Dual Flying Capacitor Active.doc

Dual Flying Capacitor Active-Neutral-PointClamped Multilevel Converter Abstract—Hybrid multilevel converters combine features of conventional multilevel topologies to provide an acceptable tradeoff between the advantages and disadvantages of these converters. For many industrial applications, common dc link is a requirement that limits the choice of topologies to neutral point clamped (NPC) and flying capacitor multicell (FCM) hybrid types. This paper investigates the operation of a hybrid five-level topology and proposes a modulation method that takes the advantage of the combined features of NPC and FCM. The dual flying capacitor (FC) active-neutral-point-clamped (DFC-ANPC) converter provides certain advantages such as natural soft switching of line frequency switches, elimination of the transient voltage balancing snubbers, and a more even loss distribution. Simulation results and experimental verification of the five-level DFC-ANPC converter are presented to validate the performance of the converter as well as the applied modulation technique.

CHAPTER 1 I. INTRODUCTION MEDIUM-VOLTAGE multilevel converters have found wide use in applications such as motor drive, grid-tied inverter and rectifier, and medium voltage dc (MVDC) [1], [2]. The main motivation for the use of multilevel converters is to achieve higher voltage capability with commercially available lower voltage semiconductor devices. Typical fast switches such as IGBTs and IGCTs with voltage ratings up to 6.5 kV can reliably operate at about 4 kV [3], [4]. One way to handle higher voltage applications is direct series connection of switching devices; however, this solution has inherent transient voltage balancing and high dv/dt issues. Multilevel converters thus rose to the occasion. They also have the added advantages of better waveform quality, lower total harmonic distortion (THD), and lower electromagnetic interference (EMI) over their two-level counterparts [5]. For applications such as MVDC, a common dc link among the three phases is a requirement. For some other applications such as motor drive, a common dc link offers the option of eliminating or reducing the complexity of phase-shift transformer at the passive front end. For certain configurations such as active front end, typically a common dc link is required. In addition, less protection and clamp circuit is required for a single common dc link in contrast to several dc links [5]–[9]. Conventionally, two multilevel converter topologies, neutral point clamped (NPC) [10] and flying capacitor multicell (FCM) [11], [12], are known to provide a common dc link. NPC and its enhanced variety, active NPC (ANPC), are widely used in industry for three-level applications [7]. For higher levels, however, NPC encounters the critical dc-link voltage balancing problem, excessive number of clamping diodes, and unbalanced loss distribution among semiconductor devices. FCM stands out as an alternative for higher levels with balanced flying capacitor (FC) voltage and excellent loss distribution [9]. The disadvantage of FCM topology, however, is the excessive number of capacitors in higher levels. Capacitors are usually avoided due to high initial price, maintenance and replacement surcharges, and low reliability [13]. Hybrid multilevel converter topologies with a common dc link combine some features of NPC and FCM that opens the possibilities to take advantages of both topologies. Among hybrid topologies, the five-level FC active NPC (FC-ANPC) provides an acceptable compromise between cost and performance and consequently, found its way to industrial applications [14]. FC-ANPC provides a balanced dc-link voltage by using only one FC at the cost of four additional switches per phase compared to the conventional basic topologies [15]. It offers a good tradeoff compared to the disadvantages of FCM and NPC

converters. One drawback of the FC-ANPC topology is the four pairs of series-connected switches, which, although operating at the line frequency, require transient voltage balancing snubbers [16]. The uneven loss distribution among semiconductor devices is another major issue that limits the nominal power of this converter

CHAPTER-2 POWER ELECTRONICS The application of solid state electronics in which the electric power is controlled and converted is called power electronics. As it deals with designing, computation, control, and integration of electronic systems where energy is processed with fast dynamics which is non linear time varying, it is referred in electrical and electronic engineering as a research subject. Mercury arc valves are the first electronic devices with high power. The conversion is performed in modern systems with tryistors , diodes, transistors which are the semiconductor switching devices, pioneered in the 1950s by R.D.Middle Brook and others. In power electronics processing of substantial amounts of electrical energy is done in contrast to electronic systems concerned with transmission and processing of signals and data. The most typical power electronics device found in many consumer electronic devices,such as battery chargers,peronal computers,,television sets,etc is an AC/DC converter. Its power ranges from tens of watts to several hundred watts. The variable speed drive which is used to control an induction motor is the common application in industry.VSDs power ranges from few hundred watts to tens of mega watts.

