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Traction transformers are one of the most important equipment in the traction power supply system. Thermal aging of transformer insulation system determines its reliable life. To achieve an estimation of the insulation life of traction power transformers operated under real load condition, the effects of traction loads on the thermal aging of oil-paper insulation in traction transformers were investigated. First, the design principles for the accelerated thermal aging experiment were studied and proposed by considering magnitude (i.e., load rate) and frequency (i.e. spare time and lasting time) of the traction load. Then, an experimental system was established in the laboratory to conduct extensive accelerated thermal aging experiments, in which the traction loads were mimicked by using positive temperature coefficient (PTC) heating plates and a programmable logic controller (PLC). Finally, the experimental results were analyzed and discussed to reveal the effects of traction load on the thermal aging of oil-paper insulation in traction transformers.

Transformer insulation paper is a key indicator for transformer remaining operational life. Paper decomposition is evaluated using the degree of polymerization (DP) which calls for samples of insulation paper from operating transformers. Since collecting such paper samples is extremely difficult, other indicators have been used to indirectly reveal the DP value of insulation paper. This includes dissolved gases in transformer oil such as carbon oxides and hydrocarbon gases, furan compounds, methanol, ethanol, and moisture along with some oil characteristics such as interfacial tension. However, for the same oil sample, these individual parameters lead to different DP values. This is attributed to the lack of accuracy of the established mathematical and artificial intelligence models correlating DP with each of the above mentioned individual parameters. This paper presents a self-learning method to estimate the DP value of transformer insulation paper based on multiple transformer oil aging parameters. The proposed method comprises data processing, fuzzy c-means and linear regression. Results reveal that estimating the DP value based on multiple aging parameters is more accurate than estimating it using one single parameter as per the current practice. The proposed method not only helps to understand the correlation between multiple oil aging parameters and the DP value of paper insulation, but also promotes the establishment of more accurate life assessment models for power transformers based on these oil aging parameters.

In the last decade, much effort has been invested in using methanol as an oil-soluble chemical marker for assessing the condition of insulating paper. The use of this marker as paper life index presents many advantages particularly with new transformers insulated with thermally upgraded papers. However, until now, no interpretation model has been available for its extensive use by the transformer community. In this paper, a methanol-based interpretation model is presented for the first time using postmortem paper analysis on core-type transformers. This model allows the evaluation of the average degree of polymerization of a transformer's cellulose winding. Furthermore, threshold values based on the methanol concentration are given using this approach. Finally, model validation was performed on a limited number of papers taken from transformers being assessed in the repair shop.

Frequency Domain Spectroscopy (FDS) is proposed as a suitable diagnostic method in this paper for implementation on energized oil-paper insulation systems. In present FDS tests, the measurement method is only designed for offline applications. To overcome the deficiencies with the offline testing, this work investigates the feasibility of performing FDS measurements on energized transformers with the view to developing an online diagnostic tool using neutral connections as injection and measurement paths. In the proposed technique, a low-pass filter system is designed to remove the high frequency noise as well as the power frequency signals present in the measured neutral currents. The technique has been successfully applied in a laboratory environment on transformers with oil-paper insulation systems with different operational ages and is feasible for single-phase as well as 3-phase transformers with the accessible neutral points. Compared with the conventional FDS results, a nonlinear dielectric phenomenon is observed from the energized insulation systems as higher dissipation factor Tanδ values primarily due to the increased losses under power frequency voltage excitation. The degree of the nonlinearity is found to correlate well with the degradation status of insulation systems under test.

This paper presents a wavelet texture analysis of microscopy images acquired from artificially-aged samples of power transformer winding insulation paper. Oil-impregnated Kraft paper samples are prepared for experimental tests where they are thermally stressed in an oven at temperatures above what would normally occur in operation. This accelerated ageing test arrangement is used to produce a set of paper samples with varying levels of insulation deterioration. From the paper samples, microscopy images of the paper surface are obtained using a standard optical microscope. These microscopic images are analyzed using a texture analysis method that utilizes a two-dimensional wavelet transform to extract detailed information from the wavelet decomposition coefficient matrices. These features are analyzed to assess how the texture of paper changes in response to thermal deterioration and ageing. The results suggest that the texture features are correlated to the commonly-used degree of polymerization test. Additionally, statistical classification is performed on the wavelet texture features using both supervised and unsupervised machine learning methods. The results demonstrate that differentiation between oil-impregnated paper samples with different levels of thermal degradation using wavelet texture analysis is effective.

Aging of transformer paper insulation is commonly investigated through indirect measurements conducted in the oil, e.g. furanic compounds. In addition to the conventional indicators, recent investigations showed that alcohols like methanol and ethanol in oil could be used as paper aging indicators. This paper focuses on the applicability of these new paper aging indicators in a novel, gas-to-liquid (GTL) technology based transformer oil through an accelerated laboratory aging experiment. The experiment was conducted at 120 °C for up to 280 days. A conventional mineral oil was also tested as a reference. Kraft paper aged in both oil types showed similar reductions in degree of polymerization (DP) and tensile strength (TS). Amount of methanol in oil is higher than that of 2-FAL in oil when DP is over 375, confirming the promise of using methanol as an early paper aging indicator. Both the new paper aging indicator, methanol and the conventional paper aging indicator, 2-FAL are applicable for the GTL oil without any modifications. In addition, oil only aging experiments showed that oil does produce methanol but it is negligible compared with oil paper aging experiment; however the amount of ethanol measured in the present oil paper aging experiment is believed to be originated from oil oxidation rather than from paper aging.

