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This work investigates the effect of alternating current partial discharge (AC PD) byproducts on the dielectric frequency response (DFR) of oil-paper insulation comprising of a small cavity, in the frequency range of 1.0 mHz to 1.0 kHz. The investigations were done by utilizing experimental results in combination with a Finite Element Method (FEM) model. The results presented in this paper show that ageing by-products in the cavity (PD stressed volume) alone will not result into the change in DFR of oil-paper insulation. According to simulation results, the significant change in the DFR results is mainly caused by PD by-products (positive and negative charges, and electrons) which tend to conduct or diffuse into the bulk of oil-paper insulation. The change in a dissipation factor curve due to PD by-products was observed to occur mainly in the middle and low frequency regions. The results indicate that the dissipation factor curves in the middle and low frequency regions depend on the level of PD by-products in oil and paper, respectively. [ABSTRACT FROM AUTHOR]

In the paper by Jin et al. (IEEE Trans. Ind. Electron., vol. 62, no. 3, pp. 1363–1371, Mar. 2015), the authors present an expansion of Ohm's law in the nonsinusoidal ac circuit, based on which a distinct instruction for analyzing and designing nonsinusoidal circuit including ac/dc link circuit is provided. This paper is very interesting and a good work. However, some equations and expressions may be improved. In this paper, they are discussed and modified. [ABSTRACT FROM AUTHOR]

The integration of the dynamic line rating (DLR) of overhead transmission lines (OTLs) in power systems security constrained unit commitment (SCUC) potentially enhances the overall system security as well as its technical/economic performances. This paper proposes a scalable and computationally efficient approach aimed at integrating the DLR in SCUC problem. The paper analyzes the case of the SCUC with AC load flow constraints. The AC-optimal power flow (AC-OPF) is linearized and incorporated into the problem. The proposed multi-period formulation takes into account a realistic model to represent the different terms appearing in the Heat-Balance Equation (HBE) of the OTL conductors. In order to include the HBE in the OPF, a relaxation is proposed for the heat gain associated to resistive losses while the inclusion of linear approximations are investigated for both convection and radiation heat losses. A decomposition process relying on the Benders decomposition is used in order to breakdown the problem and incorporate a set of contingencies representing both generators and line outages. The effects of different linearization, as well as time step discretization of HBE, are investigated. The scalability of the proposed method is verified using IEEE 118-bus test system. [ABSTRACT FROM AUTHOR]

Future power systems will contain more converter-based generation, among which are the voltage-source converter–high-voltage direct-current (VSC-HVDC)-connected offshore wind power plants (WPP). Their interaction with the onshore system influences power system dynamics in the transient stability timeframe. The respective protection and control methods which cause this interaction must be taken into account in grid-integration studies performed today. This paper gives insight into the effect of typically required fault ridethrough (FRT) and post-FRT measures of VSC-HVDC-connected offshore WPPs on the combined ac and HVDC system dynamics. Several important sensitivities are addressed, among which are: 1) FRT implementation, 2) the postfault active power-recovery rates, 3) the ac network dynamic characteristics, and 4) the HVDC topology. The analysis is first performed as a proof of concept on a small benchmark system, and subsequently generalized to a realistic dynamic model of the future Northwestern European power system. The results of this paper can be used as a reference for understanding the effects of large-scale VSC-HVDC-connected offshore WPPs on the stability of the onshore interconnected power systems. [ABSTRACT FROM AUTHOR]

This paper jointly addresses two major challenges in power system operations: 1) dealing with non-convexity in the power flow equations, and 2) systematically capturing uncertainty in renewable power availability and in active and reactive power consumption at load buses. To overcome these challenges, this paper proposes a two-stage adaptive robust optimization model for the multi-period AC optimal power flow problem (AC-OPF) with detailed modeling considerations, such as reactive capability curves of conventional and renewable generators and transmission constraints. This paper then applies strong second-order cone programming (SOCP)-based convex relaxations of AC-OPF combined with the use of an alternating direction method to identify worst-case uncertainty realizations, and also presents a speed-up technique based on screening transmission line constraints. Extensive computational experiments show that the solution method is efficient and that the robust AC OPF model has significant advantages both from the economic and reliability standpoints as compared to a deterministic AC-OPF model. [ABSTRACT FROM PUBLISHER]

This paper describes two calculable thermal ac voltage standards developed at the Silesian University of Technology in Gliwice, Poland. The design and mathematical model of these standards made calculation of their ac–dc transfer differences with very low uncertainty possible, comparable to the uncertainty of the best available calculable ac voltage standards in the 10 kHz–1 MHz frequency range. The correctness of the results of the ac–dc transfer difference calculation was also confirmed experimentally. [ABSTRACT FROM AUTHOR]

