Dip Events Research Articles

Determining the Source of Dips Using Data of One Monitor in the MV Network

This work discusses about a method that could be used to determine the source of voltage dips based on voltage and power measurements from a single monitor in the MV distribution network. The method uses information of active powers of each feeder connected to the primary substation measured before, during and after the dip events. The ratios of during-dip and post-dip powers to the pre-dip power are used to distinguish the origin and cause of the disturbance. As a power quality problem related to the highest financial losses for industrial customers, knowing the origin and cause of voltage dips is very important in order to identify the weakest part(s) of the network and to take corrective measures that improve the power quality levels and reduce the potential impact of dips to customers.

Low-voltage ride-through capability improvement of Type-3 wind turbine through active disturbance rejection feedback control-based dynamic voltage restorer

Abstract Disconnections due to voltage drops in the grid cannot be permitted if wind turbines (WTs) contribute significantly to electricity production, as this increases the risk of production loss and destabilizes the grid. To mitigate the negative effects of these occurrences, WTs must be able to ride through the low-voltage conditions and inject reactive current to provide dynamic voltage support. This paper investigates the low-voltage ride-through (LVRT) capability enhancement of a Type-3 WT utilizing a dynamic voltage restorer (DVR). During the grid voltage drop, the DVR quickly injects a compensating voltage to keep the stator voltage constant. This paper proposes an active disturbance rejection control (ADRC) scheme to control the rotor-side, grid-side and DVR-side converters in a wind–DVR integrated network. The performance of the Type-3 WT with DVR topology is evaluated under various test conditions using MATLAB®/Simulink®. These simulation results are also compared with the experimental results for the LVRT capability performed on a WT emulator equipped with a crowbar and direct current (DC) chopper. The simulation results demonstrate a favourable transient and steady-state response of the Type-3 wind turbine quantities defined by the LVRT codes, as well as improved reactive power support under balanced fault conditions. Under the most severe voltage drop of 95%, the stator currents, rotor currents and DC bus voltage are 1.25 pu, 1.40 pu and 1.09 UDC, respectively, conforming to the values of the LVRT codes. DVR controlled by the ADRC technique significantly increases the LVRT capabilities of a Type-3 doubly-fed induction generator-based WT under symmetrical voltage dip events. Although setting up ADRC controllers might be challenging, the proposed method has been shown to be extremely effective in reducing all kinds of internal and external disturbances.

Solar farm voltage anomaly detection using high-resolution [formula omitted]PMU data-driven unsupervised machine learning

The usual means of solar farm condition monitoring are limited by the typically poor quality and low-resolution data collected. A micro-synchrophasor measurement unit has been adapted and integrated with a power quality monitor to provide the high-resolution, high-precision, synchronised time-series data required by analysts to significantly improve solar farm performance and to better understand their impact on distribution grid behaviour. Improved renewable energy generation at large solar photovoltaic facilities can be realised by processing the enormous amounts of high-quality data using machine learning methods for automatic fault detection, situational awareness, event forecasting, operational tuning, and planning condition-based maintenance. The limited availability of existent data knowledge in this sector and legacy performance issues steered our exploration of machine learning based approaches to the unsupervised direction. A novel application of the Clustering Large Applications (CLARA) algorithm was employed to categorise events from the large datasets collected. CLARA has been adapted to recognise solar site specific behaviour patterns, abnormal voltage dip and spike events using the multiple data streams collected at two utility-scale solar power generation sites in England. Fourteen days of empirical field data (seven consecutive summer days plus seven consecutive winter days) enabled this analytical research and development approach. Altogether, ∼725 million voltage measurement data points were investigated, and automatic voltage anomaly detection demonstrated.

Prestack seismic data interpolation and enhancement with common‐reflection‐surface–based migration and demigration

ABSTRACTThe standard common‐reflection‐surface stacking method simulates high‐quality zero‐offset stacked data from multi‐coverage prestack data and, as by‐products, provides three kinematic wavefield attributes in two dimensions that can be applied to solve reflection seismic problems. One of the most significant applications of those attributes is for interpolation and enhancement of prestack data through the partial common‐reflection‐surface stack approach. Because this interpolation method is based on the stacking process, it usually introduces spurious noises that can create artefacts in the migration results. In order to overcome these limitations, a new prestack data interpolation and enhancement method is presented by applying the attributes and the common‐reflection‐surface operator based on two fundamental seismic imaging operations: the Kirchhoff‐type time migration and demigration. The diffraction curves required to apply these two fundamental operations are particular cases of the common‐reflection‐surface stack operator, which better fits the diffraction events and, consequently, is the best approximation for the Kirchhoff migration operator. From synthetic and real data, impulse responses of the migration and demigration operations are shown to demonstrate how this new approach regularizes and interpolates prestack data. A simple synthetic example shows good accuracy for reconstructed reflection events and well‐resolved conflicting dip events. The application example in real data reveals that the proposed approach provides cleaner regularized sections with better reconstructed events than the results of the well‐known partial common‐reflection‐surface stack approach. It has been successfully shown that the proposed approach is a well‐founded alternative for interpolation and denoising of seismic data and produces better preconditioned data for prestack migration.

