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High voltage-lift Quasi-Z-Source fed modified cascaded H-Bridge Multilevel Inverter.


In modern research activities, multilevel inverters have absorbed ever-increasing applications in renewable energy sources such as Photovoltaic (PV) and wind energy systems [1]. CHB multilevel inverter has more advantages over diode clamped and capacitor clamped multilevel inverters for a given number of levels[2-4]. The Z-source based multilevel inverters have attracted much interest in recent research works due to its rewarding advantages compared with traditional voltage source inverters and Z-source Inverters (ZSI) [5]. The quasi-Z-source concept has been applied to cascaded H-bridge multilevel inverter (CHBMLI) that provides the combined advantages of traditional CHBMLI and qZSI [6].The ZSI makes use of shoot-through and non-shoot-through states of the inverter bridge to provide boosted output voltage with single-stage power conversion capability, thus providing high immunity to the EMI noise [7-10]. Conversely, it has perceptible defects including high voltage stress across the capacitors and switching devices, high inrush current, and limited voltage boosting capability. The major limitation of the ZSI is the discontinuous input current, which may lead to low utilization and permanent damage of the dc source. To overcome the problems associated with the ZSI, a variety of improved circuit topologies were presented in [11-15].

A new extended-boost ZSIs is presented in [11], where a diode or capacitor is included to provide increased boost factor and continuous input current, but it has complex circuit structure. Quasi-Z-source inverter (qZSI) is presented in [12], achieving the advantages of lower number of power devices with lower power rating, continuous input current and reduced current stress in dc source. Quasi-Z-source cascaded multilevel inverter (qZS-CHB-MLI) provides plentiful merits over traditional CHB multilevel inverter in distributed generation applications. A new quasi-Z-source Modified Cascaded H-Bridge multilevel inverter with voltage-lift cell (VLqZS-MCHB-MLI) is presented in this paper to further increase the boost ability of the qZS-CHB-MLI, and to decrease the number of switching devices. The new proposed topology provides higher boost ability with the same shoot-through duty ratio, while maintaining the advantages of the qZS-CHB-MLI. The proposed topology is reliable against short circuits, and has lower THD, higher efficiency and higher voltage boosting capability as compared to traditional inverters. The performance analysis of the proposed topology in terms of the THD for a seven level output voltage is presented with the simulated results using MATLAB.

The traditional qZS-CHB-MLI is explained in section 2 with circuit topology and characteristics of boost factor and shoot-through duty ratio. The proposed VL-qZS-MCHB-MLI is explained in section 3. The PWM control strategy of the proposed VL-qZS-MCHB-MLI is given in section 4. The simulation results of the proposed topology are presented in section 5. Finally, the conclusion is given in section 6.

Traditional Qzs-Chb-Mli:

The traditional qZS-CHB-MLI is shown in Fig 1. It has qZ source network consisting of two inductors ([L.sub.1], [L.sub.2]), capacitors ([C.sub.1], [C.sub.2]) and one diode ([D.sub.1]). The qZ source network shares the common ground with inverter, and the current drawn by the dc source is continuous. The boost factor(B) of the qZS-CHB-MLI is given in (1)

B = [v.sub.dc]/[] = 1/(1 - 2[]) (1)

Proposed Vl-Qzs-Mchb-Mli:

To improve the voltage boost capability of the traditional qZS-MCHB-MLI, a voltage-lift cell is incorporated by replacing inductor [L.sub.2] in that topology to form a new proposed topology VL-qZS-MCHB-MLI as shown in Fig. 2. In the proposed topology, lower number of switches are utilized for generating seven level output voltage as compared with traditional qZS-CHB-MLI for the same voltage level.

3.1 Shoot-through control state:

The equivalent circuit representing the shoot-through control state is shown in fig.3. In this circuit, [L.sub.2], [L.sub.3] and [C.sub.3] are connected in parallel where the diodes [D.sub.2] and [D.sub.3] are in ON state and [D.sub.1] is in OFF state. The capacitor [C.sub.3] is charged, while [C.sub.1] and [C.sub.2] are discharged. The inductors [L.sub.1], [L.sub.2] and [L.sub.3] store energy during this state. During this state, the inductor voltages ([V.sub.L1], [V.sub.L2], [V.sub.L3]) and capacitor voltages ([V.sub.C1], [V.sub.C2], [V.sub.C3]) are as given below in (2)


3.2 Non-shoot-through control state:

The equivalent circuit of the non-shoot-through control state is shown in figure.4. In this circuit, [L.sub.2], [L.sub.3] and [C.sub.3] are connected in series, where the diodes [D.sub.2] and [D.sub.3] are in OFF state and [D.sub.1] is in ON state. The capacitor [C.sub.3] is discharged, while [C.sub.1] and [C.sub.2] are charged. The inductors [L.sub.1], [L.sub.2] and [L.sub.3] transfer energy from the dc voltage source to the load during this state. The inductor voltages and capacitor voltages as given in (3). Fig. 5 depicts the boost ability of the proposed topology which is drastically higher when compared with traditional topology for the same shoot-through duty ratio. Simple boost control strategy is utilized for the proposed topology, which is well known for its easy implementation and reduced current stress across the components.


