AC Line Research Articles

Supercapacitive performance analysis of low cost and environment friendly potato starch based electrolyte system with anodized aluminium and teflon coated carbon cloth as electrode

For device fabrication, materials needs to satisfy 3Es (economical, environment friendly and easy to prepare) along with technical characteristics. Present paper reports an electrolyte synthesized by modifying potato starch by solution cast technique which is easy to synthesize, stretchable, bendable and twistable low cost (∼ INR 15/cm2) free standing electrolyte film having wide electrochemical stability window (ESW ∼ 2.4 V), low equivalent series resistance (ESR ∼ 1.7 Ω) and small ion relaxation time (15 μs). Supercapacitors are fabricated using this electrolyte with two different electrodes (teflon coated carbon cloth and anodized aluminium). With laboratory prepared anodized Al as electrode, the electrochemical stability window is >4 V, ESR is <2 Ω and capacitance of ∼mF is obtained. Approximately −80° phase angle is obtained which remains constant from 1 Hz to 10 kHz range which covers the frequency requirement of ac line filtering application. The coulombic efficiency estimated from charging/discharging curves have values > 98% which approximately remains constant in studied number of cycle. All these indicate that prepared electrolyte is a potential candidate for electrochemical device applications.

A Mathematical Model for the Transient Stability Analysis of a Simultaneous AC–DC Power Transmission System

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.

Three-Port Bridgeless PFC-Based Quasi Single-Stage Single-Phase AC–DC Converters for Wide Voltage Range Applications

Quasi single-stage single-phase ac–dc power converters based on a three-port bridgeless power factor corrector (TPB-PFC) are proposed in this paper. The TPB-PFC, which is able to provide an ac input port, a dc load port, and a dc bus port simultaneously, is derived by introducing a new dc load port into a conventional bridgeless PFC. The load, whose voltage can be either lower or higher than the peak amplitude of the ac line voltage, is connected to the dc load port of the TPB-PFC directly. Therefore, the ac input power can be fed to the dc load with the single-stage power conversion when the instantaneous ac voltage is lower than the voltage of the dc load. Although another dc–dc converter is required to accomplish the power transmission between the dc bus port and the dc load port of the TPB-PFC, the power rating and losses of the dc–dc converter are reduced significantly, in comparison with a two-stage ac–dc converter. Furthermore, the switching losses of the TPB-PFC are reduced thanks to the three-level characteristic of the proposed converter. The topology derivation, operation principles, modulation strategies, and characteristics of the proposed ac–dc converter are analyzed and verified experimentally with a 2-kW prototype.

Efficient method for the real‐time contingency analysis of meshed HVDC power grids fed by VSC stations

An efficient method for the real-time contingency analysis of meshed high-voltage direct current (HVDC) power grids fed by voltage source converter (VSC) stations is introduced here. A linearised AC/DC grid model is initially determined considering the control strategies of the various VSC units. This lays the foundations for the determination of linear sensitivity factors with which the contingency analysis is carried out to evaluate the real-time N−1 criterion in AC/DC grids, as demanded by system control centres. Distribution and power-injection factors are subsequently derived for efficiently assessing the impact of AC/DC transmission line outages and load/generator disconnections on the HVDC grid. Conversion factors are also derived to estimate the impact of the loss of VSC stations on the AC/DC network, this being another inherent contribution of this work. The efficiency and validity of this timely approach, which finds practical applicability to the real-time operation of HVDC power grids, is confirmed using a meshed DC network fed by three VSC stations. The disconnection of AC and DC transmission lines, generators, and VSC stations are dynamically simulated using Simulink and their post-disturbance steady-state conditions are compared against those computed by the introduced method where it is confirmed that both solutions concur very well with each other.

Application of Resonance Analysis to AC–DC Networks

Medium to long-distance underground ac lines and both short- and long-distance HVdc are frequently integrated into network expansion plans. In conjunction with growing numbers of power-electronic converters in the grid, there is a rising risk related to the effects of harmonic currents. Resonance analysis should therefore be part of power system expansion planning. In this paper, network harmonic modeling and analysis are discussed, in particular, power transformer and power converter frequency models are proposed. Frequency scanning and resonance mode analysis are compared and discussed. The changes in resonance response due to transmission network expansion are drawn. The methods are applied to the investigation of the impact of one link reinforcement, both ac and ac solutions, as well as the impact of the power transformer disconnection. The performed analysis enables finding the resonance location at each studied case. The frequency analysis methods presented in this paper enable forseeing network frequency behavior that is valuable for both network analysis and network expansion planning.

