Paper on DC Microgrids for rural electrification, especially using distributed micro generation using solar or wind energy.

1. DC Micro-Grid with Distributed Generation for
Rural Electrification
Md Junayed Sarker B. Asare-Bediako J.G Slootweg W. L. Kling B. Alipuria
Eindhoven University
of Technology
Eindhoven University
of Technology
Eindhoven University
of Technology
Eindhoven University
of Technology
Eindhoven University
of Technology
j.sarker@student.tue.nl B.Asare.Bediako@tue.nl j.g.slootweg@tue.nl w.l.kling@tue.nl B.Alipuria@student.tue.nl
Abstract- This paper investigates the use of low voltage DC
distribution network for rural electrification within an
intelligent grid concept. The goal is to provide local communities
in sparsely populated areas with electricity supply generated
from renewable energy sources. Since these communities subsist
with no grid connectivity, they require a concept of micro-grid
whereby individual Solar Home Systems (SHS) can be
connected. The excess power required by any SHS is supplied
from the grid to make the system more reliable. Furthermore,
people who cannot afford Solar Home System can connect
themselves with the grid as well and get access to the basic need
of electricity. In this work a power flow supervision system has
been investigated based on domestic and grid level DC electrical
systems with MATLAB/Simulink as software supporting
platforms. The length of the grid and the number of the Solar
Home System deeply affect the load behavior of the network. So
it is suggested that a detailed study has to be performed before
implementing this new concept
Index Terms-- Micro-Grid, Distributed Generation, DC,
Solar Home Systems, PV
I. INTRODUCTION
The world is facing fundamental changes in the energy
sectors at a global level. On one hand conventional energy
resources are exhausting and becoming expensive to extract;
while on the other hand the addition of green house gases
especially carbon dioxide in the atmosphere has accelerated
the global warming. These two challenges have put the world
leadership to devise strategy for adopting renewable energy
resources to have clean sustainable energy source and to have
energy transformation at point of usage to avoid inefficiencies
of energy transport and conversion. One of the sustainable
strategies is to create clusters of areas around the rural areas
of the world where local renewable energy will be utilized to
meet their own energy demand. The major renewable energy
source for decentralized generation within rural areas is likely
to be small scale solar home system. As a result the
integration of such systems in a low voltage DC network will
be a noteworthy step towards powering these societies as an
efficient approach [1, 2]. Efficiency, protection, power
quality and the relative advantages of DC compared with AC
are investigated. Although the current technology is not
efficient enough to reach to the goal but recent development
and attention over the issue suggest that the changes in the
energy structure will take place soon. Interestingly, Solar
Home Systems (SHS) are already widely in use in the rural
areas of South Asia and Africa, where unreliable electricity
grid is forcing the policy makers to move into the self reliant
renewable energy based electricity generations. So these areas
have a lot of potential for new business development relying
on this micro-grid concept.
II. BACKGROUND
From the very early age of electricity, AC has been
regarded as the most appropriate option for power
transmission and distribution. DC was invented by Thomas
Edison and he opted for its use but due to the limitations in
voltage transformation and controlling, DC option could not
be made feasible in the electricity network. Later
Westinghouse made the proposal for AC distribution and it
came out really popular. Nikola Tesla developed transformer
module to stepping up and down the voltage, which solved
the problem of transmission losses in the network. Since then
vast advancement has been taken place in the field of AC
transmission and distribution. However, in recent days, due to
the development in power electronics converters and DC
energy sources, loads and storage systems; interest in DC has
returned. In the rural areas of developing countries where the
inhabitants have no access to public electricity grid, DC based
Micro-grid can be developed from locally generated
electricity using renewable sources.
A. Advantages of DC over AC for Rural electrification
a. There will be significant amount of reduction in the
energy loss.
b. PV can easily be interconnected with the system
which is the only source of electricity generation in
the rural areas where there is no grid connection.
c. Storage system can also be easily coupled with the
DC network.
d. Good utilization of existing solar home systems of
the considered area can be made.
e. Existing practice of using DC appliances in the Solar
Home systems in the developing countries.
B. Optimal Voltage level for DC Micro grid
The DC Micro grid is connected with different solar home
systems in a particular rural area. The SHSs generate
electricity at 24V range for the home appliances. The voltage
level is be boosted up for micro-grid connection. So an
optimal voltage level has to be decided for the DC micro grid.

2. It is decided for 120 V after the loss calculation in the
network [3]. It is really an important factor to decide for the
following system parameters:
Overall system Efficiency: The overall system efficiency
depends on the power conversion stages between the
generation and the load. The less no. of conversion results in
a less power loss. In case of DC micro grid there will be no
loss in DC to AC conversion. And voltage has to be boosted
in the grid end to minimize the power losses in the
distribution line.