AC voltage regulation is required in both the domestic and industrial sectors to avoid undesired effects from random voltage variations of the power supply. The paper introduces an ac voltage stabilizer/converter (ACVS) that is based on a controllable autotransformer technology. The proposed ACVS offers a specified strategy of voltage regulation, less harmonics, and low cost. The paper explains the operating principle of the ACVS and derives its nonlinear mathematical model. To ensure the desired performance of the ACVS while it is subject to uncertain input voltage and load variations, an optimal control strategy is designed. It is achieved via transforming the ACVS model extending with fictive axis emulation into a rotating reference frame and the linearization of the model via specific orientation of the reference frame and introducing a linear control action. Operation of the ACVS is simulated under different disturbances due to load and grid voltage changes, and compared to voltage stabilization with application of I and PI controllers. Experimental results are presented to demonstrate the voltage regulation technology. [ABSTRACT FROM PUBLISHER]

Conventional air dc circuit breakers face the problems of complex arc motion control and arc back-commutation. In order to simplify the arc motion control, solve the arc back-commutation, and shorten the breaking time, this paper proposes an arc squeeze method, which stretches the arc in the insulating slit to make the arc voltage rise. The arc voltage acts as counter voltage to the system voltage and causes the current to cross zero, and then, current interruption is completed. For the experimental study, a test device was designed and made. The effects of different structural parameters (penetration depth of insulating plate, slit cross section circumference, and area) and external parameters (operating speed of insulating plate and peak circuit current) on the arc voltage increase during the breaking process were studied and analyzed. The interruption time of fault current with maximum value within 12–22 kA is quite short (about 5 ms). [ABSTRACT FROM AUTHOR]

This paper presents a new and very simple strategy for torque and flux control of ac machines. The method is based on model predictive control and uses one cost function for the torque and a separate cost function for the flux. This strategy introduces a drastic simplification, achieving a very fast dynamic behavior in the controlled machines. Experimental results obtained with an induction machine confirm the drive's very good performance. [ABSTRACT FROM AUTHOR]

This paper focuses on the droop-based dc voltage control design for multiterminal VSC-HVDC grid systems, considering the ac and the dc system dynamics. The droop control design relies on detailed linearized models of the complete multiterminal grid, including the different system dynamics, such as the dc grid, the ac grid, the ac connection filters, and the converter inner controllers. Based on the derived linear models, classical and modern control techniques are applied to design the different controllers, including a multivariable frequency analysis to design the grid voltage droop control. In combination with the droop control, a dc oscillation damping scheme is proposed in order to improve system performance. The control design is validated through simulations of a three-terminal system. [ABSTRACT FROM AUTHOR]

AC losses in superconductors are generated every time a time-varying current/field is present. Engineers must be able to predict these losses as accurately as possible during the design phase of power applications. The electrodynamics of superconductors can be formulated as a nonlinear eddy current problem in which the resistivity of the superconducting region is a highly nonlinear function of the current density. In 3-D finite-element simulations, it leads to time-consuming simulations and convergence issues. In this paper, we compare two different $E$ – $J$ constitutive equations, namely: 1) power law model and 2) the percolation model (PM), programmed within both the $H$ – $\phi $ and $T$ – $\phi $ formulations. Based on the 3-D case of a three-filament twisted superconducting wire, the numerical performance of all these formulations/material models is compared in terms of accuracy, computation times, number of time steps, and number of Newton iterations for different relaxation methods. It is shown that the combination of the $T$ – $\phi $ formulation and the $E$ – $J$ PM works fine and should be further developed, as it seems to constitute the best modeling option from both a numerical and physical point of view. [ABSTRACT FROM AUTHOR]

In this paper, an analytical model for calculating the ac resistance due to the proximity effect in thick coil integrated spiral inductors is proposed. Based on 2-D Maxwell’s equations, the redistribution of magnetic field and current in the coils caused by the proximity effect is modeled, and the ac resistance of the inductor is extracted. The analytical model is compared with both numerical simulations and experimental results. The ac resistance calculated by the model is good (within 14.8% discrepancy) up to the frequency of the peak quality factor for thick coil integrated spiral inductors. [ABSTRACT FROM AUTHOR]

Within an alternating current (AC) optimal power flow (OPF) market clearing framework, market-settlement surplus (MS) is inevitable due to network losses, congestion, and reactive power. For a general AC-OPF model, decomposition of MS into its constituents is a challenge. Therefore, this paper proposes a solution for decomposing MS into its constituents, i.e., network losses, congestion, and reactive support services, by deriving analytical expressions for each of the components. An optimization problem, named pseudo-IV-OPF, is set up around the existing AC-OPF solution to derive expressions. The central idea involved in current and voltage OPF, that the current flow at sending end is exactly equal and opposite at receiving end, is exploited to derive the elements of MS by using Karush–Kuhn–Tucker conditions of optimality. It is important to note that a converged OPF solution, obtained from the standard AC-OPF model used by the utility, is an input to the proposed model. The efficacy of the proposed expressions is illustrated on various benchmark systems. [ABSTRACT FROM AUTHOR]

The outcome of recent numerical simulation shows that the conversion of an AC transmission line to a simultaneous AC–DC line improves the system stability. This paper presents a mathematical model, suitable for the stability analysis of a simultaneous AC–DC transmission system, transmitting both AC and DC powers simultaneously through an existing AC line. The model is based on the principle of equal area criterion. The salient feature of this model is the introduction of a solution technique of an integral component of the model, which avoids the trial and error approach for the analysis. The model development process considers a single machine-infinite bus system with a transmission line connecting the machine and the infinite bus. The process also considers the severe most fault at the sending end bus. The validation of the model is executed through two different approaches; comparison of the results obtained applying the proposed model to the power system with the published ones in the literature and with the results obtained using standard software, MATLAB–Simulink, based circuit simulations. The model is also applied to a realistic system and IEEE benchmark systems to verify its capability of evaluating the benefits of a simultaneous AC–DC transmission system. [ABSTRACT FROM AUTHOR]

