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Aerodynamic instability plays an important role in compressor design and may cause performance degradation and fatigue damage. In this paper, an experimental study on the evolution of aerodynamic instability is carried out on a compressor that combines the performance benefits of an axial stage and centrifugal stage. The spatiotemporal characteristics of unsteady wall pressure were obtained using fast-responding pressure transducers over a range of operating conditions. The results show that the axial stage works on the positive slope of the performance characteristic curve from choke to stall at low-speed operating conditions, and mainly features rotating instability. Rotating stall is also observed in the impeller (IMP) and diffuser passages. At medium-speed operating conditions, the centrifugal stage suffers a high-frequency mild surge, alternating with rotating stall. With the increase in back pressure, the mild surge diminishes, and rotating stall persists. This behavior is similar to a two-regime-surge, which has been reported for centrifugal compressors. At high-speed operating conditions, the compressor directly reaches surge without other instabilities. Further analysis of the spatial pattern of the rotating stall revealed the existence of a high-pressure region near the volute tongue, resulting in obvious pressure distortion along the circumferential direction at the volute inlet. This induced the amplitude difference of stall cells in corresponding diffuser passages. The disturbance caused by stall cells propagates upstream through the blade passage, and the largest pressure disturbance induced by the stall cell propagation appears in a circumferential position 45 deg downstream of the volute tongue at the impeller inlet and the axial stage inlet. [ABSTRACT FROM AUTHOR]

A variable pacing profile is common in different endurance events. In these races, several factors, such as changes in elevation or race dynamics, lead participants to perform numerous surges in intensity. These surges are so frequent that certain events, such as cross-country (XC) skiing, mountain biking (MTB), triathlon, and road cycling, have been termed "intermittent endurance events". The characteristics of these surges vary depending on the sport: MTB and triathlon require athletes to perform numerous short (<10 s) bouts; XC skiing require periods of short- and moderate-(30 s to 2 min) duration efforts, while road cycling is comprised of a mix of short-, moderate-, and long-duration (>2 min) bouts. These bouts occur at intensities above the maximal metabolic steady state (MMSS), with many efforts performed at intensities above the athletes' maximal aerobic power or speed (MAP/MAS) (i.e., supramaximal intensities). Given the factors that influence the requirement to perform surges in these events, athletes must be prepared to always engage in a race with a highly stochastic pace. The aim of this review is to characterize the variable pacing profile seen in endurance events and to discuss how the performance of multiple maximal and supramaximal surges in intensity can affect how athletes fatigue during a race and influence training strategies that can lead to success in these races. [ABSTRACT FROM AUTHOR]

At Tanis, a unique bi‐directional sediment package occurs precisely at the Cretaceous‐Paleogene (K‐Pg) boundary, recording the first hours of the Paleogene in uncommonly fine temporal detail. The impact ejecta‐bearing sediment package was rapidly emplaced by two massive, ∼10‐m‐high, potentially impact‐triggered surges, that inundated a steep, deeply incised paleo river valley from the direction of the contemporaneous Western Interior Seaway (WIS). Intermingling of fresh‐ and salt‐water fossils at Tanis, coeval brackish water indicators in the nearby region, and historical tsunami observations, suggest that the WIS paleoshoreline was nearby Tanis at K‐Pg time. The interpreted timing for deposition (including ejecta infall) of ∼1–2‐hr immediately post‐impact precludes a direct tsunami from the Chicxulub impact site, which would have required much more than 10 hr to reach Tanis. Seismic waves from the Mw ∼ 11 Chicxulub earthquake, arriving just minutes post‐impact, might have triggered the surge, for example, via seismic excitation of large water waves in the WIS, as proposed by DePalma et al. (2019, https://doi.org/10.1073/pnas.1817407116). Here, we explore this mechanism via a simple mathematical model of seismic excitation and propagation of a water wave into a shallow river and upstream. Matching the observations implies a relatively long source process time of many minutes, such as generated by Chicxulub crater rebound processes, in order to explain sufficient upriver amplitudes and advective transport. Atmospheric waves due to the expanding Chicxulub ejecta curtain might have provided a smaller, secondary contribution during triggering. Thus, the mechanism(s) for the surges at Tanis are now better‐constrained, yet remain incompletely resolved. Plain Language Summary: A remarkable geological record at the "Tanis" site (North Dakota) shows that two ∼10 m high tsunami‐like surges occurred on a point bar along a river upstream from the then existing Western Interior Seaway (WIS). The surge deposits are interpreted to have occurred within 1–2 hr of the Chicxulub impact associated with the end‐Cretaceous mass extinction including the dinosaurs, and are challenging to explain via traditional tsunami mechanisms because they occurred too soon after the impact to have been tsunami waves traveling from the impact site. Alternative explanations include excitation of large water waves in the WIS due to seismic shaking from the magnitude ∼11 impact‐generated earthquake or atmospheric effects from the expanding impact ejecta curtain. Here, we explore one potential triggering mechanism via a simple mathematical model of direct seismic excitation and propagation of a water wave into a shallow river and upstream. This model shows the difficulty of generating the observed surge deposits unless the timescale for impact‐generated seismic waves is much longer than is commonly presumed. Thus the actual triggering mechanism(s) of the Tanis depositional surges remains incompletely resolved, while the sediment package bears a rich source of information about the global effects of the Chicxulub impact. Key Points: An ∼10 m surge event coincident with the Chicxulub impact transported mixed freshwater/marine organisms up the Tanis river valleyNew modeling confirms limitations on the inland transport of marine fossils and helps to constrain the impact‐related triggering mechanismOther potential triggering mechanisms and strategic avenues of future study are discussed [ABSTRACT FROM AUTHOR]

