Mica Tape Research Articles

Thermal conductive network construction and enhanced thermal conductivity in mica tape composites for large generator insulation

The rapid development of large generators to high voltage and large capacity calls for high thermal conductivity and excellent insulation for their insulation. In this work, a hexagonal boron nitride (h-BN) thermal conductive network is firstly constructed. The thermal conductivity network in mica paper exhibits a three-dimensional structure, and mica tape composites was prepared with glass fabric, epoxy and mica paper above. Accordingly, a typical mica paper and mica tape have thermal conductivities of 1.07 W/(m·K) and 0.416 W/(m·K), respectively, which are 157.21 % and 75.53 % higher than those without filling. A modified thermal conductivity model (LN-SSS model) is proposed to analyze the thermal conductivity of the mica tape composites, which is found to fit the mica tape's experiment value and tendency accurately, and the mica tape composites have excellent insulation properties. The design approach in this paper also provides new ideas for insulating materials and structures with high thermal conductivity.

Monitoring of Stator Winding Insulation Degradation through Estimation of Stator Winding Temperature and Leakage Current

Switched Reluctance Motors (SRMs), Permanent Magnet Synchronous Motors (PMSMs), and induction motors may experience failures due to insulation-related breakdowns. The SRM rotor is of a non-salient nature and made of solid steel material. There are no windings on the rotor. However, the stator is composed of windings that are intricately insulated from each other using materials such as enamel wire, polymer films, mica tapes, epoxy resin, varnishes, or insulating tapes. The dielectric strength of the insulation may fail over time due to several environmental factors and processes. Dielectric breakdown of the winding insulation can be caused by rapid switching of the winding current, the presence of contaminants, and thermal aging. For reliable and efficient operation of the SRMs and other electrical machines, it is necessary to take into account the physics of the winding insulation and perform appropriate diagnostics and estimations that can monitor the integrity of the insulation. This article presents the estimation problem using a Genetic Algorithm (GA)-optimized Random Forest Regressor. Empirical properties and measurable quantities in the historical data are utilized to derive temperature and leakage current estimation. The developed model is then combined with a moving average function to increase the accuracy of prediction of the stator winding temperature and leakage current. The performance of the model is compared with that of the Feedforward Neural Network and Long Short-Term Memory over the same winding temperature and leakage current historical data. The performance metrics are based on computation of the Mean Square Error and Mean Absolute Error.

Circuit Integrity Assessment of Fire-Resistant Cables

In the electrical industry, fire accidents may happen due to overheating of electrical equipment, short circuit faults and any external source of fire. Polymeric Electrical Cables are prone to spread fire along the cable run as the insulation and sheathing materials are of flammable nature. Hence various flame retardants and low-smoke polymers are being used so that the flame spread and toxic gas release can be controlled. There are some special application cables called Fire Resistant cables which need to maintain the circuit integrity of the cable even during fire conditions. The construction of these fire-resistant cables requires fire-resistant insulation to achieve the fire-resistance property. They are such as Glass- Mica tape, Silicone Rubber, mineral insulation, metal insulation and fire retardant coating. There is various International and National standard assessing methods to assess these special cables. In this paper, the fire-resistant insulating materials and their performance during the assessment in achieving the required fire-resistant property are discussed.

Arcjet Evaluation of Thermocouple Performance in Flexible Thermal Protection System Materials

The flexible thermal protection system (FTPS) on NASA’s low-Earth-orbit flight test of an inflatable decelerator vehicle was instrumented with thermocouples to measure the in-depth thermal response during entry into Earth’s atmosphere. Accurate flight temperature measurements are critical for verifying vehicle performance during the flight test and reducing uncertainties in the thermal models, but the deployable nature of inflatable decelerator technology presents challenges. The thermocouples need to be compactable, cannot damage the FTPS or the inflatable structure, and need to be able to withstand high temperatures and large thermal gradients. The present paper details the arcjet testing that was performed to examine the potential error sources associated with instrumenting NASA’s FTPS with different thermocouple Types (Type K versus Type N), wire gauges (30 versus 28 American wire gauge), insulations (glass versus ceramic), and barriers (mica versus no mica). Several error sources were identified, including 1) conductive deposits on the thermocouple insulation electrically shorting the thermocouple leads; 2) melting of the glass thermocouple insulation, allowing for electrical shorting of the thermocouple leads; 3) thermocouple leads melting, resulting in loss of signal; and 4) oxidation of the positive thermocouple lead (known as green rot), generating a drift in the measured output. A mica tape wrap around each individual thermocouple lead was successfully used to mitigate the first error source, and switching to ceramic insulation eliminated the second error source. The third and fourth error sources were examined by exploring the effect of the thermocouple type and wire gauge. Thermocouple solutions that minimize the potential error sources at each layer in the FTPS are presented. Finally, the arcjet test results are compared to the previously reported ground-based tube furnace test results, and their applicability to the flight environment is discussed.

