Download analysis of pwm strategies for a three phase quasi z

Ind. J. Sci. Res. and Tech. 2014 2(5):82-89/Seyezhai et al
Online Available at: http://www.indjsrt.com
Research Article
ISSN:-2321-9262 (Online)
ANALYSIS OF PWM STRATEGIES FOR A THREE PHASE QUASI Z-SOURCE
INVERTER FOR PV APPLICATIONS
*
R. Seyezhai, K. Abinaya, V. Akshaya and U. Induja
Department of EEE, SSN College of Engineering, Chennai, India
*Author for Correspondence
ABSTRACT
Recently, Quasi Z-source Inverter (QZSI) has gained attention due to its advantages of lower component ratings,
reduced source stress, reduced component count and simplified control strategies. This paper presents the complete
analysis of the three main pulse width modulation strategies namely simple boost, maximum boost and maximum
constant boost for a three phase quasi z source inverter. Voltage stress and voltage gain are compared for the three
strategies for various modulation indices with a constant boost factor. It has been verified using simulation results
that the maximum constant boost technique gives a reduced voltage stress and higher voltage gain compared to the
other two PWM techniques.
Key Words: Quasi z- Source Inverter, Simple Boost, Maximum Boost and THD
INTRODUCTION
QZSI is a special recent development in the field of inverters used predominantly for photovoltaic applications.
Quasi impedance (Z) source inverter as shown in Fig.1, derived from Z source inverter (Peng, 2003 and Shen et al.,
2005), has an impedance network connected in between the source and the bridge circuit of a traditional VSI. This
impedance network permits the inverter to operate in an extra switching state. As far as a conventional voltage
source inverter is considered, there are six switching states and the conduction of the switches belonging to the same
limb is prohibited as this will result in the shorting of the source. However, in case of QZSI, this switching state,
where the switches of the same leg are switched on, called the shoot through zero state is permitted. During this state,
as depicted in Fig.1, the source energizes the L and C of the impedance network. Thus the voltage across the energy
storing elements will be summed up and given to the load during the non-shoot through switching states, thereby
increasing the voltage gain of the inverter (Joel & Peng, 2008 and Yuvan et al., 2009).
Figure 1: Circuit Diagram of Quasi Z-source Inverter
There are different modulation strategies that can be employed for operating the switches of the bridge network.
Pulse width modulation strategy is the most predominant method. In this, pulses for the IGBT s are produced by
comparing a sinusoidal reference signal and a high frequency triangular carrier signal. The shoot through switching
state is realized by comparing the reference sine wave with a constant signal called the shoot through line. The
voltage amplitude of the constant signal varies with the modulation strategy (simple boost, maximum boost and
maximum constant boost).This paper analyses the simple boost, maximum boost and maximum constant boost
techniques for different modulation indices with a constant boost factor. Voltage gain, voltage stress across the
switches, total harmonic distortion are calculated and compared. The results are verified and presented using
MATLAB simulation.
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Ind. J. Sci. Res. and Tech. 2014 2(5):82-89/Seyezhai et al
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Research Article
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BASIC OPERATION OF QZSI
The quasi z source inverter comprises two inductors (L1, L2) and two capacitors (C1, C2) as shown in fig. 1 & 2
which makes it unique. Unlike the traditional voltage fed inverters which have six vector states and two zero states
(conduction of the upper devices only or the lower devices only), the qZSI has an additional switching state called
the shoot through state. In this switching state, both the upper and the lower devices of the same leg are switched on
for a very short duration. This state is not applicable for the conventional VSI (Shen et al., 2005 and Thangaprakash
& Krishnan, 2010) as it would lead to a short circuit damaging the devices. The arrangement of the inductors and
capacitors in qZSI enables its operation in shoot through mode, by which an output greater than the input voltage is
obtained.
During the shoot through state which is for a very short duration, the circuit is complete via the inductors, capacitors
and the switches. These passive components store energy during the shoot through state and release it back during
the vector states. Thus, the output voltage can be boosted without the need of a dc-dc converter. In order to reduce
the voltage stress on the switches, the devices of all the three phase legs are switched on during the shoot through
state. Also, the output waveform is quasi shaped which reduces the total harmonic distortion and is of better spectral
quality.