Higher levels of renewable electricity generation increase uncertainty in power system operation. To ensure secure system operation, new tools that account for this uncertainty are required. In this paper, we adopt a chance-constrained AC optimal power flow formulation, which guarantees that generation, power flows, and voltages remain within their bounds with a predefined probability. We then discuss different chance-constraint reformulations and solution approaches for the problem. We first describe an analytical reformulation based on partial linearization, which enables us to obtain a tractable representation of the optimization problem. We then provide an efficient algorithm based on an iterative solution scheme which alternates between solving a deterministic AC optimal power flow problem and assessing the impact of uncertainty. The flexibility of the iterative scheme enables not only scalable implementations, but also alternative chance-constraint reformulations. In particular, we suggest two sample-based reformulations that do not require any approximation or relaxation of the AC power flow equations. In a case study based on four different IEEE systems, we assess the performance of the method, and demonstrate scalability of the iterative scheme. We further show that the analytical reformulation accurately and efficiently enforces chance constraints in both in- and out-of-sample tests, and that the analytical reformulations outperforms the two alternative, sample-based chance constraint reformulations. [ABSTRACT FROM PUBLISHER]

A small signal model of a multi-infeed high-voltage direct current (HVdc) transmission system containing a line commutated converter (LCC) and a voltage source converter (VSC) is developed. This model represents the LCC and VSC converters as operational impedances as seen from the converter ac busbar. This permits the converters to be included in the effective short-circuit ratio (ESCR) calculations. The resulting ESCR is referred to in this paper as the “Impedance based Effective Short Circuit Ratio” (IESCR). It is shown that the maximum power transfer limit (referred to as the maximum available power or MAP) of the converters is better predicted by this index compared to the conventional ESCR which ignores the operational impedances of the converters. The question also arises as to how close to this theoretical maximum power transfer limit can the HVdc system operate. Using the small-signal model, it is shown that with commonly used control strategies, the predicted MAP can only be achieved by reducing the controller gains. The results are validated using detailed electromagnetic transients simulation of the multi-infeed VSC-LCC system. [ABSTRACT FROM PUBLISHER]

This paper presents a transformerless high-voltage dc–dc converter based on cascaded sub-modules. It is intended for interconnecting high-voltage or medium-voltage dc grids. The proposed dc–dc converter consists of two phase legs, each of which consists of an upper arm and a lower arm with their middle nodes crossly connected through a branch of active sub-modules. By properly controlling the output voltage of the cross-connected branch, a trapezoidal ac current is induced to interact with the ac voltage components in the upper and lower arm for rebalancing the power amongst converter arms. The features of modular design, single-stage power conversion, and transformerless structure make the proposed dc–dc converter gain the outstanding merits of wide voltage ratio range, high system efficiency, and light converter weight. A control scheme is also elaborated for guaranteeing the normal operation of the proposed dc–dc converter. A 100-kV 100-MW simulation model performed in MATLAB/Simulink verifies the feasibility of the proposed dc–dc converter. Experimental results obtained from a 100-V 1-kW laboratory setup also confirm the validation of the proposal. [ABSTRACT FROM PUBLISHER]

Voltage ripple in single-phase ac–dc converters is usually disregarded to design the dc-link voltage control system, since large dc-link electrolytic capacitors are typically employed. However, replacement of electrolytic capacitors by film capacitors has been widely considered for increasing reliability. Consequently, due to cost constraints, such replacement usually employ low capacitance and may be combined with fast dc-bus voltage controllers. This paper shows that the conventional linear time-invariant (LTI) model may not represent the real converter behavior for reduced capacitance and/or faster controllers, leading to poor system performance or even instability. In this way, for any of these cases, the linear time-periodic (LTP) model is highly encouraged for stability and transient analysis as a tool for the controller design. Experimental results confirm that stability margins are precisely obtained from the LTP model but may not from the LTI model. Finally, this paper also compares both LTI and LTP models while considering the control gain and the dc-bus capacitance size. It is graphically revealed that the phase margin of the LTI model diverges from the LTP model for low dc-bus capacitances and high control gains. [ABSTRACT FROM PUBLISHER]

A branch flow model (BFM) is used to formulate the AC power flow in general networks. For each branch/line, the BFM contains a nonconvex quadratic equality. A mathematical formulation of its convex hull is proposed, which is the tightest convex relaxation of this quadratic equation. The convex hull formulation consists of a second-order cone inequality and a linear inequality within the physical bounds of power flows. The convex hull formulation is analytically proved and geometrically validated. An optimal scheduling problem of distributed energy storage (DES) in radial distribution systems with high penetration of photovoltaic resources is investigated in this paper. To capture the performance of both the battery and converter, a second-order DES model is proposed. Following the convex hull of the quadratic branch flow equation, the convex hull formulation of the nonconvex constraint in the DES model is also derived. The proposed convex hull models are used to generate a tight convex relaxation of the DES optimal scheduling problem. The proposed approach is tested on several radial systems. A discussion on the extension to meshed networks is provided. [ABSTRACT FROM PUBLISHER]

