Self-healing high voltage electrical insulation materials

by Anujith.T roll no. 12 S7-F

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CONTENT❖ INTRODUCTION❖ SELF-HEALING TYPES❖ ELECTRICAL TREE❖ HOW IT WORKS❖ MICROCAPSULES❖ TESTING AND RESULTS❖ ADVANTAGE AND DISADVANTAGE❖ CONCLUSION❖ REFERENCES

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INTRODUCTION● Self-healing materials have the structurally incorporated

ability to repair damage

● Micro cracking can lead to catastrophic failure of the composites and shorten the service lifetime.

● To self-heal as a direct response to electrical degradation is very attractive, especially in challenging environments.

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WHY SELF-HEALING?★ Traditional repairs are expensive

★ Sense and respond to damage,restore performance without affecting inherent properties

★ No human intervention required

★ Provide early means of detection

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TYPES OF SELF HEALING PROCESS

❖Microcapsule embedment

❖3D-microvascular embedment

❖Electro-hydranamics

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ELECTRICAL TREE

In electrical engineering, treeing is an electrical pre-breakdown phenomenon in solid insulation.

fig 1.electrical tree

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CONTINUED....❖ Damaging process due to partial discharges and progresses

through the stressed dielectric insulation

❖ It is a common breakdown mechanism and source of electrical faults in underground power cables.

❖ Initiated at regions with high local electrical fields, contaminations in the insulation or conducting irregularities/protrusions or voids

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CONTINUED….Local partial discharges will cause chemical degradation and disintegration of the polymer, thus further extending the tree channels until final electrical breakdown occurs.

fig 2. microscopic image of an tree(500nm)

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OCCURENCE❖ Occur at the interface between insulation and conductor or

within the insulation system.

❖ Bulk or surface defects create excessive electrical stress that initiates dielectric breakdown in a small region.

❖ Occurs through as additional small electrical breakdown events (called partial discharges).

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TREE GROWTH❏ The insulation system will never be perfect

❏ Cumulative long term degradation of the insulation may cause inception of electrical trees

❏ A tree or a bush grows depending on the electrical field strength, frequency and voltage waveform

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TYPES OF ELECTRICAL TREEBow-tie trees

● trees which start to grow from within the dielectric insulation and grow symmetrically outwards

toward the electrodes.

● no free supply of air which will enable continuous support of partial discharges.

● discontinuous growth, which is why the bow-tie trees usually do not grow long enough to fully

bridge the entire insulation between the electrodes, therefore causing no failure in the insulation.

Vented trees

● initiate at an electrode insulation interface and grow towards the opposite electrode.

● Having access to free air is a very important factor for the growth of the vented trees.

● grow continuously until they are long enough to bridge the electrodes, therefore causing failure in the

insulation

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HOW IT WORKS

➢ Microcapsules filled with a monomer (healing agent) are added to the insulation materials (epoxy) prior to casting.

➢ When cracks propagate in the material the microcapsules will rupture, releasing liquid healing agent into the crack.

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CONTINUED...

❏ One or more of the branches of the electrical tree will likely break a capsule filling the electrical tree with the liquid monomer.

❏ The final step is the polymerization of the monomer occurs upon contact with a catalyst also added to the epoxy resin.

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WORKING...

fig3.a. healing of an electrical tree explained b.

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CONTINUED..

● As the tree structure is interconnected, most of the tree structure is likely to be filled.

● Depends on the partial pressure and viscosity of the monomer and the surface tension of the hollow tubes

● Also on the availability of monomer relative to the dimensions of the hollow tubes

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CONTINUED● The filling itself should extinguish critical discharges,

making further growth less likely.

● Upon polymerization, further development of the electrical tree should halt, or at least be significantly delayed

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MICROCAPSULESConstituents which combine to form the MICROCAPSULES

✓ Healing Agent- DICYCLOPENTADIENE(DTP)

✓ Microcapsule Shell- UREA-FORMALDEHYDE(UF)

✓ Chemical Catalyst -BIS(TRICYCLOHEXYLPHOSPHINE)

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CONTINUED..POLYMER MATRIX- EPOXY POLYMERFIBER REINFORCEMENT - CARBON FIBER

fig 4.microscopic image of ruptured microcapsule

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CAPSULES

fig 5. microscopic image of capsule

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TESTING TESTING SETUP FOR A EPOXY POLYMER1-optical microscope connect-ed to computer2-sample specimen3-voltage source4- variac5-current sensing resistor

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RESULTAn optical micrograph taken at 200X magnification reflecting a range of characteristics for the interaction of electrical trees and microcapsules as embedded in the epoxy matrix is shown 1. 2.

fig4.Optical micrograph obtained in a sample containing 1.no catalyst 2.catalyst21

RESULT AND DISCUSSION● the predominant trend for the present dataset is for the

electrical trees to be attracted by the microcapsules

● The electrical properties of the capsule material (complex permittivity and conductivity) have not been measured as part of this study, but will be so in the future.

● electrical trees were repeatedly observed to enter microcapsules and subsequently seemed to stop growing.

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ADVANTAGES● anything that cannot be reached at the moment of

damage can be repaired

● less cost in maintenance

● lifetime of the insulation material is increased

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DISADVANTAGES

❖ this method self healing is very expensive

❖ only 70-80% strength was obtained after self-healing process

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CONCLUSION

❖ Electrical trees formed by faults/contaminants inside the insulation material can be stopped/delayed by the introduction of microcapsules containing a healing agent.

❖ They may be healed before growing to sizes that could sustain partial discharges and thus the inception of electrical trees.

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REFERENCES[1] B. J. Blaiszik, S. L. B: Kramer, S. C. Olugebefola, J. S. Moore, N. R. Sottos, S. R. White, "Self-Healing Polymers and Composites," in Annual Review of Materials Research, Vol 40. vol. 40, D. R. Clarke, et al., Eds., ed Palo Alto: Annual Reviews, 2010, pp. 179-211. [2] S. R. White, N. R. Sottos, P. H. Geubelle, J. S. Moore, M. R. Kessler, S. R. Sriram, E. N. Brown, S. Viswanathan, "Autonomic healing of polymer composites," Nature, vol. 409, pp. 794-797, Feb 2001. [3] S. Van der Zwaag, Self Healing Materials: an Alternative Approach to 20 centuries of Materials Science. Dordrecht: Springer, 2007. [4] L. A. Dissado and J. C. Fothergill, Electrical Degradation and Breakdown in Polymers. London, United Kingdom: The Institution of Engineering and Technology, 1992. [5] R. Schurch, "Techniques for Electrical Tree Imaging," IEEE International Conference on Imaging Systems and Techniques (IST), pp. 409-414, 2012 [6] J. Holton and E. Ildstad, "Electrical tree growth in extruded polypropylene," presented at the 2010 International Conference on Solid Dielectrics, Potsdam, Germany, 2010. 244

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