International Journal of Electrical and Electronics
Engineering Research (IJEEER)
ISSN 2250-155X
Vol. 3, Issue 3, Aug 2013, 201-212
© TJPRC Pvt. Ltd.
SIMULATION, ANALYSIS AND DEVELOPMENT OF PV FED QUASI IMPEDANCE
SOURCE INVERTER
R. SEYEZHAI1, ABINAYA K2, AKSHAYA V3 & INDUJA U4
1
Associate Professor, Department of EEE, SSN College of Engineering, Chennai, Tamil Nadu, India
2,3,4
Final Year EEE, Department of EEE, SSN College of Engineering, Chennai, Tamil Nadu, India
ABSTRACT
In recent days, emphasis is laid on improving the efficiency and applicability of the inverters that are currently
used in photovoltaic systems so that a pure sinusoidal waveform is obtained which can be directly used for a number of
applications. This paper presents the design and development of a Quasi Impedance Source Inverter (QZSI) whose apex
application is with the Photo voltaic power conditioning system. This paper analyzes the different existing modulation
strategies and proposes a new modulation strategy to reduce the harmonics in the output. A comparative evaluation of the
various control methods is presented in terms of Total Harmonic Distortion (THD), inductor current ripple, capacitor
voltage ripple, voltage gain and voltage stress. Simulations of the circuit configuration of these control methods are
performed in MATLAB/SIMULINK. Hardware prototype of the proposed three-phase quasi Z-source inverter for PV is
built to validate the results.
KEYWORDS: Quasi Z-Source Inverter, PV, THD, PWM Technique & Voltage Gain
INTRODUCTION
The entire world is moving towards the effective utilization of renewable energy resources due to the shortage of
fossil fuels. One of the most common methods of electrical energy generation is from the solar energy which is readily
available in tropical countries like India. However, the output voltage of the photovoltaic system is very low and varies
depending on solar irradiation and temperature. Hence, in conventional systems an additional DC-DC converter is required
to step up the voltage to the desired level before feeding it to an inverter to convert the DC to AC. The proposed QZSI is an
advanced topology which is best suited for photovoltaic applications as it offers a wide voltage gain and draws a constant
current from the source.
Unlike the traditional VSI and CSI which can either buck or boost the DC link voltage, the qZSI can buck as well
as boost in a single stage without the need of an additional DC-DC converter [1-3]. Also because of the impedance network
incorporated in between the source and the bridge circuit, the simultaneous conduction of the switching devices in the same
limb is possible. This is responsible for the boosting ability of QZSI which can be varied by adopting different control
strategies. In this paper, three PWM techniques namely simple boost, maximum boost and maximum constant boost are
analyzed.
The performance parameters for all the three modulation techniques are computed and compared. The effect of
voltage gain and voltage stress for various modulation indices is studied and the results are verified in MATLAB. The most
optimum strategy is chosen for which the QZSI is designed. Also, the PV array is modeled using modified Perturb &
Observe method in MATLAB at different temperatures and different irradiation conditions and is finally interfaced with
the quasi Z-Source inverter. The simulation results are validated by building a prototype of the proposed inverter.
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PV MODELING
Solar energy is one of the non-conventional sources of energy and the most common way to harvest this energy is
to use a photovoltaic panel which receives the photons from sun and convert it to electrical energy. PV cells are made of
semiconductor material which has the capability to produce electrical energy when light is incident on them. However, the
output obtained from the PV cell is very small. So, a number of cells are connected in series or parallel to form PV
modules which are further inter-connected to form PV arrays or PV generators. So the major building block of PV module
is a PV cell which is a p-n junction that directly converts light energy into electrical energy. Its equivalent circuit is shown
in Figure1.
