1,720,990 research outputs found
Combined Electrical and Thermal Stress on Twisted Pairs: Study of the Variation over Time of the Partial Discharges Inception Voltage
No full textThe electrical insulation of small power electrical
machines is a critical element especially when they are powered
by means of electronic power supplies using techniques such as
PWM. Power supplies based upon pulsed voltage trains can
introduce, under certain conditions, overvoltage and this
overvoltage can produce the inception of partial discharge
phenomena. Enameled wires, even in case of high insulating
thermal classes, cannot resist for a long time to the erosive action
of the partial discharges and therefore, inevitably, reaching the
total discharge conditions. In addition, the erosion of the enamel
causes a decrease of the inception voltage, aggravating the
situation and quickly leading to the total breakdown. In this
study enameled wires thermal class W-240 °C have been
considered. These wires have been used to prepare twisted-pair
specimen following the IEC 60851-5 standard. They have been
subjected to thermo-electric ageing using a sinusoidal test voltage
waveform, varying its amplitude and frequency. The obtained
results show that, during the degradation, the PDIV values
continuously decrease following an inverse power law
relationship. Furthermore, the obtained data evidence that the
PDIV derivatives do not depend on the test voltage amplitudes
and frequencies but rather seems to depend on the temperatur
Al-TEATRO: Drammaturgie didattiche per l'apprendimento dell'arabo letterario. Messer Torello e il Saladino, volume 3
No full textlo studio analizza il terzo progetto di pratica teatrale per l'apprendimento dell'arabo presso la scuola interpreti e traduttori di Forlì, università di Bologna. In questo volume, dopo qualche cenno sul contesto in cui si realizza l'esperienza e sui progetti che hanno preceduto, si procede con l'esposizione del progetto Messer Torello e il Saldino, le sue specifiche caratteristiche, l'analisi dell'itinerario teatrale, l'approfondimento culturale e letterario dei testi scelti (novella nona, giornata decima del Decameron di Giovanni Bocaccio, una maqamah "prosa rimata" di al-Hamdhani, il raccontastorie e la falconeria). L'a parte finale del volume contiene il copione teatrale (arabo e italiano) e tutte le schede lessicali
Effect of Thermal Stress over the Partial Discharge Inception Voltage on Twisted Pairs
No full textPartial discharges are one of the main causes for what concerns the degradation of the electrical machines insulation system and their onset can bring the devices in question to their end of life precociously. In addition, although an electric machine does not show partial discharges when it is put into service, it is possible that this phenomenon develops over time due to the aging of the insulating material caused by the electrical stress to which it is subjected. However, depending on the application in which these machines are used, electrical aging may not be the only cause that leads to partial discharges. Some electrical machines with type I insulating system may in fact be used in more prohibitive applications that involve high temperatures, therefore in such cases a possible cause of the onset of partial discharges may be the thermal ageing. In this study, twisted pairs with thermal class W-240 °C composed of copper wires covered with a polyimide-based enamel were analyzed. Thermal ageing tests were carried out on several specimens at different temperatures in order to measure the trend of their Partial Discharge Inception Voltage (PDIV) over time. To prevent electrical ageing phenomena from modifying and influencing the results, the repeated PDIV measurements were made in a very short time. In the here considered case, the study demonstrated that at both test temperatures the specimens showed a rapid decrease in the PDIV values in the first 65 hours while subsequently there was a flatter trend. From the reported graphs it is clear how a higher temperature can lead to a more marked decrease of the PDIV values
Electrical Aging Tests on Conventional and Nanofilled Impregnation Resins
No full textThe main insulating system components of stator windings of rotating electrical machines are organic enamel and impregnating resin. When power electronic converters supply motors, the stator insulation system is subjected to voltage waveforms such as PWM composed by trains of pulses. Due to reflection phenomena, overvoltage can be associated to pulsed waveforms and partial discharge (PD) activity can incepts
leading to the insulating system failure in short time. In this work two types of conventional and nanofilled impregnating resins, based on unsaturated polyester and solvented alkydic resin matrices, have been compared. Suitable twisted pair samples prepared by means of wires insulated with conventional enamel and by two impregnating resins have been tested. The twisted pair samples have been subjected to
