Indian Journal of Science and Technology, Vol 8(32), DOI: 10.17485/ijst/2015/v8i32/87764, November 2015
ISSN (Print) : 0974-6846
ISSN (Online) : 0974-5645
Buck Boost Inverter based Photovoltaic Power
Generation System
G. Rohini1 and A. Jaffar Sadiq Ali*2
Department of Electrical and Electronics Engineering, Jerusalem College of Engineering, Chennai - 600100,
Tamil Nadu, India; rohinimuhunthan@gmail.com
2
Department of Electrical and Electronics Engineering, Bharath University, Chennai - 600073,
Tamil Nadu, India; jaffarsadiqali.eee@bharathuniv.ac.in
1
Abstract
This paper deals with using buck boost converter principle for producing AC voltage from PV array. It uses simple buck boost
converter with switches, to invert D.C. in one single stage. The filter inductor is added to get a near sinusoidal waveform
with low THD. The buck boost chopper initially boosts up the DC voltage from PV array to required level. The operation
is similar to buck boost chopper. Then this voltage is converted to AC using switches and filters. Both the operation of
stepping up and conversion takes place simultaneously thus forming a single stage converter system.
Keywords:
1. Introduction
Development in new trends and technologies has put
immense pressure and severe exploitation of fossil fuels.
The increased use of it may lead to complete usage of
available fossil fuels. In order to have sustainable use,
we go for renewable sources of energy. One such source,
which is available free of cost is solar energy. Photovoltaic
(PV) source has achieved global attention and use of
PV arrays along with Power Converter Systems (PCS) is
becoming common nowadays.
In order to transfer the energy from PV array to utilities
PCS have to fulfill the following three requirements:
• To convert the DC voltage into AC voltage.
• To boost the voltage.
• To ensure maximum power utilization of the PV
module.
Inverters are used to convert DC voltage from PV
array to AC of 50 Hz in frequency. Boosters are used in
stepping up the voltage to required level. The boosters
may be DC choppers at DC side or step up transformers
at the AC side. Universal Single Stage Grid Connected
Inverter1 can be switched between buck, boost and
buck boost configurations by altering the Pulse Width
* Author for correspondence
Modulation (PWM) control. Discontinuous mode of
operation is used here to facilitate the shuffling. Multilevel
(three levels) half bridge diode-clamped inverter2
produces three different voltage levels: VC1, 0 and −VC2.
A Novel Single Stage Full Bridge Buck Boost Invertering
gives the basic inverter circuit with inductor series with
the switch. The switches here operate at fixed frequency
with PWM. It is basically used in UPS3. A Single-Stage
Single-Phase Transformerless Doubly Grounded GridConnected PV Interface can be used to connect the
power from PV array to grid directly. It employs double
grounding features4. MPPT with Capacitor Identifier
for PV Power System uses 2 split PV sources which is
in similar operation as previous one. Here to track the
maximum power point, the perturbation and observation
method is adapted. It moves the operating point towards
the maximum power point by periodically increasing or
decreasing the photovoltaic array voltage. To increase or
decrease its voltage, the duly ratio of on-state of switching
device is changed in the pulse width modulation5. Single
stage transformer-less PV inverter topologies for single
phase and three phases are analyzed and compared in6. A
transformerless, voltage-boosting inverter for AC modules
uses only one PV array. Here two switches are modulated
Buck Boost Inverter based Photovoltaic Power Generation System
by PWM. The rest are switched synchronizing with low
frequency. The drawback here is that double grounding
cannot be achieved7. A Novel High Performance Utility
Interactive Photovoltaic Inverter System uses a topology
that expresses buck characteristics. So it can be used only
where input voltage is higher than output. Here each PV
array should have its own GCC8.
Most of the above converters employ two stages of
conversion while the proposed converter employs a single
stage conversion using buck boost chopper circuit and
also the proposed converter can have double grounding
features for connection with the grid.
Because of single stage operation, the efficiency of the
system can be greatly improved by preventing losses.
In summary, the proposed system:
• Has only one stage to realize boost and inversion.
• Employs only one PV array so the effective utilization
of PV array can be improved.
• The average output increases and has few ripples.
• The THD is within the IEEE standards.
2. System Description
The Figure 1 shows the block diagram of the proposed
converter. It consists of PV array, a converter block, which
does both the operation of stepping up of voltage and
inversion in single stage, a filter for reducing harmonics
and load9.
