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Renewable energy sources such as wind turbine generators and photovoltaics are intermittent in nature, thus resulting in fluctuating electric power. Leveling of their electric power is indispensable to realize large-scale renewable energy sources and to avoid severe power quality problems when they are interconnected into the grid. A battery energy storage system (BESS), combining batteries with a power converter and digital control, should be installed in the vicinity of an intermittent energy source. The fluctuating power is compensated by appropriately controlling active power stored in, or released from, the battery. This paper describes a 6.6-kV transformerless battery energy storage system based on a cascade multilevel PWM (pulse-width-modulation) converter, with focus on a control method for active power and SOC (state-of-charge) balancing. A down-scaled 200-V, 10-kW, 3.6-kWh (13-MJ) laboratory model using nine NiMH (Nickel Metal Hydride) battery packs was designed, constructed, and tested to verify the viability and effectiveness of the 6.6-kV system. © 2011 Wiley Periodicals, Inc. Electr Eng Jpn, 177(1): 43-54, 2011; Published online in Wiley Online Library (). DOI 10.1002/eej.21127 [ABSTRACT FROM AUTHOR]

The low harmonic component elimination method, a method of PWM control, is effective for miniaturizing filters, or for decreasing losses by lowering the switching frequency. However, this method is only applicable to specific uses such as static var generators, because it can only read a switching angle decided beforehand from memory and can only control the fundamental wave. In this paper a PWM method to control low harmonic components is proposed. The proposed method uses a curved triangular wave that was developed in this investigation as the carrier wave. The proposed method proves to decrease the total harmonic distortion better than the conventional low harmonic component elimination method and the subharmonic method, based on experimental results employing a single-phase inverter. © 2011 Wiley Periodicals, Inc. Electr Eng Jpn, 175(4): 10-23, 2011; Published online in Wiley Online Library (). DOI 10.1002/eej.21103 [ABSTRACT FROM AUTHOR]

Household cogeneration systems using proton exchange membrane fuel cells (PEMFC systems) have attracted attention due to their high energy efficiency. To bring out a PEMFC system's performance, it is crucial to use effectively both the electric power and hot water produced from a PEMFC system. Therefore, the optimal scheduling problem of a PEMFC system is very important. However, there are difficulties due to uncertainty of household energy demand. A difficulty to which little attention has been paid so far lies in the difference between the scheduled electric power output and the actual electric power output of a PEMFC system caused by rapid fluctuations in electric power demand. In this paper, we propose a new method of dealing with this problem. In our method, we treat the energy demand as a random variable and calculate its probability density at each time step. Then a constraint that takes account of the energy demand fluctuations is defined through the probability density and is incorporated into the original optimal scheduling problem of the PEMFC system. Using our method, we can deal with rapid energy demand fluctuations in settings of the scheduling problem with long time intervals. © 2011 Wiley Periodicals, Inc. Electr Eng Jpn, 175(1): 8-17, 2011; Published online in Wiley Online Library (). DOI 10.1002/eej.21064 [ABSTRACT FROM AUTHOR]

Using an industrial lithium-ion battery that has higher energy density than conventional valve-regulated lead-acid batteries, a rack-mounted DC power supply system was assembled and tested at a base transceiver station (BTS) offering actual services. The nominal output voltage and maximum output current of the system are 53.5 V and 20 A, respectively. An 80-Ah lithium-ion battery composed of 13 cells connected in series was applied in the system and maintained by the floating charge method. The DC power supply system was installed in a 19-inch power rack in the telecommunications equipment box at BTS. The characteristics of the 80-Ah lithium-ion battery, the specifications of the DC power supply system, and field-test results are presented in this paper. \copy 2010 Wiley Periodicals, Inc. Electr Eng Jpn, 174(3): 1-8, 2011; Published online in Wiley Online Library (). DOI 10.1002/eej.21067 [ABSTRACT FROM AUTHOR]

In this paper, a new method of estimating the critical generation margin (CGM) in power systems is proposed based on transient stability diagnostics. The proposed method can directly compute the stability limit output for a given fault based on the transient energy function method (TEF). Since CGM can be directly obtained by the limit output using estimated P-θ curves and is easy to understand, it is more useful than the conventional critical clearing time (CCT) of the energy function method. The proposed method can also estimate CGM as a negative value, which means instability in the present load profile, and so a negative CGM can be directly used as a restriction on generator output. The accuracy and fast solution ability of the proposed method are verified by applying it to a simple 3-machine model and IEEJ EAST10-machine standard model. Furthermore, the usefulness of the method for severity ranking of transient stability for various fault cases is discussed by using CGM. 2010 Wiley Periodicals, Inc. Electr Eng Jpn, 173(3): 10–19, 2010; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ eej.21035 [ABSTRACT FROM AUTHOR]

This paper deals with a method for the design of a fault-tolerant control system based on the Generalized Internal Model Control (GIMC) structure, consisting of a standard outer loop feedback controller and an extra inner loop controller. The distinguishing feature of the GIMC structure is that controller design for performance and robustness may be performed separately. The outer loop controller is designed for nominal performance using some controller synthesis to meet the (nominal) control specification, while the inner loop controller is designed to make a trade-off between robustness and performance. This feature is suitable for fault-tolerant control. The outer loop controller is designed for the fault-free case, and the inner loop controller for the fault-time case. In the conventional methods, the inner loop controller is designed to maximize the robust stability margin without fault information. Therefore, the performance in the fault-time case tends to become conservative. In this paper, the inner loop controller is directly designed from experimental data collected from the faulty system. Since the collected data contains information on the fault, conservativeness in the conventional methods is decreased. The inner loop controller is designed by Virtual Reference Feedback Timing (VRFT). VRFT is a method of direct design from input-output data without identifying any models. Since the complexity of the controller can be specified by the designer, no complexity reduction is necessarily required, which is advantageous in implementation. The effectiveness of the proposed design method is confirmed by an experiment. © 2010 Wiley Periodicals, Inc. Electr Eng Jpn, 171(4): 53–62, 2010; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20929 [ABSTRACT FROM AUTHOR]