Figure 1: Equivalent Circuit of a PV Cell
This circuit is also known as single diode model of a solar cell [4,5]. The PV mathematical model used to simplify
the PV array is represented by the equation:
(1)
where Io is the PV array output current; V is the PV array output voltage; Ns is the number of cells in series and
Np is the number of cells in parallel; The factor A in equation determines the cell deviation from the ideal p-n junction
characteristics; it ranges between 1-5 but for our case A=2.15,Irs is the cell reverse saturation current that varies with the
temperature. It is found that the output of PV depends on the temperature and irradiation level in the PV cell. Table :1
provides the details about the different parameters used in the modeling of a PV array:
Table 1: PV Array Parameters
The different characteristics of PV array are obtained by considering all the above equations. The output
parameters Voltage (V), Current (I), Power (P) of PV array is plotted for different irradiation levels and temperature and
the following curves are obtained as shown in Figures 2&3:
Simulation, Analysis and Development of PV Fed Quasi Impedance Source Inverter
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Figure 2: Characteristic Curves of PV Array for Varying Irradiation and Constant Temperature (25⁰C)
Figure 3: Characteristic Curves of PV Array for Varying Temperature and Constant Irradiation (1000 W/sq m)
The maximum power point tracking algorithm is used to obtain maximum power from the PV array. The simple
perturb and observe technique is used to obtain the maximum power point from the PV characteristics. In this method
voltage is sampled and algorithm changes the voltage in the required direction. If change in power is positive, then the
algorithm increases the voltage value towards the MPP until change in power is negative.
This iteration is continued until the algorithm finally reaches the maximum power point. For interfacing the
algorithm with the inverter, the duty cycle of QZSI is varied with respect to the changes in the power and constant output
voltage is obtained from PV[6].
QUASI Z-SOURCE INVERTER
QZSI is a recently developed topology derived from the conventional Z-source inverter. The major difference
between these two topologies is the inclusion of the passive components (L&C). A slight change in the front end
impedance network makes the QZSI (Figure 4) more advantageous when compared with the ZSI [7-9]. The major
shortcomings of ZSI are eliminated with the help of QZSI topology.
These are few reasons for preferring QZSI over ZSI: the size and the rating of the passive components required
for QZSI is reduced to many folds, it has a unique feature of drawing constant current from the source and QZSI topology
has a wide voltage gain with improved spectral quality.
Photovoltaic panel has a large variation in its output voltage depending on the irradiation level and temperature.
The features of QZSI make it suitable for converting the dc voltage available from photovoltaic panels to boosted ac
voltage.
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Figure 4: Circuit Diagram of Quasi Z-Source Inverter
The QZSI topology has a passive network and an inverter bridge with six switches (S 1,S2,S3,S4,S5,S6). The passive
network has inductors (L1 and L2) and capacitors (C1 and C2) arranged as shown in Figure 4. The constant current drawing
capability from the source is mainly due to the inductor L1. The operation of QZSI is similar to ZSI, it has two switching
states: non shoot-through states (six active states and two zero states) and shoot-through state. During shoot through state,
the devices in the same limb conduct for a very short duration. By doing so, the supply gets shorted through the switches
and the passive components. Because of this short circuit the energy from the supply gets stored in these passive
components. During the non- shoot through state, the energy accumulated is wheeled back to the load. Therefore, the
output voltage gets boosted up at the single stage with the help of the passive components in the front end network. The
different switching states of QZSI are shown in Table: 2.
Table 2: Switching Table of QZSI
MODULATION STRATEGIES
The modulation technique adopted for the quasi Z-source inverter is different from the conventional VSI because
of the additional zero state called the shoot through state. Modifications are to be made in the traditional PWM technique
so as to include the shoot through states [10-12]. This can be achieved with the help of an additional constant line called
the shoot through line whose magnitude is responsible for the three modulation strategies namely simple boost, maximum
boost and constant maximum boost [13]. The switching pattern can be generated with the help of logical OR and AND
gates.
Simulation, Analysis and Development of PV Fed Quasi Impedance Source Inverter
205
Each switching cycle will have a shoot through period (Ts) and a non-shoot through period (To).With T as the
time period, we have
T= Ts+To
(2)
Ds=Ts/T
(3)
Do=To/T
(ie) Ds+Do=1
(4)
Where Ds is the shoot through duty ratio and Do is the non-shoot through duty ratio.
Simple Boost PWM Technique
The simple boost PWM technique of QZSI uses three sinusoidal waves with a phase difference of 120 degrees.
