SINGLE-STAGE NON-ISOLATED SEMI Z-SOURCE INVERTER FOR RENEWABLE SYSTEMS

Transcription

1 SINGLE-STAGE NON-ISOLATE SEMI Z-SOURCE INERTER FOR RENEWABLE SYSTEMS 1 S. Swapnil, 2 A... Sudhakar, 3 Lokesh. N 1 P.G Scholar, 2,3 Associate Professor 1,2,3 ept. of EEE, SR Engineering College, Ananthasagar,Warangal Swapnil06229@gmail.com Abstract: This paper presents an economic and doubly grounded non-isolated semi Z-source inverter for a single phase photovoltaic (P) systems. This semi Z-source inverter employs only two switches to get the same output voltage of the traditional single phase Z-source/quasi Z-source inverter. The shoot-through zero state is not applicable to the proposed semi Z-source inverter unlike the traditional Z-source inverter. Because of doubly grounded feature of semi Z-source inverter, the leakage current in input dc source is reduced and so rectifies the safety and electromagnetic interference problems, which are major issues in traditional Z-source inverter. Thus it is a preferred feature for non-isolated grid-connected inverters, especially in P applications. A revised nonlinear sinusoidal pulse width modulation method is also proposed to generate the desired duty cycle to achieve a sinusoidal voltage reference. A single phase semi Z-source inverter is simulated using MATLAB and its results are provided to verify the features of the proposed circuit. Keywords: Non-isolated inverter, Photovoltaic, quasi Z-source, semi Z-source, Grid-connected, sinusoidal pulse width modulation. I. INTROUCTION Electricity is a major commodity for the socio-economic development of any country. The major part of electricity is developed mainly from the fossil fuel. These fossil fuels have severe impact over the atmosphere and even these are limited. ue to these limitations renewable energy resources are becoming more and more popular, nowadays. Many renewable energy distributed power generators give dc output voltage, so inverter has to be interfaced with the ac grid. Many inverter topologies have been proposed and reviewed recently [1]-[2]. These inverters can be categorized into two: isolated inverters and non-isolated inverters. For low voltage grid or power levels below 20 kw, non-isolated inverters are being used. These non-isolated inverters can again be classified into two types: Single stage inverter and two stage inverter. Single stage inverter topologies are preferred because of its reduced complexity and reduced cost. If the input dc source of non-isolated inverter and grid do not have same ground, then the input dc source may have large leakage current, which will cause safety and electromagnetic interference problems [3], [4], [5]. In order to solve the aforementioned problem, either extra switches have to be used which will increase the cost and the complexity of the system or doubly grounded converters have to be used [3], [4], [5]-[7]. For the considerations of safety, cost and system simplicity, Z-source and quasi Z-source inverters are proposed [8-9]. The Z-source converter employs a unique impedance network (or circuit) to couple the converter main circuit to the power source, thus providing unique features that cannot be obtained in the traditional voltage-source (or voltage-fed) and current-source (or current-fed) converters where a capacitor and inductor are used respectively. The Z-source converter overcomes the conceptual and theoretical barriers and limitations of the traditional voltage-source converter (abbreviated as -source converter) and current-source converter (abbreviated as I-source converter) and provides a novel power conversion concept. The proposed semi Z-source inverter can achieve the same output voltage of the traditional Z-source inverter, 17

