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Abstract-- DC microgrid is a novel power system using dc distribution in order to provide a super high quality power. This system is suitable for dc output type distributed generations and energy storages. The dc power can be converted to the proper ac or dc voltages for the consumers by converters placed near loads, and these converters do not require transformers. This distributed scheme of load side converters also contributes to provide the high quality power supplying. In this research, we assumed a dc microgrid applied for residential houses. All houses have a cogeneration system such as gas engine or fuel cell. The outputs of those distributed generations are connected to the dc distribution line, and the power from the generations can be shared among the houses. The hot water from distributed generations are used in each house. We constructed a small scale experimental in our laboratory, and examined the fundamental characteristics of the dc microgrid by the experimental system when it was connected to the bulk power system. Experimental results demonstrated that the system could supply high quality power to the loads stably against sudden load variations, voltage sags of the bulk power system and short circuits of the load.

Index Terms-- microgrid, distributed generation, high quality power, dc distribution, gas engine cogeneration

I. INTRODUCTION N RECENT YEARS, a large number of distributed generations such as photovoltaic cells, fuel cells, wind

turbines and gas engine cogenerations has been installed into the bulk power system under the background of environmental problems, etc. However, it could cause problems such as voltage rises and protection problems when many distributed generations are connected to the conventional power distribution systems. One of the solutions

This research was partially supported by a grant for the Global COE Program, Center for Electronic Devices Innovation, from the Ministry of Education, Culture, Sports, Science and Technology of Japan.

H. Kakigano is with the Division of Electrical, Electronic, and Information Engineering, Osaka University, Osaka, Japan (phone: +81-6-6879-7730; fax: +81-6-6879-7730; e-mail: kakigano@eei.eng.osaka-u.ac.jp).

Y. Miura is with the Division of Electrical, Electronic, and Information Engineering, Osaka University, Japan (miura@ eei.eng.osaka-u.ac.jp).

T. Ise is with the Division of Electrical, Electronic, and Information Engineering, Osaka University, Japan (ise@ eei.eng.osaka-u.ac.jp).

T. Momose is with the Energy Technology Laboratories, Osaka Gas CO., LTD., Japan (toshinari-momose@osakagas.co.jp).

H. Hayakawa is with the Energy Technology Laboratories, Osaka Gas CO., LTD., Japan (hayakawa@osakagas.co.jp).

is to construct a new electric power system [1]-[3]. Especially, microgrids are being researched all over the world. Most microgrids adopt ac system as a main distribution [4]. In this case, dc output type distributed generations (photovoltaic cells, fuel cells) and energy storages (Li-ion secondary battery, supercapacitor) have to need inverters. In addition, some gas engine cogenerations and wind power generators also need inverters because the output voltages and frequencies of those generators are different from the bulk power systems one. Therefore, in the case of ac microgrid, if a blackout or voltage sag occurs in the bulk power system, most inverters might be tripped. So, it is difficult for ac microgrids to keep a super high quality power supplying continuously in islanding operation.

On the other hand, there is a variety of customers' needs for electric power quality. High quality power is required for a dependable society. The security of the electric power is becoming more important for our daily life.

We have proposed dc microgrid which is a novel power system using dc distribution in order to provide a super high quality power [5], [6]. This dc system is suitable for dc output type distributed generations and energy storages such as secondary batteries and supercapacitors. The dc power is converted to required ac or dc voltages by load side converters, and these converters do not require transformers by choosing proper dc distribution voltage. The distributed scheme of load side converters also contributes to provide supplying high quality power. For instance, even if a short circuit occurs at one load side, it does not affect other loads. Moreover, it is easier than ac microgrid to disconnect it from the bulk power system and change into an intentional islanding mode without any problems.

In this research, we propose one type of dc microgrids for residential houses (apartment house or housing complex). In this system, each house has cogeneration system such as gas engine or fuel cells, and those generations are connected to a dc power line. The electricity from the generations can be shared among the houses. The hot water from a distributed generation is used in each house. An experimental system based on this concept was constructed in our laboratory, and the fundamental characteristics have been studied. In this paper, the experimental results when the system is connected to the bulk power system are shown. Those results

Fundamental Characteristics of DC Microgrid for Residential Houses with

Cogeneration System in Each House H. Kakigano, Y. Miura and T. Ise, Member, IEEE, T. Momose and H. Hayakawa, Non-member

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2008 IEEE.

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demonstrate that the system is able to supply a high quality power against disturbances.

