Smart Grid and Renewable Energy, 2012, 3, 73-82 Published Online May 2012 ( 1
Simulation and Analysis of a Compact Electronic
Infrastructur e fo r D C Mi cro -G ri d: Necessity and
Mohammad Ali Tavakkoli, Ahmad Radan, Houshang Hassibi
Department of Electrical Engineering, K. N. Toosi University of Technology, Tehran, Iran.
Email:, {radan, hassibi}
Received February 14th, 2012; revised March 20th, 2012; accepted March 28th, 2012
Complex circuitry of electronic infrastructure of compact micro-grids with multiple renewable energy sources feeding
the loads using parallel operatio n of inverters acts as a deterrent in developing such systems. This paper deals with ap-
plicable techniques reducing the driving circuits in parallel power inverters used in micro-grid system (MGS), mainly
focused on the distributed generation (DG) in islanded mode. The method introduced in this paper, gives a minimal and
compressed circuitry that can be implemented very cost-effectively with simple components. DC micro-grids are pro-
posed and researched for the good connection with DC output type sources such as photovoltaic (PV), fuel cell, and
secondary battery. In this paper, the electronic infrastructure of micro-grid is expressed. Then discussed the reasons for
its complexity and the possibility of reducing the elements of electronic circuits are investigated. The reason for this is
in order to compact DC micro-grid system for electrification to places like villages. Digital Simulation in Matlab Simu-
link is used to show the effectiveness of this novel driver topology for parallel operating inverters (NDTPI).
Keywords: Drive Circuitry; Integrated Power Electronics Modules; Micro-Grid
1. Introduction
Distributed Generation (DG) is an approach that employs
small-scale technologies to produce electricity close to
the end users of power. Today’s DG technologies often
consist of renewable generators (i.e. solar PV, wind tur-
bines, micro turbines), and offer a number of potential
benefits. In many cases, distributed generators can pro-
vide lower-cost electricity and higher power reliability
and security with fewer environmental consequences than
traditional bulk power generators [1].
“Smart grid” generally refers to a class of technology
people are using to bring utility electricity delivery sys-
tems into the 21st century, using computer-based remote
control and automation. The “grid” amounts to the net-
works that carry electricity from the plants where it is
generated to consumers. The grid includes wires, substa-
tions, transformers, switches and much more. Much in
the way that a “smart” phone these days means a phone
with a computer in it, smart grid means “computerizing”
the electric utility grid [2].
The interest in Distributed Resources (DR), including
both Distributed Generation (DG) and energy storage
resources, is increasing due to their technical, economical,
reliability and environmental merits. Local aggregation
of DR systems and electrical loads results in a Micro-grid.
The Micro-grid concept has provided a new paradigm for
future distribution p ow er systems [3-6].
Micro-Grid is a small-scale grid that is designed to
provide power for local communities. A Micro -Grid is an
aggregation of multiple distributed generators (DGs)
such as renewable energy sources, conventional genera-
tors, in association with energy storage units wh ich work
together as a power supply network. Typically, a Mi-
cro-Grid operates synchronously in parallel with the
main grid. However, there are cases in which a Micro-
Grid operates in islanded mode, or in a disconnected
state. Integration of renewable energy into the utility g rid
can be at either the transmission level or the distribution
level, depending on the scale of generation. Large re-
newable energy generation such as wind farms are di-
rectly interconnected to the transmission system. Small
scale distributed generation is generally interconnected to
the medium or low voltage distribution systems. This
small network can be a residential building, commercial
building, is a market or even a v illage. Micro-grids oper-
ate mostly interconnected to the higher Voltage Distribu-
tion network, but they can also be operated isolated from
the main grid, in case of faults in the upstream network.
Copyright © 2012 SciRes. SGRE
Simulation and Analysis of a Compact Electronic Infrastructure for DC Micro-Grid: Necessity and Challenges
The flexibility of micro-grids comprises important bene-
fits, but their efficient implementation poses very chal-
lenging problems, as listed next [7]:
The benefits Micro-grids provide to power system
operation and planning need to be quantified and in-
corporated into an appropriate commercial and regu-
latory framework, so that a level playing field for all
energy technologies can be established. In order to
achieve the full benefits from the operation of Mi-
cro-grids, it is important that the integration of the
distributed resources into th e LV grids, and their rela-
tion with the Medium Voltage (MV) network up-
stream, will contribute to optimize the general opera-
tion of the system.
