A degaussing system can be used to reduce the detectability of the magnetic signature of a ship. Commonly, a degaussing system consists of a set of onboard copper coils that produce a magnetic field to compensate for the magnetic signature. High-temperature superconductive degaussing coils are considered an alternative to copper degaussing coils because of a reduction in energy losses, weight, volume, and costs. The losses of a high-temperature superconductor (HTS) degaussing system can be reduced even further by powering it with a cryocooled converter with parallel mosfets. A low-duty cycle and smaller current leads can be used. These solutions eliminate most of the power source losses. This article investigates such a cryocooled converter. The effect of the low switching frequency on the converter performance is tested. A prototype that can operate at cryogenic temperatures was built. The converter powers an HTS coil. It was found that a load current of 50 A can be achieved with a duty cycle of just 0.025 at an input voltage of 3.5 V while still meeting the requirement of a maximum current ripple of 0.5%. At a switching frequency higher than 100 Hz, the converter's performance deteriorates. Also, oscillations were observed in the circuit. This is a problem due to the low blocking voltage of the mosfets. The parasitic inductances in the circuit have a high impact on the performance because the resistance in the circuit is very low.

@en

The Dutch Climate Agreement reflects the ambition to shift to the use of renewably generated electricity by the year 2050. Pursuing this objective, a diverse range of stakeholders like energy companies, governmental organizations and network operators are mobilized for action to deliver their specific contribution. These contributions, the actions by stakeholders, are understood as a result of the plan; this equates to a plan-based understanding of the energy transition. The electricity networks need to be strengthened and extended in the years to come to facilitate the energy transition. For that reason, realizing investment projects is considered as part of the specific contribution to the energy transition network operators have to deliver.

Drawing on an in-depth case study within network operator TenneT, the current chapter foregrounds the practices by which TenneT professionals collectively give rise to the energy transition. These practices move beyond a plan-based understanding of the energy transition. The chapter demonstrates that the practices are open-ended in nature, affecting multiple stakeholders beyond the boundaries of the TenneT organization. Ultimately, the energy transition happens by the simultaneous enactment of a manifold of practices within the organizational contexts of network operators like TenneT and many other stakeholders as well.@en

The present paper discusses probability and diagnostics from a hazard rate bathtub curve perspective. We employed a basic bathtub model representing a single teething, random and wear-out phenomenon. The teething and wear-out processes lead to extreme hazard rates at the start and end of product life. Customers are supposed to endure none of these. The extremes are prevented by quality control (burn-in and testing) respectively timely servicing and replacement. Often Period and Condition Based Maintenance (PBM, CBM) are applied to plan servicing or replacement before failures occur. Hazard rates play a key role in PBM and CBM. These are different hazard rates. PBM uses the actual hazard rate of the asset group failure distribution. In contrast, CBM focuses on individual assets and employs diagnostics to estimate their individual failure probability. The employed prognostic hazard rate is based not only on the asset itself, but also on the particular diagnostic and interpretation of the results. If diagnostics and interpretation can forecast a reasonably accurate time until failure, the prognostic hazard rate is increasing with time. In other cases, a degradation is unambiguously detected, but the method does not allow to forecast a time (well). The prognostic is viewed as more or less constant suggesting random failure. The relationship between Asset Health Index (AHI) and the prognostic failure probability and hazard rate is discussed. From this, it can be shown that the AHI needs a definition on probability in addition to time to failure. Furthermore, three scenarios were studied: normal ageing; accelerated ageing associated with severe conditions like climate change; and disruptive ageing by an impact as with natural phenomena like earthquakes. Finally, the methodology of hazard rate curves deserves more credit than to be seen as a carrier of doom and gloom. The present paper made abundant use of the hazard rate and the bathtub curve to demonstrate the concept and its utility for understanding PBM, CBM and associated diagnostics.@en

