Enhancement of Multilevel Inverters Characteristics by Using PWM Techniques
Ramy Mohamed Hossam Eldin Ibrahim Abdelhamid;
Abstract
One of the biggest problems in electrical power systems is the harmonic contents. Power electronics converters used in many applications are the main sources of harmonics in the electrical grids. Harmonics can cause problems for power system network, such as overheating in the magnetic cores of transformers and motors, electromagnetic interference, and pulsating torque in induction motors.
Multilevel Inverters (MLIs) have drawn remarkable interests in the recent years and have been studied for high-power applications. MLIs synthesize desired output AC voltage waveform from several DC sources. As the number of DC sources increased, the number of output voltage levels increased and hence the obtained output voltage waveform is closer to the sinusoidal shape. There are many applications for MLIs such as AC drives, static VAR compensation, and uninterruptible power supplies. The main topologies of MLIs are diode-clamped MLI, flying-capacitor MLI, and cascaded MLI with separated DC sources. The cascaded MLI is superior to other types due to its modular topology based on H-bridge inverters.
Many switching strategies are applied to control the MLI output voltage and harmonic contents such as Space Vector Modulation (SVM) and Pulse Width Modulation (PWM). There are many PWM techniques such as sinusoidal PWM and Selective Harmonic Elimination (SHE) PWM technique. The SHE-PWM technique is widely used for eliminating pre-selected lower order harmonics from the output voltage and for controlling the fundamental voltage component.
There are two types of SHE-PWM technique, Optimized Harmonic Stepped Waveform (OHSW) PWM technique and Stepped Selective Harmonic Elimination (SSHE) PWM techniques. The main difficulty of SHE-PWM techniques is how to solve the voltage set of equations derived by using Fourier analysis to calculate the required switching angles. These equations are transcendental in nature and multiple solutions may be reached.
In this thesis, the Newton-Raphson algorithm and the Genetic algorithm methods are proposed to find the optimal switching angles for the MLI using either OHSW or SSHE-PWM techniques. The MATLAB software package is used to calculate the switching angles by any of the proposed methods. The optimal switching angles are stored in sort of look up tables which are used for simulation of the complete system.
The Newton-Raphson method is applied for OHSW-PWM in case of 5, 7, 9, 11, and 13 levels single and three-phase MLIs with equal input DC sources. Moreover, the Newton-Raphson method is applied for SSHE-PWM technique in case of 5 and 7 levels for single-phase MLI for many different values of chops per quarter cycle at each inverter level with equal and non-equal input DC sources. It has been shown that the solution ranges of the modulation index (M) become narrower with higher number of levels. It is not worthy that no solution is found after 13-level for single-phase MLI based on OHSW-PWM technique. The Genetic algorithm is applied to optimize the required switching angles for the OHSW-PWM technique in case of 5, 7, 9, 11, and 13-levels MLIs. Solution ranges of the modulation index that are obtained from the Genetic algorithm are wider than that of the Newton-Raphson method.
Simulink models are designed for different MLIs controlled by OHSW-PWM and SSHE-PWM, where the switching angles is calculated using either the Newton-Raphson or the Genetic algorithm. The simulation results validated the achieved theoretical results and proved that the obtained switching patterns can be practically implemented.
The distributed generators (DG) connected to the grid by using cascaded MLI interface based on SHE-PWM technique. The proposed interface systems controls both real and reactive power to meet power system needs with appropriate closed loop current control technique, and eliminates selected lower order harmonics from the output voltage of the MLI, and hence, reduce the total harmonic distortion. The interface system is based on a three-phase seven-level cascaded MLI. Simulation results are provided to prove the effectiveness of the proposed interface system.
Experimental prototypes for single-phase cascaded 5 and 7 levels MLI circuits are designed and built. The aim of the experimental work is to validate and to verify the theoretical and simulated results. The experimental results demonstrate the effectiveness of the proposed Newton-Raphson and Genetic algorithm methods to obtain optimal switching angles for the MLI based on either OHSW or SSHE PWM techniques.