Pwm Control Strategy Of The Proposed Sl-Qzs-Mchb-Mli:

The phase disposition pulse width modulation (PD-PWM) technique is used to generate switching signals for the proposed topology. In PD-PWM, the triangular carriers are in phase with each other, having same frequency [f.sub.c] and same amplitude [A.sub.c]. In this technique, the reference signal with an amplitude [A.sub.m] and frequency [f.sub.m] is compared with each of the triangular carrier signals to generate gate pulses. Shoot-through states can be introduced by simple, maximum and constant boost control methods. In this paper, the shoot-through states are introduced in the generated pulses by using Simple boost control method. The shoot-through states enables extended boost operation of the proposed topology. The simple boost control method employs a straight line, whose amplitude is equal to or greater than the peak value of the modulating signal in order to produce the shoot-through states which are introduced at the zero states.

For simple boost control method, the shoot-through duty ratio is given in (4)

[] = []/T = 1 - M (4)

The relationship between modulation index M and shoot-through duty ratio is given in (5)

M < 1 - [] (5)

The boost factor of VL-qZS-MCHB-MLI is shown in (6)

Boost Factor, B = 2/1 - 3[] (6)

The proposed topology utilises lower shoot-through duty ratio when compared with qZS-CHB-MLI for attaining the same boost gain. This advantage of the proposed topology provides the larger improvement in the output voltage. This can be well understood from figure 6, which describes the relationship between gain and modulation index.

Results And Analysis:

The FFT analysis of output voltage and the fundamental output voltage of the new voltage-lift quasi-Z-Source Modified Cascaded MLI is evaluated in this section. The simulation results are presented in Figs. 7, 8 and 9and their simulation parameter are provided in Table 1. As shown in Fig. 9 during the steady state, DC link voltage [V.sub.dc] is boosted to 120V for the input dc voltage of 40V.


This paper has presented a new voltage-lift quasi-Z-Source Modified Cascaded MLI with higher boost ability compared with traditional quasi-Z-source Cascaded MLI, which improves the quality of the output waveform with reduced number of switches and lower THD. The performance of the proposed topology is verified by the simulation results using simple boost control method.


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(1) G. Prem Sunder, (2) Dr. B. Shanthi, (3) Dr.Alamelu Nachiappan, (4) Dr. S. P. Natarajan

(1) Associate professor, Department of Electrical and Electronics Engineering, Mailam Engineering College.

(2) Professor, Centralized Instrumentation and Service Laboratory, Annamalai University.

(3) Professor & Head, Department of Electrical and Electronics Engineering, Pondicherry Engineering College.

(4) Professor & Head (Retd), Department of Instrumentation Engineering, Annamalai University.

Received 7 June 2016; Accepted 12 October 2016; Available 20 October 2016

Address For Correspondence:

G. Prem Sunder, Associate professor, Department of Electrical and Electronics Engineering, Mailam Engineering College.


Caption: Fig. 1: Traditional qZS-CHB-MLI

Caption: Fig. 2: Proposed VL-qZS-MCHB-MLI

Caption: Fig. 3: Equivalent circuit of the proposed VL-qZS-MCHB-MLI under shoot-through

Caption: Fig. 4: Equivalent circuit of the proposed VL-qZS-MCHB-MLI under non-shoot-through

Caption: Fig. 5: Comparison of boost ability between qZS-CHB-MLI and VL-qZS-MCHB-MLI

Caption: Fig. 6: Voltage conversion ratios of the traditional and proposed topology.

Caption: Fig. 7: Load Voltage of the VL-qZS-MCHB-MLI

Caption: Fig. 8: FFT Spectrum of the Load Voltage

Caption: Fig. 9: DC Link Voltages of the VL-qZS-MCHB-MLI
Table 1: Simulation parameters of the VL-qZS-MCHB-MLI

Input dc voltage      []     40V

VL-qZS network        L=[L.sub.1]=   40 mH
                      C=[C.sub.1]=   6000 [micro]F
Switching frequency   [f.sub.s]      5 KHz
Resistive Load        R              50 [ohm]
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Author:Sunder, G. Prem; Shanthi, B.; Nachiappan, Alamelu; Natarajan, S.P.
Publication:Advances in Natural and Applied Sciences
Article Type:Report
Date:Sep 15, 2016
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