2.1 History Power electronics had started with the development of the mercury arc rectifier which was invented by Peter Cooper Hewitt in 1902.It converts alternating current(AC) to direct current(DC).A research had started and continued on applying of thyratrons and grid controlled mercury arc valves for power transmission from 1920’s. A valve with grading electrodes was developed by Uno Lamm which made mercury arc valves useful for transmission of high voltage direct current. In 1947 Walter H.Brattain and john bardeen invented the bipolar point contact transistor at Bell labs under the direction of William Shockley. The bipolar junction transistor which was invented by shockley, improved the performance and stability of transistors,and reduced the costs of transistors. In the 1950s,semiconductor power diodes were invented

which replace the vaccum tubes. In 1956 there was great increase in the range of power electronics applications with the introduction of the silicon controlled rectifier(SCR) by general electric. The switching speed of bipolar junction transistors was allowed for high frequency DC/DC converters in1960s. power MOSFET became available from 1976 and the Insulated gate bipolar transistor(IGBT) was introduced in 1982

2.2 Types Of Systems The power conversion systems are classified based on the type of input and output power as follows: AC to DC (rectifier) DC to AC(inverter) DC to DC (DC to DC converter) AC to AC(AC to AC converter)

2.2.1 Devices The capabilities and economy of power electronics system are determined by the active devices that are available. Their characteristics and limitations are a key element in the design of power electronics systems. Formerly, the mercury arc valve, the high-vacuum and gas-filled diode thermionic rectifiers, and triggered devices such as the thyratron and ignitron were widely used in power electronics. As the ratings of solid-state devices improved in both voltage and current-handling capacity, vacuum devices have been nearly entirely replaced by solid-state devices. Power electronic devices may be used as switches, or as amplifiers. [3] An ideal switch is either open or closed and so dissipates no power; it withstands an applied voltage and passes no current, or passes any amount of current with no voltage drop. Semiconductor devices used as switches can approximate this ideal property and so most power electronic applications rely on switching devices on and off, which makes systems very efficient as very little power is wasted in the switch. By contrast, in the case of the amplifier, the current through the device varies continuously according to a controlled input. The voltage and current at the device terminals follow a load line, and the power dissipation inside the device is large compared with the power delivered to the load. Several attributes dictate how devices are used. Devices such as diodes conduct when a forward voltage is applied and have no external control of the start of conduction. Power devices such as silicon controlled rectifiers and thyristors (as well as the mercury valve and thyratron) allow control of the start of conduction, but rely on periodic reversal of current

flow to turn them off. Devices such as gate turn-off thyristors, BJT and MOSFET transistors provide full switching control and can be turned on or off without regard to the current flow through them. Transistor devices also allow proportional amplification, but this is rarely used for systems rated more than a few hundred watts. The control input characteristics of a device also greatly affect design; sometimes the control input is at a very high voltage with respect to ground and must be driven by an isolated source. As efficiency is at a premium in a power electronic converter, the losses that a power electronic device generates should be as low as possible. Devices vary in switching speed. Some diodes and thyristors are suited for relatively slow speed and are useful for power frequency switching and control; certain thyristors are useful at a few kilohertz. Devices such as MOSFETS and BJTs can switch at tens of kilohertz up to a few megahertz in power applications, but with decreasing power levels. Vacuum tube devices dominate high power (hundreds of kilowatts) at very high frequency (hundreds or thousands of megahertz) applications. Faster switching devices minimize energy lost in the transitions from on to off and back, but may create problems with radiated electromagnetic interference. Gate drive (or equivalent) circuits must be designed to supply sufficient drive current to achieve the full switching speed possible with a device. A device without sufficient drive to switch rapidly may be destroyed by excess heating. Practical devices have non-zero voltage drop and dissipate power when on, and take some time to pass through an active region until they reach the "on" or "off" state. These losses are a significant part of the total lost power in a converter. Power handling and dissipation of devices is also a critical factor in design. Power electronic devices may have to dissipate tens or hundreds of watts of waste heat, even switching as efficiently as possible between conducting and non-conducting states. In the switching mode, the power controlled is much larger than the power dissipated in the switch. The forward voltage drop in the conducting state translates into heat that must be dissipated. High power semiconductors require specialized heat sinks or active cooling systems to manage their junction temperature; exotic semiconductors such as silicon carbide have an advantage over straight silicon in this respect, and germanium, once the main-stay of solid-state electronics is now little used due to its unfavourable high temperature properties. Semiconductor devices exist with ratings up to a few kilovolts in a single device. Where very high voltage must be controlled, multiple devices must be used in series, with networks to equalize voltage across all devices. Again, switching speed is a critical factor since the slowest-switching device will have to withstand a disproportionate share of the

