Abstract—This paper focuses on the design of a hybrid power system based on renewable energy sources for Low Voltage Distribution system suitable for remote areas. Rural electrification has been a challenging issue for many third world countries. The available national voltage distribution systems are only limited to areas with good road networks. Consequently, this means rural and remote areas with poor road networks are isolated from the national grid. Research is to be conducted so as to design a feasible islanded power system that can furnish adequate power to such areas.

1. 1
Design of a Hybrid Power System for a Low DC
Voltage Distribution system suitable for remote areas.
Itai Geavas Hamunakwadi (29221359)
Electrical Electronic and Computer Department
University of Pretoria
Pretoria, South Africa
itaigh@gmail.com
Abstract—This paper focuses on the design of a hybrid power
system based on renewable energy sources for Low Voltage
Distribution system suitable for remote areas. Rural
electrification has been a challenging issue for many third world
countries. The available national voltage distribution systems are
only limited to areas with good road networks. Consequently, this
means rural and remote areas with poor road networks are
isolated from the national grid. Research is to be conducted so as
to design a feasible islanded power system that can furnish
adequate power to such areas.
Keywords— Hybrid, micro-grid, PV, DC, Maximum Power
Point Tracking, DER
I. INTRODUCTION
Isolated power systems have posed as a potential solution to
alleviate the challenges of expanding voltage distribution
networks to remote and rural areas [1]. Such islanded power
systems are fed by energy resources such as wind power, solar
energy, biomass as well as fuel cells. In 2013, it was reported
that an estimate of 1.3 billion people worldwide have no
access to any form of electricity [2]. A major portion of the
estimate includes areas in sub-Saharan Africa and South Asia.
Electrification rates for these areas are estimated to be 25.8%
and 51.8% respectively [2]. India is amongst such affected
areas where 44% of the households have no access to
electricity. Most of these areas are affected because of poor
road networks which poses challenges in trying to expand
network distribution. Economic feasibility of network
expansion to remote areas is another major reason why such
areas have no electricity access.
Since electricity plays an important role for accelerated
economic growth, many third world countries have embarked
on electrification development programs which will see a 10%
reduction of such affected areas [3]. The abundance of
renewable energy resources and as well as advancements in
power electronic technology has since made it possible for
remote areas to have access to adequate electricity [4].
Renewable energy resources allow for the generation of clean,
reliable and high quality electricity within good economical
boundaries. Since most of the renewable resources such as
solar energy and wind energy are stochastic in nature, it then
requires that a combination of such resources are used to
ensure continuous and reliable supply of power. Hybrid power
systems have received a lot of attention as an economic and
environmental-friendly solution for rural electrification [5].
II. LITERATURE REVIEW
A hybrid power system is an off-grid Distributed Energy
Resource (DER) technology that allows for the satisfaction of
demand directly [6]. Such a system incorporates a
combination of different but complementing renewable energy
resource based systems such as PV and wind. Using DERs
that complement each other allows for maximum reliability
and as maximum utilization of the available resources. In
seasons where insolation levels are low, wind speeds are
typically high and vice versa [6]. Hybrid power system can be
used to realize a voltage distribution system that can be used
to furnish both AC and DC loads with adequate power. Figure
1 shows a diagram of a typical DC distribution system.
Figure 1: Conceptual diagram of a DC distribution system [7].
Hybrid power systems can be implemented as an islanded
system or grid connected system. In situations where there is
more generation than demand, excess power can be ushered to
the national AC grid. Hybrid power systems enjoy numerous
advantages which makes them an attractive solution to
implement in remote locations. One of the major advantages
by such a system is that it can furnish power to off grid
locations where there are high related costs of network