HO Puppis: Not a Be Star, but a Newly Confirmed IW And-type Star

Abstract HO Puppis (HO Pup) was considered as a Be-star candidate based on its γ Cassiopeiae-type light curve, but lacked spectroscopic confirmation. Using distance measured from Gaia Data Release 2 and the spectral-energy-distribution fit on broadband photometry, the Be-star nature of HO Pup is ruled out. Furthermore, based on the 28,700 photometric data points collected from various time-domain surveys and dedicated intensive-monitoring observations, the light curves of HO Pup closely resemble those of IW And-type stars (as pointed out by Kimura et al.), exhibiting characteristics such as a quasi-standstill phase, brightening, and dips. The light curve of HO Pup displays various variability timescales, including brightening cycles ranging from 23 to 61 days, variations with periods between 3.9 days and 50 minutes during the quasi-standstill phase, and a semiregular ∼14 day period for the dip events. We have also collected time-series spectra (with various spectral resolutions), in which Balmer emission lines and other spectral lines expected for an IW And-type star were detected (even though some of these lines were also expected to be present for Be stars). We detect Bowen fluorescence near the brightening phase, and that can be used to discriminate between IW And-type stars and Be stars. Finally, despite only observing for four nights, the polarization variation was detected, indicating that HO Pup has significant intrinsic polarization.

Two‐dimensional dictionary learning for suppressing random seismic noise

ABSTRACTDictionary learning is a successful method for random seismic noise attenuation that has been proven by some scholars. Dictionary learning–based techniques aim to learn a set of common bases called dictionaries from given noised seismic data. Then, the denoising process will be performed by assuming a sparse representation on each small local patch of the seismic data over the learned dictionary. The local patches that are extracted from the seismic section are essentially two‐dimensional matrices. However, for the sake of simplicity, almost all of the existing dictionary learning methods just convert each two‐dimensional patch into a one‐dimensional vector. In doing this, the geometric structure information of the raw data will be revealed, leading to low capability in the reconstruction of seismic structures, such as faults and dip events. In this paper, we propose a two‐dimensional dictionary learning method for the seismic denoising problem. Unlike other dictionary learning–based methods, the proposed method represents the two‐dimensional patches directly to avoid the conversion process, and thus reserves the important structure information for a better reconstruction. Our method first learns a two‐dimensional dictionary from the noisy seismic patches. Then, we use the two‐dimensional dictionary to sparsely represent all of the noisy two‐dimensional patches to obtain clean patches. Finally, the clean patches are patched back to generate a denoised seismic section. The proposed method is compared with the other three denoising methods, including FX‐decon, curvelet and one‐dimensional learning method. The results demonstrate that our method has better denoising performance in terms of signal‐to‐noise ratio, fault and amplitude preservation.

Layer-steered filter for enhancing seismic reflection interpretability

Seismic artifacts due to random and linear noises, low fold coverage, statics, and spatial aliasing are frequently affecting uncertainties in seismic interpretation. Several conventional methods, such as median filter, have been implemented to reduce random noises. However, this method can not be utilized for the area in which rich with stratigraphic features such as clinoforms and in the area with strong dips. We implemented layer-steered filter in order to attenuate random noises in this kind of situation. Layer-steered filter has ability to attenuate random noises but still respects to local dip events; therefore, the method provides better preservation of events and stratigraphics compared to other conventional methods such as median filter and dip-steered filter.

Statistical Study of Energetic Electron Butterfly Pitch Angle Distributions During Magnetic Dip Events

AbstractThe magnetic dips can create the electron butterfly pitch angle distribution (PAD) in the inner magnetosphere. However, its universality is still unconfirmed. We statistically investigate the global distribution of the occurrence rate of the magnetic dip‐related butterfly PAD for 466 keV and 2.1 MeV electrons using the measurements of the twin Van Allen Probes spanning from September 2012 to November 2018. The statistical results show that the magnetic dip‐related butterfly PADs are mainly confined to duskside to midnight within 4.5 < L < 6, and the peak occurrence rate (30% for 466 KeV and 70% for 2.1 MeV) occurs at the duskside equator around L = 5. The parametric study results suggest that the butterfly PADs preferably occur for high energy electrons with a negative initial radial flux gradient and a nearly isotropic initial PAD, which is consistent with the statistic results. This work confirms the vital importance of the magnetic dips on the evolution of the electron PADs.

