The widespread use of SF/sub 6/ by the electric power and other industries has led to increased concentrations of SF/sub 6/ in the atmosphere. This causes concern as to possible effects on global warming, because SF/sub 6/ is a potent greenhouse gas. This paper first touches on this issue and then documents the behavior of high pressure gases such as N/sub 2/ and SF/sub 6//N/sub 2/ mixtures that can be realistically considered as acceptable intermediate or long-term replacements for pure SF/sub 6/ in some HV applications. The possible use of dilute SF/sub 6//N/sub 2/ mixtures as an alternative to pure SF/sub 6/ for some of industry's insulation needs (albeit at higher pressure) is documented, and existing knowledge on these mixtures and on the individual components (N/sub 2/ and SF/sub 6/), both basic and applied, is compiled. A guide to existing literature is provided. >

SF/sub 6//N/sub 2/ mixtures: basic and HV insulation properties

A dynamic model that computes the flashover voltages of polluted insulators energized with DC voltage is presented. The salient feature of this model is that it takes into account the configuration of the insulator profile at every instant, which plays an important role in the flashover process of the DC polluted insulators. A number of practical insulator geometries have been studied, and the validity of the model is verified by comparing the computed results with the experimental results of previous researchers. >

Dynamic arc modeling of pollution flashover of insulators under DC voltage

Compressed-gas insulated transmission lines or cables have been in service since 1968, with over 13 km (circuit length) in operation worldwide at voltages from 145 to 550 kV. The design philosophy and development are reviewed for the "rigid", flexible, and semi-flexible isolated phase lines, the three-conductor lines and the prototype dc gas-insulated cable system. The various designs of insulators, conductors, enclosures, particle traps, and terminations are described, together with the techniques for grounding and corrosion protection, factory and site testing, fault diagnostics, and calculation of current ratings.

Gas-Insulated Cables

In order to establish a method for monitoring contamination in insulators based on leakage current waveforms and their frequency characteristics, the leakage current waveforms and frequency characteristics of an artificially polluted 180 mm diameter cap and pin type insulator were investigated by the wet contaminant and the clean fog methods at fixed applied voltages. As a result it was found that leakage current waveforms become similar to the symmetrical wave when strong local arcs occur; hence, the intensity of the odd order of harmonic components, e.g. 50, 150, and 250 Hz, is high. Furthermore, it was clarified that the transition of the leakage current waveforms, until flashover occurs, is classified into six stages and that a threshold exists by which the occurrence of flashover can be predicted.

Frequency characteristics of leakage current waveforms of an artificially polluted suspension insulator

In order to establish a method for monitoring whether flashover occurs or not in a string of insulators based on leakage current waveforms and their frequency characteristics, the leakage current waveforms and the frequency characteristics of a string of 120-kN suspension insulators were investigated with artificial contamination tests and field tests. As a result, it was found that leakage current waveforms become nearly the symmetrical wave when strong local arcs occur; hence, the intensity of the odd order of harmonic components, e.g., 50, 150, and 250 Hz, is high. Furthermore, it was clarified that the transition of the leakage current waveforms until flashover occurs is classified into six stages and that a threshold exists in the magnitude of peak leakage current and prominent odd-order harmonic components by which the occurrence of flashover can be predicted.

Frequency characteristics of leakage current waveforms of a string of suspension insulators

The paper constitutes a systematic theoretical and experimental investigation into impulse breakdown voltage time curves of coaxial-cylinder gaps in SF 6 and 50% SF 6 -50% N 2 mixture. The breakdown time delay probability is treated as a bivariate distribution in statistical and formative time lags. A method is developed to deduce the marginal breakdown voltage probability curve from statistics of time delay. The experimental work was performed on a 2.54/7.0 cm diameter 60 cm long coaxial-cylinder electrodes using SF 6 or SF 6 -N 2 mixture pressures of 0.1-0.3 MPa subjected to 0.5/50 ??s and 1.2/50 ??s impulses of both polarities, covering breakdown voltages up to 550 kV and times to breakdown in the range 0.2-7 ??s. Whenever possible, the theory was checked against direct measurements and, considering the complexity of the problem, agreement is judged satisfactory.

Impulse Breakdown Voltage-Time Curves of SF 6 and SF 6 - N 2 Coaxial-Cylinder Gaps

A dynamic model of ac source-insulator interaction during the critical arcing period of polluted high-voltage insulators is developed. The simulation accounts for arc dynamics, arc motion over the contaminated insulator surface, dielectric recovery, and restrikes as well as the most relevant parameters of the insulator and the source. The effect of the source parameters on the leakage current waveform, dynamic voltage drop and critical flashover voltage are systematically investigated. Whenever possible the simulation results are compared with experiments and the results are found quite satisfactory.

AC Source-Insulator Interaction in HV Pollution Tests

The paper comprises a systematic experimental investigation into the influence of AC test source parameters on the leakage current pulses, voltage fluctuations and flashover voltages of polluted insulators. The test techniques include both salt-and clean-fog methods, while the test insulators comprise long-rod and cap and pin strings of different lengths as well as support columns.

Influence of AC Source Parameters on Flashover Characteristics of Polluted Insulators

This paper provides general guide-lines for the design of a feedback-controlled cascade rectifier source for testing heavily polluted insulators. The basic elements of the circuit are investigated with respect to both technical operating constraints and overall cost. The steady-state and transient analyses are based on analog modelling and digital simulation and have been verified on full-scale 50-kV and 100-kV sources.

Feedback-Controlled Cascade Rectifier Source for HV Testing of Contaminated DC Insulators

The paper comprises a theoretical and experimental investigation into the impulse breakdown voltage-time characteristics of a spherical-particle-contaminated coaxial-cylinder gap in compressed SF 6 and SF 2 -N 2 mixture. The experimental work was carried out on a 25/70-mm diameter coaxial-cylinder gap with a 3.2-mm spherical particle attached to the high-voltage electrode, at a gas pressure of 0.3 MPa under 0.5/50-??s and 1.2/50-??s impulses of both polarities. Whenever possible the results were compared to those previously obtained on a clean system from which the particle effect on impulse breakdown voltage and delay time was identified. The theoretical and experimental results of breakdown delay time distributions, 5% and 95% probability voltage-time curves and breakdown voltage probability functions were found to be in satisfactory agreement.

Farouk A. M. Rizk

Papers

Impulse Breakdown Voltage-Time Curves of SF 6 and SF 6 - N 2 Coaxial-Cylinder Gaps

AC Source-Insulator Interaction in HV Pollution Tests

Influence of AC Source Parameters on Flashover Characteristics of Polluted Insulators

Feedback-Controlled Cascade Rectifier Source for HV Testing of Contaminated DC Insulators

Voltage-Time Characteristics of Particle-Initiated Impulse Breakdown in SF 6 and SF 6 -N 2