Download Low-Voltage Bipolar-Type DC Microgrid for Super High Quality

3066
IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 25, NO. 12, DECEMBER 2010
Low-Voltage Bipolar-Type DC Microgrid for Super
High Quality Distribution
Hiroaki Kakigano, Yushi Miura, and Toshifumi Ise, Member, IEEE
Abstract—Microgrid is one of the new conceptual power systems
for smooth installation of many distributed generations (DGs).
While most of the microgrids adopt ac distribution as well as
conventional power systems, dc microgrids are proposed and researched for the good connection with dc output type sources such
as photovoltaic (PV) system, fuel cell, and secondary battery. Moreover, if loads in the system are supplied with dc power, the conversion losses from sources to loads are reduced compared with ac
microgrid. As one of the dc microgrids, we propose “low-voltage
bipolar-type dc microgrid,” which can supply super high quality
power with three-wire dc distribution line. In this paper, one system for a residential complex is presented as an instance of the dc
microgrid. In this system, each house has a cogeneration system
(CGS) such as gas engine and fuel cell. The output electric power
is shared among the houses, and the total power can be controlled
by changing the running number of CGSs. Super capacitors are
chosen as main energy storage. To confirm the fundamental characteristics and system operations, we experimented with a laboratory
scale system. The results showed that the proposed system could
supply high-quality power under several conditions.
Index Terms—DC microgrid, distributed generation (DG), highquality power, intentional islanding, three-wire dc distribution.
I. INTRODUCTION
NERGY and environmental problems are remarkably concerned in recent years, such as greenhouse gas, growth of
energy demand, and depletion of energy resources. Against the
background of these problems, a large number of distributed
generations (DGs) are being installed into power systems. It is
well known that if many DGs are installed into a utility grid, they
can cause problems such as voltage rise and protection problem.
To solve these problems, new conceptual electric power systems
were proposed. As one of the concepts, microgrids are especially
researched all over the world [1]– [10]. For example, in Japan,
microgrid projects are promoted by several private companies
and The New Energy and Industrial Technology Development
Organization (NEDO). Four popular NEDO’s projects (Aichi
Expo, Kyotango, Hachinohe, and Sendai) were undertaken from
FY2003 to FY2007 (Sendai: FY2004–FY2007), and the details
are reported in [9] and [10]. NEDO also promoted other microgrid projects in Asian countries to research under several
conditions.
E
Manuscript received July 1, 2010; revised August 20, 2010 and August 31,
2010; accepted September 4, 2010. Date of current version December 27, 2010.
This work was 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. Recommended for publication by
Associate Editor J. M Guerrero
The authors are with the Division of Electrical, Electronic, and Information
Engineering, Osaka University, Osaka, Japan (e-mail: [email protected]; [email protected]; [email protected]).
Digital Object Identifier 10.1109/TPEL.2010.2077682
Including those projects, most microgrids adopt ac distribution as well as conventional power systems. In this case, dc
output type sources, such as photovoltaic (PV) system, fuel cell,
and energy storages (e.g., Li-ion secondary battery and super
capacitor) need inverters. In addition, some gas engine cogenerations (GECs) and wind turbines also need inverters because
the output voltages and the frequencies are different from those
of the utility grids. Therefore, dc distribution-type microgrids
(dc microgrids) were also proposed and researched in order to
reduce conversion losses from the sources to loads [11]– [15].
The advantages of dc microgrids are summarized as follows.
1) The system efficiency becomes higher because of the reduction of conversion losses of inverters between dc output
sources and loads [15].
2) There is no need to consider about synchronization with
the utility grid and reactive power.
3) When a blackout or voltage sag occurs in the utility grid, it
does not affect the dc bus voltage of dc microgrid directly
due to the stored energy of the dc capacitor and the voltage
control of ac/dc converter. Therefore, DGs in dc system are
not easy to trip against these disturbances. In other words,
dc microgrid already has fault-ride-through capability of
its own.
On the other hand, there are some drawbacks to put dc microgrid to practical use as follows.
1) It is needed to construct private dc distribution lines for dc
microgrid.
2) The protection in dc system is more difficult than that
of the ac system because there is no zero cross point of
voltage in dc system.
