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Multi-stage SRV fracturing in horizontal wells is a new technology developed at home and abroad in recent years to effectively develop shale gas or low-permeability reservoirs, but on the other hand makes the mechanical environment of fracturing strings more complicated at the same time. In view of this, based on the loading features of tubing strings during the multi-stage fracturing of a horizontal well, mechanical models were established for three working cases of multiple packer setting, open differential-pressure sliding sleeve, and open ball-injection sliding sleeve under a hold-down packer. Moreover, mathematical models were respectively built for the above three cases. According to the Lame formula and Von Mises stress calculation formula for the thick-walled cylinder in the theory of elastic mechanics, a mathematical model was also established to calculate the equivalent stress for tubing string safety evaluation when the fracturing string was under the combined action of inner pressure, external squeezing force and axial stress, and another mathematical model was built for the mechanical strength and safety evaluation of multi-stage fracturing strings. In addition, a practical software was developed for the mechanical safety evaluation of horizontal well multi-stage fracturing strings according to the mathematical model developed for the mechanical calculation of the multi-packer string in horizontal wells. The research results were applied and verified in a gas well of Tahe Oilfield in the Tarim Basin with excellent effects, providing a theoretical basis and a simple and reliable technical means for optimal design and safety evaluation of safe operational parameters of multi-stage fracturing strings in horizontal wells.

The earthquake usually causes multi-dimensional vibration for the large span grid structure, how to effectively isolate and mitigate the vibration response of structure is an important problem in engineering. In this paper, a novel viscoelastic bio-inspired multi-dimensional vibration isolation (VBM-VI) device is proposed under the inspiration of bionic design of the bird legs, which possesses multi-dimensional vibration isolation performance by constituting of viscoelastic core pad and VE-LLS device together. The property experiment of the VBM-VI device is performed to explore the mechanical performance at vertical and horizonal directions, the hysteretic curves and characteristic parameters are discussed to reveal the influence of the excitation frequency and amplitude on the mechanical properties of the VBM-VI device, the experimental results show that the mechanical properties express significant frequency and amplitude dependence at vertical and horizontal directions. To further describe the mechanical proprieties of the VBM-VI device, the mathematical models are established at vertical and horizontal directions respectively, and the comparison analysis between experimental and theoretical results indicates that the established mathematical models can precisely describe mechanical performance of the VBM-VI device. [ABSTRACT FROM AUTHOR]

The viscoelastic damping materials (VDMs) used in the viscoelastic (VE) damping equipment may undergo thermal-oxidative aging during long-time exposure to complicated working environments and exhibit significant property degradation in their lifespan. Therefore, it is imperative to comprehend the mechanical performance of VDMs after aging. In this paper, the molecular chains of VDMs are categorized into elastic chains and free chains, and the effects of variable physical–chemical reactions in the thermal-oxidative aging process on these two kinds of chains are analyzed and then mathematically described based on the theory of chemical kinetics. By introducing the molecular chain statistical model to consider the influence of microstructure change on the macroscopic mechanical behavior of VDMs, a hyperelastic hybrid molecular chain model for thermal-oxidative aging VDMs is addressed in conjunction with the tube constraint model and filler reinforcement theory. Combined with the test data, the accuracy and applicability of the proposed model are comparatively evaluated, and the sensitivity analyses on the mechanical parameters and aging parameters of the proposed model are also conducted. The verified results suggest that the proposed model can reflect the mechanical behavior of aging VDMs during thermal-oxidative conditions with remarked accuracy. The results of the parameters analyses display that the macroscopic mechanical property of VDMs is prominently affected by their macromolecular chain structure, and the physical–chemical reactions occurring in the aging process synthetically play a collaborative influence on the mechanical performance of aging VDMs. [ABSTRACT FROM AUTHOR]

Viscoelastic (VE) damper is usually vulcanized by mixing polymer matrix, additives, and fillers, all of which have a great influence on the viscoelastic response of the material. In this study, a series of novel samples of VE materials have been produced by adding different kinds and amounts of organic small molecule modifiers (AO1035, AO60, and AO80) to neat nitrile-butadiene rubber (NBR). The mechanical properties of VE materials have been tested on a dynamic mechanical analyzer and universal testing tensile machine, and the most excellent formulation was selected for manufacturing VE damper. The dynamic mechanical properties of the VE material in the damper with amplitude and frequency are tested and analyzed. The results indicate that the VE damper has a high energy dissipation capacity. To clarify the dynamic mechanical properties of the novel VE material, a high-order fractional derivative model is proposed, which considers the effects of amplitude and frequency simultaneously based on the energy dissipation mechanism of the novel VE material. Finally, the calculated results using this model are compared with the tested data, which has verified the correctness of the mathematical model. [ABSTRACT FROM AUTHOR]