Photovoltaic generation systems must have protection devices and islanding detection devices if they are connected to utility lines of the electric power company. This is regulated in national technological requirement guidelines and the electric equipment technology standards. Islanding detection devices are installed to detect blackouts due to events such as ground faults and short circuits in utility lines. When an islanding detection device detects a power blackout, the photovoltaic generation system must be stopped immediately. It is known that islanding detection devices cannot detect islanding if induction motors are present among the loads. We investigated the influence of induction motors on islanding detection devices and found that when the frequency decreases after a utility line failure, induction motors change over to generator mode, thus compensating the active power imbalance and suppressing voltage variation. In addition, if the postfault frequency is adjusted by PLL control so as to maintain a fixed power factor, induction motors suppress this frequency change, thus extending islanding operation. It is clear from the above results that induction motors are more likely to produce islanding phenomena than static loads. © 2010 Wiley Periodicals, Inc. Electr Eng Jpn, 171(4): 8–18, 2010; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20962 [ABSTRACT FROM AUTHOR]

Pole-mounted transformers are especially vulnerable to lightning damage. The progress of the information society imposes increasingly stringent requirements for the reliability of electric power supply, and this in turn necessitates a reduction in lightning damage to pole-mounted transformers. Lightning protective devices (surge arresters) are now being installed around the primary bushing of the transformers, which has decreased the number of disconnections around the primary bushing due to lightning. But surge arresters installed on the primary side of the transformer cannot protect it against backflow lightning entering the secondary side of the transformer. The characteristic of transformer damage by backflow lightning is that the electromagnetic force produced by the current flowing into the secondary side deforms the transformer windings. This paper elucidates the mechanism of transformer damage by lightning flowing into the secondary side by comparing actual lightning damage cases with the results of verification tests using a short-circuit generator. Effective countermeasures against transformer damage by backflow lightning are examined by EMTP calculations, which indicate that neutral grounding on the low-voltage distribution line is the most effective way of decreasing the current flowing into the transformer. The lower the grounding resistance, the less current flows into the transformer. In addition, decreasing the voltage on the secondary side is important in order to protect the secondary-side bushing. The calculation results indicate that surge arresters installed around the secondary side of the transformer are effective in decreasing the voltage on the secondary side. © 2010 Wiley Periodicals, Inc. Electr Eng Jpn, 171(2): 1–11, 2010; Published online in Wiley InterScience (www.interscience. wiley.com). DOI 10.1002/eej.20921 [ABSTRACT FROM AUTHOR]

Power generation using natural energy contains electric power fluctuations. Therefore, in order to put such power generation systems to practical use, compensation for system power fluctuations is needed. In this paper, we propose a power compensation method using a biomass gas turbine generator and flywheel energy storage equipment. The gas turbine generator is used for compensation of low-frequency power fluctuations in order to decrease the required flywheel capacity. The usefulness of the proposed system is confirmed by experiments using a test plant. © 2009 Wiley Periodicals, Inc. Electr Eng Jpn, 170(3): 1–8, 2010; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20896 [ABSTRACT FROM AUTHOR]

Dispersed generators such as wind power systems, photovoltaic systems, and cogeneration systems are expected to mitigate the environmental burden of energy consumption, and their installation has been promoted recently. Micro-Grid is focused on as a method to solve some problems in a commercial electric power line when installing a large number of dispersed generators, and some demonstrative research on Micro-Grid for large-scale systems is being carried out now. Also, small cogeneration systems for houses, such as gas engines and fuel cells, are expected to improve CO2 emissions. However, if the power and heat demand of a family are relatively small or are unbalanced, the cogeneration system does not operate effectively. The authors have studied the application of Micro-Grid for home energy supply, and have developed a control system to solve this problem. The system achieves a reduction of CO2 emissions and energy costs by sharing electric power and heat among some houses with cogeneration systems. This paper presents an outline of the newly developed system, and in particular describes the effect of the reduction in CO2 emissions compared with a conventional energy supply method, and the case in which dispersed generators are installed in some houses and operate independently. © 2009 Wiley Periodicals, Inc. Electr Eng Jpn, 170(3): 19–27, 2010; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20879 [ABSTRACT FROM AUTHOR]

This paper describes a novel straight and circular road driving control scheme for electric power-assisted wheelchairs. “Electric power-assisted wheelchair” which assists the driving force by electric motors is expected to be widely used as a mobility support system for elderly people and disabled people, however, the performance of straight and circular road driving must be further improved because the two wheels drive independently. This paper proposes a novel driving control scheme based on fuzzy algorithm to realize stable and reliable driving on straight and circular roads. The suitable assisted torque of the right and left wheels is determined by fuzzy algorithm based on the posture angular velocity of the wheelchair and the human input torque proportion of the right and left wheels. Some experiments on the practical roads show the effectiveness of the proposed control system. © 2009 Wiley Periodicals, Inc. Electr Eng Jpn, 170(1): 36–44, 2010; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20846 [ABSTRACT FROM AUTHOR]

In order to carry out energy saving service, it is indispensable to calculate the energy saving effect quantitatively by presuming the energy cost at the time of not executing the energy saving measure. However, it is difficult because the energy cost changes when environment (energy demand, unit prices of fuel or electric power, and so on) changes even if an energy saving measure is not executed. Although the effect has been conventionally evaluated by the difference between the energy costs before and after the energy saving measure, the effect cannot be calculated correctly when there is environmental change. In this paper, we propose the method of estimating the energy saving effect by presuming the energy cost after an energy saving measure, assuming that the measure is not executed, and show the results of its application to the practical power plants. © 2009 Wiley Periodicals, Inc. Electr Eng Jpn, 168(4): 14–20, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20880 [ABSTRACT FROM AUTHOR]

