Summary

Electric double-layer capacitors (EDLCs) have the potential to replace aluminum electrolytic capacitors for alternating current line-filtering, aligning with the trend of device miniaturization. However, it is challenging for EDLCs to simultaneously achieve rapid ion migration, electrical response, and high areal and volumetric capacitances. Here, we demonstrate that three-dimensional, structurally integrated multi-layer carbon tube (3D-MLCT) frameworks are used as electrodes for high-performance filtering EDLCs. By simply increasing the number of tube layers, a high specific areal capacitance of 3.08 mF cm−2 at 120 Hz is achieved with a phase angle of −80.1°, exhibiting excellent capacitance and fast frequency response. The outstanding performance of the 3D-MLCT is attributed to the high density, high orientation, and high integrality of carbon tube arrays that facilitate and accelerate ion distribution onto the electrode surface. The findings of this work provide a solution for developing next-generation miniaturized filter capacitors with high capacitance and fast frequency response.

Introduction

Converting alternating current (AC) into direct current (DC) is essential for powering electronics.1,2,3 In the process, filter capacitors play a crucial role in smoothing the ripples in the rectified DC signal.4,5,6,7 Aluminum electrolytic capacitors (AECs) dominate this field but occupy the most significant volume in circuits.8,9,10 Electric double-layer capacitors (EDLCs), storing energy physically through reversible ion adsorption at the electrode-electrolyte interface, have been anticipated for fast frequency response applications.11,12,13,14,15 EDLCs are considered promising alternative devices for AC line-filtering to meet the demands of miniaturized electronic devices owing to their several-orders-of-magnitude-higher specific capacitance than commercial AECs.16,17,18,19,20,21,22,23 However, conventional EDLCs behave more like resistors than capacitors and possess poor frequency response performance at an indicator frequency of 120 Hz because of their tortuous and complicated pore structure as well as the high resistances of the electrodes.24,25,26,27

The first filtering EDLC was reported by Miller et al.,1 with discrete vertically oriented graphene (VOG) nanosheets grown on nickel substrates as electrodes, demonstrating an outstanding AC line-filtering performance. Subsequently, various carbon-nanomaterial-based electrodes with high orientation, thin thickness, and macro-porous structures have been explored for filtering EDLCs.28,29,30,31,32,33,34,35 However, the specific areal (CA) and volumetric capacitances (CV) are still at a low level—the bottleneck of miniaturized filter devices.36,37,38,39

Results and discussion

The fabricating process of 3D-MLCT is schematically illustrated in Figure 1. A 3D-AAO template with interconnected vertical and lateral channels was synthesized by anodizing impure aluminum foil in phosphoric acid electrolyte at 0°C under a voltage of 195 V (Figure S1).40,41 The template was placed into a horizontal tube furnace and heated to 1,000°C at 10°C/min under an Ar atmosphere. The first CT layer was deposited on the pore walls of the 3D-AAO template with a flow of acetylene gas under vacuum conditions for 50 min. Subsequently, an AlOx layer was deposited via an ALD process at 250°C on the inner wall of the first CT layer using trimethyl aluminum (TMA) and ozone (O3) as the precursors.42 The second CT layer was deposited on the AlOx surface via a second-time CVD process. Similarly, repeating the ALD and subsequent CVD processes can prepare a third CT layer. Finally, the 3D-MLCT was obtained after removing the AlOx layer(s) and 3D-AAO template in hydrofluoric acid solution, and the spacing of adjacent CT layers can be precisely controlled by the thickness of the AlOx layers. Furthermore, the size of the 3D-CT films can be adjusted by varying the area of the 3D-AAO as needed, and we prepared a 3D-CT film with an area of approximately 24 cm2 (see Figure S2).