Objective: Early stage diagnosis of sepsis without overburdening health services is essential to improving patient outcomes. Methods: A fast and simple-to-use platform that combines an integrated circuit with paper microfluidics for simultaneous detection of multiple-metabolites appropriate for diagnostics was presented. Paper based sensors are a primary candidate for widespread deployment of diagnostic or test devices. However, the majority of devices today use a simple paper strip to detect a single marker using the reflectance of light. However, for many diseases such as sepsis, one biomarker is not sufficient to make a unique diagnosis. In this work multiple measurements are made on patterned paper simultaneously. Using laser ablation to fabricate microfluidic channels on paper provides a flexible and direct approach for mass manufacture of disposable paper strips. A reusable photodiode array on a complementary metal oxide semiconductor chip is used as the transducer. Results: The system measures changes in optical absorbance in the paper to achieve a cost-effective and easily implemented system that is capable of multiple simultaneous assays. Potential sepsis metabolite biomarkers glucose and lactate have been studied and quantified with the platform, achieving sensitivity within the physiological range in human serum. Conclusion: We have detailed a disposable paper-based CMOS photodiode sensor platform for real-time simultaneous detection of metabolites for diseases such as sepsis. Significance: A combination of a low-cost paper strip with microfluidic channels and a sensitive CMOS photodiode sensor array makes our platform a robust portable and inexpensive biosensing device for multiple diagnostic tests in many different applications.

Several miniaturized viscometers, or microviscometers, have been developed exploiting numerous rapid prototyping techniques. Among them, paper microstrips, famously known as microfluidic paper-based analytical devices ( $\mu $ PADs), have become popular due to their cost-efficacy, simple fabrication, fast response, and easily disposable. Many fabrication methods are existing to develop paper microstrips. Herein, an alternative fabrication method is proposed where fused deposition modeling (FDM)-based 3-D printer (3DP) has been employed using polycaprolactone (PCL) filament. F, image processing has been utilized to measure viscosity in such microfluidic domain. Viscosity was calculated by measuring the time taken by the fluid to cover a fixed length between two spots in the microchannel based on the programed and color-coded regions-of-interest. The image processing program was developed considering the change in the gray scale in the virtual region of interests (ROIs) in the microchannel during the fluid flow in the paper microstrips. A 3-D printed handheld platform, containing raspberry pi with on-board camera and display, was developed to execute the image processing and automate the entire work flow. In the proposed device, the accuracy was measured to be >92%.

Cellulosic insulation materials are very widely used in power transformers, in designs up to 1,500 MVA [1]. Cellulosic insulation is formed of either wood, pressboard, or Kraft paper. The wood and thick sections of pressboard are used to manufacture the base of the transformer; pressboard is also used for the spacers and barriers of the windings, and the paper is wrapped around the current carrying conductors. The water content of this cellulosic insulation must be monitored, because if it becomes too high, a transformer can fail on overload, and will reach the end of its functional life sooner because the paper insulation will age faster [2]. Eventually, the paper will become brittle, and will either tear or lift off the conductor. The water content of paper (WCP) is the ratio of the mass of adsorbed water to the mass of dry paper, usually expressed as a percentage. Water can migrate out of the wood and pressboard and around the system, eventually increasing the WCP. If the WCP becomes too high, the transformer may be dried out by the relevant utility [3].

Flexible In2O3 nanowire transistors gated by microporous SiO2-based solid electrolytes are fabricated on paper substrates at room temperature. Low-voltage (1.0 V) operation of these devices is realized owing to the large electric-double-layer capacitance of (1.73 μF/cm2 at 20 Hz) of the microporous SiO2 solid electrolytes, which were deposited at room temperature. The subthreshold swing, current on/off ratio, and field-effect mobility of the paper-based nanowire transistors are estimated to be 74 mV/decade, 1.7×106, and 218.3 cm2/V·s, respectively. These low-voltage paper-based nanowire transistors show promise for use in portable flexible paper electronics and low-cost portable sensors.

Contamination of water bodies due to the disposal of natural and anthropogenic pollutants is one of the major global concerns in the recent times. Regular on-spot monitoring of water quality has become mandatory wherein diverse classes of materials such as organic, inorganic, heavy metals, or biological wastes are detected employing standard protocols. However, most of the existing laboratory techniques are either costly and/or time consuming owing to the process involving the expert driven sophisticated analysis techniques. Herein, we report the development of three paper based colorimetric sensors for the on-spot quantitative detection of the levels of fluoride (F−), lead (Pb2+), and pH in drinking water. The variations in the color-intensities of the paper sensors with the variation in the concentration of fluoride (F−), lead (Pb2+), and pH were converted into electronic signals to enable the development of a point-of-care-testing (POCT) device. For this purpose, initially, the sensors were characterized and calibrated with known concentrations of contaminates before successful validations with real samples. Thereafter, the sensors were illuminated with a light emitting diode from one side while the transmitted rays passing through the sensor was captured by a light dependent resistor (LDR) from the other side. The variation in the color intensity of the paper-sensor with the contaminant loading was found linearly varying with the resistance of the LDR. The sensor was then translated into a specific, stable and user friendly POCT device, which enabled inexpensive spot detection of pollutants in drinking water.