2. 2
connection with respect to long distance transmission of
electricity [8]. Furthermore, they allow for optimization use of
primary energy resources thus ensuring reliable and flexible
generation of power. However, hybrid power systems have
drawbacks associated with them. There is need to design
complex power flow control schemes to make sure that
mismatches between generation and demand are kept as
minimal as possible. In addition to that, voltage quality might
be compromised as the system has a large number of
integrated power electronic converters, which will result in
severe voltage distortion. Another challenge to realize such a
system is the need to develop proper protection schemes. Also
one other problem faced in realizing such as system is the
need to design a collector bus in which to interface all of the
DERs units.
Many researchers have proposed designs for hybrid power
systems based on renewable energy resources [9]-[11]. Most
of them focused on the techniques to interface the DERs
systems to a common DC bus. Challenges they had to content
with is power flow control and well as optimal sizing of PV,
Wind and Storage systems. Research done in [12] proposes an
optimal design of renewable energy based hybrid power
systems. Areas of interest which were focused on were ways
to optimize power generation output of DERs. One such
method used was a battery cycling system which improved
efficiency of the generator. Other researchers such as [13]
focus on implementing algorithms to greatly reduce sizes of
PV and Wind systems to optimize cost.
Many designs of hybrid power systems involved only using
one DC bus to interface loads and renewable energy power
systems. Linking two DC buses has been reported to increase
reliability of the overall system [14].
III. WORK TO BE DONE
Research to do be done will be concluded by the design of a
hybrid power system with two DC buses linked together.
Different renewable energy resources and storage systems will
be interfaced on both buses. The system will allow power
transfer capability between the two buses. This will ensure
reliability and flexible power control. Figure 2 shows the
block diagram of the proposed system. Three phase loads will
be connected to the high voltage DC bus while singe phase
loads will be connected on the low voltage side. The high
voltage DC bus magnitude is to be set at 800V whilst the low
voltage DC bus magnitude is to be set at 450V. A total of 6kW
must be available at both buses at all times.
Deliverables at the end of the research will include design of
converter modules for interfacing the renewable energy
resources to the DC buses, design of storage systems,
simulations of sub-systems as well as simulations of the entire
system. Simulations will be carried out using PSIM or
MATLAB.
Figure 2: Proposed system
From figure 2 shown above, shows the proposed power flow
of the system. Both DC buses will supplement each other’s
power requirements in the event that either buses does not
meet the total power requirement.
IV. FUNCTIONAL ANALYSIS OF THE SYSTEM
The system functions of the Low Voltage distribution system
are addressed in figure 3 below.
Figure 3: Functional Block diagram of the system
Functional units (FU1), (FU8) and (FU11) represent
renewable energy resources that will feed the distribution
system. A string of Photovoltaic (PV) panels (FU1) will
harvest solar energy and an appropriate DC/DC converter
(FU2) will interface the PV to the 450 VDC bus. Wind energy
will be harvested by the AC wind generator (FU11) and an
appropriate AC/DC power processor will convert the
generated AC to a DC voltage so as to be compatible with the
800 VDC bus. In situations when there excess generation of
energy, a storage device (FU3) will store energy and dispatch
it in situations where demand is more than generation.
The storage device (FU3) is connected to a bi-directional
DC/DC (FU4) to allow for proper charging and discharging