This paper provides a methodology to estimate power versus distance envelops of high voltage alternating current (HVAC) transmission systems for subsea applications. Alternating current (AC) technology is mature and is proven for land-based applications and relatively short offshore tiebacks. However, for long tiebacks, due to increased conductor capacitance, large reactive power needs to be supplied, leading to higher cable current rating, losses, and expensive umbilical. Subsea ac cables are limited in their capability to transmit power beyond a certain distance, depending on cable characteristics, installation conditions, and system operating mode. Power transfer boundary charts for ac technology based on analytical methods are presented. Analytical calculations and computer simulations of HVAC power cables were performed on more than 30 cables with rated voltage between 35 and 150 kV and cable cross sections between 95 and 400 \mm^2. Effects of reactive compensation on system tieback distance have been analyzed, as well as low-frequency transmission and different modes of system operation. Cable models employing multiple pi sections and distributed parameters were used. Well-known power flow and simulation tools were used for validation. AC transmission boundaries were estimated based on the voltage and current limits for various cables under various operating modes. [ABSTRACT FROM AUTHOR]

This paper presents a comparison of the small-signal stability properties for virtual synchronous machines (VSMs) with dynamic and quasi-stationary representation of the internal synchronous machine (SM) model. It is shown that the dynamic electrical equations may introduce poorly damped oscillations when realistic stator impedance values for high-power SMs are used. The quasi-stationary implementation is less sensitive to the impedance of the virtual machine model, but depends on filtering of the measured d- and q-axes components of the ac-side voltage to avoid instability or poorly damped oscillations. It is demonstrated how both implementations can be made stable and robust for a wide range of grid impedances. However, the dynamic electrical model depends on a high virtual resistance for effectively damping internal oscillations associated with dc components in the ac currents during transients. Thus, when using SM parameters with low virtual stator resistance for decoupling the active and reactive power control, the quasi-stationary VSM implementation is preferable. [ABSTRACT FROM PUBLISHER]

Paper compares three most widely used methods in insulating liquid breakdown prediction: cumulative stress method, stressed electrode area and stressed liquid volume. Breakdowns were measured on Midel 7131 on OTS100AF oil tester with two different electrodes at three gap distances. Prediction capabilities of three methods were tested with respect to results of conducted measurements and research done by other authors. The utilization of these methods is enhanced by application of numerical calculation of electric field. It is shown how these methods are interrelated and a proposal for their unification is given. Unification model is confirmed on measurements conducted in this and other research papers. [ABSTRACT FROM AUTHOR]

In this paper, a two-terminal VSC-HVDC system embedded in a weak grid ac environment is considered, emphasizing modeling, controller design, and small-signal stability analysis. Traditionally, the dc cable is modeled by \Pi-sections, implying that care has to be taken when using the model for higher frequencies or in cases of higher cable impedance density, such as submarine cables. Here, a distributed parameter cable model is used to overcome this problem. The VSC-HVDC system can be described as two cascaded blocks. The first block is a transfer function that will differ depending on what input and output variables are considered, but which is in all realistic cases stable. The second block is a feedback loop, where the forward path is a rational function and the return path is a dissipative infinite dimensional function, remaining the same in all cases. The stability is then analyzed, using the Nyquist criterion, in a straightforward manner. Numerical examples are given by the use of MATLAB. The result is that if the VSC-HVDC system using a single \Pi-section cable model is stable, so is the VSC-HVDC system using a distributed parameter cable model. [ABSTRACT FROM AUTHOR]

Realistic representations of contingencies in AC optimal power flow (ACOPF) often challenge traditional bus-branch network models. Derived from nodal analysis, such approaches represent network constraints in terms of the familiar bus admittance matrix, $Y_{\text{bus}}$. A fixed $Y_{\text{bus}}$ is unable to represent common circuit breaker actions such as bus splitting. Work-arounds for $Y_{\text{bus}}$ -based analysis typically rely on topology processing, switching between different $Y_{\text{bus}}$ matrices depending on breaker settings. In this paper, we propose a very general node-breaker approach, employing multi-port element models and using a sparse tableau formulation (STF) for network constraints. Instead of treating breaker action as altering network topology, and hence changing the structure of Kirchhoff's voltage law (KVL) and Kirchhoff's current law (KCL) equations, this approach represents a breaker's position as impacting only constraints associated with a single component, thereby maintaining fixed structure in the KVL and KCL constraints. While larger numbers of variables are required, STF proves sparser than $Y_{\text{bus}}$ formulations. Numerical case studies herein demonstrate that STF provides computational efficiency comparable to a $Y_{\text{bus}}$ -based ACOPF at the scale of several hundred buses, and lower computational cost in an example of over one thousand buses. [ABSTRACT FROM AUTHOR]