Modern compressor design targets require high performance and a wide operating range in order to reduce the environmental impact. To understand the fluid dynamics mechanisms that trigger instability, studying the system at the stability limit is required. In this work, a two-stage back-to-back centrifugal compressor for refrigerant applications has been simulated with computational fluid dynamics (CFD) techniques using unsteady calculations in different operating points close to surge. These models have been validated by comparing numerical performance with experimental data. An in-depth fluid dynamics analysis combined with the monitoring of several pressure signals, postprocessed with FFT, identified different flow phenomena in the two stages toward the surge limit. The key role of the volute that induces a stronger upstream counterpressure in the first stage has been highlighted. This effect causes the formation of high entropy (low momentum) rotating cells in the diffuser that involve a higher channel portion with respect to the flow structure in the second diffuser. This phenomenon affects the upstream flow conditions at the impeller. In addition, the interaction between the inlet guide vane (IGV) and the inducer has been analyzed, observing that in the second stage, due to the flow nonuniformity after the intermediate compressor pipe, non-negligible separations occur. Starting from the peaks detected in the FFT analysis of the pressure signals, all the above flow mechanisms have been detected and discussed. [ABSTRACT FROM AUTHOR]

A lumped-parameter dynamic performance model for a single-spool turbojet engine is presented in this paper. This model can handle pre and poststall transients under forward and reverse-flow conditions. The inter-component volume technique is employed instead of the standard matching technique to be able to handle high-frequency transients and reverse-flow conditions. Inspired by Greitzer's lumped-parameter surge model, momentum (duct) and volume elements are placed within the flow path to handle surge dynamics. Compressor and turbine maps are extended to low-flow and reverse-flow regions using a combination of the guidelines presented by Kurzke, the cubic axisymmetric characteristics of Moore and Greitzer, and a quadratic function guess for in-stall characteristics. Combustor efficiency, stability limits, and delay are taken from the literature. Poststall behavior of the model is validated using the data available in the literature for a Rolls-Royce Viper engine. A good match is observed with a correct prediction of poststall behaviors, which transition from surge after locked stall to multiple surge cycles around 80% speed and multiple surge cycles to surge after flameout around 95% speed. The effects of different modeling choices and modeling parameters on the obtained results are discussed. The produced model can be calibrated for a specific engine with surge tests, and it can be used for hard-to-test scenarios like surge after shaft breakage. Different surge/stall-causing events, such as fuel spiking, in-bleeding, and shaft breakage, are simulated to see the capabilities of the model. [ABSTRACT FROM AUTHOR]

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Acoustic resonators are widely used to dampen the sound radiation of centrifugal compressors of automobile turbochargers. Their advantages are simplicity, robustness, ease of integration, and acoustic tunability. Interestingly, some resonators mounted to the inlet side improved compressor performance at low mass flow rates significantly. In this paper, this phenomenon was investigated experimentally by an extensive study of resonator geometries on a turbocharger test rig and numerically by uRANSs of selected compressor resonator configurations near the surge limit. A detailed description is given on how the resonator can limit the disturbance of the incoming flow and improve the impeller's operating conditions by ingesting the backflow coming from the blade tips. The backflow's kinetic energy and angular momentum are greatly reduced inside the resonator before the fluid flows back into the inlet duct. Concluding, design guidelines for a resonator geometry, which improve performance as an addition to its acoustic properties, are shared. [ABSTRACT FROM AUTHOR]

A variable pacing profile is common in different endurance events. In these races, several factors, such as changes in elevation or race dynamics, lead participants to perform numerous surges in intensity. These surges are so frequent that certain events, such as cross-country (XC) skiing, mountain biking (MTB), triathlon, and road cycling, have been termed “intermittent endurance events”. The characteristics of these surges vary depending on the sport: MTB and triathlon require athletes to perform numerous short (2 min) bouts. These bouts occur at intensities above the maximal metabolic steady state (MMSS), with many efforts performed at intensities above the athletes’ maximal aerobic power or speed (MAP/MAS) (i.e., supramaximal intensities). Given the factors that influence the requirement to perform surges in these events, athletes must be prepared to always engage in a race with a highly stochastic pace. The aim of this review is to characterize the variable pacing profile seen in endurance events and to discuss how the performance of multiple maximal and supramaximal surges in intensity can affect how athletes fatigue during a race and influence training strategies that can lead to success in these races.

The study of the electrical phenomena that originate during the operation of the transformers is of vital importance to predict the consequences that these will bring and to improve the design of the system and of the transformer itself. To carry out these studies it is necessary to use existing simulation tools. The accuracy of the response of the simulation process depends on the degree of detail and the veracity of the data, as well as the type of mathematical model used, so it is necessary to obtain these data and their application in the most suitable model for the simulation of each phenomenon. This paper presents the results of a project that consisted in obtaining, through simulation, the impulse voltage distribution within the transformer windings from a detailed model with the final objective of verifying the effectiveness of the insulating systems used in the transformers prototypes used. [ABSTRACT FROM AUTHOR]

An accurate prediction method of cavitation surge is desired to design a reliable turbopump for rocket engines that allow complex operational sequences such as controlled reentry and landing. Therefore, the paper aims to develop a novel model that enables accurate predictions of both frequency and onset of cavitation surge by considering dynamic characteristics of cavitation compliance K and mass flow gain factor M. The paper conducts both experimental and numerical studies in an inducer known to cause cavitation surge. First, characteristics of cavitation surge including frequency and occurrence conditions are experimentally surveyed. Secondary, K and M for the studied inducer are numerically obtained by steady RANS simulations. Then, the one-dimensional model is initially established based on the premises acquired by performed experiments and applied to cavitation surge predictions using quasi-statically evaluated K and M. As a result of comparisons with experiments, the cavitation surge frequency is adequately evaluated by the established model, whereas the cavitation surge onset fails to be estimated only using quasi-static parameters. The new model is thus proposed by including dynamic characteristics of K and M, and it is mathematically clarified that phase properties of K and M may play a key role to trigger cavitation surge. By presuming adequate values of dynamic characteristics based on past literatures, consequently, the developed model succeeds in accurate predictions of the cavitation surge onset in the experiment. This evidences that dynamic characteristics of K and M are essential to predict cavitation surge quantitatively. [ABSTRACT FROM AUTHOR]

