Core Insulation Research Articles

Quantitative inner heat transfer for cable fire: a numerical study and effects of core diameter and insulation thickness

Quantitative inner heat transfer for cable fire: a numerical study and effects of core diameter and insulation thickness

Flame-retardant wire burning behavior by jet flame heating: Ignition, charring, and secondary flame spread

This research presents an in-depth examination of the combustion characteristics of flame-retardant electrical wires, contrasting behavior with non-flame-retardant counterparts under jet flame heating. The study systematically investigates the pyrolysis process, ignition patterns, charring behavior, and the development of secondary flames in wires with different core diameters and insulation materials. Findings indicate the heightened susceptibility of thinner wires to rapid heating, pyrolysis and charring, leading to faster ignition and more intense secondary flame development. This insight is crucial for tailoring flame-retardant formulations to specific wire dimensions. The research also delves into the thermal dynamics within the wires, highlighting how the core diameter influences axial heat conduction and, consequently, the overall flame spread behavior and pyrolysis rate. A critical discovery is the relationship between heating duration and flame sustainability, establishing a specific range of heating times for sustaining secondary flames in flame-retardant wires. Theoretical models used in the study explained the critical heat flux heating time for wire ignition, offering insights into improved fire prevention strategies, particularly in prolonged heat exposure scenarios. These findings not only advance our understanding of flame-retardant wire behavior under fire conditions but also provide guidance for selecting and using electrical wires, thereby optimizing fire safety in diverse applications.

A review on the various factors influencing the structural behaviour of TRC sandwich elements

This review article discusses the structural performance of Textile Reinforced Concrete (TRC) sandwich structures consisting of two outer TRC faces and a rigid core insulation. Sandwich structures offer advantages such as lightweight construction and thermal insulation. Hence, the application of the non-corrosive TRC as the facings of sandwich panels helps in reducing the overall thickness and weight of the panels by replacing the reinforced concrete faces and eliminating the thick concrete cover. The strength, stiffness and load-carrying capacity of TRC sandwich panels are governed by several factors such as thickness of the face, density of the core material, reinforcement ratio, the use of shear stiffeners, etc. In this review article, the influence of various parameters such as face thickness, type of textile, number of textile layers, core thickness and core density are addressed. The effect of shear connectors on enhancing the composite behaviour of TRC sandwich panels is also discussed.

An experimental investigation of flames spread interaction over two parallel electrical wires of various separations

This paper investigated experimentally the interaction behaviors of flames spreading over two parallel adjacent electrical wires at various separation distances. Experiments were carried out with thin electrical wires of copper inner core and polyethylene (PE) insulation. The separation distances between the two parallel electrical wires (their core center distance) were ranged from 1 mm (flames merge) up to 20 mm (flames completely separate with behavior identical to that of one single electrical wire) to reveal the transition of the flame behaviors with increasing separation distance. The flame base width along the wire (Wf), flame height (Hf), average mass loss rate (AMLR, m˙) and flame spread rate (FSR, Vf) were measured to quantify this transition. Results showed that the flame height, the AMLR and the FSR all increased first and then decreased with increasing of separation distance. The FSR and AMLR were maximized at the same separation distance during the transition of flames separating from its base to complete separation with increasing separation distance. Five regimes were identified for the variation transition of flame spread rate with increasing separation distance, which can be interpreted physically by the change of convection and radiation feedback with separation distance. The maximum FSR of the wire-pair, which occurred at the separation of about 8∼10 times of the wire outer diameter, was around 12% ∼22% greater than that over one single wire. Such FSR increment was more remarkable if the wire size was larger. This experimental study provides fundamental knowledge, quantitative data and physical understanding of the phenomenon of flame spreading interactions over two parallel adjacent electrical wires.