3) The loads adapted for dc power supply are required for
high system efficiency.
As an instance of dc microgrids, the system described in [14]
adopted dc 380 V as dc bus voltage. The system has PV systems
(2 × 10 kW), wind generator systems (10 kW + 2 kW), and
storage battery (97 kW), but there are no controllable DGs such
as gas engine or fuel cell. The system is normally operated
in islanding mode. When the storage energy becomes low, the
system is supplied power from the utility grid, and charges the
battery. It is also unique that the system can be changed into ac
microgrid by the switches in order to compare dc system with
ac system.
On the other hand, high-quality power is essential for some
customers such as banks, hospitals, and semiconductor factories because the downtime related to voltage sag or blackout
becomes a great concern. Besides, high-quality power is also
requested in our dependable society. Security of electric power
is becoming more important in our daily life.
0885-8993/$26.00 © 2010 IEEE
KAKIGANO et al.: LOW-VOLTAGE BIPOLAR-TYPE DC MICROGRID FOR SUPER HIGH QUALITY DISTRIBUTION
Fig. 1.
3067
Concept of low-voltage bipolar-type dc microgrid.
To satisfy high efficiency and high-quality power supply, we
proposed “low-voltage bipolar-type dc microgrid” [16]. In this
system, dc power is distributed through three-wire lines, and it is
converted to required ac or dc voltages by load-side converters.
When blackout or voltage sag occurs in the utility grid, the dc
microgrid can supply high-quality power stably, while inverters
of DGs in ac microgrids should be tripped unless they have
fault-ride-through capability.
In this paper, a dc microgrid for a residential complex is presented based on the dc microgrid concept [17]. Each house has a
cogeneration system (CGS) such as gas engine or fuel cell. The
electric power from CGSs is shared among the houses with dc
distribution line, and the total power can be controlled by changing the number of the running CGSs. As main energy storage,
super capacitors can be chosen in spite of its low energy density.
To confirm the fundamental characteristics and system operation, we did some experiments with a laboratory scale system.
The results show that the proposed system could supply highquality power under several situations. Additionally, smooth
disconnection and reconnection with the utility grid were also
demonstrated.
The composition of this paper is as follows. Section II describes the configuration and features of low-voltage bipolartype dc microgrid. Section III shows a concrete configuration of the dc microgrid for a residential complex, and explains the characteristics and operation methods. Section IV
explains the circuit and parameters of the experimental system.
Section V shows the experimental results that demonstrated the
high-quality power supply of our proposed system. Finally, we
summarize the outcomes in Section VI.
II. LOW-VOLTAGE BIPOLAR-TYPE DC MICROGRID
Fig. 1 shows a concept of the low-voltage bipolar-type dc
microgrid. The utility grid voltage 6.6 kV is converted into dc
340 V by a transformer and a rectifier. It is the characteristic
of the system to adopt three-wire dc distribution that consists
of +170 V line, neutral line, and −170 V line. The three-wire
composition contributes that the voltage to ground becomes
low, and one of the single-phase 100-V output lines becomes a
grounded neutral line as well as Japanese standard. In addition,
load-side dc/dc converters can choose the source voltage from
340 V, +170 V, or −170 V. Moreover, if one wire snaps out, it
is possible that the power is supplied by the other two lines and
an auxiliary converter.
When there are dc/dc converters for loads, and the source
voltages of them are either +170 V or −170 V, dc voltage
balance control is essential [18]. Hence, a voltage balancer is
placed near a rectifier to balance positive and negative voltages.
It is also possible the voltage balancer is placed near load side.
As energy storages, a secondary battery and a super capacitor
[electric double-layer capacitor, (EDLC)] are connected to dc
distribution line. A PV system and a GEC are also connected
through dc/dc converter and ac/dc converter, respectively. At the
load side, dc power is converted into required ac or dc voltages
by each converter. Characteristics of the system are summarized
as follows.
3068
IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 25, NO. 12, DECEMBER 2010
1) Three-wire bipolar dc distribution contributes to lower the
distribution voltage to ground. In addition, it allows dc/dc
converters in load side choose source voltage from 340 V,
+170 V, or −170 V.