Oil soluble chemical markers such as methanol, ethanol and 2-furaldehyde for assessing the condition of insulating paper still present many challenges for an accurate interpretation in real transformers. Indeed, many conceptual parameters such as design (shell vs. core) or type of cooling are needed for a more accurate interpretation of the data. Moreover, similarly to water, the measured marker concentrations in oil are temperature-dependent, i.e. an existing partition phenomenon between the oil and the solid insulation modify the solubility of the markers, thus changing their apparent concentrations in the oil. Consequently, to follow the real trend of these species during the transformer’s service life, it is crucial to correct their concentrations at a specific temperature, as is done for the water content. Knowing these facts and in order to calibrate a predictive model, Hydro-Quebec decided to access equipment when dismantled, which enables a large amount of paper to be sampled from different sections of the windings. This allows for a more accurate representation of the transformer paper conditions in relation to transformer design. The paper condition obtained by measuring the degree of polymerization in accordance with the presence of chemical markers is a valuable process. We believe that it is possible to better understand the behavior of the paper insulation and to assess markers concentration thresholds using the oil analysis. This article discusses the recent experience in this field with specific cases.

Transformer oil, insulating paper, high voltage direct current (HVDC) bushings and on-load tap changer (OLTC) are essential parts of converter transformers. When the power flow of the HVDC system is reversed or a sudden outrage occurs, the charge field will deform the dielectric internal electric field, thus easily leading to the occurrence of discharge or insulation damage. In this case, it is necessary to carry out the characterization of the oil–paper interface to avoid these consequences. This paper analyses the interface charge accumulation and dissipation characteristics of an oil–paper insulation system under DC and polarity reversal voltages. In this paper, induced electrification was used to charge the composite oil–paper insulation. The amplitude of DC and polarity reversal voltages applied in this paper was 4 kV, and paper with thicknesses of 0.08 and 0.13 mm was used. This research offers a theoretical exploration to optimize the transformer oil– paper insulation structure and prevent damage to its insulation systems. From the research on different layers and thicknesses of paper, it can be concluded that the decay is fast in the initial period and then becomes slower with the lapse of time until finally the decay curve tends to become flat. With an increase in the number of the paper layers, the interface charge density decreases. So in real life multilayer paper is used to avoid failures owing to its structural weakness. Regarding the DC polarity reversal effect, increasing the thickness of the paper is equivalent to adding paper layers. The dissipation rate of the positive interface charge is quicker than that of the negative one. In the polarity reversal test, the dissipation rate becomes smaller as the reversal time gets longer.

Aviation is often seen as an innovative sector eager to seek technological solutions. It is often assumed that aircraft cockpits have panels with electronic displays and many buttons, some labeled "auto." While this might be the case for commercial aviation (Figure 1), the vast majority of noncommercial civil aircraft cockpits look nothing like these modern cockpits. Non-commercial civil aircraft, known as general aviation (GA), typically have cockpits that mainly consist of analog instruments that show the heading of the aircraft, the altitude, and the air speed (Figure 2). While flying, the pilots make calculations using a compass and an aviation calculator. Due to the slow entry rate of new, glass-cockpit aircraft [1], the composition of the GA aircraft fleet is not going to change any time soon.

This special section presents an updated version of the paper that won the first prize award in the ITU (International Telecommunication Union) Kaleidoscope conference (K-2020).

Paper-basedpentacene organic thin-film transistors (OTFTs) with spin-coated gelatin (G) stacked gate dielectric layers, the Au/pentacene/G/G matrix-embedded iron (FeG)/Al/paper structure, were fabricated. The proposed composite-stacked bio-dielectric layer can be implemented using solutions with the degradable biomaterials. These materials enable a large-area printing of use-and-throw devices. Control devices (Au/pentacene/G/Al and Au/pentacene/FeG/Al structure) were also fabricated for comparison. High-performance paper-based OTFT constructed from the stacked gate dielectric layer exhibited a carrier mobility of 8 cm2/Vs, an ON/OFF current ratio of approximately 103, a subthreshold swing of 0.6 V/decade, and a threshold voltage of −1.4 V. These results are compatible to those OTFTs fabricated on other substrates. Therefore, the emerging biomaterial-based transistors on paper substrates may help in developing low-cost, environment-friendly devices.

Polymer electret shows a good electrostatic polarization effect, resulting in very high-density charge accumulation in semiconducting channel layer at low gate voltage. In this letter, graphene oxide enhanced poly(vinyl alcohol) film was used as the gate dielectric for flexible low-voltage indium-gallium-zinc-oxide thin-film transistors (TFTs) on paper substrates. High performance with a high current ON/OFF ratio of $\sim 1.8\times 10^{\textsf {7}}$ , a large field-effect mobility of ~42 cm2 V−1 s−1, and a low subthreshold swing of 106 mV/decade was obtained. In addition, such flexible TFTs exhibit a good stability under bending condition due to the reinforcement effect of graphene oxide. Such flexible TFTs on paper substrates have potential application in next-generation low-cost flexible paper electronics.

Flexible electronics based on the paper have attracted significant attention. A paper-based triboelectric nanogenerator (TENG) consisting of commercial and low-cost materials, e.g., printer paper, polytetrafluoroethylene tape, and conductive ink, was proposed in this study. This TENG unit can directly illuminate 24 blue commercial LEDs. Furthermore, compound paper-based TENGs were developed and a considerable enhancement of the electrical output performance was thereby realized. These compound TENGs can also serve as a height sensor.

