Benzen-EJSR_68_3_14

European Journal of Scientific Research ISSN 1450-216X Vol.68 No.3 (2012), pp. 440-452 EuroJournals Publishing, Inc. 2012 http://www.europeanjournalofscientificresearch.com

Switched Quasi Z Source Inverters with Extended

Boost Capability

V. Saravanan Department of EEE, Arunai Engineering College

Tiruvannamalai, Tamilnadu, India E-mail: [email protected]

R. Ramanujam

Former Professor, Department of EEE, College of Engineering Anna University, Chennai, India

E-mail: [email protected]

M. Arumugam Director (R&D) & Professor/ EEE, Arunai Engineering College

Tiruvannamalai, Tamilnadu, India E-mail: [email protected]

Abstract

Switched Inductor and switched capacitor Z Source inverters are recent inverter topologies that have attracted many researchers as they have better boosting ability and near sinusoidal output wave forms. The technique of cascading similar cells to improve the boosting capability thereby extending the networks of passive components is also a recent area of interest. This paper deals with the analysis of switched inductor/capacitor Z Source inverter topologies with/without quasi Z Source network. Further new extended topologies are also proposed with the addition of a few passive component networks are simulated in MATLAB/SIMULINK environment and their simulated responses are discussed. Keywords: Switched Inductor/Capacitor, Simple boost control, extended boost inverters,

Quasi Z Source Inverters. 1. Introduction Impedance (Z) Source inverters are recent topologies that can perform both buck/boost functions as a single unit. A unique feature of the Z Source inverter is the shoot through state, by which two semiconductor switches of the same phase leg can be turned on simultaneously. This feature is not available in the traditional voltage source and current source inverters [1].

Z Source Inverters are mainly applied for loads that demand a high voltage gain, such as motor drives and as a power conditioning unit for renewable energy sources like solar, fuel cells, etc to match the source voltage difference. In addition Z Source Inverters can perform the function of Maximum Power Point Tracking (MPPT) [2], [3] and [4] can feed a near sinusoidal waveform to the load with less Total Harmonic Distortion (THD).

Switched Quasi Z Source Inverters with Extended Boost Capability 441

The prospects of Z Source inverters have led to many new topologies showing a consecutive enhancement in voltage gain and output waveforms. A tradeoff between the boosting capability and component count is always a major concern to keep the cost stable. It is also to be noted that increase in the passive components with suitable modifications can improve the performance of the inverter. Switched inductor (SL) Z Source Inverter topologies are one in which only a very short shoot-through zero state is required to obtain high voltage conversion ratios. Voltage buck inversion ability is also provided in the inverter for those applications that need low ac voltages.

These features of the (SL) Z Source Inverter have urged to go for certain topological modifications that can have enhanced features. The paper begins with the discussion of a (SL) Z Source inverter in section I and the SL quasi and SL embedded Z Source inverters are discussed in section II. A proposed switched capacitor quasi Z Source inverter and the technique of embedding the dc source is discussed in section III. Extension of the topologies with various combinations is discussed in section IV. Section V deals with the simulation results and comparison of the THD values for each topology. 2. Switched Inductor Z Source Inverter An impedance-type power inverter termed as switched-inductor Z-source inverter is designed to enlarge the voltage adjustable ability. This inverter also increases the voltage boost inversion ability significantly, and only a very short shoot-through zero state is required to obtain high voltage conversion ratios. In addition, the voltage buck inversion ability is also provided for those applications that need low ac voltages [5].

The Switched Inductor (SL) Z Source inverter shown in Figure: 1 totally differs from other topologies from the viewpoint of circuit structures and operation principles. The main characteristics are summarized as follows:

The basic X-shape structure is utilized. Only six diodes and two inductors are added compared with the classical Z-source inverter. The boost factor has been increased from 1/(1-2D) to(1+D)/(1-3D). The new structure is extensible for the further development using the coupled-inductor

techniques.

Figure 1: SL Z Source Inverter

From Figure: 1, the SL Z source inverter consists of four inductors (L1, L2, L3 and L4), two capacitors (C1 and C2), and six diodes (D1, D2, D3, D4, D5 and D6). The combination of L1-L3-D1-D3-D5 and the combination of L2-L4-D2-D4-D6 performs the function of the top SL cell and the bottom SL

442 V. Saravanan, R. Ramanujam and M. Arumugam

cell, respectively. Both of these two SL cells are used to store and transfer energy from capacitors to dc bus under the switching action of the main circuit.