Cost: Cost is another important issue to be considered on.
Power electronics converters and voltage distribution cables
are expensive. They consumed major portion of system
budget. Some optimal voltage level will leverage the system
design parameters.
Safety: 120V is hazardous to human. Typically, costs of
safety and protective devices increase with the increment in
voltage level. Safety devices have to be designed based on
this issue
III. LAB SETUP
The principles of modeling the micro-grid are formulated
and a model of the grid is developed using MATLAB/
Simulink. SimPowerSystems feature of MATLAB is used to
design and build the model to simulate the power grid. The
test network is simulated with all the possible scenarios to
evaluate the system behavior. After validation and analysis of
the system, necessary changes are brought in.
In Simulink, a simple model of a solar home system is
simulated in Simulink platform. Two voltage levels are
considered for the total system, 24V is maintained at the
domestic level and 120V is maintained at grid level. The
optimal voltage level is decided after examining the load
flows and power losses in the micro-grid. The voltage level is
boosted up through DC-DC converters before providing it to
the grid to connect all the solar home systems of a particular
area to form a DC network.
A. Solar Irradiance Generator
A Simulink model has been developed to provide the
accurate irradiance data for a particular area based on the
climate data input from 1974 to 2000. It is necessary to use
the irradiance data for a specific location over a certain period
of time. This model is capable of providing irradiance and
temperature data for any location of the world. This model is
developed by Adrianus Wilhelmus Maria van Schijndel
during his PhD thesis work in 2007. The model is partially
used as to adjust with the requirement of the thesis work.
B. PV array
The PV array block represents a combination of solar cells.
A simple PV cell consists of an ideal current source in
parallel with an ideal diode. Due to the photons received on
PV panels current is generated which is represented by
current source. Short circuit current and the open circuit
voltage are the two important parameters of PV cell.
The current at the terminal of PV cell can be expressed by
following equation
Where,
I= Output current of PV, amp (A)
V= Voltage across the PV cell, volt (V)
Isc = Iph = Short circuit current/Photon current, (A)
Id = Current through intrinsic diode, amp (A)
Io= Reverse saturation current of the diode, amp (A)
q= Electron charge (1.602 ), coulomb (C)
Vd = Voltage across the diode, volt (V)
k= Boltzmann’s constant (1.381 )
T= Junction temperature, Kelvin (K)
After derivations and applying Newton’s method,
The first step is to simulate the system is to get the
irradiance data for a particular year. The solar generator block
used in the model can generate irradiance and temperature
data for a particular location. The Fig 2 shown below is
irradiance data for year 2000 in The Netherlands.
Fig 1. Equivalent circuit of a simple PV cell
Fig 2. Solar Irradiance for a year
0 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
0
200
400
600
800
1000
Time(Year)
Irradiance(W/m2)

3. The same block can generate temperature profile. Solar
irradiance and temperature data are used in the PV array
block to generate PV current. The Fig 3 shown below is
representing the temperature profile for one year.
Due to the solar irradiance on the surface of PV panel,
power is generated in the PV panel. The following curve
shows the power generation trend by the PV panel for three
days. During the simulation 120 W PV array is considered.
B. Battery
The battery is modeled based on a lead-acid battery
structure. Lead acid battery cell has two plates; positive and
negative plates are deepened in a diluted sulfuric acid
solution. Anode and cathode are made of lead dioxide (PbO2)
and lead (Pb) respectively. The battery model is designed to
have two mode of operation of charging and discharging. The
following parameters are used to model the battery block.
Where,
SOC1 = Initial State of Charge/available, %
SOCm=Max. SOC /Max. battery capacity, (Wh)
SOC(t)=Current state of charge, %
ns= No. of 2V series cells
K=Charge/discharge battery efficiency
D= Battery self discharge rate, Hour-1
(h-1
)
=Terminal voltage of the Battery, Volt (V)
=Terminal volt of Battery while charging, (V)
=Terminal volt of Battery while discharging,(V)
=PV current, amp (A)
= Battery internal resistance, (R)
= Battery current, (A)
= Load current, (A)
= Battery resistance while charging, (R)
=Battery resistance while discharging, (R)
The value of battery current ( ) is positive when battery is
in charge mode and negative when it is in discharge mode.
So, the battery terminal voltage is:
In case of charging:
In case of discharging:
Now the relationship between SOC and Battery current and
voltage,
So, the current value of SOC (t), in given time t is found by
looping the result in Simulink.
The battery current determines the State of Charge (SOC)
trend of the battery. When battery is discharging the SOC will
decrease and when the battery is charging it will increase. For
safe operation and long durability of the battery, SOC limit is
set in such a way that it cannot discharge below 50%.
C. Charge Controller
This is basically a charger block, which is necessary to keep
the battery from getting overcharged or under discharged.