The distribution of magnetic fields of positive and negative polarities over the surface of the Sun was studied on the basis of synoptic maps presented by the NSO/Kitt Peak (1978 – 2016). To emphasize the contribution of weak fields the following transformation of synoptic maps was made: for each synoptic map only magnetic fields with a modulus less than 5 G (| B | ≤ 5 G ) were left unchanged, while larger or smaller fields were replaced by the corresponding threshold values +5 G or − 5 G . Cyclic variations of the magnetic-field polarity have been observed associated with two types of magnetic-field flows in the photosphere. Rush-to-the-poles (RTTP) appear near the maximum of solar activity and have the same sign as the following sunspots. The lifetime of RTTP is ∼ 3 yr, during which time they drift from latitudes 30 – 40 ∘ to the pole, causing the polarity change of the Sun's polar field. Our aim is the study of another type of variation that has the form of series of flows with individual flows of 0.5 – 1 yr with alternating polarity. These flows, called "ripples" by Ulrich and Tran (Astrophys. J.768, 189, 2013), are located in time between two RTTP and drift from the equator to latitudes of ∼ 50 ∘ . The period of variation of ripples was shown to be 1.1 yr for the northern hemisphere and 1.3 yr for the southern hemisphere. It was found that the amplitude of the variation was higher for the time intervals where the polar field had a positive sign. Within the same flow, fields of positive and negative signs developed in antiphase. Two types of flow – RTTP and ripples – together formed a unique structure that had a close connection to the magnetic solar cycle. [ABSTRACT FROM AUTHOR]

This article proposes a flexible formation tracking control protocol (FFTC) for multiple unmanned surface vessels (USVs) to pass through narrow water channels with unknown curvatures. An observer is developed to estimate the curvatures of channels. A flexible formation tracking control protocol is, thereby designed to steer USV fleets to pass through narrow channels in a flexible serial formation. Furthermore, the asymptotically stable conditions of the closed-loop multi-USV systems are theoretically derived. Finally, both numerical simulation and experimental results with a fleet of three HUSTER-0.3 USVs are reported to substantiate the effectiveness of the proposed FFTC protocol for navigating through narrow channel in a laboratory context. [ABSTRACT FROM AUTHOR]

The prediction of the aerodynamic damping during compressor surge is a challenging task, because the flow is continuously evolving along the four surge cycle phases: pressurization (PR), flow-breakdown (FB), reversed flow (RF), and regeneration (RG), and complex flow conditions such as shocks and separations occur. Damping predictions with current existing methods typically consist of two steps. In the first step, a modified numerical model is used to simulate transient surge cycles. In the second step, damping analyses are performed for multiple timesteps along the surge cycle phases, which are then assumed as quasi-steady. The damping simulation can be performed using nonlinear or linear approaches. If shocks or separations occur, the latter yields inaccuracies in the flow and thus in the damping predictions. A new approach was developed to take into account and improve these inaccuracies. This new method includes the damping prediction within the transient surge simulation. Thus, all surge cycle phases and the continuously evolving flow conditions are considered, and nonlinear simulations are performed to account for shocks and separations. The results of this new method are presented and compared to the former method. [ABSTRACT FROM AUTHOR]

This article addresses the dynamic positioning control problem of a nonlinear unmanned marine vehicle (UMV) system subject to network communication constraints and deny-of-service (DoS) attack, where the dynamics of UMV are described by a Takagi–Sugeno (T-S) fuzzy system (TSFS). In order to save limited communication resource, a new intelligent event-triggering mechanism is proposed, in which the event triggering threshold is optimized by a $Q$ -learning algorithm. Then, a switched system approach is proposed to deal with the aperiodic DoS attack occurring in the communication channels. With a proper piecewise Lyapunov function, some sufficient conditions for global exponential stability (GES) of the closed-loop nonlinear UMV system are derived, and the corresponding observer and controller gains are designed via solving a set of matrix inequalities. A benchmark nonlinear UMV system is adopted as an example in simulation, and the simulation results validate the effectiveness of the proposed control method. [ABSTRACT FROM AUTHOR]

Rogowski coil is sensitive to current surges in nearby systems. Hence, if it is used to measure a current in a system and a current surge occurs in a nearby location to this system, the coil senses this current in some degree. This means that the performance of the coil is affected by this current surge. Therefore, this article presents a low-cost shielding method depending on using aluminum foil sheets. The effectiveness of this shielding is studied through experimental results. The experimental study is carried out by building three similar coils. One of them is designed without shield with a return winding (RC1), the second coil is designed without shield with no return winding (RC2) and the third one is shielded using aluminum foil sheet (Rc3). Also, an impulse generator is used to generate high impulse current in a system that locates at different distances from the shielded and unshielded coils. The induced voltage signals in the three coils are captured using a digital oscilloscope. The captured voltage signals are used to evaluate the shielding performance of the coil. It is found that using shielding in Rogowski coil changes the coil model parameters. So, a shielded coil model is developed and presented. The validity of the proposed model of the shielded coil is examined by using impedance–frequency measurements which carried out using an accurate LCR meter. Finally, the efficacy of the proposed shielding method and the validity of the shielded coil model are validated through the obtained measurements. [ABSTRACT FROM AUTHOR]

It is well known that for high voltage transmission lines in the range of 245 kV–525 kV, particularly in regions of high ground resistivity, lightning back flashover problems could be a major concern. Line arresters have been used successfully to address the problem. Underbuilt ground wires have been proposed as a viable alternative. The present paper shows that the full potential of additional ground wires could be achieved by optimizing their position on the tower. Such optimization often results in a preferred position at the level, or higher than the attachment height of the lower phase conductors; therefore the designation Embedded Ground Conductor. An essential requirement is that such positioning does not infringe in any way on the dielectric integrity of the electrical clearances and, under the influence of induced charge, does not become a source of positive streamers that may lead to additional radio or audible noise. It is found that a streamer- inhibited conductor, when properly placed, fully satisfies the above requirements. It is determined that for 245 kV, 345 kV and 420 kV lines, a streamer-inhibited embedded conductor could improve the back flashover rate by a factor of 10 or more [ABSTRACT FROM AUTHOR]