Structural behavior of FRP connector enabled precast geopolymer concrete sandwich panels subjected to one-side fire exposure

Precast concrete sandwich panel (PCSPs), consisting of two reinforced concrete (RC) wythes (i.e., interior and exterior), a core insulation layer and connectors, can help to improve the building energy efficiency. This study is concerned with the fire and post-fire residual performance of an innovative PCSP system, in which fiber-reinforced polymer (FRP) reinforced geopolymer concrete is used as the two wythes and FRP tubes are used as the connectors. Six PCSP specimens were fabricated and five of them were tested under one-side fire exposure. The experimental parameters included the concrete type (geopolymer and conventional concrete), the reinforcement type (basalt FRP reinforcement and steel reinforcement), the connector type (plate-type and hexagonal tubular type), and the RC wythe thickness. After the fire exposure, all the specimens were tested under concentric loading to evaluate their residual axial load capacity. In addition, numerical analysis was conducted to facilitate a better understanding on the experienced temperature field of the PCSPs. It was found that the PCSPs with geopolymer concrete exhibited a decrease of the axial load capacity by 88.5% after 4 h one-side fire exposure while they performed better than that with conventional concrete. Besides, the reinforcement type, RC wythe thickness and the connector type are key factors influencing the post-fire structural performance of the PCSPs.

Research on in-situ condition preserved coring and testing systems

As shallow resources are increasingly depleted, the mechanics’ theory and testing technology of deep in-situ rock has become urgent. Traditional coring technologies obtain rock samples without retaining the in-situ environmental conditions, leading to distortion of the measured parameters. Herein, a coring and testing systems retaining in-situ geological conditions is presented: the coring system that obtains in-situ rock samples, and the transfer and testing system that stores and analyzes the rocks under a reconstructed environment. The ICP-Coring system mainly consists of the pressure controller, active insulated core reactor and insulation layer and sealing film. The ultimate bearing strength of 100 MPa for pressure-preservation, temperature control accuracy of 0.97% for temperature-retained are realized. CH 4 and CO permeability of the optimized sealing film are as low as 3.85 and 0.33 ppm/min. The average tensile elongation of the film is 152.4% and the light transmittance is reduced to 0%. Additionally, the pressure and steady-state temperature accuracy for reconstructing the in-situ environment of transfer and storage system up to 1% and ±0.2 is achieved. The error recorded of the noncontact sensor ring made of low-density polymer is less than 6% than that of the contact test. The system can provide technical support for the deep in-situ rock mechanics research, improving deep resource acquisition capabilities and further clarifying deep-earth processes.

Thermal Performance and Insulation Structure Design of ± 400kV Valve Side Bushing of Converter

The design features of inner and outer insulation structure of valve side bushing of ± 400kV converter transformer are representative. Its main insulation structure type and design method are basically consistent with those of ± 800kV converter transformer valve side bushing in China, but its design margin is lower than that of UHV converter transformer bushing, and the valve side bushing of ± 400kV converter transformer is used more in converter valve hall, so its insulation structure design and engineering should be It is necessary to analyze and discuss the insulation structure design of valve side bushing of ± 400kV converter transformer. In this paper, ± 400kV converter transformer valve side bushing insulation accident is analyzed in detail, from which the specific causes of insulation failure are summarized, providing reference for the design of internal and external insulation structure of converter transformer bushing. Based on this, this paper first analyzes the heating mechanism of the double guide rod structure of the converter bushing, gives the design size of the double guide rod structure from the theoretical analysis point of view, further optimizes the core insulation structure of the converter bushing, and gives the external insulation design scheme of the converter bushing from the perspective of internal and external insulation coordination, and checks and calculates the overall electric field distribution The radial electric field strength is controlled at 3.11kV/mm under working voltage and 0.51kV/mm under power frequency withstand voltage, which meet the requirements of electric field strength control in the design of ± 400kV converter bushing. Furthermore, the valve side bushing of the developed ± 400kV converter transformer is tested, and typical type tests such as power frequency dry withstand voltage test and partial discharge measurement, lightning impulse dry withstand voltage test and partial discharge measurement and temperature rise test are passed. The research results of this paper theoretically give the control requirements for the field strength design of the valve side bushing of converter transformer, and optimize the design size of its double guide rod structure, and develop a prototype to prove the effectiveness and reliability of the theoretical analysis. The above research results have good theoretical guidance value and practical significance for the insulation structure design and operation state maintenance of valve side bushing of ± 400 kV converter transformer.

