High-Voltage Technology

Multi-layer insulations are commonly used in HVDC applications, giving rise to physical and chemical interfaces in the insulation systems. Space charges at the interface between insulators play crucial roles on the electric field distribution of such insulation systems. The issue arises in the design of high voltage direct current (HVDC) insulations. How to understand the behaviors, mechanisms and effects of interface charges on the electric field distribution? In the past two decades, this specific topic has attracted much interest. Much experimental evidence has shown that interface charges may not follow the classic Maxwell-Wagner-Sillar (MWS) theory. In this paper, the measurement techniques and the phenomena of space charge behaviors at the interfaces between different insulating materials are summarized. It is indicated that surface states should be included in the numerical simulation of the charge and field distributions.

Interface charges between insulating materials

The growth of cities’ population increased the interest in the opportunities and challenges that Information and Communication Technology (ICT) have on carbon footprint reduction, which fosters their environmental sustainability. Using Principal Component Analysis (PCA), six ICT-related variables from European Union (EU) cities were combined into a single two-dimensional ICT index. Then, through cluster analysis, cities were clustered into four groups based on the ICT index and Carbon dioxide (CO2) emissions. Using ICT as an indicator of smartness and CO2 emissions as an indicator of sustainability, we show that it is possible for a city to be smart but not sustainable and vice versa. Results also indicate that there is a gap between cities in northern Europe, which are the top performers in both categories, and cities in south-eastern Europe, which do not perform as well. The need for a common strategy for achieving integrated smart, sustainable and inclusive growth at a European level is demonstrated.

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Assessing the Gap between Technology and the Environmental Sustainability of European Cities

Emissions in a decarbonised economy? Global lessons from a carbon footprint analysis of Iceland

Incineration is widely adopted in municipal solid waste management, which produces large amounts of municipal solid waste incineration (MSWI) fly ash. The harmless treatment of MSWI fly ash requires the appropriate disposal of heavy metals and dioxins that are enriched in fly ash. This review summarizes recently developed harmless disposal methods for MSWI fly ash including solidification/stabilization, thermal treatment, and separation/extraction. In addition, we discuss heavy metal and dioxin fixation, and the removal capacity of fly ash via solidification/stabilization (including cement solidification, chemical stabilization, hydrothermal processes, and mechano-chemical methods), thermal treatment (including sintering, fuel-burning, or electric melting/vitrification), and separation/extraction (including water-washing, chemical reagent leaching, biological leaching, electrodialysis separation, chemical reagent extraction, and nanomaterials extraction). The advantages and disadvantages of different harmless treatment methods are compared and future research prospects and suggestions are summarized. This review provides general guidelines for the harmless treatment of MSWI fly ash in the future.

Review of harmless treatment of municipal solid waste incineration fly ash

In November 2017, the first ±1100 kV high-voltage direct-current power transformer in the world, which was made by Siemens in Nurnberg, passed its type test. Meanwhile, in early 2017, a ±1000 kV ultra-high voltage (UHV) substation was officially put into operation in Tianjin, China. These examples illustrate that the era of UHV power transmission is coming. With the rapid increase in power transmission voltage, the performance requirements for the insulation of power transformers are getting higher and higher. The traditional mineral oils used inside power transformers as insulating and cooling agents are thus facing a serious challenge to meet these requirements. In this review, the basic properties of traditional mineral insulating oil are first introduced. Then, the variation of electrical properties such as breakdown strength, permittivity, and conductivity during transformer operation and aging is summarized. Next, the modification of mineral insulating oil is investigated with a focus on the influence of nanoparticles on the electrical properties of nano-modified insulating oil. Recent studies on the performance of mineral oil at molecular and atomic levels by molecular dynamics simulations are then described. Finally, future research hotspots and notable research topics are discussed.