We used the method of moments (MoM), which is a numerical electromagnetic field computation method, to study the surge characteristics of twisted pair single-phase electric power lines. The results show that wires stranded with less than several turns per meter have almost the same surge characteristics as parallel lines, but the surge impedance decreases in tens of % when the number of strands increases. To verify the simulation results, we measured the surge characteristics of a model line of twisted pair experimentally, and there is good accordance between them. It is possible to simulate the surge in twisted pair covered with polymer dielectric insulators by MoM. Moreover, we studied the surge characteristics of twisted pair which is inserted into a metal tube. According to the results, the effect of the strand is smaller than that in free space without the metal pipe. There are two propagation modes in common mode: fast surge and slow surge. Few influences of the number of strands on the surge velocity occur in the fast propagation surge, but the increase of the surge impedance and the slowdown of the propagation speed are caused by twining in the slow propagation mode. © 2009 Wiley Periodicals, Inc. Electr Eng Jpn, 167(3): 1–9, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20737 [ABSTRACT FROM AUTHOR]

Until recently, SF6 gas has widely been used as the best insulating medium in substation equipment. However, SF6 gas was specified to be a greenhouse gas at COP3 in 1997 because of its high global warming potential (GWP), and alternative insulation gases to SF6 have been sought for a long time. Alternatives using the natural gases are considered to be suitable, but none of them show better properties for insulation as well as good environmental compatibility. Therefore, it is necessary to rationalize the equipment insulation level and reduce the test voltage of electric power apparatus to as low a level as possible. The actual lightning surge waveform (so-called nonstandard lightning impulse waveform) occurring in the actual field is different from the standard lightning impulse waveform (1.2/50 μs). There are many cases in which the actual lightning surge waveform has a steep rise and large decay of overvoltage, and the insulation requirements are not as severe as those for the standard lightning impulse waveform. In this paper, we focused our research on N2 gas as an SF6 substitute and investigated the insulation characteristics of N2 gas for a single-frequency oscillatory waveform with various frequencies from 2.7 to 20.0 MHz and damping ratios. Based on the experimental results, it might be possible to reduce the test voltage of N2 gas insulation by evaluating the crest value of the actual lightning surge waveform that has been converted into an equivalent standard lightning impulse waveform. © 2009 Wiley Periodicals, Inc. Electr Eng Jpn, 167(2): 10–20, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20754 [ABSTRACT FROM AUTHOR]

In order to keep power supply reliability at a certain level, electric power utilities have a certain amount of reserve capacity. When no generator outage or no unexpected large power demand occurs, however, the reserve capacity is regarded as surplus facility. To reduce the reserve capacity, some margin is reserved in tie lines between utilities. This margin is called the capacity benefit margin (CBM). In this study, a method of calculating optimal CBM in tie line under deregulated environment is described and two kinds of optimal CBM are proposed. As a result, it is shown how the deregulation affects the optimal CBM by using numerical simulation for the IEE Japan West 30 test system. © 2009 Wiley Periodicals, Inc. Electr Eng Jpn, 167(1): 35– 48, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20654 [ABSTRACT FROM AUTHOR]

In this paper, we assume 2 models for securing reserve capacity. One is “Commitment-based Security Model” and the other is “Reserve Market-based Security Model.” In Commitment-based security model, ISO commits procurement of reserve energy to a particular generation company. Meanwhile, in Reserve market-based security model, ISO procures reserve energy through reserve market. The main object of this research is to investigate which model will be preferable for the viewpoint of consumer's cost. To compare these models, two things are considered in this paper. One is bidding behavior of agents which bids to energy market and reserve market. To consider this, Q-Learning of multi-agent model is used. Also, the Unit Commitment (UC) is considered to calculate generation cost. This is to calculate the cost for securing reserve power more precisely. © 2009 Wiley Periodicals, Inc. Electr Eng Jpn, 167(1): 18– 25, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20705 [ABSTRACT FROM AUTHOR]

This paper proposes a novel current-source multilevel inverter, which is based on a current-source half-bridge topology. Multilevel inverters are effective for reducing harmonic distortion in the output voltage and the output current. However, the multilevel inverters require many gate drive power supplies to drive switching devices. The gate drive circuits using a bootstrap circuit and a pulse transformer can reduce the number of the gate drive power supplies, but the pulse width of the output PWM waveform is limited. Furthermore, high-speed power switching devices are indispensable to create a high-frequency power converter, but various problems, such as high-frequency noise, arise due to the high dv/dt rate, especially in high-side switching devices. The proposed current-source multilevel inverter is composed of a common emitter topology for all switching devices. Therefore, it is possible to operate it with a single power supply for the gate drive circuit, which allows stabilizing the potential level of all the drive circuits. In this paper, the effectiveness of the proposed circuit is verified through experimental results. © 2008 Wiley Periodicals, Inc. Electr Eng Jpn, 166(2): 88–95, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20475 [ABSTRACT FROM AUTHOR]

In this paper, we show an autonomous dispersed control system for independent micro grid of which performance has been substantiated in China by Shikoku Electric Power Co. and its subsidiary companies under the trust of NEDO (New Energy and Industrial Technology Development Organization). For the control of grid interconnected generators, the exclusive information line is very important to save fuel cost and maintain high frequency quality of the electric power supply, but it is relatively expensive in such small micro grids. We contrived an autonomous dispersed control system without any exclusive information line for dispatching control and adjusting supply control. We have confirmed through the substantiation project in China that this autonomous dispersed control system for an independent micro grid has a very satisfactory characteristic from the viewpoint of less fuel consumption and high electric quality. © 2008 Wiley Periodicals, Inc. Electr Eng Jpn, 166(1): 28–35, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20618 [ABSTRACT FROM AUTHOR]