3. 3
processes. A suitable DC/DC converter (FU2) will interface
the fuel cell stack (FU8) to the 800 VDC bus. Single phase
loads (FU6) will be fed by the 450 VDC bus via the single
phase inverter (FU5) whilst the three phase loads (FU10) will
be fed by the 800 VDC bus via the three phase inverter (FU9).
To ensure reliable operation of the entire system, the two DC
buses are linked by a bi-directional DC/DC converter (FU7).
The two DC buses have to supplement each other in terms of
power. The power flow control module (FU13) will ensure
that all the loads will receive the required power irrespective
of the power generation status of the buses.
A. POWER FLOW SPECIFICATIONS
To ensure that the power flow within the system is balanced,
the following assumptions will be made:
• Total power to be available at all times at both the
450 VDC and 800 VDC buses should be 3 kW.
• Based on the first assumption, it means that both
single phase and three phase load at either DC buses will
receive a total of 3kW
• The two buses will supplement each other to meet the
power requirements for each bus. This means that if the power
generating units of one of the buses cannot generate a total of
3 kW to furnish the load, the other bus will transfer the
remaining balance of the power via the bi-directional DC-DC
converter.
• The power flow magnitude and direction via the bi-
directional DC-DC converter will depend on the power status
of the both the DC buses.
• Maximum power to be flowing in either direction of
the buses via the bi-directional converter at one point in time
should not exceed 3kW.
• Constant hydrogen will be supplied to the fuel cell
stacks and thus the fuel cell system will continuously supply
the rated 3kW.
B. STORAGE SYSTEM SIZING
A battery will be used as the storage device. Batteries find
application that require evening out mismatches between solar
energy harvesting and demand. The lithium ion battery is to be
used as the storage device due to the high recommendations it
has despite the cost associated with it.
The battery is supposed to supply 3kW for 4hrs:
kWhrs_required = (3) (4) = 12kWh = 12 000Wh
In order to prolong the service life of the battery bank, a Depth
of Discharge (DOD) of 50 % will be used. De-rating using the
DOD selected:
Derated_kWh =
.
= 24 000Wh
Assuming the battery pack will be located close to the bi-
directional DC-DC converter, we can say that it will be
subjected to ambient temperatures of 60°� due to heat
generated by loss mechanism within the converter. A
temperature of 60° will correspond to a de-rating factor of
1.11.
De-rating the battering bank at 60°� = 24 000 × 1.1 =
26 640Wh
To avoid a high boost ratios for the bi-directional converter, a
standard 48V voltage was chosen for the battery bank.
Obtaining the rating of the battery bank:
Rating = = 555Ah
The battery bank will be realized by three strings connected in
parallel. Thus we need to identify a battery unit which has a
third in capacity of the total rating of the battery bank.
Rating of battery unit = 0.3333 × 555 = 185Ah
The battery found with the closest ratings was the Lithion-ion
24V 180Ah battery.
Two 24V battery will form a string to give a 48V system.
Connecting three strings in parallel will result in a battery
capacity of 540Ah
Figure 4: battery bank of the storage system
C. PV SYSTEM SIZING
The assumption made is that the PV system is to generate
3kW per hour during favourable insolation periods.
Total kW = 3kWh
To account for losses with the PV systems, we have to
calculating a de-rating factor based on these facts:
 15 % for temperatures above 25%
 5% for losses due to rays not striking the panel
 10% for losses due to the absence of MPPT features
 5% in case of dust/particles on the panel surface
 10% allowance for panel below specific age.

4. 4
 Total de-rating factor for losses =
0.85×0.95×0.90×0.95×0.90 =0.62
 In South Africa, solar radiation levels usually vary
between 4.5 – 6.5 kWh/m2
 Design will be done using the worst case insolation
levels of 4.5kWh/m2
 Obtaining the panel generation factor:
Panel Generation Factor = 0.62×4.5 = 2.79
 Total Wp of PV panel capacity needed
Wp =
.
= 1075W
The Monocrystalline SW 280 MONO Module manufactured
by Sun Electronics will be used for the panels. Ratings of the
panels are shown in the table below:
Max. Power (Pmmp) 280W
Max. voltage (Vmpp) 31.2V
Max. current (Impp) 9.07A
Open circuit voltage (Voc) 39.5V
Short circuit current (Isc) 9.71A
Table 1: ratings of SW 280W MONO panel
 No of panels = = 3.8 ≈ 4 panels
 Connecting the 4 panels in a series will produce the
following ratings:
 Voltage across the series string = 4 × 31.2 = 124.8V
 Total power generated by the series string = 124.8 ×
9.07 = 1131.94W
 Connecting the three strings in parallel with result in
a total capacity of:
Wp = 2 × 1131.94 = 2263.87W < 3kW
 Connecting 4 strings in parallel will result in a total
capacity of :
Wp = 3 × 1131.94 = 3395.8W > 3kW
 To ensure that the PV system generates at least 3kW,
3 strings in parallel will be used to realize the solar
array as shown below:
Figure 5: Solar Panel array for the PV system
 In total 12 panels will used to realise the solar PV
array.
 The PV system is overdesigned by :
Percentage overdesign =
. −
= 13.1%
 This will cater for converter losses so that at the
converter output, at least 3kW is generated.
D. FUEL CELL SYSTEM
For the Fuel Cell system design, the PEM fuel cell will be
used based on its associated advantages. Two fuel cell
modules where considered. The NEXA 1200 PEM fuel
cell and the H-3000 PEM fuel cell. The NEXA 1200 is
rated at 1.2kW and the H-3000 PEM is rated at 3kW.
Assuming a boost converter topology with an efficiency
of 85% is used, the required output from the fuel cell
stack to meet the power requirement of the load should
be:
 Pfuel_stack = .
= 3.52kW
3 modules of the NEXA 1200 PEM are to be connected in
series to give a total power of 3.6kW. Table 3 shows the
specifications of one NEXA 1200 fuel cell module.
Type of Fuel Cell PEM
Net efficiency 50%
Rated Power 1200W
Rated Performance 20-36VDC
Reactance Hydrogen and Air
Output current Max 60 A
Ambient Temperature 5 - 40℃
Dimensions 220×400×550mm
Humidification Self-humidified
Table 3: NEXA 1200 PEM fuel cell module specifications.
Connecting the three fuel cell modules in series will result in
new operating voltage of the resultant stack to be:
Vstack = 3 (20 -36) = 60 – 108 V (the 20-36 voltage of one
module satisfies the linear operation range of the fuel cell, thus