This paper considers a caching system of a single server and multiple users. We aim to characterize the memory-rate tradeoff for caching with uncoded cache placement, under nonuniform file popularity. Focusing on the modified coded caching scheme (MCCS) recently proposed by Yu, etal., we formulate the cache placement optimization problem for the MCCS to minimize the average delivery rate under nonuniform file popularity, restricting to a class of popularity-first placements. We then present two information-theoretic lower bounds on the average rate for caching with uncoded placement, one for general cache placements and the other restricted to the popularity-first placements. By comparing the average rate of the optimized MCCS with the lower bounds, we prove that the optimized MCCS attains the general lower bound for the two-user case, providing the exact memory-rate tradeoff. Furthermore, it attains the popularity-first-based lower bound for the case of general $K$ users with distinct file requests. In these two cases, our results also reveal that the popularity-first placement is optimal for the MCCS, and zero-padding used in coded delivery incurs no loss of optimality. For the case of $K$ users with redundant file requests, our analysis shows that there may exist a gap between the optimized MCCS and the lower bounds due to zero-padding. We next fully characterize the optimal popularity-first cache placement for the MCCS, which is shown to possess a simple file-grouping structure and can be computed via an efficient algorithm using closed-form expressions. Finally, we extend our study to accommodate nonuniformity in both file popularity and size, where we show that the optimized MCCS attains the lower bound for the two-user case, providing the exact memory-rate tradeoff. Numerical results show that, for general settings, the gap between the optimized MCCS and the lower bound only exists in limited cases and is very small. [ABSTRACT FROM AUTHOR]

Ships docking and traveling in a branched lower approach channel system are at risk from surges caused by multi-lane locks during emptying operations. For this reason, water-level variations in the lower approach channel in response to discharge, interval running time, and outlet location of lock operations were studied using a 2-D hydrodynamic model validated by physical model tests, and the impact of water level variation on navigation safety under extreme operation scenarios of a quadruple-lane lock group was identified. Results indicated that discharge and interval running time of lock emptying had the greatest impact on the water level variation at the lock head. Water level variation at the lower lock head of the ship lift exhibited a trend of first decreasing and then increasing with the increment of the discharge from the lock chamber into the outer river. Specifically, the surge height at the lock head of the ship lift reached the minimum when approximately 40% of the discharge generated by dual-lane locks during emptying operations was released into the outer river. Overall, the simultaneous operation of quadruple-lane lock group and unit load rejection should be avoided in engineering applications. HIGHLIGHTS A 2-D hydrodynamic model was utilized to analyze the water-level fluctuations in a branched approach channel system.; Water-level fluctuations in response to discharge, interval running time, and emptying outlet location of lock operations were revealed in details.; Extreme multi-lane lock operating scenarios were determined considering the requirement of safe docking of ships at the lower lock head of the ship lift.;

The imbalance of vehicle supply and demand is a common phenomenon that influences the efficiency of online ride-hailing systems greatly. To address this problem, a three-level hierarchical mixed deep reinforcement learning method (HMDRL) is proposed to reposition idle vehicles. A manager operates at the top level, where action-abstraction is conducted from the time dimension and is adaptive for spatially scalable and time-varying systems. Coordinators locate at the middle level and a parallel coordination mechanism that is independent of the decision order is designed to improve the efficiency of the repositioning. The bottom level is composed of executive workers to reposition vehicles with mixed states and the states contain spatiotemporal information of agents’ neighbor areas. Two reward functions are designed for the manager and the coordinators, respectively, aiming to improve the training effect by avoiding sparse rewards. A simulator based on real orders is designed and HMDRL is compared with six methods. Experimental results demonstrate that HMDRL outperforms all the other methods. In three comparison experiments, the order response rate is increased by 0.62% to 8.29%, 1.5% to 7.78%, 1.18% to 4.75%, respectively. [ABSTRACT FROM AUTHOR]

Improving endurance is important for autonomous underwater vehicles (AUVs) as it affects the operational cost and application range of the vehicle. In this article, we propose an economic model predictive control (EMPC)-based controller to reduce the control energy of AUVs while performing waypoint tracking. The proposed EMPC controller optimizes stage costs capturing the control energy consumed within the prediction horizon and a terminal cost approximating the energy-to-go, the energy required to reach the desired waypoint from the end of the prediction horizon. To approximate the energy-to-go, we partition it into the dynamic and static segments based on the operational characteristics of the optimal vehicle maneuver obtained from off-line trajectory optimization using direct collocation (DC). To account for the disturbances caused by ocean currents, we adopt the energy-to-go to a virtual Earth-fixed frame that transforms the drift in the vehicle location to the drift in the desired waypoint. Theoretical and numerical analyses of the approximated energy-to-go reveal that the proposed controller can balance the tradeoffs among energy components spent for vehicle surge, heave, and yaw controls in consideration of vehicle dynamics. Simulations under different flow conditions are conducted to compare the proposed approach with DC and a line-of-sight (LOS) guidance-based approach that optimizes vehicle surge speed for energy minimization. Through simulations, it is shown that the proposed approach achieves near-optimal performance as DC and outperforms the LOS-based approach. [ABSTRACT FROM AUTHOR]

In Versatile Video Coding (VVC), reference picture resampling (RPR) is supported to change resolution without inserting an instantaneous decoder refresh (IDR) picture or intra random access picture (IRAP). The introduction of RPR enables VVC to adapt to unstable network conditions, which benefits real-time application scenarios a lot. However, the existing bitrate leap and the low quality of the upsampled frames are the bottlenecks of promoting its application. In this paper, we propose an efficient framework to alleviate bitrate surges and further improve the coding performance. The contributions of this paper mainly include the following three aspects. Firstly, it is proved theoretically and statistically that the temporal long-distance high-resolution (HR) reference pictures can provide better prediction than temporal short-distance low-resolution (LR) reference pictures when the temporal distance between them is less than a threshold. Secondly, motivated by the theoretical analysis, existing HR reference pictures in the reference pictures list (RPL) are used to replace LR reference pictures and an adaptive decision on the number of HR reference pictures kept in RPL is applied. Thirdly, an adaptive upsampling filter (AUF) is proposed to restore pictures with the original resolution more accurately. Simulation results show 15.09% BD-rate savings on average can be achieved for the 2:1 scaling ratio under RPR configurations. For the 1.5:1 scaling ratio, the BD-rate saving is 8.14%. Besides, the bitrate surge in RPR is reduced greatly. About 25% bits reductions on average at the leaping point can be observed and the bits decrease rate is up to 50% for sequences with little and slow motion. [ABSTRACT FROM AUTHOR]