Highlights

Abstract

Keywords

Abbreviations

1. Introduction

2. Microgrid model

3. Description of the proposed multi-functional algorithm

4. Simulation results

5. Comparison with other existing techniques

6. Conclusion

CRediT authorship contribution statement

Declaration of Competing Interest

References

Abstract

A large part of the current microgrids (MGs) operate in grid-connected mode and act as slaves following the voltage and frequency dictated by the main grid. Therefore, these MGs are not expected to be operated in parallel as islands. Hence, according to the IEEE 1457-2018 standard, the power generation units in these grid-connected MGs (GCMGs) have to be disconnected in less than 2 s if islanding occurs. On the contrary, if the islanding operation (IO) does not occur, these MGs exchange power with the electrical grid. This paper explores the feasibility of a multi-functional algorithm (MA) for these GCMGs in an attempt to address three tasks simultaneously; (i) power sharing, (ii) voltage support and, (iii) islanding detection (ID). The MG object of study comprises an electronically interfaced photovoltaic (PV) unit supplemented by a battery energy storage system (BESS) equipped in combination with a bidirectional charger. The model of the GCMG has been implemented in MATLAB, where an extensive set of simulations has been performed. The results demonstrate the effectiveness and benefits of the proposed MA, which fulfils several functions all at once.

1. Introduction

Conceptually, MGs are self-sufficient entities formed by an aggregation of generation units, energy storage systems and a set of controllable loads [1]. These MGs can be either connected to the grid (i.e., the abovementioned GCMGs) or operating in stand-alone mode [2]. Even though these GCMGs can be powered by different technologies (Wind, PV, mini-hydro, etc.), rooftop PV units are becoming very popular in residential and industrial applications due to their multiple benefits. Even though these GCMGs require the main grid, if both generation and storage devices are properly coordinated and sized, the amount of energy exchanged with the grid can be significantly reduced. Therefore, these GCMGs are becoming less grid-dependents in terms of exchanged energy.

In this regard, multi-functional algorithms are gaining momentum in MGs. For example, a fully distributed MA control has been proposed in [3], while references [4], [5], [6] used MAs for optimal MG management. If the peak power is shaved and the energy supplied by the grid is curtailed, this will imply a reduction in the electricity cost [7], [8]. Undoubtedly, this aspect is crucial for these MGs; for instance, see the techno-economic analysis carried out in [9].

Albeit these MGs offer several advantages for the users and the DNs (e.g., losses reduction and voltage support) [10], [11], some issues may occur due to their inherent intermittency. In particular, voltage regulation is among the most concerning issues for the distributor operators (DSOs). Although these PV-based distributed generations (DGs) are commonly operated at unity power factor, some grid codes require reactive power support by the voltage source converters (VSCs). Therefore, optimal Volt/VAR strategies have been investigated for such purposes, see [12]. Presently, the electronically-interfaced grid-connected generation units which provide grid support are also known as grid-following VSCs, see [13].

1- Introduction

2- Circuit configuration

3- Theoretical analysis

4- SPICE simulation

5- Experiments

6- Conclusion

References

Abstract

For vibration energy harvesting, we propose an inductor-less step-up ac/dc converter in this paper. To realize the inductor-less design, the proposed ac/dc converter consists of two converter blocks: Cockcroft–Walton circuit and charge pump. Unlike existing ac/dc converters for vibration energy harvesting, the proposed ac/dc converter can achieve less electro-magnetic interference (EMI), because no magnetic component is necessary. Furthermore, owing to the Cockcroft–Walton circuit, a fullbridge circuit is not necessary to convert vibration energy. Therefore, small vibration energy, namely, as 0.3 V@1 MHz, can be converted directly to dc voltage. Through theoretical analysis and simulation program with integrated circuit emphasis (SPICE) simulation, the performance of the proposed converter with 6× voltage gain is investigated, where the proposed converter is designed by assuming 0.18 µm CMOS process. The proposed converter demonstrates that about 62% power efficiency can be provided, where the output power is 30 µW, the output voltage is about 1.6 V, and the ripple factor is 0.8%. Furthermore, the feasibility of the proposed ac/dc converter is confirmed by breadboard experiments. The inductor-less design provides us to integrate the proposed ac/dc converter into a hybrid IC chip.

