Effect of intermittent voltage source converter faults on the overall performance of wind energy conversion system

The doubly fed induction generator (DFIG) is interfaced to the AC network through voltage source converters (VSCs) which are considered to be the core of the DFIG system. This paper investigates the impact of different intermittent VSC faults on the overall performance of a DFIG-based wind energy conversion system (WECS). The fault ride through capability of the DFIG under various VSC faults is also investigated. Faults such as open circuit and short circuit across the switches, when they occur within the grid side converter (GSC) and rotor side converter (RSC), are considered and compared in this paper. Short circuit and open circuit across the DC-link capacitor are also considered in this study as common VSC problems. Simulation results indicate that the short circuit faults have a severe impact on the overall performance of the DFIG, especially when they occur within the GSC. This is attributed to the fact that the GSC directly regulates the point of common coupling voltage. The open circuit faults have less impact on the performance of the DFIG-based WECS. A proper controller along with flexible AC transmission device should be available to compensate the required active and reactive power during these faults. A protection technique is necessary to detect these faults in advance to protect the VSC switches and the machine winding from any catastrophic failure

Transient Stability Assessment of Smart Power System using Complex Networks Framework

In this paper, a new methodology for stability assessment of a smart power system is proposed. The key to this assessment is an index called betweenness index which is based on ideas from complex network theory. The proposed betweenness index is an improvement of previous works since it considers the actual real power flow through the transmission lines along the network. Furthermore, this work initiates a new area for complex system research to assess the stability of the power system

DFIG fault ride through improvement during VSC faults

The sensitivity of the doubly fed induction generator (DFIG) to external faults has motivated researchers to investigate the impact of various grid disturbances such as voltage sag and short circuit faults on the fault ride through (FRT) capability of the DFIG. However, no attention has been given to the impact of internal faults within voltage source converters (VSCs) that interface the DFIG with the grid, on the dynamic performance of the machine. This paper investigates the impact of various VSC faults on the dynamic performance and the FRT capability of the DFIG. Faults such as fire-through and flashover within the VSC switches are considered in this paper. Moreover, faults across the DC-link capacitor are included in this study as a common problem in the VSCs. The impact of these faults when they occur within the grid side converter (GSC) and rotor side converter (RSC) are investigated. A proper STATCOM controller to mitigate the effects of these faults on the FRT is proposed. The DFIG compliance with numerous and recently released FRT grid codes under these faults with and without the STATCOM are examined and compared. Furthermore, the capability of a proposed controller to bring the voltage profile at the point of common coupling (PCC) to the nominal steady-state level under five possible VSC faults cases is examined. The proposed controller is efficient, simple, and easy to implement

Complex Network Framework Based Comparative Study of Power Grid Centrality Measures

New closeness and betweenness based centrality measures have been evaluated in this paper. Power grid is modeled as a directed graph. The graph is analyzed in terms of complex network theory to identify influential nodes which control power flow pattern throughout the whole grid and as a result can create cascade if removed unintentionally or targetedly. Various measures of impacts have been analyzed to show that power grid has scale-free network characteristics, i.e., it is very much vulnerable to targeted node removal. Measures of impacts include characteristic path length, connectivity loss and blackout size. Rank similarity analysis have been carried out to show that nominal condition of power system gives critical nodes which remain critical with changes in system operating conditions as well.DOI:http://dx.doi.org/10.11591/ijece.v3i4.331

The SLUGGS Survey: Kinematics for over 2500 Globular Clusters in Twelve Early-type Galaxies

We present a spectrophotometric survey of 2522 extragalactic globular clusters (GCs) around 12 early-type galaxies, nine of which have not been published previously. Combining space-based and multicolour wide-field ground-based imaging, with spectra from the Keck/DEep Imaging Multi-Object Spectrograph (DEIMOS) instrument, we obtain an average of 160 GC radial velocities per galaxy, with a high-velocity precision of ∼15 km s−1per GC. After studying the photometric properties of the GC systems, such as their spatial and colour distributions, we focus on the kinematics of metal-poor (blue) and metal-rich (red) GC subpopulations to an average distance of ∼8 effective radii from the galaxy centre. Our results show that for some systems the bimodality in GC colour is also present in GC kinematics. The kinematics of the red GC subpopulations are strongly coupled with the host galaxy stellar kinematics. The blue GC subpopulations are more dominated by random motions, especially in the outer regions, and decoupled from the red GCs. Peculiar GC kinematic profiles are seen in some galaxies: the blue GCs in NGC 821 rotate along the galaxy minor axis, whereas the GC system of the lenticular galaxy NGC 7457 appears to be strongly rotation supported in the outer region. We supplement our galaxy sample with data from the literature and carry out a number of tests to study the kinematic differences between the two GC subpopulations. We confirm that the GC kinematics are coupled with the host galaxy properties and find that the velocity kurtosis and the slope of their velocity dispersion profiles are different between the two GC subpopulations in more massive galaxies

The Fornax Deep Survey with VST. I. The extended and diffuse stellar halo of NGC~1399 out to 192 kpc

[Abrigded] We have started a new deep, multi-imaging survey of the Fornax cluster, dubbed Fornax Deep Survey (FDS), at the VLT Survey Telescope. In this paper we present the deep photometry inside two square degrees around the bright galaxy NGC1399 in the core of the cluster. We found a very extended and diffuse envelope surrounding the luminous galaxy NGC1399: we map the surface brightness out to 33 arcmin (~ 192 kpc) from the galaxy center and down to about 31 mag/arcsec^2 in the g band. The deep photometry allows us to detect a faint stellar bridge in the intracluster region between NGC1399 and NGC1387. By analyzing the integrated colors of this feature, we argue that it could be due to the ongoing interaction between the two galaxies, where the outer envelope of NGC1387 on its east side is stripped away. By fitting the light profile, we found that it exists a physical break radius in the total light distribution at R=10 arcmin (~58 kpc) that sets the transition region between the bright central galaxy and the outer exponential stellar halo. We discuss the main implications of this work on the build-up of the stellar halo at the center of the Fornax cluster. By comparing with the numerical simulations of the stellar halo formation for the most massive BCGs, we find that the observed stellar halo mass fraction is consistent with a halo formed through the multiple accretion of progenitors with a stellar mass in the range 10^8 - 10^11 M_sun. This might suggest that the halo of NGC1399 has also gone through a major merging event. The absence of a significant number of luminous stellar streams and tidal tails out to 192 kpc suggests that the epoch of this strong interaction goes back to an early formation epoch. Therefore, differently from the Virgo cluster, the extended stellar halo around NGC1399 is characterised by a more diffuse and well-mixed component, including the ICL.Comment: Accepted for publication in ApJ; 25 pages and 14 figures. An higher resolution file is available at the following link https://www.dropbox.com/s/fvltppduysdn6pb/NGC1399_fin_2c.pdf?dl=