Machine Insulation Research Articles

Online Condition Monitoring for Diagnosis and Prognosis of Insulation Degradation of Inverter-Fed Machines

Real-time winding insulation condition monitoring is becoming an important research topic in response to increase in high availability and reliability demands in modern drives. In this paper, a novel method is proposed to monitor ground-wall insulation of low-voltage inverter-fed machines based on a multifrequency measurement of equivalent insulation ground-wall capacitance and dissipation factor. The monitoring is applied to four machine stator samples subjected to accelerated aging until failure. Insulation degradation is continuously tracked via the capacitance and dissipation parameters. A link between ground-wall insulation capacitance and final lifetime is established. The relationship between capacitance progression and its value at the final machine failure is used to develop a method for the prognosis of the final failure time.

Temperature Influence on Dielectric Response of Rotating Machine Insulation and Its Correction

Insulation condition is an essential aspect for the operational reliability of high voltage rotating machines in power plants and industrial applications. Insulation resistance (IR) and line-frequency dissipation-factor / power-factor (tanδ) measurement are often performed for the assessment of stator insulation condition. These measured values need to be normalized to a reference temperature (e.g. 40 °C) for comparison and trending and this is traditionally achieved by multiplying the results with a certain factor. However, this correction could be subject to error for an individual device since the correction factors recommended by various standards are average values of a certain number of machines at different conditions. In addition to that, insulation condition also has some influence on the temperature dependent property.With the introduction of Dielectric Frequency Response, DFR and Polarization/Depolarization Current, PDC as more advanced insulation diagnostic methods, with proper modelling, temperature correction can be done based on the insulation condition of an individual device and thus accuracy is considerably improved. In this paper, the background of DFR and its superiority in temperature correction are introduced. After that, the numerical Fourier and Inverse Fourier Transformation algorithm is applied to correct the time domain measurement (IR and PDC).

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.

Parameters influencing the dielectric loss of new winding insulation of electric machines

Deviations in the dielectric loss of electric machine insulation can be identified with the help of PD measurements based on the knowledge of the involved materials and the manufacturing process.

Heat Recovery for a Textile Stenter: CFD Analysis of Air Curtain Benefits

Modern textile stenters are designed to reduce the inefficiency of the process and to recover the flow stream, which still contains a relatively high energetic value. In recent years, research has focused on the recovery of the energy content of the low-temperature exhaust flow; nonetheless, another important aspect that may increase the efficiency of the process is the reduction of the ambient air suction. In the present research, an innovative way to improve both machine insulation and energy savings, by using preheated air, was numerically evaluated. The proposed solution utilizes an air stream transverse to the fabric (generally called air curtain), either preheated or not, to create soft gates both at the inlet and at the outlet section of the drying machine. Several valuable advantages can be listed when using this solution: reduction of the dispersion of heat and humid polluted air to the work environment, limitation of air ingestion from outside, and effective heat recovery coupled to a uniform temperature profile around the textile fabric. To analyze the insulation capability of the air curtains in terms of mass and energy transfer, a two-dimensional CFD model of the machine was realized. A test matrix including three possible fabric speeds (20, 40 and 60 m/min), three tilt angles (−15°, 0° and 15°), four mass flow rates (0% with no air curtains and 3%, 5% and 7% of the total flow rate through the machine, where the 5% case is equivalent to the flow rate ingested from the ambient) and two temperatures (15 °C and 70 °C) of the plane jets exiting from the air curtains was considered, thus covering a wide range of possible practical applications. The obtained results demonstrate that warm air curtains at both the inlet and outlet are very effective in a fabric speed range up to 40 m/min; at higher fabric speed, entrainment of warm gases from inside the machine at the fabric outlet becomes relevant, and the adoption of a cold air curtain (capable of better insulation) can be recommended in this position.