The coordinated control of a large number of distrib-
uted sources with probably conflicting requirements
and limited communication imposes the adoption of
mostly distributed intelligence techn iqu es.
The design of Micro-source Controllers enhanced with
advanced frequency and voltage control capabilities
and possessing ride-through capabilities is essential
for the stable operation of Micro-grids, especially in
islanded mode of operation.
The design of smart Storage and Load Controllers able
to face the stringent requirements posed by the islanded
operation and especially during transition from inter-
connected to islanded mode is also crucial.
2. DC Micro-Grid or AC Micro-Grid?
A micro-grid can be built using AC or DC current net-
work [8]. Figure 1 shows the overview of network con-
nectivity to distributed generation sources [9]. Despite the
fact that the AC micro-grid syste m has a benefit to utilize
existing AC grid technologies, protections and standards
ts application involves some problem of low efficiency i
Figure 1. Types of micro-grid (a) DC micro-grid; (b) AC micro-grid.
Copyright © 2012 SciRes. SGRE
Simulation and Analysis of a Compact Electronic Infrastructure for DC Micro-Grid: Necessity and Challenges 75
due to the number of power conversions required within
the crucial current path from the main grid to the loads.
One solution of this problem is the application of a DC
micro-grid as an efficient method to combine high reli-
ability and the possibility to reduce the losses. It can eli-
minate DC/AC or AC/DC power conversion stage and
thus has advantages in the stand of efficiency, cost and
system size. DC micro-grid is inspired by the absence of
reactive power, the possibility of the efficient integration
of small-distributed generation units and the fact that,
internally, all the loads operate using a DC voltage [10].
AC micro-grid imposed the inverters on both sides, gen-
eration and load. Therefore, in recent years has raised the
idea of using DC distribution network. DC micro-grid is
suitable for home loads which are mainly of DC loads.
This method eliminates the ac/dc interface. DC micro-
grid, also by one or more inverters is connected to the
utility grid and industrial loads (ac loads). However, some
references [11] have introduced a hybrid network that, the
DC link and AC link is used. The DC micro-grid has the
following adva nt ages over the AC micro-grid:
There is no need to consider about synchronization
with the utility grid and reactive power [12].
When a blackout or voltage sag occurs in the utility
grid, it does not affect dc bus voltage of dc micro-grid
directly due to the stored energy of the dc capacitor
and the voltage control of AC/DC converter. There-
fore, DGs in DC system are not easy to trip against
these disturbances. In other words, DC micro-grid al-
ready has fault-ride through capability in its own [12].
No need for frequency and phase control: Because DC
micro-grid, through a DC/AC converter connects to
utility grid. Therefore, any change in frequency net-
work does not affect the operating frequency and other
ac loads of micro- grid.
Number of converters used in the DC bu s is lower than
the AC bus [9]. In [13] has shown that the connecting
energy system combining fuel cell and solar cell us ing
DC bus, inject THD less into the network.
In DC micro-grid because, AC loads connected to the
DC bus this opportunity exists in a condition that is
generation power more than load, especially in islanded
mode, some non-critical loads out of orbit and was
feeding sensitive loads [14].
In [15] losses comparison between AC micro-grid and
DC micro-grid. Shown that losses in DC micro-grid,
15% lower than the AC Micro-grid.
In the same conditions, power transmission in DC sys-
tem is higher than the AC system [16].
In DC micro-grid, DC cables can be used to reduce
the investment cost [16].
In DC micro-grid voltage only affected by the active
power therefore, it is simple to control [17].
Lack of electrical hazards, other advantages of using
a DC bus [18].
Of course, there are some drawbacks to put dc micro-
grid to practical use as follows [19]:
It is needed to construct private dc distribution lines
for dc mi cro -grid.
The protection in dc system is more difficult than the
AC system’s because there is no zero cross point of
voltage in DC system.
The loads adapted for dc power supply are required
for high system efficiency.
3. Prerequisite Micro-Grid
The smart-grid will incorporate a variety of technologies
and tools, allowing the grid to work far more efficiently.
The US Department of Energy has listed five fundamen-
tal technologies that will drive the smart grid; these tech-
nologies include [1]:
Integrated communications, connecting components
to open architecture for real-time information and con-
trol, allowing every p art of the grid to both “talk” and
Sensing and measurement technologies, to support
faster and more accurate response, such as remote
Advanced components, to apply the latest research in
superconductivity, storage, power electronics and di-
Advanced control methods, to monitor essential com-
ponents, enabling rapid diagnosis and precise solu-
tions appropriate to any event.