Introduction

Over the past few decades, wireless sensor network (WSN) systems [1] are receiving much attention for monitoring various environment parameters. The bottleneck of the WSN systems is the limited lifetime of WSN nodes, because the WSN nodes are usually driven by a battery source. For this reason, in order to extend lifetime of WSN nodes, some of wasted energy are captured by energy harvesting collectors and are used as a power supply source in recent years. There are several types of ambient energy sources, such as solar, wind, vibration, thermoelectric. Among others, we focused on vibration energy, which is a kind of kinetic energy, because it widely exists in the ambient, including the vibration of industrial machinery, transportation vibration, body movement, and so on. In the energy harvesting collectors, the ambient energy is converted into electrical energy by power converters. To utilize vibration energy, an ac/dc converter is necessary as a vital component of energy harvesting collectors. For this reason, several types of ac/dc converters have been proposed in recent years. For example, Shi et al. [2] suggested the ac/dc converter by combining a full-bridge circuit with two buck-boost converters. Wang et al. [3] designed the ac/dc converter by combining a full-bridge circuit with a synchronized switching harvesting on inductor. Shareef et al. [4] realized the ac/dc converter without full-bridge circuits. However, due to the magnetic component, such as inductors, these existing converters becomes bulky. Furthermore, these converters are applied only for low frequency ac input. To solve these problems, we undertake the development of an inductor-less ac/dc converter for vibration energy harvesting.

1- Introduction

2- System description

3- Principle of DPC

4- Proposed control strategy for LVRT and enhanced reactive power support

5- Simulation and experimental results

6- Case I: simulation and experimental results of AC/DC converter control strategy for three different modes of operation (Inversion, Shut Down and Rectification Modes)

7- Case II: study of the LVRT

8- Conclusion

References

Abstract

This paper describes an advanced control strategy of a bidirectional three-phase PWM AC/DC converter embedded between a micro-grid and utility. This converter proves an ability to control a bidirectional power flow. Firstly, the improved control strategy is based on direct power control using a new switching lookup table obtained from the derivative analysis of the active and reactive power. Furthermore, the improved control strategy is verified by simulation. In addition, many operating modes are established due to verify the capability of a bidirectional power flow of the converter. Particularly, authors focus on three operating scenarios which are cited as follow: Inversion Mode (IM), Shut-Down Mode (S-DM) and Rectification Mode (RM). Moreover, the authors show the capability of the DPC with respect to the LVRT for Tunisian grid. Finally, the performances of this converter are validated experimentally using a Dspace 1104 board, a bidirectional AC/DC converter and the wind turbine using a SM connected with the rectifier. The simulation results compared to the experimental ones demonstrated the good performances of the proposed control strategy.

Introduction

In order to offset the growing demand for electricity in the world, the majority of countries are investing in the development of new renewable sources such as Wind Energy, Fuel Cells and Solar Energy [1–3]. For this purpose, the integration of a bidirectional AC/DC converter is necessary for the interfacing between the renewable energy sources and grid. However, its control strategy has attracted the attention of researchers since it has a THD ratio meeting the standards IEEE Std 519™ ‒ 2014 [4]. Several control approaches of the bidirectional AC/DC converter are cited in the literature [5–7], Voltage Oriented Control (VOC) [8], Virtual-Flux Oriented Control (V-FOC) [9], Voltage-based Direct Power Control (V-DPC) [9], Virtual-Flux-based Direct Power Control (VF-DPC) [10]. In this paper, we introduce an improved DPC approach since it has several advantages [9,11]. First, the PWM voltage modulation block is not separated. In contrast, the active and reactive power control is decoupled and the control is activated without a current regulation loops. In addition, this control technique gives good dynamic performances. Consequently, the improvement of power factor and efficiency is accomplished. Because of its main role in the majority of applications, various studies are being raised around the bidirectional AC/DC converter. Thus, in the literature, three operating scenarios are studied. Firstly, an improved control strategy for bidirectional AC/DC converter is developed in Ref. [1]. Then, Rakesh et al. described the three modes of operation (Inversion Mode, Shut-Down Mode and Rectification Mode). Moreover, they practically justified simulation results on an OPAL-RT test bench. However, this work has been done for a bidirectional single phase AC/DC converter. In addition, in order to reduce switching losses, a new control strategy has been addressed for a bidirectional AC/DC converter in reference [12]. So, Yi L. discussed only two modes of operation (rectifier and inverter). Finally, they have completed this work with an implementation of the control law on a Spartan-3E XC3S250E FPGA board. Furthermore, three operating modes such as Grid-Connected Mode (Inversion Mode and Rectification Mode) and Isolated Mode (Shut-Down Mode) are described in Ref. [13]. Indeed, Xiong et al. intercalated a bidirectional three-phase converter between AC Micro-grid and DC Micro-grid. Moreover, they simulated these three operating scenarios in Matlab/Simulink. Finally, however the interest of this work, the authors not validated experimentally these three operating modes.

