1,166 research outputs found
The effect of the integration interval on the measurement accuracy of RMS values and powers in systems with nonsinusoidal waveforms
In this paper the possibility of errors in the measurement of average values (in particular rms values or active powers) in power systems under nonsinusoidal conditions are discussed. The errors considered are either due to the fact that the measurement time interval is not an exact multiple of the fundamental period of the voltage and current signals, or due to the presence of interharmonics or subharmonics. The errors are calculated and the results are illustrated by means of simple examples
Scale Effects, An Error of Aggregation Not Specification: Empirical Evidence
In a set of influential papers, Charles Jones (1995a, 1995b, 1999) argued that R&D based endogenous growth models are inconsistent with the data. He showed, in a very striking manner, that the scale effects prediction of early endogenous growth models (e.g. Romer, 1986 and 1990, Grossman and Helpman, 1991, and Aghion and Howitt, 1992) is not borne out in the data. Standard endogenous growth models attribute constant or increasing returns in the stock of knowledge or technology to the aggregate level of resources. This assumption leads to the counterfactual prediction that the rate of productivity growth should be increasing in the aggregate amount of resources devoted to accumulating knowledge. This paper presents empirical evidence in support of R&D based endogenous growth models without scale effects (e.g. Young, 1998, Howitt, 1999, Thompson, 2001, and Peretto and Smulders, 2002). In these models the average level of workers or R&D workers per firm drives growth as opposed to the aggregate level and do not share the scale effects property in the limit. Using data for the US covering 1964-2001, we show that when the number of employees or scientists/engineers are scaled down on a per establishment basis, the empirics support the latter version of endogenous growth models. Specifically, the long-run size of establishments is stable, neither declining or growing in the long-run, where size is measured in two ways: by workers per establishment and R&D workers per establishment. Second, we demonstrate a positive effect running from average establishment size to productivity growth as predicted by the theories.
Tightening the belt: Constraining the mass and evolution in SDC335
Recent ALMA observations identified one of the most massive star-forming
cores yet observed in the Milky Way; SDC335-MM1, within the infrared dark cloud
SDC335.579-0.292. Along with an accompanying core MM2, SDC335 appears to be in
the early stages of its star formation process. In this paper we aim to
constrain the properties of the stars forming within these two massive
millimetre sources. Observations of SDC335 at 6, 8, 23 and 25GHz were made with
the ATCA. We report the results of these continuum measurements, which combined
with archival data, allow us to build and analyse the spectral energy
distributions (SEDs) of the compact sources in SDC335. Three HCHII regions
within SDC335 are identified, two within the MM1 core. For each HCHII region, a
free-free emission curve is fit to the data allowing the derivation of the
sources' emission measure, ionising photon flux and electron density. Using
these physical properties we assign each HCHII region a ZAMS spectral type,
finding two protostars with characteristics of spectral type B1.5 and one with
a lower limit of B1-B1.5. Ancillary data from infrared to mm wavelength are
used to construct free-free component subtracted SEDs for the mm-cores,
allowing calculation of the bolometric luminosities and revision of the
previous gas mass estimates. The measured luminosities for the two mm-cores are
lower than expected from accreting sources displaying characteristics of the
ZAMS spectral type assigned to them. The protostars are still actively
accreting, suggesting that a mechanism is limiting the accretion luminosity, we
present the case for two different mechanisms capable of causing this. Finally,
using the ZAMS mass values as lower limit constraints, a final stellar
population for SDC335 was synthesised finding SDC335 is likely to be in the
process of forming a stellar cluster comparable to the Trapezium Cluster and
NGC6334 I(N).Comment: 10 pages, 5 figures. Accepted for publication in A&
The initial conditions of stellar protocluster formation. II. A catalogue of starless and protostellar clumps embedded in IRDCs in the Galactic longitude range 15<l<55
We present a catalogue of starless and protostellar clumps associated with
infrared dark clouds (IRDCs) in a 40 degrees wide region of the inner Galactic
Plane (b<1). We have extracted the far-infrared (FIR) counterparts of 3493
IRDCs with known distance in the Galactic longitude range 15<l<55 and searched