Improved interfaces and decision support, to amplify
human decision-making, transforming grid operators
and managers into visionaries when it comes to see-
ing into their systems.
Because this paper focuses on power electronics devices
and electronic infrastructure, therefore, the more details
about the third technology that is related to this subject are
Different distributed energy systems require various
power electronics topologies for converting the generated
power to the utility compatible power. The photovoltaic
(PV) and fuel cell systems generate DC power which
needs to be converted to single-or three-phase AC for
utility connection. Additionally, an isolated DC-DC con-
verter is often used before the DC-AC inverter in order to
avoid bulky line frequency transformers for isolation and
voltage boosts. Wind micro turbine systems generate vari-
able frequency AC output which must be converted into
50 Hz AC for utility connection. The use of a back-to-
back converter is the most efficient way to utilize the g en-
erated power. Typically, most internal combustion (IC)
engines are interconnected to the utility through a fixed-
speed synchronous generator that has protective relays. A
Copyright © 2012 SciRes. SGRE
Simulation and Analysis of a Compact Electronic Infrastructure for DC Micro-Grid: Necessity and Challenges
back-to-back voltage source converter is the most suitable
choice for IC engine applications. Inclusion of storage in
the distributed energy system actually provides the user
with dispatch capabilit y of its distribu ted resour ces, which
are generally renewable energy sources like PV and wind ,
having no dispatch abi l ity on t heir own . The stored energy
can then be used to provide electricity during periods of
high demand. Depending on the type of storage, PE con-
verters are required for utility connection. The m ost unique
aspect to power electronics for energy storage is that they
must be bidirectional, both taking power from the grid
during charging and providing power for the grid during
discharge. For the battery energy storage system (BESS),
a bidirectional DC-DC converter followed by a DC-AC
inverter is the most general choice, whereas the flywheel
system can utilize a back-to-back converter for utility
4. Electronic Infrastructure of DC
The power electronic systems play an important role in
DC Micro-grid that to use them; appropriate electronic
infrastructure should be designed. For example, isolation
circuit, drive circuit, sensors, etc. collection of electronic
circuits as the infrastructure, these networks will be com-
plex. For electrification to villages, and also where there
is not enough land to install the system, the compact sys-
tem will be very important. Using the system properties
and circuits that have similar functions , electronic infras-
tructure can be compressed. IGBT driver circuit and is-
olation circuit are capable of compression.
4.1. IGBT Driver Circuit
4.1.1. Parallel Inverter
To reduce the number of driver circuits, first the different
methods of driver are reviewed. . Driver Ci rcuit with Is ol ation Transform e r
Circuit of this method is shown in Figure 2, where the
signal after passing through a transformer is connected to
the IGBT gate. To turn the two switches with a command,
two modes are consider: the first case, the source of the
two circuits with different voltage that in this case the
gate-source voltages of the two switches must be isolated.
Therefore, must use a transformer with two separate se-
condary windings Figure 3. In the second case is assumed
that the source potential of the two switches is the same.
This situation occurs for upper switch in the parallel in-
verter. As can be seen in Figure 4, a driver circuit to sup-
ply both the switches is used.
So we conclude that, when a load at DC micro-grid is
fed by two parallel inverters; if the above method is used
the transformer for high-side switches will be reduced
Figure 2. Combined transformer level shifter and gate driver
L11 L12
H11 H12
L21 L22
H21 H22
Figure 3. Drive circuit of two separate switches of two par-
allel inverters.
#1 #2
Gate Drive1.0 [ohm]1.0 [oh m]
Figure 4. Drive circuit of two switches in parallel inverters
with same source.
three. It should be noted that generally the use of trans-
formers, the system will be large. But if there is an insis-
tence that the isolation transformer is used to drive switches;
method described reduces the cost and the electronic in-
frastructure will be smaller.
Copyright © 2012 SciRes. SGRE
Simulation and Analysis of a Compact Electronic Infrastructure for DC Micro-Grid: Necessity and Challenges 77 Bootstrap Technique
In voltage inverter switches, the two switches, in a leg,
cannot be turn on at the same time, on the contrary one is
on and the other one is off, and vise versa. The bootstrap
connection provides the advantage of using two similar
switching circuitry for high and low sides; on the other
hand a very low supply voltage is needed to drive these
two switches at the same time Figure 5.
