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Magliano in Toscana, Italy

Testa L.,TechImp Spa | Serra S.,TechImp Spa | Montanari G.C.,University of Bologna
Journal of Applied Physics

This paper deals with aging phenomena in polymers under electric stress. In particular, we focus our efforts on the development of a novel theoretical method accounting for the discharge process (partial discharge) in well known defects present in polymers, which are essentially tiny air gaps embedded in a polymeric matrix. Such defects are believed to act as trigger points for the partial discharges and their induced aging process. The model accounts for the amplitude as well as the energy distribution of the electrons during their motion, particularly at the time in which they impact on the polymer surface. The knowledge of the number of generated electrons and of their energy distributions is fundamental to evaluate the amount of damage caused by an avalanche on the polymer-void interface and get novel insights of the basic phenomena underlying the relevant aging processes. The calculation of such quantities would require generally the combined solution of the Boltzmann equation in the energy and space/time domains. The proposed method simplifies the problem, taking into account only the main phenomena involved in the process and provides a partial discharge (PD) model virtually free of adjustable parameters. This model is validated by an accurate experimental procedure aimed at reproducing the same conditions of the simulations and regarding air gaps embedded in polymeric dielectrics. The experimental results confirm the validity and accuracy of the proposed approach. © 2010 American Institute of Physics. Source

Serra S.,TechImp Spa | Fattori M.,TechImp Spa | Morshuis P.,Technical University of Delft | Montanari G.C.,University of Bologna
Proceedings of the 2014 IEEE 18th International Conference on Dielectric Liquids, ICDL 2014

In order to answer technologically to the challenge of measuring and monitoring oil corrosivity and corrosive sulphur dissolved in oil, a new sensor has been designed to measure quantitatively the corrosivity of oil during service operation. The sensor has an intelligent measuring head which manages the whole measuring process according to ASTM D1275 A and B and IEC 62535 for spot measurements (within 24 h up to 72 h), or also through permanent monitoring of the transformer. In both cases, the measurements are done in situ (with the sensor installed on the transformer), avoiding cumbersome and expensive procedures and oil sampling, since the sensor can be easily installed at the transformer valves. The sensor has two active parts which highlight different aspects of the oil corrosion process: the first measures directly the corrosion level on the copper through a suitable sensor, while the second displays the threat level of the byproducts of the copper corrosion, by measuring the change of conductivity of the dielectric as a consequence of such byproducts accumulation. In this way a robust and accurate method for the assessment of the overall corrosion process and its effect on the transformer is obtained, lowering considerably the possibility of false alarms. In this paper a number of real case tests of the sensor are presented, and results are discussed, providing evidence of the effectiveness of this new technological device. © 2014 IEEE. Source

Serra S.,TechImp Spa | Montanari G.C.,TechImp Spa | Montanari G.C.,University of Bologna | Dissado L.A.,University of Leicester
Journal of Applied Physics

A model is presented that addresses the energy stability of localized electron states in insulating polymers with respect to delocalized free electron-like states at variable charge densities. The model was derived using an effective Hamiltonian for the total energy of electrons trapped in large polarons and spin-paired bipolarons, which includes the electrostatic interaction between charges that occurs when the charge density exceeds the infinite dilution limit. The phase diagram of the various electronic states with respect to the charge density is derived using parameters determined from experimental data for polyethylene, and it is found that a phase transition from excess charge in the form of stable polarons to a stable state of bipolarons with charge = 2 and spin number S = 0 is predicted for a charge density between 0.2 C/m3 and ∼2 C/m3. This transition is consistent with a change from low mobility charge transport to charge transport in the form of pulses with a mobility orders of magnitude higher that has been observed in several insulating polymers. © 2014 AIP Publishing LLC. Source

Montanari G.C.,University of Bologna | Dissado L.A.,University of Leicester | Serra S.,TechImp Spa
IEEE Electrical Insulation Magazine

While HVDC assets are impetuously expanding worldwide, the design of HVDC polymeric insulation systems still contains unknowns. It has been ascertained that space charge plays a fundamental role in DC insulation reliability because of modifications it may induce in the (Poissonian) electric field distribution, especially when heterocharge is formed during voltage application. Heterocharge (charge close to an electrode of opposite polarity to that of the electrode) can increase the electric field at the interface between electrode and polymer, thereby enhancing injection from the electrode. This can cause so large a field concentration as to increase the conductivity abruptly and lead to insulation failure due to the electrical discharge process called, usually, thermal instability breakdown [1], [2]. © 2014 IEEE. Source

Cavallini A.,University of Bologna | Lindell E.,University of Bologna | Montanari G.C.,University of Bologna | Tozzi M.,TechImp Spa
IEEE Transactions on Dielectrics and Electrical Insulation

This paper reports and discusses qualification and type test procedures for random wound motors as specified in IEC Technical Specification 60034-18-41. Since laboratory tests dealing with off-line partial discharges measurements followed by life tests showed that motors qualified as good can fail in very short times, an extensive investigation on the causes of such discrepancy and, in particular, on the differences existing between the stresses applied during off-line tests and in service is described. In addition, the results of partial discharge measurements performed on crossed pairs are presented, highlighting how the differences in the applied voltage waveform can influence the Repetitive Partial Discharge Inception Voltage. Suggestions to improve the off-line test procedure and related equipment are eventually discussed. © 2010 IEEE. Source

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