Preface
The output voltage of a power inverter is a pure sinusoidal waveform with minimum distortion. However, for practical inverters, the output voltage is a series of rectangular waveforms. The major issue for the control of the power inverters is obtaining suitable modulation methods to control the output rectangular waveforms in order to synthesize the desired waveforms. Therefore, a modulation control method is required to get a desired fundamental frequency voltage and to eliminate higher-order harmonics as much as possible.
Higher frequencies are employed in traditional pulse width modulation (PWM) methods because the undesirable harmonics present at higher frequencies, which can be filtered easily and several kilohertz, is well above the acoustic noise level. However, the traditional PWM methods cause electromagnetic interference (EMI). The rapid change in voltage (dv/dt) is the cause of EMI. A high dv/dt produces common-mode voltages across the windings of motor and leads to damage it.
In multilevel inverters, as the switching involves several small voltages, the rapid change in voltage is smaller. Further, switching at the fundamental frequency will also result in a decrease in the number of times these voltage changes occur per fundamental cycle. However, harmonic elimination is a major issue for multilevel inverters. The harmonic elimination in multilevel inverters has been proposed in this book for the following reasons:
i.Harmonics in output voltage create power losses in equipments.
ii.Harmonics are the source of EMI. Protecting devices like snubber circuits and filters have to be incorporated in the designed circuits to eliminate harmonics. Hence, the cost of the circuits increases.
iii.EMI can interfere with signals used to control power electronic devices and radio signals.
iv.Harmonics can create losses in power equipments. Harmonic currents in an induction motor will dissipate the power in the stator and motor windings.
v.Harmonics can lower the load power factor.
As mentioned earlier, multilevel inverters result in a better approximation of the sinusoidal waveform because of the increased number of DC voltage levels. The increased number of DC voltage levels provides an opportunity to eliminate more harmonic contents. The remaining harmonic content can be easily eliminated by less expensive smaller filters; because of large number of DC voltages used in multilevel inverters to block smaller voltages, several switches are needed. Since switch stress is reduced and lower switch ratings are used, if any component fails in the inverter, it will be still usable at reduced power level. In a multilevel inverter, there will be more than one way to generate the desired voltages due to switching redundancies. This will allow for the utilization of smaller and more reliable components. One disadvantage of multilevel inverters is that they require more devices than traditional inverters. The system cost may increase. The probability of system failure increases and the control of the switches is also more complicated because of more devices.
There are four kinds of control methods for multilevel inverters: traditional PWM control method, selective harmonic elimination method, space vector control method, and space vector PWM method. Space vector PWM is considered a better technique of PWM implementation owing to its associated advantages such as better fundamental output voltage, better harmonic performance, and easier implementation in digital signal processor and microcontrollers.
For these reasons, in this book, space vector PWM-based algorithms are proposed and implemented for neutral point-clamped multilevel inverter fed induction motor. These space vector-based algorithms generate not only the desired fundamental frequency voltages, but also eliminate the harmonics up to the maximum possible extent and results in reduces total harmonic distortion (THD).
The work presented in this book offers a general approach for PWM techniques and multilevel inverter topologies. The main objective of this book is to provide detailed analysis and implementation of space vector PWM technique applied to neutral point-clamped multilevel inverter. This book is extremely useful for undergraduate students, postgraduate students, industry people, and especially for research scholars working in the area of multilevel inverters.
This book presented various space vector PWM-based algorithms for multilevel neutral point-clamped inverter fed induction motor. The performance of these algorithms are evaluated in terms of inverter output voltage, current waveforms, THD, speed of induction motor, and torque ripples, and the results have been analyzed and presented.
Chapter organization of the book
This book is organized in the following manner:
Chapter 1 discusses pulse width modulation, various basic pulse width modulation techniques, advanced modulation techniques, space vector pulse width modulation technique, and the advantages of pulse width modulation techniques.
Chapter 2 presents the features of space vector pulse width modulation, space vector concept, and the two-level inverter. The implementation of space vector pulse width modulation technique is explained in detail and is applied to a two-level inverter.
Chapter 3 presents the introduction to multilevel inverter, multilevel inverter topologies, and their working principles. The advantages and disadvantages of various topologies are discussed in detail.
Chapter 4 presents the space vector pulse width modulation (SVPWM) algorithm for a three-level inverter fed induction motor. This SVPWM algorithm provides high-safety voltages with fewer harmonic components compared with two-level structures. The results and analysis of this method have been presented and analyzed in this chapter.
Chapter 5 presents a method for the generation of space vector PWM for multilevel inverters based on fractal approach for three- and five-level inverters. The fractal approach reduces algorithm complexity and execution time. The results and analysis of this method have been presented and analyzed in this chapter.
Chapter 6 explains a qualitative space vector pulse width modulation algorithm for neutral point-clamped multilevel inverter. In this method, the duty cycles of reference voltage vectors are corrected accordingly to identify the location of the reference voltage vector in each region. The appropriate switching sequence of the region and calculation of the switching ON times for each state are estimated. The results of this qualitative space vector pulse width modulation method have been presented and analyzed in this chapter.
Chapter 7 describes a space vector pulse width modulation algorithm using the decomposition method for seven-level inverter. In this method, the space vector diagram of the seven-level inverter is decomposed into six space vector diagrams of four-level inverters. In turn, each four-level inverter is decomposed into three-level inverters, and finally, the three-level inverter is decomposed into six space vector diagrams of two-level inverters. The proposed method reduces algorithm complexity and execution time. It can also be applied to the multilevel inverters above the seven-level.
Chapter 8 presents an analytical space vector pulse width modulation for multilevel inverters, which improves inverter performance. This method is based on the intrinsic relation between multilevel and two-level space vector pulse width modulation, and the dwelling time of vector calculation is derived from a two-level inverter. This method is applied up to the eleven-level inverter, which can be extended to the n-level inverter as well. The results have been presented and analyzed.