ahmed2018

2018 China International Conference on Electricity Distribution
Tianjin, 17-19 Sep. 2018
Operation planning of wind-PV-battery hybrid
system
Abdulla Ahmed, Member, IEEE and Tong Jiang
State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources, School of Electrical
and Electronic Engineering, North China Electric Power University, Beijing - China

Abstract—This paper explains the operation principles of two
renewable energy sources (RES) hybrid system consists of solar
photovoltaic (PV) and wind power (WP) which are connected to
the battery energy storage (BES) system and load demand. The
system has been designed for the city in Sudan and the results are
presented for variable wind speeds and different solar radiations.
From the results, it can be seen that the maximum power is being
extracted for all wind speeds from rated speed to average speed.
The location of the solar power station is suitable to install near to
the wind farm station since the sun and wind powers usually
complement each other, this could prove to be a good
combination. In addition, most importantly the solar energy and
wind power generation systems also produce no harmful waste
and deplete no resources. The proposed system is important for
the backup system during the changes in solar radiation and wind
speed under the assumption that the load demand is fixed as well
as helpful for power system management and designing.
Index Terms— Solar energy, wind energy, battery energy storage
system, modeling and design.
I. INTRODUCTION
I
N the world, some regions such as Sudan has a great wealth
of RES, especially solar and wind energies. These energies
do not utilize to compensate for the increasing demand for
electrical power. Also, the burden on transmission network is
increasing at an unexpecting manner so that the transmission
network becomes economically cheap. Furthermore, the
depletion of fossil fuels and the rampant increase in the price of
these fuels have resulted in increased interest to include RES
for power productions. As a result some areas far from the
Manuscript received July 15, 2018. This work was financially supported by
the Science and Technology Project of State Grid Corporation of China
No.SGHE0000KXJS1700086.
Abdulla Ahmed was with the Department of Electrical and Electronics
Engineering, Faculty of Engineering Science, University of Nyala, Nyala –
Sudan. He is now with the School of Electrical and Electronic Engineering;
North China Electric Power University, Beijing-China. (Corresponding author
phone: 008613241448448; e-mail: [email protected]).
Tong Jiang is a professor with the School of Electrical and Electronic
Engineering; North China Electric Power University, Beijing-China. (Phone:
008613691338016; e-mail: [email protected]).
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Paper No. 201805280000367
978-1-5386-6775-0/18/$31.00 ©2018 IEEE
national electricity network there is a great need for including
RES like wind and solar sources as major contributors to the
electrical power system.
Control and power management of a hybrid system that
consists of WP/PV and subsystems and BSS connected to AC
residential load and the power grid (PG) which is used as a
backup source are presented to manage the electricity between
its different generation sources and the system load demand
profile [1],[2]. An optimization system aiming to obtain the
overall costs, which including fuel costs and maintenance costs
by taking an independent micro-grid of a certain school as a
case study for the design is introduced in the work [3]. Two
algorithms are presented for optimal operation of the hybrid
system consist of wind/PV and BESS in the grid-connected
mode of a microgrid. The first one is called sources sizing
algorithm to obtain the optimal sizes of RES and the second one
is called battery sizing algorithm to obtain the optimal capacity
of BESS [4]. For the Stand-alone mode of a microgrid, hybrid
system configuration consist of wind/PV/diesel/storage based
techno-economic analyses to plan the optimal scheduling and
solve the problem for the future development planning of island
power supply is presented in [5],[6]. An artificial intelligent
method to design the stand-alone energy containing PV and WP
with BESS is pretested for reducing the yearly operating costs
while meeting the basic requirements of the stand-alone energy
system design [7]. The performance analysis of a PV-connected
grid system with compressed air energy storage system (CAES)
is proposed and investigated in terms of thermodynamic and
exergy and the results show that the efficiencies of the CAES
system are improved from 35% to 65% during the operation
period in the year [8]-[10].
The main contribution of this work is to model and design the
distributed hybrid generation system based on PV system and
WP farm connected to the BESS. Implementation of distributed
generation will reduce the CO2 emission as well as minimize
the generation cost. The rest of the paper is organized as
follows: Section II presents a general description and
mathematical modeling of the system. The case study is dealt in
Section III. Section IV describes the results and discussions.
Finally, the conclusion is presented in Section V.
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II. GENERAL DISCRIBTION AND MATHEMATICAL
MODELING OF THE SYSTEM
The proposed system is constituted of two kinds of RES,
the first is WP with rectifiers and the second is PV with DC/DC
converter system; the system load demand and lead-acid
battery are placed in order to mitigate the variability of the of
RES as shown in figure 1.
Fig.1. Schematic diagram of the proposed system.
A. Modeling of Solar System
The solar cell is basically a p-n junction fabricate in a thin
wafer or layer of semiconductors. The radiations of solar
energy can be directly converted to electricity through the
photovoltaic effect. There are many types of solar cell models.
This paper will discuss the ideal photovoltaic model and
photovoltaic model with Single-diode and series resistance.
1- Ideal Photovoltaic Model
The ideal equivalent circuit of a PV cell is a current source in
parallel with a single-diode. The configuration of the simulated
ideal solar cell with single-diode is shown in figure 2.
Tianjin, 17-19 Sep. 2018
𝐼 = 𝐼𝑃𝑉 − 𝐼0 [exp (
𝑉
𝐴𝑉𝑇
) − 1]  