1- Introduction

2- Proposed system

3- Switching technique

4- Losses calculation

5- Simulation results

6- Experimental results

7- Conclusion

References

Abstract

A modified cascaded multilevel DC-AC converter using switched diodes is presented in this paper. This DC-AC converter produces great amount of voltage levels with minimized components by using hybrid modulation. It has two DC-AC converters coupled in cascade. First one is high frequency DC-AC converter and second one is low frequency DC-AC converter. The high frequency DC-AC converter is only capable to generate two levels with high frequency but the low frequency DC-AC converter is capable to produce great amount of voltage level by utilizing switched diodes. It reduces switching losses, size, cost and volume. Simulation of the circuit is done through MATLAB. The switching algorithm is realized by Spartan 6 FPGA processor board. Simulation outcomes are validated through experimental setup.

Introduction

As multi-level DC-AC converters have potential to generate waveforms with good harmonic range and voltage, they are getting increased attention. Many applications like AC power supplies, drive systems, power grid, and solar system uses multilevel DC-AC converter. The multilevel commences from three levels. Total Harmonic Distortion of the output waveform draws near to zero, if the amount of levels extent to infinity. One of the important favours of multilevel configuration is without curtailing the output power of DC-AC converter it can reduce harmonics in the output waveform. But, the issues like voltage unbalance, requirement of voltage clamping, circuit packaging and design constraints limit the attainable voltage level. Numerous traditional multilevel DC-AC converters is available out there. (i) Neutral Point Clamp (NPC) (ii) The Cascaded H-Bridge (CHB) (iii) The Flying Capacitor (FC) multilevel DC-AC converter [1]-[3]. Traditional multilevel DC-AC converters has unique demerits like high amount of switches for getting greater amount of output voltage levels that may demand excessive effort for the installation, the switching complexity, increased quantity of gate driver circuits and the size. Various multilevel DC-AC converter configurations brought together to curtail the switch count, gate drivers, dc sources and the voltage rating of the switches [4] – [9]. Quite a few control methods studied to curtail the harmonics in output [10]. Pulse Width Modulation (PWM) is broadly in a job to regulate the DCAC converter’s output.

I- INTRODUCTION

II- PROPOSED CONVERTER

III- COMPARATIVE ANALYSIS

IV- PERFORMANCE EVALUATION

V- CONCLUSION

REFERENCES

Abstract

This paper introduces a novel single-phase buck-boost (non-)inverting variable-frequency AC-AC converter that offers a higher efficiency compared to the competitors. This converter utilizes a lower number of semiconductors. A simple and flexible switching strategy is also proposed, which generates the desired output waveform avoiding unnecessary high-frequency switching operation of semiconductor devices. A high reliable operation due to the elimination of the input source shoot-through risk, an inherent commutation capability that mitigates the voltage spikes across the semiconductors, a lower semiconductors rating requirement, an improved input current waveform quality and a smaller required input filter inductor are the main advantages of the proposed converter. Thus, the proposed converter can be successfully applied to many industrial applications such as medium-frequency transformer-isolation (MFT) for traction and wind turbine converters, AC-DC high-voltage conversion based on Cockcroft-Walton circuit and induction heating systems. The theoretical achievements and claims are all confirmed through extensive experimental tests on a 200-W laboratory setup.

