Insulation Of Electrical Machines Research Articles

Optimum Scheme for Insulation System in HV Generator Based on Electromagnetic Analysis

Electrical insulations are one of the basic parts of electrical machinery in any sizes and characteristics. Focusing on insulating, studies on the operation of industrialelectrical machinery came to the fact that the most important part of a machine is the Stator. This fact reveals the requirement for inspection of the electrical machine insulation along with the electromagnetic tensions. Therefore with respect to insulation system improvement of stator, the HV generator can be optimized. Dielectric parameters such as insulation thickness, spacing, material types, geometry of winding and slot are major design consideration. A very powerful method available to analyze electromagnetic performance is Finite Element Method (FEM) which is used in this paper. The analysis of various stator coil and slot configurations are used to design the better dielectric system to reduce electrical stresses in order to increase the power of generator in the same volume of core. These processes of optimization have been done according the proposed algorithm. In this algorithm the technical constraints have been considered. This paper describes the process used to perform classical design and improvement analysis of stator slot's insulation with respect to objective function and constraints.

Evaluation of the state of the insulation system of a stator winding of high-voltage electric machines based on measurements of statistical characteristics of partial discharges

The statistical characteristics of amplitude and amplitude-phase spectra of PDs in the insulation of model specimens with artificial flaws in a semiconducting coating are investigated. The algorithm for distinguishing the flaws in the stator winding insulation of high-voltage electrical machines based on statistical parameters of the PD spectra is developed and practically implemented.

Prevention of marginal corona discharges in the insulation of high-voltage electric machines

The problem of preventing corona glowing discharges at the junction of slot and end-winding semiconductive coatings is investigated. Allowable electrical stress and current density are determined and used to calculate the two-step stress-grading coating. The proposed construction of end-winding coatings was used to produce 20-kV turbogenerators and checked by long-term sample testing.

The improvement of the electrical insulation of high voltage electrical machines

Comparative parameters are presented for the electrical insulation systems of high voltage electric machines. Composite insulation materials produced by different manufacturers are studied. The influence of the structure and composition of electrical insulation materials on the main operation parameters (electric strength, dielectric losses, the mechanical characteristics, and resistance to thermal cycling) of the insulation systems of electric machines is shown.

Surface modification of imide containing polymers II: Co-reactive groups

This paper describes basic studies of the surface modification of polyimide covered wires for insulation of electrical machines. By introduction of surface reactive groups mechanical interlocking during the curing step should improve the life cycle. Kinetic analysis of ring opening reactions by aminolysis of low molecular model compounds for polyimides proved fast modification reactions under mild conditions. The co-reactivity of various functional groups with unsaturated polyesterimide, acrylate and epoxy resins was investigated by DSC. Aminolytic treatment of Kapton ® sheets was followed by ATR-IR. Mandrel bend test of agglutinations of unmodified and amine-treated Kapton ® sheets with different resins proved successful bonding and significantly improved adhesion.

Surface modification of imide containing polymers I: Catalytic groups

This paper describes basic studies of the surface modification of polyimide covered wires for insulation of electrical machines. Drain-off of the impregnating resin during production should be reduced by introduction of surface catalytic groups. 1H NMR kinetic analysis of aminolytic ring opening reaction of low molecular model compounds for polyimides showed very fast modification reactions. The catalytic effect of various functional groups on unsaturated polyester imide, acrylate and epoxy resins was investigated by DSC. Co(II)-catalysts and tertiary aliphatic amines proved highest activity for double bond containing systems and epoxy resins, respectively. Aminolytic treatment of Kapton ® slides was followed by ATR-IR spectroscopy. Plate–plate rheometer measurements of epoxy resins employing tertiary amine-treated Kapton ® slides proved significantly reduced gelling temperature.

The effect of voltage distortion on ageing acceleration of insulation systems under partial discharge activity

This paper focuses on the features of harmonic distortion which may affect significantly the reliability of typical ac-power network equipment, such as low-voltage self-healing capacitors used for reactive power and harmonic compensation. Moreover, the effect of high-frequency pulse-like voltage generated by Adjustable Speed Drives (ASD) on electrical machine insulation is also investigated, resorting to life tests carried out on different insulating materials of the standard and corona resistant type, at electrical field levels able to incept partial discharges (PD).

Failure mechanism of the interturn insulation of low voltage electric machines fed by pulse-controlled inverters

Partial discharges (PDs) occur in the air-filled gaps of enameled wires that are touching. They erode the insulation and consequently lead to an interturn breakdown. This paper describes the failure mechanism of low voltage interturn insulation as a consequence of PDs and shows why and how it is influenced by the insulation design, temperature, and the applied voltage. Understanding the failure mechanism leads to better founded maximal permissible stresses and a proper design of the interturn insulation to avoid premature drive failures.

