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Paper accepted for presentation at 2003 IEEE Bologna PowerTech Conference, June 23-26, Bologna, Italy

A Review of Islanding Detection Methods for Distributed Resources T.Funabashi, Senior Member, IEEE, and K.Koyanagi, Member, IEEE, and R.Yokoyama, Member,

IEEE

Abslrac-Islanding detection methods for distributed resources are reviewed. Special emphasis is put on some of the detecting systems based on "active methods" which have been developed in Japan for rotating machine type distributed generations. Then, reduction of the islanding detection sensitivity with multiple distributed generations is discussed. Finslly, a new concept of an islanded operation is introduced.

Index Term-Distributed generation, distributed resources, distribution system, islanding detection, islanded operation, power quality 1. INTRODUCTION

D

ISTRIBUTED generations have been broadly used and are expected to be an important element in the future electric power systems [I]. These generation systems have characteristics which are different from those of conventional large capacity fossil and nuclear generation systems. Distributed generations are relatively small and many of them make use of renewable energy such as a wind power or a hydraulic power. And, when the distributed generation systems are operated in parallel with utility power systems, especially with reverse power flow, the power quality problems become significant. Power quality problems include frequency deviation, voltage fluctuation, harmonics and reliability of the power system. In addition, most important problem is an islanding protection. When a distributed generation system with some loads is disconnected from the utility power system, the distributed generation is going to supply the loads and, although this is rare, continue an islanded operation of power system. The islanded operation should be avoided because of safety reasons for maintenance man and power quality reasons of distributed lines. To solve these problems, islanding detectors are used to detect an islanded operation and trip the circuit breaker between the power system and the distributed generation [2],[3]. By this time, many methods for the islanding protection have been proposed [4-221. These techniques can be devided into two categories; passive and active methods. The passive systems include undedover T. Funabashi is with Meidensha Cornration. 36-2. Nihonbashihakozakicho, Chueku, Tokyo, 103-8515 jAPAN. (e-mil [email protected]). K. Koyanagi is with TEF'CO Systems Corporation, 19-15, Shinbashi bchame. Minato-ku. 105-0004 Tokvo. ~. JAPAN (e-mail [email protected]) R. Yokoyama is with Tokyo Metropolitan University, 1-1, Minmi+sawa, Hachioji-shi, 1926397 Tokyo, JAPAN (e-mail: [email protected])

0-7803-7967-51031$17.0002003 E E E

frequency and undedover voltage relays. The most widely recognized active methods are the Reactive Export Error Detector (REED) [5],[6] and the system fault level monitor 161. In this paper, special emphasis is put on the active methods that have been developed in Japan for rotating type distributed generations [23]-[26]. Then, reduction of the islanding detection sensitivity with multiple distributed generations is also discussed. Finally, a new concept is discussed on an islanded operation that is fully using power electronics devices and information technologies. 11. ISLANDING DETECTION

In this chapter, brief introduction to the islanding detection is provided. Then, islanding detection methods developed so far are reviewed briefly, with special emphasis on five "active" methods developed and made in use in Japan. A. What is an Islanded Operation?

A fault occurring in the power distribution system is generally cleared by the protective relay that is located closest to the faulty spot. As a result, a distributed generation tries to supply its power to part of the distribution system that has been separated from the utility's power system. In most cases, this distributed generation assumes an overloaded condition, where its voltage and frequency are lowered and it is finally led to stoppage. However, though this is a rare case, a generator (or a group of generators) connected to this islanded system is provided with a capacity that is large enough to feed power to all the loads accommodated in the islanded system. When the loads are fed power only from the distributed generations even after the power supply is suspended from the power company, such a situation is called an "islanded operation" or "islanding". B. Hazardfrom an Islanded Operation

If a condition of islanded operation is continued, there can be concern ahout physical injury because of the inspection and restoration personnel or the public coming in contact with the live parts. In addition, when the power is supplied from the distributed generations, the quality of the fed power may be lowered as compared with the cases when the power is fed from the power company. It is often considered that the lowered quality may affect the loads adversely. At the power company, programs have been established so that the relevant circuit breaker or a switch is automatically closed at the substation affer the lapse of the predetermined time period, in order t o achieve prompt restoration from a service intermption.