Fig 7. State of charge (SOC) for three days
0 1 2 3 0
0.7
0.75
0.8
0.85
0.9
0.95
1
Time (Day)
SOC
Fig 3. Temperature for a Year
0 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
-10
0
10
20
30
40
Time (Year)
Temperatute(degC)
Fig 6. Battery current for three days
0 1 2 3
-3
-2
-1
0
1
2
3
4
5
Time (Day)
Current(A)
Fig 5. Battery model

4. Generally a deep cycle battery should not exceed the state of
charge (SoC) 100% and go below 20%. The charger module
designed in the solar home system consists of two switches.
TABLE I
SWITCHING OPERATION IN CHARGE CONTROLLER
Condition A’ A Condition B’ B
V>= 25.40 0 0 V<= 22.75 0 0
V>= 25.40 1 0 V<= 22.75 1 0
23.92<V<25.40 0 0 22.75<V<24.03 0 0
23.92<V<25.40 1 1 22.75<V<24.03 1 1
V<=23.92 0 1 V>=24.03 0 1
V<=23.92 1 1 V>=24.03 1 1
From the Fig 19, it can be depicted that switch A is connected
between PV array and rest of the system and switch B is
connected between load and rest of the system. Switch A in
the PV side is opened when battery voltage becomes larger
than the desired value and will remain open until the voltage
gets dropped down. Whereas switch B is opened when the
battery voltage drops down below the desired level and will
remain in that stage until it rebounds back to the preferred
level. These switching functions are implemented in
Simulink. Here the battery voltage is being compared with
different voltage levels using ‘compare to constant blocks’ of
Simulink which will return 1 if it is true otherwise 0.
D. Load
The energy demand is very low in the rural areas of
developing countries. Their basic electricity demand is for
lighting purpose. Recently lot of development has been taken
place in energy efficient bulbs. LED has made a huge
advancements and it is more efficient than the standard
incandescent bulbs. Moreover, most of the electrical
appliances use DC power as well. The prices of these home
appliances are not very expensive and available in the market.
Cell phones have become an important tool in everyday’s life
of all sorts of people. So the option of mobile phone charging
is really attractive for them. In Simulink, the controllable load
is modeled using resistors and switches.
E. DC-DC Converter
In the model, two types of DC-DC converter are used. The
converter shown in Fig 21, only changes the output current of
PV array to the current of battery voltage. In the real
hardware setup DC-DC converter is used with MPPT to
regulate the voltage output of PV. The DC-DC converter
block demonstrated in Fig 3, is used to change the voltage
from 24 V to 120 V and vice versa. Mathematical modeling
approach has been performed here to design the converter. It
is Switched-mode Boost (step-up) DC-DC averaged converter
model with input current control and efficiency model.
In DC-DC converter topologies, the output voltage is
dependent on the input voltage. It is also related with the duty
cycle of the switch. The relationship among these parameters
can be shown as follows:
Where,
= output voltage of the converter
= input voltage of the converter
= duty cycle for the converter
One problem with DC-DC converters is that, the efficiency
tends to fall when operated below the rating. So in the
implemented model of DC-DC converter, the efficiency was
optimized for the optimal operation.
PV
Array
Battery
Load
Switch A Switch B
Fig 5. Block diagram of a solar home system
SW Control
Micro
Grid
= =
= =
Mic
ro
Gri
d
= =
Mic
ro
Gri
d
= =
120V Grid
Line
Central
Control UnitConsumer
Type B
Consumer
Type A
Battery
Load
PV Array
DC-DC
Converter
DC-DC
Converter
Battery
PV Array
SW Control
Fig 7. Layout of the DC Micro-grid

5. F. Central Control Unit
In the network, there are two kinds of consumers. One
type of consumer can generate their own electricity from
Solar Home Systems but in case of additional need of
electricity or during the night time when the consumption is
high, they can consume the electricity from the micro-grid.
These type of consumers are defined as ‘Type-A’ consumers.
Another type of consumer has no generation system of his
own. They consume electricity from the grid all through the
day. And they are ‘Type-B’ consumers. So it is quite difficult
to bring the balance in between production and consumption
and maintain a stable voltage in the grid. A control unit
having a large battery bank with PV can ensure the
production of electricity and make the system balance. DC-
DC converter is used to boost the voltage level and send the
energy to the grid. At the type- A consumer end, when
thebatery voltage of local battery will go down below some
certain level then the system will switch all the loads from its
own system to grid.
G. Grid Connection
Every Solar Home system will be connected to the grid.
The grid network can be star or mesh or loop configured.
Since the grid will be installed in rural areas in a small region
encompassing 10-15 homes so loop network is considered to
be the best.
The main focus was to design a solar home system and then
connect them in the network. Type B consumer can easily
consume electricity from the grid whereas the type A, solar
home system users will only consume the electricity when
they don’t have enough storage in their local system.