2) The distribution of the load-side converters contributes
to provide a super high quality power supplying. For instance, even if a short circuit occurs at one load side, it
does not affect other loads.
3) Various forms of electric power like single-phase 100 V,
three-phase 200 V, dc 48 V can be obtained. These converters are transformerless, therefore, it contributes to the
downsizing and high efficiency.
4) DC distribution system is suitable for dc output type DGs
and energy storages. If dc power can be supplied to loads
directly or through dc/dc converters, the system efficiency
becomes high.
5) When an accident occurs in a utility grid, this system
can be disconnected from the utility grid seamlessly and
supply electric power continuously. The reconnection to
the utility grid is also smooth.
6) When a temporary overload occurs at a single-phase load,
electric power can be shared between load-side converters
by using additional electric power lines.
It is also possible to form dc loop configuration at dc distribution part [19] and share power between other dc microgrid
systems.
Fig. 2.
System configuration of the dc microgrid for residential complex.
Fig. 3.
Interconnected operation.
III. DC MICROGRID FOR RESIDENTIAL COMPLEX
A. System Configuration
As a concrete system, we propose a dc microgrid for a residential complex shown in Fig. 2. There are around 50–100 houses
in the system, and each house has a CGS (gas engine or a fuel
cell). The CGSs are connected to dc distribution line (three-wire,
±170 V), and the electric power is shared among houses. Form
this configuration, it is expected that the total CGSs operation
period increases and it leads to effective utilization of primary
energy [20]. In order to keep high efficiency, these CGSs should
not be operated by a partial load condition, but operated by a
start/stop control. Cogenerated hot water is used in each house or
shared with next houses. This system connects to the utility grid
by a rectifier. At load side, various forms of electric power (ac
100 V, dc 48 V, etc) can be obtained by the converters. Despite
of low energy density, EDLCs are used as main energy storage
because of the fast response, the safeness (especially compared
with Li-ion battery), the easy measurement of the stored energy,
and no toxicity of the inner materials.
B. System Operation
The total generated power is controlled by changing the number of running CGSs. When the system connects to the utility
grid, the deficient power is compensated from the utility grid, as
shown in Fig. 3. We call this state interconnected operation. In
this operation, the rectifier controls the dc distribution voltage,
and the supervisor computer changes the number of the running
CGSs so that the power from the utility grid is within the con-
tract demand. In other words, the generated power from CGSs
does not flow to the utility grid in the interconnected operation.
When the system disconnects from the utility grid, the surplus
or deficient power is compensated by the EDLC, as shown in
Fig. 4. We call this state as intentional islanding operation. In this
operation, the converter of the EDLC controls the dc distribution
voltage, and the number of the running CGSs is determined by
not only the load consumption, but also the stored energy of
EDLCs. When the stored energy becomes over a maximum
limit, the system stops one of the operating CGSs. Then, the
total output of CGSs becomes less than the load consumption,
and the EDLC discharges until the stored energy becomes less
than the minimum limit. On the contrary, when the stored energy
becomes under the minimum limit, the system starts a CGSs.
Then, the total output of CGSs becomes more than the load
consumption, and the EDLC charges until the stored energy
becomes more than the maximum limit. These two modes are
repeated alternately in the intentional islanding operation.
This system should choose the CGSs that can start up for
a few minutes, so that energy storage does not need a large
capacity. Therefore, EDLC can be used as main energy storage
in this system. The output capacity of EDLC and its converter are
KAKIGANO et al.: LOW-VOLTAGE BIPOLAR-TYPE DC MICROGRID FOR SUPER HIGH QUALITY DISTRIBUTION
Fig. 4.
3069
Intentional islanding operation.
designed to compensate the assumed maximum load variation.
However, if larger load variation occurs in the system, the output
of EDLC could reach the maximum charge or discharge level. If
the charge is limited, the supervisor computer stops the required
number of CGSs. If the discharge is limited, the supervisor
computer stops power supplying to specific loads that do not
need high-quality power, and runs CGSs to make the output of
EDLC become within the limits.
Fig. 5.
Flowcharts of (a) disconnection and (b) reconnection.