overall voltage. Mercury valves were once available with ratings to 100 kV in a single unit, simplifying their application in HVDC systems.

The current rating of a semiconductor

device is limited by the heat generated within the dies and the heat developed in the resistance of the interconnecting leads. Semiconductor devices must be designed so that current is evenly distributed within the device across its internal junctions (or channels); once a "hot spot" develops, breakdown effects can rapidly destroy the device. Certain SCRs are available with current ratings to 3000 amperes in a single unit. Solid-state devices Device

Description Uni-polar, uncontrolled, switching device used in applications such as rectification and circuit directional current control. Reverse voltage blocking device, commonly modeled as a switch in series

Diode

with a voltage source, usually 0.7 VDC. The model can be enhanced to include a junction resistance, in order to accurately predict the diode voltage drop across the diode with respect to current flow.

Ratings Up to 3000 amperes and 5000 volts in a single silicon device. High

voltage

requires series

multiple silicon

devices.

This semi-controlled device turns on when a gate pulse is present and the anode is positive compared to the cathode. When a gate pulse is present, the Silicon-controlled

device operates like a standard diode. When the

rectifier(SCR)

anode is negative compared to the cathode, the device turns off and blocks positive or negative voltages present. The gate voltage does not allow the device to turn off.[4] The thyristor is a family of three-terminal devices that include SCRs, GTOs, and MCT. For most of the devices, a gate pulse turns the device on. The device

Thyristor

turns off when the anode voltage falls below a value (relative to the cathode) determined by the device characteristics. When off, it is considered a reverse voltage blocking device.[4]

Gate

turn-off The gate turn-off thyristor, unlike an SCR, can be

thyristor(GTO)

turned on and off with a gate pulse. One issue with

Up to 3000 amperes, 5000

volts

in

a

single silicon device.

the device is that turn off gate voltages are usually larger and require more current than turn on levels. This turn off voltage is a negative voltage from gate to source, usually it only needs to be present for a short time, but the magnitude s on the order of 1/3 of the anode current. A snubber circuit is required in order to provide a usable switching curve for this device. Without the snubber circuit, the GTO cannot be used for turning inductive loads off. These devices,

because

of

developments

in

IGCT

technology are not very popular in the power electronics realm. They are considered controlled, uni-polar and bi-polar voltage blocking.[5] The triac is a device that is essentially an integrated pair of phase-controlled thyristors connected in inverse-parallel on the same chip.[6] Like an SCR, when a voltage pulse is present on the gate terminal, the device turns on. The main difference between an Triac

SCR and a Triac is that both the positive and negative cycle can be turned on independently of each other, using a positive or negative gate pulse. Similar to an SCR, once the device is turned on, the device cannot be turned off. This device is considered bi-polar and reverse voltage blocking.

Bipolar

junction The BJT cannot be used at high power; they are

transistor(BJT)

slower and have more resistive losses when compared to MOSFET type devices. To carry high current, BJTs must have relatively large base currents, thus these devices have high power losses when compared to MOSFET devices. BJTs along with MOSFETs, are also considered unipolar and do not block reverse voltage very well, unless installed in pairs with protection diodes. Generally, BJTs are

not utilized in power electronics switching circuits because of the I2R losses associated with on resistance and base current requirements.[4] BJTs have lower current gains in high power packages, thus requiring them to be set up in Darlington configurations in order to handle the currents required by power electronic circuits. Because of these multiple transistor configurations, switching times are in the hundreds of nanoseconds to microseconds. Devices have voltage ratings which max out around 1500 V and fairly high current ratings. They can also be paralleled in order to increase power handling, but must be limited to around 5 devices for current sharing.[5] Power MOSFET