Preventing effect of different interstitial materials on thermal runaway propagation of large-format lithium iron phosphate battery module

Thermal runaway propagation (TRP) has become an urgent problem in the field of lithium-ion battery (LIB) fire safety, bringing potential risks to their large-scale applications. In this work, a novel strategy to prevent TRP of large-format lithium iron phosphate battery (LFP) module using aerogel, polyimide foam (PIF) and mica tape composite insulation cotton (MTCC) is proposed and investigated experimentally under two modules. One module consists of three batteries with insulation placed in every other battery (Individual Insulation Module, IIM), and the other module has four batteries with insulation placed in every third battery (Spacer Insulation Module, SIM). The prevention effect of the interstitial materials is analyzed by changing the thickness and configuration type in two modules. In addition, the heat transfer from the front batteries to the last battery under SIM is calculated. The results show that both the aerogel and 2 mm PIF can prevent TRP, and the latter can reduce the heat transfer power from the penultimate cell to the last cell from 785.28 W to 314.2 W. However, PIF and MTCC with 1 mm thickness can only prolong the TRP time by 1351 and 1462 s, respectively. Modules with SIM configuration are less dangerous than those with IIM. The heat from the penultimate battery dominates the temperature rise of the last battery, while the heat transferred from other parts to the last battery only accounts for 10–19 % of the total heat.

Enhanced thermal conductivity and insulation properties of mica tape with BN coating via electrostatic spraying technology

AbstractThe bottleneck problem of the development of high‐voltage motors to a larger capacity can be solved if the thermal conductivity is improved without sacrificing the insulation characteristics of the main insulation material. In this paper, a spraying technology based on electrospinning is proposed that can evenly spray boron nitride (BN) with high thermal conductivity onto the surface of mica tape. With the increase in BN content, the insulation and thermal conductivity of mica tape are optimized to varying degrees. When the BN content reaches 2.25 wt%, the in‐plane conductivity increases from 1.6 to 2.7 w/(m K), while the out‐of‐plane conductivity increases from 0.17 to 0.26 w/(m K). Additionally, the direct‐current (DC) breakdown strength has increased from 90.6 to 96.9 kV/mm and the dielectric constant and dielectric loss are almost unchanged. Finally, through simulation verification, if the modified mica tape is applied to the main insulation of a synchronous motor, the operating temperature of the motor insulation will be reduced from 73.5 to 66.1°C. This study provides a new research idea for the preparation of high thermal conductivity insulating materials.

Enhanced Mechanic Strength and Thermal Conductivities of Mica Composites with Mimicking Shell Nacre Structure

As the main insulation of high-voltage motors, the poor mechanical and thermal conductivities of mica paper restrict the motor’s technological advances. This paper prepared multilayer toughening mica composites with a highly ordered “brick-mud” stacking structure by mimicking the natural conch nacre structure. We investigated the mechanical, thermal, and breakdown properties by combined study of tensile strength, stiffness, thermal conductivity, and breakdown strength at varying mica and nanocellulose contents. The results show that thermal conductivity of the mica/chitosan composites were gradually enhanced with the increase in mica content and the composite shows the optimal synthetic performance at 50 wt% mica content. Further addition of the nanocellulose can extremely enhance the thermal conductivities of mica/chitosan composites. The composite with 5 wt% nanocellulose obtained the maximal thermal conductivity of 0.71 W/(m·K), which was about 1.7 times that of the mica/chitosan composite (0.42 W/(m·K)) and much higher than normal mica tape (0.20 W/(m·K)). Meanwhile, the breakdown strength and tensile strength of mica/chitosan/nanocellulose composite also demonstrated substantial improvement. The application of the mica/chitosan/nanocellulose composite is expected to essentially enhance the stator power density and heat dissipation ability of large-capacity generators and HV electric motors.