However, if the above-mentioned islanded operation is continued longer, a condition of asynchronous closure is assumed and the fault may be evolved further. This results in a further delay in the restoration from the failure. For the reasons described above, the distributed generations and the protktive devices applied to the connecting point of their system are required to trip the circuit breaker located at this connecting point, by sensing such a condition when the power supply 60m the system is lost. This function is referred to as the "islanding detection" or "loss-of-mains protection." C. Conventional Techniquesfor Islanding Defecfion As one of the meaSuTeSto be taken for the detection of islanded operation, variations in the voltage and frequency are detected at the time of modal transition to islanded operation, When a variation signal is sensed, the circuit breaker at the interconnecting point is tripped. This is a well-known practice

detect the occurrence of islanded operation by monitoring a change in the system's frequency or voltage. -If there is Ay change in the generator load at the time of separation 6om the system, the frequency and voltage of the generator are changed to produce a new energy balance. In most cases when small-capacity generators are used, this method is often used to detect the occurrence of islanded operation. In this case, however, detection is impossible to cany out unless the load change at the time of separation 60m the system exceeds the range of compensation to be accomplished by the generator control system. If there is a good balance beheen the generator output and the load in the islanded system as described above, variations in voltage and frequency are generally small even though the condition of islanded operation is assumed. Therefore, it is impossible for these relays to prevent the islanded operation. In the case of interconnection for "reverse power flow not present," it is possible to prevent the occurrence of islanded operation by means of the "reverse power relays" that have long been used. These relays can not detect the islanded operation when the power flow is directed 60m the distributed generation to the distribution system side. The transfer trip system is available. This system is applicable to the distributed generations in the case of interconnection "with reverse power flow." This system transfers a signal to a circuit breaker to hip it on the distributed generation side. Actually, the data transferred are the output signal for circuit breaker tipping, sent from the power distribution substation. Since this system calls for a data transmission path, the price of the entire equipment is inevitably raised and this is a disadvantage for this system. There are some more disadvantages such that it is impossible to detect the islanded operation due to a fault in the higher system, and that the system is not applicable to any service interruption that occurs in the part of the power distribution lime. The SCADA system is also available. This system is applicable even in the case of a failure caused in the higher power system. This system is used to supervise the ON-OFF status of all circuit breakers that are liable to assume the conditions of islanded operation. Therefore, when islanded

operation occurs, a data signal is transferred and the relevant circuit breaker in interconnection is tripped.

D. Passive and Active Methods If a unit with the function of detecting an islanded operation is installed on the distributed generation side and the occurrence of islanded operation can be exactly detected on this side, it is then possible to omit the transfer trip system and the SCADA equipment to enable economic structuring of a distributed power system. Such a self-end detection type (using information on the distributed generation side only) of the islanding detection system can be roughly categorized into a passive system and an active system. Table I shows the features of the passive and active methods. TABLE 1 PASSIVE AND ACTlVE METHODS