The mechanical properties of viscoelastic (VE) dampers directly affect the aseismic performance of viscoelastically damped structures; therefore, it is of great significance to accurately describe the nonlinear mechanical characteristics of VE dampers in the aseismic design and analysis of structures. However, most of the existing mathematical models for VE dampers have been established from a macroscopic perspective, and there is a general lack of a comprehensive connection to the microstructure characteristics of VE materials and external influence factors such as loading frequency, ambient temperature, and strain amplitude. In this paper, inspired by the molecular chain network models and fractional derivative theory, a microstructure-based equivalent visco-hyperelastic model is proposed for VE dampers with consideration of temperature dependence and the filler reinforcement effect. To verify the characterization capacity of the proposed model, laboratory experiments on the dynamic property of VE dampers were carried out with varying frequencies, temperatures, and strain amplitudes, and the proposed model was then employed to predict the experimental results. Finally, model parameter analysis was conducted to clarify the relationship between material microstructure and its macroscopic performance. The experiments indicate that the VE damper possesses an excellent energy-dissipation capability, and characteristic parameters of VE dampers tend to be more sensitive in the low ranges of frequency and temperature than in the high ranges. Comparisons between the experimental and numerical results suggest that the proposed model can describe the mechanical properties of VE dampers at different frequencies, temperatures, and strain amplitudes with good accuracy. Parameter analysis demonstrates that the proposed model can reflect the influence of material microstructure on the macroscopic mechanical properties of VE dampers. [ABSTRACT FROM AUTHOR]

Viscoelastic (VE) dampers are one of the most promising techniques for reducing vibration in engineering structures caused by earthquakes and wind. This work aims to develop a kind of high-dissipation VE damper for civil structures at low frequency and large amplitude in shear mode. First, nitrile rubber (NBR)/organic small-molecule composite VE materials are optimized and then made into VE damper. In order to test the mechanical performance and energy dissipation performance of the VE damper, the dynamic mechanical performance experiments at different temperatures, frequencies, and amplitudes were implemented. The experimental results show that the VE damper exhibits great stiffness and excellent energy dissipation capacity under different loading conditions. Second, a fractional derivative model based on Gauss microchain, Williams–Landel–Ferry (WLF) equation, and internal variable theory is proposed to accurately describe the effects of temperature, frequency, and amplitude on the dynamic mechanical properties of VE dampers. Finally, the accuracy of the mathematical model of VE damper is verified by comparing the calculated results with the experimental results. The study provides a theoretical basis for effective vibration reduction of civil structures with VE dampers at low frequency. [ABSTRACT FROM AUTHOR]

Existing converter models are normally reduced as simplified electrical components, whereas it overlooks the impacts of fast dynamics of converter current control loops. Therefore, the dc fault current calculation with the traditional simplified converter models becomes invalid when the simulation time is longer. To overcome this problem, this article incorporates the control effects of converters during the transient process and proposes three dc fault analysis models of classical three converter topologies for the high-voltage dc (HVdc) application including line communicated converter (LCC), two-level voltage-source converter (VSC), and modular multilevel converter (MMC). For an LCC-based rectifier, the nonlinear part of original model is first linearized with the least square method due to the large dc-link voltage deviation during dc fault, and the proportional integral (PI) current regulator with the reduced linearized model can be equivalent as an RC circuit for dc fault analysis. However, for two-level VSC or MMC, the dc-link voltage deviation does not drop that much with a large dc-link capacitor discharging process during transient dynamics. As a result, the original model is linearized relying on the small-signal method based on prefault operation points, and the dynamics of fast inner current loop with the linearized model can be represented as an RL circuit for dc fault current calculation. for dc fault analysis The proposed dc fault analysis models of three converters as equivalent RLC circuits are well testified under different fault resistances and dc reactors, control parameters, and operating conditions. Moreover, dc fault simulations of a hybrid HVdc system have verified the effectiveness of the proposed converter models. [ABSTRACT FROM AUTHOR]