Research on Defect Detection Technology of Composite Insulator Sheath

The composite insulator sheath plays an important role in protecting the core rod and insulation. The thickness of the sheath must meet the corresponding standard requirements. During the insulator manufacturing process, the core rod decentration leads to uneven thickness of the sheath, which is easy to cause the risk of composite insulators premature failure during operation. The phased array detection technology can accurately detect the thickness of the sheath, so that defects such as uneven thickness of the sheath (decentration of the core rod) can be detected, and the thickness of the sheath at the end of the composite insulator can also be effectively detected. DR detection technology can effectively identify the interface between the mandrel and the sheath, but there is a risk of the sheath thickness distortion due to inappropriate transillumination parameters.

Analytical Evaluation of Core Losses, Thermal Modelling and Insulation Lifespan Prediction for Induction Motor in Presence of Harmonic and Voltage Unbalance

Electrical motor are the ubiquitous workhorses of the industry, working over wide range of conditions and applications. Modern motors, designed to exact ratings using new materials and improved manufacturing techniques, are now much smaller but have higher loadings. They are being operated much closer to the point of overload than ever before. To ensure a satisfactory lifespan for the motor, temperature rise must be limited to the safe values. This paper proposes an analytical approach to estimate core losses of induction motor supplied by either harmonic content or voltage unbalance source. A model based on the thermal lumped parameters is introduced and used to predict the insulation lifespan of induction motors. Lumped parameters network of the motor is developed based on dimensions of the motor, thermal resistances, thermal capacitances, and loss sources. Then, the model is used to estimate the temperature in different parts of the machine and its insulation lifespan. Finally, the predicted results are verified by experiments.

Electrical and Mechanical Condition Assessment of Low Voltage Unshielded Nuclear Power Cables Under Simultaneous Thermal and Mechanical Stresses: Application of Non-Destructive Test Techniques

Low voltage cables play a crucial role in the safe and reliable operation of nuclear power plants. This paper discusses the effect of simultaneous combined thermal and mechanical stresses on the overall insulation of unshielded low voltage power cables used in nuclear power plants. Cable samples based on XLPE/CSPE insulation were exposed to thermal-mechanical combined accelerated aging tests for an extended period lasted to 907 hours. Time and frequency domain dielectric spectroscopy were applied to reveal the degradation level of the core insulation and jacket due to combined stresses. The former technique was conducted through the measurement of the decay and return voltage slopes, while the latter technique was based on the measurement of the complex permittivity. Moreover, the mechanical properties of the cable insulation were investigated by applying the measurement of the Shore D hardness. The results showed that the combined aging affects both the electrical and mechanical properties of the overall insulation. The complex permittivity at 100 Hz, the decay voltage slope, and the hardness had an upward trend with aging, while the return voltage slope showed an inverse trend. In addition, the results of the electrical and mechanical measurements showed strong correlation with aging time and further correlation was observed between the dielectric and mechanical properties. The correlation between aging time and test results suggests that the presented test methods are promising tools for in situ condition monitoring of low voltage unshielded cables in nuclear power plants.

Development of sandwich panels for multi-functional strengthening of RC buildings: Characterization of constituent materials and shear interaction of panel assemblies

This paper presents an experimental and numerical study aiming at the development of a sustainable and multifunctional composite sandwich panel for the rehabilitation of reinforced concrete (RC) buildings from the 1960s to the mid-1980s. The sandwich panel, which was designed for the structural, thermal and acoustic refurbishment of building facades, comprises three main components: (i) thin outer layers of Recycled Steel Fibre Reinforced micro–Concrete (RSFRC) that fulfil the strength, ductility and durability requirements of the panel; (ii) a lightweight core made of polystyrene that provides thermal insulation; and (iii)internally distributed glass fibre reinforced polymer (GFRP) connectors that join the different layers of the panel, providing an adequate structural behaviour to the composite system. The mechanical characterization tests highlighted the viability of using RSFRC for the production of structural sandwich facade panels, as relatively high post-cracking tensile capacity was obtained for thin RSFRC layers. Pushout and pullout tests were carried out on intermediate-scale specimens representative of the sandwich panel solution for assessing the overall composite behaviour of the sandwich panels and analysing the influence of the type of core insulation layer (expanded/extruded polystyrene cores, with different surface finishing), of the anchoring conditions (25 and 35 mm of embedment depth) and diameter of the GFRP connectors (8 and 12 mm). These tests showed that the structural GFRP connectors with diameters of 8 and 12 mm are able to ensure shear load transfer between RSFRC layers, exhibiting better composite behaviour when combined with anchorage depths of 25 and 35 mm, respectively. The numerical part of this study aimed at modelling the failure mechanisms observed at the interface between RSFRC and polystyrene, showing good agreement between experimental and numerical results, with important conclusions being drawn regarding cohesion and friction angle between these materials.