Principle

1 Monitor changes in voltage, tiequency,

I Aoply disturbances frakdistrihuted

Possible, such as

'1

system In the Passive system, Various values are always supervised, such as variations in voltage, fiWenCY, harmonic distottion, etc., measured at the interconnection point of a distibuted generation. When the state of islanded operation O C C ~ S , these values are greatly changed from the regular values observed during interconnection with the utility's power system. Utilizing this fact, the occurrence of an islanded operation is detected. The passive system offers an outstanding feature that highly sensitive and high-speed detection is possible. A disadvantage is that a misoperation Can Occur if there is a sudden load change. In addition, detection is impossible unless the load change at the time of the system separation exceeds the compensation range that is assured by the generator conh-ol System. The ROCOF relay [21,[41 is generally accepted as the standard method for islanding protection of the distributed System. These relays monitor the voltage waveform, and operate when the rate-of-change of frequency exceeds a setting and duration exceeds a time delay setting. COROCOF relay 1161 is the new -e. In Principles, it compares the rateof-change of frequency at the generator with that of the rest of the System. The Phase displacement monitor [21,[41 operates when there are changes in the phase displacement in the system voltage waveform. These phase displacements are direct results of changes in the System 6equenV. In the power fluctuation method [8]-[1 I], changes in the power output &om the generator are used. Under normal conditions, the transfer function reflects the characteristic of both the distributed generation and the utility System. With islanding operation, the transfer function reflects the distributed generation only. Dr. Salman proposed many Passive methods [12]-[151, including the method for monitoring power factor in addition

to the rate-of-change of frequency, and that is called the elliptical trajectory (EET) techniques. The basic principle of EET is that the changes in current and voltage introduced by the fault occurred on a transmission line &e related by elliptical trajectory. If the islanded operation detector makes a misoperation, the circuit breaker at the interconnectionpoint is tripped and it takes plenty of time till restoration. For this reason, careful consideration is needed for the operation setting of islanding detector. In a distributed generation where an inverter or power conditioner is used, flexible operation is possible with the passive system. In such a case, the inverter may be stopped when an islanded operation is sensed. If a misoperation takes place, the auto-reset function of inverters may be used. 2) Active System In the case of the active system, the status of islanded operation can be detected even under the perfect equilibrium state, which cannot be sensed by the passive system. In this case, an external turbulence is always given fiom the distributed generation to the utility's power system so that the voltage and kquency are assuredly changed when the condition of islanded operation is assumed. In contrast to the passive system, the active system requires time to give an external turbulence and to detect kequency changes, etc. due to the external turbulence. As a result, the detection time tends to be prolonged. Theoretically, however, there is an advantage of fieedom from a dead zone. In conclusion fiom the above, it is recommended to use a combination of both passive and active systems to ensure the perfect prevention of an islanded operation. The REED relay makes the distributed generator control system generate a level of reactive power flow at the relaying point [2],[5],[6]. This power flow can only be maintained only when the utility source is interconnected with the distributed generation. If the level of reactive power flow is not maintained at the setting value, islanding is detected. System fault level monitor uses measurements of power system source impedance. This is measured from the short circuit current and the supply voltage reduction monitored when a shunt inductor is briefly connected across the supply voltage using thyristor switch 161. In radio fiequency signal method, the radio signal is injected into the system and monitored by a receiver tuned to separate the superimposed signal fiom the power system wave[7]. 3) Five active methods developed in Japan Table I1 shows the relationship between the typical active detection systems and the applicable power generating facilities. (In this table, rotating machines are categorized into synchronous generators and induction generators.) Reactive power fluctuation method [18]-[20] is a typical active type method, which is applicable to synchronous machine and inverter interfaced DC sources type distributed resources, but is not applicable to induction machine interfaced distributed resources. As shown in Fig.1, small fluctuation signals are periodically applied to automatic voltage regulator of synchronous generators. Islanding relay detects frequency deviation which becomes significant when the system moves to the islanded situation.

TABLE 11 APPL'CAENLITY OF ACT'VE

Fig.1 A power system wiih disiributed

QC mode frequency shift method [21] is also one of active methods for synchronous generators. It also applies fluctuation signal to an automatic voltage regulator, but in this method a signal does not have a constant fiequency, but a frequency that is related to the power system kequency fluctuation. The method also detects an islanded operation by watching a power fiequency fluctuation. Reactive power compensation method is making reactive power fluctuation by reactive power compensator rather than voltage regulator signal. So, it is applicable to induction machine type distributed resources. Load fluctuation method is a method to insert some load such as resistor to the point of common coupling. The impedance seen fiom the distributed resources side changes when the islanded operation starts. Load fluctuation islanding relay detects the islanding by monitoring the impedance. This method is applicable to rotating machine type distributed resources. Inter-harmonics is a non-integer harmonics. Interharmonics injection method is always injecting the noninteger harmonics to the power system. The response of the power system to the inter-harmonics will change when the islanded operation of the distributed resources starts [22].