Continuous monitoring of breathing flow is an essential but often poorly utilized predictor that is causing poor patient outcome. A low-cost, light-weight, easy-to-use, reliable, and disposable breathing sensor is required to bypass the limitations of existing conventional sensors, which are bulky, expensive, and often require experts to handle. In this article, a low-cost, inkjet-printed graphene sensor on a disposable glossy photo-paper substrate is presented for breathing-rate monitoring. The sensor architecture consists of a graphene nanoparticle based thin functional layer on top of a silver nanoparticle based interdigitated conductive pattern. A standard office inkjet-printer was configured with nanoparticle inks and a printed circuit board design software was utilized for the layouts. The sensor was tested in a laboratory environment, and its data were analyzed for different breathing patterns. An empirical model of the sensor was developed using the Cole–Cole impedance model. Test results showed successful detection of breathing rates for different breathing patterns. The prototype sensor provides a low-cost, disposable, and practical solution for frequent breathing pattern recognition.

In this paper, we present a practical application of dirty paper coding (DPC) for the Gaussian cognitive Z-interference channel. A two stage transmission scheme is proposed in which the cognitive transmitter first obtains the interference signal from the primary transmitter and then uses DPC to improve the performance of the cognitive link. Numerical results show that causal knowledge of the interference provides more than 3 dB improvement in performance in certain scenarios over a scheme that does not use interference cancellation. Results are also shown when the cognitive transmitter operates in both half-duplex and full-duplex modes.

Insulators or bushings are one of the most common cause of transformer failures especially those with fire and collateral damage. This can be prevented by application of off-line and on-line monitoring, especially by measuring of capacitance C and dielectric dissipation factor tanδ, but current physical understanding of these parameters should be improved. A complex research of temperature dependence of C and tanδ, which is based on measurements on a specially designed oil impregnated paper bushing models with different moisture contents in the insulation and during artificial ageing, is presented in the paper. Useful relationships, which enable calculation of tanδ as a function of observed influencing parameters, are also presented.

Oil-paper compound insulation has been widely used in power transformers for quite a long time because of its good performance. The paper insulation degrades under the combined stresses of thermal, electrical and chemical stresses during routine operations, resulting in the reduction of its degree of polymerization (DP) and hence its mechanical and electrical strength. Space charges in a dielectric material have a close relation to its electrical performance. Therefore, in this work, the effect of paper-aged states (with different DP values) on the space charge behavior in oil-impregnated paper insulation is investigated, and the pulsed electro-acoustic (PEA) method is used for the space charge distribution measurement. In order to achieve this purpose, new oil-impregnated paper samples are thermally-accelerated aged for different time interval to obtain different DP values, and then the residual oil in each sample is removed, and the Kraft paper samples are dried in a vacuum dry oven. After that, all paper samples are impregnated with new oil, and PEA measurements are performed at room temperature. The experimental results show that, when the external dc voltage is applied on the oil-paper sample, homo-charge injection occurs for all samples. The samples with different aging states exhibit quite different space charge characteristics for both voltage-on and voltage-off conditions. When the aged states are worse, for voltage-on condition, more charges are induced at cathode and anode, as well as more space charges are accumulated in the bulk. For voltage-off condition, the space charge decay rate shows no obvious change for different aged samples.

The electrophoretic display (EPD) has been widely applied on electronic readers (E-Readers) for displaying image/text contents. With a touch panel, the user can put annotations on the contents at will. However, none of the available E-Readers support the function of paintbrush, which allows the user to plot gray tone figures. This is because the EPD is a bistable display whose driving method is quite complicated. In this paper, we first discuss the issues of realizing paintbrush function on the EPD display controller and then propose a new signal processing engine for the paintbrush function. The engine contains four stages: 1) screening the detected pixel coordinates; 2) interpolating pixels; 3) performing image halftoning to generate more visual gray levels; and 4) driving the pixels to the target states. The experimental results show that the engine meets the real-time requirement to complete the pixel interpolation as well as produce the gray-tone traces.

Solution-processed graphene oxide/chitosan composite electrolyte film showed a large specific gate electric-double-layer capacitance of $\sim 3.16~\mu \text{F}$ /cm $^{{\mathsf {2}}}$ at 1 Hz. Indium–zinc-oxide (IZO) homojunction thin-film transistors (TFTs) on paper substrates using such composite electrolytes as gate dielectrics showed a good electrical performance and a high stability. Flexible IZO-based homojunction TFTs showed a high drain current ON/OFF ratio of $\sim \textsf {1.8}\times \textsf {10}^{{\textsf {7}}}$ , a large field-effect mobility of >30 cm $^{{\textsf {2}}}\text{V}^{{-\textsf {1}}}\text{s}^{{-\textsf {1}}}$ , and a low subthreshold swing of 90 mV/decade. At last, a resistor-loaded inverter with a maximal conversion factor of 6.7, and a dual in-plane gate NAND logic operation were also demonstrated on such flexible IZO-based TFTs. Such oxide-based homojunction TFTs on paper substrates have potential applications in next-generation low-cost and portable new-concept electronics.

Accumulation of interfacial space charge in oil-paper interface is a critical issue in insulation diagnostics of transformers. This interfacial charge mainly accumulates due to the conductivity difference of oil and paper. Accumulation of interfacial charge leads to localized field enhancement, which further leads to partial discharges and acceleration in the aging of insulation. Therefore, from the point of view of transformer insulation diagnostics, assessment of interfacial charge is very important. However, it is not easy to estimate interfacial space charge behavior from the transformer diagnostics methods currently in use. In case of Polarization-Depolarization Current (PDC) measurement, a well known method for transformer condition monitoring, the effect of interfacial charge is reflected in the non-linearity of current response during polarization and de-polarization. During de-polarization process, a part of the interfacial charge accumulated during polarization period is absorbed by the electrodes producing a current, which is difficult to separate using conventional linear dielectric theory. In this paper, an attempt has been made to separate this current component from de-polarization current through considering charge de-trapping mechanism. Terming this current component as de-trapping current, its relationship with other parameters of transformer insulation is discussed. The developed methodology has been applied on several practical transformers. It was observed that the time constant of de-trapping current is related to the paper conductivity, oil conductivity, dissipation factor and age of the insulation.