Calculation of circulating current in transposition bars is important to the transposition-type design of stator windings in large generators. The circuit network model of the stator transposition bar, including only resistance and reactance parameters, is proposed in this paper, and the calculation method for parameters is given. The analytical algorithm of circulating current in stator transposition bars is proposed based on the circuit network model. Contrary to the existing calculation method for circulating current, the presented analytical algorithm is a general and fast method for different transposition types, which can avoid establishing complex 3-D models. In order to validate the proposed analytical algorithm, the strand current and circulating current of less than 540 ^\circ with a void transposition bar in a water-cooled turbogenerator are calculated by analytical algorithm and 3-D finite-element method. [ABSTRACT FROM PUBLISHER]

We describe a numerical approach to determine the magnetic power losses caused by alternating magnetizations in thin electrical steel sheets. This paper deals with the evaluation of different loss models which are suitable for postprocessing strategies and distinguishes between analytical equations and numerical procedures. We include improved approximations of the hysteretic and excess loss behavior of ferromagnetic materials into the 1-D partial differential equation that governs the field distribution in thin sheets. The focus of application of the developed loss model lies on nonsinusoidal magnetizations covering high induction values to at least 1.9 T in case of nonoriented steels and a wide frequency range from 25 Hz to at least 2 kHz. We modified existing descriptions of the field contributions due to hysteresis and excess loss to circumvent the limitation to low inductions of the original formulations. Our approach is experimentally verified by testing a variety of nonsinusoidal waveforms which cover considerable minor hysteresis loops and a wide span of induction and frequency values. [ABSTRACT FROM PUBLISHER]

This paper presents a three-phase ac–dc–ac drive system to cope with applications in which high performance and reliability are required with reduced dc-link and switch blocking voltages. The subsystem at the grid-side can be implemented into two alternatives, i.e., topologies G1 and G2. While G1 presents a sole three-phase transformer connected in series between the three-phase grid and converter $Ag$, whose secondary is connected to the converter $Bg$, G2 has a three-phase transformer with two taps at the secondary. The transformers in G1 and G2 process part and total system power, respectively. The subsystem at the machine-side is composed of the machine and two converters that allow series- and parallel-modes for the three-phase machine. The interesting characteristic of the system is to adapt its machine-side configuration as a function of the speed. The proposed configurations guarantee low harmonic distortion for both high speed (high voltage) and low speed (low voltage) operations with a higher modulation index. In addition, the system allows the machine's operation with high current at low speed, which is interesting for applications with constant mechanical load. A comprehensive model of the proposed systems and modulation strategies are presented. Simulated and experimental results validate the theoretical expectations. [ABSTRACT FROM AUTHOR]

Under the effects of alternating current (AC), the resistance and reactance of electric machine windings are different from their static state. As electric machine windings usually comprise many conductors, it makes the modeling and analysis very cumbersome using the finite element method. This paper presents an analytical method for computing AC resistance and reactance of electric machine windings placed in ferromagnetic slots, which is more convenient and efficient, and could be extended to the slots with different complex structures. First, the slot is divided into a number of layers based on the concept of discretization. Then, the magnetic field governing equation is established, and the distributions of magnetic field intensity and the current density of conductors under AC conditions are solved. By Poynting's theorem, the complex power flowing into the conductors is calculated, and then the effective resistance and reactance of the windings are obtained. Finally, the deduced analytical solutions are compared with the finite element method, and the results show that they have a good agreement for both single-layer winding and the double-layer windings with current phase difference. The presented analytical model is useful for understanding the AC effects and optimization of electric machine windings placed in ferromagnetic slots. [ABSTRACT FROM AUTHOR]

This paper presents a novel single-stage ac/dc rectifier with high-frequency isolation and bidirectional power flow capability, which is feasible to connect dc grids to ac ones. A thorough analysis is performed based on the development of an actual model as well as a fundamental one using the double-sideband amplitude modulation equations. The derived models allow the determination of the active and reactive power flow through the transformer as well as defining the operating regions for which soft commutation is achieved. Besides, the current stresses of the semiconductors devices are analyzed in detail. Experimental results obtained from a 5 kW prototype are presented and discussed in order to validate the proposed model as well as demonstrate the converter's performance and claimed advantages. The proposed concept can also be extended to dc/dc, ac/ac, and multilevel topologies. [ABSTRACT FROM AUTHOR]

Surface-mounted permanent magnet synchronous machine with concentrated windings (cwSPMSM) is a high-performance drive machine and has been adopted in many applications. The difficulty of implementing its sensorless control at low and zero speeds is its multiple saliencies, which is much more significant than most other ac machines. The traditional decoupling methods provide successful results only under the condition that high-order saliencies are not stronger than half of the primary saliency. Furthermore, the behavior of the multiple saliencies is principally frequency dependent. Based on the characteristics of such machines, this paper proposes a multisignal injection method for realizing sensorless control. This method injects multiple high-frequency signals with different frequencies and magnitudes into the machine. Different frequency components in the response current signals are demodulated and then combined together to get the clear primary saliency signal, which is used to identify the rotor position. This new method was validated using a cwSPMSM at low speed. The experimental results proved the effectiveness and accuracy of the new method. [ABSTRACT FROM PUBLISHER]