A fast, innovative matching method for the spot power market, considering network constraints, has been developed. In this method, buy and sell order bids are respectively divided into the aggregated volume of several band prices. Then, the aggregated volume and the center of each band price are used to calculate an index band clearing price, which contains the real clearing price. The division and calculation process is iterated until the band price is less than the tick size of the bidding price. This method is applied to a real problem in the Japan power market, with 9 bidding areas, 10 area-connecting lines, and 9000 orders (volume/price pairs). Numerical simulation results show that the new method is ten times faster than conventional linear programming, demonstrating the effectiveness of this method. © 2008 Wiley Periodicals, Inc. Electr Eng Jpn, 165(4): 21–29, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20694 [ABSTRACT FROM AUTHOR]

In a distribution system containing a step voltage regulator (SVR), the maximum capacity of distributed generators (DGs) is calculated for DGs completely dispersed on a distribution line. The maximum capacity of the DGs is calculated under the constraint of an upper or lower voltage regulation value and an allowable current value by using voltage and current profiles expressed analytically in terms of our proposed power density model. As the voltage control method for the SVRs, we consider the conventional SVR, whose transformation ratio is fixed to 1 if it detects reverse power flow, and a reverse power flow SVR which operates appropriately even if it detects reverse power flow. Calculation of the maximum capacity of DGs with respect to the power factor of the DGs indicates which parameters, including the power factor of the DGs, the distribution of the DGs, and the load, influence the maximum DG capacity. Calculation of the maximum capacity of DGs versus the system length indicates that the constraints can be subdivided into two modes in the conventional SVR and four modes in the reverse power flow SVR. The maximum DG capacity in the system with a reverse power flow SVR is larger than that in a system with the conventional SVR. © 2008 Wiley Periodicals, Inc. Electr Eng Jpn, 165(4): 41–51, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20591 [ABSTRACT FROM AUTHOR]

In this paper, the authors present the system configuration and control scheme of a dispersed power source equipped with quick backup function, which acts as a dispersed power source when no fault exists in a commercial power network and functions as a backup power supply in case of a fault of the network. If a fault, such as a voltage interruption on the network, occurs, the dispersed power source disconnects critical loads from the network in an instant and continues supplying power to the critical loads. We also discuss the method of disconnecting a power network sufficiently quickly when some fault occurs on the network, adopting non-gate-turn-off devices such as thyristors for the AC interconnecting switches. Since it takes only 2 ms to disconnect the loads from a faulted network, there are no negative effects on the loads. Simulations and experimental tests confirm the practicability of the proposed method. © 2008 Wiley Periodicals, Inc. Electr Eng Jpn, 165(1): 74–81, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ eej.20700 [ABSTRACT FROM AUTHOR]

A super-distributed energy system is a future energy system in which a large part of its demand is fed by a huge number of distributed generators. At one time some nodes in the super-distributed energy system behave as load, whereas at other times they behave as generator—the characteristic of each node depends on the customers' decision. In such situation, it is very difficult to regulate the voltage profile over the system due to the complexity of power flows. This paper proposes a novel control method of distributed generators that can achieve autonomous decentralized voltage profile regulation by using multi-agent technology. The proposed multi-agent system employs two types of agent: a control agent and a mobile agent. Control agents generate or consume reactive power to regulate the voltage profile of neighboring nodes and mobile agents transmit the information necessary for VQ-control among the control agents. The proposed control method is tested through numerical simulations. © 2008 Wiley Periodicals, Inc. Electr Eng Jpn, 164(4): 43–52, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20484 [ABSTRACT FROM AUTHOR]

This paper describes a techique of calculating voltage and power flow state of a distribution system using the information measured with two or more switches with sensors installed in a distribution system. In the proposed technique, a set of the power distribution section where two or more switches with a sensor are connected is defined as the large section, and the active power and reactive power consumption in the large section are calculated based on measurement information, voltage (RMS), current (RMS), and power factor. Using the simple distribution system model consisting of the large sections, the power consumption of the large section is calculated by the power flow calculation to separate power consumption and power distribution loss. It is distributed to the small sections that constitute the large section, and detailed power flow calculation is performed. Verification of the proposed technique and basic estimation of the calculation error were performed using the simple power distribution system model. © 2008 Wiley Periodicals, Inc. Electr Eng Jpn, 164(4): 33–42, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20416 [ABSTRACT FROM AUTHOR]

This paper proposes a hybrid method of hierarchical optimization and parallel tabu search (PTS) for distribution system service restoration with distributed generators. The objective is to evaluate the optimal route to recover the service. The improvement of power quality makes the service restoration more important. Distribution system service restoration is one of the complicated combinational optimization problems that are expressed as nonlinear mixed integer programming. In this paper, an efficient method is proposed to restore the service in a hierarchical optimization with PTS. The proposed method is tested in a sample system. © 2008 Wiley Periodicals, Inc. Electr Eng Jpn, 164(4): 15–23, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ eej.20463 [ABSTRACT FROM AUTHOR]

In order to devise countermeasures for harmonic disturbances and harmonic suppression in power systems effectively, it is necessary to develop a harmonic analysis approach with high accuracy. The major harmonic analysis approach is to re-create harmonic distribution in a power system model by using a simulation method. However, in order to carry out high-accuracy estimation of the harmonic distribution using the simulation method, after creating a load model which consists of several parameters associated with the measured harmonic impedance, the optimal load model parameters must be determined. So far, appropriate load model parameters have been determined by trial and error. Therefore, a systematic approach to determine the optimal load model parameters is needed to estimate the measured harmonic impedance with high accuracy. In this paper, a determination method for the optimal load model parameters to estimate the measured harmonic impedance is proposed. The proposed method is based on Particle Swarm Optimization (PSO), which is one of the optimization methods using the concept of swarm intelligence. In order to check the validity of the proposed method, the load model parameters estimated by the proposed method are evaluated using test data and field data of Hokuriku Electric Power Co. © 2008 Wiley Periodicals, Inc. Electr Eng Jpn, 164(2): 44–53, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20415 [ABSTRACT FROM AUTHOR]