Development of Epoxy with Nano and Micro Fillers for Core Insulation of Composite Insulators

Epoxy composites are extensively used in different applications in high voltage engineering. However, the use of these composites for outdoor insulators has been challenging due to complexities arising out of the conditions of operation of the insulators. Pollution performance of composite insulators is influenced by the nature of the pollution existing, in addition to the combination of stresses acting on the insulators. The use of nano and micro combination of fillers in the epoxy composites is an ideal choice for achieving a proper balance of electrical, thermal and mechanical properties. This paper considers the merits of using combination of micro and nano fillers in an epoxy matrix to demonstrate that such insulation systems would provide better alternatives to the conventional epoxy systems for use as central rod in composite insulation systems for severe contaminated conditions.

Testing the applicability of LDPE/HNT composites for cable core insulation

Abstract Composites prepared from low-density polyethylene (LDPE) and tubular halloysite nanotubes (HNTs) with different contents of HNTs were comprehensively tested. HNTs were utilized to suppress the intensity of heat release from the ignition of LDPE. The results revealed that the large polarity discrepancies and high levels of chemical inertness of LDPE and HNTs led to a reduced level of HNT dispersion in the polymer matrix. Nevertheless, even for small quantities of HNTs, differential scanning calorimetry (DSC) confirmed the decrease in the intensity of heat release during the LDPE/HNT thermal decomposition. Mechanical tests demonstrated that the presence of HNTs did not significantly influence the yield strength but did contribute to toughening of the LDPE matrix. Measurement of the dielectric properties revealed that the presence of HNTs also contributes to an increase in the number of permanent dipoles and charge carriers. However, the non-polar nature of the LDPE matrix can effectively suppress this disadvantage.

Diamond enriched lamination and winding insulation for electrical machines

PurposeThe thermal management of electrical insulations poses a challenge in electrical devices as electrical insulators are also thermal insulators. Diamond is the best solid electrical insulator and thermal conductor. This can lead to a paradigm change for electrical machine winding and lamination insulation design and thermal management. The paper introduces these techniques and discusses its effect for the design of electrical machines and its potential consequences for electromagnetic analysis, for example, in multi-physics modelling. The diamond winding insulation is patent-pending, but the diamond enriched lamination insulation is published for the benefit of the scientific community.Design/methodology/approachThe windings of electrical machines are insulated to avoid contact between the coil and other conductive components, for example, the stator core. The principle of using mica tape and resin impregnation has not changed for a century and is well established to produce main insulation on a complex conductor shape and size. These insulations have poor heat-conducting properties. Similarly, the insulation of laminated steel sheets comprising the stator and rotor restrict heat flow. Diamond-based insulation provides a new path. Increased thermal conductivity means reduced temperature rise and the reduced thermal time constants in multi-physics simulations and system analysis.FindingsThe largest benefit of a diamond-based core insulation is in electrical machines in which the losses are conducted axially to the coolant. These are machines with radial ducts and effective cooling in the end regions. The main benefit will be in reducing the number of radial ducts that positively affect the size, production costs and the copper losses of the machine. The increased thermal conductivity of the diamond insulation system will reduce the thermal constants noticeably. These will affect system behavior and the corresponding simulation methods.Originality/valueDiamond insulation can lead to a paradigm change for electrical machine winding and lamination insulation design and thermal management. It might also lead to new modeling requirements in system analysis.