111. ISLANDING DETECTION OF MLnTIPLE DISTRBUTED GENERATIONS A big challenge in islanding detection is an assessment for the c a e with multiple distributed resouTces in the same distribution line 1231-1261. In this chapter, problems of the islanding detection sensitivity reduction are discussed for the case with multiple rotating-type dishibuted generations. A. Classification of Active Method The active islanding detection system developed in Japan, to be applied to the distributed generations of the rotating machine system, can he classified according to the turbulence signals and the detected signals, as specified in Table 111. TABLE Ill CLASSIFICATIONOF ACTIVE ISLANDING DETECTION METHODS

(I)

(2)

(3)

1 (4) I

(5)

Method Reactive power fluchlation QC-mode frequency shiR Reactive power compensation Load fluctuation Inter-harmonics injectron

Twbulence signal AVR voltage setting

AVR voltlge setting SVC Reactive power Load insertion Inter-harmonic current

1 l",ectlo"

I

Detection signal Frequency deviation Rate of frequency change . Frequency deviation Current dividing ratio Susceptance

The detecting systems of (1) to (3) in the Table I11 utilize the frequency sway in the generator, caused in compliance with the active turbulence in the state of an islanded operation. The detecting systems of (4) and ( 5 ) examine the difference in impedance between interconnection with the commercial power system and an islanded operation. B. Pending in Multiple Generations lnterconnection As described previously, the active islanded operation detecting system always gives a turbulence fiom the distributed generation to the utility's power system in order to change voltage or frequency when the state of islanded operation is assumed. If multiple dishibuted generations are installed in the same commercial power system, there is concern about the reduction of islanded operation detecting sensitivity and an increase in the influence upon the utility's power system when an active system is used. There are a variety of combinations of multiple distributed generations to be installed in the same power system, according to the type of generation, the presence of reverse power flow, the principle of active system, the generator capacity ratio, etc. According to each combination, it is necesswy to evaluate the detecting sensitivity and the degree of influence upon the utility system in order to establish the effective countermeasures and defme the limits of applications.

C. Reduction of the Detecting Sensitivity due to Mutual Interference In regard to the reduction of the detecting sensitivity due to mutual interference in the active system, the degree of influence can differ according to the combination of the systems and the ratio of generator capacities. For some