II. DC MICROGRID Figure 1 shows a concept of the system. In the dc

microgrid, there are around 50-100 houses, and the every house has a gas engine or a fuel cell cogeneration. The outputs of all distributed generations are connected to the dc distribution line, and the electrical power is shared among other houses through the dc line. Therefore, it can be expected that the operation chances of cogeneration become increased, and it leads to higher utilization of the cogeneration [7]. To keep the high efficiency, those distributed generations should not be operated by a partial load condition, but operated by a start/stop control. The hot water is used in each house, but it is also possible to share with the houses. The system is connected to the bulk power system through a rectifier at one point, and various forms of electrical power like single phase 100 V, 3-phase 200 V, DC 100 V, etc. can be obtained by converters placed near loads. These converters do not need transformers, therefore it contributes to the downsizing and high efficiency.

Secondary batteries and supercapacitors can be connected to the dc distribution line through dc/dc converters. If a blackout occurs in the bulk power system, the dc microgrid is able to disconnect it rapidly, and the power supply will be continued without any voltage sags at the load, resulting in the intentional islanding mode. There are some energy control operations at the intentional islanding. We adopted a

following method that a supervisor computer controls the number of distributed generations in the system, and the excess or deficiency power is compensated by an electric double layer capacitor (EDLC). This operation might cause that an energy storage in the system need a huge capacity, because most commercial distributed generations take a few minutes (or a few hours) for enabling and disabling power. In this research, though, we assumed that the enabling and disabling times of distributed generations are short in the system (a few second). Then, it is possible that the surplus or shortage power is compensated by only EDLC storage system.

In this paper, to examine the basic characteristic when the system is connected to the bulk power system, all experiments were verified without an EDLC storage.

III. EXPERIMENTAL SETUP

A. Configuration of the Experiment System The configuration of the experimental system is shown in

Figure 2. The experimental setup and the gas engine cogeneration system are shown in Figure 3 and Figure 4. By using the experimental system, we verified the fundamental

Fig. 1. The concept of the proposal dc microgrid.

Fig. 2. Configuration of the experimental system.

Fig. 3. Experimental setup dc microgrid.

Rectifier

Single Phase Inverter

Line Impedance

Control Boards

DC Circuit Breaker

DC/DC Converters

Hot Water Tank

Gas Engine Unit

Hot Water Tank

Gas Engine Unit

Fig. 4. Configuration of the 1 kW cogeneration system.

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TABLE I MAIN PARAMETERS

Rectifier

Ls 2 [mH] Cs 10 [F] Crec 1500 [F] C1 4700 [F]

switching frequency 10 [kHz]

Single phase inverter

Linv 2 [mH] Cinv 18.3 [F] C2 12200 [F] switching frequency 9 [kHz]

DC/DC converter

Lcon 5 [mH] Ccon 220 [F] C3 440[F] switching frequency 10 [kHz]

characteristics: the electric power sharing by the dc power line among the houses, the behavior at the accident in the bulk power system or the dc microgrid, and the power quality to the loads. The 3-phase AC 200 V of the bulk power system is converted to DC 170 V by the rectifier, and the power is supplied to each house through the dc distribution line. This system applied the bi-pole 3-wire system (two 170 V lines and one neutral line) to the dc distribution. It contributes that the one side of the output of the single phase 100 V can be a grounded neutral line as well as a Japanese standard power system, and the dc voltages from the ground become lower level. It is assumed that there are three households in the experimental system, and each house has a gas engine cogeneration. In this research, one of them is a real gas engine cogeneration, and other two houses use dc power supplies as gas engine cogenerations. It was confirmed by the experiment that the dc power supply could imitate as the gas engine cogeneration. The dc/dc converters (buck chopper) are connected between distributed generations and dc lines. At the load side, the dc power is converted into the single phase 100V with the inverter at one house, and other two houses are supplied the dc power directly. A variable resistor and an electric load device are used as a load of each house. To check the influence by the distance of the dc distribution line, the distance from the rectifier to two houses was assumed to be 100 m, and the resisters 0.5 and the reactors 30 H were installed in the dc distribution line as a line impedance (VV-F cable, 5.5 mm2, it calculated used the data of [8]). It is also assumed to be about 100 m between two houses and one house, resisters 1 and reactors 30 H were installed (VV-F cable, 2 mm2). Moreover, a dc circuit breaker is connected to each house (the rated voltage: DC 600 V, and the rated current: 15 A).

B. Element Devices The circuit of the system is shown in Figure 5, and the

main parameters are shown in Table 1. Outlines of the devices which composed the system are described below. As a gas engine cogeneration system, we chose a

commercial one (the rated capacity 1 kW). The ac output of the generator is converted into DC 390-400 V by the rectifier. Nomaly, the dc power is converted to a single phase 200 V and connected to the bulk power system, but

Fig. 5. Circuit of the experimental system.