These reference sine waves are compared with a high frequency triangular wave. Whenever the magnitude of each sine
wave is found to be greater than the carrier wave, the corresponding switching device in the three upper limbs is switched
on. Its complement is given to the lower limb devices. The shoot through state in simple boost strategy is incorporated
using a constant shoot through line whose magnitude is greater than the amplitude of the reference sine wave. Whenever
the carrier wave is greater than the shoot through line, the inverter operates in the shoot through mode i.e. all the switching
devices in the inverter circuit are switched on. The switching pattern for this control strategy is illustrated in the following
figure (Figure 5)
Figure 5: Switching Pattern for Simple Boost Control Strategy of QZSI
The boost factor for this control is calculated by
B= 1/(2ma-1)
(5)
where ma is the modulation index
ma= (B+1)/2B
Voltage gain(G) is calculated by
(6)
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G= 2Vrms/Vdc
(7)
Where Vrms is the peak ac output voltage and Vdc is the dc input voltage.
Voltage stress is calculated as
Vstress=2-(1/G)
(8)
It is found that as the value of modulation index increases, the voltage gain decreases. This reduces the voltage
stress across the switching devices when the modulation index is increased. However, due to the presence of the traditional
zero states in the switching pattern of this control strategy, the voltage stress is relatively higher when compared to the
other modulation strategies.
Maximum Boost PWM Technique
This modulation strategy employed for the operation of QZSI is similar to that of the simple boost. In addition to
the normal PWM technique, it employs two constant dc shoot through line whose magnitude is equal to the peak of the
reference sine wave. When the carrier wave is greater than the maximum of the reference sine wave, shoot through pulses
are generated otherwise the circuit is in the non-shoot through state. This technique converts all the traditional zero states
into shoot through states thereby reducing the voltage stress across the switches. Also, the boosting capability of the
inverter is improved and the output voltage is found to be the maximum in this modulation technique. However, there is
low frequency inductor current ripple due to the variation in the shoot through duty cycle. The switching pattern of this
PWM technique is illustrated in Figure 6.
Figure 6: Switching Pattern for Maximum Boost Control Strategy of QZSI
The boost factor for this control strategy is given by,
B=п/(3√3ma-п)
ma= п(B+1)/( 3√3B)
(9)
(10)
From these equations, we can find that for a fixed modulation index, the boost factor is greater in maximum boost
control technique. But, there is low frequency inductor current and capacitor voltage ripple which can be eliminated by
adopting constant maximum boost PWM technique.
Simulation, Analysis and Development of PV Fed Quasi Impedance Source Inverter
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Maximum Constant Boost PWM Technique with Third Harmonic Injection
In constant boost technique, the shoot through duty cycle should be maintained constant. In order to maintain
constant duty cycle, the upper and lower shoot through values should be periodical. This control strategy is suitable for low
frequency applications as the ripple in the capacitor voltage and the inductor current is highly reduced. The total harmonic
distortion (THD) in the output voltage can be further reduced by means of third harmonic injection. By injecting a sine
wave with a frequency of thrice that of the reference sine wave, all the integral multiples of third harmonics are eliminated.
The reference wave with the third harmonic injection can be expressed as
Va'= Msin(θ) +aMsin(3θ)
(11)
The reference wave for the constant boost technique with third harmonic injection and its switching pattern is
illustrated in Figure7.
Figure 7: Switching Pattern for Constant Boost with Third Harmonic Injection of QZSI
It can be seen that for a specified modulation index, the boost factor is greater than the simple boost technique but,
is lower than the maximum boost strategy. However, unlike the maximum boost technique, there is no inductor current
ripple and the voltage stress across the capacitor is also reduced to a great extent. By employing third harmonic injection,
the THD of the output voltage is least in this technique. Therefore, this technique can be considered as the optimum
strategy for implementation of quasi z source inverter.
SIMULATION RESULTS
The quasi z-source inverter topology was simulated for all the three modulation strategies namely simple boost,
maximum boost and constant maximum boost with third harmonic injection in MATLAB and the simulation parameters
are shown in Table.3.