When the low-side switch M2 is on, the bootstrap di-
ode, D, conducts and charges the storage capacitor. If we
assume the saturation voltage drop of the low-side power
device M2 and the forward volt drop of diode D to be
negligible, the capacitor will charg e to approximately the
low voltage supply potential. When the high-side switch
M1 is on low side switch M2 is off, D is reverse-biased
and the high-side circuitry is powered from C. In this
condition, the voltage on C droops as it discharges when
supplying the high-side circuitry [20 ].
Novel driver topology for parallel operating inverters
(NDTPI) proposed and described in detail in the previous
papers of the authors [21]. In Figure 6, two inverters are
parallel that driver circuit is optimized. The implemented
switches in this figure have two different types: the low-
side uses IC-drive and high-side uses the bootstrap tech-
nique. In this case either of inverter have a common
Figure 5. Bootstrap technique employed for creating a float-
ing supply [20].
Figure 6. Parallel inverters with common ground [21].
ground and inverters commands are same. If each switch,
a driver is used; then the number of the drive circuit will
be twelve. However, this method has been used only three
drivers. Thus, the parallel capabilities of the inverters in
Micro Grid can be used to reduce the number of drive to a
quarter; of course for two parallel inverters. This method
can develop for more than two parallel inverters [21].
As discussed above, the bootstrap technique has the
capability which can be use this feature and brief drive
circuit. In addition to reducing costs and also reduce the
size of the electronic infrastructure. Optocoupler
Using an optocoup ler, can be fed the switch that its source
is float. The main structure of the optocoupler circuit is
shown in Figure 7. Optocoupler is requires a separate
supply both the input and output that necessary to use a
DC/DC isolated with driver circuit. Despite difficulties,
due to the very good compatibility with the IC, this device
now is the best way to dri ve fl oati ng swi t c hes.
In parallel inverter that use optocoupler, for drive high-
side switches is needed use of a separate optocoupler. But
according to Figure 8, for the lower switch can be used
from an optocoupl er.
So we conclude, as the isolation transformer in parallel
inverter number three optocoupler reduced . But the main
problem with this method, the IC is too costly.
4.1.2. Centralized Driver Circuit of Three-Phase
In DC micro-grid, for supply three-phase loads, three-
phase inverter is used. In this section, centralized driver
Figure 7. Main structure of optocoupler [20].
sig out
Opt-Co upler
sig out
Opt-Co upler
sig out
Figure 8. Driver circuit optocoupler in parallel inverter.
Copyright © 2012 SciRes. SGRE
Simulation and Analysis of a Compact Electronic Infrastructure for DC Micro-Grid: Necessity and Challenges
circuit is introduced. In this type of inverter for high-side
switch, in each leg, used one bootstrap circuit. If high-
side switches, in three legs, turned on together, then re-
move any bootstrap drive circuit is not possible. But it
should be noted that, switch on together three high-side
switches means of the output voltage is zero. That, equi-
valent to three low-side switches can be turned on. Fig-
ure 9, shows the method for avoid to turn on the high-
side switches at the same time. The commands of the
driver circuit with the proposed switching method illus-
trated in Figure 10. The switching process of the pro-
posed method is shown in Figure 11.
4.2. Isolation Circuit
In DC micro-grid each sources of energy such as solar,
fuel cell and battery reserve through a converter dc/dc
Buck Boost can be connected to the network. In Figure
12, Buck Boost topology is shown. As can be observed
due to the floating source voltage, to drive any of the
switches an isolation circu it is needed.
Considering the significant number of dc/dc converter
in solar energy sources as MPPT, removing driver circuit
isolation can be reduced circuit size and cost. Cuk con-
verter can be used to solve this problem that its topology
is shown in Figure 13, in this topology, source of switch
connected to the ground. Af ter using cuk converters, iso-
lation circuit can be eliminated.
Figure 9. Method for avoid to turn on the high-side switches
at the same time.
Figure 10. The commands of the driver circuit with the pro-
posed switching method.
Figure 11. The proposed algorithm for controlling the driver
Figure 12. Buck boost topology.
Figure 13. Cuk topology.
5. Simulation Results
5.1. Drive Circuit of Parallel Inverter
To analyze the presented structure correctness, two par-
allel inverters connected to a common load at Simulink
MATLAB is simulated whose results are given in next
section. In this case both inverters have common ground.
Copyright © 2012 SciRes. SGRE
Simulation and Analysis of a Compact Electronic Infrastructure for DC Micro-Grid: Necessity and Challenges 79
In part of the drive circuit, because the emphasis is on
bootstrap technique then simu lation results this techniqu e
will be shown that described in detail in the previous
papers of the authors [21].
