Breakdown of CF3I Gas and its Mixtures under Lightning Impulse in Coaxial-GIL Geometry

SF6 is widely used in modern transmission and distribution networks because of its outstanding dual qualities: arc quenching and dielectric insulation. As a gas medium, SF6 is chemically inert, non-toxic, and non-flammable, which makes possible the construction of compact SF6 switchgear. One major known disadvantage of the gas is that it has a global warming potential which is 23,900 times higher than CO2. This has led to research into alternative gases with a much lower environmental impact, and one of the emerging candidates is CF3I. The high boiling temperature of CF3I means that it has to be used as part of a mixture inside gas-insulated equipment. To carry out the investigation on CF3I, a scaled-down coaxial system that replicates the maximum electric field of a 400 kV GIL system was designed and fabricated. The insulation performances of CF3I/CO2 and CF3I/N2 gas mixtures were then examined by measuring the 50% breakdown voltage, U50, using a standard lightning impulse waveform (1.2/50) under absolute pressures of 1 to 4 bar. The experimental results show that CF3I gas mixtures have promising potential as an insulation medium for application in gasinsulated lines

Lightning caused overvoltages on power transformers recorded by on-line transient overvoltage monitoring system

Transient overvoltages generated by lightning strikes or switching operations represent a significant risk to bushings and windings of power transformers. They cause stress on the insulation system and can, over time, cause dielectric failure and damage to power transformers. Many transformer failures are reported as dielectric failures and they are not necessarily linked to any particular event when they occur but may be the result of prior damage from transient overvoltage events. Lightning and switching overvoltage waveforms appearing at transformer terminals in real operating conditions may significantly differ from standard impulse voltage waveforms used during laboratory testing. The number and amplitudes of overvoltages which stress the insulation depend on various parameters such as the lightning strike density in the considered area, since it determines how often the transformer is stressed by lightning overvoltages. Since the overvoltage amplitudes at transformer terminals are usually unknown, an on-line overvoltage transient recorder can be used with the ability to sample, analyse and store transients in real-time. In this paper, an on-line transient overvoltage monitoring system (TOMS) for power transformers is presented that is capable to continuously record in real-time various kinds of transient overvoltages such as lightning or switching overvoltages. Special attention is paid to lightning caused transient overvoltages recorded at the terminals of 150 MVA power transformer. Recorded waveforms originating from lightning strikes to overhead lines are correlated with data from the lightning location system (LLS) and supervisory control and data acquisition (SCADA) system. Collected data about overvoltage stresses can be used as the basis for the assessment of the transformer insulation condition, estimation of health index and for analysis of various kinds of events such as faults or equipment failures

Lightning caused overvoltages on power transformers recorded by on-line transient overvoltage monitoring system

Transient overvoltages generated by lightning strikes or switching operations represent a significant risk to bushings and windings of power transformers. They cause stress on the insulation system and can, over time, cause dielectric failure and damage to power transformers. Many transformer failures are reported as dielectric failures and they are not necessarily linked to any particular event when they occur but may be the result of prior damage from transient overvoltage events. Lightning and switching overvoltage waveforms appearing at transformer terminals in real operating conditions may significantly differ from standard impulse voltage waveforms used during laboratory testing. The number and amplitudes of overvoltages which stress the insulation depend on various parameters such as the lightning strike density in the considered area, since it determines how often the transformer is stressed by lightning overvoltages. Since the overvoltage amplitudes at transformer terminals are usually unknown, an on-line overvoltage transient recorder can be used with the ability to sample, analyse and store transients in real-time. In this paper, an on-line transient overvoltage monitoring system (TOMS) for power transformers is presented that is capable to continuously record in real-time various kinds of transient overvoltages such as lightning or switching overvoltages. Special attention is paid to lightning caused transient overvoltages recorded at the terminals of 150 MVA power transformer. Recorded waveforms originating from lightning strikes to overhead lines are correlated with data from the lightning location system (LLS) and supervisory control and data acquisition (SCADA) system. Collected data about overvoltage stresses can be used as the basis for the assessment of the transformer insulation condition, estimation of health index and for analysis of various kinds of events such as faults or equipment failures