Appropriate shielding methods can reduce the lightning induced voltage and current of cables. In this article, different lightning surge currents (8/20 and 5/320 μs) are coupled to the tested multi-core cable with single shielding layer and double shielding layers. The shielding effectiveness (SE) on the lightning surge current is compared for different cable shielding methods. The experimental results show that the cable induced current is effectively reduced when the shielding layer, the shielding spare core wire or the metal tube are grounded at two ends. A better shielding effect for the cable with double shielding layers is achieved by grounding both ends of the outer shielding layer and one end of the inner shielding layer, reducing the induced current to about 5% when the shielding layer is not grounded. Moreover, the SE of the spare core wire, used as the shielding wire in the multi-core cable, is better than that of the external shielding wire of the cable. The SE of the metal tube increases more and more slowly with the increase of the tube coverage percentage. Furthermore, the descending order of the SE of shielding methods involved here are grounding both ends of double shielding layers, grounding both ends of single shielding layer, spare core wire shielding and metal tube shielding. Finally, some engineering recommendations are proposed through the comparison between the shielding methods in this article and relevant works. [ABSTRACT FROM AUTHOR]

A high-frequency electromagnetic plane wave arises owing to a high-altitude electromagnetic pulse (HEMP), which may cause overvoltages in distribution lines and insulation failure. To protect distribution lines from such overvoltages properly, it is necessary to evaluate the effectiveness of countermeasures. Nowadays, the finite-difference time-domain (FDTD) method, which is one of the full-wave numerical approaches, has become a powerful tool for analyzing electromagnetic transient phenomena. In this article, first, we propose a simplified approach to modeling an impinging plane wave by taking into account homogeneous soil parameters in the three-dimensional FDTD method on the basis of a total field/scattered field technique for the analysis of HEMP-based plane-wave coupling. Second, using the proposed technique, we carry out the statistical analysis of overvoltages in a three-phase distribution line and evaluate the effectiveness of a multipoint grounded overhead shield wire and surge arresters, which are commonly used as lightning protection measures, while modeling the reinforcing bars of concrete utility poles, earth electrodes, and vertical grounding wires to simulate a realistic condition. [ABSTRACT FROM AUTHOR]

Lightning electromagnetic (EM) fields are typically computed by assuming the current propagation within a straight vertical channel of negligible cross section. However, it is well known that the lightning path is tortuous and this can significantly affect the radiated fields and the lightning-induced voltages on power lines. Lupò et al. (2000) proposed an analytical method evaluating the EM fields radiated by a tortuous channel assuming a step current source; to consider an arbitrary (realistic) current source, a slow-converging convolution integral was applied. The aim of this article is to reduce the computation time of such procedure by extending to the tortuous channel case the approach proposed by Brignone et al. (2021) for the evaluation of the lightning EM fields with arbitrary current source, originally developed for a straight vertical channel. The performances of the present method are evaluated by means of comparison with the solution proposed by Lupò et al. (2000), showing an excellent agreement with a significant reduction of the CPU effort. This in principle will allow the computation of lightning-induced overvoltages at both individual and statistical level without a prohibitive computational effort. [ABSTRACT FROM AUTHOR]

The reignition probability of switching overvoltage caused by vacuum circuit breaker (VCB) switching off shunt reactor (SR) in offshore wind farm (OWF) is studied. Firstly, a model of OWF is developed in the environment of electromagnetic transient program. According to the switching time of VCB, a current period is divided into non-reignition zone, reignition zone and current chopping zone. Based on the analysis of overvoltage mechanism, a method for calculating critical arcing time and reignition probability is proposed. Then, the effects of resistor-capacitor (RC) snubber on reignition probability are discussed. The capacitance of RC snubber is optimized based on switching dispersibility of VCB and critical arcing time, and a new structure of RC snubber is developed. Finally, the hybrid method of RC snubber with new structure and phase-controlled switching is presented to suppress the switching overvoltage. The results indicate that the method can eliminate the reignitions and weaken the impact of overvoltage on the insulation of power apparatus. [ABSTRACT FROM AUTHOR]

This paper presented an assessment of lightning energy absorption by dedicated surge arresters (SADT), which are provided to protect the distribution transformers on 10kV overhead distribution lines (OHLs). A hybrid Partial Element Equivalent Circuit-Multi-conductor Transmission Line (PEEC-MTL) code was applied to evaluate lightning surges on SADT. This method has the capability of modeling both short and long wires in OHL systems and is verified numerically and experimentally. A Monte Carlo procedure was adopted to assess the fault rate (FR) of SADT. It is found that the direct lightning strokes to the phase wire are the primary threat to the SADT, compared with the strokes to the poles as well as indirect strokes to the ground or structure nearby. Several protection schemes are identified, and their performance is evaluated from the aspects of lightning protection and economic cost. The full shielding wire (SW) scheme performs the best, but with the most expensive protection cost. The lightning energy absorption by SADT can be suppressed significantly by installing extra SAs on the adjacent poles. The scheme of two or four extra SAs together with a partial SW can achieve a balanced performance of protection effect and economic cost for the OHL. The SW for a section of the OHL alone is not enough to the lightning protection for SADT. [ABSTRACT FROM AUTHOR]

Multiple strokes are the main factors affecting the safe and stable operation of power equipment. As the core lightning protection equipment, arresters are also frequently thermally collapsed due to multiple strokes. This paper investigates thermal effects of polymer-housed metal-oxide surge arrester (PMOSA) under multiple strokes. The comparative experiments of PMOSA lightning impulse are carried out through multiple strokes of experiment platform, including continuous lightning stroke and multiple strokes. Furthermore, the temperature distribution of the housing surface is obtained by infrared imager for thermal effect analysis. Subsequently, some typical DC parameters are tested to explore the association of impulse aging and thermal effects of the arrester under multiple strokes. Finally, a thermal-electric coupling model (TECM) is constructed to investigate the influence of some factors on the temperature of PMOSA from the perspective of materials and structure. The research in this paper can provide a theory reference and brief data support for improving the heat dissipation performance of arresters under multiple strokes based on structural optimization design and material modification. [ABSTRACT FROM AUTHOR]

In this work, two heuristic techniques used to reduce the number of simulations of lightning-induced voltages, which are required to assess the lightning performance of distribution lines according to the Monte Carlo methodology, are presented. The first uses the Rusck Formula to discard the events that do not cause damage to the line. The second is based on a multi-nonlinear regression of the previously simulated events. Both techniques were applied to estimate the flashover rate of straight and complex lines above flat ground with finite conductivities. As a result, between 80 % to 90 % of the total number of simulations were eliminated with a reduced error. [ABSTRACT FROM AUTHOR]