5. 5
numerical manipulation such as addition of the voltages of the
operating ranges is feasible)
Since the modules are connected in series, the current
operating range will remain the same: Istack = 0 – 60A
Since fuel cells do not accept reverse currents, diodes are
connected between the fuel cell modules to prevent this.
Figure 5 shows the configuration of the fuel cell stack to be
used.
Figure 6: Fuel cell system configuration
E. WIND SYSTEM SIZING
The output of the permanent magnet synchronous generator of
the wind turbine generates AC and thus to interface it to the
800VDC bus, a full bridge rectifier will be utilized. Assuming
a full bridge rectifier with an overall efficiency of 85% will be
used and to boost the output voltage of the rectifier to
800VDC, a boost DC-DC converter will be utilized. Assuming
an overall efficiency of the boost converter to be 85% as well,
then a generator will the following ratings will be used:
 Prating =
. × .
= 4.152kW ≈ 4kW
A wind turbine with a permanent magnet synchronous
generator will be used for the Wind power generating system
based on the associated advantages it offers. The 4kW
RICHUAN wind turbine with an integrated permanent
magnet synchronous generator will be used. The rating of the
wind turbine and generator are shown in table 2 below:
OPERATING DATA
Rated Capacity 3.1 kW
Working Wind
Speed
3-24m/s
Cut-in wind speed 3m/s
GENERATOR
Type Permanent magnet synchronous
generator
Output Voltage 24-380VLL
Protection class IP 55
Table 4: Wind Turbine ratings
F. CONVERTER SELECTION AND SIZING
1. PV SYSTEM
To ensure low input and output ripples, and ensure high boost
ratios a two level interleaved boost DC-DC converter will
interface the solar arrays to the 450 VDC bus. The output
power used for the design will be 3.3950kW to cater for any
losses within the converter.
Figure 7: two level interleaved boost DC-DC converter
The input design parameters of the interleaved boost converter
are as shown in table 5.
Parameter Design calculation
VIN 214.8V
VOUT 450V
POUT 3.395kW
D 0.7266

6. 6
fsw 100kHz
IO 7.55A
C 6.087µF
L1 = L2 =L 328.99µH
Table 5: input design parameters for the interleaved converter
2. FUEL CELL SYSTEM
For the fuel cell system, a tapped coupled inductor boost
converter was used to interface the fuel cell stack to the
800VDC bus. It was chosen as it provides high voltage step-up
ratios at low duty cycles thus minimizing the downfalls of the
conventional boost converter. Table 6 shows the input design
parameters of the converter.
Figure 8: tapped inductor boost converter
Parameter Design calculation
VIN 214.8V
VOUT 450V
POUT 3.395kW
D 0.7266
fsw 100kHz
IO 7.55A
C 6.087µF
L1 = L2 =L 328.99µH
Table 6: input design parameters for tapped boost converter
3. WIND SYSTEM
Initially a diode rectifier was chosen to rectify the ac voltage
generated by the 3 phase permanent magnet synchronous
generator of the wind-turbine. Preliminary simulations that
where carried show that the input current was highly distorted
resulting in harmonics being injected to the generator. This
will result in overheating of the generator and poor
performance operation. To mitigate the issues of harmonics,
an active rectifier cascaded with a boost converter was used as
the power interface of the system. Input design parameters are
shown in table 7.
Figure 9: cascaded active rectifier for the wind turbine
ACTIVE RECTIFIER
VS 388VLL
IS
Vo,avg 514V
Idc 5.836A
fs 50Hz
fsw 10kHz
Cf 2.85mF
D 0.558
BOOST CONVERTER
VIN 514V
VOUT 800V
D 0.358
C 839nF
L 3.151mH
IOUT 3.75A
IL,avg 5.841A
∆Ipk-pk 0.584

