Performance and maximum load capacity of uninterruptible power system using double‐layer capacitor

Power service interruptions cause problems in various facilities. Even an instantaneous voltage drop may give rise to serious problems in computer systems or electronic equipment. The uninterruptible power system (UPS) has been used to compensate for the power service interruptions. Also, the instantaneous voltage drop compensator using the electrolytic capacitor has been developed for the instantaneous voltage drop. Recently, the double‐layer capacitor has been considered as a new energy storage element. This capacitor has many advantages such as no maintenance, long lifetime, and quick charge/discharge characteristics with large current and it has higher energy density than the electrolytic capacitor. Therefore, we developed the UPS using the double‐layer capacitor. In this paper, the performance of the UPS using the double‐layer capacitor is shown by simulated and experimental results. Furthermore, the discharge characteristics of the double‐layer capacitors are investigated on the basis of the equivalent circuit including the capacitors and a voltage booster. Finally, the maximum load capacity to compensate is clarified for the system. © 2005 Wiley Periodicals, Inc. Electr Eng Jpn, 154(3): 73–81, 2006; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.20190     

Energy storage system utilizing large‐capacity electric double‐layer capacitors for peak‐cut of power demand

Recently, due to a rapid increase of demand for air conditioning in summer, peak power demand is becoming increasingly acute. Therefore, the load factor has a tendency to drop every year. The drop of the load factor is leading to a drop in the utilization factor of the power facilities and an increase in the cost of installation. In this paper, we propose an energy storage system for peak‐cut of power demand, in which we use large‐capacity electric double‐layer capacitors. This energy storage system has some distinctive characteristics, including long life span, maintenance‐free operation, preservation of environment, high efficiency at charge/discharge, and so on. This paper deals with the circuit arrangement of the proposed energy storage system, the charge equalization method of the capacitors, and the control method of the converter at charge/discharge. Finally, the operating characteristics of this system are evaluated by simulation analysis. © 2000 Scripta Technica, Electr Eng Jpn, 133(3): 83‐92, 2000     

Flywheel‐based AC cache power for stand‐alone power systems

Stand‐alone power systems (SPS) are attracting more and more interest with the global move toward distributed generation (DG). Without strong support from the power grid, they suffer from poor load‐following capability at varying loads. A cache power that has fast response and high energy efficiency is demanded. As a solution, this paper provides an AC power technology based on flywheel energy storage. Different from the other DC generation technologies such as electric double layer capacitor (EDLC) or superconducting magnetic energy storage (SMES), the proposed flywheel system generates AC power and therefore can be directly connected to the power line without any power semiconductors. Furthermore, the proposed technology realizes power in/out automatically in response to the frequency/voltage variation of the power line. Therefore, this system has the advantages of robustness, simplicity, and fast response. Besides, by getting rid of power semiconductors, the proposed flywheel system has a good overload capability as high as two to three times. We prove by simulation and experimentation the validity and effectiveness of the proposed technology to provide cache power for stand‐alone power systems. © 2013 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.     

A Comparison of Electric Power Smoothing Control Methods for Distributed Generation Systems

Renewable forms of energy such as solar energy have attracted attention as alternative sources of fossil fuel. The output power of photovoltaic generation systems changes steeply. Changes in output power have an effect on the electric power quality of the power system. For that reason, a system which can smooth fluctuations of output power is required. In this paper, the moving average, modified moving average, single exponential smoothing, and double exponential smoothing methods are applied to electric power smoothing control schemes for distributed generation systems with photovoltaic generation. The reduction rate of power fluctuations and the maximum stored energy of electric double layer capacitors are adopted as system evaluation measures. To confirm the effectiveness of the power smoothing control methods, a distributed generation power system with photovoltaic generation systems is simulated with the power electronics circuit simulation software PSIM. The methods are then compared by evaluating the reduction in the capacity of electric double layer capacitors without loss in the power smoothing effect.     