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Review of Research Progress on the Electrical Properties and Modification of Mineral Insulating Oils Used in Power Transformers

The formation of space charge in the oil/paper insulation under HVDC condition could influence the electric field distribution. In the case of HVDC converter transformer, the distortion of the electric field may affect its performance. It is, therefore, important to analyze factors that can affect space charge formation and dissipation characteristics such as temperature, moisture, electric stress and ageing process. This paper mainly focuses on the effect of electric stress and oil ageing on space charge characteristics in oil/paper insulation system. The pulsed electroacoustic technique (PEA) method was used to monitor space charge dynamics. This data was further incorporated into a model established using COMSOL software to estimate space charge impact on electric field distribution, with an emphasis on the electric field after polarity reversal operation. The results show that the electric field arising from the space charge formation is much higher compared to the average electric field after the polarity reversal operation. Moreover, the electric field caused by space charge decreases faster for the aged oil-impregnated pressboard compared to the fresh one, which could be related to different space charge dissipation characteristic related to the condition of the oil.

Space charge characteristics in oil and oil-impregnated pressboard and electric field distortion after polarity reversal

This paper focuses on the space charge behavior in oil-paper insulation systems used in power transformers. It begins with the importance of understanding the space charge behavior in oil-paper insulation systems, followed by the introduction of the pulsed electrostatic technique (PEA). After that, the research progress on the space charge behavior of oil-paper insulation during the recent twenty years is critically reviewed. Some important aspects such as the environmental conditions and the acoustic wave recovery need to be addressed to acquire more accurate space charge measurement results. Some breakthroughs on the space charge behavior of oil-paper insulation materials by the research team at the University of Southampton are presented. Finally, future work on space charge measurement of oil-paper insulation materials is proposed.

Progress of Space Charge Research on Oil-Paper Insulation Using Pulsed Electroacoustic Techniques

Space charge build-up has been considered as the major issue in the development of HVDC apparatus such as the converter transformer. The existence of space charge can distort the local electric field, which could lead to the degradation and even breakdown of insulation materials. Therefore, it is vital to investigate factors that can affect space charge formation and dissipation characteristics such as the temperature, moisture, ageing, thickness, multi-layered structure and electric fields. This paper mainly focuses on the effect of multilayers and thickness on space charge behaviour of oil and pressboard insulation system. Space charge was measured using the pulsed electroacoustic technique (PEA) method. The space charge results are quantitatively analysed to establish the relationship between interfacial charge density and different pressboard and oil thickness ratios. A new space charge interpolation methodology is utilized to input space charge into the multilayers oil and pressboard model using COMSOL software. The local electric field of multilayers oil-impregnated pressboard and oil could be simulated, with the emphasis on the electric field after the polarity reversal operation. From space charge results, they indicate that the increased thickness of pressboard could prohibit the interfacial charge increase while the increased oil thickness could facilitate interfacial charge increase. Moreover, from the electric field simulation results, they indicate that there is the electric field gradient caused by the space charge for multilayers oil and oil-impregnated pressboard structure. After the polarity reversal, the maximum electric field of the oil caused by the space charge is higher than electric field calculated based on the Maxwell-Wagner theory.

Multilayers oil and oil-impregnated pressboard electric field simulation based on space charge

In order to evaluate the acoustic wave behavior within the Pulsed Electro Acoustic (PEA) cell, a simulation model has been developed in this work. The model, implemented in Matlab environment, is based on the analogy between acoustic and electrical quantities. Therefore, it was possible to model the PEA cell as cascade connected lossy transmission lines. The model has been validated theoretically by making a comparison with a simulation result found in literature. The experimental validation has also been made by using the PEA cell of the LEPRE high voltage lab. In addition, four graphs have been realized. Two of them can be used to establish in easy and fast way to obtain the minimum ground electrode and absorber thicknesses that should be used for a specific sample, in order to avoid reflections in the main PEA output signal. For the same aim, two more graphs are provided to select correctly the sample thickness that can be tested in a given PEA cell.

Bo Huang

Papers

Review of Research Progress on the Electrical Properties and Modification of Mineral Insulating Oils Used in Power Transformers

Space charge characteristics in oil and oil-impregnated pressboard and electric field distortion after polarity reversal

Progress of Space Charge Research on Oil-Paper Insulation Using Pulsed Electroacoustic Techniques

Multilayers oil and oil-impregnated pressboard electric field simulation based on space charge

The Acoustic Wave Behavior Within the PEA Cell for Space Charge Measurement