Compared to HVAC, the application experience of composite/polymer insulators in HVDC transmission lines is limited. However, the tendency to utilize them in HVDC transmission lines, where pollution flashover is a major problem, is growing due to their water-repellent feature. This trend demands understanding insulator flashover performance and recognizing key parameters in the design and dimensioning of insulators. This paper presents the measurement and analytical results for dc pollution flashover of silicon rubber insulators at extra heavy pollution conditions. Two approaches are employed to model flashover voltage gradient and quantify the contribution of salt deposit density, specific leakage distance, average diameter, shed spacing to shed depth ratio, and form factor to insulator pollution performance. Developing a nonlinear regression model, the effect of the mentioned parameters on pollution flashover is studied. Then, the artificial neural network (ANN) and randomization method are used to develop a model to interpret parameters contribution to pollution flashover, and the results are validated with those of the regression method. Finally, by applying an ANN-based randomization approach, a multi-input model is created to quantify the significance of different parameters in flashover voltage gradient. The model ranks specific leakage distance and average diameter, respectively, as the most and least important geometrical characteristics in pollution flashover performance of SiR insulators. The method and results can be a source of information to optimize the design and dimensioning of HVDC SiR insulators especially in severe pollution conditions. [ABSTRACT FROM AUTHOR]

In this paper, we propose a flexible mixed-integer linear programming formulation of the AC OPF problem for distribution systems, using convexification and linearization techniques. The proposed formulation allows the representation of discrete decisions via integer decision variables, captures the nonlinear behavior of the electrical network via approximations of controllable accuracy, and can be solved to global optimality with commercial optimization solvers. The formulation is based on conventional variables that describe network behavior, which ensures its flexibility and the possibility of application to various distribution system problems, as we indicate with case studies. [ABSTRACT FROM PUBLISHER]

This paper proposes a new design procedure for a bidirectional dc/dc LCL converter for potential megawatt range applications, including mobile mining equipment. This type of dc/dc converters employ two dc/ac converters and a passive LCL filter instead of a traditional high-frequency transformer. The exclusion of the transformer eliminates the core losses, while the switching losses are minimized by using soft switching operation. The proposed design method is different from previous publications as it ensures zero reactive power circulation, minimizes the size of the biggest capacitor, ensures stable and robust operation, and offers a good tradeoff between the capacitor size and the fault current in the converter under a dc fault condition. In addition to the proposed design, a simple and effective control strategy is proposed, which is based on single phase $pq$ theory. The converter design and control have been evaluated by a digital simulation in EMTP-RV software environment for a 100 MW 300 kV/20 kV converter. Experimental validation has been carried out on a laboratory 200W 100V/20V prototype converter. [ABSTRACT FROM AUTHOR]

AC loss has become a key problem for practical applications of high-temperature superconductors. It is well known that mechanical deformation will affect the critical current and ac loss as the superconductor is subjected to the magnetic field or transport current. In this paper, the ac losses in Bi2Cr2Ca2Cu 3O10 (Bi2223)/Ag tapes under bending deformation are investigated. Bi2223 tape consists of Ag matrix and Bi2223 filaments, and the ac losses are generated by transport current and external field. By considering the plastic deformation of Ag matrix and residual strain, the simplified mechanical fracture model is established. Then, the finite element method analysis is used to investigate the relation of critical current and ac loss with bending deformations, and the losses are presented for different transport current and external magnetic field conditions. The influences of filament fracture on the ac loss are discussed for transport current and external magnetic field cases, respectively. [ABSTRACT FROM PUBLISHER]

In this paper, the performance of the dual stator-winding induction generator (DWIG) variable frequency ac (VFAC) generating system with unbalanced loads is analyzed, and an asymmetrical operation analysis method based on a three-port load network model is proposed. This method has four key steps: (1) deriving the three-port load network model for three-phase four-line unbalanced loads; (2) establishing the positive sequence, negative sequence, and zero sequence equivalent circuits for the DWIG; (3) combining the above model and circuits together to obtain the system whole equivalent model; and (4) writing the corresponding equations and solving them. The experimental results and the calculation results are almost consistent, which proves that the proposed method is correct and valid, and the DWIG VFAC generating system with unbalanced loads has a good performance. [ABSTRACT FROM PUBLISHER]

Interfacing of electrical machine models with ac power networks has a significant impact on numerical accuracy and efficiency in state-variable-based transient simulation programs. This paper continues the recent work in this area by proposing a new explicit constant-parameter voltage-behind-reactance (VBR) induction machine model that includes main flux saturation and allows a direct interface to any external network. The proposed model uses numerical approximations to achieve a decoupled interfacing circuit with constant RL branches that is easy to use in many simulation programs. Computer studies demonstrate that the proposed model provides high numerical accuracy for machines with a diverse range of parameters, even at fairly large integration step sizes. [ABSTRACT FROM AUTHOR]

A nonlinear control able to perform voltage and power stability in direct current (dc) microgrids for an efficient renewable energy integration that is capable of providing frequency support to an alternate current grid is introduced in this work. A generic dc microgrid modeling is developed, and the introduced controllers are shown to ensure grid stability, both by a theoretical analysis with a formal proof of stability using Lyapunov techniques, than by simulations performed in MATLAB. A comparison with classical proportional integral control techniques is described to show the performance improvements. [ABSTRACT FROM AUTHOR]