MODULATION STRATEGIES FOR QZSI
The modulation techniques that are adopted for quasi Z-source inverter are different from the conventional Voltage
Source Inverter (VSI) because of the addition of an extra state called the shoot through zero state. The switching
pattern for quasi Z-source inverter must incorporate this shoot through state to get a voltage gain greater than the
conventional VSI. Sinusoidal pulse width modulation technique is employed here as this gives a reduced power loss
in the switches. Shoot through state is added by using a constant pulse called the shoot through line voltage. Based
on this shoot through line voltage, three modulation techniques namely Simple boost control and maximum boost
control and maximum constant boost are presented and discussed briefly.
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 -------------- (1)
Ds=Ts/T, Do=To/T
(ie) Ds+Do=1 ------------- (2)
Where Ds is the shoot through duty ratio and Do is the non-shoot through duty ratio.
Simple Boost Control
As per the conventional PWM technique, three sinusoidal reference signals with 120° phase shift is compared with
triangular carrier signal to generate the switching pulses for the non-shoot through states. Whenever the magnitude
of the sine wave is greater than the triangular wave, the upper devices in the limb are switched on; else the device is
switched off. The complement of this control is given to the lower set of switching devices in the limb. The
additional shoot through state at which the devices in the same limb are switched on is obtained by comparing the
carrier signal with the constant dc line. Simple boost control uses two straight lines also called as shoot through line
whose magnitude is always greater than the reference sinusoidal signal amplitude. When the triangular wave
exceeds the shoot through line the inverter goes into the shoot through state, otherwise the circuit operates as a
normal PWM based inverter (Thangaprakash & Krishnan, 2010). The switching pattern of the simple boost control
is shown in Fig. 2.
Figure 2: Switching pattern for simple boost
The boost factor for this control is calculated by
B= 1/(2ma-1) --------(3)
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Ind. J. Sci. Res. and Tech. 2014 2(5):82-89/Seyezhai et al
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Where ma is the modulation index
ma= (B+1)/2B ---------(4)
Voltage gain(G) is
G= 2Vrms/Vdc=ma*B --------(5)
Where Vrms is the peak ac output voltage and Vdc is the dc input voltage
Voltage stress is
Vstress=2-(1/G) ------- (6)
From equations (3) to (6), it is clear that to get a high voltage gain, modulation index must be small. As the
modulation index decreases the voltage stress impressed on the switch increases. Even though this is the simplest
control strategy the voltage stress across the switches is relatively high. The presence of some traditional zero states
in the switching pattern increases the voltage stress across the switches. This is overcome by the maximum boost
control technique.
Maximum Boost control
By adopting maximum boost control strategy the drawbacks of the simple boost control technique is eliminated and
a swell in the voltage greater than the simple boost technique is obtained. A slight modification in the shoot through
line is responsible for this increase in the output voltage. The maximum amplitude of the reference sinusoidal signal
is considered as the shoot through line magnitude in this control technique. 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. Due to the change in the value of shoot through constant line all the traditional zero states are
converted into shoot through state. As a result for the same modulation index (m a) the boosting capability is
improved and the voltage stress is minimized when compared with the simple boost technique (Thangaprakash &
Krishnan, 2010; peng et al., 2005) .The switching pattern for the maximum boost is shown in Fig.3 (Joel & Peng,
2008; Silver et al., 2011). The boost factor for this control strategy is
B=п/(3√3ma-п)
ma= п(B+1)/( 3√3B) (Yuvan et al., 2009)
From the above equations, it is clear that for a fixed modulation index the boost factor is greater in maximum boost
control technique (Shahparasti et al., 2010 and Liske et al., 2011).
Figure 3: Switching pattern for maximum boost
Figure 4: Switching pattern for constant boost with third harmonic injection
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Maximum Constant Boost Control Method
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.
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
Simulations were carried out in MATLAB to verify the analysis of the simple boost and maximum boost modulation
strategy. The simulation parameters are shown in table1. The voltage gain and the voltage stress across the switches
is computed for different values of modulation indices for both the techniques and tabulated in table2 and table3.
Also, a graph is plotted for the comparison of the two modulation techniques. Figs. 5, 6 & 7 show the output voltage
waveform for simple and maximum boost control methods.
