Simulation Of A Space Vector Pwm Controller For A Three-Level Voltage-Fed Inverter Motor Drive

ABSTRACT

Multilevel voltage-fed inverters with space vector pulse width modulation strategy are gained importance in high power high performance industrial drive applications. This paper proposes a new simplified space vector PWM method for a three-level inverter fed induction motor drive. The three- level inverter has a large number of switching states compared to a two-level inverter. In the proposed scheme, three-level space vector PWM inverter is easily implemented as conventional two-level space vector PWM inverter. Therefore, the proposed method can also be applied to multilevel inverters. In this work, a three-level inverter using space vector modulation strategy has been modeled and simulated. Simulation results are presented for various operation conditions using R-L load and motor load to verify the system model.

CHAPTER ONE

1.0                                                        INTRODUCTION

The inverters are used to converts dc power into ac power at desired output voltage and frequency. The waveform of the output voltage depends on the switching states of the switches used in the inverter. Major limitations and requirements of inverters are harmonic contents, the switching frequency, and the best utilization of dc link voltage. Pulse width modulation (PWM) inverters are studied extensively during the past decades. In this method, a fixed dc input voltage is given to the inverter and a controlled ac output voltage is obtained by adjusting the on and off periods of the inverter components. The most popular PWM techniques are the sinusoidal PWM and space Vector PWM. With the development of DSPs, space-vector modulation (SVM) has become one of the most important PWM methods for three-phase voltage source inverters. In this technique, Space-vector concept is used to compute the duty cycle of the switches. It is simply the digital implementation of PWM modulators. Most advanced features of SVM are easy digital implementation and wide linear modulation range for output line-to-line voltages.

In this paper, first a model for Pulse Width Modulation (PWM) based Inverter and Space vector PWM (SVPWM) based Inverter will be made and simulated using MATLAB/SIMULINK software and then the performance of both the techniques will be compared using MATLAB/SIMULINK.

1.2                                             OBJECTIVE OF THE PROJECT

SVPWM aims to generate a voltage vector that is close to the reference circle through the various switching modes of inverter. Space Vector Pulse Width Modulation (SVPWM) is one of the most used techniques to generate sinusoidal voltage and current due to its facility and efficiency with low harmonic distortion. This algorithm is special, used in power electronic applications. This paper describes simulation algorithm of SVPWM using MATLAB/SIMULINK. SVPWM based Inverter are broadly classified to reduce the switching losses and harmonics in the system.

1.3                                         SIGNIFICANCE OF THE PROJECT

The space vector pulse width modulation (SVPWM), as a new kind of vector control technique, has the advantages of high voltage utilization, less distorted output, simple control method, etc. In this paper, the control technique is applied to three-phase photovoltaic (PV) grid-connected inverter system. It uses double loop control method combined with PI control technique. The simulation model of PV grid-connected inverter system is established in Matlab/Simulink. The simulation results show that the application of SVPWM technique effectively improves the dynamic performance of PV grid inverter system and the THD of grid-connected current is low.

1.4                                                 SCOPE OF THE PROJECT

The Space Vector Pulse Width Modulation (SVPWM) refers to a special switching sequence of the upper three power devices of a three-phase voltage source inverters (VSI) used in application such as AC induction and permanent magnet synchronous motor drives. It is a more sophisticated technique for generating sine wave that provides a higher voltage to the motor with lower total harmonic distortion. Space Vector PWM (SVPWM) method is an advanced; computation intensive PWM method and possibly the best techniques for variable frequency drive application. In SVPWM technique, instead of using a separate modulator for each of the three phases, the complex reference voltage vector is processed as a whole. Therefore, the interaction between the three motor phases is considered. SVPWM generates less harmonic distortion in the output voltages and currents in the windings of the motor load and provides a more efficient use of the DC supply voltage in comparison with sinusoidal modulation techniques. Since SVPWM provides a constant switching frequency; the switching frequency can be adjusted easily.

1.5                                              PROBLEM OF THE PROJECT

The main problems in PWM viz. harmonic distortion and switching speed are overcome by implementing the Space-Vector PWM (SVPWM) technique by using the Xilinx tool VHDL (Verilog High Speed Integrated Circuit (VHSIC) Hardware Description Language) and tested in programmable Integrated Circuits of Field Programmable Gate Array (FPGA). The results are provided along with simulation analysis in terms of hardware utilization and schematic, power report, computing time and usage of memory.

1.6             ADVANTAGES OF SVPWM COMPARED TO SINUSOIDAL PWM

1. Since the triplen order harmonics are appeared in the phase-to-centre voltage of SVPWM, it has higher modulation index compared to the Sinusoidal PWM. When the modulation index increases the THD of the output voltage decreases. Hence SVPWM has less amount of current and torque harmonics than those of sinusoidal PWM.

2. For Sinusoidal PWM (SPWM) Vmax = Vdc/2

For Space Vector PWM Vmax = Vdc/√3

Where, Vdc is DC-Link voltage. From this it is clear that Space Vector PWM can produce about 15 percent higher than Sinusoidal PWM in output voltage.

3. In SPWM different phases may switch simultaneously. But in SVPWM only one phase is switch at a time. Hence SVPWM has reduced switching losses compared to SPWM.

4. The SPWM inverter can be thought of as three separate driver stages which create each phase waveform independently. But Space Vector Modulation treats the inverter as a single unit.