Table 3: Simulation Parameters for PV Fed Quasi Z Source Inverter
Input Voltage
Inductors L1,L2
Capacitors C1,C2
18V
5mH
1150µF
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Table 3:Contd.,
Inductor resistance rL
0.0005Ω
Capacitor resistance rC
0.005Ω
Boost factor
2
Switching frequency fS
20 kHz
Equivalent resistor Rload
25Ω
40
Output voltage(Volts)
30
20
10
0
-10
-20
-30
-40
0.11
0.12
0.13
0.14
0.15
0.16
0.17
0.18
0.19
0.2
Time(sec)
Figure 8: Load Voltage Waveform of PV Fed Quasi Z Source Inverter
with Simple Boost Strategy for Modulation Index, ma=0.75
Figures 8, 9 & 10 shows the simulated load voltage for simple boost, maximum boost and maximum constant
boost with third harmonic injection.
60
Output voltage(volts)
40
20
0
-20
-40
-60
0.1
0.11
0.12
0.13
0.14
0.15
0.16
0.17
0.18
0.19
Time(sec)
Figure 9: Load Voltage Waveform of PV Fed Quasi Z Source Inverter
with Maximum Boost Strategy for Modulation Index, ma=0.9
Figure 10: Load Voltage Waveform of PV Fed Quasi Z Source Inverter with Constant
Maximum Boost - Third Harmonic Injection for a Modulation Index, ma=0.9
0.2
Simulation, Analysis and Development of PV Fed Quasi Impedance Source Inverter
209
Figures 11 &12 clearly depict the comparison of the three strategies (simple boost, maximum boost, maximum
constant boost with third harmonic injection) with respect to voltage stress and voltage gain.
Figure 11: Graph Depicting the Relationship between Vgain and ma for all the Three Modulation Strategies
Figure 12: Graph Depicting the Relationship between Voltage Stress and ma for all the Three Strategies
EXPERIMENTAL RESULTS
It is clear from the analysis and simulation results that maximum voltage gain with lesser THD and comparatively
lesser stress is obtained for the maximum constant boost modulation strategy with third harmonic injection. Thus, a
prototype of this topology is developed, to validate the results experimentally. The successful operation of the QZSI
demands proper gating of the switches belonging to the three limbs. A program is coded in PIC 18f4550 microcontroller
using MPLAB and C18 compiler. The control logic is thoroughly studied and delay functions are given at proper time
intervals so as to generate pulses. The gate of each switch (IGBT) is triggered using these pulses. These pulses cannot be
given directly to the gate terminal because of the variation in the voltage level of the two circuits. Thus, these pulses are
given to anode-cathode terminals of the opto-coupler which in turn triggers the switches. Optocoupler provides isolation
between the inverter circuit and the triggering circuit. The basic block diagram representing the connection is shown in
Figure13.
Figure 13: Block Diagram Representing Gating Circuit
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R. Seyezhai, Abinaya K, Akshaya V & Induja U
Six such circuits are implemented for triggering each IGBT. The components are soldered in a general purpose
board with proper grounding connections. An impedance network is developed in separate board by properly placing the
inductors and capacitors. The inverter network using the IGBT switches is also developed and interfaced with the
impedance circuit and driver circuit as shown in Figure 14.
Figure 14: Hardware Implementation (Prototype) of the QZSI
For the purpose of experimentation, the circuit is tested with an input of 3V from a regulated power supply, for
the boost factor 2,we obtain 6V as output which is shown in the Figure15.
Figure 15: QZSI Output Waveform as Obtained from the Prototype
CONCLUSIONS
In this paper, QZSI and its various modulation techniques are studied and analyzed. Further, a novel control
strategy called maximum constant boost with third harmonic injection has been designed and developed. It has been
verified by simulation results that the new control technique provides a higher voltage gain with reduced harmonics and
stress on the switches. By implementing a prototype for the same, the simulation results are validated and presented. In
addition, a PV array is modeled in MATLAB. Using modified Perturb and Observe method of MPPT algorithm, maximum
point of power production is determined and the simulation results highlighting the characteristics of PV array is presented.
Therefore, QZSI is a suitable topology for photovoltaic applications.
ACKNOWLEDGEMENTS
This work was supported by the SSN Management under Student’s internally funded project. So, we acknowledge
Simulation, Analysis and Development of PV Fed Quasi Impedance Source Inverter
211
the support from the management for the successful completion.
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