Kerr Electro-optic Field Mapping Measurements Research Articles

Space charge injection behaviors and dielectric characteristics of nano-modified transformer oil using different surface condition electrodes

Electrodes are damaged by external forces, mechanical wear, and electrochemical corrosion during long-term operation in power systems, resulting in rough electrode surfaces. Space charge injected into the liquid by a rough electrode under a strong electric field causes electric field distortion and leads to breakdown. Moreover, the charge injection is associated with the energy barrier at the electrode-liquid interface. The combined effect mechanism of nanoparticles and electrode surface condition on the insulating characteristics of transformer oil was investigated using various surface condition aluminum plate electrodes. The breakdown voltages and space charge patterns in pure transformer oil and Al2O3 nano-modified transformer oil under impulse voltage were measured in this study through Kerr electro-optic field mapping measurement. The test results indicate that the breakdown voltage of nano-modified transformer oil depends on voltage polarity and electrode surface conditions and the charge injection shows the same mode in pure transformer oil using different surface condition electrodes, but the magnitude of charge injection differs considerably. Due to nanoparticle’s ability of shielding space charges of different polarities and the charge injection of different surface condition electrodes, they have different effects on the breakdown process of transformer oil between different surface condition electrodes. An interfacial model was formed to explore the mechanism of the effect of nanoparticles and electrode surface conditions on the insulation performance of liquid dielectrics and analyze the breakdown process using the space charge injection behaviors.

Asymmetric injection and distribution of space charges in propylene carbonate under impulse voltage

Space charge can distort the electric field in high voltage stressed liquid dielectrics and lead to breakdown. Observing the evolution of space charge in real time and determining the influencing factors are of considerable significance. The spatio-temporal evolution of space charge in propylene carbonate, which is very complex under impulse voltage, was measured in this study through the time-continuous Kerr electro-optic field mapping measurement. We found that the injection charge from a brass electrode displayed an asymmetric effect; that is, the negative charge injection near the cathode lags behind the positive charge injection near the anode. Physical mechanisms, including charge generation and drift, are analyzed, and a voltage-dependent saturated drift rectification model was established to explain the interesting phenomena. Mutual validation of models and our measurement data indicated that a barrier layer, which is similar to metal-semiconductor contact, was formed in the contact interface between the electrode and propylene carbonate and played an important role in the space charge injection.

Effect of Fe3O4 nanoparticles on space charge distribution in propylene carbonate under impulse voltage

Addition of nanoparticles of the ferromagnetic material Fe3O4 can increase the positive impulse breakdown voltage of propylene carbonate by 11.65%. To further investigate the effect of ferromagnetic nanoparticles on the space charge distribution in the discharge process, the present work set up a Kerr electro-optic field mapping measurement system using an array photodetector to carry out time-continuous measurement of the electric field and space charge distribution in propylene carbonate before and after modification. Test results show that fast electrons can be captured by Fe3O4 nanoparticles and converted into relatively slow, negatively charged particles, inhibiting the generation and transportation of the space charge, especially the negative space charge.

Time-continuous Kerr electro-optic field mapping measurement under impulse voltage using array photodetector

Space charge affects the electric field distribution in high voltage-stressed liquid dielectrics and can cause insulation degradation. This letter reports on research using a Kerr electro-optic measurement system with an array photodetector for Kerr electro-optic field mapping measurements in propylene carbonate under application of switching impulse voltages to investigate the electric field and space charge effects in high voltage-stressed liquid dielectrics. The electric field distribution between parallel aluminum electrodes is measured, and the results are compared with those measured using a charge-coupled device measurement system. The array photodetector measurement system has the advantages of higher sensitivity and higher time resolution because simultaneous optical Kerr measurements can be taken at multiple positions using a multi-element array photodetector. The results show that bipolar homo-charge injection occurs between the parallel aluminum electrodes. In the near future, this measurement system will be used in continuing research into the measurement of space charge effects using low Kerr constant liquids such as transformer oil.

Kerr electro-optic field mapping study of the effect of charge injection on the impulse breakdown strength of transformer oil

The smart use of charge injection to improve breakdown strength in transformer oil is demonstrated in this paper. Hypothetically, bipolar homo-charge injection with reduced electric field at both electrodes may allow higher voltage operation without insulation failure, since electrical breakdown usually initiates at the electrode-dielectric interfaces. To find experimental evidence, the applicability and limitation of the hypothesis is first analyzed. Impulse breakdown tests and Kerr electro-optic field mapping measurements are then conducted with different combinations of parallel-plate aluminum and brass electrodes stressed by millisecond duration impulse. It is found that the breakdown voltage of brass anode and aluminum cathode is ∼50% higher than that of aluminum anode and brass cathode. This can be explained by charge injection patterns from Kerr measurements under a lower voltage, where aluminum and brass electrodes inject negative and positive charges, respectively. This work provides a feasible approach to investigating the effect of electrode material on breakdown strength.