A novel microelectromechanical-systems (MEMS)-controlled paper-like transmissive flexible display device was modeled by a combination of a cantilever with a flat plate and was realized by roll-to-roll printing process for the first time. This model provides predictions as well as improvement suggestions to both mechanical and electrical designs. A newly developed roll-to-roll printing process which was composed of flexography, gravure, lift-off, and lamination techniques used to manufacture this device was proved applicable on flexible electronics with high-volume, low-cost, and large-area solutions. This 20 V-driven device provided distinguishable three primary colors with averaged transmittance of 50% in visible region for full color flexible display applications and showed commercialization compatibility. Its electrical, mechanical, and optical characteristics excelled previous similar works. The proved major advantages of mechanical reliability, low operation voltage, and process simplicity done by this work made the MEMS flexible display an important alternative to electrophoretic, electrowetting, and electrochromic systems.

A femtosecond mode-locked soliton erbium-doped fiber laser (EDFL) is demonstrated using a commercially available nonconductive graphene oxide paper as a saturable absorber. The paper that is sandwiched between two fiber ferrules acts as a mode-locker. The EDFL generates a soliton mode-locked pulse train with a repetition rate of 15.62 MHz and pulsewidth of 680 fs. It is observed that the fiber laser has a low pulsing threshold as well as low damage threshold. The pulse energy and peak power are 0.0085 nJ and 11.85 W, respectively. The easy fabrication of graphene oxide paper should promote its potential application in ultrafast photonics.

This paper reports on the use of cellulose paper simultaneously as electrolyte, separation of electrodes, and physical support of a rechargeable battery. The deposition on both faces of a paper sheet of metal or metal oxides thin layers with different electrochemical potentials, respectively as anode and cathode, such as Cu and Al, lead to an output voltage of 0.70 V and a current density that varies between 150 nA/cm2 and 0.5 mA/cm2, subject to the paper composition, thickness and the degree of OHx species adsorbed in the paper matrix. The electrical output of the paper battery is independent of the electrodes thickness but strongly depends on the atmospheric relative humidity (RH), with a current density enhancement by more than 3 orders of magnitude when RH changes from 60% to 85%. Besides flexibility, low cost, low material consumption, environmental friendly, the power output of paper batteries can be adapted to the desired voltage-current needed, by proper integration. A 3-V prototype was fabricated to control the ON/OFF state of a paper transistor.

Conduction and polarization mechanisms in the oil and paper of a power transformer are discussed in this article. Some of the electrical characteristics of an insulating oil sample can be determined accurately at room temperature using the IEC 61620 test procedure. High voltage stress and high temperature may modify these characteristics. Deterioration products, both acid and non-acid types, influence the measured conductivity, but moisture or contaminants do not do so. Reasonable upper limits have been suggested for new and in-service oil. Pressboard moisture content and conductivity of the oil in the main ducts of a transformer may be measured using dielectric response techniques, which also give information on the state of the insulation at interfaces. Measurement of polarization and depolarization currents in a transformer without oil allows conduction and polarization phenomena occurring in the insulating paper to be identified. The depolarization current characterizes the type of aging in the paper, provided the applied voltage stress is very low.

Oil-paper insulation system is an extraordinarily essential system of converter transformer as well as HVDC bushing. This paper studies the interface charge accumulation and dissipation characteristics of oil-paper insulation system under DC voltage. In this paper, an induced electrification was used to charge the oil-paper samples. The amplitudes of the DC voltage varied from 1 to 4 kV and the paper was 0.13 mm in thickness. From the research of different voltage amplitudes and charging times of the paper, conclusions can be drawn that the decay is fast initially, and then becomes slower with the lapse of time. At last the dissipation trend tends to be flat. With the increase of the voltage amplitudes, the surface potential rises more rapidly, and higher voltage would also cause the charge to dissipate more quickly. Longer charging times would cause the charge to dissipate a bit more slowly. The decay rate of the negative voltage is a little smaller than that of the positive charge. Moreover, when it comes to the effect on the interface charge of the paper layers, the surface potential is more proportional to the increase of paper layers. When the amplitude of the voltage applied in the test is small or the charging time is short enough, the interface charge can be hardly accumulated on the bottom layer. In addition, the tdV/dt and the decay time curves under the positive voltage usually have two peak values, while there is only one peak value in most of the curves under the negative condition.

Since power transformers are valuable components of power networks, reliable determination of the remaining lifetime of a transformer is very important for power utilities. Power transformers consist of oil, a core, wire, and paper (cellulose), and their performance and remaining useful lifetime depend on the state of the oil-impregnated paper. Changing the oil over the lifetime of a transformer is possible, but it is not possible to change the insulation paper. Thus, the end of the paper lifetime will also be the end of the transformer lifetime (typically more than 40 years). The health condition of an in-service power transformer can be assessed by chemical and electrical tests of the oil and the paper [1].