A power decoupling control method for an isolated single‐phase AC‐to‐DC converter based on a high‐frequency cycloconverter

This paper proposes a new power decoupling method for a high‐frequency cycloconverter which converts the single‐phase line‐frequency ac input to the high‐frequency ac output directly. The cycloconverter consists of two half‐bridge inverters, two input filter capacitors, and a series‐resonant circuit. The proposed power decoupling method stores the input power ripple at double the line frequency in the filter capacitors. Therefore, the proposed method achieves a unity power factor in ac input and a constant current amplitude in the high‐frequency output without any additional switching device or energy storage element. This paper theoretically discusses the principle and operating performance of the proposed power decoupling method, and the viability is confirmed by using an experimental isolated ac‐to‐dc converter based on the high‐frequency cycloconverter. As a result, the proposed power decoupling method effectively improved the displacement power factor in the line current to more than 0.99 and reduced the output voltage ripple to 4% without any electrolytic capacitor.     

Discharger using cascaded switched capacitor converters and selectable intermediate taps for electric double‐layer capacitors

Electric double‐layer capacitors (EDLCs) offer several advantages over traditional batteries, such as a long cycle life, high power capability, and good low‐temperature performance. However, their major drawbacks, such as low specific energy and large voltage variation due to charge/discharge cycling, necessitate the use of high‐efficiency power conversion electronics that can be used to efficiently discharge EDLCs and thus completely utilize the precious stored energy. In this study, we propose a novel discharger for EDLCs; this discharger uses cascaded switched capacitor converters (SCCs) and selectable intermediate taps. Although the voltage conversion ratio of SCCs is fixed, the load voltage can be maintained within a desired range by the selectable intermediate taps. The circuit configuration, operating principles, and procedure for designing SCCs and selectable intermediate taps are presented. Experimental tests were performed using an EDLC module and a 200‐W prototype of the discharger. The obtained results showed that the 60‐V EDLC could be discharged to 30 V with an average efficiency of 96% when the load voltage was maintained within the range of 30 to 40 V. © 2013 Wiley Periodicals, Inc. Electr Eng Jpn, 183(3): 37–45, 2013; Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/eej.22281     

Interior structure of UPQC‐type QCC at the low‐voltage side in the FRIENDS

Recently, the worldwide deregulation of the electric power industry is progressing. Further, it is expected that a many distributed generation facilities and energy storage systems will be installed at the end point of the power systems. Therefore, the future power system must be able to cope with these various qualities of power in the deregulated situation. The authors have proposed the FRIENDS (Flexible, Reliable, and Intelligent ENergy Delivery System) concept as a new distribution system that copes with a new framework of the electric power business in the near future. The most typical aspect of FRIENDS is the introduction of new equipment; Quality Control Centers (QCC) between distribution substations and customers. In this paper, the authors propose a concrete interior structure and investigate some control methods for realizing unbundling power quality supply and power conditioning using the proposed UPQC‐type QCC. The effectiveness of the proposed method is confirmed through the instantaneous value analysis using PSCAD/EMTDC. © 2003 Wiley Periodicals, Inc. Electr Eng Jpn, 146(3): 26–38, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.10243     

Moving pick‐up‐type contactless power transfer systems and their efficiency using series and parallel resonant capacitors

The equivalent circuit, the efficiency, and the important characteristics of moving pick‐up type contactless power transfer systems are described. If the primary series capacitor and the secondary parallel capacitors are chosen correctly and the winding resistances are ignored, the equivalent circuit of the transformer with these capacitors becomes the same as an ideal transformer at the resonant frequency. This simple approximation helps to understand the phenomena occurring with load changes. Because the circuit analysis becomes simple, the approximate value of the power transfer efficiency can be derived. This paper describes the determination of the capacitor values, the derivation of the equivalent circuit and the efficiency, and test results. © 2010 Wiley Periodicals, Inc. Electr Eng Jpn, 172(2): 47–54, 2010; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.20970     

