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CHAPTER 1
INTRODUCTION
1.1
BACKGROUND
Power electronic inverters are the essential equipments to convert
and control the electrical power in the wide range of milli watts to giga watts
with the help of semiconductor devices. These inverters are finding increased
attention in recent years. Hence, highly efficient power electronic
technologies and reliable control strategies are needed to reduce the loss of
energy and to improve power quality. High power electronic devices are
being used increasingly to control and facilitate flow of electric power while
meeting stringent-operating conditions of present heavily loaded networks.
In recent years, numerous industrial applications have begun to
demand higher power equipment. Medium voltage motor drives and their
applications require medium voltage to megawatt power level. It is hard to
connect a single power semiconductor switch directly to a medium voltage
grid. As a result, a multilevel power inverter structure has been introduced as
an alternative in high power and medium voltage situations.
One of the most significant recent advancements in power
electronics is the multilevel inverter. Multilevel inverters are nowadays the
most preferred choice for high-voltage and high-power applications in
industry. The unique structure of multilevel voltage source inverters allows
them to reach high voltages with low harmonics without the use of
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transformers or series-connected synchronized switching devices. The general
function of the multilevel inverter is to synthesize a desired voltage from
several levels of dc voltages. By increasing the number of levels in the
inverter, the synthesized output voltages have more steps, generating a
staircase waveform, which has a reduced harmonic distortion. However, a
high number of levels increases the control complexity and introduces voltage
imbalance problems.
Multilevel inverter technologies are receiving increased attention
recently in high voltage-high power applications due to its high voltage
handling and good harmonic rejection capabilities. A multilevel inverter not
only achieves high power ratings, but also enables the use of renewable
energy sources. Renewable energy sources such as photovoltaic, wind, and
fuel cells can be easily interfaced to a multilevel inverter system for a high
power application.
Multilevel inverters have also emerged as an attractive choice in the
field of medium-voltage to high-voltage industrial drive applications, which
include motor drives, power distribution, power quality, and power
conditioning applications. It is important that the multilevel inverter is
introduced as a solution to increase the inverter operating voltage above the
voltage limits of classical semiconductors. Using this concept, the power
conversion is performed in small voltage steps, resulting in better power
quality. These inverters are suitable for high-voltage applications because of
their ability to synthesize output voltage waveforms with a better harmonic
spectrum and attain higher voltages with a limited maximum device rating.
Additionally, the harmonic content of output waveform decreases
significantly as the number of inverter levels increases.
This research work focuses on minimizing the most significant
harmonic components of the generated waveform while keeping other
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harmonic components within the acceptable range for the cascaded multilevel
inverter with unequal or varying dc sources.
An important issue in designing an effective multilevel inverter is
to ensure the increase of power, the reduction of voltage stress on the power
switching devices, and generation of high-quality output voltages and
sinusoidal currents.
In recent years, multilevel inverters have been developed for
several reasons.
1.
Few Industries have begun to demand higher power
equipment, which now reaches the MW level. Controlled AC
drives in the megawatt range are usually connected to the
medium-voltage network. Today, it is hard to connect a single
power semiconductor switch directly to medium voltage grids
(2.3- 6.9 kV) (Jose Rodriguez et al 2002).
2.
Multilevel inverters can solve problems with some present
two-level PWM adjustable-speed drives (ASDs). Usually
Adjustable-Speed Drives (ASDs) which operate in highpower range are connected to medium-voltage network. ASDs
usually employ a front-end diode rectifier and an inverter with
PWM-controlled switching devices to convert the DC voltage
to variable frequency and variable voltage for motor speed
control. Motor damage and failure have been reported by
industry as a result of some ASD inverters’ high-voltage
change rates (dv/dt), which produce a common-mode voltage
across the motor windings. High-frequency switching can
exacerbate the problem because of the numerous times this
common mode voltage is impressed upon the motor each
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cycle. The main problems are reported as “motor bearing
failure” and “motor winding insulation breakdown” because
of circulating currents, dielectric stresses, voltage surge, and
corona discharge Bell and Sung (1997), (Erdman et al 1995)
and (Bonnett 1997). The failure of some ASDs is because the
voltage change rate (dv/dt) sometimes can be high enough to
induce corona discharge between the winding layers (Tolbert
et al 1999) and (Tolbert et al 1998).