Development and Evaluation of Fire Resistant Railway Signalling Cable

The railway signalling cable is designed for remote control and teletransmission in underground railway networks. It is very critical situation that the railway signalling cable continues to transmit data in the event of a fire. In this study, the fire resistant test performance of railway signalling cable has been developmented and evaluated via experimental research. A standard railway signalling cable generally consists of conductor, insulation, laying-up, wrapping, screen, inner sheath, armour and outher sheath. The mica tape was added as one layer with 30% overlapping to produce desired fire resistant cable. First step, the railway signalling cable produced used only mica tape used, results show that the cable with mica tape is successful at 110 V. Also, it is not bad for the 250V as it seen the test results. After having non satisfied results, glass tape was added additionally over inner sheath (instead of PP tape) for better results for higher voltage values. After redesign of the cable with addition of glass tape over inner sheath, better results were expected but the results were not as good as expected. Only at 250V appropriate results were achieved according to both IEC 60331 – 21 & 60331-2 standard. When we examined the situation, we saw that the cable was flattened a bit after the new-design. Then, the re-production of the new design cable (mica + glass tape) is been planned without any rework process. All results show that the new design railway signalling cable is resistant to fire tests according to IEC 60331-21, IEC 60331-1&2 up to 1000V as shown in the paper.

Particularities of measuring partial discharges in stator windings insulation systems of high-voltage electrical machines

The purpose of the article is to provide potential tools that can make a significant contribution to the identification of partial discharges (PD). Different types of partial discharges occur in stator winding insulation and a few partial discharges may occur simultaneously. Internal partial discharges are electrical discharges that occur in voids in the insulation of the stator winding. In typical stator insulation systems that use epoxy bonded mica tapes, insulation degradation due to internal partial discharges is usually slow (many years or decades). External partial discharges (slot PD and surface PD in the end-winding) are more dangerous and lead to the destruction of the insulation in a short time (several months or years). Therefore, the identification of insulation defects is essential. The analysis of existing methods for identification of defects in the insulation of high-voltage electrical machines using the results of measuring the partial discharges characteristics is carried out. The advantages and disadvantages of each of the groups of identification methods are characterized. It is shown that among the models of knowledge representation in solving problems of diagnostics of insulation systems for high-voltage electrical machines, identification methods, including field tests using training samples, are among the most suitable ones. It is noted that detection of insulation defects and their identification cannot be carried out only by direct measurements of PD characteristics and other dielectric parameters (electrical resistance, dielectric loss, polarization index). For this, special computing programs based on pattern recognition methods should be used. Results are presented of identification of technological defects in the insulation of the stator winding at the stage of factory testing, obtained using the PD identification method developed by the authors

A new insight into more effective viscosupplementation based on the synergy of hyaluronic acid and phospholipids for cartilage friction reduction

Osteoarthritis is a joint disease characterized by progressive damage to the cartilage. For a long time, viscosupplements have been based on hyaluronic acid (HA), which is known to relieve the pain of osteoarthritic patients. Recently, however, it has been hypothesized that phospholipids and HA may be collectively responsible for the extremely low friction observed in joint models (mica tape), suggesting that such a combination might lead to more effective viscosupplements. Using a pin-on-plate tribometer, we have tested this hypothesis by analyzing the coefficient of friction of conventional HA viscosupplements compared to the new viscosupplement formula of phospholipids and HA. Our findings show that phospholipids and HA do indeed lead to the lowest friction in the joints at both cartilage-mica tape and cartilage-glass interfaces (joint models). Furthermore, these results also hold true when both viscosupplements are mixed with synovial fluid simulating the real conditions after viscosupplement injection into joints. Therefore, we have validated the synergic effect of phospholipids and HA on lowering the cartilage friction and we further hypothesized that this effect is due to hydration lubrication. We believe these findings might contribute to development of new viscosupplement which would significantly improve the lives of person suffering from osteoarthrosis.