Partial discharge (PD) detection is an important tool for evaluating the insulation condition of high-voltage power equipment. This paper investigates a diagnostic system consisting of a magnetic sensor with dedicated electronic circuitry. The investigated sensor is based on the giant magnetoresistance (GMR) effect, which enables the possibility of contactless detection of magnetic fields caused by PD currents. However, the bandwidth of the circuitry can be a limiting factor. In order to exploit the bandwidth of an extra high-sensitivity adapted giant magneto-resistive (xMR) sensor, this paper describes dedicated designs for compensation and signal conditioning of the xMR signals. Typical performance has been evaluated, including frequency response and time-domain response to fast calibrator pulses by the xMR sensor with the implemented circuit. The results show that the xMR sensor with the implemented circuit covers a relatively broad bandwidth and can detect PD pulses with a minimum amount of 10 pC, preliminarily verifying its possibility to detect PDs. Using this xMR system, magnetic signals from an experimental model producing surface discharges have been detected and compared with simultaneously measured signals from a high-frequency current transformer (HFCT). The results show that the xMR system can measure the magnetic fields produced by the PD currents.@en

Partial discharge (PD) detection is essential for evaluating the insulation condition of power equipment. In this paper, the use of a high-sensitivity giant magnetoresistance (GMR) sensor with designed circuits for non-contacting PD detection is presented. Using this GMR system, magnetic signals have been successfully measured that are emitted in laboratory setups of a cross-linked polyethylene (XLPE) cable section and of a gas-insulated system (GIS) with artificial discharging defects. Comparison with simultaneously detected signals from a high-frequency current transformer (HFCT) demonstrates the high sensitivity and performance of the xMR system for PD detection.@en

Partial discharges (PD) detection is an effective diagnostic method to assess the insulation condition of electrical power equipment in the high-voltage laboratory or field tests. This paper presents a non-contacting PD detection method for power equipment. The method is based on an extra high-sensitivity adapted giant magneto-resistive (xMR) sensor that measures the magnetic field produced by the PD currents. Firstly, this paper describes the sensor’s relevant principle and signal conditioning circuit. Next, the sensor’s typical performance, including the frequency response and time-domain response to calibrator PD pulses, is measured and compared with our previous work. The results indicate that the xMR system’s bandwidth is improved to the MHz range. Finally, PD experiments are carried out and compared with measurements using a commercially available high-frequency current transformer (HFCT), which allows for verification of the coherence of the results concerning the PD pulses and phase-resolved PD (PRPD) patterns. The results show that PD in a cross-linked polyethylene (XLPE) cable or a gas-insulated system (GIS) with artificial discharging defects is successfully measured, demonstrating the sensitivity and performance of the xMR system for PD detection.@en

Partial discharge (PD) detection is a standardized technique to qualify the insulation condition in power equipment. The main purpose of the article is to evaluate the performance of an extra high-sensitivity adapted giant magneto-resistive (xMR) sensor for non-contacting PD detection. First, compensation and signal conditioning circuits of the sensor are designed. Frequency response and time-domain response to fast calibrator pulses of the sensor with the implemented circuit are measured. Besides, PD experiments based on corona and surface models are carried out and compared with measurements using a high-frequency current transformer (HFCT). The results show that the xMR system can measure the magnetic fields produced by the PDs at distances up to 50 cm. The correlation between the HFCT and xMR signals is proportional under different voltages, showing that PDs can be effectively detected and evaluated by this method. PDs in a cross-linked polyethylene (XLPE) cable with an artificial discharging defect are successfully measured, demonstrating the sensitivity and performance of the xMR system.

@en

This paper discusses the background of maintenance activities and styles in relation to Industry 4.0 and Reliability 4.0 capabilities with a focus on cables and associated insulators. First the 4.0 wave is introduced in comparison to the preceding waves. Next the basic of maintenance strategies are discussed in terms of the hazard rate bathtub curve stages: teething, random failure, early failure (i.e., fast wear) and normal wear. Subsequently, the maintenance activities inspection, servicing and replacement are discussed for the three main styles corrective, period based and condition based maintenance (respectively CM, PBM and CBM). The principle of renewal by resetting the hazard rate with PBM and CBM and their limitations are addressed. It is argued that none of the maintenance styles is superior to the other styles. The merits of each are situation dependent. The utility task is not confined to managing individual assets, but to maintain a power supply system. In this respect, repair strategies and redundancy are important aspects that have to be considered with the choice of maintenance. Reliability Centered Maintenance (RCM) is a methodology to optimize the maintenance mix given the system configuration, criticality and capabilities. The concept of the 4.0 wave and RCM is then discussed in relation to three prime failure modes in cable systems: water treeing, partial discharge and thermal breakdown. The final discussion addresses the lifetime of auxiliary components that are required for the 4.0 wave capabilities, but may have a shorter lifetime than the planned lifetime of smart assets.@en