Corona deterioration reduction in large electrical machine insulation

Partial discharge or corona activity in the insulation system of large electric machine stators has caused deterioration of these machines, and in some cases resulted in premature failures. Development work is being done to better understand the cause and effects of this corona activity. Efforts are centering around the increased use of corona-resistant material, and the reduced use of corona-sensitive materials. Some of the developments are described in this paper. >

An acoustic diagnostic technique for use with electric machine insulation [comment with reply

The authors' original approach [see ibid.,vol. 1, no. 6, p. 1186-93, 1994] of employing ultrasonic nondestructive testing techniques to search for stator bar insulation defects is commented upon. The comments provided are very welcome since the authors feel this is an important new technique in the armory of diagnostic tools available for assessing the health of generator stator insulation.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

An acoustic diagnostic technique for use with electric machine insulation

Many of the electrical failures of large electric machines have mechanical precursors. This premise has led to the attempts described here to detect mechanical changes, such as groundwall delamination, by acoustic methods as a means of early detection of incipient failure. By optimally launching an ultrasonic wave into a stator bar and using the conductor as a waveguide, it has been shown possible to interrogate the groundwall insulation and the critical interface region adjacent to the conductor. Laboratory studies using an epoxy mica stator bar subjected to thermal cycling have demonstrated that the acoustic signatures obtained with this method are mirrored by conventional corona spectroscopy conducted simultaneously, although earlier damage is clearly discernible. Based on the results of this study, the acoustic technique appears to have substantial promise as an advanced detection tool. >

Partial Discharge Energy Measurements on Electrical Machine Insulation When Energized at Frequencies between 0.1 Hz and Power Frequency

The paper describes a very-low-frequency (VLF) high-voltage test facility and includes the results of partial-discharge (corona) measurements undertaken on samples of electrical machine insulation. The samples were energized by means of a high-voltage generator, capable of producing alternating Voltages from 0.1 Hz up to power frequency. In order to eliminate spurious signals, partial-discharge measurements were undertaken using a pulse discrimination system (PDS). The output of the PDS was fed to a pulse height analyzer which enabled the distribution of discharge magnitudes to be displayed. This information could also be transferred to an x-y recorder to provide a permanent record. The pulse height analyzer also gave integrated values of both energy and discharge magnitude. The results demonstrate the variation of discharge-inception voltage with frequency and show how it is affected by the properties of the insulation under test. The inception voltage is constant over the range 0.1 Hz to 50 Hz for low-conductivity materials, but increases at lower frequencies when there is significant conduction across the void surfaces. If tests are undertaken at a specified level above the appropriate inception voltage, then the integrated discharge magnitudes and energy values have been found to be similar to those obtained at power frequency. The paper recommends the basis for a test procedure in which discharge energy/voltage measurements would be undertaken at a specified frequency within the range 0.1 Hz to 1 Hz, the exact value depending upon the type of insulation.

Distribution of switching surges in the line-end coils of cable-connected motors

When specifying the winding insulation of electrical machines it is necessary to know the electrical stresses to which the winding will be subjected. In the case of interturn insulation the most severe stresses are caused by steep-fronted voltage waves produced by circuit-breaker closure. To some extent such steepfronted waves are reduced in severity by the time they reach the machine terminals by distortion and attenuation in the cable connecting the machine to the circuit breaker. In the studies described the interturn voltages in the line-end coils of cable-connected machines are evaluated. The equivalent circuits for the machine winding, the cable, and the energising source are obtained and compounded into an equivalent circuit for the complete system. The interturn voltages in the line-end coils of the machine are obtained from solutions of this equivalent circuit. Studies are made with a number of different cable lengths and types, and for a number of energisation source representations. Single-pole and 3-pole simultaneous closure of the circuit breaker are considered. The use of series inductors at the circuit breaker, or shunt capacitors at the motor terminals, for reducing the severity of the interturn voltages is also investigated. The results obtained indicate that the type of cable insulation and the length of cable are both significant in influencing the magnitude of interturn voltages. Energisation source impedance has a large effect. Comparison of seriesinductance and shunt-capacitance methods of surge protection reveals that both methods are effective, but that both produce other unwanted effects.

The Insulation of Electrical Machines

The Insulation of Electrical Machines

The Insulation of Electric Machines . Turner and Hobart. Pp. vi + 297. 146 illustrations. New York, The Macmillan Company. 1905. Price, $4.50.

<i>The Insulation of Electric Machines</i> . Turner and Hobart. Pp. vi + 297. 146 illustrations. New York, The Macmillan Company. 1905. Price, $4.50.

The Insulation of Electric Machines

The Insulation of Electric Machines. By H. W. Turner H. M. Hobart. Pp. xvi + 297. (London: Whittaker and Co., 1905.) Price 10s. 6d. net.