C. Voltage Clamp Control
When the system is in the interconnected operation, the EDLC
does not charge or discharge unless the dc distribution voltage
exceeds a limited range. We propose a voltage clamp control
by the EDLC. When the dc distribution voltage increases over
the upper limit (360 V), the dc/dc converter of the EDLC is
operated to clamp the dc voltage at the level. This clamp control
contributes to prevent overvoltage of the devices connected to
the dc line. The converter of the EDLC is also operated to clamp
at the lower limit (320 V) when the dc voltage decreases to the
lower limit, e.g., the current from the utility grid is limited by the
capacity of the rectifier. In addition, this clamp control assists
the disconnection and reconnection process, as described in the
following section.
D. Disconnection and Reconnection With Utility Grid
We propose disconnection and reconnection procedures with
the voltage clamp control. Fig. 5(a) shows the flowchart of the
disconnection procedure. When a problem of the utility grid
is detected, the system stops the grid interface rectifier. Then,
the dc distribution voltage decreases, and the converter of the
EDLC clamps the voltage at the lower limit (320 V). After this,
the converter of the EDLC changes its operation from the clamp
control to the dc voltage control. Then, the voltage reference is
gradually changed from the lower clamping limit to the nominal
dc distribution voltage (340 V).
Without the clamp control, the voltage control has to be immediately moved from the rectifier to the converter of the EDLC.
However, there should be some delay in the communication net-
TABLE I
MAIN PARAMETERS
work. Therefore, this clamp control plays an important role to
complete the disconnection certainly.
Fig. 5(b) shows the flowchart of the reconnection procedure,
which is opposite to the disconnection process. After the utility
grid is recovered, the controller of the rectifier detects the phase
of the utility grid by phased-lock loop (PLL). Then, the system
changes the operation of EDLC from voltage control to clamp
control, and dc distribution voltage decreases to the lower limit
(320 V). After this, the system starts the rectifier, and the voltage
reference is changed gradually from the lower clamping limit to
the nominal voltage.
In the experiment described later, it is judged by the voltage
drop level and the period whether the problem occurs in the
system or not. If the voltage decreases less than 30%, the system
judges there is a problem in the utility grid. If the voltage drop
is between 30% and 80%, and the period is more than 1 s, the
system also judges that the utility grid has a problem. In this
case, this system does not disconnect from the utility grid if
voltage sag occurs, because the period of the most voltage sag
is usually within 0.5 s.
3070
IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 25, NO. 12, DECEMBER 2010
Fig. 6.
V–I curve of constant power load.
Fig. 7.
Simplified dc distribution circuit.
Fig. 8.
Stability condition of simplified circuit.
Fig. 9.
Target model in this study.
E. Evaluation of System Stability
It is well known that negative incremental impedance of converters can cause stability problems in dc power supply system [21]– [23], because these converters are operated like constant power load. Fig. 6 shows a V–I curve of constant power
load. The negative incremental impedance r can cause a voltage oscillation on the distribution line. Fig. 7 shows a simplified
dc distribution circuit. In case that the source voltage is instantaneously changed, the current and voltage dynamics can be
Fig. 10.
Simulation circuit.
Fig. 11.
Equivalent circuit and control block of rectifier.
represented by the following equations
Δv = r Δi
ΔVs = RΔiL + L
ΔiL = C
(1)
dΔiL
+ Δv
dt
dΔv
+ Δi
dt
(2)
(3)
where ΔVs , ΔIL , and Δv are the instantaneous variations of
source voltage, inductor current, and load voltage, respectively.
KAKIGANO et al.: LOW-VOLTAGE BIPOLAR-TYPE DC MICROGRID FOR SUPER HIGH QUALITY DISTRIBUTION
Fig. 12.
3071
Circuit of the experimental system.
Using (1)–(3), we obtain the following state equation
⎡ P
1 ⎤
d Δv
Δv
0
2
Cv
C
⎦
⎣
+
ΔVs .
