Multilevel Inverter(3-level) topologies(Diode and capacitor clamped) and controlscheme(spwm). PDF

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CHAPTER 1 1. INVERTER INTRODUCTION: A dc-to-ac converter whose output is of desired output voltage and frequency is called an inverter. Based on their operation the inverters can be broadly classified into Voltage Source Inverters(VSI) Current Source Inverters(CSI) A voltage source inverter is one w...

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CHAPTER 1

1. INVERTER INTRODUCTION: A dc-to-ac converter whose output is of desired output voltage and frequency is called an inverter. Based on their operation the inverters can be broadly classified into Voltage Source Inverters(VSI) Current Source Inverters(CSI) A voltage source inverter is one where the independently controlled ac output is a voltage waveform. A current source inverter is one where the independently controlled ac output is a current waveform. On the basis of connections of semiconductor devices, inverters are classified as Bridge inverters Series inverters Parallel inverters Some industrial applications of inverters are for adjustable- speed ac drives, induction heating, stand by air-craft power supplies, UPS(uninterruptible power supplies) for computers, hvdc transmission lines etc.

1.1Comparison of the 2-level and multilevel inverters In 2-level inverter output voltage waveform is produced by using PWM with two voltage levels. This causes the output voltage and current to be distorted and the THD of the voltage is poor(Figure 1, left). In 3-level inverter output voltage and current is much more sinusoidal and the THD is better (Figure 1, right).

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Figure 1. Comparison of the 2-level and 3-level inverter output voltages and currents .

Comparison of conventional two level inverters and multilevel inverters S.No Conventional Inverter 1 Higher THD in output voltage 2 More switching stresses on devices

Multilevel Inverter Low THD in output voltage Reduced switching stresses on Devices

3

Not applicable for high voltageapplications

Applicable for high voltage Applications

4

Higher voltage levels are not produced

Higher voltage levels are Produced

5

Since dv/dt is high, the EMI from system is high

Since dv/dt is low, the EMI from system is low

6

Higher switching frequency is used hence switching losses is high Power bus structure, control schemes are simple

Lower switching frequency can be used and hence reduction in switching losses control scheme becomes complex as number of levels increases

7

Table 1 Comparison of conventional two level inverters and multilevel inverters

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CHAPTER 2

2. MULTI LEVEL INVERTERS 2.1 INTRODUCTION: Multilevel Converters has been attracted a large interest in the power industry in the recent years. Industry has started to involve in higher power equipment, which already reaches megawatt level. Conventional power electronic converters are only able to switch each individual input or output link between two possible voltage levels, especially those of the internal DC voltage link. The general structure of the multilevel converter is to generate a sinusoidal voltage from several levels of voltages which are usually obtained from capacitor voltage sources. Three different topologies have been projected for multilevel converters: Diode clamped converter; Flying capacitor converter (Capacitor Clamped); and lastly cascaded converter. Several modulation and control strategies have been developed or being used for multilevel converters including the following: Multilevel sinusoidal pulse width modulation (PWM), multilevel selective harmonic elimination, and space-vector modulation (SVM).

2.2 Advantages of multilevel converters 1. They are able to generate output voltages with very low distortion and lower dv/dt. 2. They are able to bring in input current with very low input distortion. 3. They are able to produce smaller common mode (CM) voltage, therefore, reducing the stress in the motoring bearings. In addition, using complicated modulation methods, CM voltages can be eliminated. 4.They can be functioned with a much lower switching frequency.

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The only disadvantage of the multilevel converter is that it required a huge amount of semiconductors switches. It should be pointed out that lower voltage rated switches can be used in the multilevel converter and as a result the active semiconductor cost is not considerably increased when compared with the two level cases. On the other hand, each active semiconductor added requires associated gate drive circuitry and adds further complication to the converter mechanical layout. Another disadvantage which is to be mention is that the small voltage steps are typically formed by isolated voltage sources or a bank of series capacitors. Isolated voltage sources may not always be readily available and series capacitors require voltage balance. To some extend, the voltage balancing can be addressed by using an uncalled-for switching states, which exist due to the high number of semiconductor devices. Nevertheless, for a complete solution to the voltage-balancing problem, another multilevel converter maybe is required. A multilevel converter can be implemented in many different ways, each with advantages and disadvantages. The simplest techniques which involve the parallel or series connection of conventional converters to form the multilevel waveforms. Complicated structures actually insert converters within converters. Whatever approach is being chosen, the subsequent voltage or current rating of the multilevel converter will becomes a multiple of the individual switches, and therefore the power rating of the converter can exceed the limit imposed by the individual switching devices. Power-electronic inverters are becoming popular for various industrial drives applications. In recent years also high-power and medium-voltage drive applications have been installed. To overcome the limited semiconductor voltage and current ratings, some kind of series and/or parallel connection will be necessary. Due to their ability to synthesize waveforms with a better harmonic spectrum and attain higher voltages, multi-level inverters are receiving increasing attention in the past few years. The multilevel inverter was introduced as a solution to increase the converter operating voltage above the voltage limits of classical semiconductors. The multilevel voltage source inverter is recently applied in many industrial applications such as ac power supplies, static VAR compensators, drive systems, etc.