Where 𝐼𝑃𝑉 is the current generated by the incidence of
sunlight; 𝐼0 is the diode reverse bias saturation current and 𝑉𝑇 is
the thermal voltage of a PV module and can be expressed by:
𝑉𝑇 =
𝑁𝑆 ∗𝑘∗𝑇
𝑞



Where Ns is cells connected in series; q is the electron
charge; k is the Boltzmann constant; T is the temperature of the
p-n junction and A is the diode ideality factor.
A PV cell can at least be characterized by the short circuit
current (ISC ), the open circuit voltage (VOC ) and the ideality
factor A.
The output of the current source is directly proportional to
the light falling on the cell. For the same irradiance and p-n
junction temperature conditions, the short-circuit current (ISC )
is the greatest value of the current generated by the cell. The
short current is given by:
For V=0,
ISC = I = IPV 


Likewise, for the same irradiance and p-n junction
temperature conditions, the open circuit voltage (𝑉𝑂𝐶 ) is the
greatest value of the voltage at the cell terminals and it can be
written as:
𝑉 = 𝑉𝑂𝐶 = 𝐴 ∗ 𝑉𝑇 ln [1 +
𝐼𝑆𝐶
𝐼0
] 



In Eq. 2, for I = 0. Thus,
ISC = IPV 
And at the same conditions, the output power is given by:
Fig. 2. Ideal PV cell with single-diode.
𝑉

The output current of an ideal solar cell can be expressed by:
𝑃 = 𝑉 {𝐼𝑆𝐶 − 𝐼0 [𝑒𝑥𝑝 (
𝐼 = 𝐼𝑃𝑉 − 𝐼𝐷 
2. Photovoltaic Model with Single-diode and Series
Resistance
To build the photovoltaic model with single-diode and series
resistance can only adding a series resistance to the previous
model to be more accuracy and complexity. The circuit of this
model is shown in figure 3.


The current passed through the diode (𝐼𝐷 ) can be given by:
𝐼𝐷 = 𝐼0 [exp (
𝑉
𝐴𝑉𝑇
) − 1]


𝐴∗𝑉𝑇
) − 1]}
By substituting Eq. 2 into Eq.1, The output current of an ideal
solar cell is expressed as:
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Tianjin, 17-19 Sep. 2018
The power in moving air is the flow rate of kinetic energy per
second. Therefore:
𝑝𝑜𝑤𝑒𝑟 =
1 𝑑𝑚
2 𝑑𝑡
𝑣2 


The power extracted by the blades is customarily expressed
as a fraction of the upstream wind power as:
𝑝0 =
Fig. 3. PV circuit model with single-diode and series resistance.
For the same irradiation and temperature conditions, the
inclusion of a series resistance in the model implies the use of a
recurrent equation to determine the output current in function of
the terminal voltage. The I-V characteristics of the solar cell are
given by:
𝐼 = 𝐼𝑃𝑉 − 𝐼0 [exp (
𝑉+𝐼∗𝑅𝑆

𝐴∗𝑉𝑇


The short circuit current 𝐼𝑃𝑉 is given by:
𝐼𝑠𝑐 = 𝐼 = 𝐼𝑃𝑉 − 𝐼0 [exp (
𝐴∗𝑉𝑇
) − 1]

For V = 0 and ISC = IPV
Normally the series resistance is small and negligible. The
open circuit voltage 𝑉𝑂𝐶 can be given by:
𝑉 = 𝑉𝑂𝐶 = 𝐴 ∗ 𝑉𝑇 ln [1 +
𝐼𝑆𝐶
𝐼0
] 

For I = 0, and the output power is given by:
𝑃 = 𝑉 {𝐼𝑆𝐶 − 𝐼0 [exp (
𝑉+𝐼∗𝑅𝑆
𝐴∗𝑉𝑇
) − 1]}

B. Modeling of Wind Power
Wind energy can be defined as the process by using the wind
turbines to convert the kinetic energy in the wind into
mechanical power. Once a wind turbine has converted the
kinetic energy in the wind into rotational mechanical energy,
that rotational energy is usually converted by a generator into
electricity.
The kinetic energy in the air of mass “m” moving with speed
v is can be given by:
𝐾𝑖𝑛𝑒𝑡𝑖𝑐 𝐸𝑛𝑒𝑟𝑔𝑦 =
CICED2018
1
2
2
𝑚𝑣