=
R
1
Δi
L
Δi
dt
−
−
L
L
(4)
From (4), the stability condition can be calculated by Hurwitz
criterion. For example, when the parameters are set to be L =
0.1(mH) and C = 10(mF), and VS = 340(= 170 + 170)(V),
the stability condition can be drawn, as shown in Fig. 8. The
stable condition is in the shaded area. There are two unstable
areas. The upper one is an area, where the power cannot be
supplied to the load because of the voltage drop of resistance
R. The lower one is an area, where the resonance is caused by
inductance L and a smoothing capacitor C. The oscillation in
an upper unstable area does not happen in a practical system
because proper sectional size conductor will be selected based
on the system capacity. On the other hand, there is a possibility
that the oscillation in a lower unstable region happens when the
resistance R or the capacitance C is small, or the inductance L
is large. Therefore, the stability of proposed dc microgrid was
examined by the computer simulation (MATLAB/Simulink).
Fig. 9 shows the target model. It is an apartment of ten floors
with two houses per one floor. The rectifier is controlled to keep
the distribution voltage constant (dc 340 V). Fig. 10 shows the
simulation circuit. The main parameters are shown in Table I.
The line resistances and line inductances are calculated based on
the impedance of unit length (1 km) of the CV cable. Each house
that includes inverter and CGS was assumed to be a constant
power load. Fig. 11 shows the circuit and control block of the
rectifier. The rectifier was represented by a current source and
a smoothing capacitor. The current minor loop of the control
block was simulated as a first-order delay. The time constant
Tim was set to be 2 ms.
From the simulation results, voltage oscillation did not occur
even when the line resistances were set to be smaller than 1/200
of the original value shown in Table I. From the economical
point of view, it is not practical the line resistances are so small.
In addition, it was confirmed that the oscillation did not occur
under considerable situations such, as the looped distribution
line, power sharing among houses (P was set to be less than 0
at some houses), etc.
IV. EXPERIMENTAL SYSTEM
To examine the fundamental characteristics and the proposed
system operations, we constructed a laboratory scale experimental system. The circuit and main parameters are shown in Fig. 12
and Table II. It is assumed that there are three households, and
each house has a GEC. Fig. 13 shows the system appearance,
and Fig. 14 shows a GEC used in house 1. We use a commercial
GEC with the rated capacity of 1 kW. Fig. 15 shows the inside
configuration of the GEC. The generator outputs AC 340 V,
307.5 Hz, and the ac power is converted into dc 390–400 V by
3072
IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 25, NO. 12, DECEMBER 2010
TABLE II
MAIN PARAMETERS OF EXPERIMENTAL SYSTEM
a thyristor-diode rectifier. Normally, the dc power is converted
to single-phase AC 200 V and flown to the utility grid, but we
modified to take the dc power directly. In the other two houses,
we substitute dc power supply for the real GEC, which output
dc 400 V constantly.
Buck choppers are connected between GECs and dc distribution lines because the voltage of the gas engine (400 V) is
different from the distribution voltage (340 V). We designed the
circuit to be symmetry against the neutral line, because this converter is supposed to be worked as a voltage balancer under an
appropriate control. For switching devices of this converter, we
adopted super junction MOSFETs (SPW16N50C3, Infineon)
with due regard to the converter efficiency. The output power is
changed gradually from 0 to 1 kW or from 1 to 0 kW for 1 s.
In house 1, dc power is converted into single-phase 100 V
by a half-bridge inverter, and voltage feedback control with a
current minor loop is adopted. In houses 2 and 3, dc power is
supplied to each electronic load directly, which substituted for
an inverter and loads.
The rectifier connected to the utility grid is controlled to keep
the dc voltage constant (340 V) in the interconnected operation.
The circuit is a conventional two-level voltage source converter,
and current control based on d–q decoupling control is adopted.
The current reference is calculated from the dc voltage reference and the feedback value. The control time constant of the
voltage control was set to be 15 ms. To balance positive voltage
(+170 V) and negative voltage (−170 V), a voltage balancer is
connected at the dc side of the rectifier, as shown in Fig. 6.
As energy storage, EDLC is connected through dc/dc converter. The rated voltage is 160 V, and the capacitance of one
EDLC is 4.5 F. Four EDLCs are connected directly in parallel
without any additional circuits, so the total capacitance is 18 F.
We use two digital controllers (PE-Expert2, Myway Plus)
for this system. The sampling frequency is 10 kHz. One of
the controllers is for rectifier, dc voltage balancer, single-phase
inverter, and converter of EDLC. Another controller is for three
dc/dc converters for GEC. Those controllers can communicate
each other.