Features of thermal ageing of cables with different type of insulation

A wide range of temperature and duration of cable routes operation requires the development of objective methods for assessing the properties of cable insulation. While working, preparing and repairing cable routes, it is important to use express methods to determine the technical condition of insulation, to search for diagnostic parameters indicating the process of crack formation. An ongoing monitoring of cable insulation with standard devices controlling electrical resistance and leakage currents is not sufficiently effective for evaluating the performance and predicting the properties of cable insulation. The study is aimed at further testing of the technique by analysing the properties of materials used in cable engineering, as well as comparing the results obtained by various devices manufactured by national and foreign companies. Tests were carried out to assess the quality of insulation based on the monitoring of hardness of the hose insulation and the conductor insulation of KNR and KNREk cables. In the KNREk type cables, the core insulation is also made of the RTI type rubber, but the hose insulation is created on the basis of a material from polyvinyl chloride compound. The method of assessing the quality of insulation and assessing the degree of ageing of cable material is implemented by measuring the hardness of insulation material using the Shore method using portable hardness testers of the following types: TH-200, NOVOTEST TS D-A (TS D-D) up to 100 HSА (HSD). It is shown that the proposed technique makes it possible to identify objective statistical parameters of the material hardness and to diagnose the quality of insulation in operation without dismantling cable routes. The hardness parameters of hose insulation from polyvinyl chloride compound can be controlled by TS D-D devices. The heat resistance of hose insulation of the KNREk type cables is higher than that of the cables of the KNR type, which creates opportunities for their longer operation at high temperatures.

Detection of Insulation Defects in Automated In-Process Testing of Electric Wire during its Extrusion

The paper describes the electro-capacitive method for monitoring wire capacitance, which is implemented using the CAP-10 device, employed for in-process testing of the single core electric wire capacitance. Focus is made on the operating principle of the CAP-10 device. The possibility of using the CAP-10 device for detecting local defects in wire insulation is proved. Insulation defects such as foreign inclusions in the form of copper shavings, air cavities inside insulation and those at the core–insulation boundary are modeled. The impact of the defect geometric parameters on the wire capacitance measured during in-process testing is investigated through numerical simulation. Mathematical simulation results are validated through the physical model using the CAP-10 device.

The interaction between fuel inclination and horizontal wind: Experimental study using thin wire

Forward flame spread behavior changes significantly with both the fuel inclination and the wind velocity, but interactions between the inclination and horizontal wind is still not well understood. In this work, a controlled laboratory experiment is conducted in a wind tunnel providing a horizontal wind ranged from 0 (quiescent) to 2.5 m/s, and a thin electrical wire as fuel that can be inclined from a horizontal (0°) to a vertical (90°). An electrical wire of 0.8-mm diameter with copper core and polyethylene insulation was used as the characteristic thin fuel. The flame-spread rate, as well as the flame geometrical characteristics, was quantified as a function of the wind velocity and wire inclination. Results show that as the horizontal wind velocity is increased, the flame-spread rate over the wire for all inclination angles first increased to a maximum, and then, slightly decreased until blow-off. The critical inclination for flame acceleration decreases from 45° to 15° as the wind velocity increases. The fastest flame spread along the inclined wire occurs when the wind pushes the flame parallel to the fuel. The interaction between the fuel inclination and wind velocity are quantified using the Froude number. Moreover, the blown-off wind velocity decreases as the fuel inclination is increased, which can be explained by a critical strain rate (536 s−1). This work provides valuable information for fire behaviors under wind not only on the inclined wires and roofs in structure fires, but also on inclined branches in wildland fires.

Influence of dehydration on the dielectric and structural properties of organically modified montmorillonite and halloysite nanotubes

