Water trees

R. Vogelsang, R. Brütsch, T. Farr, K. Fröhlich: „Electrical tree propagation along barrier-interfaces in epoxy resin“

Electrical tree propagation along barrier-interfaces in epoxy resin R. Vogelsang1 (Student member IEEE), R. Brütsch2, T. Farr1 and K. Fröhlich1 (Fellow IEEE) 1 High Voltage Laboratory at the Swiss Federal Institute of Technology Zürich, Switzerland 2 Von Roll Isola AG Breitenbach, Switzerland

Abstract: The propagation of electrical trees along interfaces in epoxy resin is described. The electrical trees grew in needle-plane samples without and with internal barriers up to the final breakdown. The barrier materials glass, mica and PTFE had been chosen because they have significantly different bond mechanisms to the epoxy resin as well as being common materials in various composite high voltage insulation systems. The mechanical strength of the systems barrier material – epoxy resin was determined for the materials tested. The results show that the propagation of the tree along the barrier is dependent on the type of chemical bond and the shear strength between the epoxy resin and the barrier material. The higher the bond strength between the barrier and the epoxy the higher the resistance to electrical tree propagation. Furthermore it appears that the wettability of the barrier materials plays a minor role in tree propagation along them. The results give a better understanding of the failure mechanism of certain composite insulating materials exposed to high voltage.

Introduction Composite insulation systems are widely used in high voltage equipment. Typical composites are mica-epoxy systems of high voltage rotating machines or glass fibre reinforced materials. During their service the composites may be exposed to partial discharges and electrical treeing for long periods. The electrical treeing, a process in which fine erosion channels propagate through the material, is often referred to as the most important degradation mechanism in solid polymeric insulation [1–3]. Much work has been done over the years to investigate tree inception and tree growth in homogeneous materials. A comprehensive collection is that of Dissado [1]. In samples with barriers, Varlow et al. [4] and Cooper [5] have pointed out the importance of mechanical factors on the propagation of electrical treeing. Sweeney et al. [6] and Farr et al. [7] have mainly concentrated on the simulation of treeing in samples without and with barriers.

To the authors it seemed to be interesting to investigate the electrical tree propagation at the interface by concentrating on the chemical bonds between the barrier and epoxy resin and their strength.

This approach is expanded by the investigations of Auckland et al. [8] who found that adhesion mechanisms may play an important role in tree growth at barriers. In these investigations, the electrical tree could travel along as well as through the barriers. In contrast, the present authors ensured that the electrical tree had to propagate along the barriers and could not penetrate them. The electrical breakdown values are compared to the externally measurable values of the shear strength at the interface and the wettability of the barrier material.

Bond mechanisms between the different barrier materials and epoxy resin When two materials are brought very closely together the binding forces of the atoms are responsible for the interfacial strength [9]. Between the barrier materials and the epoxy resin different types of bonds can occur.

Each type of bond has different binding energies, which determine the strength of the interface between the barrier material and the epoxy resin. The strength of the interface is composed of the strengths of the different bond types.

To investigate the influence of interface properties on tree growth, barrier materials with a significant difference in the bond strength to the epoxy resin have been chosen. Their bonds can be classified as follows: Glass – epoxy resin: The bonds between the glass and the epoxy are most likely determined by strong covalent or hydrogen bonds between the OH-groups of the glass and the amine-groups of the epoxy resin. In addition, van der Waals forces between the glass and the epoxy resin can also occur but are generally considered to contribute less to the adhesive strength than covalent or hydrogen bonds.

Mica – epoxy resin: Since there are far fewer OHgroups at the surface of the mica compared to glass, there are also fewer covalent or hydrogen bonds between them and the amine-groups of the epoxy resin.

In addition van der Waals forces occur between the mica and the epoxy resin.

PTFE – epoxy resin: The bonds between the polytetrafluorethylene (PTFE) and the epoxy resin are assumed to be determined by van der Waals forces only.