Power Quality Events Segmentation in a Voltage Waveform using Joint Triggering Point Detection Scheme

Accurate and timely detection of power quality (PQ) events is imperative for adequate corrective measures to be taken. This paper presents a method of PQ event detection development called joint triggering point detection (JTPD) scheme with a view to achieving a more accurate PQ event detection in a voltage waveform. The JTPD combines the advantage of cumulative sum (CUSUM) algorithm for the statistical distribution of a signal waveform and the discrete wavelet transform (DWT) for change-point detection. The performance of JTPD scheme using detection rate was compared with CUSUM and DWT schemes and based on the analysis, the detection rate of JTPD, CUSUM and DWT were 100%, 95% and 50% respectively. With this, the proposed approach outperformed CUSUM and DWT schemes by 5% and 50% respectively. Hence, the proposed approach is suitable for accurate detection of voltage dip, swell and interruption PQ events Keywords: Cumulative Sum, Discrete Wavelet Transform, Joint Triggering Point Detection, Power Quality, Voltage Waveform

DISCOVERY OF RAPIDLY MOVING PARTIAL X-RAY ABSORBERS WITHIN GAMMA CASSIOPEIAE

Gamma Cassiopeiae is an enigmatic Be star with unusually strong hard X-ray emission. The Suzaku observatory detected six rapid X-ray spectral hardening events called "softness dips" in a ~100 ksec duration observation in 2011. All the softness dip events show symmetric softness ratio variations, and some of them have flat bottoms apparently due to saturation. The softness dip spectra are best described by either ~40% or ~70% partial covering absorption to kT ~12 keV plasma emission by matter with a neutral hydrogen column density of ~2-8e21 cm-2, while the spectrum outside of these dips is almost free of absorption. This result suggests the presence of two distinct X-ray emitting spots in the gamma Cas system, perhaps on a white dwarf companion with dipole mass accretion. The partial covering absorbers may be blobs in the Be stellar wind, the Be disk, or rotating around the white dwarf companion. Weak correlations of the softness ratios to the hard X-ray flux suggest the presence of stable plasmas at kT ~0.9 and 5 keV, which may originate from the Be or white dwarf winds. The formation of a Be star and white dwarf binary system requires mass transfer between two stars; gamma Cas may have experienced such activity in the past.

The Origin of X-ray Emission from the Enigmatic Be Star γ Cassiopeiae

AbstractGamma Cassiopeiae is an enigmatic Be star with unusually hard, strong X-ray emission compared with normal main-sequence B stars. The origin has been debated for decades between two theories: mass accretion onto a hidden compact companion and a magnetic dynamo driven by the star-Be disk differential rotation. There has been no decisive signature found that supports either theory, such as a pulse in X-ray emission or the presence of large-scale magnetic field. In a ~100 ksec duration observation of the star with the Suzaku X-ray observatory in 2011, we detected six rapid X-ray spectral hardening events called “softness dips”. All the softness dip events show symmetric softness ratio variations, and some of them have flat bottoms apparently due to saturation. The softness dip spectra are best described by either ~40% or ~70% partial covering absorption to kT ~12 keV plasma emission by matter with a neutral hydrogen column density of ~2 − 8 × 1021cm−2, while the spectrum outside of these dips is almost free of absorption. This result suggests that two distinct X-ray emitting spots in the γ Cas system, perhaps on a white dwarf companion with dipole mass accretion, are occulted by blobs in the Be stellar wind, the Be disk, or rotating around the white dwarf companion. The formation of a Be star and white dwarf binary system requires mass transfer between two stars; γ Cas may have experienced such activity in the past.

Real-Time Multiple Event Detection and Classification Using Moving Window PCA

This paper proposes a method for the detection and classification of multiple events in an electrical power system in real-time, namely; islanding, high frequency events (loss of load), and low frequency events (loss of generation). This method is based on principal component analysis of frequency measurements and employs a moving window approach to combat the time-varying nature of power systems, thereby increasing overall situational awareness of the power system. Numerical case studies using both real data, collected from the U.K. power system, and simulated case studies, constructed using DigSilent PowerFactory, for islanding events, as well as both loss of load and generation dip events, are used to demonstrate the reliability of the proposed method.