Table 1: Simulation Parameters for QZSI
Input Voltage
Inductors L1,L2
Capacitors C1,C2
Inductor resistance r L
Capacitor resistance r C
Boost factor
Switching frequency fS
Equivalent resistor Rload
150V
5Mh
1150µF
0.0005Ω
0.005Ω
2
20 kHz
25Ω
Figure 5: Output Waveform for simple boost
modulation Strategy
Figure 6: Output Waveform for maximum boost
modulation strategy
Table 2: Performance parameter for Simple Boost
Ma
Vo
Vac
Vgain
0.7
360
150
2
0.75
305
145
1.933333
0.8
290
141
1.88
0.9
270
138
1.84
0.95
260
137
1.826667
85
Vstress
1.5
1.482759
1.468085
1.456522
1.452555
Ind. J. Sci. Res. and Tech. 2014 2(5):82-89/Seyezhai et al
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300
Output voltage(volts)
200
100
0
-100
-200
-300
1.7
1.71
1.72
1.73
1.74
1.75
1.76
1.77
1.78
1.79
1.8
Time(sec)
Figure 7: Output Waveform for maximum constant boost with Third Harmonic injection based modulation strategy
Table 3: Performance parameter for Maximum
Boost
Ma Vo Vac Vgain
Vstress
0.8
460 245 3.266667 1.693878
0.9
290 141 1.88
1.468085
0.95 245 121 1.613333 1.380165
Table: 4 Constant maximum boost with third
harmonic injection
Ma Vo Vac Vgain Vstress
0.85 285 140 1.8666 1.464
0.9
265 135 1.8
1.444
0.95 240 125 1.667
1.4
From Tables 2, 3 & 4 it is found that maximum constant boost gives a higher output and high voltage gain for QZSI.
Figs. 8, 9, 10 & 11 depicts the graph for voltage gain and voltage stress for different modulation indices. From the
results, it is observed that maximum constant boost gives a better performance compared to simple boost.
Figure 8: Voltage gain versus Ma
Figure 9: Voltage stress versus Ma
Figure 10: Voltage gain and ma for constant boost
with third harmonic injection
Figure 11: Voltage stress and ma for maximum
constant boost with third harmonic injection
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APPLICATION OF QUASI Z-SOURCE INVERTER FOR PV
The special features of QZSI make it suitable for photovoltaic applications. PV array output voltage is dc in nature
and is very low and it cannot be connected to the grid directly. Conventional configuration of connecting
photovoltaic array to the grid is a combination of dc/dc converter and an inverter. DC to DC converter can act as a
buck boost converter and the function of inverter is to deliver power to the grid. The drawbacks of this conventional
system are increased cost, complicated control system, and reduced efficiency. All these drawbacks are eliminated if
QZSI is used to connect the PV to the grid. It will boost the voltage obtained from solar panel and provide the
required power to the grid. It acts as an intermediary stage between the PV array and three phase grid and it is used
for standalone applications of PV. QZSI draws continuous current from the source. This feature is very much needed
by the PV array, as its output voltage, current and power depends on many factors like temperature, irradiation level.
PV array is modelled and simulated by considering all these factors in matlab (Seyezhai et al., 2013). The efficiency
of a solar cell is very low. In order to increase the efficiency, methods are to be undertaken to match the source and
load properly. One such method is the Maximum Power Point Tracking (MPPT). This is a technique used to obtain
the maximum possible power from a varying source. In photovoltaic systems the I-V curve is non-linear, thereby
making it difficult to be used to power a certain load. In this project we use QZSI and vary its duty cycle to obtain
maximum power from the solar cell.
There are many methods used for maximum power point tracking a few are listed below:
• Perturb and Observe method
• Incremental Conductance method
• Parasitic Capacitance method
• Constant Voltage method
• Constant Current method
A detailed analysis of perturb and observe method alone is carried out and interfaced with qZSI.
Perturb and observe method
This method is the most common and simple method. 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.
Figure 12: PV simulation with perturb and observe algorithm and QZSI
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300
Output voltage (volts)
200
100
0
-100
-200
-300
1.6
1.61
1.62
1.63
1.64
1.65 1.66
Time(sec)
1.67
1.68
1.69
1.7
Figure 13: Waveform of PV interfaced QZSI
The output waveform of the PV interfaced QZSI with third harmonic injection is as shown in Fig.13.
CONCLUSION
The detailed analysis of the two prominent PWM strategies, simple boost and maximum boost and maximum
constant boost is presented with simulation results. It is seen that for a specific input voltage and modulation index,
the stress impressed on the switches is considerably lower for maximum constant boost than the other two PWM
techniques, the reason being the conversion of a zero state to shoot through state. Thus, quasi Z source inverted
operated in the maximum constant boost PWM technique will provide better results when practically implemented
for photovoltaic applications.
ACKNOWLEDGEMENT
The authors wish to thank the SSN management for funding the proposed work.
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