2

Paper No. 201805280000367

𝜌. 𝐴 . 𝑉 3 . 𝐶𝑝

Where p0 is the output power (KW); ρ is the air density
(Kg/m3 ); A is the area swept by the rotor blades (m2 ) and V is
the velocity of the air (m/s 2 ). 𝐶𝑝 is the fraction of the upstream
wind power, which is captured by the rotor blades. The
remaining power is discharged or wasted in the downstream
wind. The factor 𝐶𝑝 is called the power coefficient of the rotor
or the rotor efficiency and can be determined by:

) − 1]
𝐼𝑃𝑉 ∗𝑅𝑆
1
𝑉
𝐶𝑝 =
𝑉
2
(1+ 𝑉0 )(1−( 𝑉0 ) )

2

For a given upstream wind speed, the value of 𝐶𝑝 depends
on the ratio of the downstream to the upstream wind speeds,
that is (𝑉0 /V). The maximum value of 0.59 when the (𝑉0 /V) is
one-third. The maximum power is extracted from the wind at
that speed ratio when the downstream wind speed equals
one-third of the upstream speed. Under this condition, the
power output can be written as:
𝑝𝑚𝑎𝑥 =
1
2
𝜌. 𝐴 . 𝑉 3 . (0.59)


The theoretical maximum value of 𝐶𝑝 is 0.59. In practical
designs, the maximum achievable 𝐶𝑝 is below 0.5 for
high-speed.
III. CASE STUDY
This work is a part of project developing in Sudan. We used
data from the developing wind farm and solar PV system in the
small city in Sudan as the case study. The ministry of electricity
and dams of the Republic of Sudan is developing a wind farm
with a final capacity of 20 MW and PV power station with final
capacity of 5MW. These plants are planned to be integrated into
an existing isolated Diesel power station. A wind speed and
solar radiations measurements has been finished and identified
areas of high wind speed and solar radiations and investigated
the feasibility of generation electricity by wind and solar
energies. The mechanical conversion efficiency of the wind
turbine is taken as 95% and the electrical conversion efficiency
of the generator is taken as 90%.
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IV. RESULTS AND DISCUSSIONS
For the PV system, the power generated is a function of the
temperature and the sunshine values of the site where the solar
cell is placed. This power produced can increase or decrease
according to any variations in the temperature and/or the
shining. In this system, the temperature is considered as
constant and equal to 25°C, and the sunshine changed from
zero to 1000𝑤/𝑚2 . It can be seen that from the table I, the
maximum power rating is 300wp for the one section and the
maximum input power into the inverter is 600kwp as shown in
Table III.
For the wind power, the value of the power generated
depends on the performance coefficient CP and the available
wind speed. To maximize this output power, and as the wind
speed is varying from time to time, the performance coefficient
must be maximized and controlled. Table II shows the results
of the specific parameters of the wind turbine and power rated
is 850kw.
For the battery energy storage system conditions, the battery
system should not be discharged to the minimum limit and
charged to the maximum limits; because maximum charging
and minimum discharging will cause a decrease in the life cycle
of the battery system. Thus, in order to overcome this problem,
the SOC should be controlled. Therefore, SOC must be lies
between 0.3<SOC<0.8. The initial value of SOC is considered
to be equal to 0.3. For the load system demand conditions, the
load is manifested by the current it needs from the source and
both the voltage of the wind turbine and the solar panels should
be higher than the voltage of the battery system.
TABLE I
SIZING INFORMATION RELATED TO THE PV FARM
Power rating
Module efficiency
Rated voltage
Rated current
open circuit voltage
Short circuit current
Max reverse current
Max system voltage
300wp
15.15%
37.03V
8.12A
44.98V
8.46A
20A
1000V
TABLE II
POWER GENERATED FROM THE VARIABLE WIND SPEED
Wind Turbine Parameter
Value
Cut-in wind speed
34m/s
Cut-out wind speed
32 m/s
Rated power
850kw-1MW
Type
3 blade upwind
Rotor diameter
54m
Rotor swept area
2123.7 m2
Nominal wind speed
6.5m/s
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Paper No. 201805280000367
Tianjin, 17-19 Sep. 2018
TABLE III
INPUT PARAMETERS FOR THE INVERTOR
Max input power
600kwp
DC voltage range.mpp
450 to 750V
Max DC voltage
900V
Max DC current
1145A
Voltage rippl
˂ 3%
V. CONCLUSION
The work chooses a small city in Sudan as the case study to
designs a hybrid system containing solar PV and wind power
connected with BESS aiming to reduce the operation cost. The
paper explained the modeling and design of the proposed
system to obtain the behavior and characteristics performance
of the system, which can be helpful in the impact of RES into
the power grid system with BESS to produces clean energy as
well as minimizing the use of fossil fuels and reduce emissions.
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