Evaluating the reliability and sensitivity of the Kerr electro-optic field mapping measurements with high-voltage pulsed transformer oil

Transformer oil is the most widely used dielectric liquid for high voltage insulation. Measurements of the electric field distribution in high voltage pulsed transformer oil are of both practical and theoretical interests. Due to its low Kerr constant, previous electro-optic measurements with transformer oil rely on a technique called ac modulation, which works only for dc steady-state electric field mapping. With the help of a high-sensitivity charge-coupled device, we directly measure the Kerr electro-optic effect between parallel electrodes in transformer oil stressed by high voltage pulses. This work aims at demonstrating the reliability and evaluating the sensitivity of the measurements for three cases with identical electrodes: space-charge free, uniform electric field in the mid-region of the gap; space-charge free, non-uniform fringing electric field; and space charge distorted electric field in the mid-region of the gap. Future directions to improve accuracy by identifying and handling various sources of error and noise are suggested.

Preliminary kerr electro-optic field mapping measurements in propylene carbonate using point-plane electrodes

Propylene carbonate is used in Kerr electro-optic field mapping measurements because of its non-toxic chemical nature and very high Kerr constant. We describe the experimental design and chemical purification procedure that minimizes reactions with the electrodes and removes particles and moisture. We measure the optical characteristic parameters using the ac modulation method with point-plane electrodes that allows calculation of the electric field magnitude and direction at any point even though the electric field direction varies along the light path. The “onion peeling” method of radial discretization of concentric circles centered on the axisymmetric axis of the point electrode at various heights above the ground plane is used to recover from light intensity measurements the radial and axial components of the electric field which are compared to the theoretical space charge free solutions.

Computation methods for one-dimensional bipolar charge injection

Kerr electrooptic field-mapping measurements have shown strong space charge effects in high-voltage stressed dielectrics, where the magnitude and sign of injected charge depends on electrode material. With appropriate choice of electrode material combinations and voltage polarity is possible to have uncharged, unipolar positively or negatively charged, or bipolar charged dielectrics. The bipolar homocharge case lowers the electric field at both electrodes due to space-charge shielding, and thus allows a higher voltage without breakdown. Using charge-transport analysis the authors consider the general case of charge injection from both electrodes and solve time and space dependences of the electric field and charge distributions and the time dependences of the terminal voltage and current. This analysis extends earlier work by including nonequilibrium charge dissociation and recombination so that the effective ohmic conductivity is not constant but varies with time and position due to local net charge density. Specific cases discussed are the open-circuit voltage decay of an initially charged capacitor, the charging and discharging by a Marx generator, and alternating voltage excitations.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Kerr electro-optic field mapping measurements in electron-beam irradiated polymethylmethacrylate

Kerr electro-optic field mapping measurements in electron-beam irradiated polymethylmethacrylate

Kerr Electro-Optic Field Mapping Measurements in Electron Beam Irradiated Polymethylmethacrylate

Kerr electro-optic field mapping measurements are presented in electron beam irradiated polymethylmethacrylate (PMMA) where the accumulated trapped charge results in large self-electric fields of the order of 1 to 2.5 MV/ cm. The resulting numerous light maximum and minimum recorded on photographic film and videotape allow accurate measurement of the time dependence of the electric field and space charge distributions.

Dielectric properties of water and water/ethylene glycol mixtures for use in pulsed power system design

