ABSTRACT:
Multiplicity of the triangular carrier signals is a criterion for the extension of sinusoidal pulse width modulation, SPWM, to a number of output voltage levels per phase-leg in cascaded H-bridge (CHB) multilevel inverter (MLI). Considering medium and high voltage applications where appreciable number of output voltage levels from CHB MLI is needed, commensurate high number of carrier signals in either classical level- or phase-shifted SPWM scheme for this inverter is inevitable. High-quality output waveforms from CHB MLI system demands precise synchronization of these multi-carrier signals. Sampling issues, memory constraints and computational delays pose difficulties in achieving this synchronization for real-time digital implementation. This study presents a PWM template for CHB MLI. The developed control concept generates adequate modulation templates for CHB inverter wherein a sinusoidal modulating waveform is modified to fit in a single triangular carrier signal range. These templates can be used on CHB inverter of any level with no further control modification. Nearly even distribution of switching pulses, equal sharing of the overall real power among the constituting power switches and enhanced output voltage quality were achieved with the proposed modulation. For a 3-phase, 7-level CHB, simulation and experimental results, for an R-L load, were presented.
KEYWORDS:
1. Cascaded
H-bridge inverter
2. Sinusoidal
pulse-width modulation
3. Total
harmonic distortion
SOFTWARE: MATLAB/SIMULINK
BLOCK DIAGRAM:
Figure 1. Per Phase Block Diagram Of The
Multilevel Waveform Template, Mwt, Generation.
EXPECTED SIMULATION RESULTS:
Figure 2. Simulated Output Voltage And Current
Waveforms Of The 7-Level Chb Mli With The Proposed Pwm Scheme. (A) Phase A
Individual H-Bridge Output Voltages, (B) Phase-Leg Voltages, (C) Line Voltages,
(D) Line Currents.
Figure 3. Simulated Dc-Link Voltages, Fft Analyses
Of The Phase-Leg And Line Voltage Waveforms And Real Output Power Waveforms.
(A) Dc-Link Voltages For The Whole Phases, (B) Fft Analysis Of The Phase-Leg
Voltage Waveform From Ipd, Ps And Proposed Modulation Schemes, (C) Fft Analysis
Of The Line Voltage Waveform From Ipd, Ps And Proposed Modulation Schemes, (D)
Real Output Power Waveforms Of The Individual H-Bridges With The Proposed Spwm
Scheme.
Figure 4. Inverter Conduction And Switching Losses For Modulation Index Range Of 0.6 To 1.
CONCLUSION:
Presented
in this paper is a hybridized single carrier-based pulse width modulation
scheme for cascaded H-bridge multilevel inverter. Its operational concept
wherein a sinusoidal modulating waveform is modified to fit in a single
triangular carrier signal range in order to generate the desired output waveform
template for the MLI has been explained in detail. The principle of generating
the modulating templates is a furtherance of earlier established modulation
approaches for multilevel inverters. It has been shown that the generation of
the modulating templates is a clear demonstration of the extension of the
well-known bipolar PWM to multi-cascaded H-bridge units. Once the templates are
generated, it can be used on CHB inverter of any level with no further control
modification; only the parameter N need to be specified. From industrial point
of view, the presented concept of MWT will find its application in large number
of cascaded H-bridge systems because with the proposed modulation, the inverter
control system becomes insensitive to the traditional concept of multiplicity
of carrier waves as the number of inverter level increases. This will be highly
advantageous since the extra control effort of carrier synchronization will be
by-passed in the control algorithm. The proposed SPWM ensures nearly even
distribution of switching pulses among the constituting power switches using a
reverse-voltage-sorting comparison algorithm. Consequently, the real power
variations in the entire cascaded H-bridges are kept within a very narrow band.
From our findings, the proposed control approach results in a hybrid modulation
scheme that mediates between the phase and level-shifted carrier-based SPWM
techniques; thereby inheriting the good features in these two modulation
schemes. The performance of the proposed SPWM scheme has been presented through
scaled down simulations and experiments on a 3-phase, 7-level CHB inverter;
results have been adequately presented.
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