Load‐following control for dispersed generators using a PI controller

Recently, there has been growing interest in utilizing dispersed generation systems, which are grouped into micro turbine systems, co‐generation systems, and so on, as a substitute for fuel oil energy and a technology to prevent global warming. Since start‐up time of dispersed generation systems is short, it is possible to operate systems to supply load power corresponding to a demand. Moreover, PPSs (Power Producers and Suppliers) can participate in a power retail sales company, since deregulation of electric utilities was instituted in March 2000. However, PPSs have to keep instantaneous generating power commissioning rule, to maintain supply‐and‐demand balance between customer and supplier. Therefore, in this paper, we examine instantaneous generating power commissioning for dispersed generators where start‐up time is short and it is possible to operate systems to supply a power load corresponding to a demand. We adopt a PI controller as a controller. The system is composed of double control loop in inner loop and in outer loop. In inner loop electric power is controlled and in outer loop electric energy is controlled. The controller parameters are designed using the pole‐placement technique. The effectiveness of the proposed control system is confirmed by simulations. © 2003 Wiley Periodicals, Inc. Electr Eng Jpn, 145(2): 58–66, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.10200     

Pulse current regenerative resonant snubber‐assisted two‐switch flyback‐type ZVS PWM DC‐DC converter

In this paper, a two‐switch high‐frequency flyback transformer‐type zero voltage soft‐switching PWM DC‐DC converter using IGBTs is proposed. Effective applications for this power converter can be found in auxiliary power supplies of rolling stock transportation and electric vehicles. This power converter is basically composed of two active power switches and a flyback high‐frequency transformer. In addition to these, two passive lossless snubbers with power regeneration loops for energy recovery, consisting of a three‐winding auxiliary high‐frequency transformer, auxiliary capacitors and diodes are introduced to achieve zero voltage soft switching from light to full load conditions. Furthermore, this power converter has some advantages such as low cost circuit configuration, simple control scheme, and high efficiency. Its operating principle is described and to determine circuit parameters, some practical design considerations are discussed. The effectiveness of the proposed power converter is evaluated and compared with the hard switching PWM DC‐DC converter from an experimental point of view, and the comparative electromagnetic conduction and radiation noise characteristics of both DC‐DC power converter circuits are also depicted. © 2005 Wiley Periodicals, Inc. Electr Eng Jpn, 152(3): 74–81, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.20081     

A 0.75‐V, 4‐μW, 15‐ppm/°C, 190 °C temperature range,voltage reference

A low‐voltage, low‐power, low‐area, wide‐temperature‐range CMOS voltage reference is presented. The proposed reference circuit achieves a measured temperature drift of 15 ppm/°C for an extremely wide temperature range of 190 °C (?60 to 130 °C) while consuming only 4 μW at 0.75 V. It performs a high‐order curvature correction of the reference voltage while consisting of only CMOS transistors operating in subthreshold and polysilicon resistors, without utilizing any diodes or external components such as compensating capacitors. A trade‐off of this circuit topology, in its current form, is the high line sensitivity. The design was fabricated using TowerJazz semiconductor's 0.18‐µm standard CMOS technology and occupies an area of 0.039 mm². The proposed reference circuit is suitable for high‐precision, low‐energy‐budget applications, such as mobile systems, wearable electronics, and energy harvesting systems. Copyright © 2015 John Wiley & Sons, Ltd.     

A module‐integrated isolated solar micro‐inverter without electrolytic capacitors

This paper presents a module‐integrated isolated solar micro‐inverter. The studied grid‐tied micro‐inverters can individually extract the maximum solar power from each photovoltaic (PV) panel and transfer to the AC utility system. A harmonic suppression technique is used to reduce the DC‐bus capacitance. Electrolytic capacitors are not needed in the studied solar micro‐inverter. High conversion efficiency, high maximum power point tracking accuracy and long lifespan can be achieved. The operation principles and design considerations of the studied PV inverter are analyzed and discussed. A laboratory prototype is implemented and tested to verify its feasibility. Copyright © 2012 John Wiley & Sons, Ltd.     