Gassing problems and premature insulation failures in wind turbine transformers indicate reliability concerns about the insulation of such components. This fact necessitates investigating degradation of transformer insulation systems under high-dV/dt pulse voltages that exist in today's wind energy plants. For this purpose, PD activity, generally assumed to be the main long-term cause of insulation deterioration, needs to be analyzed. This work evaluates the effects of impulse voltage parameters such as rate of rise and repetition frequency on inception voltage and intensity of PD activities. PD parameter measurement under high-dV/dt voltages is challenging due to interferences from fast oscillations, and difficulties of PD energy measurements. To avoid such issues, which are related to electromagnetic detection methods under pulse energization, this work uses a chemical approach to compare PD activities, based on the rate of hydrogen generation in a controlled test chamber with oil/paper samples. Gas monitoring of the oil containing impregnated paper samples reveals that a faster rate of rise increases PD energy as well as inception voltage. Results based on experiments conducted in small oil chambers show good correlation between the amount of hydrogen detected and PD energy level; hence it can be used as a benchmark for comparing PD energy levels under various impulse waveforms.

This paper investigates an optimization of the conventional relay selection for multirelay environments. In contrast with previously reported selection schemes, where a selected relay accesses the channel in a dedicated cooperative slot, the proposed scheme recovers the bandwidth loss of the half-duplex constraint by allowing two relays to simultaneously access the channels. Based on an appropriate dirty-paper coding (DPC) technique among relays, the proposed scheme enables a relay to establish communication with the destination at the same time that another relay forwards the data from the source. It is proven that the interplay between relay selection and the superposition DPC weight factor provides a tradeoff between relaying and new data performance. Hence, an appropriate codesign of the superposition DPC parameter and opportunistic relay selection can achieve efficient communication for the new data without affecting the relaying performance. The proposed scheme is compared with conventional relaying approaches, and its enhancements are provided through theoretical studies and numerical results.

The oil-paper insulation in a transformer gradually ages due to the combined effects of thermal and electrical stress that occur during its operation. In this paper, for investigating the changes that occur within the oil-paper during the aging process with different aging stresses, partial discharge (PD) characteristics and trap parameters are measured. An internal discharge cavity model is designed for the electrical stress, and the thermal stress is generated by a baking oven simultaneously. The experiments are designed such that the dominant factor on the aging process is either thermal, electrical, or a unity of both stress factors. The phase resolved partial discharge (PRPD) spectrograms are recorded at different aging time, and the trap parameters are extracted by thermally stimulated depolarization current (TSDC) method. Additionally, the surface morphologies of these samples are observed by using a scanning electron microscope (SEM). The results show that the trap energy presents a decreasing tendency when the electrical stress plays the dominant role, and the discharge energy becomes larger over the aging time until pre-breakdown. When the thermal stress dominates the aging process, the trap energy shows an increasing tendency over the aging time, and there exists a process of ‘material conditioning’ that occurs at the initial stage of the aging process. The change of trap parameters in aged oil-paper will further affect PD characteristics. Correlation analysis of PD and trap characteristics can promote more accurate condition estimation of aged oil-paper.

Due to its excellent performance, oil-paper insulation has been widely used in many power apparatuses including converter transformer of high voltage direct current (HVDC) system. Space charge accumulation phenomena may significantly distort local electric field inside transformer and cause oil-paper insulation overstressing. As a result of long term operation, oil-paper gradually degrades. In most severe cases, the breakdown of insulation system may occur. In this work, the effect of space charge on breakdown characteristics of aged oil-paper insulation is investigated. Oil-paper samples with different aging states are obtained by thermally accelerated aging for various periods of time, categorized by the degree of polymerization (DP) of 1107 (new), 725, 504, 376 and 241, respectively. Experimental results show that the breakdown voltage of oil-paper samples without pre-stressing remains almost constant with thermal degradation. After the homo-polarity pre-stress, when the DP value is higher than ∼300 (from 1107 to 376), the breakdown voltage has an increasing tendency with an increase in thermal degradation. On the contrary, the breakdown voltage presents a decreasing tendency after the hetero-polarity pre-stress. When the DP is lower than ∼300 (DP241), the changes of breakdown voltage does not remain the same trend as previous samples after pre-stressing. Meanwhile, space charge distributions in different aged samples are measured by the pulsed electro-acoustic (PEA) method, and the trap parameters are correspondingly evaluated. It has been experimentally demonstrated that special attention should be paid to space charge accumulation and aging state of oil-paper insulation, which may lead to severe insulation accidents.

Oil-paper insulation system is an extraordinarily essential system of converter transformers as well as HVDC bushings. When the system changes its state, such as switching the thyristors, operating over-voltage appears, and the charge will deform the dielectric internal electric field, thus easily leading to the occurrence of discharge or insulation failure, so charge accumulation and dissipation on oil-paper insulation under pulse voltage need more investigation. This paper studies the surface charge behavior on double-layer oil-paper insulation under pulse voltage and inducing electrification was used to charge the oil-paper composite. Studies of various voltage amplitudes, pulse frequencies, numbers and polarities showed that the decay is fast initially, and then becomes slower with the time. Eventually the dissipation of charge behaviors tends to be flat. With the increase of voltage amplitudes, the surface potential rises, and less positive charges are accumulated on the surface than negative charges. The increase of the pulse frequency causes the initial values of surface potential a little smaller and the charge to dissipate more quickly. Also, with the increase of pulse numbers, the initial value gets bigger and the surface charge would be slowly dissipated. From the tdV/dt and decay time curves, the figures of positive voltage usually have two peaks, while there is only one peak in most of the curves under the negative one. With the increase of voltage amplitudes, the charge will dissipate more quickly. Additionally, the peak value and the characteristic time increase with the reduction of pulse frequency and the addition of numbers.