Shoot-through state: This state corresponds to the additional zero state produced by the shoot-through actions of the top and bottom arms of the main circuit. During this sub-state, S is ON, while both Din and Do are OFF. For the top SL cell, D1 and D2 are ON, and D3 is OFF. L1 and L3 are charged by C1 in parallel. For the bottom SL cell, D4 and D5 are ON, and D6 is OFF. L2 and L4 are charged by C2 in parallel.

Non-shoot-through state: This state corresponds to the six active states and two zero states of the main circuit. During this sub-state, S is OFF, while both Din and Do are ON. For the top SL cell, D1 and D3 are OFF, and D5 is ON. L1 and L2 are connected in series, and the stored energy is transferred to the main circuit. For the bottom SL cell, D4 and D5 are OFF, and D5 is ON. L3 and L4 are connected in series, and the stored energy is transferred to the main circuit. At the same moment, to supplement the consumed energy of C1 and C2 during the shoot-through state, C1 is charged by Vin via the bottom SL cell, and C2 is charged by Vin via the top SL cell.

Boost Inversion Ability Analysis of the Whole Inverter: The voltage conversion ratio of the whole inverter G can be expressed by

/ 2pn

in

vG MB

V (1)

Where G the peak value of the output phase voltage. The maximum voltage conversion ratio Gmax versus any desired modulation index M, can be expressed by

max 0 _1

(2 )3 2 sD M

M MG MB G

M (2)

Where G0_s is maximum voltage conversion ratio of the traditional Z-source inverter and its expression depend on M is given by

0 _ 2 1sM

GM

(3) The average duty ratio of the shoot through zero state is expressed by

0 2 3 32

T MD

T

(4)

The equivalent boost factor B under the condition of variable duty ratios is 1 4 3 31 3 9 3 4

D MB

D M

(5)

The maximum voltage conversion ratio Gmax versus any desired modulation index M, approximates to

max 0 _

(4 3 3

/ 2 9 3 4pn

m

in

M MvG MB G

V M

(6) The maximum voltage conversion ratio of the classical Z-source inverter and its expression

depend on M is given by as

0 _3 3 3

m

MG

M

(7) Simple Boost Control Simple boost control was adopted to control all the topologies discussed in this paper [6], [7].

The waveforms for the switching pulses are shown in the Figure: 2. The switching scheme or the logical implementation of the PWM generation is shown in Figure: 3.


Figure 2: Switching pulse generation.

Figure 3: Logical implementation of PWM.

3. Enhanced Switched Inductor Z Source Inverter A. Switched Inductor quasi Z Source Inverter

Figure: 4 shows that the Switched Inductor quasi Z Source Inverter topology that provides inrush current suppression, unlike the SL-ZSI topology, because no current flows to the main circuit at start up; however, the inductors and capacitors in the proposed switched-inductor quasi Z Source Inverter still resonate. Compared with a conventional continuous current quasi-Z-source inverter, the proposed inverter adds only three diodes and one inductor and the boost factor increases from 1/(1-2D) to (1+D)/(1-2D-D2) [8], [9].

Figure 4: Switched inductor quasi Z Source Inverter

The inverter has similar states as that of the switched inductor Z Source Inverter. The control strategy adopted for SL Z Source Inverter can be implemented for SL quasi Z Source Inverter also.


The peak dc-link voltage across the inverter main circuit is expressed in and can be rewritten as

1 2 2

11 2PN C C dc dc

DV V V V BVD D (8)

The boost factor of the inverter is given as 0

220 0

111 2

1 2

TD TB

D D T TT T

(9)

B. Embedded Switched Inductor quasi Z Source Inverter. The need for embedding the dc sources is because, this procedure can directly connect the

sources to the inductors of impedance network therefore the dc input current flows smoothly compared to that in a traditional Z-source inverter [10], [11].

The embedded Z-source inverter assumes the two sources can produce the same voltage gain as the traditional Z-source inverter. The SL-qZSI can be implemented in the embedded Z-source inverter that operates with either one or two dc sources. Figure: 5 shows the embedded switched-inductor quasi-Z-source inverter with two isolated dc sources.