The dc circuit breaker plays an important protection role in the dc distribution systems. Hybrid dc circuit breaker (HCB) offers low conduction losses and reasonably fast response times. In this article, a high power density, modular, and scalable power electronic interrupter (PEI) design is introduced for medium-voltage (MV) HCB platforms. Following this concept, the single low-voltage module is first investigated in terms of the high pulse current ratio and the voltage clamping circuit. After the comparison and test among different devices, a compact module with 1.7-kV discrete IGBTs are selected as main switches. While the voltage clamping circuit is carefully designed to guarantee no tail current bump and sufficient turn-off voltage margin. Next the whole system with series modules is optimized on the aspects of the turn-off strategy, power supply structure, and layout of modules. An improved staged turn-off scheme is proposed, which can not only reduce the peak current and total metal oxide varistor (MOV) energy, but also help balance the MOV energy distribution. A 12-kV and 1-kV breaking capable PEI was demonstrated based on the proposed PEI design, achieving a peak power density of 7.4 kW/cm3, much higher than the IGBT module-based solution. The breaking capability and effectiveness of the modular PEI is demonstrated in a 6-kV MVDC HCB device. [ABSTRACT FROM AUTHOR]

This article proposes a novel ultra-fast self-powered bidirectional solid-state circuit breaker (SSCB) with voltage overshoot suppression that uses gallium nitride switches with very low conduction losses. This SSCB can be used in low and medium ac/dc networks for both short-circuit protection and overcurrent protection. An exhaustive methodology for RCD snubber design applied to SSCBs is provided. A comprehensive review of the technical literature is presented exhibiting that the proposed SSCB reduces conduction losses and detection time in comparison with other similar SSCBs. The performance features of the proposed SSCB are verified through the functional tests that are carried out employing a hardware-implemented prototype. Experimental results confirm that our proposal is able to detect and process the short-circuit failure in 282 ns; the authors have not found any faster results in the literature. [ABSTRACT FROM AUTHOR]

In this article, in order to improve the power quality and decrease the consumed power of an induction motor (IM), which is supplied by an alternate battery-powered system, an algorithm that can be used to save its energy consumption has been proposed. The control mechanism is used to reduce the wasted energy caused by IMs start-up current based on bounding its optimal starting voltage. The proposed algorithm includes two parts: First, use branch-and-bound theory to derive the control rule of the IMs start voltage and give corresponding offline calculation procedure framework. Second, according to the control rule, adjust online voltage of the zinc–bromine flow battery, which is used to feed the IM. Due to the proposed approach, the minimum start-up voltage of the battery-powered system can be predicted and by which one the IMs surge current has been suppressed online. Moreover, the relationship of the output voltage of the battery and inverter supply to the load current has been derived by modeling the battery-powered system. The experiments demonstrate that compared with before optimization, the amount of energy consumption of the IM supplied by zinc–bromine flow battery decreases by 8.5% and corresponding indicator parameters of the power quality can reach the standard IEC TS62749-2015. [ABSTRACT FROM AUTHOR]

This article is dealt with the problem of trajectory tracking with prescribed performance for a family of underactuated surface vessels (USVs) under model uncertainties and disturbances. The prescribed performance means that the USV tracks a given trajectory with the arbitrarily predefined speed of response and accuracy. The existing prescribed performance control (PPC) solutions and the traditional robust control approaches for USVs may have the singularity issue or cause a discontinuous control signal. Thereby, a new-type adaptive PPC strategy is put forward in this article. The adaptive technique is devoted to tackling model imperfections as usual, whereas the constraint-handling technique is adopted in a novel way. Herein, we first construct an auxiliary variable instead of using the approach angle or azimuth angle. Then, we impose constraints on the position error, not the tracking error, and the auxiliary variable, simultaneously. In this way, the predefined performance is achieved by moreover a singularity-free continuous control action. These theoretical findings are illustrated via a comparative simulation study. [ABSTRACT FROM AUTHOR]

Deployment of photovoltaic generation in smart grids has introduced overvoltage problems. Cyberattackers can take advantage of this situation and cause intentional overvoltages in distribution systems using stealthy false data injections (FDIs). To protect distribution grids from these FDIs, the first step is to model cyberattacks that can cause overvoltages. An accurate nonlinear optimization model is developed for stealthy FDI cyberattacks that can result in overvoltages on targeted nodes in distribution grids without being detected by bad data detection algorithms. Compared to the existing research, which mainly focused on dc state estimation for transmission systems, the proposed model formulates cyberattacks that cause overvoltages using ac state estimation in distribution grids. The proposed nonlinear model is then approximated to a convex optimization model using second-order cone programming relaxation formulation to guarantee that a global optimum exists for the model. Several case studies are considered to validate the effectiveness of the proposed attack model. It is shown that by limiting the FDIs on targeted buses to 20% of their nominal load, multiple buses can experience severe overvoltages in a distribution grid. [ABSTRACT FROM AUTHOR]

Abstract In this study, the implementation of different corona discharge models in a transient program for overhead multiconductor lines based on the implicit Crank‐Nicolson Finite Difference Time Domain method is presented. Among different corona models available in the literature, two empirical models (Gary's and Suliciu's models) and a physics‐based model (Malik's model) are discussed, which are broadly used in EMT programs, and their predictions under fast‐front lightning surges and slow‐front switching impulses are assessed. The authors critically review the dynamic capacitance approach in terms of numerical accuracy, when dealing with slowly varying voltages, and applicability, regarding dispersive models assuming a non‐instantaneous charge‐voltage relation. The alternative voltage‐controlled generator approach is proposed, which is successfully included in the implicit Crank‐Nicolson scheme. One of the objectives of this study is to show how different corona models may predict overvoltages with huge discrepancies, depending on the transient waveform and the propagation distance, despite the similarity of the q‐v hysteresis curves. More evident discrepancies between models under fast‐front voltage waves are reported, rather than for switching impulses. The consistency of these results in terms of the models' features and q‐v curves is analysed, highlighting the necessity for new engineering tools for corona simulation of general predictive capability.