The time-to-failure for oil-impregnated paper (OIP) insulation is governed by two primary aging mechanisms: electrical and thermal. The electrical life can be represented as an Inverse Power Law, where lifetime is inversely proportional to applied electric field. The process of thermal aging on the other hand is established by Arrhenius Law, which relates the rate of aging exponentially to temperature. Due to thermal aging, the structure of insulation is altered owing to chemical changes like oxidation, polymerization, and cellulose degradation. For life estimation of a service-aged high-pressure gas filled (HPGF) cables, electrical endurance tests are normally performed at controlled voltage levels to estimate the time to breakdown. However, it is equally necessary to investigate how thermal aging influence changes in the electrical life of insulation. Therefore, in this paper, firstly short-term ramped stress tests are carried out on elevated thermal aged OIP samples extracted from already field-aged HPGF to find a rough estimate of breakdown voltages at different temperatures. Then, long-term electro-thermal step stress tests are performed on the samples to establish a correlation of temperature on the electrical life of the OIP insulation. The long-term stress tests produce reliable breakdown statistics and Maximum Likelihood Estimation of Inverse Power Law fitted on 2-parameter Weibull distributed breakdown data indicate a reduction of model parameter, n from 13.61 to 7.38 with an increase in temperature from 45 to 75 °C and a constant shape factor, beta of 1.50. The dissipation factor, tandelta related to the aging also shows an increase with temperature across a wide frequency range and is inversely proportional to the breakdown voltage.

@en

Elevated thermal stress-related aging is significant on the oil-impregnated paper (OIP) used as insulation in high-pressure gas cables (HPGCs). The aim of this article is to develop a cheap alternative for lab dielectric measuring and characterizing temperature-dependent parameters for OIP. First, this article derives the operating thermal conditions of the grid-aged cable based on IEC standards after analyzing the loading data using machine learning techniques to determine the elevated temperature levels for the experiments. Second, a novel lab-fabricated inexpensive electronics circuit is developed for polarization and depolarization current (PDC) measurements which can be adapted for such measurements over expensive commercial devices. From the measured parameters, an extended three-branch Debye model is optimized using a developed error function approach based on the Akaike information criterion (AIC) and goodness of fit. The model indicated a reduction in the branch resistance with temperature elevation and aging, whereas the branch capacitance revealed an increasing trend. The resultant relaxation time (RC) showed a decrease overall. Last, a short-duration frequency domain spectrum was analyzed and extrapolated to obtain parameters for a wide range of frequencies and fit in a Cole-Cole model, derived for oil-paper insulation. The time constants obtained from this model also confirmed a reducing trend across the temperature and aging variations and the model parameter, the alpha coefficient showed a decreasing trend. Last, the effect of the measured dielectric parameters is reflected with breakdown values to investigate the effect of temperature on the electrical life of insulation.

@en

This paper describes Weibull data analytics in support of grid resilience as well as interpreting test and R&D results. The target was to develop a widely accessible spreadsheet IDA application that can be used on relatively light computing devices such as mobile phones and tablets. This became possible by the use of adjusted ranking with suspended data and development of high accuracy approximations of plotting positions and linear regression weights. A coherent methodology was developed for plotting, parameter estimation and various types of inferences in addition to data analytics. These are: quantifying the similarity of failure processes by their distributions, optimization of period based maintenance cycles, evaluation of expected time and confidence limits for a next failure. These are all integrated into IDA which is made available as freeware for common benefit.@en

Ships can avoid to be detected by magnetic mines by reducing their magnetic signature with degaussing coils. Degaussing currents are provided by switched mode power supplies which impose a current ripple on top of the degaussing current. The ripple might be visible in the magnetic signature which would increase the detectability of the ship. A way to reduce the ripple in the magnetic field is to use a switching modulation scheme in the degaussing power supplies. In this paper, a magnetic model of a ship with degaussing coils is described. It is used to find the magnitude of the ripple in the magnetic signature. Also the effect of reducing the current ripple by frequency modulation is investigated. Several modulation schemes are modelled. It is found that the ripple in the magnetic signature is often, but not always, negligible due to attenuation by the ship’s hull. For low frequency switching applications, like high temperature superconductor degaussing systems, the ripple is visible in the magnetic signature. It is found that switching frequency modulation is a very effective technique to reduce the ripple of degaussing currents. Of the tested schemes, random lead lag and random switching frequency are the most effective.@en

The here reported work is part of a project on supporting grid resilience by asset management techniques. The present work focuses on support of decision-making after a few failures occurred that may be the start of many more. Methods are reviewed and new algorithms developed where the present IEEE/IEC standard does not provide. Two cases of early failures and wear-out are analyzed as examples for the data analytics.