Fundamentals of condenser bushings

This article focuses on the concept of capacitance-graded, i.e. condenser bushings – both oil-impregnated and resin-impregnated paper bushings – for extra-high voltage transformers. After a description of the dielectric loss, the article goes on to describe the selection, testing, transportation, storage and installation of these bushings, so as to avoid a bushing failure which may lead to a catastrophic failure of transformers

Effect of water on electrical properties of Refined, Bleached, and Deodorized Palm Oil (RBDPO) as electrical insulating material

This paper describes the properties of refined, bleached, deodorized palm oil (RBDPO) as having the potential to be used as insulating liquid. There are several important properties such as electrical breakdown, dielectric dissipation factor, specific gravity, flash point, viscosity and pour point of RBDPO that was measured and compared to commercial mineral oil which is largely in current use as insulating liquid in power transformers. Experimental results of the electrical properties revealed that the average breakdown voltage of the RBDPO sample, without the addition of water at room temperature, is 13.368 kV. The result also revealed that due to effect of water, the breakdown voltage is lower than that of commercial mineral oil (Hyrax). However, the flash point and the pour point of RBDPO is very high compared to mineral oil thus giving it advantageous possibility to be used safely as insulating liquid. The results showed that RBDPO is greatly influenced by water, causing the breakdown voltage to decrease and the dissipation factor to increase; this is attributable to the high amounts of dissolved water

Investigation on the feasibility of trifluoroiodomethane (cf3i) for application in gas-insulated lines

It is widely acknowledged that the world needs to reduce the level of greenhouse gas emissions. It is proposed to use potentially cleaner renewable energy sources to replace fossil fuels, and thus reduce greenhouse gas emissions. A significant challenge facing renewable energy sources, however, is that the power generation facilities are often located far from the load centres, meaning that new high capacity long-distance transmission systems would need to be built. This is a particular issue since there are increasing difficulties in obtaining approval to construct new overhead lines (OHL). An alternative is gas-insulated lines (GIL), a system for the transmission of electricity over long distance and is considered as a viable technical solution in places where OHL cannot be constructed. The currently adopted gas medium in GIL, however, is sulphur hexafluoride (SF6), which is a potent greenhouse gas. Trifluoroiodomethane (CF3I) has been proposed as an alternative insulation medium to SF6 in GIL, and this thesis investigates the potential of using a CF3I gas mixture in GIL applications. It is hoped that the research can lead to a new form of environmentally friendly power transmission system that could cope with the increasing power demand in large metropolitan areas, and contribute to the reduction of SF6 usage in the high-voltage industry. The literature survey reviewed the research work on CF3I gas and its mixtures to date. Several research gaps were identified, and these informed the investigations carried out in this research. Reduced-scale coaxial test systems with the electric field properties of a full-scale 400 kV GIL were designed, developed and fabricated. The designs were simulated using COMSOL to ensure that the highest field would be along the centre of the conductor. The effective ionisation coefficients of various CF3I gas mixtures were calculated using BOLSIG+, which provided estimated values for the critical reduced field strength of each gas mixture. Extensive laboratory tests using a standard lightning impulse (1.2/50) were conducted on the fabricated prototypes using various CF3I/CO2 and CF3I/N2 gas mixtures to determine the 50% breakdown voltage. The breakdown characteristics of CF3I gas mixtures were examined for pressure, geometric ratio, impulse polarity, buffer gas and mixture content. Based on the measured breakdown voltage and calculated critical reduced field strength of various CF3I gas mixtures, a two-stage streamer/leader mathematical model was developed to evaluate the reduction in field strength at higher pressures. A comparative study was carried out on CF3I gas mixtures in a rod-plane electrode configuration under standard lightning impulse and steep-front square impulse waveforms. This investigation focused on the V-t characteristics of CF3I gas mixtures in this particular configuration. A phase equilibrium experiment was also carried out to determine the boiling point of various CF3I gas mixtures