Abstract Background The primary aim of this study was to assess the outcome of elderly intensive care unit (ICU) patients treated during the spring and autumn COVID-19 surges in Europe. Methods This was a prospective European observational study (the COVIP study) in ICU patients aged 70 years and older admitted with COVID-19 disease from March to December 2020 to 159 ICUs in 14 European countries. An electronic database was used to register a number of parameters including: SOFA score, Clinical Frailty Scale, co-morbidities, usual ICU procedures and survival at 90 days. The study was registered at ClinicalTrials.gov (NCT04321265). Results In total, 2625 patients were included, 1327 from the first and 1298 from the second surge. Median age was 74 and 75 years in surge 1 and 2, respectively. SOFA score was higher in the first surge (median 6 versus 5, p

In capacitive pulsed power supply (CPPS), semiconductors, such as crowbar diodes, can be damaged by surge energy in worst cases, which reduces the reliability of CPPS. The generation mechanism of surge energy is discussed initially. A surgesuppression scheme is presented to reduce the breakdown risks of semiconductors. Surge voltages and currents are simulated and analyzed based on the classical circuit models of CPPS and railgun. Then, two installation positions of the surgesuppression device are systematically investigated. The simulated results show that installing the surge suppression device at the railgun muzzle can suppress the surge voltages induced by an armature leaving the railgun barrel, and installing the surge suppression device at the railgun breech can reduce overcurrents of semiconductors in reverse recovery phases. Ultimately, experimental results validate the theoretical analysis. [ABSTRACT FROM AUTHOR]

Different from typical circuit breakers, breakers used to switch AC filters in convertor stations have to operate frequently according to the real-time operation mode and daily load changes. Therefore, the performance of filter breakers decreases very quickly, which leads to frequent accidents. Both preinsertion resistors and controlled switching devices can suppress switching transients. In this paper, a comparative analysis of whether a controlled switching device and preinsertion resistor should be adopted is carried out using PSCAD. The transient is calculated as the combined result of the switching angle and preinsertion resistor value, and the optimal configurations are proposed. After determining the resistor value and whether a controlled switching device should be included, the reference voltage of the arrester should be reevaluated. [ABSTRACT FROM AUTHOR]

Ground wires (GW) struck by lightning are often exposed to transient impulse voltages in the MV range. This results in intense negative streamer-type corona, causing a considerable increase in the effective wire capacitance and a corresponding reduction in its surge impedance. Such variables have a significant influence on the tower top potential and on the electromagnetic coupling between the ground wire and phase conductors. Presently there is considerable uncertainty regarding the impulse corona parameters to be used in back flashover calculations. In the present paper a new method is formulated for determination of GW impulse corona parameters. The new approach is compared with two existing methods and with available experimental results. It is found that the method usually used within the IEEE has little physical justification and is excessively conservative. It is also shown that such corona parameters significantly depend on the GW height above ground, which has been totally ignored in the CIGRE method. This and other unjustified simplifying assumptions explain the surprisingly large discrepancy between values of the negative impulse corona constant recommended by CIGRE and IEC. [ABSTRACT FROM AUTHOR]

This paper presents a novel methodology to estimate the saturation curve parameters of single-phase transformers, using minimum information from inrush and no-load tests. Only the value of the first current peak and the decay time are used from a unique inrush test, and only the third and fifth current harmonics from a unique no-load test. The proposed methodology is based on the existence of a signature for each transformer which can be easily obtained and characterize all possible inrush currents. This is the main contribution of the paper. Different transformers with different parameters have been tested in the laboratory. Experiments and simulations are carried out to validate the methodology. First, the respective no-load and inrush tests are made. Second, the parameters of each transformer are estimated. Third, the performance and accuracy of the proposed methodology are verified by a close agreement between the measured and the estimated saturation curves and between the simulated and the recorded inrush waveforms. Some remarkable advantages of the proposed methodology are: (a) considers the deep saturation, (b) avoids the need to know the residual flux (or to demagnetize the transformer) and the switching angle from the inrush test, and (c) it is computationally straightforward. [ABSTRACT FROM AUTHOR]

This paper is concerned with the intelligent event-triggering-based positioning control of networked unmanned marine vehicle (UMV) systems with hybrid attacks, where the UMV and control station is connected by a communication network and the DoS attack and Deception attack are studied. Firstly, a stochastic switched Takagi-Sugeno (T-S) fuzzy system model is proposed for the networked nonlinear UMV systems subject to aperiodic DoS attack and random Deception attack. Then, a novel asynchronous advantage actor-critic (A3C) learning-based event-triggering approach is introduced to alleviate the communication load. By using the Lyapunov stability theory and switched system analysis method, the mean-square exponential stability condition of the closed-loop system and the design method of observer-based controller are devised. Finally, an example of a networked UMV system is given to verify the effectiveness of the proposed resilient control strategy. [ABSTRACT FROM AUTHOR]

This paper proposes an integrated dispatching framework for matching drivers with riders in ride-hailing systems. The goal is to compute matching solutions that maximize social welfare and benefit both sides of the market, such that the sustainable growth of the ride-hailing system is ensured. The proposed framework integrates data-driven proactive guidance strategies with batched matching optimization to increase social welfare, improve matching rate and reduce rider wait time. Proactive guidance strategies are computed by leveraging short-term demand forecasts based on historical data. Taken the resulting guidance strategies as inputs, the batched matching algorithm computes optimal bipartite matching between drivers and riders in a batch. Using New York City taxi data from 2016 March 1st to March 31st as input, we conduct a numerical study to evaluate the performance of the proposed framework and compare it with existing approaches in the literature. Our results show that the proposed framework improves social welfare for up to 50%. It also increases the matching rate by an average of 20% and reduces the average rider wait time by over 15%. This implies a strong potential for the proposed dispatching framework to improve service quality in ride-hailing systems. [ABSTRACT FROM AUTHOR]

