Project Title:

Optical Tomography Measurements of High Voltage Conduction and Breakdown Phenomena in Dielectrics

Principal Investigator:

Markus Zahn

Research Staff:

Sponsor:

Program Areas:

Continuum Electromechanics
High Voltage

Duration:

Abstract:

The research continues analytical, computational, and experimental study using optical tomography measurements of high voltage insulation, conduction, prebreakdown and breakdown characteristics in dielectrics. The methodology uses electric field induced birefringence (Kerr effect) with an improved sensitive optical measurement system and a new advanced mathematical formulation that allows calculation of electric field magnitude and direction in any electrode geometry from optical intensity measurements. Because the physics of high voltage charge injection and transport, prebreakdown and electrical breakdown are not known for most metal/dielectric systems, the electric field distribution cannot be calculated from knowledge of system geometries alone. Optical measurements provide a direct approach to determining electrical constitutive laws and learning the physics of the electrical breakdown process and so offers a research methodology for major advances in increasing the breakdown strength of dielectric systems.

In the past, Kerr effect measurements have been limited to one or two dimensional parallel plate, coaxial cylinder, or parallel cylinder electrode geometries where the electric field direction does not change along the light path. However, for most controlled electric breakdown tests, a "point" electrode is usually used to create high electric fields at modest voltages and to localize the region of the resulting spark discharge. Kerr effect measurements could not be used with such geometries because the electric field direction varies along the light path and there was no physical and mathematical model that would relate a measured optical signal to the magnitude and direction of the applied electric field distribution.

This limitation has been removed by new results  whereby a rigorous mathematical formulation was derived for Kerr effect measurements so that the electric field magnitude and direction everywhere can be calculated from optical measurements. The theory has been verified with measurements using point/plane electrodes with low Kerr constant transformer oil and high Kerr constant propylene carbonate. Using matricant analysis with an "onion-peeling" method of radial discretization of thin circular layers the spatially varying electric field magnitude and direction for the first time was calculated from Kerr measurements of light intensity.

To improve measurement sensitivity for weakly birefringent dielectrics an AC voltage superimposed on a high voltage DC level is often used. The non-linear optical response results in an optical signal which has a DC level, a component at the fundamental frequency of the AC voltage as well as a double frequency harmonic. By utilizing a frequency sensitive lock-in amplifier tuned to the double frequency harmonic it is possible to measure the Laplacian space charge free field. Measuring the fundamental frequency of the optical signal gives the DC electric field including the effects of space charge.

The research program has involved both graduate and undergraduate students to try to further understand charge injection, conduction, aging, degradation, prebreakdown and breakdown mechanisms in high field stressed gaseous, liquid, and solid dielectrics using new modern optical, electronic, and computer instrumentation to give "eyes" inside materials to see prebreakdown and breakdown field and charge distributions that could not be measured before.

Specific work tasks are: 1) Extend and complete the mathematical formulation of the Kerr effect where the applied electric field magnitude and direction change along the light path for any three dimensional geometry; develop computational algorithms to convert measured optical signals to electric field distributions; and to verify analysis with experiments; 2) Develop a computer interfaced camera system with an optical array detector distributed over an area to automate sensitive Kerr measurement data acquisition and processing without mechanical motion; 3) Perform sensitive Kerr electro-optic field and charge mapping measurements in a well controlled and monitored test cell for various liquid/solid material combinations in the volume and within the electrical double layer near interfaces as a function of DC voltage amplitude and polarity, AC voltage amplitude and frequency, direction of interface with respect to applied electric field, temperature, moisture, conductivity, and concentration of moisture and trace additives; 4) Extend the sensitive Kerr effect technique and perform measurements for weakly birefringent dielectrics in pulsed, ramped, and other time varying applied electric fields; 5) Relate the results of the modeling, optical measurements, and dielectrometry measurements to a better understanding of high voltage conduction, prebreakdown and breakdown phenomena in dielectrics in order to reliably extend the operation of high voltage apparatus to higher voltages; and 6) Construct a compact optical sensor measurement system that will be applied to electric field and space charge measurements in an operating transformer.