Efficiency estimation method of three-wired AC to DC line transfer

The development of power semiconductor converters technology expands the scope of their application to medium voltage distribution networks (6-35 kV). Particularly rectifiers and inverters of appropriate power capacity complement the topology of such voltage level networks with the DC links and lines. The article presents a coefficient that allows taking into account the increase of transmission line capacity depending on the parameters of it. The application of the coefficient is presented by the example of transfer three-wired AC line to DC in various methods. Dependences of the change in the capacity from the load power factor of the line and the reactive component of the resistance of the transmission line are obtained. Conclusions are drawn about the most efficient ways of converting a three-wired AC line to direct current.

Stability and control of mixed AC–DC systems with VSC‐HVDC: a review

Voltage-source converter-high-voltage direct current (VSC-HVDC) systems have become an attractive option for integrating remote and far-from-shore renewable energy resources to main AC grids. The desire for greater power transfer capability and the difficulty in securing right-of-way for new AC lines in many countries is also resulting in the increased use of embedded VSC-HVDC systems operating in parallel with existing AC lines. It has been stated that the control and operation of VSC-HVDC systems are of particular concern for weak grids with fewer large synchronous generation units (a highly probable case for many grids in future). If the anticipated proliferation of VSC-HVDC links continues, several aspects of system stability will be significantly impacted. This study presents an overview of the effects of VSC-HVDC control and operation on power system stability. The structure, control, control tuning, and modelling of VSC-HVDC is briefly summarised to provide context for subsequent discussion of the system dynamics. An extensive critical review of the previous research into mixed AC-DC systems incorporating VSC-HVDC is then provided including voltage stability, small and large-disturbance angle stability, high-frequency interaction, and frequency stability. Finally, recommendations are presented to guide critical future research.

MXene-Based Flexible Supercapacitors for AC Line-Filtering with Ultrafast Frequency Response

Slow charging-discharging rate of flexible electrochemical capacitors (FECs) is a critical challenge of current self-powered portable electronics. Mostly, FSCs function as a power source at direct current, but quit functioning of pulse energy harvesting and current ripple filtering, like conventional capacitors. Thus, portable electronics calls for high frequency response FECs. Motivated from this requirement, we demonstrate ultrafast frequency response of FECs integrating conducting polymer/MXene hybrid electrodes and polymer gel electrolyte. The composite ratio and thickness of material matrix dramatically influences the frequency response behavior of FECs. The FECs with optimized electrode material features accomplished excellent volumetric capacitive and frequency response with bending functionality. Additionally, the real-life capability of fabricated FECs for AC line filtering was demonstrated through LED as DC indicator with bent ECs. Therefore, the high volumetric capacitance and robust frequency response of fabricated FECs offers the most requisites of self-powered portable electronics.

A Generalized Approach to the Load Flow Analysis of AC–DC Hybrid Distribution Systems

This paper proposes a unified load flow (LF) model for AC–DC hybrid distribution systems (DSs). The proposed model can be applied in hybrid DSs with mixed configurations for AC/DC buses and AC/DC lines. A new classification of DS buses is also introduced for LF analysis. A set of generic LF equations has been derived based on comprehensive analysis of the possible AC–DC hybrid system configurations. Three binary matrices, which are used as a means of describing the configuration of the AC and DC buses and lines, have been employed in the construction of the unified power equations. These matrices enable a single configuration at a time to be activated in the power equations. The proposed LF model is generic and can be used for both grid-connected and isolated hybrid DSs. The new model has been tested using several case studies of hybrid DSs that include different operational modes for the AC and DC distributed generators. As a means of evaluating the effectiveness and accuracy of the proposed model, the LF solution was compared to the solution produced by PSCAD/EMTDC. A comparison of the results reveals the efficacy of the proposed model.

The Hybrid Transmission Grid Architecture: Benefits in Nodal Pricing

Recently, we proposed a capacity expansion approach for transmission grids that combines the upgrade of transmission capacity with a transition in system structure to improve grid operation. The key to this concept is a particular hybrid AC/DC transmission grid architecture, which is obtained by uprating selected AC lines via a conversion to HVDC. We have shown that this system structure improves optimal power flow (OPF) solvability and that it can reduce the total generation costs. In this work, we study the benefits of this hybrid architecture in the context of a deregulated electricity market. We propose an efficient and accurate nodal pricing method based on locational marginal prices (LMPs) that utilizes a second-order cone relaxation of the OPF problem. Applicability of this method requires exactness of the relaxation, which is difficult to obtain for conventional meshed AC transmission grids. We prove that the hybrid architecture ensures applicability if the LMPs do not coincide with certain pathological price profiles, which are shown to be unlikely under normal operating conditions. Using this nodal pricing method, we demonstrate that upgrading to the hybrid architecture can not only increase the effective transmission capacity but also reduce the separation of nodal markets and improve the utilization of generation.