2 with only two active switches. Both proposed and traditional circuits share the same form of impedance network. But it is placed at ac side in the proposed circuit, which is smaller in size than the impedance network placed at dc side in the traditional Z-source inverter. The modulation strategy of the proposed circuit is also different, which does not use sinusoidal voltage reference with extra shoot through reference to achieve the sinusoidal output voltage. But the traditional circuit uses the above said modulation strategy to get the sinusoidal output voltage. The proposed circuit uses a modified voltage reference (which is explained and derived in the further sections) instead of the sinusoidal reference to achieve the desired form of output voltage. II. TRAITIONAL Z-SOURCE INERTER AN ITS MOULATION STRATEGY A. Principle of Operation Fig. 1 shows the traditional single-phase Z-source H-bridge inverter and its modulation method. Simple boost control is used as an example. The correct conduction time of each switch of two-phase legs is generalized by two sinusoid voltage references compared with a triangle carrier voltage. The two sinusoid voltage references v A * and v B are 180 degree phase shift from each other. Two straight lines v P and v N are used to generalize the shoot-through zero state. When the carrier is higher than the upper straight line, phase leg A goes to shoot-through state, whereas phase leg B goes to shoot-through state when the lower straight line is greater than the carrier [10]. By controlling the shoot-through duty cycle, the traditional Z-source inverter can achieve the different voltage gain. III. PROPOSE SEMI Z-SOURCE INERTERS A. Principle of Operation Fig. 2 shows the two states equivalent circuit in one switching period using semi-quasi-z-source inverter as an example for analysis. Fig. 3(a) shows state I when switch S1 is conducted. uring this period, capacitor C1 and the input voltage source charge the two inductors, and the inductor current is increased. Fig. 3(b) shows state II when switch S2 is conducted. uring this period, the two inductors become the source and the inductor current is decreased. The inductor current reference and the capacitor voltage reference direction are marked in the figure for the following steady-state equation derivation. According to the inductor voltage second balance and the capacitor charge balance equations, we can have the following steady-state equations: voltage 0 _ gain C1 in I L in 2...(1)...(2) 2 I 0...(3) I L 1 I 0...(4) Fig.2 proposed single-phase semi-z-source inverters. (a) semi z-source inverter. (b) semi-quasi-z-source inverter Fig.1. Traditional single-phase Z-source H-bridge inverter and its modulation method. Fig.3.semi-quasi-z-source operation modes in one switching period (a) state I S1 is ON.(b) state II S2 is ON. 18

3 the duty cycle cannot be changed in a sinusoid manner. So a new duty cycle reference has to be used to generate the correct sinusoid output voltage. B. Modulation Strategy Fig.4. uty cycle operation region of the proposed semi-z-source inverters. The notation stands for duty cycle of switch S1 and the switches S1, S2 are conducted in complementary manner. Fig. 4 shows the voltage gain curve of the proposed semi-z-source inverters. By operating switch S1 with duty cycle changing from 0 to 0.667, the proposed inverters are able to output the same voltage range (+in to -in ) as the full-bridge inverter, as shown in Fig. 5 with red solid line. When the duty cycle of S1 changes from (0 0.5), the inverter can output the positive output voltage; when the duty cycle of S1 changes from ( ), the inverter can output the negative output voltage. When the duty cycle is equal to 0.5, the semi-z-source inverters are able to output zero voltage. Modulation methods for the proposed semi-z-source inverters and the traditional single-phase Z-source inverter are compared in this section. Fig. 8 shows the proposed modified SPWM method of semi-z-source inverters. Instead of using the sinusoid voltage reference, a modified voltage reference as derived in (8) is used as the reference signal for the conduction of switch S2 in order to output the sinusoid voltage. When the reference is greater than the carrier, switch S2 is turned ON; otherwise, S2 is turned OFF. And the gate signal of S1 is complementary with switch S2. The modified voltage reference as derived in (7) can be also used directly to generate the gate signal of S1. But in real implantation, the gate signal generation of S2 needs less calculation of SP, which is usually preferred. So Fig. 8 uses the generation of gate signal S2 as an example. The modulation index of the modified SPWM method is also in the range of 0 1. Fig. 8 shows the situation when the modulation index M = 2/3 as an example. Fig. 9 shows the duty cycle operation region with different output voltages when the modulation index is equal to 1. It can be concluded from Fig. 9 that in order to get the sinusoidal output voltage, the duty cycle is limited in the region (0 2/3). The output voltage of the inverter can be represented by 0 sin wt...(5) And the modulation index can be defined as M in...(6) Combining (5) and (6) into (1), we can get M sin wt...(7) 2 M sin wt 1 = 1 is the duty cycle of S2 and is derived as (8) 2 M sin wt The sinusoid output voltage can be achieved by using a sinusoidal changed duty cycle, because of linear relation between input and output voltage of the full-bridge inverter. But the output voltage and the input voltage of the semi-z-source inverter are no longer in a linear relation. In order to achieve the sinusoid output voltage, Fig 5. Proposed Modified SPWM Method for Semi Z-Source Inverters I. SIMULATION RESULTS The word data is plural, not singular. The subscript for the permeability of vacuum µ 0 is zero, not a lowercase letter o. The term for residual magnetization is remanence ; the adjective is remanent ; do not write remnance or remnant. Use the word micrometer instead of micron. A graph within a graph is an inset, not an insert. The word alternatively is preferred to the word alternately (unless you really mean something that alternates). Use the word whereas instead of while (unless you are referring to simultaneous events). o not use the word essentially to mean approximately or effectively. o not use the word 19