Basically, the existing solar home system users are having
very few number of low power home appliances in their
houses as they generate very less energy by their system. If
they want to consume more energy then they will have to buy
bigger system for their houses. The bigger system needs
bigger storage capacity which increases the system price
drastically. The proposed DC micro-grid will allow them to
get the benefit of having the facility of consuming more
energy without spending much amount of money for buying
the new system. They can just get the electricity from the
micro-grid operators and pay them monthly.
As mentioned in the earlier section, a central control unit is
considered in the grid which is basically consists of PV
panels, DC-DC converters and batteries. The central control
system will calculate the demand in the micro-grid and install
PV arrays and batteries as per the requirement.
IV. CASE STUDY
PV systems are being encouraged all over the world and
the concept of LVDC grid would open up the door to a
number of possible business opportunities. Moreover a
number of under developed countries in Asia, Africa and
South America are heavily suffering with the problem of
unreliable electricity supply. Hence introducing of LVDC
grid in the rural area can alleviate the problem of these
regions to a great extent. The rural electrification system
should be designed in such a way that they are very energy
efficient and reliable. Also, already existing structure which
are less efficient in energy usage are needed to be retrofitted
to reduce the amount of energy consumed by them. Doing
this requires engineering knowledge and skills from various
disciplines; energy engineering, electrical engineering,
mechanical engineering, architectural engineering etc.
Fig 7. Power demand for a particular variable load
0 1 2
0
10
20
30
40
50
60
70
80
90
Time (Day)
Power(W)
Fig 9. Power supplied to the variable load by local system
0 1 2
-20
0
20
40
60
80
100
Time (Days)
Power(W)
Fig 7. Grid voltage during a day
0 2 4 6 8 10 12 14 16 18 20 22 24 0
60
70
80
90
100
110
120
130
140
150
X: 8.46e+004
Y: 116.6
Time (Hrs)
V(Volt)
X: 5.65e+004
Y: 117.1
X: 2.91e+004
Y: 117.9
X: 100
Y: 118.1
Fig 9. Power supplied to the variable load by central control unit
0 1 2
-10
0
10
20
30
40
50
60
70
80
Time (Day)
Power(W)

6. V. FAULTS CHARACTERISTICS IN A DC NETWORK
Another important aspect of modeling of a LVDC grid is to
design its protection system. In most cases, this system use
grid connected breakers and current limiting capability during
DC faults.
The DC system has different fault characteristics than an
AC system. The protection system has to be designed based
on these characteristics. There are basically two methods to
clear up the fault current, the first is to take some action to
detect and clear the fault before the system gets affected-first
tripping of the circuit and the other one is to delay the
tripping till the transient gets passed away.
There is number of potential devices available in the
market which are capable of giving protection to the DC
network. Fuse is the simplest of all protection devices. It is
very much suitable at the residential end. The mostly used
protection devices are Circuit breaker but due to their zero
crossing of the current issue they are not suitable for DC [8].
And circuit breaker for DC system is not commercially
available. Another important protection device can be solid
state switches. The advantage of such devices is, it can
operate in micro-second range and already in use in water
vehicles. But recent development in the power electronics and
DC distribution systems create a great hope for this
technology to come alive in mass use.
VI. BUSINESS OPPORTUNITIES
PV systems are being encouraged all over the world and
the concept of LVDC grid would open up the door to a
number of possible business opportunities. Moreover a
number of under developed countries in Asia, Africa and
South America are heavily suffering with the problem of
unreliable electricity supply. Hence introducing of LVDC
grid in the rural area can alleviate the problem of these
regions to a great extent [9]. The financial plan of the
entrepreneurship depends on the market sizing and then
capturing the share of the market. Since the potential market
size for solar home system in the Bangladesh is 500000. For
the analysis of financial plan, only this customer segment is
considered. Inflation rate and interest rate are considered as
9.15% and 13.7%. The tax rate is assumed as 35%.
VI. CONCLUSION
In this paper, a LVDC micro-grid is presented to meet the
basic energy need of the people of rural areas, which
integrates the existing individual renewable energy and
storage modules into a single grid. There were some
difficulties in delivering the data in the simulink model as
maximum accuracy was tried to achieve. Despite these facts
the model performs nicely and helps to get the feasibility
status of the proposed concept. Some components suggested
in the model are not currently available in the market which is
another drawback of the whole idea. There are a variety of
possible improvements that could be made to the model and
the proposed concept. Finally, this grid can provide more
efficiency to the system and quality power to the 24V DC
loads. Concept and idea of the low voltage DC grid is
discussed and simulated, which proves the validity of the
proposed idea.
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Fig 8. Simulink model of the micro-grid
Continuous
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