Conference on Electrical Insulation and Dielectric Phenomena, CEIDP 2002, Cancun, Mexico

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R. Vogelsang, R. Brütsch, T. Farr, K. Fröhlich: „Electrical tree propagation along barrier-interfaces in epoxy resin“

Experimental setup The tests have been carried out with a needle-plane arrangement incorporating barriers. The samples were 40 mm by 40 mm by 40 mm in size. The distance of 5 mm between the point and the plane was chosen to study tree growth in a wider volume considering the higher thickness of typical high voltage insulation. The different barriers in the insulation were plates of 40 mm length, 20 mm width and a thickness of 0.2 mm. They were placed centrally between the two electrodes perpendicular to the axis of the system. The samples were fully moulded in epoxy resin of type Araldite D (relative permittivity of 4.0). The samples have been prepared in a vacuum process. For easy detection of the trees a fully transparent formulation was chosen. To promote treeing from the beginning of the voltage application, needles (Ogura Jewel) with a tip radius of 1 µm were used. The electrical arrangement is mainly a conventional test circuit consisting of a 50 Hz high voltage transformer and a 1:1000 capacitive voltage divider. The treeing experiments were carried out with a constant ac voltage of 28 kV rms. High voltage was applied to the needle. The plane surface of the sample opposite the needle tip was coated by conducting silver paint and grounded. The electrical treeing was observed with a CCD-camera via telephoto lenses (Figure 1).

The breakdown time values were analysed assuming a two parameter Weibull-distribution. The number of samples in each test was n = 8.

To determine the shear strength of the barrier materials to the epoxy resin a shear peel test has been carried out.

The wetting of the barrier materials has been determined by measuring the contact angle of the epoxy resin to the different barrier materials used. high voltage source

sample

up to the final value. In all samples tested the tree inception time was less than 30 seconds. Once the first branch had been created, the electrical tree grew in the form of small branches to the ground electrode. When the first branch reached the plane and bridged the electrodes, the final breakdown did not occur immediately. After the first branch has reached the opposite electrode, additional branches were created and more volume between the needle tip and the plane was taken up by the tree. The existing branches expanded into hollow pipe-shaped channels and the final breakdown occurred when the first channel between the electrodes attained a significant diameter. The tree growth characteristic through the material as well as a 3-stage tree growth model have been described in detail

by Vogelsang et al. [10].

The 63%-value of the total breakdown time of the measured 8 samples without a barrier is approx. 1 h.

Tree growth with and along different barrier materials Since the same voltage and needle tip geometry was used as in the samples without barrier, the tree inception occured along the same lines as described before. After inception the tree grew in direction to the barrier.

Because the barrier used in the experiments cannot be penetrated by the tree, it spreads on the surface of the barrier material in order to find a favourable way arround it (Figure 3). The spreading of the tree on the barrier follows a circular pattern and can be seen as burned marks on the surface of the barrier (Figure 4).

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Figure 3: Treeing around a barrier that cannot be penetrated (glass). lamp

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traces of circular tree spreading on the mica

Figure 1: Sample and test setup for the treeing experiments.

Results Tree growth without barrier Due to the high electric field at the needle tip the inception of the first branch (minimum length of 100 µm) always occurred during the initial voltage rise

main breakdown path

1 cm

Figure 4: Traces of the circular tree spreading on the surface of a barrier (dark areas) with the breakdown path (black area).

Conference on Electrical Insulation and Dielectric Phenomena, CEIDP 2002, Cancun, Mexico

947

R. Vogelsang, R. Brütsch, T. Farr, K. Fröhlich: „Electrical tree propagation along barrier-interfaces in epoxy resin“

In the experiments it was observed that the tree always grew along the interface of the barrier material and the epoxy resin. This may be explained by the electric field in the arrangement, which is forcing the tree to proceed tangentially to the barrier. In addition, it can be assumed that the tree would prefer to move along the interface when this is electrically weaker compared to the bulk of the epoxy resin. A discharge in a tree channel from the needle to the plane and its spreading along the surface of the barrier are shown in Figure 5.

The introduction of a barrier that can not be penetrated results in a prolongation of the total breakdown time. The difference between the breakdown time of the samples without and with the chosen barrier materials glass, mica and PTFE is significant as regards the 63%-values because the confidence intervals do not overlap (Figure 6). The total breakdown time is longest for samples with barriers of glass followed by the samples with barriers of mica. The shortest breakdown time has been measured with the samples with a barrier of PTFE. The differences in the results are significant as regards the 63%-values because their confidence intervals do not overlap (Figure 6).