Degradation of stator insulation of high-voltage asynchronous machines in gamma and neutron radiation field

This paper presents the results of an examination of function stability of high-voltage asynchronous motors exposed to ambient strain caused by combined neutron and gamma radiation. This problem appears in practice when a high-voltage asynchronous motor is used in nu- clear power plants where it can be exposed to this type of ambient strain. The failure of the engine's operation under such conditions may have unexpected consequences. As more than 50 % of failure (malfunction) of high-voltage asynchronous motors is caused by damage to stator insulation, the focus of the paper was on testing the effects of combined neutron and gamma radiation on stator insulation. The tests were carried out under well-controlled laboratory conditions on samples taken from both new and used factory coil windings. Two-layer samples were used to record partial discharge threshold voltage and breakdown voltage. By comparing the experimentally obtained results with the applicable mathematical-statistical procedure, an estimate was made of the aging acceleration of stator insulation and the time duration of reliable operation of high-voltage asynchronous motor was determined by life-time exponent.

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.

Insulation condition assessment of high‐voltage rotating machines using hybrid techniques

The enduring life span of the machine insulation will be decided based on degradation level in motor and generator stator windings. The non-destructive diagnostic tools like dielectric loss and capacitance test and partial discharge (PD) analysis, recognized to access the deterioration in the insulation system of rotating machines. The experiments reveal various characteristic parameters such as leakage current, dielectric dissipation factor, the capacitance value, and PD magnitude. The integrity of the rotating machine can be find out by analyzing these parameters. This research study shows the hybrid method for prediction of insulation condition in the stator winding by utilizing the artificial neural network (ANN) with gravitational search algorithm in comparison with ANN and ANN–genetic algorithm. The advent of expert systems ensures the quality assurance and service life assessment of the high-voltage assets. It offers a predictive maintenance solution to personnel dealt with power utilities thereby increasing the uptime, reliability, and productivity, which in turn reducing the operating costs, downtime and unplanned outages. For testing and predicting the insulation status, several 11 kV machines are considered. The predicted results using hybrid techniques extend a close agreement with reference to the data obtained from the experiments performed. The proposed method indicate the competent and trustworthy, by the presented test results.

Identification of slot discharges in rotating machine insulation system using variable frequency PD measurement

Slot discharge studies are very important from the insulation diagnosis perspective. Further in practice, testing of rotating machines with power frequency requires sources which are quite bulky and costly. Instead if the diagnosis is carried out at lower frequencies (<50 Hz), the size and cost of the test source can be reduced significantly. Therefore, it is important to know whether the study at low frequencies can be considered equivalent to study at power frequency. Hence, in this study, a slot discharge phenomenon occurring in a rotating machine insulation system was investigated experimentally at different frequencies (50, 10, 1 and 0.1 Hz). Slot discharges are known to have a distinct phase resolved partial discharge pattern for power frequency (50/60 Hz) excitation. These distinct patterns are useful in identifying the presence of slot discharge activity in rotating machines. Slot discharges were created on a 6.6 kV mica-epoxy stator coil and the slot discharge characteristics measured at different frequencies are compared. Slot discharge measurements were repeated by varying the air gap between the insulation surface and grounded steel plate. The changes in slot discharge characteristics due to the increase in air gap were found similar at all frequencies. The comparison suggests that the identification of slot discharge is also possible from the partial discharge (PD) measurement at low frequencies.

Choosing Expert Evaluations of Diagnostic Tests of Electric Insulation for High-Voltage Electric Machines

This article provides the results of studying the influence of test voltage frequency on the dynamic parameters (dielectric losses and characteristics of partial discharges) of micaceous insulation of stator winding for high-voltage (HV) electric machines. The tests have been conducted using mockup samples of stator insulation with an artificial defect in the front area. It has been established that the low-frequency (0.1 Hz) test voltage increases the dielectric loss tangent and, at the same time, reduces the peak apparent charge of external partial discharges compared to the tests at power frequencies. The established ratios must be taken into account when choosing expert diagnostic evaluations during high-voltage electric-machine tests.