Secondary Arc Current During DC Auto Reclosing in Multisectional AC/DC Hybrid Lines

In this paper, a contribution to the analysis of electromagnetic ac to dc interactions in complex ac/dc hybrid overhead lines was made. For accurate calculation of such coupling, standard transformations like the symmetrical components cannot be used. Therefore, a method based on adjusted modal decomposition was presented, which allows analysis of untransposed and partly transposed configurations in arbitrary switching states. The method was applied to the case of dc fault clearing using single-pole auto reclosing (dc SPAR), where parallel ac circuits could induce a secondary arc in the dc line. The magnitude of the secondary arc current was examined, which allows evaluating the necessary dead time of the dc SPAR. The influence of parameters like line configuration, load flow, and fault location was discussed. After that, a realistic study case hybrid line was analyzed. In particular, the comparison to secondary arcs in ac lines and the behavior as a function of fault location in the context of changing ac systems and line configurations along the hybrid dc corridor provide new insights into a relevant phenomenon.

Upgrading transmission line capability by AC–DC conversion

In recent times, it has been very difficult to build corridors for new overhead lines in industrialized countries like India due to various constraints. To avoid such complicated implementation, building of new infrastructure, new methods are required to upgrade conventional overhead lines. The conversion from AC to DC lines is an alternative way to upgrade the capability of power transmission in order to avoid Rights Of Way (ROW) issues. Hence, in this paper, a methodology is adapted to convert the High Voltage AC (HVAC) line and High Voltage DC (HVDC) for improved power transmission. Electric Field Intensity and Magnetic Field Intensity are evaluated and a real-time data over 400 kV transmission network is analysed to test its performance. The identified network is also analysed under steady and dynamic state with ETAP/PSS/E tool to measure the performance of power transmitted. Further, load flow analysis, short circuit and transient stability are also studied to check the efficacy of the methods adopted to convert HVAC to HVDC lines.

AC signal characterization for optimization of a CMOS single-electron pump

Pumping single electrons at a set rate is being widely pursued as an electrical current standard. Semiconductor charge pumps have been pursued in a variety of modes, including single gate ratchet, a variety of 2-gate ratchet pumps, and 2-gate turnstiles. Whether pumping with one or two AC signals, lower error rates can result from better knowledge of the properties of the AC signal at the device. In this work, we operated a CMOS single-electron pump with a 2-gate ratchet style measurement and used the results to characterize and optimize our two AC signals. Fitting this data at various frequencies revealed both a difference in signal path length and attenuation between our two AC lines. Using this data, we corrected for the difference in signal path length and attenuation by applying an offset in both the phase and the amplitude at the signal generator. Operating the device as a turnstile while using the optimized parameters determined from the 2-gate ratchet measurement led to much flatter, more robust charge pumping plateaus. This method was useful in tuning our device up for optimal charge pumping, and may prove useful to the semiconductor quantum dot community to determine signal attenuation and path differences at the device.

Analysis and Assessment of Operation Risk for Hybrid AC/DC Power System based on the Monte Carlo Method

Based on the Monte Carlo method, an improved risk assessment method for hybrid AC/DC power system with VSC station considering the operation status of generators, converter stations, AC lines and DC lines is proposed. According to the sequential AC/DC power flow algorithm, node voltage and line active power are solved, and then the operation risk indices of node voltage over-limit and line active power over-limit are calculated. Finally, an improved two-area IEEE RTS-96 system is taken as a case to analyze and assessment its operation risk. The results show that the proposed model and method can intuitively and directly reflect the weak nodes and weak lines of the system, which can provide some reference for the dispatching department.

Fast Response, Carbon-Black-Coated, Vertically-Oriented Graphene Electric Double Layer Capacitors

Development of electrical double layer capacitors using vertically oriented graphene nanosheets with fast response continues. The inherent open morphology of the nanosheets allows efficient access to charge storage surfaces, making them suitable for AC line filtering. However, since the overall surface area is only about a factor of ∼310x over the geometric area, the specific capacitance available remains limited. This work presents utilization of the conventional growth of vertically oriented graphene nanosheets on Ni substrates as the underlying architecture for coating with high surface area carbon black to substantially increase the specific capacitance while retaining the open morphology to allow good frequency response at 120 Hz. The carbon black coating was deposited on ∼1.2 μm and ∼2.5 μm high nanosheets using an aerosol spray method. Deposition times from 0–8 s, in 1 s intervals, provided coatings which translated into a specific capacitance of 2.3 mF/cm2 at 120 Hz (8 s coating) and a volumetric capacitance of 4.6 F/cc (energy storage elements). Improvements in the uniformity of the carbon black coatings suggest that much higher specific capacitances are possible. COMSOL models of high density VOGN grown to 10 μm high and covered uniformly with 100 nm of carbon black coating suggest a capacitance of ∼42 mF/cm2 with acceptable frequency response at 120 Hz can be achieved.