Multilevel Inverters (MLIs) have drawn remarkable interests in the recent years and have been studied for high-power applications. MLIs synthesize desired output AC voltage waveform from several DC sources. As the number of DC sources increased, the number of output voltage levels increased and hence the obtained output voltage waveform is closer to the sinusoidal shape. There are many applications for MLIs such as AC drives, static VAR compensation, and uninterruptible power supplies. The main topologies of MLIs are diode-clamped MLI, flying-capacitor MLI, and cascaded MLI with separated DC sources. The cascaded MLI is superior to other types due to its modular topology based on H-bridge inverters.
Many switching strategies are applied to control the MLI output voltage and harmonic contents such as Space Vector Modulation (SVM) and Pulse Width Modulation (PWM). There are many PWM techniques such as sinusoidal PWM and Selective Harmonic Elimination (SHE) PWM technique. The SHE-PWM technique is widely used for eliminating pre-selected lower order harmonics from the output voltage and for controlling the fundamental voltage component.
There are two types of SHE-PWM technique, Optimized Harmonic Stepped Waveform (OHSW) PWM technique and Stepped Selective Harmonic Elimination (SSHE) PWM techniques. The main difficulty of SHE-PWM techniques is how to solve the voltage set of equations derived by using Fourier analysis to calculate the required switching angles. These equations are transcendental in nature and multiple solutions may be reached.
In this thesis, the Newton-Raphson algorithm and the Genetic algorithm methods are proposed to find the optimal switching angles for the MLI using either OHSW or SSHE-PWM techniques. The MATLAB software package is used to calculate the switching angles by any of the proposed methods. The optimal switching angles are stored in sort of look up tables which are used for simulation of the complete system.
The Newton-Raphson method is applied for OHSW-PWM in case of 5, 7, 9, 11, and 13 levels single and three-phase MLIs with equal input DC sources. Moreover, the Newton-Raphson method is applied for SSHE-PWM technique in case of 5 and 7 levels for single-phase MLI for many different values of chops per quarter cycle at each inverter level with equal and non-equal input DC sources. It has been shown that the solution ranges of the modulation index (M) become narrower with higher number of levels. It is not worthy that no solution is found after 13-level for single-phase MLI based on OHSW-PWM technique. The Genetic algorithm is applied to optimize the required switching angles for the OHSW-PWM technique in case of 5, 7, 9, 11, and 13-levels MLIs. Solution ranges of the modulation index that are obtained from the Genetic algorithm are wider than that of the Newton-Raphson method.
Simulink models are designed for different MLIs controlled by OHSW-PWM and SSHE-PWM, where the switching angles is calculated using either the Newton-Raphson or the Genetic algorithm. The simulation results validated the achieved theoretical results and proved that the obtained switching patterns can be practically implemented.
The distributed generators (DG) connected to the grid by using cascaded MLI interface based on SHE-PWM technique. The proposed interface systems controls both real and reactive power to meet power system needs with appropriate closed loop current control technique, and eliminates selected lower order harmonics from the output voltage of the MLI, and hence, reduce the total harmonic distortion. The interface system is based on a three-phase seven-level cascaded MLI. Simulation results are provided to prove the effectiveness of the proposed interface system.
Experimental prototypes for single-phase cascaded 5 and 7 levels MLI circuits are designed and built. The aim of the experimental work is to validate and to verify the theoretical and simulated results. The experimental results demonstrate the effectiveness of the proposed Newton-Raphson and Genetic algorithm methods to obtain optimal switching angles for the MLI based on either OHSW or SSHE PWM techniques.
Other data
| Title | Enhancement of Multilevel Inverters Characteristics by Using PWM Techniques | Other Titles | تحسين أداء عاكس متعدد المستويات بإستخدام تقنية تعديل عرض النبضة أسم | Authors | Ramy Mohamed Hossam Eldin Ibrahim Abdelhamid | Keywords | Multilevel Inverter, Pulse Width Modulation, Distributed Generator, Genetic Algorithm, Newton Raphson | Issue Date | 2014 |
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