The simulation of shallow flows over obstacles is an important problem in environmental fluid dynamics, including exchange flows over seabed sills, atmospheric flows past steep mountains and water flows over river bedforms. A common mathematical treatment consists in using vertically-averaged models instead of vertically-resolved ones by introducing a suitable shallow water approximation. The dispersionless Saint Venant equations are a useful tool, albeit accuracy is not enough in many circumstances. The next approach consists in resorting to the Serre–Green–Naghdi theory, which is well known to produce good solutions for long non-breaking waves. However, a common feature of flows over obstacles is the generation of breaking waves at its lee side, which are important to model, given their role in the mixing and transport of passive scalars downstream of the terrain barrier. The Serre–Green–Naghdi theory fails to model these flows, producing unrealistic trains of undular waves. A widely used practice consist in resorting to a patching approach in a numerical setting where the solutions of Serre–Green–Naghdi and Saint Venant equations are assembled once wave breaking is detected by case-dependent empirical parameters. In this work an alternative method to dealt with wave breaking over obstacles within the Boussinesq-type approximation is proposed. The exact depth-averaged equations for flows over uneven beds are developed and presented as function of the vertical acceleration and non-uniformity of velocity with elevation. By introducing a suitable kinematic field, a new high-order phase resolving system of non-hydrostatic equations is presented, containing the usual dispersive corrections of Serre–Green–Naghdi theory plus high-order corrections for velocity profile modeling. It is found that the new theory allows the simulation of both breaking and non-breaking waves in shallow flows over obstacles without introducing any case-dependent calibration parameter. The new shallow water approximation is thus an alternative method to deal with wave breaking in Boussinesq type models. [ABSTRACT FROM AUTHOR]

The pressurizer is a crucial component for the safe operation of pressurized water reactors (PWRs). The pressure and level control systems of the pressurizer interact strongly with each other. This cross-coupling between the inputs and outputs makes it difficult for conventional control methods to achieve optimal control effects on both the pressure and level. A simple state feedback control for the pressurizer with a decoupling pressure level precompensator realized using the diagonal matrix method is therefore proposed in this study. A detailed three-zone nonequilibrium dynamic model that reflects the stratification and mass and energy exchange phenomena inside the pressurizer is applied, and a state feedback control that utilizes the physically measurable states of the pressurizer is employed to simplify the state feedback controller to a form that is equivalent to the output feedback control. The improvement of the existing dominant pole placement technique in the feedback controller is described, and the correlation between the parameters and automated tuning methods is also given. The performance of the control system is evaluated through simulations under different operating scenarios to verify the proposed method. [ABSTRACT FROM AUTHOR]

In conventional medium-voltage solid-state circuit breakers (SSCBs), series-connected semiconductor switches are used with metal oxide varistors (MOVs) in parallel with each of them to extend the voltage ratings of the breakers without incurring the dynamic voltage unbalance. Besides, for isolating the faults at faster speeds, the clamped surge voltages of the MOVs are always much higher than the system voltages. For those reasons, the voltage ratings of all the switches in the SSCBs have to be overdesigned according to the surge voltage values, which causes high material costs along with nonideal conduction losses in the SSCBs employed for medium-voltage dc distribution systems. To solve this dilemma, a surgeless diode-clamped multilevel SSCB is proposed in this article. It offers an ultrafast isolation speed with no significant surge voltages or dynamic unbalanced voltage across the semiconductor switches. With these attributes, the proposed SSCB can potentially use the semiconductor switches rated at lower voltages compared to conventional SSCBs and have higher efficiency along with faster isolation speed. The simulation and experimental results are presented to validate the technical feasibility and practical values of the proposed SSCB. [ABSTRACT FROM AUTHOR]

This paper presents an adaptive neural fault-tolerant control algorithm for the path-following activity of the underactuated surface vehicle (USV) using the novel output-based triggering approach. In the algorithm, the event-triggered mechanism is designed utilizing the attitude states from the kinematics aspect of USV. Both the control inputs and the related calculation thread are implemented only at the trigger instants. Furthermore, neural networks (NNs) are employed to remodel the model uncertainties, and the adaptive observer is developed to estimate and compensate for the effect of the unknown actuator faults. With the direct Lyapunov theorem, the semi-global uniformly ultimately bounded (SGUUB) stability can be guaranteed for the closed-loop system in aspects of both the trigger instant and the continuous interval. The comparison experiment has been illustrated to verify the effectiveness of the proposed algorithm, which can effectively improve the information transmission performance and the fault-tolerant capability. [ABSTRACT FROM AUTHOR]

Surge is one of the main factors affecting the stability of the centrifugal compressor. This article investigates surge disturbance suppression of active magnetic bearing (AMB)-rotor systems in magnetically suspension centrifugal compressors. An active disturbance control method is adopted to make the rotor stably levitate under surge disturbance. First, the dynamic model of the magnetic bearing rotor is established and simplified into a single-channel model. Second, a third-order linear extended state observer (LESO) and a linear active disturbance rejection controller are designed. The performance analysis of the controller shows that the estimation error accumulation of the observer is bounded and the estimation errors decrease with the increase of the observer bandwidth. Furthermore, the variable differential parameter added in the differential link of the controller can flexibly adjust the system damping. A method of tuning parameters for linear active disturbance rejection control (LADRC) is proposed to suppress surge disturbance while satisfying the stability of the system. Finally, the results of the experiment demonstrate that the proposed scheme is effective in suppressing the large-scale vibration effect of the surge disturbance on the rotor, and the magnetically suspended centrifugal compressor (MSCC) can run stably with low amplitude under the modified surge and mild surge. [ABSTRACT FROM AUTHOR]

Recommending applications (apps) to improve health or educational outcomes requires long-term planning and adaptation based on the user feedback, as it is imperative to recommend the right app at the right time to improve engagement and benefit. We model the challenging task of app recommendation for these specific categories of apps—or alike—using a new reinforcement learning method referred to as episodic multi-armed bandit (eMAB). In eMAB, the learner recommends apps to individual users and observes their interactions with the recommendations on a weekly basis. It then uses this data to maximize the total payoff of all users by learning to recommend specific apps. Since computing the optimal recommendation sequence is intractable, as a benchmark, we define an oracle that sequentially recommends apps to maximize the expected immediate gain. Then, we propose our online learning algorithm, named FeedBack Adaptive Learning (FeedBAL), and prove that its regret with respect to the benchmark increases logarithmically in expectation. We demonstrate the effectiveness of FeedBAL on recommending mental health apps based on data from an app suite and show that it results in a substantial increase in the number of app sessions compared with episodic versions of $\epsilon _n$ ε n -greedy, Thompson sampling, and collaborative filtering methods. [ABSTRACT FROM AUTHOR]