INTRODUCTION

Many industrial applications such as the adjustable speed drives require AC-AC power conversion systems offering flexible output AC voltage properties. The most recently proposed direct AC-AC converters in [1]–[6], successfully provide a wide range gain buck-boost operation for simultaneous increase or decrease of the output voltage amplitude. These converters offer many advantages including snubber-less operation, inherent commutation, high-quality input current and output voltage waveforms and reliable and efficient operation. However, they do not have the ability to change the frequency of the output voltage waveform, which is already necessary for many industrial applications. On the other hand, the AC-DC-AC converters can generate any arbitrary waveform at the output through several power stages [7]–[10]. In addition, they need large DC-link capacitors and filter inductors, which increase their implementation cost, size and power loss. The other AC-AC converters with the ability to change the frequency are the matrix converters (MCs) that directly connect the input source to the output without needing large DC-link capacitors [11]–[14]. It is worth mentioning that the single-phase MCs, in contrary with the AC-DC-AC competitors, cannot generate pure sinusoidal output voltage waveforms, though they have recently found wide applications such as adjustable speed drives, aircraft power supplies and induction heating systems [15]–[17]. The first single-phase MC, proposed by Zuckerberger et al. in [18], can only decrease the voltage magnitude and increase its frequency with low power quality and harsh voltage spikes. In [19] and [20], a safe commutation strategy is proposed for the converter of [18]. However, the quality of waveforms is not yet acceptable. Some other single-phase MCs are presented in [15] and [20], which are not suitable for the applications requiring both step-up and step-down operation due to their limited gain range. In order to provide a wide gain range, the Z-source-based MC is proposed in [21], shown in Fig. 1(a), which offers the capability of simultaneous buck and boost operation. However, it requires a high number of energy storage components and ten semiconductors leading to low efficiency and high size and cost. Moreover, it suffers from a severe commutation problem leading to large voltage spikes.

This paper presents the development of methods and mathematical models to determine the DC/AC average efficiency of inverters that allow to optimize estimates of electricity generation of photovoltaic systems. The adaptive hybrid mathematical model of DC/AC average efficiency of inverters of photovoltaic systems proposed in this paper may be composed of three function settings–Linear, Lognormal, and Polynomial–considering the influence of the relative power, which varies with the sizing factor inverter and the DC input voltage. Determining of the average efficiency with a high degree of accuracy guarantees greater reliability in estimating electricity generation of photovoltaic systems. Depending on the specific behavior of each model and topology of inverter, the adaptive hybrid mathematical model determines the fit functions with highest coefficient R2 , and estimates with accuracy the DC/AC average efficiency that represents the operation and dynamic behavior of the inverters.