An oil-paper insulation system which will withstand DC voltage under normal conditions is a key component of converter transformers as well as high voltage direct current (HVDC) bushings. When the system changes its state, operating overvoltage appears. This kind of overvoltage will consequently affect the application of oilimmersed facilities and the safety of entire grid. Therefore, this paper studies the charge coupling behavior of double-layer oil-paper insulation under DC and pulse voltages. From the research of different pulse voltage amplitudes, pulse numbers and polarity, conclusions can be drawn that, with the increase of the pulse voltage amplitudes, DC and pulse voltages in the same polarity cause the absolute value of the initial surface potential to decrease and then rise; while in different polarities, the charge with the same polarity as the pulse voltage increases. Furthermore, for the same pulse voltage, the absolute initial value of the DC voltage applied in the same polarity is higher than that in different polarities. Considering the pulse number effect, it is obvious that the initial value increases with the increase in the pulse number, and then reaches saturation. Also, with the increase in pulse number, the decay rate decreases and thus the charge is slowly dissipated. From the tdV/dt and the decay time curves, the peak value increases with the increase in the pulse number, while the characteristic time varies by the polarity of the DC and pulse voltages.

This paper presents a novel technique to determine the optimized slope of triangular excitation in frequency domain spectroscopy that can provide better information about condition of oil-paper insulation. In conventional frequency domain spectroscopy, estimation of frictional losses due to the interactions of oscillating dipoles helps in investigating insulation condition. But, different types of dipoles in insulation cannot have similar dielectric relaxation times. Therefore, excitation having multiple frequencies is necessary for the dipoles to undergo more synchronized oscillations. This synchronized oscillation of dipoles reduces the inter-dipolar interactions and provides better information regarding insulation condition. Hence, in this work, triangular excitation is used for frequency domain spectroscopy since its harmonic contents can be varied by changing the slope. The slope of the triangular excitation is optimized to obtain more accurate information about the insulation condition. The optimized triangular excitation for insulation is determined using the proposed equivalent circuit model. To experimentally determine the effectiveness of the model, three test samples having preset moisture content in paper have been prepared in the laboratory. The optimized triangular excitations are then applied to the test samples to estimate their insulation condition. Experimental results show that the use of optimized triangular excitation provides information regarding the condition of oil-paper insulation with greater accuracy. In order to investigate the applicability of the proposed equivalent circuit model, the experiment has also been performed on real-life transformers.

—We have interfaced an array of polymer light emitting\ud diodes (OLEDs) fabricated onto a glass substrate to a sheet\ud of paper via a pressure sensitive conducting adhesive. By screen\ud printing a series of capacitive touch pads and connecting tracks\ud onto paper using a low-cost conductive graphite ink, we are able to\ud drive individual pixels in the OLED array via CMOS-based electronics\ud that are also attached to the paper. Three AA batteries are\ud used to power the CMOS electronics, touch pads and the OLED\ud array, with pixels in the array operating at a brightness of up to\ud 210 cd/m2. The work highlights a practical interface between\ud plastic- and paper-based electronics.

The oil-paper insulation of valve-side windings in a high voltage direct current (HVDC) converter transformer is subjected to both complicated voltage and severe thermal stresses. Partial discharge (PD) occurring in oil-paper bounded cavities is influenced by voltage stress type as well as temperature. This paper presents PD behaviors in an oil-paper bounded gas cavity under pulsating DC voltages. The voltage across the gas cavity and PD inception voltages at different test conditions are analyzed. Influences of temperature on statistical PD distributions and parameters are also presented. A numerical model is used to interpret PD behaviors under pulsating DC voltages and various temperatures. Experimental results and theoretical statements show that the temperature exhibits a great influence on the behaviors of cavity discharge under pulsating DC voltages. Volume resistivity, gas pressure, charge decay time constant, and electron generation rate are the primary parameters influencing PD behaviors at different temperatures.

Knowledge of the condition of power transformer winding insulation paper is fundamental to making optimum asset replacement decisions in the power industry. The ability to assess the aged condition of Kraft paper quickly and non-destructively using portable instrumentation would significantly increase the opportunities for gaining this knowledge. Insulation paper degrades over time in-service and its degree of polymerization (DP) reduces, eventually affecting its mechanical strength. At low DP levels the insulation may start to disintegrate and the risk of electrical breakdown increases. Currently-used methods of estimating DP are either approximate or destructive. The use of spectroscopy together with multivariate statistical analysis (MVSA) provides a powerful non-destructive evaluation of the condition of paper. From initial feasibility studies, we have developed a simple, portable system (TRANSPEC) using fiber-optics and broad-band spectroscopy that can measure the degree of polymerization of various aged transformer papers to a precision of approximately 30 DP units with a spatial resolution of 14 mm. The system can also measure the chemical composition and condition of the insulating mineral oil. MVSA regression models were constructed from library spectral data, and these models are used to predict the DP of other papers with parameters that fall within the range spanned by the set of calibration samples. Separating oil and moisture information from wetted paper is possible and will be reported in a separate publication. With a single TRANSPEC system, non-destructive in-situ analysis of the DP of insulating paper is possible, providing a rapid cost-effective method for transformer insulation condition assessment and monitoring, which correlates well with current destructive methods.