for the young clumps using Hi-GAL, the survey of the Galactic Plane carried out
with the Herschel satellite. Each clump is identified as a compact source
detected at 160, 250 and 350 mum. The clumps have been classified as
protostellar or starless, based on their emission (or lack of emission) at 70
mum. We identify 1723 clumps, 1056 (61%) of which are protostellar and 667
(39%) starless. These clumps are found within 764 different IRDCs, 375 (49%) of
which are only associated with protostellar clumps, 178 (23%) only with
starless clumps, and 211 (28%) with both categories of clumps. The clumps have
a median mass of 250 M_sun and range up to >10^4$ M_sun in mass and up to 10^5
L_sun in luminosity. The mass-radius distribution shows that almost 30% of the
starless clumps identified in this survey could form high-mass stars, however
these massive clumps are confined in only ~4% of the IRDCs. Assuming a minimum
mass surface density threshold for the formation of high-mass stars, the
comparison of the numbers of massive starless clumps and those already
containing embedded sources suggests an upper limit lifetime for the starless
phase of 10^5 years for clumps with a mass M>500 M_sun.Comment: accepted for publication in MNRAS. Online catalogues available soon,
please contact the authors if intereste
The initial conditions for stellar protocluster formation
Context. Galactic plane surveys of pristine molecular clouds are key for establishing a Galactic-scale view of star formation. For this reason, an unbiased sample of infrared dark clouds in the 10◦ < |l| < 65◦, |b| < 1◦ region of the Galactic plane was built using Spitzer 8 µm extinction. However, intrinsic fluctuations in the mid-infrared background can be misinterpreted as foreground clouds.
Aims. The main goal of this study is to disentangle real clouds in the Spitzer Dark Cloud (SDC) catalogue from artefacts due to fluctuations in the mid-infrared background.
Methods. We constructed H2 column density maps at ∼1811 resolution using the 160 µm and 250 µm data from the Herschel Galactic plane survey Hi-GAL. We also developed an automated detection scheme that confirms the existence of a SDC through its association
with a peak on these Herschel column density maps. Detection simulations, along with visual inspection of a small sub-sample of SDCs, have been performed to get more insight into the limitations of our automated identification scheme.
Results. Our analysis shows that 76(±19)% of the catalogued SDCs are real. This fraction drops to 55(±12)% for clouds with angular diameters larger than ∼1 arcmin. The contamination of the PF09 catalogue by large spurious sources reflects the large uncertainties associated to the construction of the 8 µm background emission, a key stage in identiying SDCs. A comparison of the Herschel
confirmed SDC sample with the BGPS and ATLASGAL samples shows that SDCs probe a unique range of cloud properties, reaching down to more compact and lower column density clouds than any of these two (sub-)millimetre Galactic plane surveys.
Conclusions. Even though about half of the large SDCs are spurious sources, the vast majority of the catalogued SDCs do have a Herschel counterpart. The Herschel-confirmed sample of SDCs offers a unique opportunity to study the earliest stages of both low- and high-mass star formation across the Galaxy
Massive 70 micron quiet clumps I: evidence of embedded low/intermediate-mass star formation activity
Massive clumps, prior to the formation of any visible protostars, are the
best candidates to search for the elusive massive starless cores. In this work
we investigate the dust and gas properties of massive clumps selected to be 70
micron quiet, therefore good starless candidates. Our sample of 18 clumps has
masses 300 < M < 3000 M_sun, radius 0.54 < R < 1.00 pc, surface densities Sigma
> 0.05 g cm^-2 and luminosity/mass ratio L/M < 0.3. We show that half of these
70 micron quiet clumps embed faint 24 micron sources. Comparison with GLIMPSE
counterparts shows that 5 clumps embed young stars of intermediate stellar mass
up to ~5.5 M_sun. We study the clump dynamics with observations of N2H+ (1-0),
HNC (1-0) and HCO+ (1-0) made with the IRAM 30m telescope. Seven clumps have
blue-shifted spectra compatible with infall signatures, for which we estimate a
mass accretion rate 0.04 < M_dot < 2.0 x 10^-3 M_sun yr^-1, comparable with
values found in high-mass protostellar regions, and free-fall time of the order
of t_ff = 3 x 10^5 yr. The only appreciable difference we find between objects
with and without embedded 24 micron sources is that the infall rate appears to
increase from 24 micron dark to 24 micron bright objects. We conclude that all
70 micron quiet objects have similar properties on clump scales, independently
of the presence of an embedded protostar. Based on our data we speculate that
the majority, if not all of these clumps may already embed faint, low-mass