The unbalanced nature of the distribution networks (DNs) and communication asynchrony pose considerable challenges to the distributed voltage regulation. In this paper, two distributed voltage control algorithms are proposed to overcome these challenges in multiphase unbalanced DNs. The proposed algorithms can be leveraged in online implementations to cope with the fast-varying system operating conditions. By adopting the linearized multiphase DistFlow model, the voltage control problem is formulated as a convex quadratic programming problem for which a synchronous distributed algorithm is developed based on the dual ascent method. To account for communication delays, an asynchronous distributed algorithm is proposed evolving from the synchronous one by incorporating an event-triggered communication protocol. Furthermore, closed-form solutions to the optimization subproblems are derived to enhance the computational efficiency, and communication complexity is reduced significantly to the extent that only neighborhood information exchange is required. Finally, the convergence of the proposed algorithms to the global optimality is established analytically. Numerical tests on the IEEE 123-bus network not only corroborate that our proposed algorithms are more efficient in eliminating voltage violations and minimizing network loss compared with two benchmarks but also validate their effectiveness for online implementations. [ABSTRACT FROM AUTHOR]

In this paper, we propose a distributed online voltage control algorithm for distribution networks with multiple photovoltaic (PV) systems based on dual-ascent method. Conventional distributed algorithms implement voltage control only when the algorithms converge. However, our proposed algorithm is able to carry out voltage control immediately. In particular, we derive a closed-form solution for PV controllers to locally update the active and reactive power set points aiming at minimizing the total loss and maintaining bus voltages within the acceptable ranges. The optimality is guaranteed and the convergence is established analytically. Moreover, our proposed algorithm only requires the information exchange between neighboring PV systems, thus reducing communication complexity. Finally, numerical tests on IEEE 37-bus distribution system verify the effectiveness and robustness of our proposed algorithm. [ABSTRACT FROM AUTHOR]

The pipeline structure is prone to multi-dimensional vibration under the effect of external excitation, fluid pulsation and water hammer, which influences transportation efficiency and leads to accidents. In this paper, the multi-dimensional vibration isolation and mitigation device (MVIMD) is developed, which controls the vibration of the pipeline structure from two aspects simultaneously: isolating vibration and mitigating vibration. The device which consists of viscoelastic (VE) bearing, VE dampers and U-shaped springs, has energy dissipation capacities both in horizontal and vertical vibration due to employing VE material with high energy dissipation capacity. In order to investigate the mechanical properties of the device in the horizontal and vertical directions, the characterization tests under different frequencies and amplitudes are carried out. To accurately describe the nonlinear characteristics of the device caused by the complex viscoelasticity, the mathematical model for MVIMD is proposed for considering the effects of frequency and displacement amplitude based on the equivalent standard solid model. Finally, the numerical results are compared with the experimental data. It can be shown from comparison results that the proposed model can describe the properties of MVIMD precisely. [ABSTRACT FROM AUTHOR]

In this paper, a hierarchical security-constrained optimal power flow (SCOPF) model is proposed for a meshed ac/multiterminal HVDC (MTDC) system with high wind penetration. The two interacting levels in the proposed model are as follows: 1) the high level is a traditional SCOPF problem in an ac system that aims to minimize total generation and security control costs; and 2) the low level is a dynamic power dispatching problem, which regulates power flow in an MTDC grid according to reference signals from the high level. Thus, the proposed method utilizes an MTDC system to provide support for the ac system by redistributing power flow across the entire grid and reducing control costs. Two modified IEEE meshed ac/dc systems are used to demonstrate the performance of the proposed method. [ABSTRACT FROM PUBLISHER]

Due to various weather perturbation effects, the stochastic nature of real-life solar irradiance has been a major issue for solar photovoltaic (PV) system planning and performance evaluation. This paper aims to discover clearness index (CI) patterns and to construct centroids for the daily CI profiles. This will be useful in being able to provide a standardized methodology for PV system design and analysis. Four years of solar irradiance data collected from Johannesburg (26.21 S, 28.05 E), South Africa are used for the case study. The variation in CI could be significant in different seasons. In this paper, cluster analysis with Gaussian mixture models (GMM), K-Means with Euclidean distance (ED), K-Means with Manhattan distance, Fuzzy C-Means (FCM) with ED, and FCM with dynamic time warping (FCM DTW) are performed for the four seasons. A case study based on sizing a stand-alone solar PV and storage system with anaerobic digestion biogas power plants is used to examine the usefulness of the clustering results. It concludes that FCM DTW and GMM can determine the correct PV farm rated capacity with an acceptable energy storage capacity, with 36 and 46 rather than 1457 solar irradiance profiles, respectively. [ABSTRACT FROM PUBLISHER]