Figure 1. Block diagram of proposed converter.
2.1 Traditional Buck Boost Converter
Here we consider a traditional buck boost chopper circuit.
The chopper consists of a buck boost inductor which is
connected through a switch alternatively to the source
and to the load. There is a diode to prevent the flow of
current to load during charging period10. A capacitor is
used to maintain the output across the load and to reduce
the ripple. The basic circuit is shown in Figure 2.
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Figure 2. Circuit diagram of traditional buck boost
chopper.
2.1.1. Operating Modes
Mode 1: When the switch S is on, the diode becomes
reversed biased thus allowing the current to flow through
the inductor. The inductor current changes linearly and
the rate of change of the current is constant. The inductor
gets charged.
Mode 2: When the switch S is off, the polarity of
inductor gets reversed to maintain the current flow. This
forward biases the diode D. Thus the energy stored in
inductor is released to the load capacitor and resistor11.
The capacitor is used to maintain the output voltage and
to reduce the ripple.
The output voltage gets either bucked or boosted
depending upon the duty ratio (k). Based on the below
formula:
Vo = Vs *
k
1- k
The output voltage will be boosted if k is greater than 0.5,
else output voltage is less than the input. The above
formula is applicable if the inductor current is continuous.
For discontinuous current this formula cannot be used.
The advantage of using inductor in discontinuous current
mode of operation is that the value of inductor gets
reduced. So in this proposed circuit, inductor current is
discontinuous. So the formula for output voltage is
obtained by interpolation.
2.2 Proposed Converter
In proposed converter the inductor current is made
discontinuous. By having input voltage and load to be
constant, a set of values for pulse width are simulated and
through interpolation the relation between output voltage
(V0) and pulse width (d) is obtained as
Vo = 174+(d-0.45)*380
Indian Journal of Science and Technology
G. Rohini and A . Jaffar Sadiq Ali
This concept is used to get a boosted DC output across
the load. This is then, converted to AC with the help of
switches12.
Figure 3. Circuit diagram of proposed boost converter.
In Mode 2, S5, S2, D1 is ON. When S1 gets turned OFF,
the inductor polarity reverses and the diode D1 gets
forward biased. As switches S5 and S2 are ON the inductor
discharges its energy to the load through filter capacitor
and inductor. This is shown in Figure 4b.
In Mode 3, S1, S5 is ON. When S1 and S5 are ON the
inductor gets charged and the polarity of inductor reverse
biases diodes D2 and D1. This is shown in Figure 4c.
In Mode 4, S3, S4, D2, D3 ON. When S3 and S4 are
ON the inductor polarity gets reversed and it forward
biases diode D2 and D4.Thus the current now flows in the
opposite direction through filter and load14. This is shown
in Figure 4d.
The converter consists of DC source, which is a PV
array, connected to the buck boost inductor through
switches as shown in Figure 3.
The capacitor Cin is decoupling capacitor and Cf is
used for filtering purpose along with filter inductor Lf.
In this converter, switches along with diodes are used for
inverting DC to AC in single stage.
(c)
2.2.1 Operating Modes
The converter has four operating modes. S1 is operated at
10 kHz with pulse width 60%. S2, S4 and S3 are operated at
50 Hz. S5 is ON for 10 ms along with S2 and is operated at
10 kHz with pulse width 60% for the next 10 ms, when S3
and S4 are ON.
(d)
(a)
Figure 4 shows the equivalent circuits for different
modes of operation. In mode 1, S1, S5, S2 is ON. The
inductor gets charged. Meanwhile the filter capacitor,
charged in previous mode, discharges and maintains the
output voltage across the load13.
Figure 4 . CFigure 4. (a) Circuit for mode 1.
(b) Circuit for mode 2. (c) Circuit for mode 3.
(d)Circuit for mode 4.
3. Design Analysis
We know that
E = P*T
Let
Emax = Vo*Io*Ts
(b)
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Indian Journal of Science and Technology
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Buck Boost Inverter based Photovoltaic Power Generation System
From basic equation for voltage across inductor, we
get the average Ton and Toff to be
Ip
T on = Lc *
Vp
Ip
T off = Lc *
Vo
But,
Ts=Ton + Toff
The simulation of the converter is done by using
MATLAB/Simulink.