3.
Modern power electronic devices can switch at higher
frequency and higher voltages, which can generate broadband
Electro Magnetic Interference (EMI). These high-speed
semiconductor
switches
allow
faster
PWM
carrier
frequencies. Although the high-frequency switching can
increase the motor running efficiency and is well above the
acoustic noise level, the (dv/dt) associated dielectric stresses
between insulated winding turns are also greatly increased
(Tolbert et al 1999) and (Tolbert et al 1998).
For these reasons, multilevel inverters are preferred for high-power
medium voltage drive applications because the power structure can be
realized with devices of lower voltage rating.
Multilevel inverter has drawn tremendous interest in high power
applications because it has many advantages: it can realize high voltage and
high power output through the use of semiconductor switches without use of
transformer and without dynamic voltage balance circuits. When the number
of output levels increases, harmonics of the output voltage and current as well
as Electro Magnetic Interference (EMI) decreases. Multilevel inverters are
receiving increased attention in industry and research is probably due to less
switching loss; less total harmonic distortion (THD) in the voltage waveform
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and small dv/dt and nearer-sinusoidal outputs. The preferred output of a
multilevel inverter is synthesized by several sources of dc voltages. With an
increasing number of dc voltage sources, the inverter voltage waveform
approaches a nearly sinusoidal waveform while using a low switching
frequency scheme. This results in low switching losses, and because several
dc sources are used to synthesize the total output voltage, each experiences a
lower dv/dt compared to a single level inverter Sinha and Lipo (1998).
One application for multilevel inverters is distributed power
systems. Multilevel inverters can be implemented using distributed energy
resources such as photovoltaic and fuel cells, and then be connected to an AC
power grid (Villanueva et al 2009). If a multilevel inverter is made to either
draw or supply purely reactive power, then the multilevel inverter can be used
as a reactive power compensator. For example, a multilevel inverter being
used as a reactive power compensator could be placed in parallel with a load
connected to an AC system. This is because a reactive power compensator
can help to improve the power factor of a load (McKenzie 2004).
Another application for multilevel inverters is to interconnect
different power grids. For example, two diode-clamped multilevel inverters
can be used to produce such a system. One multilevel inverter acts as a
rectifier for the utility interface Lai and Peng (1996). The other multilevel
inverter acts as an inverter to supply the desired AC load (Tolbert et al 1999).
Such a system can be used to connect two asynchronous systems and acts as a
frequency changer, a phase shifter, or a power flow controller.
1.2
REVIEW OF PREVIOUS RESEARCH
Most of the research work in the three-phase power inverter area
was for balanced three-phase systems, such as motor drive applications,
where three-legged power inverters are used. Usually, split DC link capacitors
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are used when the neutral connection is needed. A balance of the dc voltages
of a multilevel inverter cannot be achieved in all loading conditions.
Especially unbalanced loads are prevalent for UPS, standalone power
generation applications and there is no limitation to the percentage of
unbalance. Imbalances in load or transient loads have a significant impact on
the multilevel inverter dc voltage ripple.
Using three-legged power inverters to deal with unbalanced source
has been addressed in (Prasad et al 1989), Prasad and Shamin (1993) and
Donato Vincenti and Hua Jin (1994). However, a three-legged power inverter
is unable to deal with zero-sequence unbalance. To solve the limitation,
usually separate dc sources are used. In all the previous research, there were
neither three-dimensional space vector modulations schemes proposed for
cascaded H-bridge inverters, nor the modeling and control aspects of
cascaded H-bridge inverters discussed. When the cascaded H-bridge
multilevel inverter was introduced for motor drive applications, an isolated
and separate DC source was needed for each H-bridge cell (Hammond 1997).
Also, the cascaded H-bridge inverter is suitable for universal power
conditioning of power systems proposed (Peng et al 1997). The inverter
provides lower costs, higher performance, less EMI, and higher efficiency
than the traditional PWM inverter for power line conditioning applications,
both series and parallel compensation. Although the cascaded inverter has an
inherent self-balancing characteristic, a slight voltage imbalance can occur
because of the circuit component losses and limited controller resolution. A
simple control method has been proposed for reactive and harmonic
compensation, which ensures DC voltage balance presented in (Jose
Rodriguez et al 2002) and (Peng et al 1996).