Introduction

Interest in the integration of distributed generation units in the electric power system arose from the growth of incentives to using alternative sources of energy, the technological evolution that resulted in the fall of prices of energy generation systems [1,2], and from environmental pressures [3]. Especially, distributed generation photovoltaic systems have technological maturity, performance, reliability, economic competitiveness and harmonious architectural integration. In this scenario of inserting distributed generation photovoltaic systems, the consumer units also become electrical energy generating units (prosumer units). The DC/AC inverter is responsible for converting DC electrical energy generated by photovoltaic modules (photovoltaic array or generator) to AC electrical energy, with characteristics and quality for injection in the power grid. Since the technology was developed, the inverters have considerable increases in DC/AC efficiency and safety in energy conversion, achieving efficiencies of 98% in medium powers [4] and high efficiencies even at load levels of 10 or 20% of power nominal [5]. The inverters used in grid connected photovoltaic systems have a maximum power point tracking, anti-island protection, high conversion efficiency, automatic synchronization with the power grid, low harmonic distortion level and power factor close to the unit [5,6]. Most inverters use a two-stage power conversion topology; the first is a DC/ AC/DC or DC/DC stage, which is needed to increase the voltage to higher values. The second stage is the DC/AC conversion for power grid connection [7,8]. There are two main classifications of inverter topologies for photovoltaic systems, with or without galvanic isolation [9]. The use of inverter with low or high frequency transformer ensures galvanic isolation [10]. Galvanic isolation solutions offer safety, but also losses in the extra components, so the inverters without galvanic isolation, that is, transformersless inverters, can increase efficiencies by 1–2% [11]. As a safety measure, the first grid-connected photovoltaic systems were designed to operate at low voltages and therefore, required inverters with transformer [12]. Galvanic isolation prevents leakage of current that can cause, for example, degradation of performance, activation of protections, safety difficulties, and problems of electromagnetic compatibility [8]. However, the use of transformerless inverters has some limitations, for example, several thin film modules need protection against leakage currents caused by the parasitic capacitance. In this case, inverters with high frequency transformers are required; thus it is possible to ground the negative pole of the modules avoiding an electrical corrosion, which damages their cells, impairing their performance and service life [13,14].

I- INTRODUCTION

II- CONTROL BLOCK DIAGRAMS OF SSSC, TCSC AND POD.

III- SIMULATION MODEL

IV- RESULT AND DISCUSSION

V- CONCLUSION

VI- REFRENCES

Abstract

With the growing power demand, it is vital to expand power transfer capacity of transmission lines. But the new expansion in our present transmission system is confined due to financial problems, environmental concerns and health hazards due to electric and magnetic fields. May be its worthwhile option to use flexible AC transmission system (FACTS) for enhancement of power capacity of transmission system. This paper presents, fruitfulness FACTS devices SSSC and TCSC with POD (Power oscillation damper) as additional controller. The major function is to decrease the peaks overshooting and clearing time of inter area oscillations, hence betterment in transient stability. For the fulfillment of analysis, time domain simulation outcomes are correlated for results verification. In addition, the parallel operation of SSSC for the system has been analyzed. This paper proposes modeling and simulation on MATLAB/Simulink Software for the proposed test system. Result displays that, power oscillation are rapidly damped than traditionally PSS by employing POD with FACTS-Controllers.

Introduction

In present-day world, electrical power demand is increasing significantly over the last few decades. After all this growing rate of power demand does not pursue through the enrichment in power generating plants and transmission capacity. Thus in place of, to accommodate the increasing electric load requirement, power generation plants are operating at their maximum capacity[1]. Similarly transmission lines are also running nearer to their thermal limits. So, the power systems are seemly less sheltered and always carrying the exposure of voltage instability which has led to many major network collapses world-wide[2]. As the electric power system was not so complicated in previous times, the major controversy was to damping the local area oscillations which were simply performed by the aid of AVRs[3].Then PSS appear which associated to the generators which present sizable concurrence towards the oscillations of the network. Hence traditionally PSS was employed for damping the local zones of oscillations in electric network[4],[5]. Different preventive measures as generation and transmission of energy adjourn, carrying reserve generators online, load curtailing and VAR back up by serial or parallel capacitors are followed to conquered voltage instability controversy. However, most of them are electromechanical controller which got the demerits like sluggishness, wear and tear[6]. As an alternate key, intense consideration have been compensated to FACTS (Flexible Alternating Current Transmission System) devices which are directed from present-day components of power electronics, which could give active and reactive power accurately and keep the network stability limit[7]. In present paper FACTS devices with additional controller is employed for damping the power network oscillations. Thus by employing POD-FACTS the network power oscillations can be quickly damped as related with another commonly devices[8]. Since these controllers impact the driving voltage and thus the current and flow of power directly. For a given MVA size series controllers are manifolds robust than shunt controllers, in order to perform the desire functions such as to control the current/power flow and damping the oscillations[9].