extruded DC cable at higher voltage level or higher value of working stress.

Fig. 1. Extruded DC cables for submarine and land power transmissions

II. EXPERIMENT

A. Breakdown Cell for Material Film Study Fig. 2 shows a design of the breakdown cell. The test object

was sandwiched between two electrodes. The shape of the HV electrode was a rod with a diameter of 20 mm and that of the ground electrode was a cylinder with a diameter of 25 mm. The test object was a solid insulating film with a diameter of 310 mm and a thickness between 100 µm and 350 µm. The test object was mechanically supported with a solid barrier in a shape of disc from the bottom side of the test object. On the top side, an enclosure was built in the shape of cylinder tube around the HV electrode and formed a test chamber together with the solid plate barrier. During experiment, the test object was stressed when a lightning impulse (LI: 1.2/50 µs) voltage was applied to the HV electrode. A partial breakdown would first created in the air wedge between the HV electrode and the test object at a voltage lower than the breakdown voltage of the solid test object, which brought electric charges onto the surface of the test object, and an uniform electrical field distribution was thus created inside the test object. On the other hand, the partial breakdown created in the air wedge would not be able to flash over the surface of the test object in

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causing a test failure because the enclosure wall of the chamber would stop the streamer propagation along the surface of the test object. A test was ensured with no oil being involved. Several types of solid insulating materials were successfully tested in comparing their breakdown properties in using as materials for extruded DC cables.

Fig. 2. Electrical breakdown setup for the study of DC cable materials

B. Breakdown Test on Model Cables

Model cables were manufactured on the basis of materials selected from insulation film study. Lightning impulse breakdown results are compared and reported in this paper. The insulation thickness of the model cables was 5.5 mm, while the cross-section area of conductor was about 95 mm2.

III. RESULTS AND DISCUSSIONS

A. The Theory for Impulse Breakdowns

From the theory of dielectric physics, the heat transport equation for unit volume of insulation can be expressed as

( ) EgradTKdivdtdT

C v2γ=•− (1)

where Cv is the specific heat, T is temperature, t is the time, E is the electrical field, K and γ are thermal and electrical conductivities. For impulse voltage, the second term on the left side of the equation (1) can be disregarded supposed it is small. In principle, computer simulation can be made after the relationship of E=E(t), and γ = γ(T, E) are known. If

ttEE bb /•= (2) and

( )T/exp0 βγγ −= (3)

where Eb is a critical field caused breakdown, tb is the time to breakdown. γ0 and β are coefficients of the insulation, the breakdown equation obtained from equations (1) through (3) will be

( ) ( )( )TtTCE bvb 0020 2/exp/3 ββγ •≅ (4)

where T0 is the ambient temperature. It is seen from (4) that the electrical breakdown strength is governed by several parameters. The strength is inversely proportional to the square root of the time to breakdown tb, which, on average, is related to the thickness of the insulation and the breakdown channel propagation velocity across the insulation. With the consideration further the impact of high electrical field, a general power relationship is assumed between the breakdown strength and the thickness of the insulation

)(10 d mmEb

−≅ (5) where d is the thickness of the insulation, m0 and m1 are two parameters related to the properties of the insulation material. This relationship will be used in the analysis of breakdown data in considering the volume effect of breakdown in the following sections. B. Experimental results

Fig. 3 shows Weibull plot of the electrical breakdown strength for film test objects and model cables. The scale parameters, breakdown strength mean values, and the breakdown values at the probability of 0.1% are obtained from a Weibull program. These values are plotted in Fig. 4 against the thickness of insulation. Power curve fit was made in seeing the relationship between the breakdown strength and the thickness of the insulation at the breakdown probability values of 63.2%, 50% and 0.1%, respectively. Table I predicts the breakdown strength values at the selected insulation thicknesses of 0.1, 5.5, 9, 12, and 25 mm according to the equations obtained from the curve fit. The breakdown results show good agreement with the data reported by [10] for similar materials for films with a thickness about 100 µm. It was believed that the breakdown strength increased with improved morphology. For model cables, the mean breakdown strength at the insulation thickness of 9 mm is about 140 kV/mm at room temperature according to the equation obtained for the developed material in our study (Table I), which is higher than 105 kV/mm (at 85-90 °C) of the XLPE

Test object

Enclosure wall

Solid barrier in cylindrical disc

Solid plate barrier

Sealing insulation paste

Test chamber

Legs of test cell

Gro-und Elec-trode

Toroid

Toroid

HV Elec-trode

519

DC cable developed with the inorganic nano-filler of magnesium oxide (MgO) [11]. In considering the influence of temperature, the two data are comparable.