Modulation methods for multilevel inverter can be classified
according to the switching frequency methods. The key issues for the control
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of the inverters are to find the modulation methods to control the output
rectangular waves to synthesize the desired sinusoidal waveforms. Therefore,
a modulation method needs to generate desired fundamental sinusoidal
voltage while at the same time certain higher order harmonics are eliminated.
Pulse Width Modulation (PWM) techniques have been widely used
in the field since 1960s. Many PWM schemes have been proposed for singlephase and three-phase applications, including Sinusoidal PWM method
(SPWM) Buja and Indri (1977) and Patel and Hoft (1974) Space Vector
Control (SVC) method, selective harmonic elimination method, and Space
Vector PWM method (SVPWM) Alfred Busse and Joachim Holtz (1982) and
(Ahmet et al 1997). SPWM and SVC methods cannot be applied to multilevel
inverters with unequal DC voltages. Until now, the selective harmonic
elimination method can eliminate the number of harmonics not more than the
number of the switching angles in the transcendental equations. Due to the
difficulty of solving the transcendental equations, real-time control of
multilevel inverters with unequal DC voltages is impossible.
In this thesis, after reviewing the traditional PWM, especially twodimensional space vector modulation (2-D SVPWM), three-dimensional
space vector modulation (3-D SVPWM) methods are proposed for multilevel
inverters (Zhang et al 2002). SVPWM became more and more popular due to
its merits of good utilization of the DC link voltage, possible optimized output
distortion and switching losses, and relatively simple and easy for digital
implementation. The two-dimensional space vector modulation can be applied
to multilevel inverter with equal dc sources whereas three-dimensional space
vector modulation can be applied to both equal and unequal dc sources. All
the existing space vector modulation schemes implemented in a twodimensional space are unable to deal with the zero sequence component
caused by unbalanced loads. The three-dimensional space vector modulation
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scheme has a better DC link voltage utilization and results in a low harmonic
distortion. It is an effective solution to handle zero sequence component
caused by the unbalanced loads.
Most of the previous works on three-dimensional space vector
modulation algorithms proposed deals with the diode-clamped inverter. The
unequal dc sources cannot be applied to diode-clamped inverter. As far as the
author concern the first three-dimensional space vector modulation for
cascaded H-bridge inverter presented in (Robert 1979) previous researches on
the 3-D SVPWM algorithm shows that the reference vectors are not on a
plane, if the system is unbalanced for a multilevel inverter. But, the proposed
Optimized 3-D SVPWM (3-D OSVPWM) algorithm maintains 1200 phase for
each phase to compute switching state vectors. Since, the optimized threedimensional space vector modulation scheme has a better DC link voltage
utilization and results in a low harmonic distortion. It is an effective solution
to handle zero sequence component caused by unbalanced loads. The
SVPWM technique generates the desired fundamental frequency voltage and
also eliminates any number of the specified harmonics without the restriction
of the number of unknowns in the harmonic equations and available solutions
for the harmonic equations.
Another contribution of the SVPWM technique is that it simplifies
the optimal system performance searching by making a tradeoff between
switching frequency and harmonic distortion, since it can vary its switching
frequency for different modulation indices. Although SVPWM methods are
very powerful in reducing harmonics, finally it is accepted that SVPWM
technique becomes computationally very demanding and complexity
increases as the number of levels increases.
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1.3
PROBLEM STATEMENT
Modern power electronics can be applied to solve or mitigate
power quality problems. Harmonic content in the electrical system is one of
the biggest problems in power quality.
The key issue for multilevel inverter modulation is the harmonic
elimination. All unbalanced or nonlinear loads draw harmonic currents. Those
harmonics cause a lot of problems, such as distorted voltage, voltage flicking,
overheated transformer, high torque ripple in the generator, severe EMI noise
to communication systems and computer systems. Those problems become
more severe as more unbalanced loads are put in use in the field. In order to
address the problems caused by unbalanced loads, some standards have been
established to limit harmonics produced by unbalanced or nonlinear loads.
The most commonly cited standards are IEC 1000-3-2, and IEC 1000-3-4.