The objective of this work is to discuss the concept of back-to-back interconnection systems with energy storage, especially with a Superconducting Magnetic Energy Storage (SMES) incorporated into a back-to-back DC link. In this case, each converter of the back-to-back system is used as a power conditioning system for the SMES coils. Since the AC–DC converter can be designed independently of the frequency of the power system, a two-way switch is connected to the AC side of each converter. This two-way switch can select the interconnected power systems. By using the two-way switches, this system can provide the stored energy in the SMES system to each interconnected power system through two AC–DC converters. For instance, lower-cost power of each power network can be stored through two converters during the off-peak hours and made available for dispatch to each power network during periods of demand peak. Then this system increases the reliability of electric power networks and enables the economical operations depending on the power demand. This paper describes the unique operations of the back-to-back interconnection with SMES and discuses the optimal SMES configuration for a 300-MW-class back-to-back interconnection. © 2008 Wiley Periodicals, Inc. Electr Eng Jpn, 164(2): 37–43, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20482 [ABSTRACT FROM AUTHOR]

The “DC Micro-grid” is a novel power system using DC distribution in order to provide superhigh-quality electric power. The DC distribution system is suitable for DC output distributed generation techniques such as photovoltaic and fuel cells, and energy storage forms such as batteries and electric double-layer capacitors. The power is distributed through a DC distribution line and converted to the required AC or DC voltage by converters placed near loads. The load-side converters do not need transformers due to the choice of the proper distributed DC voltage. The spread converter scheme helps to assure high-quality power supply. Even if a short circuit occurs on one load side, it does not affect other loads. In this paper, we propose a DC micro-grid configuration and converter control methods for generation and energy storage. Computer simulation results demonstrate seamless operation during turn-on and turn-off of distributed generation, transients at the times of connecting and disconnecting operation with the bulk power system, and stability against sudden large load variations. © 2008 Wiley Periodicals, Inc. Electr Eng Jpn, 164(1): 34–42, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20641 [ABSTRACT FROM AUTHOR]

SF6 gas is widely used in electric power apparatus such as gas-insulated switchgears (GIS), because of its superior dielectric properties; however, it has been identified as a greenhouse gas at COP3 in 1997, and alternative insulation gases to SF6 have recently been investigated. One of the candidates is CO2 gas, which has lower global warming potential (GWP). However, CO2 gas has a lower withstand voltage level than SF6 gas; therefore, it is necessary to rationalize the equipment insulation level and reexamine the insulating test voltage for electric power apparatus as low as possible. From our previous investigation, in SF6 gas insulation system, we obtained that the insulation requirements of the real surges (called nonstandard lightning impulse waveform) are not as severe as those of the standard lightning impulse waveform. This paper describes the evaluation method for real surges, based on insulation characteristics of CO2 gas gaps. Furthermore, the method was applied to typical field overvoltage waveform in the lightning surge time region for 500-kV systems and it is obtained that the equivalent peak value of the standard lightning impulse waveform is possibly reduced by 10 to 15%. © 2008 Wiley Periodicals, Inc. Electr Eng Jpn, 163(3): 1– 9, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20560 [ABSTRACT FROM AUTHOR]

For the size reduction of electric power equipment, the electric field stress around solid insulators is to be carefully designed. This concern will be solved by the application of FGM (Functionally Graded Materials). In this paper, we investigated an application feasibility of FGM from numerical simulation, and the fabrication technique for gas insulated equipment. Finally, we could confirm the significant effect of FGM application for gas insulated equipment. © 2007 Wiley Periodicals, Inc. Electr Eng Jpn, 162(2): 39–45, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20363 [ABSTRACT FROM AUTHOR]

Most of the breakdowns in an electric power system are caused by lightning to the tower used for high-voltage transmission wires. To avoid accidents due to lightning, it would be desirable to develop a measurement system for the spatial electric field distribution around the tower. We have developed a new system that can measure the electric field around the steel tower for electric power transmission by using an optical electric sensor. In conventional methods, the sensor is generally illuminated by light transmitted by optical fibers. On the other hand, the new measurement system illuminates the Pockels sensor directly by using a laser to measure the far electric field. Furthermore, our newly proposed feedback system makes it possible to measure the spatial electric field without concern for the change of Pockels sensor's operation point influenced by temperature change. This system is applicable as a remote sensing system to monitor the spatial electric field around a high-voltage transmission line. © 2007 Wiley Periodicals, Inc. Electr Eng Jpn, 162(2): 10–19, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20552 [ABSTRACT FROM AUTHOR]

In order to obtain wind energy effectively, the pole-change-type induction generators are used as the wind turbine generators. Otherwise, the pole-change-type induction generator causes the voltage dips at pole changing time. To maintain the power quality, it is important to know the state change of the generator operation. Therefore, the authors have studied a state criterion of generator using the tower shadow effect, which is the active power oscillation caused by a rotation torque drop when the tower and the turbine blade overlap each other. In this paper, an improved identification method of oscillation frequency, which is the criterion of wind turbine generator operation, is proposed. The proposed method is applied to measured data and good results are obtained. © 2007 Wiley Periodicals, Inc. Electr Eng Jpn, 162(1): 25–31, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20395 [ABSTRACT FROM AUTHOR]

