1,720,975 research outputs found
Reliability of HVDC and MVDC Electrical Asset: The Challenge of Insulation Design
No full textReliability of DC electrical asset components has not been investigated as broadly as for sinusoidal AC supply. On the other hand, DC assets are more and more common, and they are forecasted to grow in number, power, and voltage in the near future. This is not only in transmission grids, but also in distribution/generation (renewables), industrial application and electrification transportation. To complicate the framework, it has to be recognized that DC asset components are not only subjected to DC steady-state voltage, but also to voltage and temperature transients, as those coming from energization, voltage polarity inversion, ripple, repetitive voltage impulses and load variations. The major issue to asset component reliability can come, in these conditions, from electrical insulation. While designing insulation under AC sinusoidal voltage is a century-long practice, with many feedbacks from field installations, the same does not hold for modulated sinusoidal (i.e., power electronics) and DC supply. Electrical stresses can be different in magnitude and distribution from AC to DC, and load variations in DC can contribute to electric stress variations much more than in AC. All of this may impact significantly on aging rate and reliability of electrical insulation.This paper investigates the difference between electric field distribution, and consequent aging mechanisms and rate, from AC sinusoidal to DC supply, considering, in particular, the real DC operating conditions during which voltage and load transients can occur frequently. The contribution of partial discharges to aging rate will be also taken into account, bringing to the derivation of a probabilistic life model that can allow reliability estimations in the design of DC insulation systems to be achieved
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
Investigating Energization Transients and the Potentiality of Partial Discharge Inception and Damage in Nanofilled Polypropylene Insulation for DC Cables and Capacitors
No full textNew nanostructured insulating materials are investigated, within the European project GRIDABLE, to be used for DC cable and capacitors. Besides electrical, thermal and mechanical properties, and life behavior, work is being done to evaluate their capability to endure highly stressing conditions as those cause by the inception of partial discharges, PD. This paper, in particular, analyzes what happens during energization of a DC cable or capacitors, when voltage goes from zero to the nominal value in a few seconds, while the internal electrical field takes longer time to reach the DC steady state configuration. During an energization transient, indeed, electrical field in insulation, and insulation defects (as cavities), is driven by permittivity, not conductivity as in steady state. Hence, PD might occur with high repetition rate, which would not occur, or at much lower repetition rate, in steady state. The way to evaluate the time constant of the transient, that is, through charging current measurement, is described and successfully fitted to the results of PD measurements performed on two types of polypropylene, PP: neat and nanostructured, and a crosslinked polyethylene, XLPE, having significantly different electrical characteristics
An Approach to Noise Rejection and Partial Discharge Separation in DC Cable Testing, during Steady State and Voltage Polarity Inversion Transients
No full textQuality control, commissioning and operation of DC cable systems (and, in general, electrical apparatus) require PD testing and monitoring technologies that are effective in rejecting noise and identifying partial discharges and their source, both under DC and AC power supply. Such technologies are not fully available at present, so that this paper presents a contribution to make them feasible. A new algorithm is suggested that may be effective to separate partial discharges from noise both under steady-state DC and during voltage transients, through an automatic and unsupervised approach which looks very promising for laboratory and on-field testing and monitoring of partial discharges in DC cable systems
Effect of Voltage Slew Rate on Partial Discharge Phenomenology during Voltage Transient in HVDC Insulation: The Case of Polymeric Cables
No full textIn this article, the partial discharge (PD) behavior inside a cavity embedded in a dc cable, upon application of a voltage step from transient till steady-state dc, is analyzed as a function of voltage slew rate. The variation of PD characteristics, namely PD charge amplitude and repetition rate, is modeled and fitted from the beginning of the voltage transient to dc steady state, and the extent of aging associated with PD at different slew rates is evaluated through a cumulative damage concept. Two possible scenarios for the electric field transient and PD repetition rate are presented, differing for the ratio of conductivity and permittivity of dielectric and cavity medium. Having demonstrated that the proposed PD repetition rate model can predict reasonably the experimental results, it is shown that slowing down voltage rise time during, e.g., cable energization or voltage polarity inversion, could be beneficial to reduce the accelerated aging extent associated with voltage transients