Fig. 6. Image of the dc power output.

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TABLE II CONDITION OF THE SYSTEMS

House 1 House 2 House 3

No. Title Load [kW]

DG Output Power [kW]

Load [kW]

DG Output Power [kW]

Load [kW]

DG Output Power [kW]

41 Load step variation 0 ON 1 1 0 1 0

4-2 Voltage sag 1.2 0 0 0 1 0

43 Short Circuit 1.2

Short Circuit 1 2 1 1 1

we modified it to be able to take out the power from the dc line directly as shown in Figure 6. The voltages of the dc power supplies to simulate the gas engine cogenerations were set to be 400V.

The rectifier connected to the bulk power system is controlled to keep the dc voltage become 340 V (= 170 V + 170 V) constant. In details, the current control based on a dq decoupling control is adopted. The current reference is calculated from the dc voltage reference and the feedback value. The control time constant for dc voltage control was set to be 15 ms.

The inverter which supplies a single phase 100 V is composed of one leg. The one side of the output lines is a neutral line. The control is a voltage feedback control with a current minor loop.

The buck choppers control the output powers of gas engine cogenerations. The circuit was designed to become symmetry against the neutral line. When they are turned on or turned off, the output power is changed gradually from 0 to 1 kW or from 1 to 0 kW, and it takes 1 second to avoid the damage to the generator.

IV. EXPERIMENTAL RESULTS Various experiments were carried out by using the

experiment system. In this paper, the experimental results of the step change of the load and the voltage sag of the bulk power system were shown. Table 2 shows the condition of each house in each experiment.

A. Step Change of the load The experiments of a sudden load variation were examined

to confirm the stability of the system. Figure 7 shows the experimental results of the load step change in House 1. Before the load change, the distributed generation of House 1 supplied 1 kW, and the power was transported to other houses. An electrical heater was used as the load of House 1 (rated power consumption 1.2 kW). The graphs on the upper left are the ac side voltage and current of the rectifier. The upper right graphs are the dc distribution voltage, the rms value of the ac output voltage and ac output current of House 1. The lower left is a dc voltage and currents of House 2. The dc voltage and the current of House 3 are shown in lower right. The voltage between the +170 V and neutral line is named the positive side dc voltage side, and the voltage between a neutral line and -170 V is named the negative side dc voltage.

The rms value of the single phase inverter output voltage changed slightly when the load was changed, but it was satisfied the voltage was within the range of Japanese allowed voltage 1016 V. The load current increasing gradually was the characteristic of the electrical heater. It was confirmed power supply was steady against the sudden load variation, and it did not cause any big disturbances.

B. Voltage Sag of the Bulk Power System The experimental results of the voltage sag at the bulk

power system are shown in Figure 8. The voltage sag was imitated by using a multipurpose power supply, and the voltage was set to decrease 20 % for 0.5 s. All distributed generations are turned off. When the voltage sag occurs, the dc voltage is controlled constant by the rectifier, and the current on the ac side of the rectifier was increased to keep the power from the bulk power system constant. As a result, it was confirmed that the voltage change of the dc power line was almost negligible, and there was no influence in the power supply to the load. Although a periodic change was seen in the rms value of the single phase inverter output voltage, but this is a control characteristics of the inverter. From these results, it was confirmed that the power supply to the dc microgrid was steady even if the voltage sag was occurred.

C. Short Circuit at the load This experiment was carried out to confirm that a short

circuit at a load did not affect other loads. To make a short circuit situation, a small resistance (2 ) was connected in parallel at the ac load terminal of the house 1. When the instantaneous output current of the inverter became 50 A, an over current protection was worked. Figure 9 shows the experimental results. At around -0.5 s, the short circuit was occurred, and the inverter output was stopped by the over current protection. It shows that the dc distribution line voltage was stable, and the other loads were not affected by the short circuit. After the inverter was stopped, the output power flow of the gas engine cogeneration at House 1 was transmitted to other houses. It was shown that this changing of the power flow was done smoothly, and the power sharing by the dc distribution line was stable against the accident.

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Fig. 7. Experimental results of a step variation of load power consumption.

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Fig. 8. Experimental results of a voltage sag of the bulk power system.

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Fig. 9. Experimental results of a short circuit of load in House 1.

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V. CONCLUSIONS In this paper, we proposed the concept of the dc microgrid

for residential houses where each house has a cogeneration system such as a gas engine, and shares the power among the houses by the dc distribution line. To examine the fundamental characteristics of the system, a laboratory scale dc microgrid was constructed. The experimental results by a small scale model demonstrated that the system was able to supply a high quality power to the loads against a sudden load variation. When a voltage sag occurred in the bulk power system, it was confirmed that the high quality power supply was continued to the load without any influences. It was also confirmed that a short circuit accident at one load did not affect the power supplying to the other loads.