LED Driver Based on Boost Circuit and LLC Converter

In order to reduce the size and cost of a medium-power light-emitting diode (LED) driver, a novel single-stage LED driver integrating a boost converter with a half-bridge LLC resonant converter has been proposed. This topology is composed of a boost power factor correction circuit operating in a discontinuous conduction mode and an isolated LLC circuit unit with soft-switching characteristics. The proposed LED driver only adopted two switch components, thus benefiting from control simple and without employing an extra switch driver and a control circuit. The operating principle and characteristics of the proposed LED driver are analyzed in detail. In addition, experimental results of a laboratory prototype for supplying a 100-W/70-V LED lighting systems from 220-V/50-Hz ac line voltage are presented to verify well the correctness of the theoretical analysis and parameters’ design.

Application of the parametric proper generalized decomposition to the frequency-dependent calculation of the impedance of an AC line with rectangular conductors

Abstract AC lines of industrial busbar systems are usually built using conductors with rectangular cross sections, where each phase can have several parallel conductors to carry high currents. The current density in a rectangular conductor, under sinusoidal conditions, is not uniform. It depends on the frequency, on the conductor shape, and on the distance between conductors, due to the skin effect and to proximity effects. Contrary to circular conductors, there are not closed analytical formulas for obtaining the frequency-dependent impedance of conductors with rectangular cross-section. It is necessary to resort to numerical simulations to obtain the resistance and the inductance of the phases, one for each desired frequency and also for each distance between the phases’ conductors. On the contrary, the use of the parametric proper generalized decomposition (PGD) allows to obtain the frequency-dependent impedance of an AC line for a wide range of frequencies and distances between the phases’ conductors by solving a single simulation in a 4D domain (spatial coordinates x and y, the frequency and the separation between conductors). In this way, a general “virtual chart” solution is obtained, which contains the solution for any frequency and for any separation of the conductors, and stores it in a compact separated representations form, which can be easily embedded on a more general software for the design of electrical installations. The approach presented in this work for rectangular conductors can be easily extended to conductors with an arbitrary shape.

A mathematical model for the loadability analysis of a simultaneous AC–DC power transmission system

Simultaneous AC–DC power transmission system can, generally, increase the load carrying capability of an existing long AC transmission line. A mathematical model of a simultaneous AC–DC transmission system, transmitting power through an existing AC transmission line, is proposed in this paper. The model is based on the development of expression for AC and DC power flow of simultaneous AC–DC system in terms of the power flow of original AC system. It incorporates the thermal constraint of the line, the allowable maximum percentage of DC voltage mix in an already installed AC line and the operating practice of a long transmission line. 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 two different standard software packages, PSpice and MATLAB simulink, based circuit simulations. The model is also applied to a realistic system to verify its capability of evaluating the benefits of a simultaneous AC–DC transmission system. The model is an analytical tool for transmission planners to take the decision whether an existing AC transmission system would increase the power transfer capability if it were converted into a simultaneous AC–DC system.

Releasing more capacity for EV integration by DC medium voltage distribution lines

With the integration of large-scale electric vehicles (EVs), more capacities of medium voltage distribution networks need to be released. However, there are limited spaces to build new lines, and voltage violation may happen due to the power fluctuation when EVs charge/discharge in a random way. This study proposes to convert some existing AC medium voltage distribution lines to DC and forms a hybrid AC/DC medium voltage distribution network through connecting existing AC lines with a voltage source converter (VSC). Transfer capacity of lines can be increased through DC distribution and flexible power shift between the AC and DC lines can be achieved, based on which more EVs can be accommodated. Configurations of hybrid AC/DC distribution networks are developed, and the capacities released are quantified. A control scheme, which includes a loss minimisation mode and a voltage regulation mode, is proposed in order to optimise real and reactive power outputs of the VSC. Simulation studies are performed to verify the proposed method.

Calculation of the ground hybrid electric field of ±800kV EHV DC and 750kV UHV AC parallel transmission lines

Based on the method of time-domain upwind difference algorithm, the ground hybrid electric field is calculated. The typical tower is selected from the ±800kV Hami- Zhengzhou DC transmission project and the 750kV Tianshui-Baoji double-loop transmission line project. In the paper, the minimum height above ground is determined when the individual AC and DC lines meet the electric field limits, respectively. Then the article analyzes the AC and DC parallel spacing and the width of the corridor, using the weighted method to determine the control values of hybrid electric field.