One class of modern pulse power generators use deionized water as an energy storage, switching and transmission dielectric. Water is chosen for its high dielectric constant and relatively high resistivity, which allows reasonably sized and efficient low-impedance high-voltage pulse lines where pulse durations are less than 100 µs. Water/ethylene glycol mixtures are being researched, so that rotating machinery, rather than the usual Marx generator, can be used as the primary energy store. The high resistivity and high dielectric constant of these mixtures at low temperature permit low-loss operation on millisecond time scales. Simple design criteria linking load parameters and charging circuit characteristics to the liquid dielectric are developed which show that the dielectric constant, breakdown strength, and relaxation time are the primary properties of interest to the pulse power engineer. On time scales greater than 100 µs, injection of space charge, with density q and mobility µ, affects the charging and discharging circuit characteristics, introduces the time constant of the time of flight for injected charge to migrate between electrodes, and increases the effective ohmic conductivity σ to σ + qµ. A drift-dominated conduction model is used to describe measured space-charge effects. Kerr electrooptic field mapping measurements show strong space-charge effects with significant distortions in the electric field distribution a few hundred microseconds after high voltage is applied. The injected charge magnitude and sign depends on the electrode material. Thus by appropriate choice of electrode material combinations and voltage polarity, it is possible to have uncharged liquid, unipolar-charged negative or positive, or bipolar-charged liquid. An important case is that of bipolar injection, which has allowed up to a 40 percent higher applied voltage without breakdown than with no charge injection, and thus a doubling of stored energy due to the space-charge shielding which lowers the electric field strengths at both electrodes. Although injected space charge increases the stored electric energy over the capacitive space-charge-free energy, (1/2)CV <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , more energy is required from a source during charging and the energy delivered to a resistive load is reduced because of internal dissipation in the capacitor as the charge is conducted to the electrodes. However, it appears that this extra dissipation due to injected charge can be made negligibly small and well worth the price if the space charge allows higher voltage operation for long charging time or repetitively operated machines.

Kerr electro-optic field mapping measurements in water using parallel cylindrical electrodes

An extensive set of Kerr electro-optic field mapping measurements are reported for highly purified water over the temperature range of 8.8–29.5 °C. The Kerr constant is independent of temperature over this range and equals B≂3.4×10−14 m/V2. Pulsed high voltages up to 140 kV across parallel cylindrical electrodes with a 1-cm gap are applied on millisecond time scales. For early times, the measured results agree with the space-charge-free electric field distribution while for times greater than 500 μsec, there is significant space-charge distortion due to positive charge injection.

High voltage electric field and space-charge distributions in highly purified water

High voltage Kerr electro-optic field mapping measurements in highly purified water show significant field distortions due to injected positive space charge over the temperature range of 8–25 °C using parallel plane stainless steel electrodes stressed to average field strengths up to 150 kV/cm. The net space charge was always positive with density of ≂2 C/m3 which is about 40% of the background value of charge density of thermally generated hydronium and hydroxyl ions. Because measurements only showed a net positive space charge, a unipolar drift dominated conduction model was developed where positive charge would migrate in an ohmic medium. This model predicts the observed propagating charge front, electric field enhancement at the noncharge injecting electrode, and field decrease at the charge injecting electrode. The time of flight of positive charge between electrodes gave an estimate of the positive ion mobility of μ≂4×10−7 m2/(V s) which is inbetween the values of the hydronium ion mobility measured at low voltages and the electrohydrodynamic mobility which accounts for fluid convection effects due to the Coulombic force putting the fluid into motion. The dielectric relaxation time at T≂8 °C (τ≂320 μsec) measured at low voltage was much less than the measured charge transport time between electrodes, τmig ≂2 msec, so that the calculated effects of space charge were much less than measured because the charge relaxes quickly as it migrates. It was found that a dielectric relaxation time of τ≂1.6 msec would provide a good fit between analysis and experiments.

Charge injection and transport in high voltage water/glycol capacitors

A water/ethylene glycol mixture to be operated at temperatures below 0 °C is being investigated for use as the high permittivity, low loss dielectric in pulse forming lines for pulsed power applications. High voltage open circuit decay curves show a dielectric relaxation time that decreases with initial voltage at room temperature, while at −10 °C, the open circuit decay has a negative second derivative with time in contrast to the expected exponential decay which always has a positive second derivative. It is hypothesized that this anomalous behavior is due to space-charge injection which increases the ohmic conductivity σ to σ+q μ, where q is the time and space dependent net charge density with mobility μ. A drift dominated conduction model is used to solve for the electric field and charge density distributions and the terminal voltage-current behavior of a lossy capacitor where one electrode injects charge proportional to the instantaneous local electric field. The specific case studied is of a parallel plate capacitor rapidly charged to an initial voltage and quickly open circuited. The calculated open circuit voltage decay curves show good agreement with measurements and yield experimental values for the charge injection parameter and the high field mobility of the injected charge. The value of mobility for the best fit of experimental data to the theory agrees with the value for the hydronium ion. Preliminary Kerr electro-optic field mapping measurements in highly purified water without glycol, confirm positive charge injection.