In this paper, tests on viscoelastic (VE) dampers are carried out, and the results indicate that VE dampers have high energy dissipation capacity and excellent anti-fatigue properties. To clarify the mechanical properties of VE dampers, a new mathematical model, named the equivalent fractional Kelvin model, which considers temperature and frequency effects simultaneously, is proposed. The numerical results using this model are in good agreement with experimental results. Then, through analysis on a five-story reinforced concrete frame structure with and without VE dampers, it is found that the seismic responses of the structure with VE dampers are significantly reduced and VE dampers have high energy dissipation capacity. [ABSTRACT FROM AUTHOR]

The ramp events of photovoltaic (PV) generation will cause severe voltage variations in a distribution network. To address this issue, a two-stage robust optimization based intra-hour dispatch model is proposed for the coordination of the on-load tap changer and smart PV systems. Hence, the distribution network security can be ensured by adjusting substation voltage and controlling PV systems. Case study on 33-bus system verifies the effectiveness of the proposed model. [ABSTRACT FROM PUBLISHER]

This letter is focused on the issue of power imbalance caused by significant fluctuations of photovoltaic (PV) generation in a stand-alone microgrid. To exploit the flexibility of microgrid in addressing such issue, a novel rolling optimization based dispatch strategy involving a microgrid flexibility check model and a PV ramp scenario generation model is proposed. Based on the proposed strategy, the optimal operation schedule for conventional generation and energy storage system can be efficiently found. Case study demonstrates the effectiveness of the proposed model. [ABSTRACT FROM PUBLISHER]

In this paper, we have designed and characterized the beam steering structure for terahertz radiation based on doubly corrugated spoofed surface plasmon polariton (DC-SSPP) architecture and analyzed the design of a high-speed terahertz analog-to-digital converter (ADC). Two DC-SSPP waveguides comprising metal-covered GaAs are utilized to build an interferometric structure with localized doping in the groove region. Active modulation of free carrier density in the doped n-GaAs will cause relative phase differences between terahertz beams in separate arms of the interferometer, giving rise to deflected radiation in the far field. In this paper, both mathematical modeling and finite-element simulations are used to verify and optimize the design. Two different architectures operating on the enhancement mode and depletion mode are studied separately in detail. Applying the beam bender in the ADC design poses challenges such as structural symmetry and multibit resolution. With a full differential implementation of the input signal and combination of nonlinear detector arrays with carefully designed gain and saturation power, a 3-bit ADC is demonstrated with higher than 60 GS/s speed and direct digitized readout. [ABSTRACT FROM AUTHOR]

Abstract: The earthquake excitation is a kind of complicated multi-dimensional motion. It is not safe and accurate enough if only considering the horizontal earthquake excitation, especially for complex and important engineering structures. Therefore, anti-earthquake measures for engineering structures considering multi-dimensional excitations have become an extremely important research topic. In this paper, a new multi-dimensional earthquake isolation and mitigation device (MEIMD) is developed and tested under different cases at both horizontal and vertical directions. Effects of excitation frequency and displacement amplitude on both horizontal and vertical properties of this device are considered. At the same time, a modified equivalent standard solid model is proposed, which can effectively describe the effects of frequency and displacement on the horizontal and vertical properties of the device. Research indicates that the MEIMD possesses high-energy dissipation capacity at both horizontal and vertical directions and the proposed model can accurately simulate the value of the storage modulus and the loss factor. [Copyright &y& Elsevier]

SUMMARY As a semi-active control device, magnetorheological (MR) dampers have been paid more attention because of their high controllability, fast response and low power requirement. When MR dampers are used for vibration mitigation, some challenge topics must be taken into account, such as design method, performance study and intelligent control algorithm. In this paper, a detailed design process of MR damper involving the geometry design and magnetic circuit design is carried out, and a multistage shear-valve mode MR damper is designed and manufactured. Then the MR damper is tested to investigate the influence of control current, displacement amplitude and excitation frequency on the damper's mechanical behavior and energy dissipation performance. At the same time, the design target values are compared with experimental results. Comparison results show that the proposed design method holds promise in designing and optimizing the MR damper. Finally, a modified Sigmoid model is proposed. Comparison results between the experimental data and the numerical data indicate that the modified Sigmoid model can accurately describe the behaviors of the MR damper. Copyright © 2012 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