Harmonics may be divided into two types: 1) voltage harmonics
and 2) current harmonics. Both the harmonics can be either generated by
source or load. Voltage and current source harmonics imply power losses and
Electromagnetic Interference (EMI) and pulsating torque in AC motor drives.
Harmonic elimination is performed for several reasons. The reason is
Electromagnetic Interference (EMI). Harmonics are a source of EMI. Without
harmonic elimination, designed circuits would need more protection in the
form of snubbers and EMI filters. As a result, designed circuits would cost
more.
The traditional PWM methods employ much higher switching
frequencies for two reasons. The first reason concerns harmonics. Undesirable
harmonics occur at much higher frequencies. Thus, filtering is much easier
and less expensive. The second reason concerns audible noise. Several kHz is
well above the acoustic noise level. Also, if the generated high frequency
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harmonics are above the bandwidth of some actual systems, there is no power
dissipation due to these harmonics (McKenzie 2004).
But as mentioned above, traditional PWM schemes have the
inherent problems of producing Electro Magnetic Interference (EMI). Rapid
changes in voltages (dv/dt) are a source of EMI. The presence of a high dv/dt
can cause damage to electrical motors. A high dv/dt produces common-mode
voltages across the motor windings. Furthermore, higher switching
frequencies can make this problem worse due to the increased number of
times and these common-mode voltages are applied to the motor during each
fundamental cycle. Problems such as motor bearing failure and motor
winding insulation breakdown can result due to circulating currents and
voltage surges (Tolbert et al 1999) and also long, current-carrying conductors
connecting equipment can result in a considerable amount of EMI.
The objective of this thesis is to develop a simple and efficient control
method for minimizing harmonic distortion. Previous works on SVPWM
technique does not takes into account the actual unbalance of the multilevel
inverter and the reference vectors are not on a plane, if the system is
unbalanced. The cost effective solution is to use an algorithm that achieves
voltage balance based on the use of redundant vectors, maintains 120 0 phase
for each phase to compute switching state vectors and the nearest switching
sequence and should be useful in systems with or without neutral and
unbalanced load. The control algorithm should be completely generalized and
can be applied to different multilevel inverter topologies and for any number
of levels.
The thesis is arranged as follows:
Chapter 2 provides a summary of the existing literature and the
state-of–art in multilevel inverter topologies and control, and modulation
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strategies. The advantages and disadvantages of various multilevel inverter
topologies and control techniques are discussed in detail. Finally, a brief
overview of the impact of unbalanced load/ source on the performance of
multilevel inverter is discussed.
In Chapter 3, the cascaded H-bridges multilevel inverter is
discussed in more detail. Also, in addition to the multilevel fundamental
switching scheme, some other switching schemes being applied to multilevel
inverters are discussed. Specifically, the Bipolar Programmed PWM,
Unipolar Programmed PWM and Virtual Stage PWM switching schemes are
presented. The idea of using unequal dc sources with multilevel inverters is
then discussed.
Chapter 4 addresses various SVPWM control methods for
multilevel inverters with equal and unequal DC voltages. Particularly, TwoDimensional (2-D) SVPWM and Three-Dimensional (3-D) SVPWM
algorithms are discussed in detail. A novel three-dimensional space vector
modulation scheme for computing the switching state vectors and the nearest
switching sequence is presented. The Optimized 3-D Space Vector
Modulation algorithm (3-D OSVPWM) for multilevel inverter with varying
dc sources and unbalanced load is developed and mathematical derivation is
given for balanced and unbalanced systems. The 3-D OSVPWM algorithm
presented for multilevel inverter is useful for the system with unbalanced load
or harmonic generation. It is discussed, how optimized 3-D space vector
modulation algorithm can be used to find solutions for reducing the Total
Harmonic Distortion (THD).
In Chapter 5, implementation of the proposed optimized 3-D space
vector modulation algorithm (3-D OSVPWM) on an 11-level multilevel
inverter. Simulation and Experimental results are given for both twodimensional (2-D) SVPWM and optimized three-dimensional (3-D)
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OSVPWM algorithms for the cascaded H-bridge multilevel inverter. All the
simulation and experimental results prove the validity of the proposed threedimensional space vector modulation schemes.
Chapter 6 concludes the thesis work and gives future research
directions.