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One of the significant advantages of multilevel configuration is the harmonic reduction in the output waveform without increasing switching frequency or decreasing the inverter power output. The output voltage waveform of a multilevel inverter is composed of the number of levels of voltages, typically obtained from capacitor voltage sources. The so-called multilevel starts from three levels. As the number of levels reach infinity, the output THD (Total Harmonic Distortion) approaches zero. The number of the achievable voltage levels, however, is limited by voltage unbalance problems, voltage clamping requirement, circuit layout, and packaging constraints. Multilevel inverters synthesizing a large number of levels have a lot of merits such as improved output waveform, a smaller filter size, a lower EMI (Electro Magnetic Interference), and other advantages. The principle advantage of using multilevel inverters is the low harmonic distortion obtained due to the multiple voltage levels at the output and reduced stresses on the switching devices used.

2.3 MULTILEVEL VOLTAGE CONCEPT: Recent advances in power electronics have made the multilevel concept practical. In fact, the concept is so advantageous that several major drives manufacturers have obtained recent patents on multilevel power converters and associated switching techniques. It is evident that the multilevel concept will be a prominent choice for power electronic systems in future years, especially for medium-voltage operation. Multi-level inverters are the modification of basic bridge inverters. They are normally connected in series to form stacks of level. The topological structure of multilevel inverter must cope with the following points. 1) It should have less switching devices as far as possible. 2) It should be capable of enduring very high input voltage such as HVDC transmission for high power applications. 3) Each switching device should have lower switching frequency owing to multilevel approach.

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There are various multilevel concepts used for various applications. Various multilevel circuits are used to generate multiple voltage levels. Some of the multilevel concepts with various voltage levels are given below.

2.4 ADVANTAGES OF MULTILEVEL VOLTAGES : In general, multilevel power converters can be viewed as voltage synthesizers, in which the high output voltage is synthesized from many discrete smaller voltage levels. The main advantages of this approach are summarized as follows: 1. The voltage capacity of the existing devices can be increased many times without the complications of static and dynamic voltage sharing that occur in series-connected devices. 2. Spectral performance of multilevel waveforms is superior to that of their two- level counterparts. 3.Multilevel waveforms naturally limit the problems of large voltage transients that occur due to the reflections on cables, which can damage the motor windings and cause other problems. 4.In very high power application especially with very high input voltage, traditional two-level VSIs could not avoid to sue the series connected semiconductor switches so as to cope with limitations of device rating utilized and it may be very cumbersome and even problematic mainly due to difficulty of device matching deteriorating utilization factor of switching devices. The multilevel topology, however, suggests a good solution for such a problem.

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2.5 APPLICATIONS: DC power source utilization: An inverter converts the DC electricity from sources such as batteries, solar panels, or fuel cells to AC electricity. The electricity can be at any required voltage; in particular it can operate AC equipment designed for mains operation, or rectified to produce DC at any desired voltage. Grid tie inverters can feed energy back into the distribution network because they produce alternating current with the same wave shape and frequency as supplied by the distribution system. They can also switch off automatically in the event of a blackout. Uninterruptible power supplies Inverters convert low frequency main AC power to a higher frequency for use in induction heating. To do this, AC power is first rectified to provide DC power. The inverter then changes the DC power to high frequency AC power‟ HVDC power transmission With HVDC power transmission, AC power is rectified and high voltage DC power is transmitted to another location. At the receiving location, an inverter in a static inverter plant converts the power back to AC. Variable-frequency drives A variable-frequency drive controls the operating speed of an AC motor by controlling the frequency and voltage of the power supplied to the motor. An inverter provides the controlled power. In most cases, the variable-frequency drive includes a rectifier so that DC power for the inverter can be provided from main AC power. Since an inverter is the key component, variable-frequency drives are sometimes called inverter drives or just inverters. Electric vehicle drives Adjustable speed motor control inverters are currently used to power the traction motors in some electric and diesel-electric rail vehicles as well as some battery electric vehicles and hybrid electric highway vehicles such as the Toyota Prius.

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Various improvements in inverter technology are being developed specifically for electric vehicle applications. In vehicles with regenerative braking, the inverter also takes power from the motor (now acting as a generator) and stores it in the batteries.

Air conditioning A transformer allows AC power to be converted to any desired voltage, but at the same frequency. Inverters, plus rectifiers for DC, can be designed to convert from any voltage, AC or DC, to any other voltage, also AC or DC, at any desired frequency. The output power can never exceed the input power, but efficiencies can be high, with a small proportion of the power dissipated as waste heat.

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CHAPTER 3

3. MULTI LEVEL INVERTER TOPOLOGIES Multilevel power conversion technology is a very rapidly growing area of power electronics with good potential for further development. The most attractive applications of this technology are in the medium- to high-voltage range (2-13 kV), and include motor drives, power distribution, power quality and power conditioning applications. There are different types of multi level circuits involved. The first topology introduced was the series H-bridge design. This was followed by the diode clamped converter, which utilized a bank of series capacitors. A later invention detailed the flying capacitor design in which the capacitors were floating rather than series-connected. Another multilevel design involves parallel connection of inverter phases through inter-phase reactors. In this design, the semiconductors block the entire dc voltage, but share the load current. Several combinational designs have also emerged some involving cascading the fundamental topologies. These designs can create higher power quality for a given number of semiconductor devices.