10 100 1000

Weibull Probability Distribution

Average Breakdown Strength [kV/mm]

0.01

0.1

0.3

0.50.63

0.9

0.99

0.001

SHLNFilm Sample

284.5398.2335.4308.3Scale (kV/mm)

6.136.0411.2810.4Shape

265.72371.28320.16294.69Mean (kV/mm)

43.9564.3837.0730.79Stand D. (kV/mm)

92.26126.9181.8158.7Breakdown 0.1%

L

S

H

NCLCBCSCable Sample

161.0171.4152.1Scale (kV/mm)

14.7214.2513.19Shape

156.0164.6147.2 Mean (kV/mm)

11.1817.1410.54Stand D (kV/mm)

100.7105.590.08Breakdown 0.1%

CS

CL

CB

Cable Film

Fig. 3. LI electrical breakdown strength of model cable and film test objects

0

100

200

300

400

500

0 5 10 15 20 25 30

Scale parameter: N

Scale parameter: L

Scale parameter: H

Scale parameter: S

Scale Parameter: BPA [10]

Scale Parameter: BPE [10]

Scale parameter: CS

Scale parameter: CL

Scale parameter: CB

Power fit scale parametrer

Power fit mean breakdown

Power fit breakdown 0.1%

Eb(63.2%) = 234.4 * d^(-0.220)

Eb(50%) = 225.34 * d^(-0.216)

Eb(0.1%) = 130.99 * d^(-0.154)

Breakdo

wn Stren

gth [kV/m

m]

Insulation Thickness [mm]

Fig. 4. Relationship between LI breakdown strength and insulation thickness at different probability values of breakdown

TABLE I: POWER CURVE FIT FOR LI BREAKDOWN STRENGTH

Insulation thickness: d 0.1 mm

5.5 mm

9 mm

12 mm

25 mm

Eb (63.2%)=234.4*d^(-0.220) (kV/mm) 389.3 161.0 144.4 135.6 115.3

Eb (50.0%)=225.34*d^(-0.216) (kV/mm) 370.3 156.0 140.3 131.8 112.5

Eb (0.1%)=130.99*d^(-0.154) (kV/mm) 186.9 100.7 93.3 89.3 79.7

It is a tricky question in talking about electrical treeing

propagation velocity in extruded cable insulation under a LI voltage [12]. However, the time to breakdown can be obtained from the waveform of LI breakdown voltage during experiment (Fig. 5). An average breakdown propagation velocity was calculated with the thickness of the insulation test object being divided by the time to breakdown. Model cable tests were made in ABB High Voltage Cables in Sweden (Fig.6). Fig. 7 shows breakdown velocity measured from the

model cables. The average breakdown velocity of the model cable CB is 1.95 km/s which is lower than 2.90 km/s of the model cable CS, while the velocity of the model cable CL is in between with the velocity of 2.19 km/s. The results indicated the influence of morphology on breakdown. Fig. 3 shows that the breakdown strength of the cable CB was higher than that of the cable CS, while that of the cable CL was in between. The breakdown strength values show agreement with the analysis of the breakdown propagation velocity. A slow propagation velocity will lift the value of breakdown strength. On the other hand, all the three types of materials are however good in breakdown strength values in saying from the view point of design working stress. They have qualified potential in using as insulation materials for extruded DC cables.

Fig. 5. A LI voltage waveform and a breakdown chopped wave

Fig. 6. Model cable tests made in the HV laboratory of ABB High Voltage Cables in Sweden

Time to breakdown

LI breakdown chopped wave

520

0.5

1

1.5

2

2.5

3

3.5

4

Model Cable CS CL CB

Average Breakdown Velocity (km

/s)

Mean (2.90 km/s)

Mean(2.19 km/s) Mean

(1.95 km/s)

Fig. 7. Breakdown velocity of model cables at LI breakdown voltages

IV. CONCLUSIONS

An oil free test setup has been successfully developed for the study of LI breakdown strength for DC cable insulating materials. Test results show good agreement with the data reported by other groups for the similar materials.