References and Links:

Publications:

1. Zahn, M., "Transform Relationship between Kerr-effect Optical Phase Shift and Nonuniform Electrical Field Distributions," IEEE Transactions on Dielectrics and Electrical Insulation, Vol. 1, No. 2, pp. 235-246, April, 1994.
2. Üstündag, A., T.J. Gung, and M. Zahn, "Kerr Electro-Optic Theory and Measurements of Electric Fields with Magnitude and Direction Varying Along the Light Path," IEEE Transactions on Dielectrics and Electrical Insulation, Vol. 5, No. 3, pp. 421-442, June 1998.
3. Gung, T.J., A. Üstündag, and M. Zahn, "Preliminary Kerr Electro-Optic Field Mapping Measurements in Propylene Carbonate Using Point-Plane Electrodes," accepted for publication in the Journal of Electrostatics, 1999. 
4. Zahn, M., "Kerr Effect Measurements with Non-Uniform Electric Field Distributions Whose Direction Changes Along the Light Path," Conference Record of the 1994 IEEE International Symposium on Electrical Insulation, June 5-8, 1994, Pittsburgh, PA, pp. 137-140. 
5. Zahn, M. and R. Hanaoka, "Kerr Electro-Optic Measurements of Transformer Oil In a Point-Plane Geometry," Proceedings of the 4th International Conference on Properties and Applications of Dielectric Materials, July 3-8, 1994, Brisbane, Australia, pp. 697-700.
6. Zahn, M. and R. Hanaoka, "Kerr Electro-Optic Field Mapping Measurements Using Point-Plane Electrodes," Proceedings of the 2nd International Conference on Space Charge in Solid Dielectrics, April 2-7, 1995, Antibes-Juan-Les-Pins, France, pp. 360-372; also Supplement à la Revue "Le Vide": science, technique et applications," No. 275, January-March, 1995.
7. Üstündag, A. and M. Zahn, "Optical Tomography of Kerr Electro-Optic Measurements with Axisymmetric Electric Field," 1996 IEEE International Symposium on Electrical Insulation, June 16-19, 1996, Montreal, Quebec, Canada, pp. 462-465.
8. Üstündag, A., T.J. Gung, and M. Zahn, "Kerr Electro-Optic Measurement Technique Determination of Nonuniform Electric Fields," 12th International Conference on Conduction and Breakdown in Dielectric Liquids, July 15-19, 1996, Rome, Italy, pp. 457-460.
9. Gung, T.J. A. Üstündag, and M. Zahn, "Kerr Electro-Optic Measurements of Nonuniform Axisymmetric Electric Field Distributions Using Point-Plane Electrodes," 1996 Annual Report for the Conference on Electrical Insulation and Dielectric Phenomena, pp. 61-63, 73.
10. Gung, T.J., A. Üstündag, and M. Zahn, "Kerr Electro-optic Measurements of Non-uniform Axisymmetric Electric Field Distributions Using Point-Plane Electrodes," 1997 Annual Report of the Conference on Electrical Insulation and Dielectric Phenomena, Minneapolis, MN, Oct. 19-22, 1997, pp. 222-225.
11. Gung, T.J., A. Üstündag, and M. Zahn, "Kerr Electro-optic Measurement Tomography of Nonuniform Axisymmetric Electric Field Distributions Using Point-Plane Electrodes," 1997 Conference on Electrical Insulation and Dielectric Phenomena, pp. 222-225, Oct. 19-22, 1997, pp. 222-225.
12. Gung, T.J., A. Üstündag, and M. Zahn, "Preliminary Kerr Electro-optic Field Mapping Measurements in Propylene Carbonate Using Point-Plane Electrodes," Electrostatics Society of America-Institute of Electrostatics Japan Joint Symposium on Electrostatics, June 23-26, 1998, pp. 95-106.
13. Üstündag, A., T.J. Gung, and M. Zahn, "A New Reconstruction Algorithm for Kerr Electro-optic Measurement of Space Charge in Arbitrary Geometries," 1998 Conference on Electrical Insulation and Dielectric Phenomena, Oct. 26-28, 1998, Atlanta, GA, pp. 364-367.
14. Zahn, M. "Optical, Electrical, and Electro-Mechanical Measurement Methodologies of Electric Field, Charge, and Polarization in Dielectrics," 1998 Conference on Electrical Insulation and Dielectric Phenomena, Oct. 26-28, 1998, Atlanta, GA, pp. 1-14.
15. Üstündag, A., "Kerr Electro-Optical Tomography for Determination of Nonuniform Electric Field Distributions in Dielectrics," MIT Ph.D. thesis, May, 1999.
16. Gung, T.J., "Kerr Electro-Optic Measurements of Nonuniform Electric Field Distributions in Dielectric Liquids," MIT Ph.D. thesis, May, 1999./li>
17. Zahn, M. "Optical, Electrical and Electromechanical Measurement Methodologies of Field, Charge and Polarization in Dielectrics," IEEE Transactions on Dielectrics and Electrical Insulation, Vol. 5, No. 5, pp. 627-650, October 1998.

This web page is maintained by Brett Klein. Email questions/comments to him at bklein@mit.edu.