INTRODUCTION The schematic of inverter system is as shown in Fig. 3.1, in which the battery or rectifier provides the dc supply to the inverter. The inverter is used to control the fundamental voltage magnitude and the frequency of the ac output voltage. AC loads may require constant or adjustable voltage at their input terminals, when such loads are fed by inverters, it is essential that the output voltage of the inverters is so controlled as to fulfill the requirement of the loads. For example if the inverter supplies power to a magnetic circuit, such as a induction motor, the voltage to frequency ratio at the inverter output terminals must be kept constant. This avoids saturation in the magnetic circuit of the device fed by the inverter.

Fig. 3.1 : Schematic for Inverter System

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As in the single phase voltage source inverters PWM technique can be used in threephase inverters, in which three sine waves phase shifted by 120° with the frequency of the desired output voltage is compared with a very high frequency carrier triangle, the two signals are mixed in a comparator whose output is high when the sine wave is greater than the triangle and the comparator output is low when the sine wave or typically called the modulation signal is smaller than the triangle. This phenomenon is shown in Fig. 3.2. As is explained the output voltage from the inverter is not smooth but is a discrete waveform and so it is more likely than the output wave consists of harmonics, which are not usually desirable since they deteriorate the performance of the load, to which these voltages are applied.

Fig. 3.2: PWM Illustration by the Sine-Triangle Comparison :

(a) Sine-Triangle Comparison (b) Switching Pulses Recent advances in power electronics have made the multilevel concept practical. In fact, the concept is so advantageous that several major drives manufacturers have obtained recent patents on multilevel power converters and associated switching techniques. It is evident that the multilevel concept will be a prominent choice for power electronic systems in future years, especially for medium-voltage operation. Multi-level inverters are the modification of basic bridge inverters.

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They are normally connected in series to form stacks of level. The number of levels in an inverter bridge defines the number of direct current (DC) voltage steps that are required by the inverter bridge in order to achieve a certain voltage level at its output. Because power semiconductor switches have limited voltage capability, the total DC bus voltage of an inverter bridge is divided into a number of voltage steps, such that each voltage step can be handled by one power switch. For high power applications, voltages and currents must be pushed up. Hence, maximum ratings of power semiconductors become a real handicap. Paralleling devices, subsystems and systems leads to higher current levels. On the other hand, series connections are the solution for dealing with larger voltages. Nevertheless, given a chain of devices connected in series, achieving static and dynamic voltage sharing among switches become a problem. This will also affect the reliability of the system. An advantage of multilevel inverters compared with the classical two-level topology, is that the output voltage spectra are significantly improved due to having a greater availability of voltage levels, Hence, the output voltages can be filtered with smaller reactive components, and additionally, the switching frequencies of the devices can be reduced. These two benefits, together with the ability to deal with higher voltage levels, confer on multilevel inverters a very important role in the field of high power applications. The intriguing feature of the multilevel inverter structures is their ability to scale up the kilovolt-ampere (KVA) rating and also to improve the harmonic performance greatly without having to resort to PWM techniques. The key features of a multilevel structure follow: •The output voltage and power increase with number of levels. Adding a voltage level involves adding a main switching device to each phase. •The harmonic content decreases as the number of levels increases and filtering requirements are reduced. •With additional voltage levels, the voltage waveform has more free-switching angles, which can be reselected for harmonic elimination. •In the absence of any PWM techniques, the switching losses can be avoided. Increasing output voltage and power does not require an increase in rating of individual device. •Static and dynamic voltage sharing among the switching devices is built into the structure through either clamping diodes or capacitors.

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•The switching devices do not encounter any voltage-sharing problems. For this reason, multilevel inverters can easily be applied for high-power applications such as large motor drives and utility supplies. •The fundamental output voltage of the inverter is set by the dc bus voltage Vdc, which can be controlled through a variable dc link.

3.1 Classification of High power Converters

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TYPES OF MULTILEVEL INVERTERS Figure.3.3 shows the classification of multilevel inverter topologies which is existing in the area of power conversions

Figure.3.3 Classification of multilevel inverters

All three multilevel inverters can be used in reactive power compensation without having the voltage unbalance problem.

3.2 Diode-Clamped Multilevel Topology : The first practical multilevel topology is the neutral-point-clamped (NPC) PWM topology. The three-level version of this topology, has several distinct advantages over the two-level topology.

The advantages are: 

Voltages across the switches are only half of the dc-link voltage. The first group of voltage harmonics is centered on twice the switching frequency.

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This topology can be generalized, and the principles used in the basic three-level topology can be extended for use in topologies with any number of levels. However, practical experience with this topology revealed several technical difficulties that complicate its application for-high power converters. Th...