@en

Detection of the magnetic signature of ships can be avoided by using a degaussing system; a set of on-board copper coils that compensates for the magnetic signature. High temperature superconductors (HTS) are currently investigated as a replacement for copper degaussing coils. By using HTS, we have to deal with higher currents and therefore with higher power supply losses. Also, large current leads are needed which introduces extra losses. This paper investigates different possible solutions to minimize these losses. Four H-bridge-based MOSFET topologies are presented that were designed to reduce the power supply and current lead losses. The first topology uses an H-bridge configuration so that the degaussing current can freewheel through the low-resistance MOSFETs. The second topology places the H-bridge inside the cryostat so that the current leads can be made smaller. The third topology includes a smoothing capacitor in the cryostat so that the current leads and input current are even smaller. The fourth topology uses a transformer so that the current leads can be eliminated. Measurements were done to determine the MOSFETs and capacitor performance in liquid nitrogen. The simulated losses of the four topologies are compared to determine the most energy-efficient option for supplying current to the HTS coils. It was found that by submerging multiple parallel MOSFETs in liquid nitrogen, the on-state resistance is decreased and the current supply can be made more efficient. Also, by placing a smoothing capacitor inside the cryostat, the current lead losses can be minimized significantly. The benefits of using a transformer do not outweigh the transformer losses.

@en

Weighted Linear Regression (WLR) can be used to estimate Weibull parameters. With WLR, failure data with less variance weigh heavier. These weights depend on the total number of test objects, which is called the sample size n, and on the index of the ranked failure data i. The calculation of weights can be very challenging, particularly for larger sample sizes n and for non- integer data ranking i, which usually occurs with random censoring. There is a demand for a light-weight computing method that is also able to deal with non- integer ranking indices. The present paper discusses an algorithm that is both suitable for light-weight computing as well as for non-integer ranking indices. The development of the algorithm is based on asymptotic 3-parameter power functions that have been successfully employed to describe the estimated Weibull shape parameter bias and standard deviation that both monotonically approach zero with increasing sample size n. The weight distributions for given sample size are not monotonic functions, but there are various asymptotic aspects that provide leads for a combination of asymptotic 3-parameter power functions. The developed algorithm incorporates 5 power functions. The per- formance is checked for sample sizes between 1 and 2000 for the maximum deviation. Furthermore the weight distribution is checked for very high simi- larity with the theoretical distribution.@en

Testing materials and devices for a specific failure mechanism usually yields a characteristic distribution of failure times. However, failure of power electronics (PE) can be due to a manifold of mechanisms. Even in a single PE device multiple failure processes may compete. For instance, modules may contain various components that fail according to different processes. When testing a batch of devices with competing processes, the resulting failure distribution is likely to consist of a combination of distributions, also called ‘entangled’ distributions. If processes compete, the time of failure according to the one process masks the time of failure according to a competing process. The observed failure time for one process leads to a so-called censored failure time of the competing process. If there is a balance between occurring failure processes, data analytics should not only involve the observed, but also the censored failure times. Another cause for distributions to blend, is the test of inhomogeneous test batches. For instance, some test objects may contain a production flaw, while other objects don’t. Such a mix of (sub) populations will also yield a total distribution consisting of entangled distributions. Enhanced stresses in a test are likely to accelerate the various aging processes at different rates. Awareness of distribution entanglement and acceleration is useful for interpreting test results and developing effective testing methods. The present contribution reviews the statistical analysis of R&D data. It discusses competing processes, mixed populations, censored failure data, accelerated aging, and how these phenomena affect analytics by graphical analysis and parameter estimation. Finally, series and parallel system configuration and reparability aspects are discussed.@en

