Gas Insulated Power Apparatus Research Articles

Green gas for grid as an eco-friendly alternative insulation gas to SF6: From the perspective of PD initiated by metallic particles under DC

Abstract This paper dealt with characteristics of partial discharge (PD) initiated by metallic particles under DC voltage in green gas for grid (g3), which is an emerging and promising eco-friendly alternative insulation gas to SF6. Experimental setup was configured to simulate PD under DC in gas-insulated power facilities. Two types of particle, namely rectangle particle and sphere particle were used. The results indicated that the discharge inception voltages in g3 gas were 90.1-92.5% of that in SF6. In two particles, PD occurred with higher average apparent charge and discharge repetition rate in g3 compared with those in SF 6 gas. The time-resolved partial discharge presented similar characteristics in g3 and SF6 gas. The time interval between two successive discharge pulses were 0.1 ms-10 ms and 5 ms-15 ms in the rectangle particle and sphere particle, respectively. Results from this paper are expected to provide fundamental material for the development of SF 6 -free gas-insulated power apparatus.

Green Gas for a Grid as An Eco-Friendly Alternative Insulation Gas to SF6: From the Perspective of Partial Discharge Under AC

This paper deals with the characteristics of partial discharge (PD) in green gas for a grid (g3), which is thought to be a promising eco-friendly alternative to substitute SF6 used in electrical power facilities and to reduce the greenhouse effect. g3 gas with 4% NOVECTM 4710/96% CO2 was used and electrode systems including a protrusion on conductor (POC) and a protrusion on enclosure (POE) were fabricated to simulate PD in a gas-insulated structure. PD characteristics in terms of partial discharge inception voltage (PDIV), pulse parameters in time and frequency, and the phase-resolved partial discharge (PRPD) pattern in SF6 and g3 were compared. From the results, the PDIVs of g3 were 76%–81% of that of SF6 in the POC and were 78%–84% of that of SF6 in the POE, depending on the gas pressure. Rising time, pulse width, and relative amplitude in the frequency domain of PD pulses in g3 gas were greater than those in SF6. In addition, the PRPD patterns indicated that both the average apparent charge and pulse count of PD in g3 were higher compared with those in SF6. The results from this paper are expected to provide fundamental material for the green manufacturing of gas-insulated power apparatus.

Surface FGM Insulator Based on BaTiO3 Magnetron Sputtering for Electric Field Grading of AC Gas Insulated Power Apparatus

Electric field optimization is a critical technique to ensure the compact structure and reliable operation for AC gas insulated power apparatus. To obtain a more uniform electric field distribution, this paper proposes the concept of the d-SFGM insulator, i.e., applying the surface functionally graded material (SFGM) with a designed thickness (d) distribution of high permittivity along the insulator surface. The magnetron sputtering technique was used to fabricate the d-uniform insulator and the d-SFGM insulator by depositing BaTiO 3 layers. The theoretical analysis indicates that the electric field distribution is optimized using the d-SFGM insulator, followed by the d-uniform insulator. The finite element simulations were employed to investigate the electric field distributions and dielectric losses of different insulators. The results show that for the case of the d-uniform insulator, a higher permittivity or larger thickness of the sputtering layer results in a more uniform electric field distribution. With the application of the d-SFGM insulator, the maximum electric field strength and dielectric loss are both significantly reduced compared to the d-uniform insulator. Finally, experimental tests were conducted to further verify the feasibility of SFGM. The AC flashover voltage of the d-SFGM insulator is much higher than those of the conventional insulator and the d-uniform insulators. Therefore, the d-SFGM insulator has great potential for improving the reliability of the AC gas insulated power apparatus.

Filling ratio control of TiO2 and SiO2 in epoxy composites for permittivity-graded insulator with low coefficient of thermal expansion

The feasibility of a permittivity-graded epoxy insulator with a low coefficient of thermal expansion (CTE) was clarified by evaluating the dielectric and thermomechanical properties of TiO2/SiO2 epoxy composites, which were epoxy composites co-filled with TiO2 and SiO2 particles. Upon varying the filling ratio of TiO2 to SiO2 while keeping the total filler volume constant, the relative permittivity of the TiO2/SiO2 epoxy composites varied in the range of 3.5–7.5 in our experiment while the CTE remained similar to that of an aluminum conductor used in gas-insulated power apparatus. The range of the relative permittivity of the TiO2/SiO2 epoxy composites with a low CTE satisfied the condition for realizing a permittivity-graded epoxy insulator. The application of an appropriate gradient of the permittivity distribution to a permittivity-graded epoxy insulator with a low CTE resulted in the relaxation of the electric field on the surface of an insulator.