The dielectric behaviours of organically modified montmorillonite (Cloisite-20) and nanosized tubular halloysite (Dragonite-HP) were investigated using broadband dielectric spectroscopy (BDS) under dehydration conditions up to 200°C. The thermal and structural properties of both tested clay minerals were also initially examined in the as-received and dehydrated samples. Dragonite-HP was shown to lose 2.2 mass% of the adsorbed and interlayer water up to 200°C. The dehydration of Dragonite-HP also caused the tightly connected tubular layers to unfold, thereby increasing the specific surface area and the total pore volume. Cloisite-20 lost only 1.1 mass% of its adsorbed water during dehydration due to the presence of an organic modifier, bis(hydrogenated tallow alkyl)dimethyl. Its presence led to decreases in the specific surface area and total pore volume of Cloisite-20 relative to those of pristine montmorillonite. BDS revealed that the dielectric constant (ε') and dissipation factor (tan δ) of the thermally treated Dragonite-HP increased and that the volume resistivity (ρv) decreased within one order of magnitude in the temperature range of −40 to 90°C. In contrast, the ε' of the thermally treated Cloisite-20 increased by two orders of magnitude, the tan δ increased by more than three orders of magnitude, and the ρv decreased by five orders of magnitude. The values of ε', tan δ and ρv measured via BDS demonstrate that the dielectric properties of Dragonite-HP at a standard industrial frequency of 50Hz and under a typical operating temperature range are more advantageous than those of Cloisite-20. This finding is very promising for the possible use of Dragonite-HP as a nanofiller for clay/polymer nanocomposites intended for cable core insulation manufacturing.

Fire Protection and Sustainability of Structural Steel Buildings with Double-Shell Brickwork Cladding

The proposed paper focuses on the research of sustainability of steel buildings in the case of a double-shell brickwork cladding, assessed under fire protection criteria and fire performance aspects. More specifically, it concerns an approach in designing and evaluating the structural components of energy-efficient steel buildings with regards to their fire performance. It is important to understand what the performance criteria are intended to be applied when selecting a particular material or method, including exposure to fire, duration, aesthetics, cost and maintenance. Application of insulating materials is one of the most common means of protecting structural steel members from fire. The knowledge and the practical outcomes about the fire resistance of the steel buildings with double-brickwork cladding have been derived on the basis of an estimation and evaluation of fire resistance of several construction details of the building's envelope. The study focuses on the thermal analysis of structural steel members, whose cross-section consists of the steel member core, the thermal and fire insulation and other layers. Because of the complexity that governs the equations describing the thermal phenomena, the structural detail models have been studied using finite element analysis. This is a thermal analysis that permits to quantify the response of the respective structural elements in terms of temperature with reference to time. The aim of the expected data is to evaluate the position of a steel member in connection with the double-shell brickwork cladding and promote those construction materials that are efficient under fire taking into account simultaneously their thermophysical, hydrothermal and environmental properties.

Метод исследования термофлуктуационных процессов в задачах диагностики и прогнозирования изоляционных материалов

Introduction. The investigation of thermal fluctuation processes in insulating materials in accordance with the thermal conductivity theory for solving the problems on diagnostics and forecasting the residual life of insulating materials on the basis of a digital recorder, as well as on the nondestructive temperature method, is described. The work objective is to improve the nondestructive diagnostics methods, namely, the development of an automated control system for the state of insulation, and a computational and experimental study. Materials and Methods. Mathematical models that describe the layer-by-layer temperature distribution of the cable line in accordance with the theory of thermal conductivity using Fourier differential equation are proposed. A generalized algorithm for the operation of the PCL parameters monitoring recorder is created. It implements the technique of nondestructive testing of thermal fluctuation processes in PCL insulation materials. A comparative analysis of the experimental and calculated characteristics of the temperature distributions is carried out. At that, different charging modes of operation and functions of the cable current variation are investigated. Research Results. Mathematical models and software for numerical simulation of the temperature field in the cable cross-section in accordance with the theory of thermal conductivity are developed. Physical properties of materials and the geometric dimensions of cable elements are considered. A comparative analysis of the experimental and calculated characteristics of the temperature distributions is made. The developed simplified mathematical model for determining the temperature of the most heated point of the cable core insulation on the basis of the measured values of the surface temperature of the power cable and the air temperature for various changes in the effective value of the cable current is validated. A method for investigating thermal fluctuation processes based on the layered temperature sensors in PCL is developed and justified. That makes it possible to combine two control techniques - prediction of the growing insulation defect and nondestructive testing of the thermal fluctuation processes of a power cable - in one measuring tool. The suggested mathematical model can be used as a base for calculating the thermal processes of power cables in real time mode, as its adequacy is confirmed by the experimental studies. Discussion and Conclusions. The obtained results can be used in the development of the theory, methods of diagnostics and prediction of the insulating materials state in complex distributed systems under various operating conditions.