Stability assessment for transmission systems with large utility‐scale photovoltaic units

Photovoltaic (PV) systems are gradually replacing conventional synchronous generators. However, reduced system inertia and lack of dynamic grid support from PV are the main issues that could have a detrimental impact on the transient response in power systems when critical contingencies arise. In this study, the authors modelled and analysed transient and small-signal stability for a representative transmission system with realistic loading scenarios and high PV penetration levels. First, system eigenvalues were calculated to identify critical modes. Thereafter, the results of the small-signal analysis were further expanded by performing transient simulations after critical contingencies. Such contingencies detrimentally excited the critical modes of the system. To carry out this analysis, they implemented a positive-sequence dynamic model of a utility-scale PV unit (USPVU) in the open programming environment MATLAB/Simulink. This dynamic model is based on a Western Electricity Coordinating Council (WECC) generic model (full converter model), which is suitable for electromechanical transient studies. Also included was the model of the PV array, dc-dc converter, and associated control systems. The most critical factors pertaining to the detrimental or beneficial impact of USPVUs on stability were the unit commitment and dispatch strategy and the protection/control strategy during voltage swell and dip events for equivalent PV penetration and loading scenarios.

Assessment of Voltage Dips based on Field Measurements in MV Networks

Voltage dip is considered as the PQ problem related to the highest financial losses for the customers. It occurs unpredictably mainly due to short-circuit faults in the networks. Network operators and customers want to know more about the occurrence and the potential impact of voltage dips. PQ monitoring is an essential tool for the assessment of voltage dips and it can ultimately contribute to the regulation of voltage dip events. In this paper, firstly the methods of voltage monitoring and voltage dips characterization are discussed. Voltage dip indices of six substations are presented in order to assess the occurrence, type and severity of dips that occur in typical Dutch MV networks. Finally, an approach, which is based on the disrupted loads because of a voltage dip event, is introduced and applied to the data at one substation in order to assess the quality of supply voltage and its implications for the end-users.

Influence of sympathetic inrush on voltage dips caused by transformer energisation

Energising a transformer with other adjacent transformers in service may induce sympathetic inrush that could lead to long duration voltage dips. This study addresses this issue through analysing voltage dip events observed on a 400/132 kV grid when energisations of several generator step-up transformers were performed. With the help of a number of field measurements of voltage dips involving sympathetic inrush, a detailed network model in alternative transients program/electromagnetic transients program has been developed and validated. Based on the network model, comprehensive simulation studies have been performed. In addition, because of the nature of multiple controlling parameters, sensitivity assessment has been carried out to identify the key influential parameters. It is found that the sympathetic inrush, although does not affect voltage dip magnitude, can prolong voltage dip duration by 143% under the worst energisation condition. The sympathetic inrush occurring in substation transformers gives rise to a network-wide sympathetic interaction, resulting in further prolonged voltage dips on the 132 kV grid. Finally, it is proven that application of static var compensator is able to speed up the voltage dip recovery.

Evidence for quiescent synchrotron emission in the black hole X-ray transient Swift J1357.2−0933

We present high time-resolution ULTRACAM optical and NOTCam infrared observations of the edge-on black hole X-ray transient Swift J1357.2-0933. Our data taken in 2012 and 2013 show the system to be at its pre-outburst magnitude and so the system is in quiescence. In contrast to other X-ray transients, the quiescent light curves of Swift J1357.2-0933 do not show the secondary star's ellipsoidal modulation. The optical light curve is dominated by variability with an optical fractional rms of ~35 per cent, a factor of >3 larger than what is observed in other systems at similar time-resolution. Optical flare events lasting 2-10min with amplitudes of up to ~1.5 mag are seen as well as numerous rapid ~0.8 mag dip events which are similar to the optical dips seen in outburst. Similarly the infrared J-band light curve is dominated by variability with a fractional rms of ~21 per cent and flare events lasting 10--30 min with amplitudes of up to ~1.5 mag are observed. The quiescent optical to mid-infrared spectral energy distribution in quiescence is dominated by a non-thermal component with a power--law index of -1.4, (the broad-band rms SED has a similar index) which arises from optically thin synchrotron emission most likely originating in a weak jet; the lack of a peak in the spectral energy distribution rules out advection-dominated models. Using the outburst amplitude--period relation for X-ray transients we estimate the quiescent magnitude of the secondary star to lie in the range V_min=22.7 to 25.6, which when combined with the absolute magnitude of the expected M4.5 V secondary star allows us to constrain to the distance to lie in the range 0.5 to 6.3 kpc. (Abridged)