Voltage-time (v-t) characteristic plays an important role during the insulation coordination of a power system. Power transformer is the crucial equipment in the design of a reliable power system. During the operation of a transformer, Oil Impregnated Paper (OIP), the widely used winding insulation of transformer, undergoes significant irreversible aging. Aging alters the dielectric properties of the OIP reducing it's withstand capability. Hence in this work, the impulse v-t characteristic of fresh OIP and 8 years service aged OIP of the same manufacturer are compared. Moreover, when the transformers are directly connected to Gas Insulated Substations (GIS) and during the switching operation of vacuum circuit breakers, OIP is stressed by Very Fast Transient Overvoltage (VFTO). In addition, transformers are subjected to repeated impulses generated during multiple lightning strokes, switching operations of circuit breakers and HVDC converters. Various studies have pointed out the adverse effect of VFTO and repeated impulses on transformer insulation. Therefore, the effects of VFTO and repeated impulses (both LI and VFTO) on the v-t characteristics of fresh and service aged OIP are quantitatively assessed and discussed with reference to the Basic Impulse Insulation Level (BIL) and protection level of the transformer insulation.

In pulp and paper plants, power transformers play a critical role in process continuity. These transformers are subject to internal short circuits, external short circuits, and abnormal operating conditions. The following protection challenges to power transformers will be explored, and methods to improve the protection are provided. First, remanence in a current transformer (CT) may cause misoperation of phase differential protection due to compromised CT performance. Heavy through-faults, sympathetic inrush, and recovery inrush all cause high current. This, combined with high remanent flux, can create a security issue. IEEE CT performance calculations will be used to support the use of dual slope differential characteristics to promote secure differential protection operation when challenged with unequal CT performance. Second, on transformer energizing, the second harmonic current has been traditionally used as a means to prevent phase differential misoperation. Certain transformers may not exhibit high enough the second harmonic, causing a dependability issue if the restraint is set too low. The use of the second and fourth harmonics for inrush detection will be shown to enhance the reliability during energizing inrush situations. Third, overexcitation can occur from abnormal operation of the utility system or the plant's excitation control. Causes of overexcitation will be outlined and the use of volts per hertz protection is explored. With overexcitation occurring from system voltage rise, the phase differential protection has been traditionally blocked using the fifth harmonic restraint. This may cause an undesired nonoperation of the phase differential protection if an internal fault occurs while the transformer is overexcited, causing a delay in tripping and severe damage to the transformer. A technique using adaptive phase differential pickup value will be illustrated to overcome this challenge. Finally, on resistance-grounded power transformers, phase differential protection sensitivity for ground faults near the neutral is decreased. The use of ground differential protection will be explored and the increased sensitivity gained will be demonstrated.

Ground faults in generator stator and field/rotor circuits are serious events that can lead to damage, costly repair, extended outage and loss of revenue. This paper explores advances in field/rotor circuit ground fault and stator ground fault protection. [1] These advanced protection strategies employ AC injection, ground differential protection and the use of hybrid grounding to reduce both internal generator ground fault levels and facility ground levels in utility-paralleled operating modes.

A portable fiber-optic spectroscopic system (TRANSPEC) has been developed for nondestructive measurement of aged transformer insulating paper. Following successful measurement of degree of polymerization (DP) of a range of transformer-aged paper samples, the system has now been shown to separate the oil and paper information for measurement of DP in oil-wetted paper samples. In addition, the system has been shown to be capable of the prediction of both oil and water content of paper to a high accuracy, and is also capable of identifying and quantifying different water species. Spectroscopic measurements have been used together with gravimetric water adsorption measurements to investigate the kinetics of uptake of water vapour into paper from air in the case of nominally dry and oil-impregnated samples. Relationships between water adsorption parameters and properties of the paper insulation have also been investigated using measurements under controlled conditions.

This paper reviews the motor temperature operating consideration for totally enclosed fan cooled (TEFC), open dripproof (ODP) and weather protected II (WP-II) type of motor through 500 HP, both low voltage (LV) and medium voltage (MV). The IEEE Standard 841 has been revised as IEEE-Std 841-2001 and is expected to be in use for at least five years. The TEFC motors covered in this standard have found wide acceptance in the pulp and paper industry. Much of the material also applies to large motors sizes. The thermal impact on the stator winding, rotor core, bearings and lubrications as specified in standards and applied in mills are considered.

Generators are very efficient and are typically quite reliable. This article describes, key preventive maintenance actions, along with recommended cycle times of the generator. Initially, the basic preventive maintenance tasks such as visual inspection of generator components, particularly the stator windings, rotors and wedge system is discussed. An extensive generator health assessment should be done to determine what problems are most severe and need priority attention.

It is a general belief that mineral oil filled power transformers are designed to operate reliably for up to 40 years. However, there are many transformers installed worldwide operating well beyond such age. Therefore, transformer end-of-life should not be simply indicated by the designed 40 years, but determined by the insulation system end-of-life.

The articles in this special section were presented at the Eighth International Telecommunication Union (ITU) Kaleidoscope Academic Conference. The conference took place in Bangkok, Thailand from 14–16 November 2016, in parallel with ITU Telecom World 2016. The topic of the conference was “ICTs for a Sustainable World” and the event was technically co-sponsored by the Institute of Electrical and Electronics Engineers (IEEE) and the IEEE Communication Society.

The articles presented in this special section were presented at the 27th USENIX Security Symposium, that was held 15–17 August 2018 in Baltimore, Maryland.