Study of Water Droplets Behavior on Electrical Machine Insulation Under the Influence of Uniform Electric Fields: the Influence of Some Parameters on Mica Sheets

Mica sheets are a vital component of electrical machine insulation. Although great attention has been paid to partial discharges and their deteriorating effect on such insulation, relatively little attention has been given to surface discharges. This paper investigates the effect of water droplets, their volume, their conductivity, their positioning as well as their number on the flashover voltage of mica sheets. It is indicated that all the above mentioned parameters influence the flashover voltage

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.

Identification of Permanent Magnets for Selecting the Drying Mode of Electrical Machine Insulation

Identification of Permanent Magnets for Selecting the Drying Mode of Electrical Machine Insulation

Parametric and Nonparametric Models for Lifespan Modeling of Insulation Systems in Electrical Machines

This paper describes an original statistical approach for the lifespan modeling of electric machine insulation materials. The presented models aim to study the effect of three main stress factors (voltage, frequency, and temperature) and their interactions on the insulation lifespan. The proposed methodology is applied to two different insulation materials tested in partial discharge regime. Accelerated ageing tests are organized according to experimental optimization methods in order to minimize the experimental cost while ensuring the best model accuracy. In addition to classical parametric models, the life–stress relationship is expressed through original nonparametric and hybrid models that have never been investigated in insulation aging studies before. These two models present the original contribution of this paper. For each material, models are computed from organized sets of experiments and applied on a randomly configured test set for validity checking. The different models are evaluated and compared in order to define their optimal use.

Optimized dielectric design of stator windings from medium voltage induction machines fed by fast front pulses

AC induction machines are exposed to fast-front pulses when fed from frequency converters. This type of excitation produces additional dielectric stress in the machine's insulation system which can result in premature deterioration or even failure. In this work, a non-uniform transmission line modeling approach is used in combination with FEM-based parameter computation to predict the transient voltage distribution in a machine winding under fast-front pulse excitation. Using this voltage distribution as input, different optimization algorithms are applied to identify and evaluate modifications to the winding design aimed at minimizing the dielectric stress. Test cases considering two different geometrical configurations show that the optimized designs considerably reduce the dielectric stress in the machine coils.

Classification of partial discharge sources in mica-epoxy-glass insulation sample using statistical analysis

Partial Discharge (PD) in rotating machine insulation system is deleterious. Even though the main insulation of rotating machine is Mica, which is highly PD resistant, the discharges taking place in slot region and end windings are highly deteriorating to insulation system. In this paper, PD measurements are taken in Mica-Epoxy-Glass insulation sample using cylindrical-plane electrode system. The sample contains a defined void on the surface. Distinguished Phase Resolved Partial Discharge (PRPD) patterns are obtained for different location of void. The different PRPD patterns obtained corresponds to internal void in the bulk of insulation, void between high voltage electrode and insulation surface, void between ground electrode and insulation surface; and surface discharge. The PRPD patterns obtained from void between high voltage electrode and insulation surface, in the actual scenario, indicates PD originating near the conductor surface inside the insulation system of rotating machines. The PRPD patterns obtained from void between ground electrode and insulation surface indicates PD originating on the surface of insulation such as slot discharge, end winding discharge in rotating machine insulation system. The variation of PD parameters like Charge magnitude (q) with respect to applied voltage is shown for different location of void. Further statistical parameters like skewness and kurtosis are calculated for charge distribution (n-q) and phase distribution (n-Φ) for different PRPD patterns. It is inferred that statistical analysis of PD parameters is a good tool to classify of partial discharge sources.

The Insulation for Machines Having a High Lifespan Expectancy, Design, Tests and Acceptance Criteria Issues