In the combined-cycle power plant generators are started by using the igniting arrangement up to the ignition rotational speed of the gas turbine. On the other hand, in the case of using the superconducting generator, it is difficult to apply the igniting arrangement used to generate electricity on the combined cycle for the structure as is. We examined the induction motor starting method for the superconducting generator by using the 70-MW-class quick-response excitation superconducting model generator and the VVVF power supply. From the examination, we confirmed the ability to raise the rotational speed from 6 to 360 rpm. Moreover, it was found to be able to start 200-MW-class superconducting generators by the induction motor starting method with the analysis. © 2007 Wiley Periodicals, Inc. Electr Eng Jpn, 160(2): 30– 38, 2007; Published online in Wiley InterScience (www.interscience. wiley.com). DOI 10.1002/eej.20283 [ABSTRACT FROM AUTHOR]

This paper presents and analyzes a new simple instant-estimation method for time-average quantities such as rms values of voltage and current, active and reactive powers, and power factor for single-phase power with the fundamental component of constant or nearly constant frequency by measuring instantaneous values of voltage and current. According to the analyses, the method can instantly estimate time-average values with accuracy of the fundamental frequency, and estimation accuracy of the power factor is about two times better than that of voltage, current, and powers. The instant-estimation method is simple and can be easily applied to single-phase power control systems that are expected to control instantly and continuously power factor on a single-phase grid by inverter. Based on the proposed instant-estimation method, two methods for such power control systems are also proposed and their usefulness is verified through simulations. © 2007 Wiley Periodicals, Inc. Electr Eng Jpn, 159(2): 34–43, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20236 [ABSTRACT FROM AUTHOR]

A new circuit of parallel-type voltage sag compensator is proposed in this paper. The proposed voltage sag compensator can reduce the necessary capacity of the energy storage capacitor for compensation by boost-type power factor correction rectifier charging the capacitor during a voltage sag. As a result the proposed system can reduce the size and maintenance of the apparatus. Copyright © 2004 Wiley Periodicals, Inc. This paper shows the principle and control system of the proposed circuit. Compensation characteristics were demonstrated by simulation and experiment. © 2007 Wiley Periodicals, Inc. Electr Eng Jpn, 159(2): 19–33, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20261 [ABSTRACT FROM AUTHOR]

Noncontact energy transfer systems are used in various fields in the industry. A high-frequency constant current must be supplied to the feeder of noncontact energy transfer systems. This paper proposes a high-frequency constant power supply for a noncontact energy transfer system using an immittance converter. When there are two or more pick-up coils on the feeder, the output of one pick-up coil does not affect other pick-up coils on the feeder if the feeder current is constant. We proposed use of an immittance converter to transform the high-frequency constant-voltage source into a high-frequency constant-current source. “Immittance converter” is a combined form of “impedance-admittance converter.” The immittance converter has input impedance that is proportional to the admittance of the load connected across 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, it converts a constant-voltage source into a constant-current source and a constant-current source into a constant-voltage source. This paper contains descriptions of the configuration of the new high-frequency constant-current power supply for noncontact energy transfer system, output characteristics versus excessive reactance and small frequency deviations for different values of the duty factor. The experimental results are also described. © 2006 Wiley Periodicals, Inc. Electr Eng Jpn, 158(3): 81–89, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20438 [ABSTRACT FROM AUTHOR]

Electronic apparatus, such as a computer, are a capacitor input type rectifier load for the power supply. The rectifier load causes distortions of the power source voltage waveform. Copyright © 2004 Wiley Periodicals, Inc. This paper describes the output voltage characteristic of the single phase PWM inverter controlled by a discrete-time model following method and connected to a rectifier load. First, the rectifier load model of a discrete-time system is considered. Next, the discrete-time model following control system used for this load is described. Then, simulation and experimental results are examined. The simulation results show that this method has robustness to the load change. The experimental system consists of an inverter controlled by a digital signal processor (DSP) and a rectifier load. Experimental results confirmed the validity of the simulation result. © 2006 Wiley Periodicals, Inc. Electr Eng Jpn, 158(2): 72–81, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20375 [ABSTRACT FROM AUTHOR]

Electric power transactions are subdivided into pool transactions and bilateral contracts, as well as security transactions. A certain transaction level of, for example, bilateral contracts may affect pool transactions by causing transmission congestion and the like. In this paper, we discuss how both the market and the bilateral transaction participants share the operation cost and charge for using a network, taking account of power transmission loss allocation among generators. Following these discussions, we present an optimal scheduling method for pool participants. To ascertain the effectiveness of the proposed scheduling algorithm and to show how the bilateral contract level affects pool operation, we will demonstrate various simulation results for a model power system of an 8-unit 44-bus power system. © 2006 Wiley Periodicals, Inc. Electr Eng Jpn, 158(2): 20–30, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20392 [ABSTRACT FROM AUTHOR]

The development of a wind turbine generator system in Japan is regarded as a promising source of clean energy, and its popularization is rapidly increasing. In Japan, there are many wind turbine generator systems installed on the Sea of Japan coast, with large wind capacity. However, since the Sea of Japan coast is a region with abundant winter lightning, a phenomenon peculiar to Japan, the wind turbine generator systems are subject to damage from winter lightning. Thus, it is important to observe the lightning currents striking wind turbine generator systems in order to provide lightning protection. The authors therefore installed currents sensors (Rogowski coils) on wind turbine generators installed in Hokkaido and Kagoshima Prefectures, and performed waveform observation of lightning stroke current in the winter of 2002 and the summer of 2003. These observations yielded a few current waveforms for lightning striking wind turbine generators. This paper describes the investigation and presents the observation results. [ABSTRACT FROM AUTHOR]