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discharge in a channel and on the barrier barrier

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Figure 5: Discharge in a tree channel from the needle to the plane and spreading along the barrier surface.

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The results show a correlation between the shear strength of the interface barrier material and epoxy resin and the total breakdown time of the samples (Figure 7).

It has to be mentioned that for the interface glass – epoxy the real shear strength is higher than the value shown in the diagram because the glass suffered internal fracture before the materials separated.

When propagating along the interface the tree must expend energy in order to destroy the bonds between the barrier material and the epoxy resin. If these bonds have a high strength, like covalent or hydrogen bonds between the glass and the epoxy, the tree must expend more energy to propagate along the interface. At a constant voltage level this means a longer breakdown time as seen in Figure 7, where the longest breakdown time values of all samples tested has been measured for glass. The high bond strength between glass and epoxy causes also a high shear strength of the interface.

Between the mica and the epoxy resin fewer covalent or hydrogen bonds exist together with weaker van der Waals forces. Therefore the tree has to expend less energy to propagate along the barrier. This results in a shorter breakdown time compared to that of a glass barrier. The lower bond strength causes therefore also a significant lower shear strength of the interface between mica and epoxy resin compared to that between glass and epoxy resin (Figure 7).

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Influence of bond types and shear strength on tree propagation along different barrier materials

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Discussion

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Figure 7: Measured breakdown time and shear strength of the different barrier materials to epoxy resin.

Conference on Electrical Insulation and Dielectric Phenomena, CEIDP 2002, Cancun, Mexico

948

R. Vogelsang, R. Brütsch, T. Farr, K. Fröhlich: „Electrical tree propagation along barrier-interfaces in epoxy resin“

At the interface PTFE – epoxy resin, where exist only weak van der Waals forces, the tree can propagate with a lower energy demand along the interface than on a glass or a mica barrier. This results also in a significant low shear strength between the PTFE and the epoxy resin than between glass or mica and epoxy resin.

At a constant voltage level this means a significantly shorter breakdown time as seen in Figure 7.

Compared to the values at the interface between mica and epoxy resin the shear strength of the interface glass – epoxy resin is much higher, whereas the shear strength of the interface PTFE – epoxy resin is much lower (Figure 7). It has, however, to be mentioned that it is very difficult to determine an exact quantitative value for the shear strength at interfaces. The values are dependent on the thickness of the epoxy layer between the barriers, elastic properties of the barrier materials, the temperature, the type of epoxy resin, the cleanliness of the samples, the production process and much else. In the experiments, it was tried to keep exactly the same production process of the samples for the electrical tests as well as for the mechanical tests. Therefore it can be assumed that the relation between the values give reliable results for the given materials and production process.

Influence of surface-wettability on tree propagation along different barrier materials The wettability of surfaces indicates how readily they are covered by the epoxy resin. It is particularly important at the microscopic roughness of the surfaces.

The wettability has been determined by measuring the contact angle of the liquid epoxy resin on the surface of the barrier materials used. A low contact angle means thereby a high wettability. The breakdown time shows no evident correlation to the wettability of the different barrier materials. (Figure 8: the contact angle is inversely proportional to the wettability).

Conclusions 1. Higher breakdown time values can be achieved by the introduction of a barrier, which cannot be penetrated by electrical treeing, between the electrodes of the sample.

2. When the tree cannot penetrate the barrier material, it propagates tangentially along the interface between the barrier and the epoxy resin in a roughly circular pattern in order to find a favourable way around it.

3. The propagation speed of the electrical tree along the interface is determined by the bond types at the interface between the barrier material and the epoxy resin. The highest resistance to tree propagation is found at interfaces with mainly covalent or hydrogen bonds as between glass and epoxy resin. The lowest tree propagation resistance is found at interfaces with mainly van der Waals bonds as between PTFE and epoxy resin.

