1,721,058 research outputs found
Crisi e declino nella manifattura italiana: note per l'agenda di ricerca e la politica industriale
Full text linkLa durezza della crisi economica e la velocità di sfaldamento della base manifatturiera del paese, così come emergono dai dati sul reddito pro-capite e la produttività, travalicano ormai il portato euristico acquisito da categorie analitiche quali “crisi strutturale”, “ristrutturazione industriale” o “delocalizzazione produttiva”. Allo stesso tempo, il termine di “declino” richiede una disambiguazione. Per la prima volta dal secondo dopoguerra, i fattori di globalizzazione economica e le dinamiche geopolitiche, impattando sugli irrisolti elementi di fragilità del sistema produttivo nazionale, possono innescare esiti di declino industriali auto-cumulativi e non reversibili. Inoltre, l’esame di alcuni dei caratteri del modello di specializzazione italiano e delle dinamiche di generazione e diffusione tecnologica (R&S e ICT) non evidenziano gli auspicabili segnali di cambiamento; questa pericolosa stasi è poi rafforzata dal protratto impasse del quadro socio-politico e dall’assenza di politiche industriali coraggiose. Il presente contributo, nel riflettere su questi fenomeni “epocali”, ripercorre alcune questioni irrisolte sul modello di sviluppo italiano, con l’ambizione di rimettere il tema del declino socio-economico e culturale del paese al centro dell’agenda di ricerca e della politica industriale.
Codici JEL: L60, O14, O30, O40, P17
HVDC and UHVDC polymeric cables: Feasibility and material development
No full textFeasibility of HVDC and UHVDC insulated cables should not be considered as obvious when planning and designing transmission DC lines. Proper cable insulation design criteria and development of new materials able to withstand higher electro-thermal stresses could be needed to reach the feasibility of the insulated cable component of an UHVDC link
A voltage threshold in operating condition of PWM inverters and its impact on reliability of insulation systems in electrified transport applications
No full textThere is a trend in industrial applications and electrified transport towards rising supply voltage and switching modulation frequencies, in order to reach better efficiency and higher specific power of rotating machines. However, the aspect of insulation system design and reliability for the whole chain of the adjustable speed drive, including inverter, cable and rotating machine, is often neglected. The major electric aging mechanism that can bring insulation to failure in short times is partial discharges, which can incept in insulation defects, being triggered by local electric stress. Understanding PD phenomenology in insulation fed by PWM is, therefore, a must to ensure adequate life to the insulation chain. This paper shows the existence of a peculiar PD behavior, associated with a threshold voltage below and above which the PD regime may change significantly in terms of amplitude and repetition rate. Inverter behavior under PD is modelled, as a function of the number of inverter levels, and the validity of the model is tested through PD and accelerated life testing. Good fitting to experimental results is obtained using twisted pairs test objects, with striking evidence about the effects on life of operating below or above the threshold on insulation reliability
Investigating aging phenomenology of type I and type II insulation systems of rotating machines fed by power converters
No full textAging of insulation systems of rotating machines controlled by voltage source converters is an issue becoming of utmost importance due to the broader and broader application of speed-controlled rotating machines. Since one of the major concerns for machine reliability is the appearance of partial discharge (PD), this paper investigated how PD behave under multi-level inverter waveforms, and what is their impact on insulation life. PD patterns under sinusoidal and repetitive impulse voltage, considering two, three and five level inverters, are presented showing that PD phenomenology is deeply different, and its impact on insulation life changes depending on number of levels and voltage impulse rise time. This holds for both Type I and Type II insulation
Modelling of supply voltage frequency effect on partial discharge repetition rate and charge amplitude from AC to DC at room temperature
Full text linkThis paper has the purpose to derive an analytical model ('continuum' model) able to describe the behavior of partial discharge (PD) repetition rate and amplitude, occurring in a cavity embedded in polymeric insulation, as a function of the frequency of the supply voltage, going from AC power supply frequency, 50-60 Hz, to DC. In the range between DC and 50-60 Hz focus is made on data coming from tests under AC sinewave with very low frequency (VLF) such as 0.1 Hz and 0.01 Hz, which are commonly used for cable testing. It is shown that the proposed 'continuum' model can provide reasonably good fit to the experimental results obtained in the range DC to 60 Hz, regarding PD repetition rate and amplitude. To reach such result, the equivalent circuit is modified from that commonly used and made by fully-capacitive or resistive components, in order to take into account the change of polarization mechanisms which, depending on dielectric material, may play a non-negligible role to establish the repetition rate from low frequency to DC power supply. In addition, the residual voltage after a PD event has to vary with frequency to reach good fitting. Also, it is shown that PD amplitude under DC and VLF can be lower than under AC 50-60 Hz due to the delay time of the firing electron, thus experimental PD amplitude varies with frequency depending on material and defect typology and location
The Effect of Superimposed Switching Impulses on Space Charge Accumulation in XLPE-insulated HVDC Cables
No full textDC Voltage Supply in TD: What about Electrical Insulation Reliability and Maintenance?
No full textThe growth of DC supply in distribution and electrical transport raises the issue of assessing the long-term behavior of electrical insulation. Type and prequalification tests are specified for cables, but ensuring a life at low failure probability may be not as obvious as under AC. Besides caring of reliable operation under DC steady-state, design must take into account that DC insulation systems are subjected, during operation, to numerous energizations and, sometimes, voltage polarity inversions, during which the electric field distribution is as under AC. This may favor partial discharge occurrence and, thus, increase degradation rate, because, in general, the partial discharge inception voltage in AC is lower than that in DC. The potential sources of accelerated degradation of DC insulation, and how they can affect reliability and life, are discussed in this paper, focusing on partial discharges. Issues related to partial discharge measurements under DC power supply and diagnostic tools are also examined, highlighting a worrisome lack of focus on DC electrical insulation
Partial discharge measurements and life estimation in DC electrical insulation during voltage transients and steady state
No full textThe design and operation of DC insulation systems is more complex than for AC sinusoidal supply. The most subtle issues are how the electric field in DC can vary with operation and affect the partial discharge inception voltage and partial discharge phenomenology. During operation, a DC insulation system is indeed subjected to voltage and load transients, the former due to energization and voltage polarity inversion. This paper sheds some lights on such issues, going through another fundamental problem, that is, how to carry out effective partial discharge detection under DC supply, both in steady state and during voltage transients. An algorithm that can enable automatic and unsupervised partial discharge measurements in DC electrical apparatus is presented and validated based on partial discharge monitoring performed on test objects representing defective insulation systems. Identification of the source generating partial discharges, which is the basis for diagnostic and condition maintenance, is achieved without the support of experts. The impact of voltage transients on the life and reliability of DC insulation systems is modelled in the presence and absence of partial discharges. It is shown that, depending on the repetition rate, the presence of partial discharges, even if only during voltage transients, can dramatically affect the insulation life and cause premature insulation breakdown
Measuring PD under Fast Slew Rate, High Voltage and High Frequency Repetitive Voltage Impulses
No full textIncreasing specific power in emerging assets, as those for electrification transport, is a must that is substantiated through the use of power electronic drives, having increasingly higher voltage, switch slew rate and frequency. This paper proposes time-domain based algorithms able to separate and identify PD pulses occurring during voltage impulse rise time, thus very hardly detectable. The paper investigates the validity of such algorithms as a function of voltage amplitude and modulation frequency. The proposed method is effective in extreme conditions where PD pulses and switching noise overlap integrally in the frequency domain, and they are not easily recognizable in the time domain even by an expert eye
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