4 issue as a euphemism for problem. When compositions are not specified, separate chemical symbols by en-dashes; for example, NiMn indicates the intermetallic compound Ni 0.5 Mn 0.5 whereas Ni Mn indicates an alloy of some composition Ni x Mn 1-x. Be aware of the different meanings of the homophones affect (usually a verb) and effect (usually a noun), complement and compliment, discreet and discrete, principal (e.g., principal investigator ) and principle (e.g., principle of measurement ). o not confuse imply and infer. Prefixes such as non, sub, micro, multi, and "ultra are not independent words; they should be joined to the words they modify, usually without a hyphen. There is no period after the et in the Latin abbreviation et al. (it is also italicized). The abbreviation i.e., means that is, and the abbreviation e.g., means for example (these abbreviations are not italicized). Input voltage An excellent style manual and source of information for science writers is [9]. Output voltage & current 20

5 GATE PULSES sincere thanks to Prof. P. ENKATESHWARLU, Principal for his support throughout my paper. I would like to thank Prof. C. ENKATESH, Head of the epartment of Electrical and Electronics Engineering for his constant motivation and support during the course of my paper. I express my gratitude and sincere thanks to my paper guide Assoc. Prof. A... SUHAKAR, and Asst.Prof N.LOKESH I truly appreciate and value their esteemed guidance and encouragement from the beginning to end of this work. I am indebted to them for having helped me shape the problem and providing insights towards the solution. S1, S2 Modulation index(m=vo/vin) Output voltage vo v v v v v v v v v 0.2 8v 0.1 4v 0 0v Table for modulation index and output voltage In the above table we can observe modulation index and output voltage are linear. CONCLUSION In this paper, an economic single stage single phase non-isolated semi Z-source inverter is proposed, which is suitable for a P panel in low voltage grid connected applications. The proposed semi Z-source inverter is able to achieve the same output voltage as the traditional converter, with only two active switches. The input dc source and the output ac voltage share the same ground, which effectively eliminates the leakage current caused by the P panel. A modified SPWM method is also proposed to solve the nonlinear voltage gain problem of the semi-z-source inverter. A single phase semi Z-source inverter has been designed and simulated using MATLAB. ACKNOWLEGMENT I am gratefully acknowledging the help of incharge for the department of library of Electrical Engineering for his support during the paper. I also thank all the teaching and non-teaching staff for their co-operation to me and I also thank my friends for their support. I wish to express my gratitude to my Parents, whose love and encouragement have supported me throughout my education REFERENCES [1] M. Calais and. G. Agelidis, Multilevel converters for single-phase grid connected photovoltaic systems An overview, in Proc. IEEE Int. Symp. [2] Y. Xue, L. Chang, S. B. Kjaer, J. Bordonau, and T. Shimizu, Topologies of single-phase inverters for small distributed power generators: An overview, IEEE Trans. Power Electron., vol. 19, no. 5, pp ,Sep [3] J. M. A. Myrzik and M. Calais, String and module integrated inverters for single-phase grid connected photovoltaic systems A review, in 2003 IEEE Bologna PowerTech Conf. Proc., Jun., vol. 2, p. 8. [4] S. Araujo, P. Zacharias, and R. Mallwitz, Highly efficient single-phase transformerless inverters for grid-connected photovoltaic systems, IEEE Trans. Ind. Electron., vol. 57, no. 9, pp , Sep [5] O. Lopez, F.. Freijedo, A. G. Yepes, P. Fernandez-Comesaa, J. Malvar,R. Teodorescu, and J. oval-gandoy, Eliminating ground current in a transformerless photovoltaic application, This is our pleasure to express our immense gratitude and 21

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