The simulation circuit consists of inductor, diode, input
DC voltage source, load resistor, MOSFET, capacitors and
filter inductor. The specification of the converter is given
in Table 1.
Table 1. Specifications of the converter
Thus on adding Ton and Toff we get
-1
æ Ts ö ïì 1
1 ïü
Ip = çç ÷÷ * ïí + ïý
è Lc ø ïîïVp Vo ïþï
Also the energy stored in inductor is
E=
4. Results and Discussions
Lc * Ip2
= E max
2
Therefore critical inductance
-2
æ 0.25Ts ÷ö ìïïæ 1
1 öüï
Lc = çç
÷÷ * íçç + ÷÷÷ïý
èçVp * Ip ø÷ ïîïçèVp Vo ø÷ïþï
Specification
Output Voltage (RMS)
Input Voltage
Duty Cycle
Decoupling Capacitor
Buck boost inductor
Filter capacitor
Filter inductor
Switching Frequency
Parameter
V0
Vin
d
Cin
L
Cf
Lf
f
Value
230 V
95 V
60%
4500 μF
160μH
20μF
660 mH
10 kHz
Figure 5 shows the open loop simulation diagram. In
open loop circuit the output voltage is 228V (RMS) for
the input voltage 95V. The output voltage is thus boosted.
A disturbance in source side is given in Figure 6. The
voltage of the source is varied from 95V to 85V as PV
Figure 5. Open loop circuit diagram with RL load.
Figure 6. Open loop circuit diagram with R load without disturbances.
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Vol 8 (32) | November 2015 | www.indjst.org
Indian Journal of Science and Technology
G. Rohini and A . Jaffar Sadiq Ali
Figure 7. Open loop circuit diagram with R load and with disturbance at source side.
Figure 8. Output voltage waveform with disturbance at 0.2s.
Figure 9. Gate pulse waveforms.
Figure 10. Output waveform with RL load.
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Indian Journal of Science and Technology
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Buck Boost Inverter based Photovoltaic Power Generation System
array voltage can have a maximum variation of 10 %. As
the voltage varies the output voltage too varies which
can be seen clearly from the output waveform shown in
Figure 7. The output voltage before the disturbance was
228V and after disturbance at 0.2 seconds, the output
voltage drops to 212V. The gate pulse for the MOSFETs is
shown in the Figure 8.
The Figure 9 shows the open loop simulation circuit
with RL load. The load is such that XL is greater than R.
The Figure 10 shows the output waveform with RL load.
Figure 11. Circuit diagram for closed loop with R load.
Figure 12. Output voltage waveform for closed loop with R load.
Figure 13. FFT analysis of output waveform.
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Vol 8 (32) | November 2015 | www.indjst.org
Indian Journal of Science and Technology
G. Rohini and A . Jaffar Sadiq Ali
Figure 14. Circuit diagram for closed loop with RL load.
Figure 15. Output voltage waveform for closed loop with RL load.
Figure 16. FFT analysis of output waveform for RL load.
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Buck Boost Inverter based Photovoltaic Power Generation System
Figure17. FFT analysis of output waveform for RL load for modified design.
The output voltage with this load is 270V.
Figure 11 shows the closed loop circuit diagram for R
load. The output voltage is compared with the reference
value. The error is given to a PI controller. It is passed
through a limiter and compared with repeating sequence
to generate required pulses15. The Figure 12 shows the
output waveform having 230 V as RMS voltage. The
Figure 13 shows the FFT analysis for the closed loop
waveform. It indicates that THD is of 4.14% which is
below than the IEEE standards of 5%. The Figure 14
shows the closed loop circuit with RL load. The output
voltage is maintained at 230V. The output waveform is
shown in Figure 15. The FFT analysis show about 8.33%
THD present in the output waveform. This is shown in
Figure 16. On further tuning of filter depending on the
load parameters the THD is reduced to 5.62% as shown
in Figure 17.
5. Conclusion
A converter prototype based on buck boost chopper
principle has been developed and investigated, basic
relationships for the proposed boost converter are derived
and experimental results are presented. The advantages
like transformerless, low THD, use of one single PV array
and provision for double grounding, if the converter is
to be connected to the grid allows us to recommend the
use of the proposed converter in high-power industrial
applications and power management.
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