With the introduction of the competitive electrical power market, large-scale customers can select electric power suppliers. Customers need to consider not only the economical efficiency but also reliability, to decide the amount of electrical power to purchase. This paper develops an economic electric power purchase strategy for customers focusing on reliability. A bilateral contract model expressing electric power suppliers as generators with a forced outage rate is proposed and introduced to assess potential outage risks of the bilateral contract between an electric power supplier and a customer. The outage-related cost, consisting of potential outage risks and the estimated outage cost, is also proposed and introduced as an index of the reliability on the customer side, and an optimal reliability level of a customer is obtained by using the index. Several numerical examples demonstrate the availability of the proposed electric power purchase decision method and reliability improvement strategies for customers are discussed. © 2006 Wiley Periodicals, Inc. Electr Eng Jpn, 156(4): 9–21, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20345 [ABSTRACT FROM AUTHOR]

The player in deregulated electricity markets can be categorized into three groups of GENCO (Generator Companies), TRANSCO (Transmission Companies), and DISCO (Distribution Companies). This research focuses on the role of Distribution Companies, which purchase electricity from the market at randomly fluctuating prices, and provide it to their customers at given fixed prices. Therefore, Distribution Companies have to take the risk stemming from price fluctuation of electricity instead of the customers. This entails the necessity of developing a certain method to make an optimal strategy for electricity procurement. In such a circumstance, this research has the purpose of proposing a mathematical model based on stochastic dynamic programming to evaluate the value of a long-term bilateral contract for electricity trade, and also a project of combination of the bilateral contract and power generation with their own generators for procuring electric power in a deregulated market. © 2006 Wiley Periodicals, Inc. Electr Eng Jpn, 156(2): 32–42, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20105 [ABSTRACT FROM AUTHOR]

“Tracing the power flow of electricity” is a concept that has become the focus of attention recently, as the deregulation or the liberalization of the electric power market proceeds. Although a number of methods, such as “tracing method by power flow contribution,” have already been published, reactive power contribution has been little studied to date. However, applying “tracing the power flow of electricity” to ancillary service, for example compensation of reactive power, is very important. In this paper, we focus on line flows and static capacitors in particular, and study how to deal with the problem. We perform a computer simulation using a simple system and show the tracing results. © 2006 Wiley Periodicals, Inc. Electr Eng Jpn, 156(1): 33–43, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ eej.20118 [ABSTRACT FROM AUTHOR]

Application of a dispersed power supply system consisting of a large-scale photovoltaic system (PV), a fuel cell (FC), and an electric double layer capacitor (EDLC) is studied in this paper. This system is operated in autonomous mode, taking account of time delay characteristics of FC. The modified Euler type Moving Average Prediction (EMAP) model is improved using short-time fast Fourier transform (ST-FFT). The Adaptive Control type EMAP (AC-EMAP) model is introduced to reduce the capacity of EDLC. This system can meet the multi-quality electric power requirements of customers, and improve voltage stability and uninterruptible power supply (UPS) function as well. Moreover, the required capacity of EDLC to compensate the fluctuation of both PV output and Load demand is clarified by a simulation based on collaborative operation method by a prediction model using software MATLAB/Simulink. © 2006 Wiley Periodicals, Inc. Electr Eng Jpn, 156(1): 13–24, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20262 [ABSTRACT FROM AUTHOR]

This paper proposes a new circuit topology of the three-phase soft-switching PWM inverter and PFC converter using IGBT power modules, which has the improved active auxiliary switch and edge resonant bridge leg-commutation-link soft-switching snubber circuit with pulse current regenerative feedback loop as compared with the typical auxiliary resonant pole snubber discussed previously. This three-phase soft-switching PWM double converter is more suitable and acceptable for a large-capacity uninterruptible power supply, PFC converter, utility-interactive bidirectional converter, and so forth. In this paper, the soft-switching operation and optimum circuit design of the novel type active auxiliary edge resonant bridge leg commutation link snubber treated here are described for high-power applications. Both the main active power switches and the auxiliary active power switches achieve soft switching under the principles of ZVS or ZCS in this three-phase inverter switching. This three-phase soft-switching commutation scheme can effectively minimize the switching surge-related electromagnetic noise and the switching power losses of the power semiconductor devices; IGBTs and modules used here. This three-phase inverter and rectifier coupled double converter system does not need any sensing circuit and its peripheral logic control circuits to detect the voltage or the current and does not require any unwanted chemical electrolytic capacitor to make the neutral point of the DC power supply voltage source. The performances of this power conditioner are proved on the basis of the experimental and simulation results. Because the power semiconductor switches (IGBT module packages) have a trade-off relation in the switching fall time and tail current interval characteristics as well as the conductive saturation voltage characteristics, this three-phase soft-switching PWM double converter can improve actual efficiency in the output power ranges with a trench gate controlled MOS power semiconductor device which is much improved regarding low saturation voltage. The effectiveness of this is verified from a practical point of view. © 2006 Wiley Periodicals, Inc. Electr Eng Jpn, 155(4): 64–76, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20207 [ABSTRACT FROM AUTHOR]

A severe fault may cause loss of synchronism of a small number of generators. And it may take many generators out of service resulting in a wide area blackout. When two groups of generators lose synchronism, the magnitude of voltage on some lines becomes zero. Therefore, an approach whereby lines with zero voltage are opened has been employed in order to prevent loss of synchronism from extension. This paper presents a new method using voltage and current measured on a line during disturbance to detect the line with zero voltage at the instant when loss of synchronism takes place. A method to detect loss of synchronism using voltage and current measured on a line was already proposed. This paper presents results of the tests performed using a power system simulator which is composed of a DC motor, a synchronous generator, artificial transmission line, circuit breaker equipment, and so on. The validity of the proposed methods was verified by finding loss of synchronism correctly. © 2006 Wiley Periodicals, Inc. Electr Eng Jpn, 155(4): 7–19, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20346 [ABSTRACT FROM AUTHOR]