4. The propagation speed of the tree along the interface can be related to the mechanical shear strength of the interface between the barrier material and the epoxy resin. The higher the shear strength at the interface, the higher its resistance to tree propagation.

5. The wettability of the barrier seems to play a minor role in tree growth compared to the types of bond between the barrier material and the epoxy resin.

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The PTFE surface has almost the same wettability as the glass surface but shows significantly lower breakdown time values. It appears from the measured results that mainly the type of bonds is important for the propagation of the tree along the interface. This means for the tested materials that few but strong bonds, like the covalent or hydrogen bonds between glass and epoxy, provide a higher resistance to tree growth than many van der Waals bonds at the interface PTFE – epoxy resin.

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This work has been sponsored by Von Roll Isola AG (Switzerland), PD Tech Power Engineering AG (Switzerland) and KTI (Swiss Commission for Technology and Innovation). The authors are most grateful for their support.

The authors also wish to express their gratitude to T. H. Teich, W. Caseri and H.-P. Burgener for their fertile and encouraging discussions.

barrier material

Figure 8: Measured breakdown time and contact angle of different barrier materials to epoxy resin.

Conference on Electrical Insulation and Dielectric Phenomena, CEIDP 2002, Cancun, Mexico

949

R. Vogelsang, R. Brütsch, T. Farr, K. Fröhlich: „Electrical tree propagation along barrier-interfaces in epoxy resin“

References [1]

Dissado, L.A., Fothergill, G.C.: “Electrical degradation and breakdown in polymers”, Peter Peregrinus Ltd., London, UK, 1992, ISBN: 0 86341 196 7

[2]

Champion, J.V., Dodd, D.J.: „Systematic and reproducible partial discharge patterns during electrical tree growth in an epoxy resin“, Journal of Appl. Phys., 29, 1996, pp. 862–868

[3]

Dissado, L.A., Dodd, S.J., Champion, J.V., Williams, P.I., Alison, J.M.: „Propagation of electrical tree structures in solid polymeric insulation”, IEEE Trans. on Dielectrics and Electrical Insulation, Vol. 4, No. 3, June 1997, pp. 259–79

[4]

Varlow, B.R., Auckland, D.W.: “The Influence of Mechanical Factors on Electrical Treeing”, IEEE Trans. on Dielectrics and Electrical Insulation, Vol. 5, No. 5, October 1998

[5]

Cooper, J.M.: “The effect of barriers on electrical treeing” in IEE International Conference on Dielectric Materials and Measurements and Applications, Conference Proceedings, Canterbury, 1986, pp. 238-241

[6]

Sweeney, P.J.J., Dissado, L.A., Cooper, J.M.: “Simulation of the effect of barriers upon electrical tree propagation”, J. Phys. D: Appl. Phys. 25, 1992 pp. 113-119

[7]

Farr, T.; Vogelsang, R.; Fröhlich, K.: „A new deterministic model for tree growth in polymers with barriers“, Conference on Electrical Insulation and Dielectric Phenomena, CEIDP 2001, Toronto, Canada, pp. 673-676

[8]

Auckland, D.W.; Kabir, S.M.F.; Varlow, B.R.: „Effect of barriers on the growth of trees in solid insulation“, IEE Proceedings-A, Vol.139, No. 1, January 1992, pp. 14 – 20

[9]

Habenicht, G.: „Kleben“, Ch. 6, Springer, Berlin, 1990, ISBN: 3-540-51878-9

[10] Vogelsang, R.; Fruth, B.; Farr, T.; Fröhlich, K.: „Detection of electrical tree propagation by partial discharge measurements“, to be published at the 15th International Conference on Electrical Machines, ICEM 2002, August 2002, Brügge, Belgium

Authors’ addresses: Ruben Vogelsang, High Voltage Laboratory of the Swiss Federal Institute of Technology, Physikstrasse 3, ETH Centre, 8092 Zürich, Switzerland, Email: vogelsang@eeh.ee.ethz.ch Rudolf Brütsch, von Roll Isola AG Switzerland, Passwangstrasse 20, 4226 Breitenbach, Switzerland Email: rudolf.bruetsch@vonroll-isola.com

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