protostellar cores. If these clumps are to form massive stars, this must occur
after the formation of these lower mass stars.Comment: 44 pages, 11 Figures. Accepted for publication in MNRA
Uncertainty analysis of a test bed for calibrating voltage transformers vs.Temperature
The paper addresses the evaluation of the uncertainty sources of a test bed system for calibrating voltage transformers vs. temperature. In particular, the Monte Carlo method has been applied in order to evaluate the effects of the uncertainty sources in two different conditions: by using the nominal accuracy specifications of the elements which compose the setup, or by exploiting the results of their metrological characterization. In addition, the influence of random effects on the system accuracy has been quantified and evaluated. From the results, it emerges that the choice of the uncertainty evaluation method affects the overall study. As a matter of fact, the use of a metrological characterization or of accuracy specifications provided by the manufacturers provides respectively an accuracy of 0.1 and 0.5 for the overall measurement setup
Smart characterization of rogowski coils by using a synthetized signal
With the spread of new Low-Power Instrument Transformers (LPITs), it is fundamental to provide models and characterization procedures to estimate and even predict the LPITs\u2019 behavior. In fact, distribution system operators and designers of network models are looking for all forms of information which may help the management and the control of power networks. For this purpose, the paper wants to contribute to the scientific community presenting a smart characterization procedure which easily provides sufficient information to predict the output signal of a Low-Power Current Transformer (LPCT), the Rogowski coil. The presented procedure is based on a synthetized signal applied to the Rogowski coil. Afterwards, the validity of the procedure is assessed within the Matlab environment and then by applying it on three off-the-shelf Rogowski coils. Simulations and experimental tests and results involving a variety of distorted signals in the power quality frequency range and by adopting a quite simple measurement setup demonstrated the effectiveness and the capability of the procedure to correctly estimate the output of the tested device
Low-Power Instrument Transformers and Energy Meters: Opportunities and Obstacles
Low-Power Instrument Transformers (LPITs) are becoming the preferred measurement device in the medium voltage (MV) distribution network (DN). They have several benefits compared to legacy solutions. However, the adoption of LPITs results in the need for adapting the grid and its assets to accept them. One practical example is using LPITs as the current and voltage source for energy meters (EMs), which are also used for billing purposes. The resulting measurement chain introduces several metrological challenges that must be studied and investigated. Therefore, in this work, the scenarios of LPITs and energy meters are introduced along with the latest relevant international standards. Afterwards, the opportunities and obstacles due to the implementation of the LPIT plus energy meter measurement chain are discussed. The discussion focuses on metrological requirements, accuracy evaluation, target uncertainty, and influence quantities affecting the performance of the devices
Accuracy Type Test for Rogowski Coils Subjected to Distorted Signals, Temperature, Humidity, and Position Variations
Low-Power Instrument Transformers (LPITs) are becoming the first choice for distributed measurement systems for medium voltage networks. However, there are still a lot of challenges related to their operation. Such challenges include their accuracy variation when several influence quantities are acting on them. Among the most significant influence quantities are temperature, electromagnetic field, humidity, etc. Another aspect that increases the importance of studying the LPITs’ accuracy behavior is that, once installed, they cannot be calibrated for several years; hence, one cannot compensate for in-field conditions. Hence, this work aims at introducing a simple type test for a specific LPIT, the Rogowski coil. First, an experimental setup to assess the effect of temperature, humidity, and positioning on the power quality accuracy performance of the Rogowski coil is described. Second, from the results and the experience of the authors it has been possible to design a specific type test. The test has the aim of finding the limits of the accuracy variations of a single Rogowski coil. Afterwards, such limits can be used to compensate for the in-field measurements, obtaining an overall higher accuracy. The results of this work may contribute to the alwaysevolving standardization work on LPITs
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