Partial Discharge Measurement under an Oscillating Switching Impulse: A Potential Supplement to the Conventional Insulation Examination in the Field

Partial discharge (PD) detection under oscillating switching impulse (OSI) voltage was performed on three types of insulation defects, including a protrusion on a conductor, a particle on an insulator surface, and a void in an insulator, which are three kinds of the common potential insulation hazards in gas insulated power apparatus. Experiment indicated that the PD sequences under OSI were composed of various combinations of the single pulse, the multiple pulses, and the reverse polarity pulse. The difference between the PD inception voltage (PDIV) and the breakdown voltage (BDV) under OSI voltage was greater than that under alternating current (AC) voltage in some cases, which can provide a more sufficient margin below the BDV for PD diagnosis. The OSI voltage also showed a better performance for exciting PDs with detectable magnitudes from small-scale defects, of which the AC voltage was incapable under our test conditions. The different PD activities with different interfaces under an impulse and a slowly varying voltage were speculated to be associated with the gradient of the background electric field and the space-charge mobility.

Fabrication and simulation of permittivity graded materials for electric field grading of gas insulated power apparatus

An innovation for the higher performance and reliability of electric power apparatus can be brought about by a novel technique on electrical insulating materials. The application of functionally graded materials (FGM) with spatial distribution of dielectric permittivity in solid insulator is an effective technique to contribute to the above innovation, because FGM can control the electric field distribution with a simple shape. In this paper, we investigated the fabrication and simulation techniques of FGM with various spatial distributions of dielectric permittivity using centrifugal force. Firstly, we developed a simulation technique to estimate the dielectric permittivity distribution of FGM by considering the particle movement in a viscous fluid under centrifugal force. Secondly, based on the simulation technique, we fabricated FGM samples with various relative permittivity distribution and confirmed the validity of the simulation technique. Consequently, the fabrication results provided a good agreement with the simulation results.

Finite Element Analysis of Thermal Problems in Gas-Insulated Power Apparatus With Multiple Species Transport Technique

Thermal analysis is paramount in the design process of gas-insulated power apparatus. In this paper, a multiple species transport technique is proposed to deal with the heat transfer problem of gas-insulated power apparatus, in which the ambient air as well as SF6 gas is included in the solution region. The FEM is employed to investigate the coupled eddy current, fluid, and thermal fields. Temperature-dependent electric and thermal properties of the materials are considered. The power losses calculated from electromagnetic analysis are used as heat input for fluid and thermal analysis, in which the need for convective boundary condition is eliminated. The proposed method is applied to the thermal analysis of both single- and three-phase bus bars and is validated by comparison with the analytical method and experimental results reported in the literature.

Breakdown mechanism in C<sub>3<.sub>F<sub>8</sub>/CO<sub>2</sub> gas mixture under non-uniform field on the basis of partial discharge properties

This paper investigates simultaneous measurements of partial discharge (PD) current and the corresponding light emission phenomena of C3F8/CO2 gas mixture under a non-uniform field with AC high voltage application in order to discuss the PD properties and breakdown mechanism of new gaseous dielectrics as SF6 substitutes. Perfluoro carbon (PFC) gas mixture is a possible candidate as SF6 substitute because it almost fulfills the environmental aspects, dielectric properties, boiling point, and other requirements of SF6 substitute. The authors simultaneously measured PD phenomena such as time-sequential PD current pulses and corresponding light emission images before breakdown during the positive half cycle of the applied AC voltage where most breakdowns occurred. Consequently, it was found that there were three kinds of current pulse patterns, which depended on the gas pressure and the applied voltage amplitude. It was also confirmed that breakdown occurred around the voltage peak, i.e. 90 degrees. of the applied voltage, irrespective of the PD pattern. Furthermore, the simultaneous measurements enabled the authors to classify the discharge area in the N-shaped characteristic of the breakdown voltage based on discrimination of discharge types between streamer and leader discharges. Namely, the location where leader discharge is generated by the transition from streamer discharge in the N-shaped characteristic was indicated. These experimental results and the simultaneous measurement technique will help clarify the breakdown mechanism for new gases or gas mixtures as SF6 substitutes. The obtained results will also contribute to insulation diagnosis for next-generation gas-insulated power apparatus with the newly developed gaseous dielectrics.