The windings insulation of electrical machines will remain a topic that is updated frequently. The criteria severity requested by the electrical machine applications increases continuously. Manufacturers and designers are always confronted with new requirements or new criteria with enhanced performances. The most problematic requirements that will be investigated here are the extremely long lifespan coupled to critical operating conditions (overload, supply grid instabilities, and critical operating environments). Increasing lifespan does not have a considerable benefit because the purchasing price of usual machines has to be compared to the purchasing price and maintenance price of long lifespan machines. A machine having a 40-year lifespan will cost more than twice the usual price of a 20-year lifetime machine. Systems which need a long lifetime are systems which are crucial for a country, and those for which outage costs are exorbitant. Nuclear power stations are such systems. It is certain that the used technologies have evolved since the first nuclear power plant, but they cannot evolve as quickly as in other sectors of activities. No-one wants to use an immature technology in such power plants. Even if the electrical machines have exceeded 100 years of age, their improvements are linked to a patient and continuous work. Nowadays, the windings insulation systems have a well-established structure, especially high voltage windings. Unfortunately, a high life span is not only linked to this result. Several manufacturers’ improvements induced by many years of experiment have led to the writing of standards that help the customers and the manufacturers to regularly enhance the insulation specifications or qualifications. Hence, in this publication, the authors will give a step by step exhaustive review of one insulation layout and will take time to give a detailed report on the standards that are linked to insulation systems. No standard can provide insurance about lifespan, nor do any insulation tests incorporate all of the operating conditions: thermal, mechanical, moisture and chemical. Even if one manufacturer uses the standards compliance to demonstrate the quality of its realization; in the end, the successful use in operation remains an objective test. Thereafter, both customer and manufacturers will use the standards while knowing that such documents cannot fully satisfy their wishes. In one 20-year historical review, the authors will highlight the duration in insulation improvements and small breakthroughs in standards writing. High lifespan machines are not the main interest of standards. A large part of this publication is dedicated to the improvements of the insulation wall to achieve the lifespan. Even if the choice of raw materials is fundamental, the understanding of ageing phenomena also leads to improvements.

Characterizing the in situ Thermal Behavior of Selected Electrical Machine Insulation and Impregnation Materials

This paper presents results from an analysis of thermal behavior for alternative slot liner insulation and varnish impregnation materials used in the construction of electrical machines. These materials are typically characterized by the individual material properties altered to suit a particular application. However, the manufacturer provided material data are usually inadequate when comparing the complete insulation systems. This research is focused on the conductive heat transfer from the winding body into the machine periphery in context of the complete insulation system. An experimental approach using the stator-winding segment subassembly has been adopted here, for the representative in situ heat transfer measurements. The effects of impregnation “goodness,” in volume manufacture repeatability, and individual versus in situ material physical properties on the stator-winding thermal behavior are discussed in detail. The results suggest that the use of a particular slot liner insulation and varnish impregnation has a prominent impact on the winding heat transfer, as well as appropriate manufacture and assembly processes used. The experimental work has been supplemented with theoretical analysis to provide a more comprehensive insight into the winding heat transfer phenomena; in particular the winding-to-slot contact thermal resistance.

Application of EDA methodology for assessment of the rotating machines insulation system condition

The insulation system of rotating machines of high importance has always been the object of thorough screening with certified test methods that are getting constantly improved. Low-power machines and 'less important' machines like high voltage motors, are rarely subjected to detailed electrical tests because of low resources allocated for their maintenance. The introduction of the EDA methodology in practice creates the conditions for reliable and complete diagnostics of stator windings of big machines, as well as the fast, easy and inexpensive screening for low-power machines (i.e. HV motors). The aim of the paper is to present the EDA methodology and its possibilities, including the solutions within the hardware and software.

Improvement of Insulation for Rotating Machine by Dry Ice Method

This paper presents the improvement of rotating machine insulation by the powerful blast-cleaning, dry ice method. In this work, the large synchronous generator, 16.25 MVA 6.9kV and 78,240 running hour, was investigated and analyzed. The characteristic of insulation such as polarization and depolarization current (PDC), polarization index (PI) and dielectric loss tangent (tan δ) were measured according to the IEEE standard 43-2013. After cleaning with dry ice blasting, the insulation of generator has improved remarkably. The charging current is decreased due to removal of conductive layers from the insulation surface. For other parameters, polarization index and loss tangent are increased 52.03% and decreased 11.66% respectively. Furthermore, the operation time of dry ice method is very short when compared with traditional cleaning methods, resulting in reduced maintenance downtime and increased more productivity.