It is assumed that the distance between the rectifier and two
houses is 100 m, and single conductor cable (5.5 mm2 ) is used
Fig. 13.
Laboratory scale dc microgrid experimental system.
Fig. 14.
Gas engine unit and hot water tank.
Fig. 15.
Image of the dc power output.
TABLE III
CONDITION OF EACH EXPERIMENT
KAKIGANO et al.: LOW-VOLTAGE BIPOLAR-TYPE DC MICROGRID FOR SUPER HIGH QUALITY DISTRIBUTION
Fig. 16.
3073
Experimental results of voltage sag (50 %, 0.5 s).
as the dc distribution line. To simulate the influence, we placed
resisters (0.5 Ω) and reactors (30 μH). It is also assumed that
single conductor cable (2.5 mm2 , 100 m) is used between two
houses and one house, and we placed resisters (1 Ω) and reactors
(30 μH).
V. EXPERIMENTAL RESULTS
Various experiments were carried out with the experiment
system such as load variation, GEC operation, short circuit at a
load, and etc [6]. In this paper, we show three kinds of experimental results: voltage sag of the utility grid; disconnection procedure; and reconnection procedure. Table III shows the loads
and GECs conditions in each experiment.
A. Voltage Sag of the Utility Grid
The experimental results of the voltage sag in the utility grid
are shown in Fig. 16. The voltage sag was simulated by a multipurpose power supply, and the voltage sag was programmed to
decrease 50% for 0.5 s. When the voltage sag occurred, the dc
voltage was controlled constant by the rectifier, and the current
on the ac side of the rectifier increased to keep the power from
the utility grid. As a result, it was confirmed that the dc voltage
fluctuation was almost negligible, and the power supplied to
all loads stably. There were the voltage drops by the line resis-
tances, but the line resistances and inductances did not affect
the system operation.
B. Disconnection From the Utility Grid
Fig. 17 shows the experimental results of the disconnection
procedure. At the initial condition, the system was in the interconnected operation. When the system detected that the voltage
of the utility grid was lower than the 30% of the nominal voltage, the rectifier was stopped. Then, the dc distribution voltage
decreased, and the converter of EDLC clamped it at the lower
limit (320 V). After this, the converter control was changed from
the clamp control to the dc voltage control. Finally, the voltage
increased to 340 V gradually for 1 s, and the system was in the
intentional islanding operation. In this period, the rms value of
the single-phase inverter voltage, which was shown as “house 1
inverter output voltage (rms),” was not affected, and the smooth
disconnection was confirmed from the results. It was also confirmed the line resistances and inductances did not affect the
disconnection process.
C. Reconnection With the Utility Grid
Fig. 18 shows the experimental results of the reconnection
procedure. When the system detected the utility grid was recovered from a problem, the controller of the rectifier detects the
3074
IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 25, NO. 12, DECEMBER 2010
Fig. 17.
Experimental results of disconnection from the utility grid.
Fig. 18.
Experimental results of reconnection from the utility grid.
phase of the utility grid by PLL. Then, the control of EDLC’s
converter was changed from the dc voltage control to the clamp
control. Then, the dc voltage decreased to the lower clamp limit,
and the rectifier started and controlled the dc voltage. Finally,
the voltage gradually increased to 340 V for 1 s, and the system
was in the interconnected operation. In this period, the rms value
of the single-phase inverter voltage was not affected as well, and
the smooth reconnection was also confirmed. In addition, it was
also confirmed that there are no affects by the line resistances
and inductances.
In this paper, we show the disconnection and reconnection
results under the condition that all GEC were turned off so
that all power was supplied from the utility grid. Surely, we
confirmed in the case that some GEC were tuned on, and those
results also showed the smooth disconnection and reconnection.
for residential complex, where each house has a CGS such as
a gas engine, and the power is shared among the houses by
dc distribution. To confirm the fundamental characteristics and
the proposed operations, a laboratory scale experimental system
was constructed. Several kinds of experiments were carried out
such as a sudden load variation, short circuit at a load, interconnected operation, intentional islanding operation, supplying
commercial home appliances, etc. As a part of the results, we
showed the results of voltage sag in the utility grid, disconnection procedure, and reconnection procedure in this paper. These
results demonstrated that the system could supply high-quality
power to loads in those conditions. The mathematical analysis
of the system stability during proposed disconnection or reconnection process is a next study.