I

combinations of the active systems, no interference may occur theoretically. However, if the same systems are combined or the ratio of generator capacities is almost unity, it is necessary to P . Y. .attention to the possible reduction of the detecting sensitivity. The descriptions below indicate the possible problems caused by the combination of generations in a case where multiple generations are mixed in the same system. 1) Generator capacity ratio When two distributed generations with different capacities are connected to the same power system, the behavior of an islanded operation tends to he dominated by that of the generator that has the larger capacity. This tendency is intensified, as the ratio of capacities becomes larger. 2) Generator without reverse power flow When a generator without a reverse power flow and the other generator with a reverse power flow are mixed in the same system, the reverse power relay are used for the generator without a reverse power flow and the active islanding detector is applied to the generator with a reverse power flow. In this case, the generator without a reverse power flow restrains the active fluctuation irrespective of the principle of detection used for the active system. Since the difference in impedance is decreased between a grid interconnection and an islanded operation, there is some concern about the reduction of the detecting sensitivity. This tendency becomes more eminent, as the capacity of the generator without a reverse power flow is greater. For an active system, like the Reactive power fluctuation system, where the islanded operation is sensed in terms of frequency, the frequency maintaining capability is raised by the effect of the synchronizing force of the generator without a reverse power flow. Therefore. the frequency variation is ~. decreased during the islanded operation. In regard to the interhannonic injection system, there has been a report that the detected susceptance value is not influenced if the capacity of the generator with a reverse power flow is smaller than or equals to the capacity of generators without a reverse power flow. This report indicates that the reduction of the detecting sensitivity caused by the generators without a reverse power flow is not a problem. If there is a generator without a reverse power flow in utilizing the load fluctuation system, the currents on the utility's system side contain those from the bus line and those from the generator without a reverse power flow. Therefore, compared with the interconnection of a single generator unit, the reduction of the detecting sensitivity turns out to be a greater problem. 3) Interference among the same active systems In the Reactive power fluctuation system, the detecting sensitivity becomes lowest if the AVR voltage setting variation is made at the same frequency in the reverse phase. Therefore, if multiple generators with almost the same characteristics are installed in the same power system and the active signals can be synchronized there, it is preferable to make the AVR voltage setting variation at the same frequency in the same phase. If multiple generators belonging to different owners are connected to the same power system and it is difficult to synchronize the active signals, the above-

mentioned setting should be made independentlyof each other to cope with the frequency variation. The same thing can be said if the generator characteristics are mutually different and the effect of synchronization is canceled for the AVR voltage setting variation. The authors has established an analytical program that assures the efficient accomplishment of performance verification for the Reactive power fluctuation system under the condition of the interconnection of multiple generators. This program is useful for the evaluation of the application to the islanded operation detector unit and the decision of setting values. It is generally believed that the Reactive power compensation system shows a similar tendency as that of the Reactive power fluctuation system. Unlike the Reactive power fluctuation system, the QCmode frequency shift system is capable of automatic synchronization because the variation in the AVR voltage setting value is determined based on the measured frequency changing rate. Accordingly, the reduction of the sensitivity is hardly caused by mutual interference. In the load fluchlation system, some effective measures can be taken, such that loads are inserted among different phases, the load insertion intervals are kept unchanged but the insertion timing is shifted, or the load insertion intervals are changed. However, for the same reason as in the case of generators without a reverse power flow, reduction of the sensitivity can occur if there are multiple generators in the same power system. In the interharmonic injection system, it is necessary to change the order of the injected harmonics for each generator. However, since this system measures harmonic currents and voltages of the injection order, it is necessary to use the handpass filter that has the same passband 6equency as that of the injected harmonics. In addition, each setting of harmonics order is required to be different from each other so that the injected harmonics of a generator does not affect the result of measurement for the other generator. 4) Inferferenceamong diferenf active sysfems In a case where different systems are used, the behavior of the generator with the larger capacity becomes dominant in regard to the behavior of the islanded operation if the generator capacities are greatly different from each other. This is due to the reasons of I ) described previously. Therefore, concern about the interference among the active systems becomes greater if the generator capacities are similar to each other. In the Reactive power fluctuation system, the QC-mode frequency shift system, and the Reactive power compensation system, fluctuations are positively given to the generator and detection i s based on the ffequency. In the load fluctuation system and the interharmonic injection system, the fluctuations are not given positively. If the former and the latter are combined, the latter may suppress the fluctuations in the former and the detecting sensitivity of the former may be deteriorated. When the Reactive power fluctuation system and the QCmode ftequency shift system are combined, the latter system is automatically synchronized with the fluctuations caused by

the former system. Accordingly, the reduction of the sensitivity is hardly caused by mutual interference. For the same reasons as the interconnection with a generator without a reverse power flow as stated in 2) above, the combination of the load fluctuation system and any other system can give rise to deterioration in the detecting sensitivity in the load fluctuation system.