The power circuit for the complete inverters system is
shown in Figure 6. The DC input stage of the inverters
consists of a 500V DC source along with two inductors
have been serried with each switch, before they are con-
nected to the load, for damp out the transient current.
Figure 14, presents gate source voltage of switch H11
while drive by an IC-driver separated without bootstrap
method, Figures 15-19, presents conditions at which in-
verters are common in ground, Figure 15, shows low-
side switches gate source voltage (L21) and Figure 16,
shows high-side switch gate source voltage (H11) that
maximum voltage is 14 volt, the voltage drop is due to
bootstrap capacitor discharge at the time of (HX) switches
run up.
Figure 17 shows gate source voltage (H21) and Figure
18 shows bootstrap capacitor voltage to the ground whose
voltage is about 500 volt. Figure 19 is presented to con-
firm inverters performance wi th suggested structure.
For simplicity, in Figure 6 the d r iv er c ir cu it fo r a ph a se
is shown; but similarly to the other phases of the sample
0.108 0.10850.109 0.10950.11
Gate-s ource vol ta ges switch (H
) /V
Figure 14. H11 switch gate source voltage.
Gate-source voltages switch (L
) /V
0.1080.1085 0.1090.10950.11
Figure 15. L21 switch gate source voltage.
0.108 0.10850.109 0.10950.11
Gate-s our ce v oltages switch (H
) /V
Figure 16. H11 switch gate source voltage.
G ate-sour ce volta ges sw itch (H
) /V
00.02 0.040.06 0.14 0.16 0.180.2
Figure 17. H21 gate source voltage in common ground con-
0.108 0.1085 0.109 0.10950.11
Bootstrap ca pac itor voltage /V
Figure 18. Bootstrap capaci tor voltage to the ground.
00.05 0.10.15 0.2
Output current/A
Figure 19. Parallel inverter output current.
Copyright © 2012 SciRes. SGRE
Simulation and Analysis of a Compact Electronic Infrastructure for DC Micro-Grid: Necessity and Challenges
5.2. Isolation Circuit
In Figure 20 each source such as solar, fuel cell and bat-
tery reserve through a converter dc/dc Buck Boost or cuk
connected to DC bus that its voltag e value con stant 48 V.
Simulation results are shown in Figures 21 and 22.
5.3. Drive Circuit of Three-Phase Inverter
In Figures 23 and 24, inverter output voltage and fre-
quency spectrum it shows. Vol tage three- phase i nverter with
the conventional bootstrap circuit is quite similar. Figures
25 and 26 show the gate-source voltage switch H11. Fig-
ure 25 shows the envelope curve in any one of points; the
applied voltage to the switch H11 limit is not exceeded.
The peak voltage in Figure 26, a bootstrap capacitor dis-
charge is visible.
For the circuit is functioning correctly the bootstrap
capacitors are able to supply enough energy to turn the
switch. Figures 27 and 28 in the voltage across this cap-
acitor are shown. Be seen, voltage across bootstrap cap-
acitor at about 15 volts and the capacitor voltage ripple is
13% that is acceptable. Therefore, two circuits are well
able to supply enough energy to turn switches.
6. Conclusion
Electrification to home load, market, commercial building
or even villages and also where there is not enough land
to install the system, the compact system will be very
important. One of the ideas in this regard is the u se of the
Figure 20. DC micro-grid.
Figure 21. DC bus voltage with Buck boost converter.
Figure 22. DC bus voltage with cuk converter.
Figure 23. Output voltage of three-phase inverter with the
proposed switching method.
Figure 24. Frequency spe ctrum w ith the pr oposed sw itching
Copyright © 2012 SciRes. SGRE
Simulation and Analysis of a Compact Electronic Infrastructure for DC Micro-Grid: Necessity and Challenges 81
Figure 25. Push the H11 switch gate-source voltage.
Figure 26. H11 switch gate-source voltage.
Figure 27. Bootstrap capacitor voltage in first bootstrap cir-
Figure 28. Bootstrap capacitor voltage in second bootstrap
DC micro-grid. It can eliminate DC/AC or AC/DC power
conversion stage and thus has advantages in the stand of
efficiency, cost and system size. But the idea has been
proposed in this pape r that the DC syste m will be s maller.
Using the system properties and circuits that have similar
functions, electronic infrastructure can be compressed.
IGBT driver circuit and isolation circuit are capable of
compression. In this paper, an electronic infrastructure for
driving and isolation with less circuitry of parallel oper-
ating inverter and t h ree-p has e inverter were presented.
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