Power Management Strategy of a Standalone Hybrid
Energy Power System 1A. Ananthi Christy and
2Vara Rushiraj
1Department of EEE,
SAVITHA University, Chennai. 2Department of EEE,
SRM University, Chennai.
Abstract The objective is to analyze control strategy for proposed hybrid power
system for stand-alone applications. The system includes photovoltaic system
and a proton exchange membrane fuel cell (PEM-FC) with battery storage
system. In the proposed system PV will be taken as a primary source to
supply the load and FC will be backup source. During day time when
sunshine is sufficient PV will feed the load and excess power will be stored in
a battery storage system and when the weather conditions unfavorable say
during cloudy day or night time, FC will come into picture to supply the
load.Hence by integrating these sources we can maintain and control demand
power. A Simulation model is designed for the proposed hybrid energy
system under MATLAB/SIMULINK environment. The control strategy used
in this proposed system is fuzzy logic controller (FLC). The proposed system
is most effective one to be used in remote areas or an island where grid
supply is difficult and costly.
Index Terms:Hybrid power system, photovoltaic system, fuzzy logic
controller, maximum power point tracking.
International Journal of Pure and Applied MathematicsVolume 114 No. 12 2017, 59-69ISSN: 1311-8080 (printed version); ISSN: 1314-3395 (on-line version)url: http://www.ijpam.euSpecial Issue ijpam.eu
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1. Introduction
The demand of renewable energy technology has been increasing day by day for
fulfilling the energy deficiency. Present day challenges such as environmental
pollution, energy crises, climate change, global warming etc. can be reduced by
appropriate use of renewable energy sources. Renewable sources like solar,
wind and fuel cells are significant sources to overcome mentioned problems.
Solar system is one of the most remarkable source due to advantages such as no
moving parts, no emission or harmful gases, compact design, minimal
maintenance etc. However there are some disadvantages that makes solar
system inefficient like it is not available during night time or cloudy atmosphere
so the output will vary and it will decrease efficiency and reliability of the
system. Hence single renewable source might be unable to feed the power all
the time due to the intermittent nature of renewable sources. The possible
solution to this problem is that we can integrate two or more RES to meet the
demand. It will improve availability of power all the times therefore system will
become more efficient and reliable. Such system will be known as hybrid power
system in which solar system will be taken as a primary source and other source
such as fuel cell or ultra-capacitor will be backup source whenever solar is not
sufficient. Battery can also be used for storage purpose to store excess power
and feed the power difference. Fuel cell can be taken as a backup source. This
combination is the most effective one. The proton exchange membrane fuel cell
has advantages such as low working temperature, compactness, excellent load
following capability etc. Due to the slow response of FC we will connect battery
so it can improve dynamic characteristics of the hybrid power system. This
hybrid power is controlled by fuzzy logic controller in which membership
function and fuzzy rules are developed. The application of the hybrid power
system is for stand-alone residential purpose such as some remote location or
island where grid power is not available. Hence by integrating these sources we
can feed uninterruptable power supply. The system is developed using Mat-
lab/Simulink environment.
2. System Description
The proposed block diagram of hybrid power system is shown in the figure
1.The system consists of PV system with maximum power point tracking
(MPPT), FC system and battery system for storage purpose at the source side.
Incremental conductance(IC) algorithm is used for maximum power tracking of
PV system. Here fuel cell is used as a backup source and PV system will work
as a primary source. There are some advantages of PEM (Proton exchange
membrane) fuel cell type over others like low working temperature, fast
dynamic response, compactness etc... If PV system fails FC or Battery will
supply the power. Battery is used whenever both the sources are insufficient to
feed the power. Fuzzy logic controller is used to supply uninterruptable power
supply. cuk converter is used for step up the generated
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Voltage and the output is given to the load via inverter for -Sinusoidal voltage
waveform. Neutral point clamp inverter is used for generation of multilevel
output voltage waveform. The system is designed for standalone applications
where grid power is not available and costly such as remote area or island.
Fig. 1: Block diagram
3. System Modeling
A. PV System Modeling with MPPT Technique
The figure 1 shows one diode equivalent circuit model of PV cell. The model
contains one current source Iph, one diode, series resistance Rs and parallel
resistance Rsh.
Fig. 2: One diode model of PV cell
Where, Iph is the total current produced by the cell for a given irradiance and
temperature conditions in Amperes, Id is the diode current and V is load
voltage. PV systems have nonlinear characteristics which will vary the power
output. Hence it is necessary to get the maximum power which will be tracked
by MPPT technique. Such a point is highly dependent on temperature and
irradiance of the cell which will be varying throughout the day time. Here PV
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system rating is 1.1 kW with 250 series connected cells and 25°c temperature,
short circuit current, Isc=7.34 A, open circuit voltage, Voc=0.6v.