IV. AUTONOMOUS DISTRIBUTION SYSTEMS USING POWER ELECTRONICS DEVICES AND INFORMATION TECHNOLOGIES Islanding detection relays are not a completed technology and has some challenges are exist in addition to the problems written in the previous chapter. To overcome these challenges, various kind of new detection methods and simulation methods have been investigated [27]-[32]. Various islanding detection methods have been proposed 6om the point of view that islanding operation must be stopped. However, many new concepts of an autonomous distribution system were also proposed. New technologies are investigated such as power flow control and output stabilizing control using power electronics devices, loop type distribution system rather than radial, a hybrid system with power electronics devices, integrated protection and control system, and interactive information exchange between customers and resources. In such a new system, interface between the distributed resources and power distribution system will be highly improved and output controllability will be improved and as a result power qualities in the distribution system will be highly improved. An example is shown here. On-line information is exchanged between the distributed generations in attempt to have an autonomous operation of the distribution system dominated by the distributed generators [33]. The flexibility of the distribution system isolated from the higher voltage system i s improved by the on-line information exchange.

V. CONCLUSIONS Five active methods featured in this paper are in practical operation in the real power systems in Japan. However, they are not a completely established technology and there are some challenges, such as islanding detection for the power system connected with multiple distributed generators. In the future, a new concept of an islanded operation will be introduced. That is to be a secure, reliable and stable isolated system by using a information technologies and power electronics devices. VI. REFERENCES [I] [21

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P.D.Hopewell, N.Jenkins, A.D.Cross, “Lossaf-Mains Detection for Small Generators”, IEE Proceedings, Electrical Power Applications, ~01.143,n0.3, May 1996, pp.225-230 161 J.W.Wari& “Loss of Mains Protection”, ERA Conference on Circuit Protection for Industrial and Commercial Installations, London, 1990 171 P.J.OKane, B.Fox, D.J.Morrow, “Impact of embedded generation on emergency reserve”, IEE Proceedings, Generation, Transmission & Dislributioq vo1.146,no.2, March 1999, pp.159-163 M.A.Redfem O.Usla and G.Fielding, “Protection againlt Loss of IS] Utility Grid Supply for Dispersed Storage and Generation Unit‘‘, I E E W E S Summer Meeting, Seatde WA, July 1992 191 MARedfem, 0.Usta and G.Fielding, “Power based algorithm to provide loss of grid protection for embedded generation”, IEE Proc., Gener. Transm. Distrib., Vo1.141,No.6, 1994, PP.640-646 [IO] M.A.Redfern, O.Usta and J.I.Barren, “A New Digital Relay for Loss of Grid to Protect Embedded generation”, DPSP’93 (International Conference on Developments in Power System Protection), 1993, pp.127-130 [ I l l M.A.Redfem, J.I.Barreu and O.Usta, “ A New Microprocessor Based Islanding Protection Algorithm for Dispersed Storage and Generation Units”, IEEE Transactions on Power Delivery, Vol. IO, No.3, July 1995, pp.1249-1254 [I21 S.K.Salman, “Investigation of lhe Effect of Load Magnitude and Characteristics on the Detection of Islanding Condition”, UPEC’97 (Univenities Power Engineering Conference), 1997, pp.423426 [I31 S.K.Salman and D.J.King, “Investigation into Methods of Detecting Loss of Mains an the Interfacing Link Between a Utility and a Distribution System Containing Embedded Rotating Generation”, UPEC’97 (Universities Power Engineering Conference), 1997, pp.ll22-1125 [I41 S.K.Salman, “Detection of Embedded Generator Islanding Condition Thine Rllintical _I_..~ ~ . . .Traiectarv ~ ~ ~ , Techniaue”. . . DPSP’97 (International Conference on Developments in Power System Protection), 1997, pp. 103-106 [IS] S.K.Salman, D.J.King and G.Weller, “Detection of Loss of Mains Based on Using Rate of Change of Voltage and Changes in Power Factor”, UPEC2000(Universities Power Engineering Conference), Belfast 2000 [I61 C.G.Bright, “COR0COF:Camparison of Rate of Change of Frequency Protection. A Solution to the Detection of Lass of Mains”, DPSP2OOI(International Conference on Developments in Power System Protection), 2001, pp.70-73 1171 T.lshikawa and Y.lwai, “Outline of Islanding IXtection Technology”, T.IEE Jarran, Vol.116-B. Na.5, 1996, pp.521-524 (in Japanese) [I81 Motohashi, et. al., “Islanding detection device for synchronous generators wnnected with dismbution lines (reactive power fluchlation method)” Proc. IEE Japan Vol.ll9-B, No.1, 1999 (in Japanese) 1\91 Motohashi, et. al., “Test results of islamding detection device using real time digital simdator”. Proc. IEE Japan annual meeting, No.1550, 1996 (in Japanese) [ZO] Motohashi, et. al., “Study on islanding detection device for synchronous generators wnnected with diseibution lines wnsidering a counter measure to voltage fluctuations”, hoc. IEE Japan Workshop on Power Engineering and Power systems engineering, PE-97-36, 1995 (in Japanese) [21] T.Kato, et. al., “A Navel Islanding Protection System for Synchronous Generators”, TJEE Japan, Vol.l2O-B, No.8/9, 2000, pp. 1182-1 193 (in Japanese) Nishijimq et. al., “Islanding detection with inter-harmonics injection”, Proc. IEE Japan annual meeting, 6-304.2000 (in Japanese) Matohashi, et. al., “Application of synchronous generator with islanding deteotion to multi-machines power system (Reactive power fluctuation method)” Proc. IEE Japan Workshop on Power Engineering and Power systems engineering, PE-98-117, 1998 (in Japanese) [241 O.Tsukamoto, H.lshii, T.Okayaru and K.Yamsguchi, “Detection of Islanding of Multiple Dispersed Rotating Generators Using Conelation Techniaue”. 1CEE98 llnternational Conference on Electrical .~. Engineering), 1998, pp.508-51 I T.Kai and H.Kanedq “Study on performance of islanding detection device for multiple synchronous generators by using MATLAB”, Proc. IEE Japan Workshop on Power Engineering and Power systems engineering, PE-99.140, 1999 (in Japanese)

