ABSTRACT:

This study presents SVPWM template for CHB MLI. The developed control concept generates adequate modulation for CHB inverter and compared with the sinusoidal pulse width modulation. A LCL filter is added in order to get sinusoidal waveforms at load side. 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. The simulation is to be carried out in MATLAB/Simulink software.

INTRODUCTION:

In recent time, the field of power electronics has witnessed appreciable boost in the areas of power-conditioning circuit topologies and their corresponding control/modulation strategies. This trend is a direct response to the global quest and search for sustainable and more environmental-friendly electrical power sources and utilizations. In this scenario, inverters/dc-ac converters are among the cardinal power electronics devices deployed in various key areas such as high-voltage direct-current (HVDC) transmission, Flexible AC Transmission Systems (FACTS), renewable energy grid integration, railway and vehicle traction, energy storage systems, marine propulsion, solar water pumping system, grinding and rolling mills, compressors and extruders, to mention. Such deployment requires certain criteria to be met by the power electronics inverters of which high output voltage and a range of operational output power quality indices are crucial. The multilevel inverter, MLI, is the best device for the provision of these requirements. Awareness of the inherent potentials of MLIs and their positive impacts on series of industrial applications have led to the proliferation of multilevel inverter power circuit configurations, as reported in the literature. Critical assessment of these newly evolved MLI topologies shows that they are actually either a hybrid or an off-shoot of one of the conventional MLI configurations: cascaded H-bridge (CHB), diode-clamped and capacitor-clamped (flying capacitor) multilevel inverters. With the exception of the CHB MLI, the often-used fundamental MLI configurations have been limited to the syntheses of 3-level output voltages due to the inherent operational imbalance in the constituting capacitor banks’ voltages. These 3-level configurations have attained industrial maturity and commercially available for the past years. On the other hand, the CHB MLI topological feature allows the syntheses of feasible multilevel output voltages, depending on application. Therefore, regarding scalability, modularity and overall improved power quality, the CHB MLI is often the chosen option for high-voltage/power deployment. The most popularly used modulation scheme for the control of the amplitude and frequency of the synthesized output voltage waveform of CHB MLI is the triangular carrier-based sinusoidal pulse width modulation, SPWM. Its concept of generating gating signals is inverter-phase-leg based. This precisely implies that the same control concept is repeated in each phase of the inverter; the only modulating parameter difference is the phase angle shift. Precisely, the control concept involves the comparison a high frequency triangular carrier wave with a fundamental frequency sinusoidal modulating signal; setting the zero-sequence signal to zero. In effect, SPWM presents no rigorous and complex computational difficulties and its extension to multilevel and/or multiphase systems involves only a multiplicity of the triangular carrier and sinusoidal modulating signals. It is factual that the Level-shifted (LS) and Phase-shifted (PS) pulse width modulations are the two broad versions of the SPWM technique that have been well established in the literature, for CHB MLI. In-phase disposition (IPD) variant of the LS PWM exhibits the best harmonic performance amongst all these modulation techniques; whereas the harmonic performance of the alternative phase-opposite disposition (APOD) of the LS PWM is same with that of PS SPWM scheme. Regarding uniform distribution of switching pulses among power switches of the constituting cascaded cells and even power delivery from each of the CHBs, the PS PWM claims superior modulation performance. In order to merge these good modulation features inherent in IPD and PS PWM schemes, various approaches have been proffered for this combinational concept as demonstrated. As aforementioned, multiplicity of the triangular carrier signals is a basic criterion for the extension of SPWM to a number of output voltage levels per inverter phase-leg. In other words, each of the synthesized output voltage levels in an inverter phase-leg is tagged to a generating triangular carrier. Considering medium and high voltage applications where appreciable number of synthesized output voltage levels from CHB MLI is needed, commensurate high number of triangular carrier signals in either classical LS or PS SPWM scheme for this inverter configuration is inevitable. And high-quality output parameter waveforms from the CHB MLI system demands precise synchronization of these multicarrier signals.

PROBLEM STATEMENT:

This paper presents a Space vector PWM for cascaded H-bridge multilevel inverter. It is compared with the operation of SPWM wherein the sine modulating waveform is used with multiple triangular carrier signal in order to generate the desired output waveform for the MLI. The obvious innate drawback of SPWM (non-distribution of switching signals to power switches and unequal output power sharing) is effectively removed from the modulation scheme following a simple reverse-voltage-sorting algorithm. In effect, the proposed control approach results in vector based modulation scheme that regulates the current flow with respect to voltage even with LCL filter. The comparison tables are made between two approaches for different load setup.