Surface partial discharge in hydrogenerator stator windings: Causes, symptoms, and remedies

Partial discharges (PD) are small electrical discharges that occur within voids within the stator winding insulation system of a hydrogenerator. PD is harmful to the insulation: the repeated bombardment of electrons and ions on the void's surface gradually ruptures the chemical bonds of the insulation's organic materials and leads to electric treeing or electric tracking. In typical stator insulation systems that use mica tapes bonded by epoxy, deterioration of the insulation due to PD is usually slow (many years or decades). In hydrogenerator stator windings, PD is more a symptom of an aging process than a cause of deterioration [1].

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.

Heat of Combustion as the Key Fire Characteristics of Electrical Cables

Abstract This scientific study deals with investigation of the heat of combustion and effective heat of combustion of selected electrical cables. Two different electrical cables for rated voltage of 0.6/1 kV were investigated. Both cables were power three-core with cross-section area of each core of 1.5 mm2. The cores of both cables were made of a bar cooper wire. Insulations of conductors of both cables were made of silane cross-linked polyethylene without any inorganic filler, while the bedding and outer sheath were made of polyethylene-based copolymer (the beddings were filled with two fillers - aluminium hydroxide and calcium carbonate, while the outer sheath were filled only with aluminium hydroxide). Reaction to fire class of both cables was B2ca, s1, d0, a1. The main difference in the investigated cables was that the core of one of them was wrapped in a glass mica tape (this cable showed circuit integrity maintenance under fire conditions during 180 minutes). The heat of combustion and effective heat of combustion were determined by the oxygen bomb calorimeter according to the ISO 1716:2018 standard. The highest effective heat of combustion showed the insulation of wires (for both cables 42.47 ± 0.03 MJ/kg), lower value showed outer sheath (interval form13.61 to 15.26 MJ/kg) and the lowest value was determined for bedding (interval from 4.69 to 6.39 MJ/kg). The effective heath of combustion per unit of length of both investigated cables lies in the interval from 1.37 to 1.38 MJ/m. Therefore, there is no significant difference in effective heats of combustion of the electrical cables investigated.

EFFECT OF A DIELECTRIC BARRIER ON THE ELECTRIC FIELD DISTRIBUTION IN HIGH-VOLTAGE COMPOSITE INSULATION OF ELECTRIC MACHINES

Introduction. Modern high-voltage systems for composite insulation of electrical machines consist of tape glass mica paper materials (dry or pre-impregnated). The electrical characteristics of a multilayer composite insulation system are determined by both the fractional content of the individual components and their electrophysical properties. Purpose. The analysis of the influence of electrophysical characteristics and thickness (fraction) of the dielectric barrier on the distribution of the electric field in the composite high-voltage insulation of electrical machines. Methodology. Simulation of surface charge accumulation at the interface between the substrate and the dielectric barrier is based on the Maxwell–Wagner theory for interfacial polarization. Practical value. The influence of the electrophysical characteristics and thickness of the dielectric barrier on the distribution of the electric field has been established. In the steady state, the electric field strength in the dielectric barrier exceeds the average value by 50 %. In the region of small transition times (up to 1 s), the relative dielectric constant of the barrier has a significant effect on the distribution of the electric field. The use of a dielectric barrier with a higher dielectric constant and fractional content in comparison with the substrate leads to an increase in electric field strength by 5 % relative to the average value in composite insulation. Experimental studies of the long-term electrical strength of glass mica-belt ribbons in the cured (thermosetting) state are consistent with the simulation results. Composite insulation based on glass fiber mica tape with a high content of the mica barrier and fiberglass of smaller thickness has (8-16) % higher values of long-term electric strength.