Figure 5: Embedded Switched Inductor quasi Z Source Inverter

The peak dc-link voltage across the inverter main circuit is expressed in and can be rewritten as

1 2 2

11 2PN C C dc dc

DV V V V BVD D (10)

It could be noted that the peak dc-link voltage across the main circuit is same for both the cases showing that one- and two-source SL-qZSIs produce the same output voltage gain; however, the C1 voltage of the one-source SL-qZSI in Figure: 4 is lower than that of two-source SL-qZSI in Figure: 5, while the C2 voltage of the one-source inverter is higher than that of the two-source inverter. The average dc input current of the two inverters is the same; however, the dc input current, Iin2 that connects to switched-inductor cell of the two-source inverter, as shown in Figure: 5 contain more ripple. In the non-shoot-through state, Iin2 = IL2; in the shoot-through state, Iin2 = 2IL2. Thus, the average of Iin2 is (1+D)IL2. 4. Proposed Switched Capacitor Topologies Switched Capacitor (SC) quasi Z Source Inverter can be realized by replacing the inductor cell with a capacitor cell as shown in Figure: 6. with small modifications in the positions of the passive


components. This proposed topology has certain additional features like increased boosting capability. Also the capacitors can minimize the ripple current within the inverter and the efficiency of the system can be improved.

Also the SC quasi Z Source Inverter can be modified to operate with two dc sources by embedding the dc sources within the network. The principle of embedding the dc sources remains the same as in the previous cases but the output characteristics differs magnificently for the proposed SC quasi Z Source Inverter. The circuit is shown in Figure:7 and the circuit has an additional advantage of supplying with a smooth dc source with a reduced ripple current compared to the SL quasi Z Source Inverter.

Figure 6: Switched Capacitor quasi Z Source Inverter

Figure 7: Embedded Switched Capacitor quasi Z Source Inverter

5. Proposed Extended Boost Topologies As mentioned earlier, the switched-L and C, Z-source inverters are developed by replacing either the Z-source inductors or capacitors by the switched-L or switched-C cells. Cascading of cells can be done as shown in Figure: 8. (in other words, N cells in cascaded using n = N + 1 inductors). Beginning with the extended cell structure shown in Figure: 8, the generalized concept is to introduce more inductors in parallel during shoot-through charging and more inductors in series during non-shoot-through discharging. These orientations can indeed be guaranteed by the diode layout found within each switched-L cell [12]. The cascading procedure also reduces the voltage stress across the capacitors during start conditions [13].


Figure 8: Extended SL Z Source Inverter

Shoot-Through: This state is introduced by turning on the two switches from a phase-leg of the inverter bridge. By doing so which causes diodes D and D3n to turn off, while D3n-1 and D3n-2 starts conducting. All the inductors are then connected in shunt for charging by the two Z-source capacitors, and their common voltage expression is written as Vl = - Vc.

Non-Shoot-Through: It is represented by one of the eight traditional active and null VSI states. In this state, D and D3n conduct, while D3n-1 and D3n-2 reverse-bias, causing all the inductors to series-discharge their energy to the external ac load. The common inductive voltage expression is then expressed as Vl = (Vc-Vdc)/n.

It is obvious that the boost factor increases as the number of (n) cascaded cells increases. Besides providing high gain, the switched-L inverter might still have some convincing advantages over those existing Z Source Inverters, when they are commanded to produce the same output voltage from the same given input voltage. It is also obvious that multi-cell switched-L network results in a reduction of the capacitive voltage stress.

Similarly the SL quasi Z Source inverter can be extended as shown in Figure: 9. The two inductor cells can be added as shown with a capacitor to carry the excess voltage and also feed the inverter with a ripple free current [14].

Figure 9: Extended SL quasi Z Source Inverter First extension

The extended SC quasi Z Source Inverter can be realized with the addition of two capacitor cells as shown in the Figure: 10. There is an inclusion of an inductor in addition to the two capacitors


found in the previous topology. The inductor regulates the current as well as filters the dc supply entering into the inverter. The function of the capacitor remains the same as in the previous case.

Figure 10: Extended SC Z Source Inverter First extension

6. Simulation Results & Discussions Simulations are carried out in MATLAB/SIMULINK environment for all the discussed topologies with the given input voltage of value 80V and a resistive load of value 130. Selection of L and C values decides the required boost function [15]. In this case L1=L2=2mH; C1=C2=50F is used with a switching frequency of 20 KHz.