Power curve fit shows good correlation between LI breakdown strength and the thickness of insulation. The obtained equations matched well with breakdown data of thin films and model cables, and show a possibility to predict the breakdown strength of cable insulation.

Breakdown strength is correlated with the morphology of insulating materials. A robust insulation system may reveal a lower propagation velocity of breakdown, and thus has a higher value of LI breakdown strength.

The breakdown strength of several materials has been analyzed at different probability values of breakdown. For an extruded DC cable with an insulation thickness of 25 mm, the breakdown strength of the developed materials is expected to be at 80 kV/mm at the breakdown probability value of 0.1%, and 113 kV/mm at the breakdown probability value of 50%. The results support the development of extruded DC cables at higher voltages.

ACKNOWLEDGMENT

The authors wish to thank Mr. Magnus Klang for his responsibility in carrying out the model cable tests at ABB High Voltage Cables in Sweden.

REFERENCES [1] R. Jocteur, M. Osty, H. Lemainque, and G. Terramorsi, “Research and

development in France in the field of extruded polyethylene insulated high voltage cables,” CIGRE paper 21-07, pp. 1-22, Paris, 1972.

[2] K. Yoshida, F. Numajiri, Y. Sakamoto, M. Tsumoto, T. Tabata, and K. Kojima, “Research and development of HVDC cables in Japan,” CIGRE paper SC-21, 21-03, pp. 1-14, Paris, 1974.

[3] M. Byggeth, K. Johannesson, C. Liljegren, U. Axelsson, “Gotland HVDC Light - the world’s first commercial extruded HVDC cable system,” CIGRE paper, 14-205, pp. 1-6, Paris, 2000.

[4] B. Jacobson, Y. Jiang-Hafner, P. Rey, G. Asplund, M. Jeroense, A. Gustafsson, and M. Bergkvist, “HVDC with voltage source converters

and extruded cables for up to ±300 kV and 1000 MW,” CIGRE paper, B4-105, pp.1-8, Paris, 2006.

[5] R. Liu, M. Bergkvist, and M. Jeroense, “Space charge distribution in an extruded cable aged in tap water for 3 years,” 2007 International Conference on Solid Dielectrics, pp. 438-441, Winchester UK, July 8-13, 2007.

[6] M. Jeroense, A. Gustafsson, and M. Bergkvist, “HVDC Light cable system extended to 320 kV,” CIGRE paper, B1-304, pp.1-9, Paris, 2008.

[7] Y. Maekawa, K. Watanabe, S. Maruyama, Y. Murata, and H. Hirota, “Research and development of DC +/- 500 kV extruded cables,” CIGRE paper, 21-203, pp. 1-8, Paris, 2002.

[8] P. Nordberg, M. Bergkvist, O. Hansson, and J. Felix, “High power development raises MIND technology to 1000 MW and further,” CIGRE paper, 21-302, pp. 1-5, Paris, 2000.

[9] M. Marelli, A. Orini, G. Miramonti, and G. Pozzati, “Challenges and achievements for new HVDC cable connections,” CIGRE paper 502, SC B4 2009 Bergen Colloquium, pp. 1-11.

[10] I. L. Hosier, A. S. Vaughan, and S. G. Swingler, “The effects of measuring technique and sample preparation on the breakdown strength of polyethylene,” IEEE Trans. on Dielectrics and Electrical Insulation, Vol. 9, No. 3, pp. 353-361, June 2002.

[11] Y. Murata and M. Kanaoka, “Development history of HVDC extruded cable with nanocomposite material,” 8th International Conference on Properties and Applications of Dielectric Materials (ICPADM), pp. 460-463, Bali, Indonesia, 26-30 June, 2006.

[12] A. Ishibashi, T. Kawai, S. Nakagawa, H. Muto, S. Katakai, K. Hirotsu, and T. Nakatsuka, “A study of treeing phenomena in the development of insulation for 500 kV XLPE cables,” IEEE Trans. on Dielectrics and Electrical Insulation, Vol. 5, No. 5, pp. 695-706, October 1998.

Conductor

5.5 mm

Insulation

Semico. layers

521