A micro-computed tomography investigation of the breakdown paths in mica/epoxy machine insulation

X-ray computed tomography is a technology capable of visualizing a wide variety of materials in 3D. In this investigation, insulation samples were electrically aged and computed tomography imaging was used for analysis of the microstructure and breakdown paths. Samples consisting of mica tape wrapped around aluminum tubes and impregnated with epoxy resin were produced. Two samples were electrically aged at 40 kV rms AC and another two at 30 kV rms AC until electrical breakdown. The samples were then scanned and reconstructed using x-ray micro computed tomography. The reconstructed images were analyzed and used for rendering volumetric 3D-models of the breakdown channels in the insulation. The results of μ-CT investigation show the location of vaporized insulation as well as the directions of the breakdown paths. When inspecting the 3D models, it is possible to follow the breakdown from electrode to electrode. The results also show that the mica tape layers will act as barriers forcing the breakdown path to follow the epoxy resin layers. It is noted how the breakdown path takes a tortuous course around the mica tape layers and how resin pockets inside the insulation weaken the insulation. Conversely, an improved continuous coverage of the mica tape without wrinkles would improve the insulation properties.

Life Estimation of Thermally Stressed Insulating Papers of Rotating Machines by Using Arrhenius Model

In the present study, life estimation of accelerated aged insulating papers used in rotating machines is estimated by Arrhenius model. Various types of insulating papers used in rotating machines such as Mylar, Nomex-Mylar-Nomex(NMN), Leatheroid, Dacron-Mylar-Dacron(DMD), Mica Tape, and Kapton are thermally aged at 90°C, 130°C and 170°C with 300hrs duration of each sample. These insulating materials wrapped on copper conductor with single and double layer. Then, breakdown voltage test is performed at each temperature for both single and double layer papers to determine their dielectric strength. Further, correlation analysis between breakdown voltage and different applied temperatures is performed for all paper samples. Popular Arrhenius plot is drawn by short term test data on breakdown voltage for an extrapolation to long term performance of various types of insulating paper used in rotating machines. The results showed that the performance of double layer insulation is almost twice than single layer performance. The aging analysis and testing of insulating papers was performed in TIFAC-Core centre of NIT Hamirpur, H.P., India.

Study on VPI Manufacture Technology of Stator Bar for 200MW Air-Cooled Turbo-Generator

The dry mica tape and impregnation resin were introduced in this paper, which were main insulation materials of 200MW air-cooled turbo-generator stator bar manufactured by VPI (vacuum pressure impregnation) technology. The insulation manufacture technology, corona protection technology and properties of real products also were introduced in this paper. The results show that the electrical properties of the stator bars made by this kind of VPI technology reached the top level of the world which could be seen as follows: there is no discharge appeared during the AC high voltage potential test when the test voltage was 3UN, the dissipation factor tgδ under 0.2UN is lower than 1%, and the increment Δtgδ=tgδ0.6UN- tgδ0.2UN is lower than 0.5%, when the voltage applied is from 0.2UN to 1.2UN. The magnitude of maximum partial discharge is only 90pc to 110pc, the instantaneous breakdown strength of the bars are all higher than 34kV/mm, under the field of 8.73 kV/mm, the lest enduring time of the test bar is 1500h.

Thermal characterization of mica–epoxy composite used as insulation material for high voltage machines

The study of mica–epoxy-based composite has a great role in high voltage machines industry. Beside electric properties, this composite should present compatible mechanical and chemical, in this case thermal characteristics to insulate properly the conductor strand, avoiding short-circuits, and leading electrical current with minimal losses. Improvement of the quality is possible through the knowledge of raw materials and system. This study aims to list a complete thermal characterization of mica composite, its components, epoxy resin, anhydride methylhexahydrophtalic, mica tape and zinc naphthenate, and thermoanalytical interactions between them. These data shows intrinsic properties of the system, which is so relevant to its great electrical and thermal performance. Thermal analysis allows the detailed study of curing process and thermal decomposition, predicting and suggesting mechanisms, beside future and possible optimization to the system. Composite system glass transition (Tg) was obtained through an important and very respected methodology, presenting the value of Tg = 138 ± 2 °C, finally characterizing the material.

Monolith-1 thermosetting insulation of Class-H heat resistance

The development of the monolith-1 insulation system is considered based on untreated glass mica tape and epoxyisocyanate compound with high electrical characteristics if up to 200–220°C.

Polyestermiride compounds as insulating base of modern traction electric motors

A brief review is given of insulating systems for electric machines and impregnating compounds as one of the key insulating components is given. Possibilities of producing and processing various types of domestic compounds are considered. Properties of glass mica tapes that contain an impregnating compound for cohesion are described.