A series–parallel connected thermal converter (TC) arranged in an $m \times m$ matrix of a heater and an $m^{2}$ -series connected thermocouple are proposed to reduce low-frequency ac–dc differences. The electrical circuit theory on the proposed circuit suggests that the low-frequency ac–dc differences of $m \times m$ series–parallel connected TCs decrease in proportion to 1/ $m^{2}$ , avoiding any degradation in the output voltage and a time-consuming measurement. A significant reduction of the low-frequency ac–dc differences by a factor of 10 for the proposed circuit is experimentally demonstrated with no decrease of the output voltage, which is consistent with the electrical circuit theory. [ABSTRACT FROM AUTHOR]

Atmospheric pressure plasma jets driven by a pulsed dc power supply have several advantages over the traditional jets driven by an ac power supply. However, how the pulse rise time affects the characteristic of the plasma jets is not clearly known. In this paper, the simulation results of a plasma jet driven by pulsed dc voltages with different pulse rise time \(t_{\mathrm {\mathbf {rise}}}\) are presented. It shows that, the shorter the pulse rise time \(t_{\mathrm {{rise}}}\) is, the faster the head of the plasma jet propagates. [ABSTRACT FROM PUBLISHER]

Under transient operating conditions, especially in the case of alternating current or pulse current, the high temperature superconducting (HTS) magnet will suffer ac losses leading to changes in the temperature and critical current distribution throughout the magnet. A magneto-thermal model is needed to simulate this process. In fact, the state change of the magnet will lead to a nonlinear performance of the HTS magnet in the power system. This paper introduces a coupling simulation and modeling method for the HTS magnet based on a cosimulation model built in MATLAB and COMSOL. The HTS magnet element is a customized module created via the self-code S-Function in MATLAB. A magneto-thermal finite element model based on the PDE and Heat Transfer modules of COMSOL is built into the S-Function. This model allows the state of the HTS magnet to be monitored during the operating process. A small-scale HTS magnet including three single pancake coils with a dc excitation system is illustrated to verify the nonlinear characteristic of the HTS magnet and the effectiveness of this coupling simulation and modeling method. [ABSTRACT FROM PUBLISHER]

The purpose of this study is to compare possible superconducting 380-kV cables with existing overhead lines and underground cables. The study compares the required corridors and energy losses of different versions of a 1.7-km transmission system with a power of 6.6 GVA. Comparison covers the following setups: 4 × 380 kV high-voltage overhead lines, 8 × 380 kV conventional cables, 2 × 380 kV and 4  × 380kV superconducting cables with warm and cold dielectrics. The paper will present the description of the different setups as well as the calculation of energy losses. The required widths of the underground cable corridors are also considered in this study. Finally, the temperature profile and pressure drop of the superconducting cables are investigated. The main results can be summarized as follows. 1) For the assumed voltage level and power, overhead lines require a corridor width of 70 m, whereas a corridor of less than 7.7 m in width is necessary for superconducting 380 kV cables. 2) The 380-kV superconducting cable reduces the energy loss by approximately 8100 MWh per year in comparison to the conventional overhead lines. This yields a feasible energy loss reduction of up to 75%. [ABSTRACT FROM PUBLISHER]

AC loss is an unavoidable problem for a conduction-cooled HTS superconducting magnetic energy storage (SMES) magnet during dynamic operation, which may cause a temperature raise and affect the reliable operation of the magnet. In this paper, the anisotropic homogenization method is employed to realize the modeling of a 150 kJ hybrid SMES magnet based on H-formulation. The SMES magnet has a maximum of approximately 7000 turns. First, the homogenization method is validated that it can reasonably predict the ac losses of both YBCCO coils and BSCCO coils, but it has lower accuracy to estimate the ac losses of BSCCO coils than YBCO coils. Then, ac loss characteristics of the SMES magnet during dynamic operations are investigated. The magnetic field, current density distribution, and ac loss distribution are presented and discussed for different operation conductions. The results indicate that a steady-state current of less than 100 A may be an appropriate choice during power exchange for reducing ac loss. [ABSTRACT FROM PUBLISHER]

Process-induced variations of 10-nm node n-type FinFETs considering middle-of-line parasitics were investigated in terms of dc/ac performances using fully calibrated TCAD simulations. Variations of positive fixed oxide charge density at shallow trench isolation interface and source/drain (S/D) height influenced off-state and on-state performance variations, respectively, but slightly on RC delay variations. Fin width variations induced off-state performance and RC delay variations critically due to the fluctuation of short channel effects. But, fin height variations affected them slightly due to the preserved gate-to-channel controllability and the buffered effects by varying drain currents and gate capacitances in the same direction. Gate length, spacer length, and S/D length variations influenced dc/ac performance variations severely; 2-nm-length changes were barely acceptable to satisfy 10% RC delay margin. Thus, the process-induced variability parameters, including fin width, gate length, spacer length, and S/D length, should be controlled tightly under a few nanometers to reduce dc/ac performance variations of the FinFETs. [ABSTRACT FROM AUTHOR]

