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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. 1 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) 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) 83 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) 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 84 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) 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 Online Available at: http://www.indjsrt.com Research Article ISSN:-2321-9262 (Online) 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 86 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) 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 87 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) 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. 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