VI. CONCLUSION
ACKNOWLEDGMENT
For smooth introduction of a number of DGs, we proposed
“low-voltage bipolar-type dc microgrid” to satisfy high efficiency and high-quality power supply. We presented a system
The authors would like to thank T. Momose and H. Hayakawa
(Osaka Gas CO., LTD) for their suggestions and supports that
made it possible to complete this study.
KAKIGANO et al.: LOW-VOLTAGE BIPOLAR-TYPE DC MICROGRID FOR SUPER HIGH QUALITY DISTRIBUTION
REFERENCES
[1] “Microgrids,” IEEE Power Energy Mag., vol. 6, no. 3, pp. 26–94, 2008.
[2] M. Barnes, G. Ventakaramanan, J. Kondoh, R. Lasseter, H. Asano,
N. Hatziargyriou, J. Oyarzabal, and T. Green, “Real-World microgrids:
An overview,” in Proc. IEEE Int. Conf. Syst. Syst. Eng. (SoSE), Apr. 2007,
pp. 1–8.
[3] H. Nikkhajoei and R. H. Lasseter, “Distributed generation interface to the
CERTS microgrid,” IEEE Trans. Power Del., vol. 24, no. 3, pp. 1598–
1608, Jul. 2009.
[4] J. C. Vasquez, J. M. Guerrero, A. Luna, P. Rodrı́guez, and R. Teodorescu,
“Adaptive droop control applied to voltage-source inverters operating in
grid-connected and islanded modes,” IEEE Trans. Ind. Electron., vol. 56,
no. 10, pp. 4088–4096, Oct. 2009.
[5] E. Barklund, N. Pogaku, M. Prodanovic, C. Hernandez-Aramburo, and T.
C. Green, “Energy management in autonomous microgrid usingstabilityconstrained droop control of inverters,” IEEE Trans Power Electron.,
vol. 23, no. 5, pp. 2346–2352, Sep. 2008.
[6] Y. Abdel-Rady, I. Mohamed, and E. F. El-Saadany, “Adaptive decentralized droop controller to preserve power sharing stability of paralleled
inverters in distributed generation microgrids,” IEEE Trans Power Electron., vol. 23, no. 6, pp. 2806–2816, Nov. 2008.
[7] Y. Li and C. Kao, “An accurate power control strategy for powerelectronics-interfaced distributed generation units operating in a lowvoltage multibus microgrid,” IEEE Trans. Power Electron., vol. 24, no. 12,
pp. 2977–2987, Dec. 2009.
[8] Lawrence Berkeley National Laboratory, Microgrid Symposium Website
[Online]. Available: URL: http://der.lbl.gov/microgrid-symposiums.
[9] S. Morozumi, “Micro-grid demonstration projects in Japan,” in Proc. 4th
Power Convers. Conf., Japan, 2007, pp. 635–642.
[10] N. Yamato, A. Fukui, and K. Hirose, “Effect of breaking high voltage
direct current (HVDC) circuit on demonstrative project on power supply
systems by service level in Sendai,” in Proc. 29th Int. Telecommun. Energy
Conf. (INTELEC), 2007, pp. 46–51.
[11] Y. Ito, Y. Zhongqing, and H. Akagi, “DC microgrid based distribution
power generation system,” in Proc. IEEE 4th Int. Power Electron. Motion
Control Conf. (IPEMC), vol. 3, 2004, pp. 1740–1745.
[12] D. Salomonsson, L. Soder, and A. Sannino, “An adaptive control system
for a DC microgrid for data centers,” IEEE Trans. Ind. Appl., vol. 44,
no. 6, pp. 1910–1917, Nov./Dec. 2008.
[13] J. M. Guerrero, J. C. Vásquez, and R. Teodorescu, “Hierarchical control
of droop-controlled DC and AC microgrids: A general approach towards
standardization,” IEEE Conf. Ind. Electron. (IECON), pp. 4305–4310,
2009.