Table 1: PV system parameters
Parameters Value
Type Series connected Solar cells
Power rating 1.1 kW
Voltage 150 volt
Number of cells 250
The incremental conductance method has been developed to overcome the
drawbacks of previous method P&O. In incremental conductance method the
conductance value is compared with the incremental conductance value and
according to power curve duty ratio is increased when slope of power curve is
positive and duty ratio is decreased when slope of power curve is negative [2].
Hence this method is more complex and advanced version of P&O technique
which has drawbacks such as oscillations in MPP. The method will calculate
conductance value and flowchart is shown in figure 3 below.
Fig. 3: Flowchart of IC MPPT technique
B. Fuel Cell System Modeling
Fuel cells are electrochemical devices in which chemical energy is converted
into electrical energy. Proton Exchange membrane fuel-cell (PEMFC) is reliable
when supply is intermittent and is commercially available at large industrial
scale capacities [2]. Dynamic response of PEMFC is fast with 1-3s time of
power release.
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Operating regions of fuel cell are activation polarization, ohmic polarization,
and concentration polarization. The activation polarization can be considered as
the no load region where there is a large voltage drop for a small increase in the
output current [2].
Generally single fuel cell output voltage is low therefore group of fuel cells are
connected in series to get the high DC voltage.
Table 2: Fuel cell system parameters
Parameters Value
Type PEMFC(Proton exchange membrane fuel cell)
Power rating 1.26 kW
Voltage 24v DC
Number of cells 42
C. Battery System Modeling
Battery is a device which is used as a storage purpose and it will store energy in
electrochemical form.it has wide range of applications such as hybrid electric
vehicle and hybrid power system.in this paper the battery is integrated to PV
and FC system to develop hybrid power system.
Nickel metal hydride battery is used with a nominal voltage of 18v and initial
SOC (state of charge) 100%.
4. Power Management Strategy Using FLC
Fuzzy logic incorporates a rule-base structure in attempting to make decisions.
Fuzzy logic is an aggregation of rules, based on the input state variables
condition with a corresponding desired output. [4] In this paper the controller is
implemented in such a way that it will control the three sources to feed
uninterruptable power supply.
The controller has one reference input and one output. This fuzzy logic
controller is implemented using membership functions and 18 if…then fuzzy
rules. Error signal generated from the difference of load power and source
power is taken as input.
PV will work for first 0 to 0.04s with irradiance as 1000 w/m2 after that FC will
switch on for 0.04 to 0.07 when irradiance is 600 w/m2.lastly battery will supply
power of difference for 0.0.7 to 1s when PV and FC both are insufficient to feed
the load.
Table 3: Switching of sources
Time 0 to 0.04s 0.04 to 0.07s 0.07 to 0.1s
PV 1000 w/m2 600 w/m
2 zero
FC Off Off On
BATTERY Charge Discharge Discharge
LOAD 1405 w (1405+100)=1505w
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5. Simulation Results and Analysis
Fig. 4: Simulation diagram of PV system with IC MPPT technique
Fig. 5: Output waveforms of Irradiance, current, voltage and power respectively of PV
system
Fig. 6: Simulation diagram of hybrid PV/FC/Battery power system
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Fig. 7: Cuk converter input power
Fig. 8: Cuk converter output voltage and current
Fig. 9: Multilevel inverter three phase output voltage waveform without filter
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Fig. 10: multilevel inverter three phase output voltage waveform with filter
Figure 4 shows simulation diagram of PV system MPPT applied. Dynamic
response of the system is analyzed by step changes in irradiance and to check
how controller is worked. Irradiance changes at 1000 w/m2 to 800 w/m2 for T=
0.4 to 0.7s and it becomes zero for T= 0.7 to 1s.output waveforms of current
voltage and power are changes according to irradiance changes as shown in
figure 5.
Figure 6 shows complete simulation diagram of hybrid power system in which
PV, FC and battery are integrated. Here Incremental conductance MPPT
technique is used for maximum power generation. Outputs of these three
sources are given to cuk converter for step up the voltage. Neutral point clamp
inverter is used for sine wave generation. This system is suitable for stand-alone
applications so here load is taken is RL load and one additional load is taken for
specific period of time testing how system will work.
Figure 7 shows cuk converter input power which is being almost constant
around 1000w throughout with respect to time. There is a spike in the waveform
which shows switching of the sources at time T= 0.04s and T= 0.07s. Hence we
can say that uninterruptable power supply is maintained at the source side by
using controller.
Multilevel inverter output waveform without filter and with filter are shown in
figure 9 and figure 10.Here three phase voltages are shown with proper sine
wave voltage coming around 450volts which is appropriate for residential
applications.
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6. Conclusion
A hybrid power system consisting of PV system and FC system with battery as
a storage purpose is used. The proposed system is most effective for stand-alone
applications such as island or some remote areas where grid power is not
available. MPPT technique is used to track the maximum power from PV.
Fuzzy logic controller is implemented for management of power. The system
draws maximum power when sun-light is available and FC will supply power
during night time or when PV is switched off. Battery will be used to store the
excess power and feed power when both PV and FC are insufficient to supply
demand. It is seen that by integration of different sources uninterruptable power
supply can be feed.
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