Case i) Switched Inductor Z Source inverter (SL ZSI) An output phase voltage of 230V and phase current of 1.8A are obtained shown in Figure: 11

when the inverter operated in boost mode. Buck operation of the inverter is also possible with suitable modifications.

Figure 11: Output voltage & current waveforms

Case ii) Switched Inductor quasi Z Source Inverter (SL qZSI) An output phase voltage of 500V and phase current of 3.8A are obtained shown in Figure: 12.



Case iii) Embedded Switched Inductor quasi Z Source Inverter (ESL qZSI) An output phase voltage of 1000V and phase current of 8A are obtained which are shown in

Figure: 13. The output voltage can be varied with the variations in the two input dc sources.

Figure 13: Output voltage & current waveforms, when Vin1=Vin2= 80V

Case iv) Switched Capacitor quasi Z Source Inverter (SC qZSI) An output phase voltage of 190V and phase current of 1.5A are obtained shown in Figure: 14.



Case v) Embedded Switched Capacitor quasi Z Source Inverter (ESC qZSI) An output phase voltage of 380V and phase current of 3A are obtained shown in Figure: 15.

The output voltage can be varied with the variations in the two input dc sources.

Figure 15: Output voltage & current waveforms when Vin1=Vin2= 80V

Case vi) Extended SL Z Source Inverter (XSL ZSI) An output phase voltage of 300V and phase current of 2.8A are obtained shown in Figure: 16.

The addition of passive component networks has shown a considerable rise in voltage and current waveforms when compared with Figure:11.


Case vii) Extended quasi SL Z Source Inverter (XSL qZSI) An output phase voltage of 650V and phase current of 5.5A are obtained shown in Figure: 17.

The addition of passive component networks has shown a considerable rise in voltage and current waveforms.



Case viii) Extended quasi SC Z Source Inverter (XSC qZSI) An output phase voltage of 250V and phase current of 1.8A were obtained and are shown in

Figure: 18. The addition of passive component networks has shown a considerable rise in voltage and current waveforms. It is also evident that further increase of the number of cells can also be done i.e the topologies can be extended to higher levels to obtain higher voltage boosting capability.


Table 1: VTHD values of various topologies

Sl. No Topology VTHD in % 1 Switched Inductor Z Source inverter 4.92 2 Switched Inductor quasi Z Source Inverter 4.94 3 Embedded Switched Inductor quasi Z Source Inverter 4.93 4 Switched Capacitor quasi Z Source Inverter 4.82 5 Embedded Switched Capacitor quasi Z Source Inverter 4.86 6 Extended SL Z Source Inverter 5.44 7 Extended quasi SL Z Source Inverter 5.46 8 Extended quasi SC Z Source Inverter 5.44

Table 1 shows the VTHD values of the various topologies obtained through FFT analysis listed

above. The SL and SC topologies have nearly similar values of THD. However the topology with extended boosting capability has higher THD values than the previous cases. This is obvious because of the increase in the number of passive components has a direct effect THD values of the extended boost topologies. A tradeoff between the boosting capability and THD has to be made to have reliable output characteristics. Table 2: Voltage gain comparisons

SL ZSI SL qZSI ESL qZSI SC qZSI ESC qZSI XSL ZSI XSL qZSI XSC qZSI SL ZSI -- 2.17 4.34 0.82 1.65 1.30 2.82 1.08 SL qZSI 0.46 -- 2.0 0.38 0.76 0.6 1.3 0.5 ESL qZSI 0.23 0.5 -- 0.19 0.38 0.3 0.65 0.25 SC qZSI 1.2 2.6 5.26 -- 2.0 1.5 3.4 1.3 ESC qZSI 0.6 1.3 2.6 0.5 -- 0.7 1.7 0.6 XSL ZSI 0.7 1.6 3.3 0.6 1.2 -- 2.1 0.8 XSL qZSI 0.3 0.7 1.5 0.2 0.5 0.4 -- 0.3 XSC qZSI 0.9 2.0 4.0 0.7 1.5 1.2 2.6 --