As a semiactive control device, magnetorheological dampers have been paid more attention due to their high controllability, fast response, and low power demand. One of the important characteristics for magnetorheological dampers is magnetic saturation, that is, the maximum damping force will reach some value and no longer vary with the increasing input current, especially in the presence of large magnetic flux density. In order to take this problem into account fully, tests on a shear-valve mode magnetorheological damper are carried out to consider the effects of input current, displacement amplitude, and loading frequency on the properties of the magnetorheological damper during magnetic saturation situation first. Then, the magnetic saturation phenomenon of the magnetorheological damper is simulated using the finite element method, and the numerical simulation results are compared with the experimental results. Finally, a magnetic saturation mathematical model is proposed to describe the properties of the magnetorheological damper, and the numerical hysteresis curves of the proposed magnetic saturation mathematical model, the Bingham model, and the Bouc–Wen model are compared with the experimental results. It can be concluded that the magnetic saturation mathematical model can describe the influence of input current, displacement amplitude, and excitation frequency on the properties and the magnetic saturation property of the magnetorheological damper. [ABSTRACT FROM AUTHOR]

Classical spot pricing theory is based on multipliers of the primal problem of an optimal market dispatch, i.e., the solution of the dual problem. However, the dual problem of market dispatch may yield multiple solutions. In these circumstances, spot pricing or any standard pricing practice based on multipliers cannot generate a unique clearing price. Although such situations are rare, they can cause significant uncertainties and complexities in market dispatch. In practice, this situation is solved through simple empirical methods, which may cause additional operations or biased allocation. Based on a strict extension of the principles of spot pricing and surplus allocation, we propose a new pricing methodology that can yield unique, impartial, and robust solution. The new method has been analyzed and compared with other pricing approaches in accordance with spot pricing theory. Case studies support the results of the theoretical analysis, and further demonstrate that the method performs effectively in both uniform-pricing and nodal-pricing markets. [ABSTRACT FROM PUBLISHER]

Many kinds of dampers are invented to reduce the dynamic responses due to earthquake or strong wind. Viscoelastic damper is a kind of commonly used passive control device for vibration mitigation of structures. Its energy dissipation characteristics are affected by environmental temperature and excitation frequency. In order to describe the complex characteristics of viscoelastic dampers changing with temperature and frequency, a new model — the equivalent standard solid model, which is based on the standard linear solid model and temperature-frequency equivalent theory, is proposed. The comparison results between experimental and numerical data of viscoelastic dampers show that the equivalent standard solid model can precisely describe energy dissipation behavior of viscoelastic dampers under different temperatures and frequencies. At the same time, 52 viscoelastic dampers are applied for seismic retrofit design on Xi’an Petrol Hotel, and the finite analysis for structures with and without dampers are carried out under frequent-earthquake and infrequent-earthquake excitations. The dynamic responses of structures with and without dampers are compared. Analytical results show that viscoelastic dampers can reduce the earthquake responses effectively, and they are high-performance energy dissipation devices. [ABSTRACT FROM PUBLISHER]

• A user-programmable hybrid simulation platform is established. • The methods for solving equation for viscoelastically damped structures are proposed. • The reliability of the hybrid simulation platform to test nonlinear systems is verified. • A series of hybrid simulation experiments for viscoelastically damped structures are implemented. To truly reflect the influence on structural seismic response control using viscoelastic dampers with strong nonlinear properties and reduce the cost and difficulty of experimental research, a new user-programmable hybrid simulation platform is developed. The core implementation details of this platform, including the core architecture, software system, and testing setup, are described. To better realize accuracy and stability of the hybrid simulation experiment, methods for solving the equivalent force equilibrium equation for hybrid simulation experiments on viscoelastically damped structures are proposed. Then, the mechanical properties of the experimental substructure (viscoelastic damper) are tested under different temperatures, amplitudes and frequencies, and the mathematical model parameters of the experimental substructure are identified. A seismic response comparison of viscoelastically damped structures obtained by the hybrid simulation experiment and the time history analysis verifies the robustness and reliability of the hybrid simulation platform to test nonlinear systems. Lastly, a series of hybrid simulation experiments for viscoelastically damped structures under different temperatures, seismic intensities and seismic waves are implemented using this hybrid simulation platform for the first time. The experimental results reveal the influence on structural seismic response control by viscoelastic damper under different temperatures, seismic intensities and seismic waves. [ABSTRACT FROM AUTHOR]