Wind farm voltage dip measurements

This paper is based on continuous measurements of voltages and currents from three wind farms for a period of 1 year, and the focus is on voltage dips. The purpose is to get an overview of the characteristics and rate of voltage dips, which occur in the wind farms and to study the wind turbine responses to voltage dips. In each of the wind farms there is one measurement point at a single wind turbine and one for measuring the contribution from the whole wind farm. Different wind turbine technologies are used in the three wind farms; fixed speed turbines with directly connected induction generators in wind farm 1 and variable speed turbines with power electronics converters and synchronous generators in wind farms 2 and 3. Voltage dips are evaluated according to the standard EN 50160, by considering the durations and residual voltages of the positive sequence component voltage dips. Some examples of voltage dip events with corresponding responses in active and reactive power are shown and discussed with a view to the different technologies. Copyright © 2013 John Wiley & Sons, Ltd.

How earthquake properties vary with depth

A new study shows systematically how seismic properties vary with depth. Lay et al. analyzed recent large and great earthquakes, including the 2004 Sumatra‐Andaman (Mw 9.2), 2010 Chile (Mw 8.8), and 2011 Tohoku (Mw 9.0) earthquakes. The researchers define four domains of seismogenic behavior along megathrust faults according to depth. In domain A, the shallowest, reaching to about 15 kilometers below sea level, large tsunami‐ generating earthquakes can occur. In domain B, extending from about 15‐ to 35‐kilometers depth, great earthquake events with large slip but diffuse short‐period energy occur. In domain C, from 35‐ to 55‐kilometers depth, smaller isolated megathrust patches rupture, producing bursts of coherent short‐period energy in both great ruptures and in moderatesized events. In domain D, which extends from about 30‐ to 45‐kilometers depth in subduction zones where relatively young ocean lithosphere is being underthrust with shallow plate dip, low‐frequency earthquakes, seismic tremor, and slow slip events occur. Below this zone, stabile sliding or ductile doma takes place. (Journal of Geophysical Research‐Solid Earth, doi:10.1029/2011JB009133, 2012)

Dipping in CygnusX-2 in a multi-wavelength campaign due to absorption of extended ADC emission

We report results of one-day simultaneous multiwavelength observations of Cygnus X-2 using XMM, Chandra, the European VLBI Network and the XMM Optical Monitor. During the observations, the source did not exhibit Z-track movement, but remained in the vicinity of the soft apex. It was in a radio quiescent/quiet state of < 150 microJy. Strong dip events were seen as 25% reductions in X-ray intensity. The use of broadband CCD spectra in combination with narrow-band grating spectra has now demonstrated for the first time that these dipping events in Cygnus X-2 are caused by absorption in cool material in quite a unique way. In the band 0.2 - 10 keV, dipping appears to be due to progressive covering of the Comptonized emission of an extended accretion disk corona, the covering factor rising to 40% in deep dipping with an associated column density of 3.10^{23} atom cm^{-2}. Remarkably, the blackbody emission of the neutron star is not affected by these dips, in strong contrast with observations of typical low mass X-ray binary dipping sources. The Chandra and XMM gratings directly measure the optical depths in absorption edges such as Ne K, Fe L, and O K and a comparison of the optical depths in the edges of non-dip and dip data reveals no increase of optical depth during dipping even though the continuum emission sharply decreases. Based on these findings, at orbital phase 0.35, we propose that dipping in this observation is caused by absorption in the outer disk by structures located opposite to the impact bulge of the accretion stream. With an inclination angle > 60 deg, these structures can still cover large parts of the extended ADC, without absorbing emission from the central neutral star.

Severity assessment issues for short voltage dips

This paper analyzes the impact of the systematic deviations that can be found in the measurements of voltage dips (also called sags) adopting algorithms in agreement with standards as it is commonly done by commercial instruments. This analysis is particularly interesting because the results of dip measurements are utilized for the calculation of severity levels and the assessment of site indexes that are parameters adopted in determination of the quality level of the power supply, but also to develop planning and design criteria for new electrical power grids or to select equipment with proper intrinsic immunity. The paper starts with an insight about dip phenomenon presenting the most important systematic deviations introduced by standard algorithms. Then, the remarkable effects on severity index calculations are presented with particular attention to short dip events. The conditions of non-detection, for dip shorter than 1 cycle, are analytically calculated and a great ambiguity for a dip even shorter than 2 cycles is shown.