Based on the latest experimental results of two different modern graded grain-oriented electrical steels measured up to 2 T under ac, the existing \boldsymbol 10 popular analytical approximation functions for ac magnetization curves are critically evaluated. None of them are capable of making a satisfactory approximation over the full useful range (0–2 T). Therefore, a new simple explicit empirical function is proposed that is simple to use (six parameters) and sufficiently accurate (adjusted \textR^2\boldsymbol \ge \boldsymbol 0.9992 ) to represent the ac magnetization curve. The empirical function is also shown to be suitable (adjusted \textR^2\boldsymbol \ge \boldsymbol 0.9913 ) for extrapolating the ac magnetization curve into the high-flux-density region (1.80–2 T) when only using the available experimental data in the region 0–1.80 T from steel manufacturers. [ABSTRACT FROM AUTHOR]

Objective: Currently, there is no imaging method that is able to distinguish the functional activity inside nerves. Such a method would be essential for understanding peripheral nerve physiology and would allow precise neuromodulation of organs these nerves supply. Electrical impedance tomography (EIT) is a method that produces images of electrical impedance change (dZ) of an object by injecting alternating current and recording surface voltages. It has been shown to be able to image fast activity in the brain and large peripheral nerves. To image inside small autonomic nerves, mostly containing unmyelinated fibers, it is necessary to maximize SNR and optimize the EIT parameters. An accurate model of the nerve is required to identify these optimal parameters as well as to validate data obtained in the experiments. Methods: In this study, we developed two three-dimensional models of unmyelinated fibers: Hodgkin–Huxley (HH) squid giant axon (single and multiple) and mammalian C-nociceptor. A coupling feedback system was incorporated into the models to simulate direct and alternating current application and simultaneously record external field during action potential propagation. Results: Parameters of the developed models were varied to study their influence on the recorded impedance changes; the optimal parameters were identified. The negative dZ was found to monotonically decrease with frequency for both HH and C fiber models, in accordance with the experimental data. Conclusion and significance: The accurate realistic model of unmyelinated nerve allows the optimization of EIT parameters and matches literature and experimental results. [ABSTRACT FROM AUTHOR]

A particle-fluid flow under alternating current (ac) electrokinetics was numerically simulated to investigate the three-dimensional (3-D) particle motion in a complex electric field of a high conductivity medium generated by an electrode-multilayered microfluidic device. The simulation model coupling thermal–fluid–electrical and dispersed particle problems incorporates three ac electrokinetics (ACEK) phenomena, namely, the ac electrothermal effect (ACET), thermal buoyancy (TB), and dielectrophoresis (DEP). The electrode-multilayered microfluidic device was fabricated with 40 electrodes exposed at the flow channel sidewalls in five cross sections. The governing equations of the simulation model are solved by the Eulerian–Lagrangian method with finite volume discretization. Fluid flow simulations in three cases with or without consideration of ACET and TB are performed to clarify the contributions of these phenomena. The fluid flow is found to be composed of short-range vortices due to ACET and long-range circulation due to TB based on the features of the electrode-multilayered microfluidic device. The 3-D particle trajectory influenced by the fluid flow is compared with four values of the real part of the Clausius–Mossotti (CM) factor to evaluate the DEP phenomenon. The simulation model is validated by experiments using a cell suspension. The pattern of cell trajectories in the upper part of the flow channel measured by particle tracking velocimetry agrees with the simulated pattern. By comparison of the simulation and experiment, it is found that the cells moving straight away from the electrode on the focal plane are decelerated within the region of 60 μm from the electrode by positive-DEP with ${\text{Re[K}}(\omega)] = {\text{0.08}}\hbox{--}{\text{0.11}}$. Furthermore, the 3-D DEP-effective region and the ACET and TB dominant regions for the cells are predicted by evaluating the particle-fluid relative velocity due to DEP force with ${\text{Re[K}}(\omega)] = {\text{0.10}}$. Consequently, the flow mechanism and dominant region of each ACEK phenomenon in the device are clarified from the 3-D simulation validated by the experiments. [ABSTRACT FROM AUTHOR]

The rising share of renewable generation increases uncertainty in power system operation. New methods are needed for secure operation of power systems under uncertainties. We present a robust optimal power flow algorithm for mixed ac and dc power transmission systems which considers uncertain renewable generation and/or load. The uncertain infeeds are assumed to lie inside a known norm-bounded set. The presented algorithm guarantees satisfaction of various steady-state power system constraints for all uncertainties within this set. In order to satisfy these constraints at minimal cost, the algorithm modifies both the redispatch and primary reserve. This is done through the optimization of setpoints and droop gains of generators and high-voltage dc converters. The algorithm is evaluated on a test system, the Cigre B4 DC grid, the IEEE RTS 96 grid, and on the Danish transmission system. In these examples, we show that redispatch costs can be reduced by additionally optimizing primary reserve, and demonstrate primary reserve sharing between asynchronous ac grids through dc grids. We illustrate the impact of the uncertainty set choice on the worst-case redispatch costs, perform a sensitivity analysis of the approach, and evaluate the computation time as a function of the number of uncertainties. [ABSTRACT FROM AUTHOR]