7. 7
4. DC LINK BI-DIRECTIONAL CONVERTER
For the DC bus link, a bi-directional buck boost was used to
interface the two DC buses together. Table 7 shows the input
design parameters of the converter.
Figure 10: DC link bi-directional buck boost converter
VHIGH 800V
VLOW 450V
fsw 100kHz
BOOST MODE (450V - 800V)
POUT 3kW
D 0.4375
C 410nF
L 2.951mH
IO 3.75A
IL,avg 6.667A
∆IL,pk-pk 0.667A
∆Vpk-pk 40V
BUCK MODE (800V – 450V)
D 0.5625
IO 6.667A
POUT 3kW
∆IL, pk-pk 0.667A
L 2.951mH
C 84nF
∆Vpk-pk 22.5V
Table 8: input parameters for the dc link converter
Table 8 shows the input design parameters that will realise the
desired specifications of the DC link. The mode of operation
of the bi-directional buck boost will depend on the power
status of either DC buses.
5. STORAGE SYSTEM
To do away with operating with high boost ratios in the
discharging mode and operating with low duty ratios during
the charging mode a bi-directional converter employing
cascaded active converters was used. The individual active
transformers are linked by a high frequency transformer which
can step up the voltage or step down the voltage (discharging
and charging mode) based on the number of turns configured
on the transformer.
Using this converter allows the realisation of both high and
low voltage conversion ratios while driving the switches with
gate signals with favorable duty ratios whether if it is stepping
up or stepping down the voltage. This is advantageous in that
conduction losses for the switches are greatly reduced, a result
which points out to high efficiencies. Figure 11 shows the
converter configuration that was employed to realise the
power converter of the battery system.
Figure 11: bi-directional converter for the battery system.
6. THREE-PHASE INVERTER FOR 3-PHASE LOADS
The three-phase load will be connected to the 800VDC bus
whereby power will be supplied to it via a three-phase voltage
source inverter (VSI) employing the SPWM technique.
Assumptions that where made is that both the single phase and
three phases are purely resistive and the equations below hold:
S ∅
= P ∅
S ∅
= P ∅

8. 8
A 3kVA 3-phase voltage source is to be designed and thus the
output power of the inverter will be 3kW. Since the single
phase voltage was chosen to be 230V, the line to line voltage
for the three phase loads will be calculated as follows:
VL-L(rms) = √ VL-N = 398.3V
The peak amplitude of the line to line voltage is calculated as
follows:
VL-L,peak = √ VL-L(rms) = 563.38V
Since the amplitude voltage is less than the input voltage of
800V, the inverter will operate in the linear range where by
the modulation index will be less than 1. The relationship
between the peak amplitude line voltage and modulation index
is given by the relation that is as follows:
VL-L,peak = ma√
V
From the relation above, the modulation index was calculated
to be 0.81. Using the relationship given below, we can
calculate the peak amplitude of the modulating signal given
that a peak value of 1V was assigned to the triangular signal:
ma =
V
V i
With a modulation index of 0.81 and Vtri =1V, the peak
amplitude of the modulation signal was calculated to be 0.81.
For the SPWM, in order to generate three-phases, three sine
waves and a high frequency triangular carrier wave where
used to generate the PWM. The three sine waves have 120°
phase difference with each other. Since the frequency of AC
voltage that will be furnished to the load should be 50Hz, the
frequency of the three sine waves should also be 50Hz and the
frequency of the carrier waveform is usually given by the
relation below.
2N =
f
f
 N is an integer number, representing the number of
voltage pulses per half cycle.
 fc represents the frequency of the carrier signal
 fs represents the frequency of the sinusoid signal
Choosing a value for N = 30, and an fs value of 50Hz, the
frequency of the triangular carrier waveform was calculated to
be 1500 kHz.
The switching pulses are generated by comparing the
sinusoidal waveforms with the triangular wave. When the sine
wave is greater than the triangular voltage, a pulse is generated
at the output of the comparator. Figure 13 shows the circuit
diagram of the 3-phase voltage source that was used.
Figure 12: SPWM signal generator employing SPWM
Figure 12: 3-phase voltage source converter
7. SINGLE PHASE INVERTER FOR 1-PHASE LOADS
For the single phase inverter, a voltage source employing bi-
polar SPWM was used. A 3kVA inverter was designed
making assumptions that the load will be purely resistive thus:

9. 9
S ∅
= P ∅
= 3kVA
The output voltage of the inverter was chosen to be 230V
(rms) and thus the output current was obtained using the
following relation:
Is,rms =
×
= 13.043A
Is,peak = √ ×13.043 = 18.44A
The modulation index of the inverter was calculated as
follows:
ma =
V √
V
=
× √
= 0.723
As was mentioned in the previous assignment, a DC –link
capacitor was designed so as to decouple the input power and
the inverter and the output power to the load. Electrolytic
capacitors were chosen as the DC-link capacitors. Two factors
were considered during sizing of the DC link capacitor:
 transient DC fluctuations which are caused by rapid
variation of input power flowing into the DC-link
capacitor.
 AC fluctuation of the DC-link voltage that is usually
caused by the double-line frequency ripple power
generated from the load side which might result in
distortion of the load current
To try and suppress or limit the double-line frequency voltage
to the desired level of 1% of the DC bus voltage, the DC-link
capacitor was sized as follows:
CDC =
S
ωgV V , ipp
=
×
. .
= 2.357mF
The equivalent resistance of the load was calculated using the
per phase values of the system using the following relation:
R =
VLN
P ∅
= 17.6Ω
G. RESULTS
Simulations of all converters were carried out using
PSIM. Simulations of output voltage, output current and
power will be presented.
1. PV SYSTEM
Simulations of the two level interleaved were carried out. The
converter’s function is to boost the voltage generated by the
PV panels to a voltage of 450V, a voltage compatible with the
450 VDC bus.
450V
Figure 13: output voltage of the boost converter
6.67A
Figure 14: output current of the boost converter
3.38kW
Figure 15: output power of the boost converter
2. FUEL CELL SYSTEM
Simulations of the tapped inductor boost converter were
carried out. The converter’s function is to boost the voltage
generated by the fuel cell stacks to a voltage of 800V, a
voltage compatible with the 800 VDC bus.
800V
Figure 16: output voltage of the tapped inductor converter.

10. 10
3.75A
Figure 17: output current of the tapped inductor converter
0 0.002 0.004 0.006 0.008 0.01
Time (s)
0
1000
2000
3000
4000
VP1*I2
3kW
Figure 18: output power of the tapped inductor converter
3. WIND SYSTEM
Simulations of the tapped inductor boost converter were
carried out. The converter’s function is to boost the voltage
generated by the fuel cell stacks to a voltage of 800V, a
voltage compatible with the 800 VDC bus
800V
Figure 19: output voltage of the active rectifier
3.75A
Figure 20: output current of the active rectifier
3000W
Figure 21: output power of the active rectifier
4. THREE-PHASE INVERTER FOR 3-PHASE LOAD
Vab VbcVca
Vrms =398.32V
Figure 22: output voltages of the three phase inverter
Irms =4.352A
Figure 23: line current of the single phase inverter
5. SINGLE-PHASE INVERTER FOR 1-PHASE LOAD
Vrms = 229.9V
Figure 24: output voltage of the single phase inverter

11. 11
P=2.997kW
Figure 25: output power of the single phase inverter
6. DC LINK BI DIRECTIONAL BUCK BOOST
800V
Figure 26: Output voltage at the 800VDC bus
3.748A
Figure 27: output current at the 800VDC bus
P=3kW
Figure 28: output power at the 800VDC bus
450V
Figure 29: output voltage at the 450VDC bus
6.67A
Figure 30: output current at the 450VDC bus
P=3kW
Figure 31: Output power at the 450VDC bus
7. BATTERY POWER SYSTEM
Figure 32: output voltage during the discharge mode
48V
Figure 33: output voltage during charging mode

12. 12
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13. HYBRID SYSTEMS (COMPLETE SYTEM)
Figure 34: Circuit diagram of the entire hybrid system
Figure 35: The hybrid system as captured using PSIM

14. 800V
Figure 36: Output voltage at the 800VDC bus of the hybrid system.
450V
Figure 37: Output voltage at the 450VDC bus of the hybrid system