A design methodology for power‐efficient reconfigurable SC ΔΣ modulators

This paper presents a methodology to design reconfigurable switched‐capacitor delta‐sigma modulators (ΔΣMs) capable of keeping their corresponding power efficiency figures constant and optimal for a set of resolutions and signal bandwidths. This method is especially suitable for low‐bandwidth, medium‐to‐high‐resolution specifications, which are common in biomedical application range. The presented methodology is based on an analytic model of all different contributions to the power dissipation of the ΔΣM. In particular, a novel way to predict the static power dissipated by integrators based on class A and class AB operational transconductance amplifier is presented. The power‐optimal solution is found in terms of filter order, quantizer resolution, oversampling ratio, and capacitor dimensions for a targeted resolution and bandwidth. As the size of the sampling capacitors is crucial to determine power consumption, three approaches to achieve reconfigurability are compared: sizing the sampling capacitors to achieve the highest resolution and keep them constant, change only the first sampling capacitor according to the targeted resolution, or program all sampling capacitors to the required resolution. The second approach results in the best trade‐off between power efficiency and simplicity. A reconfigurable ΔΣM for biomedical applications is designed at transistor level in a 0.18‐µm complementary metal–oxide–semiconductor process following the methodology discussed. A comparison between the power estimated by the proposed analytic model and the transistor implementation shows a maximum difference of 17%, validating thus the proposed approach. Copyright © 2014 John Wiley & Sons, Ltd.     

Internally Self‐Sustaining Electric Power Generation

The electrical double layer capacitor‐electrostatic induction electric power generation system (EDLC‐ESIG) undergoes an electric cycle consisting of three steps: energy storage by electrostatic induction, power generation, and initialization by electromechanical coupling. An internally self‐sustaining electric power can be generated by repetition of cycles with periods of the order of seconds. The objective of this paper is to show the design of EDLC‐cell configuration for practical use to produce constant power output. On the basis of energy densities of the conventional EDLCs for energy storage, the volumetric power density has been estimated to be 450 W/L for the discharge time of 10 s. Possibilities of commercial applications of the EDLC‐ESIG system to vehicles and high power generations in central station have been considered with respect to the power densities of the EDLC‐cells consolidated into a generation system.     

A High‐Efficiency Single‐Phase Utility Interactive Inverter with an Active Power Decoupling Function and its Control Method

The present paper introduces a single‐phase utility interactive inverter with a power decoupling function. In a conventional single‐phase inverter, power pulsation at twice the grid frequency appears in the input power. Hence, electrolytic capacitors having large capacitances have been connected to the DC input terminal to stabilize the input DC‐bus voltage. Because the lifetime of the electrolytic capacitor is relatively shorter than that of another component, the lifetime of the inverter is affected by the capacitor. In order to prevent such a problem, a novel single‐phase inverter circuit with an active power decoupling function is introduced. The pulsating power on the input DC‐bus line and the pulsated energy on the input DC capacitor is transferred to the energy in a small film capacitor. Hence, the extension of the inverter lifetime can be expected by substituting a small film capacitor for the large electrolytic capacitors. In addition, the loss in the power decoupling circuit is very small; hence, the reduction in the overall conversion efficiency of the inverter can be minimized. The effectiveness of the proposed method is verified using a 300 W experimental setup.     

A new variable‐speed,constant‐frequency,stand‐alone power generating system

Variable‐speed and constant‐frequency power generating systems using rotor excitation of the wound‐rotor induction machines have been used for such applications as variable‐speed pump generators and flywheel energy storage systems. However, the stand‐alone generating system of this type has only been reported and has not yet been practically used. On the other hand, the stand‐alone generating systems using diesel engines have been widely used for emergency supplies of plants or isolated islands and so on. However, in these cases, synchronous generators are usually used. If the output frequency is to be kept constant, there is the need to control the speed of the engine using a high‐performance governor. Even then, the output frequency changes in the case of a sudden load change. This paper proposes a new stand‐alone power generating system. In this system, the constant‐frequency output voltage can be obtained even though rotor speed changes by several percent. © 2003 Wiley Periodicals, Inc. Electr Eng Jpn, 146(2): 75–85, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.10191     