We are now setting up an EDLC (18 F, rated voltage 160 V) and a boost chopper as an energy storage device for the system. After installing the EDLC, we will examine the fundamental characteristics at the intentional islanding.

REFERENCES [1] T. Ise, Functions and configurations of quality control center on

FRIENDS, Transmission and Distribution Conference and Exhibition 2002: Asia Pacific. IEEE/PES, vol 1, 2002, pp. 590-595.

[2] Al. Domijan, Jr, A. Montenegro, A. J. F. Keri, and K. E. Mattern, Simulation Study of the Worlds First Distributed Premium Power Quality Park, IEEE Trans. Power Delivery, vol. 20, no. 2, 2005, pp. 1483-1492.

[3] R. H. Lasseter, and P. Paigi, Microgrid: A Conceptual Solution, 35th Annual IEEE Power Electronics Specialists Conference, Germany, 2004, pp. 4285-4290.

[4] M. Barnes, G. Ventakaramanan, J. Kondoh, R. Lasseter, H. Asano, N. Hatziargyriou, J. Oyarzabal, T. Green, Real-World MicroGrids- An Overview, System of Systems Engineering 2007, IEEE, pp.1-8.

[5] H. Kakigano, Y. Miura, T. Ise, and R. Uchida, DC Micro-grid for Super High Quality Distribution- System Configuration and Control of Distributed Generations and Energy Storage Devices , 37th Annual IEEE Power Electronics Specialists Conference, Korea, 2006, pp. 3148-3154.

[6] H. Kakigano, Y. Miura, T. Ise, and R. Uchida, DC Voltage Control of the DC Micro-grid for Super High Quality Distribution, The Fourth Power Conversion Conference, Japan, 2007, pp. 518-525.

[7] Y. Hayashi, S. Kawasaki, T. Funabashi and Y. Okuno, Power and Heat Interchange System using Fuel Cells in Collective Housing, The Fourth Power Conversion Conference, Japan, 2007, pp. 1207-1211.

[8] SWCC Showa Cable Systems Co., LTD Web Site. Available: http://www.swcc.co.jp/cs/index.htm

Hiroaki Kakigano (M06) was born in 1976. He received the B.S. and M.S. degree in nuclear engineering from Nagoya University, Japan in 1999 and 2001, respectively. In 2001, he joined Nissin Inc., where he had worked in an electrical technology. Currently he is an assistant professor in the Division of Electrical, Electronic and Information Engineering, Osaka University, Japan. His research interests include the power electronics and the new power distribution system.

Yushi Miura (M06) received doctorate in Electrical and Electronic Engineering from Tokyo Institute of Technology in 1995. From 1995 to 2004, he joined Japan Atomic Energy Research Institute as a researcher and developed power supplies and superconducting coils for nuclear fusion reactors. Since 2004, he has been an associate professor of the Division of Electrical, Electronic and Information Engineering of Osaka University. His areas of research involve applications of power electronics and superconducting technology. Currently he is interested in control of distributed generations and energy storages in the power systems.

Toshifumi Ise (M87) was born in 1957. He received the Bachelor, Master, and Dr. of Engineering degrees in electrical engineering from Osaka University, Osaka, Japan, in 1980, 1982, and 1986, respectively. Currently, he is a Professor with the Division of Electrical, Electronic and Information Engineering, Faculty of Engineering, Osaka University, where he has been since 1990. From 1986 to 1990, he was with the Nara National College of Technology, Nara, Japan. His research interests are in the areas of power electronics and applied superconductivity including superconducting magnetic energy storages (SMES) and new distribution systems. Dr. Ise is a member of the Institute of Electrical Engineers of Japan and the Japan Society for Power Electronics.

Toshinari Momose (Non-member) was born in 1969. He received the B.E. and M.E. degree in from Osaka University, Osaka, Japan in 1992 and 1994 respectively. From 1994 to 2003, he joined at the Gas Appliance Development Department of Osaka Gas Co., Ltd. and From 2003 he joined at the Energy Technology Laboratories of it. He received a Dr. degree in from Osaka University in 2005.

Hideki Hayakawa (Non-member) received the B.E. and M.E from Osaka University in 1985 and 1987, respectively, then joined the Research Center, Osaka Gas Co., Ltd. From 1990 to 1993, he was a researcher at ATR Auditory and Visual Perception Research Laboratories. He obtained a Ph.D. from Osaka University in 1994. He is now research supervisor at the Energy Technology Laboratories, Osaka Gas. Co., Ltd.

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