Developing Automated Gas Recovery System for SF6/N2 Insulation Gas Mixtures

We have already three gas insulated power lines in service applying SF6/N2 gas mixtures, the two in Europe and the rest in South Asia. To cope with the global warming issue on gas insulated power apparatus, we have been investigating the recovery of SF6 from those apparatus. This paper describes an automated gas recovery method that secures SF6 gas concentration in the recovered gas at a high and desired level. The gas mixtures investigated contain 5 - 20 vol. % of SF6 in N2. We adopt a membrane method to separate the two gas species, in which two membrane separators are connected in cascade. There are several factors that affect the concentration of SF6 in the recovered gas. As the concentration greatly depends on recovery gas flow speed, we control the speed by using a piezo-valve. For driving the piezo-valve, we use a voltage regulator together with an infrared gas concentration online monitor. This link allows PID control of the SF6 gas recovery system. Successful operation results of this automated system, as well as the basic recovering characteristics of the cascaded system, are demonstrated after a brief review on gas recovery methods.

Consideration of radiated electromagnetic waves from partial discharge based on half-wave dipole antenna model

Partial discharge (PD) may take place due to residual defects like metallic particles in SF6 gas-insulated power apparatus such as GIS. However, the signal of PD occurring in SF6 gas is very weak and susceptible to external noise in air. Moreover, because of the complicated mechanism of PD, the radiation property of electromagnetic waves from PD has not as yet been clarified. Therefore, it is hard to distinguish the PD signal in SF6 gas from external noise. From the above points of view, we have been investigating the radiation mechanism of electromagnetic waves from PD. We measured the polarization characteristics of electromagnetic waves radiated from PD in comparison with those of half-wave dipole antennas. The polarization characteristics of PD were explained by the theory of half-wave dipole antenna, rather than that of an infinitesimal dipole antenna. Moreover, we compared the power spectrum for PD measured using a biconical antenna with that received from the half-wave dipole antenna or infinitesimal dipole. It was found that the power spectrum for PD also corresponded to that for the half-wave dipole antenna with a length of 50 cm. © 1999 Scripta Technica, Electr Eng Jpn, 126(4): 40–47, 1999

Optimization of HV electrode contour with the highest gaseous insulation performance

In this paper, we developed an optimization algorithm of a HV electrode contour to improve insulation performance of gas insulated power apparatus. Under the consideration of insulation performance of gaseous dielectrics, we could obtain an optimum electrode contour with the highest breakdown voltage. Moreover, we investigated the calculation efficiency and applicability of the optimization algorithm by means of a Borda electrode profile facing a grounded square corner as the optimization example.

On electrostatic trapping of particle contamination in GITL systems

The insulation performance of gas-insulated transmission line (GITL) systems can be substantially affected by the presence and movement of contaminating metallic particles. The dynamics of wire and spherical particles in a coaxial-electrode system under AC voltage have been studied under different conditions. At voltages just above the lifting voltage the particle moved near the outer enclosure. Under appropriate conditions the particle can move into the high field region and eventually cross the interelectrode gap, thus causing a potential insulation hazard. Contaminating particles in a GITL system move randomly in the electric field. Control of contaminating particles is crucial for reliable operation of GITL systems. A numerical model of the contaminating particle's motion in the interelectrode gap in a GITL system was used to study particle control methods in gas-insulated power apparatus. >

New Gaseous Insulation

Studies on gas mixtures are described over the past 15 years, Much work has been performed in universities, research institutes, and industries, to account for the synergistic effect of gas mixtures and to apply gas mixture insulation to electric power apparatus. From a basic point of view, various swarm parameters have been measured and are proposed for Boltzmann equation analysis to understand the synergistic effect experimentally and theoretically. The propagation characteristics of discharges and flash-over properties on the surface of a spacer in gas mixtures also have been investigated. In the application area, gas mixtures have been applied to two gas-insulated electric power apparatus: gas vapor-cooled transformers and cubicle-type GIS. Advantages such as excellent cooling ability, increased breakdown properties and compact apparatus which resulted from the application of gas mixture insulation, are described.

Electrostatic field between a conducting sphere and a dielectric-coated electrode

In electrical insulation application (e.g. compressed gas insulated power apparatus) composite dielectrics are often encountered, and a knowledge of electrical field at the metal dielectric interfaces is useful design information; The paper deals with a sphere— plane geometry separated by a solid dielectric, with the sphere having a known charge but an unknown potential. A method of simulated optimized images is used to calculate the field. A very simple interpretation is obtained for the rapid enhancement of field at the point of contact between the sphere and the solid dielectric. Also it is known that at locations away from the point of contact, the electric field is not significantly altered by the presence of a dielectric ( ϵ r⩽3.