Recently, the number of distributed generators (DGs) connected to distribution systems has been increasing. System operators should know the maximum capacity of DGs that can be connected without problems to one feeder of the system in order to control the system appropriately. Many studies of the maximum capacity of the DG have been presented, but they have produced limited results calculated by a typical or average-value model. However, many DGs will access one feeder if deregulation of the electric power industry is accelerated in the near future. In order to deal with this situation, the authors have derived a general formula to calculate the range of the maximum DG capacity per feeder. Copyright © 2004 Wiley Periodicals, Inc. In order to deal with sets of DGs that are dispersed completely on the distribution line, the authors have derived a differential equation for the complex power and one for the voltage drop, which are expressed as functions of distance from the substation. The general formula to calculate the range of the maximum DG capacity connected to the system is determined by solving these equations under the constraints of the line voltage, the line current, and the power factor of the DGs. By a numerical analysis, the authors have calculated the maximum capacity of DGs depending on many parameters, such as the length of the feeder, the DG power factor, and the like. In a short-length system, the maximum DG capacity is governed by the current constraint, but in a long length system, it is governed by the upper voltage constraint. © 2006 Wiley Periodicals, Inc. Electr Eng Jpn, 155(3): 18–28, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20330 [ABSTRACT FROM AUTHOR]

Open access to electric power transmission networks has been carried out in order to foster generation competition and customer choice worldwide. When several PPSs request to simultaneously supply power to customers based on bilateral contracts, it is expected that the transmission network accepts amounts of wheeled power requested by the PPSs as much as possible. It is possible to maximize total requested wheeled power by controlling power flow through transmission lines. It is well known that FACTS device is available to control line flow flexibly. In this paper, in order to maximize total wheeled power simultaneously requested by several PPSs, the authors propose an algorithm to determine the optimal reactance of TCSC (a type of FACTS device). The proposed algorithm is based on Particle Swarm Optimization (PSO), which is an optimization method based on swarm intelligence. In the proposed algorithm, PSO is improved to enhance ability of searching global minimum by giving different characteristic to behavior of each agent. In order to check the validity of the proposed method, numerical results are shown for 6 and IEEE 30 bus system models. © 2006 Wiley Periodicals, Inc. Electr Eng Jpn, 155(2): 17–26, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20085 [ABSTRACT FROM AUTHOR]

This paper discusses the synchronization phenomena of wind turbines in a wind farm. The phenomena illustrate the situation where a blade revolution of a wind turbine is pulled into phase with that of the other turbines in a wind farm. If blades of many turbines simultaneously pass in front of their towers, output power fluctuations of the turbines due to tower shadow effects also become synchronized. This causes voltage fluctuation to increase and may give rise to voltage flicker problems. The purpose of this paper is to examine the phenomena theoretically. Obtaining a differential equation that describes the phenomena with output power, voltage, and slip fluctuations of a wind turbine, we examine dependence of the phenomena on the related parameters. We also prove the validity of the equation through simulation analyses. © 2006 Wiley Periodicals, Inc. Electr Eng Jpn, 155(1): 9–18, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20201 [ABSTRACT FROM AUTHOR]

We performed measurements of the radio frequency power and temperature of electrodes and plasma parameters of rf discharge. The purpose of this research is to check the accuracy of the measurement of energy consumed in plasmas. The heat energy in plasma was obtained by heat flux that was calculated from substrate temperature. The temperature was measured by a laser interferometer using LiNbO3 crystals. In the case of electrode spacings of 2, 3, and 4 cm, the heat energy toward electrodes was 7.7, 5.4, and 4.0 W, respectively. The current and voltage measurement was made at the driven electrode. The input power was 8.2, 5.1, and 7.7 W for electrode spacings of 2, 3, and 4 cm, respectively. The heat energy calculation based on the electron density distribution measured was conducted by estimating the Bohm flux and ambipolar diffusion flux. It was found that the heat energy toward electrodes was 1.4, 1.8, and 1.5 W for electrode spacings of 2, 3, and 4 cm, respectively, and the heat toward the wall was negligible. © 2006 Wiley Periodicals, Inc. Electr Eng Jpn, 155(1): 1–8, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20295 [ABSTRACT FROM AUTHOR]

We have developed a digital real-time simulator of Power Electronics Controllers, so-called FACTS (Flexible AC Transmission Systems) Controllers and/or Custom Power by using MATLABTM/SIMULINKTM and dSPACETM System. This paper describes the modeling and the calculation accuracy of a UPFC (Unified Power Flow Controller) model. Hence, the developed simulator operates at a large time step, in order to improve simulation accuracy, a correction processing of the switching delay is implemented into the UPFC model. Calculation accuracy of the real-time UPFC model is at the same level as EMTDCTM results. We confirm stable operation of the developed UPFC model by connecting a commercial real-time digital simulator (RTDSTM). © 2006 Wiley Periodicals, Inc. Electr Eng Jpn, 155(1): 19–26, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20200 [ABSTRACT FROM AUTHOR]

Despite substantial progress of research efforts in the field of photovoltaic (PV) power generation, some serious problems remain to be solved, including a fairly low energy generated by PV systems in comparison to the expected output, and a broad spread of output values among individual PV systems. The expected level of generated energy is usually evaluated by means of such coefficients as “the performance ratio” which is obtained as the energy actually generated by a PV array over a period of 1 year divided by the calculated output over the same period. However, this method has some limitations, such as the necessity to continuously perform data measurements over a long time, difficulties in the examination of factors reducing the level of the generated energy or improving the system efficiency, and so on. In this study, the authors propose a new method of accurate evaluation of the generated energy. The authors also examined a method for classifying the losses caused by the rise of the PV cell temperature and presented a method of data measurements over a short time period. The proposed method can also be applied to the classification and analysis of losses occurring at the time of the MPPT operation of the system (Pmax operation). © 2006 Wiley Periodicals, Inc. Electr Eng Jpn, 154(4): 24–33, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20222 [ABSTRACT FROM AUTHOR]