sinusoidal voltage waveforms, having amplitudes slightly higher than the Partial Discharge Inception Voltage. Aging tests have been performed until the insulating system breakdown, which is assumed as end-point criterion: during the tests, the PD activity has been monitored by means of a PRPDA system. For each sample, starting from the PD signals post processing, the apparent current of positive discharges has been considered: this derived quantity showed a linear behavior with the degradation time. In order to define a life model, the slopes of the straight
lines have been put in relation with the related lifetimes by means of the inverse power law
Transient Earth Voltage Sensor Measurements as a Nonintrusive Tool for Partial Discharge Analysis in Switchgears
No full textA review on the comparison of conventional and corona resistant nanofilled enamels
No full textThe low voltage rotating electrical machines, typically based on Type I random/wound windings, are normally controlled by power electronic drives. The power supply voltage waveforms are characterized by fast rise and fall time fronts and in case of resonance and reflection phenomena, overvoltage could appear at the connection terminals of the machine. These overvoltages can incept the partial discharges in the electrical insulation, leading to the total breakdown of the machine. The nanocomposite material used as enamel on the winding wires can lead to increase the performances of the insulating system. In recent years, new class of corona resistant materials based upon nano particles have been characterized. In this work, an overview of the research studies performed by the CMTEST laboratory staff of the University of Genova is presented to summarize the results obtained comparing the behavior of conventional and nano-filled enamels
Effects of a DC Offset on Lifetime and PD Activity during AC Aging Tests on Enameled Twisted Pairs
No full textStudy of the Partial Discharge Inception Voltage Variation Over Time on Enameled Wires Subjected to Thermal-Electrical Stress
No full textThe electrical insulation is a crucial element in electrical machines. The performance of the insulation system can be affected by the operating conditions that cause ageing of the insulation. The evaluation of the effects of the environment stresses is a key in the prediction of the electrical insulation breakdown. Being capable of forecasting the maintenance avoiding the out of service due to insulation breakdown is certainly an objective. It is proven that partial discharges (PD) are one of the causes of electrical insulation breakdown. The evolution of PD is an indication of the condition of the electrical insulating system. The severe lowering of the Partial Discharge Inception Voltage (PDIV) value can be a signal of the approaching to the insulation total breakdown. The continuous lowering of the PDIV is due to the degradation of the insulation properties caused by the stresses applied during the operation time. The aim of this study is to evidence and compare the effects of thermal stress on different kind of enamelled wires analysing the PDIV because is one of the accelerating factors together with relative humidity and frequency of the applied voltage [1]-[7]. In this study, twisted pairs made with copper wires covered with different enamels and having thermal class 200°C were assessed. The samples were subjected to thermo-electrical aging tests at the same voltage amplitude and frequency while different temperatures have been applied. The PDIV relevant to the samples was measured regularly to monitor its evolution over time
Comparison of PD Patterns Obtained by Conventional Conducted Signals Measurement and TEV Sensor during Electrical Ageing Tests
No full textThe conventional Partial Discharges (PDs) measurement systems, IEC60270 compliant, normally use a quadripole as coupling device to convert the current pulses to output voltage signals when the system bandwidth is in the frequency range from 40 kHz to 1 MHz. This kind of measurement usually needs an off-line measure and an invasive procedure to connect the objects under test to the measurement system. Other measurement systems based on non-invasive sensors like High Frequency Current Transformer (HFCT) and Transient Earth Voltage (TEV) could be used if the provided results give comparable response. Focusing on TEV sensors, they can be coupled to measurement systems having a frequency bandwidth out of the range defined in IEC60270 standard. The comparison among TEV measurement system and those IEC60270 compliant can be based on the derived and statistical quantities computed starting from the PD pattern. PD Patterns have been collected during electrical ageing test applied to twisted pair located in an air insulated medium voltage switchgear (AIMVS)
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