Table 3: Current gain comparisons

SL ZSI SL qZSI ESL qZSI SC qZSI ESC qZSI XSL ZSI XSL qZSI XSC qZSI SL ZSI -- 2.1 5.0 0.8 1.6 1.5 3.0 1.0 SL qZSI 0.4 -- 2.3 0.3 0.78 0.73 1.4 0.4 ESL qZSI 0.2 0.4 -- 0.1 0.3 0.31 0.6 0.2 SC qZSI 1.2 2.5 6.0 -- 2.0 1.8 3.6 1.2 ESC qZSI 0.6 1.2 3.0 0.5 -- 0.9 1.8 0.6 XSL ZSI 0.6 1.3 1.07 0.5 1.07 -- 1.96 0.64 XSL qZSI 0.3 0.6 1.63 0.27 0.5 0.50 -- 0.3 XSC qZSI 1.0 2.1 5.0 0.8 1.6 1.5 3.0 --

Table 2 and Table 3 shows the comparison of voltage and current gains between various

existing and proposed topologies, which is obtained by dividing the output phase voltages/currents in the row by corresponding ones in the column.

The proposed extended SL ZSI exhibits better voltage/current gain characteristics when compared with the other topologies which are highlighted in bold letters in seventh column, whereas the embedded SL qZSI shows better performance, if it is energized by two dc sources in the third column which is shown in the above tables. 7. Conclusions The performance of a SL Z Source Inverter, SL quasi Z Source Inverter and SL embedded Z Source inverters are studied in the simulation environment. New topologies such as SC quasi Z Source Inverter, SC embedded Z Source Inverter are proposed which exhibit satisfactory performance. Both SL and SC topologies are extended by increasing the number of SL and SC cells respectively and the boosting capability was compared.

The SC Z Source Inverters can be used for both boost and buck operations and can have reduced stress on the capacitors in start conditions and that can be applied to the entire spectrum of power conversion and it is apparent that many Z Source conversion circuits can be derived. Therefore, these inverters are reasonably competitive for buck-boost applications with their good performances confirmed by simulation. References [1] F. Z. Peng, 2003, Z-source inverter, IEEE. Industrial Applications, vol. 39, no. 2, pp. 504

510. [2] Y. Huang, et.al, Nov. 2006, A Z-source inverter for residential photovoltaic systems, IEEE

Transactions on Power Electronics, vol. 21, no. 6, pp. 17761782. [3] Q. Li and P. Wolfs, May 2008, A review of the single phase photovoltaic module integrated

converter topologies with three different DC link configurations, IEEE Transactions on Power Electronics., vol. 23, no. 3, pp. 13201333.

[4] J.Wang, Sep. 2004, Low cost fuel cell converter system for residential power generation, IEEE Trans. Power Electronics., vol. 19, no. 5, pp. 13151322.

[5] Miao Zhu, et,al., 2010, Switched Inductor Z-Source Inverter, IEEE Transactions on Power Electronics, Volume: 25 , Issue: 8, pp. 2150 2158.

[6] Fang Zheng Peng, et.al., July 2005, Maximum Boost Control of the Z-Source Inverter, IEEE Transactions on Power Electronics, Vol. 20, No. 4.

[7] Miaosen Shen, et.al., 2006, Constant boost control of the Z- source inverter to minimize current ripple and voltage stress, IEEE Transactions on Industry Applications, Vol 42 , Issue:3, pp. 770 778.


[8] Joel Anderson, et.al., 2008, Four Quasi-Z-Source Inverters IEEE Conference Proceedings, Page No. 2743 2749.

[9] Yuan Li, et.al., 2009, Quasi-Z-Source Inverter for Photovoltaic Power Generation Systems, IEEE Conference Proceedings, Page No. 918- 924.

[10] Poh Chiang Loh, et.al, 2010, Embedded EZ-Source Inverters, IEEE Transactions on Industry Applications, vol. 46, no. 1, pp. 256267.

[11] F. Gao, et.al, 2008, Operational analysis and comparative evaluation of embedded Z-source inverters, in Proc. IEEE PESC, pp. 27572763.

[12] Ding LI, et.al., 2010, Generalized Multi-Cell Switched-Inductor and Switched-Capacitor Z-source Inverters, IEEE ICSET Conference Proceedings.

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[14] Chandana Jayampathi Gajanayake, October 2010 et.al., Extended-Boost Z-Source Inverters, IEEE Transactions on Power Electronics, Vol. 25, No. 10, pp. 26422652.

[15] Rajakaruna. S, et.al, 2010, Steady-State Analysis and Designing Impedance Network of Z-Source Inverters, IEEE Transactions on Industrial Electronics, Vol.57 , Issue: 7, pp. 2483 2491

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Benzen-EJSR_68_3_14