This paper presents the investigation on surface discharge behavior of various dielectric samples under DC. It sequentially develops the knowledge base for the study and analysis of the partial discharge (PD) defect with the goal of PD defect identification under DC. In order to facilitate this, the material properties of the dielectric are measured. Finite Element (FEM) simulation is used to obtain the preliminary estimates of the electric field and dielectric properties that concern partial discharge behavior. The DC-PD tests performed on the surface dielectric samples demonstrate a highly plausible behavior based on simulation results and other literature. It also displays a great degree of similarity towards the AC surface discharge behavior. The paper concludes by presenting novel partial discharge fingerprints for the surface PD defect that will aid in defect identification under HVDC.@en

This paper discusses a method for data analysis of early failures that are typically due to defects from production and/or installation. The approach is based on Weighted Linear Regression and aims at estimating the next failure moment and the failure probability in an interval. As an additional goal, methods are intended to be suitable for light computing devices.@en

High voltage superconducting power apparatus and systems have been developed in the world, e.g. a 275 kV high temperature superconducting (HTS) cable in Japan or a 220 kV HTS fault current limiter in Russia. CIGRE SC D1 has so far established WGs D1.15 and D1.38 for the state-of-the-art R&D on superconducting power apparatus and systems and published TBs No. 418 (June 2010) and No. 644 (December 2015), respectively. Especially in WG D1.38, HTS materials, electrical insulation, and cryogenics were focused as the common and critical factors for successful R&D and practical development of superconducting power apparatus and systems. From the above technical background, a new WG D1.64 was established in 2016 to focus on the electrical insulation systems at cryogenic temperatures as one of the common and critical issues described in the above TB No. 644. Electrical insulation systems at cryogenic temperatures have been recognized as one of the key technologies for the efficient, reliable, and practical development of superconducting power apparatus. Besides the superconducting power apparatus, the electrical insulation systems at cryogenic temperatures will also be helpful for fusion reactors, accelerators, and industrial applications. This TB of WG D1.64 discusses the fundamentals and applications of the electrical insulation systems at cryogenic temperatures. Not only the dielectric properties and physical mechanisms of insulation materials at cryogenic temperatures, but also thermomechanical properties are discussed in Chapters 2, 3, and 4. The state-of-the-art R&D projects on superconducting power apparatus and their design, test, and failure experiences are introduced in Chapters 5 and 6. The main results in each chapter can be summarized as follows: Chapter 2: Dielectric properties of insulation materials at cryogenic temperatures Dielectric properties and data of insulation materials (gases, liquids, solids, and new insulation materials) at cryogenic temperatures are summarized. Cryogenic gases still follow Paschen’s law, if it is related to the respective density of the gas, instead of pressure. A new approach for universal Paschen diagrams for typical cryogenic gases is presented. For certain applications, mixtures of gases are more suitable than their pure counterparts. Liquid nitrogen is the most studied and used liquid insulation material for HTS power applications. The dielectric strength of liquid nitrogen is greatly reduced by bubbles, which move in the liquid influenced by buoyancy or electric field during the device operation. The incompatibility of room temperature solid insulation materials and techniques for cryogenic temperatures is primarily due to the additional mechanical stresses and microstructural changes present at cryogenic temperatures. Most new solid materials are polymeric nanocomposites. Significant progress has been made in preparing and improving the electrical properties of conventional polymeric materials with nanometre size particle fillers for cryogenic applications. Other composites can also improve dielectric properties, such as the combination of Tyvek with polyethylene and functionally graded materials. Chapter 3: Discharge characteristics and mechanisms The chapter discusses the discharge processes of the insulation medium, which have been identified and influence the breakdown and partial discharge characteristics and their dependence on electric field non-uniformity, which is defined with the utilization factor η: uniform (η > 0.9), quasi-uniform (0.9 ≥ η > 0.6), weakly non-uniform (0.6 ≥ η > 0.3) and strongly non-uniform (η ≤ 0.3). The chapter starts by describing how the gas breakdown behaviour changes from room temperature to cryogenic temperature and how the cryogenic gases are influenced. Although the discharge theory for liquids is less well established, the state-of-the-art discharge mechanisms of liquid nitrogen are presented in uniform, quasi-uniform, and non-uniform electric fields, respectively. The discharge processes in solid insulation materials are discussed in terms of the effects of structure, temperature, applied