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VlI. BIOGRAPHIES

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Tashihisa Funsbashi (M’1990, S M 1996) was born in Aichi, Japan. He received B.S degree from Nagoya University, Aichi, Japan in 1975 and Doctor degree 60m Doshisha University, Kyoto, Japan in 2000. He joined Meideusha Corporation in 1975 and has engaged in research on power system analysis. Currently, he is Senior Engineer of the Power Systems Engineering Division. He is a Chartered Engineer in UK, a senior member of IEEE, a member of IEE and IEE Japan. Kaoru Koyanagi has received B.S degree in applied physics from Tokyo University of Education in 1971 and Ph.D. degree in electrical engineering from Tokyo Metropolitan University in 2000. From 1971 to 1996, he was with Toshiba Corp. working on power system analytical engineering. He has been involved in various aspects of power system stability analysis and design of control systems. He is currently with TEPCO System Corporation and involved with software development in power system engineering.He is amemberoflEE ofJapan3‘EE and IEEE’

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Ryuichi Yokoyams has received the degrees of B.S.,M.S.,andPh.D. inelectrical engineeringfrom Waseda University, Tokyo, Japan, in 1968, 1970, and 1973 respectively. Since 1978 he has been working for the Faculty of Technology of Tokyo Metropolitan University, and is currently a professor of Electrical Engineering. His field of interests include wntrol and optimization of large-scale systems and application of arlificial intelligence to power systems. He is a member of the Society of lnsrmment and Control Engineers (SICE) of Japan, the IEE of Japan, CERE., and IEEE.