Zola Predosa, Italy
Zola Predosa, Italy

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Cavallini A.,University of Bologna | Montanari G.,University of Bologna | Tozzi M.,TechImp Spa | Chen X.,Xi'an Jiaotong University
IEEE Transactions on Dielectrics and Electrical Insulation | Year: 2011

This paper presents a methodology for the diagnosis of HVDC systems based on partial discharge analysis. In particular, the characteristics of partial discharge phenomena from different test objects are highlighted, with the aim of providing tools enabling automatic identification of the type of PD, as well as noise and disturbance rejection, for on-line and/or off-line PD testing of HVDC equipments. The results show that, using a fuzzy identification system based on higher-order moments of the PD magnitude and inter-time distributions, corona phenomena can be identified easily, whereas it is sometimes difficult to distinguish between internal and surface partial discharge sources. For this purpose, the characteristics of internal and surface partial discharge phenomena as a function of influence variables as humidity and temperature are reported. By exploiting these characteristics through on-line/off-line PD monitoring systems, accurate identification of the PD phenomena affecting an electrical apparatus can be attained. © 2011 IEEE.

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 | Year: 2010

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.

Cavallini A.,University of Bologna | Montanari G.C.,University of Bologna | Tozzi M.,TechImp Spa
Conference Record of IEEE International Symposium on Electrical Insulation | Year: 2010

This paper deals with partial discharge measurements in oil-filled distribution transformers. Measurements are performed using antenna sensors, bearing in mind that these transformers are almost never equipped with bushing taps, so that alternative means for partial detection need to be devised. A simplified model is presented to investigate the emission pattern associated with partial discharge phenomena inside the transformer. The model is validated through measurements and computer simulations, and serves the purpose of designing an optimum sensor. Eventually, PD measurements on field on distribution transformers are discussed. © 2010 IEEE.

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 | Year: 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.

Testa L.,TechImp SpA | Serra S.,TechImp SpA | Montanari G.C.,University of Bologna
Journal of Applied Physics | Year: 2010

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.

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 | Year: 2014

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.

Tozzi M.,TechImp S.p.A. | Cavallini A.,University of Bologna | Montanari G.C.,University of Bologna
IEEE Electrical Insulation Magazine | Year: 2011

Large numbers of motors driven by variable-speed drives have failed prematurely over the past 20 years. Increased partial discharge (PD) activity in the insulation is known to be the main reason for these failures, causing faster insulation degradation and accelerating aging, and leading to loss of reliability [1]-[13]. Whereas Type II motors (rated voltage ≥700 V and generally form wound) are designed to withstand PD activity during their working life [14], Type I motors (rated voltage <700 V and generally random wound) are not expected to experience PD activity. To test for PD in Type I motors fed by variable-speed drives, the International Electrotechnical Commission (IEC) released a technical specification (TS) in 2006 [15] describing impulsive or sinusoidal off-line tests to be carried out on Type I insulation systems (including complete stators). Although at first glance the TS appears appropriate for assessing PD in stator insulation resulting from impulses from power electronic drives, in practice it is ambiguous and inaccurate [16] and is currently under review. It is evident that a number of offline and on-line tests must still be carried out, because few correlations between off-line tests and on-line motor performance [13], [17] have been reported in the literature. In this article, the second of a series of three dealing with the correlation between off-line and on-line PD on Type I motors fed by pulse-width-modulation (PWM) drives, the results from the on-line PD monitoring of four random-wound motors are presented. The first article (Part 1 of the series) [16] reported the results obtained from application of the off-line test procedure to these four motors, in accordance with the TS [15]. In the third article some concerns regarding the current TS will be discussed and some critical aspects emphasized. The off-line type tests described in the first article [16] were carried out bearing in mind the characteristics of the PWM converter, e.g., impulse rise time, overshoot factor at the motor terminals, length of the cables between the converter and the motor, and the rated voltage and number of converter levels. One would expect that motors passing the type test would not experience PD when connected to the converter. However, one of the conclusions of the first article was that motors B1 and C1 failed the type test, whereas motors B2 and A1 passed it. © 2011 IEEE.

Tozzi M.,Techimp s.p.a. | Cavallini A.,University of Bologna | Montanari G.C.,University of Bologna
IEEE Electrical Insulation Magazine | Year: 2011

The off-line and on-line partial discharges (PD) under impulsive voltage on induction motors were investigated. The tests on crossed and twisted pairs of enameled wires have shown that the impulse width does not have a strong influence on the PD inception voltage (PDIV), that is the minimum voltage level at which PD occurs, but strongly affects the repetitive PD inception voltage (RPDIV). The results showed that the shape of the voltage impulse needs to be better specified. The impulses having a narrow width and of peak-to-peak amplitude equal to twice the dc bus voltage are required to properly simulate the overshoots. The additional phase-to-phase insulation tests using either sinusoidal or bipolar square voltage are required to detect possible defects along the stator winding. The off-line tests are type tests, which mean that PD testing of random-wound motors is performed once to validate a motor design. The manufacturing flaws such as physical damage to the wire insulation can promote PD activity in motors with good design.

Montanari G.C.,University of Bologna | Dissado L.A.,University of Leicester | Serra S.,TechImp Spa
IEEE Electrical Insulation Magazine | Year: 2014

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.

Tozzi M.,Techimp Spa | Cavallini A.,University of Bologna | Montanari G.C.,University of Bologna
IEEE Electrical Insulation Magazine | Year: 2010

Shortly after adjustable speed drives became popular for controlling induction motors, it became evident that a correlation could be established between the use of adjustable speed drives and premature motor failures [1]-[12]. These failures were mainly attributed to partial discharges (PDs) associated with the increased electrical stresses [8]. It was shown that overvoltages at the motor terminals could be two or more times greater than the dc bus voltage that develops due to the mismatch between the impedance of the motor and the cables connecting the drive to the motor. Furthermore, the nonuniform voltage distribution between the turns within a coil was deemed responsible for the PD degradation of the interturn insulation, which normally does not occur under normal sinusoidal power frequency voltage. This latter observation was found to be more significant in random-wound motors. © 2006 IEEE.

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