The Influence of Nanocomposite Filler on the Lifetime Performance of Polypropylene under Voltage Polarity Reversal
No full textMany researchers are investigating the feasibility of using polypropylene in HVDC cables. To understand the basic electrical properties of nanocomposite polypropylene, this work investigates the influence of nanostructuring on DC breakdown strength, life under periodic voltage polarity reversals and space charge accumulation properties. Preliminary results show that unfilled samples feature higher charge accumulation, lower breakdown strength, and shorter life than the nanocomposite counterpart. Based on these results, nanostructured dielectrics can give an important improvement in electrical insulation properties
A Contribution to Everlasting Electrical Insulation for DC Voltage: PD-Phobic Materials
Full text linkThis paper focuses on the design of insulating materials candidate from DC-supply applications, proposing viable solutions that may increase the defect-tolerance of an insulation system and avoid the presence of highly energetic phenomena, specifically partial discharges, which can cause premature insulation breakdown. It is shown that, in principle, polymeric materials for DC insulation can be structured, possibly through nanotechnology, to avoid partial discharge inception in steady state even in UHVDC cables and high-field design insulation systems, which would exclude partial discharge degradation from the aging factors that can affect insulation reliability and life. This approach may provide basic tools to design DC insulation having electro-thermal life and reliability of virtually unlimited extent, thus of so-called PD-phobic materials
Biaxially oriented silica-polypropylene nanocomposites for HVDC film capacitors: Morphology-dielectric property relationships, and critical evaluation of the current progress and limitations
No full textDielectric polymer nanocomposites are considered as one of the most promising insulation material candidates for future capacitive energy storage applications, providing tailorability of charge trapping and transport properties at the nanometric level which is a key for increased dielectric performance of biaxially oriented polypropylene (BOPP) for metallized film capacitors in high-voltage direct current (HVDC) applications. In this study, a comprehensive investigation of morphology and dielectric performance of pilot-scale BOPP nanocomposites with hexamethyldisilazane (HMDS)-treated hydrophobic fumed silica nanoparticles was carried out, providing critical perspectives on the performance and challenges of PNCs for thin film capacitors also in a broader context. In non-oriented cast films, incorporation of nanosilica modified the crystallization kinetics and α/β-crystalline spherulitic morphology of polypropylene and reduced the accumulation of space charge under a DC electric field. The nanocomposites exhibited promising dispersion characteristics in the nano-scale, however, the low amount of micron-sized agglomerates inherently present in commercial fumed silica persisted in the compounds which can become critical for thin film applications. Subsequently, biaxial-stretching-induced morphology development and dielectric properties of silica-BOPP nanocomposites were evaluated, highlighting the role of precursor morphology and film processing in the silica-BOPP film morphology, defects and dielectric performance. Charge trapping and transport properties of silica-BOPP films were investigated by isothermal and thermally stimulated techniques under high DC electro-thermal stresses, indicating profound modification of the trap density of states brought about by nanosilica. This resulted in more homogeneous space charge distribution and reduced temperature- and field dependent DC conductivity at 100 °C in comparison to neat BOPP under moderate field stresses (<200 V μm-1), while simultaneously maintaining low dielectric loss. However, the localized weak points caused by silica agglomerates still remain a challenge for the structural homogeneity and dielectric breakdown performance of thin BOPP films under extreme-field stress, hence emphasizing the need for further advancements in the agglomerate and PNC film morphology control to provide high-reliability nanodielectric capacitor thin films for practical HVDC film capacitor applications
Partial Discharge Inception Voltage in DC insulation systems: A comparison with AC voltage supply
No full textIn this paper the AC and DC partial discharge inception voltages, PDIV, are modeled and validated through measurements performed on multi-layer flat specimens with artificial defects. The results highlight that, depending on temperature and material conductivity, there can be a large difference between PDIV under DC and AC, which depends on insulation temperature, thus, e.g., cable loading. Indeed PDIV-DC can be much larger than PDIV-AC at room temperature, but the two values can be very close (with PDIV-DC being even smaller than PDIV-AC) at maximum operating temperature, i.e. full load
- …