Three‐phase immittance converter

The immittance converter has an input impedance that is proportional to the admittance of a load connected across its output terminals. In this converter, the output current is proportional to the input voltage and the input current is proportional to the output voltage. Consequently, a constant‐voltage source is converted into a constant‐current source and a constant‐current source into a constant‐voltage source. The immittance converters consisting of only passive elements (inductors L and capacitors C) are suitable for use in the high‐frequency links in power electronics applications. Previously, we proposed several types of immittance converters and some applications to power electronics equipment. In this paper, we propose a new three‐phase immittance converter consisting of three L and C elements each to obtain an alternating current source from a three‐phase voltage source without control. This paper presents a configuration of the new three‐phase immittance converter that operates in either anti‐phase or in‐phase modes between the input voltage and the output voltage, and its voltage–current conversion characteristics and efficiency characteristics. © 2003 Wiley Periodicals, Inc. Electr Eng Jpn, 145(1): 52– 58, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.10169     

Electric double‐layer capacitor module with series‐parallel reconfigurable cell voltage equalizers

When electric double‐layer capacitors (EDLCs) are connected in series, a cell voltage imbalance occurs due to nonuniform cell properties. Cell voltage imbalance should be minimized to prolong cycle lives and maximize the available energy of cells. In this study, we propose a series‐parallel reconfigurable cell voltage equalizer that is considered suitable for energy storage systems using EDLCs instead of traditional secondary batteries as the main energy storage sources. The proposed equalizer requires only EDLCs and switches as its main circuit elements, and it utilizes EDLCs not only for energy storage but also for equalization. An equivalent circuit model using equivalent resistors that can be regarded as an index of equalization speed is developed. Current distribution and cell voltage imbalancing during operation are quantitatively generalized. Experimental charge–discharge tests were performed on the EDLC modules to demonstrate the performance of the cell voltage equalizer. All the cells in the modules could be charged/discharged uniformly even when a degradation‐mimicking cell was intentionally included in the module. The resultant cell voltage imbalances and current distributions were in good agreement with those predicted by mathematical analyses. © 2012 Wiley Periodicals, Inc. Electr Eng Jpn, 181(4): 38–50, 2012; Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/eej.21287     

Experimental verification of a dynamic voltage restorer capable of significantly reducing energy‐storage element requirements

This paper deals with a dynamic voltage restorer (DVR) characterized by installing the shunt converter at the load side. The DVR can compensate for the load voltage when a voltage sag appears in the supply voltage. An existing DVR requires a large capacitor bank or other energy‐storage elements such as double‐layer capacitors or batteries. The DVR presented in this paper requires only a small DC capacitor intended for smoothing the DC‐link voltage. Moreover, three control methods for the series converter are compared and discussed to reduce the series‐converter rating, paying attention to the zero‐sequence voltages included in the supply voltage and the compensating voltage. Experimental results obtained from a 200‐V, 5‐kW laboratory system are shown to verify the viability of the system configuration and the control methods. © 2008 Wiley Periodicals, Inc. Electr Eng Jpn, 165(3): 73–82, 2008; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.20460     

A method for designing the power and capacitance of fuel cells and electric double‐layer capacitors of hybrid railway vehicles

A hybrid railway traction system with fuel cells (FC) and electric double‐layer capacitors (EDLC) is discussed in this paper. This system can save FC costs and absorb regenerative energy. A method for designing FC and EDLC on the basis of the output power and capacitance, respectively, has not been reported, although their design is one of the most important technical issues encountered in the design of hybrid railway vehicles. Such a design method is presented along with a train load pro?le and an energy management strategy. The design results obtained using the proposed method are veri?ed by performing numerical simulations for a running train. These results reveal that the proposed method for designing the EDLC and FC on the basis of the capacitance and power, respectively, and using a method for controlling the EDLC voltage, is su?ciently e?ective in designing e?cient EDLC and FC of hybrid railway traction systems. © 2013 Wiley Periodicals, Inc. Electr Eng Jpn, 184(3): 47–54, 2013; Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/eej.22395