[14] K. Yukita, Y. Shimizu, Y. Goto, M. Yoda, A. Ueda, K. Ichiyanagi, K. Hirose, T. Takeda, T. Ota, Y. Okui, and H. Takabayashi, “Study of AC/DC
power supply system with DGs using parallel processing method,” in
Proc. Int. Power Electron. Conf. (IPEC)-ECCE Asia, Sapporo, Japan, 22,
2010, pp. 722–725.
[15] H. Kakigano, M. Nomura, and T. Ise, “Loss evaluation of DC distribution
for residential houses compared with AC system,” in Proc. Int. Power
Electron. Conf. (IPEC)-ECCE Asia, Sapporo, Japan, 2010, pp. 480–486.
[16] 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—,” in Proc. 37th Annu. IEEE
Power Electron. Spec. Conf. (PESC), Korea, 2006, pp. 3148–3154.
[17] H. Kakigano, Y. Miura, T. Ise, T. Momose, and H. Hayakawa, “Fundamental characteristics of DC microgrid for residential houses with cogeneration system in each house,” IEEE Power Energy Soc. General Meeting,
Jul. 2008, pp. 1–8.
[18] H. Kakigano, Y. Miura, T. Ise, and R. Uchida, “DC voltage control of the
DC micro-grid for super high quality distribution,” in Proc. 4th Power
Convers. Conf., Japan, 2007, pp. 518–525.
[19] M. Saisho, T. Ise, and K. Tsuji, “DC loop type quality control center for
FRIENDS-system configuration and circuits of power factor correctors,”
in Proc. Trans. Distrib. Conf. Exhib. 2002: Asia Pac. IEEE/PES, vol. 3,
pp. 2117–2122.
3075
[20] Y. Hayashi, S. Kawasaki, T. Funabashi, and Y. Okuno, “Power and heat
interchange system using fuel cells in collective housing,” in Proc. 4th
Power Conver. Conf., Japan, 2007, pp. 1207–1211.
[21] C. M. Wildrick, F. C. Lee, B. H. Cho, and B. Choi, “A method of defining
the load impedance specification for a stable distributed power system,”
IEEE Trans. Power Electron., vol. 10, no. 3, pp. 280–285, May 1995.
[22] D. L. Logue and P. T. Krein, “Preventing instability in DC distribution
systems by using power buffering,” in Proc. IEEE Power Electron. Spec.
Conf. (PESC’01), vol. 1, pp. 33–37.
[23] X. Feng, J. Liu, and F. C. Lee, “Impedance specifications for stable DC
distributed power systems,” IEEE Trans. Power Electron., vol. 17, no. 2,
pp. 157–162, Mar. 2002.
Hiroaki Kakigano (M’06) was born in 1976. He
received the B.S. and M.S. degrees in nuclear engineering from Nagoya University, Japan, Nagoya, in
1999 and 2001, respectively, and the Ph.D. degree
from Osaka University, Osaka, Japan, in 2009.
He is currently an Assistant Professor at Osaka
University. His research interests include power electronics, microgrids, and dc distribution systems.
Yushi Miura (M’06) received the Ph.D. degree in
electrical and electronic engineering from the Tokyo
Institute of Technology, Tokyo, Japan, in 1995.
From 1995 to 2004, he was a Researcher at Japan
Atomic Energy Research Institute, where he developed power supplies and superconducting coils for
nuclear fusion reactors. Since 2004, he has been
an Associate Professor at the Division of Electrical,
Electronic and Information Engineering, Osaka University, Osaka, Japan. His areas of research include
the applications of power electronics and superconducting technology, as well as the control of distributed generations and energy
storages in the power systems.
Toshifumi Ise (M’87) was born in 1957. He received
the B.Eng., M.Eng., and Ph.D. degrees in electrical
engineering from Osaka University, Osaka, Japan, in
1980, 1982, and 1986, respectively.
Since 1990, he has been with the Division of Electrical, Electronic and Information Engineering, Faculty of Engineering, Osaka University, where he is
currently a Professor. From 1986 to 1990, he was
with the Nara National College of Technology, Nara,
Japan. His research interests include power electronics and applied superconductivity including superconducting magnetic energy storages and new distribution systems.
Dr. Ise is a member of the Institute of Electrical Engineers of Japan and the
Japan Society for Power Electronics.