voltage, magnetic field, irradiation, and interfaces of different materials. Ageing mechanisms usually for about 30 years under electrical stress and cryogenic conditions are addressed with the statistical and lifetime calculations. Chapter 4: Mechanical properties and fatigue under thermal stress Mechanisms of a thermally-induced mechanical breakdown are described in this chapter. Exact control of the thermomechanical behaviour during cool-down/warm-up cycles is essential for a normal operation of superconducting systems. Quite often, the design of the insulation system for superconducting equipment is a compromise between appropriate dielectric and thermomechanical performance determined by thermal conductivity, thermal expansion, and mechanical properties at cryogenic temperature. Thermal and mechanical properties of cryogenic materials are summarized with the fatigue under thermal stress. Practical examples are also introduced, e.g. huge heat conductivity of sapphire in fault current limiter (SFCL) technology, adjusting thermal expansion of epoxy resin for redundant superconducting coils and thermal cycling stability, and the effect of polyimide insulation on the improvement of heat transfer. Chapter 5: Experience in design of insulation systems and devices From the application viewpoint, this chapter starts with the insulation of superconducting wire and tapes and continues with superconducting devices and systems. Polyimide is used in a wide variety of cryogenic applications and in particular for insulating HTS wires and tapes. High performance polyimide may offer superior electrical and mechanical performance, easy application, and flexibility for different voltage withstand needs. Enamel made out of polyvinyl acetate, polyetherimide, or polyurethane varnishes is used for coil windings of electrical motors, transformers, MRI/NMR magnets, particle colliders, or high energy physics equipment. Design experience of superconducting devices and systems has been accumulated primarily for cables and SFCL. For superconducting cables with electrical insulation layers composed of liquid nitrogen and polypropylene laminated paper (PPLP) there are lots of design experiences for both AC and DC. Recently, gas cooled superconducting cables are investigated, where a 4 mol% GH2 and 96 mol% GHe mixture possessed about 80 % higher AC and DC breakdown strength compared to pure GHe at various pressure levels at 77 K. Shrinkage at long cable length is also described in this chapter. Among several types of SFCLs, resistive SFCLs are more demanding in terms of insulation design. When a fault current flows in a resistive SFCL, a quench of HTS materials may occur and generate bubbles in liquid nitrogen, hence so-called dynamic breakdowns can be induced by the bubbles under the operating voltage of SFCL. The assumption of continuous thermal stress could be too conservative. To rationalize the dielectric insulation design for SFCLs, the study of dynamic breakdown is therefore important. Design experience of other superconducting devices such as transformers, rotating machines, and magnets is summarized with state-of-the-art worldwide projects. Chapter 6: Dielectric testing of cryogenic insulation systems Testing methods of superconducting devices and systems are introduced mainly for cables and SFCLs. For superconducting cables, shipping tests and performance tests on site are described. The dielectric withstand of resistive SFCL has to be verified both in steady-state conditions and in the dynamic regime, i.e. during current limitation. Dielectric test experience in steady-state conditions under AC and DC SFCL is described. Validating the dielectric withstand in the dynamic regime requires power tests under short-circuit conditions. During these power tests, the internal insulation will be verified at the same time as the current limiting performance and the general withstand of the device, which has not yet been authorized in the testing methods of SFCLs. Test experience of transformers, DC reactors, fusion magnets is also described. In many cases, not limited to low-temperature insulation technology, reports on design development, experimental research, or operation of a device or equipment indicate successful results achieved according to the initial design or schedule. In contrast, few reports describe unexpected trouble. When trouble occurs, it is often difficult to investigate and identify the cause. However, for subsequent R&D, it is important to examine failure experiences. Investigations into failure experiences associated with cryogenic insulation are collected in this chapter.@en

Dielectric elastomer actuators (DEAs) are a class of electrostatic actuators that have promising applications in fields like sensors, soft-robotics, microfluidics, and energy harvesting. The crucial points in the working principle of DEA are the application of high electric field and the use of compliant electrodes. These electrodes are typically composed of a mixture of a soft polymer base filled with highly conductive particles. In this work, we show that the charged impurities, possibly present in the electrodes composite, in combination with a high electric field can cause the formation of polarization interface layers between the electrodes and the inner dielectric. These layers can, in the long term, diminish the actuation performance of the DEA.

@en