The main benefit of the power MOSFET is that the base current for BJT is large compared to almost zero for MOSFET gate current. Since the MOSFET is a depletion channel device, voltage, not current, is necessary to create a conduction path from drain to source. The gate does not contribute to either drain or source current. Turn on gate current is essentially zero with the only power dissipated at the gate coming during switching. Losses in MOSFETs are largely attributed to on-resistance. The calculations show a direct correlation to drain source onresistance and the device blocking voltage rating, BVdss. Switching times range from tens of nanoseconds to a few hundred microseconds, depending on the device. MOSFET drain source resistances increase as more current flows through the device. As frequencies increase the losses increase as well, making BJTs more attractive. Power MOSFETs can

be paralleled in order to increase switching current and therefore overall switching power. Nominal voltages for MOSFET switching devices range from a few volts to a little over 1000 V, with currents up to about 100 A or so. Newer devices may have higher operational characteristics. MOSFET devices are not bi-directional, nor are they reverse voltage blocking.[5] || These devices have the best characteristics of MOSFETs and BJTs. Like MOSFET devices, the insulated gate bipolar transistor has a high gate impedance, thus low gate current requirements. Like BJTs, this device has low on state voltage drop, thus Insulated-gate

low power loss across the switch in operating mode.

bipolar

Similar to the GTO, the IGBT can be used to block

transistor(IGBT)

both positive and negative voltages. Operating currents are fairly high, in excess of 1500 A and switching voltage up to 3000 V.[5] The IGBT has reduced input capacitance compared to MOSFET devices which improves the Miller feedback effect during high dv/dt turn on and turn off.[6]

MOS-controlled

The MOS-controlled thyristor is thyristor like and

thyristor(MCT)

can be triggered on or off by a pulse to the MOSFET gate.[6] Since the input is MOS technology, there is very little current flow, allowing for very low power control signals. The device is constructed with two MOSFET inputs and a pair of BJT output stages. Input MOSFETs are configured to allow turn on control during positive and negative half cycles. The output BJTs are configured to allow for bidirectional control and low voltage reverse blocking. Some benefits to the MCT are fast switching frequencies, fairly high voltage and medium current ratings

(around 100 A or so). Similar to a GTO, but without the high current requirements to turn on or off the load. The IGCT can be used for quick switching with little gate current. The devices high input impedance largely because of the MOSFET gate drivers. They have low resistance outputs that don't waste power and very fast transient times that rival that of BJTs. ABB has published data sheets for these devices and provided descriptions of the inner workings. The Integrated

gate-

commutated thyristor(IGCT)

device consists of a gate, with an optically isolated input, low on resistance BJT output transistors which lead to a low voltage drop and low power loss across the device at fairly high switching voltage and current levels. An example of this new device from ABB shows how this device improves on GTO technology for switching high voltage and high current in power electronics applications. According to ABB, the IGCT devices are capable of switching in excess of 5000 VAC and 5000 A at very high frequencies, something not possible to do efficiently with GTO devices.[7]

2.2.2 DC to AC converters DC to AC converters produce an AC output waveform from a DC source. Applications include adjustable speed drives (ASD), uninterruptable power supplies (UPS), active filters, Flexible AC transmission systems (FACTS), voltage compensators, and photovoltaic generators. Topologies for these converters can be separated into two distinct categories: voltage source inverters and current source inverters. Voltage source inverters (VSIs) are named so because the independently controlled output is a voltage waveform. Similarly, current source inverters (CSIs) are distinct in that the controlled AC output is a current waveform.

Being static power converters, the DC to AC power conversion is the result of power switching devices, which are commonly fully controllable semiconductor power switches. The output waveforms are therefore made up of discrete values, producing fast transitions rather than smooth ones. The ability to produce near sinusoidal waveforms around the fundamental frequency is dictated by the modulation technique controlling when, and for how long, the power valves are on and off. Common modulation techniques include the carrier-based technique, or pulse width modulation, space-vector technique, and the selectiveharmonic technique. Voltage source inverters have practical uses in both single-phase and three-phase applications. Single-phase VSIs utilize half-bridge and full-bridge configurations, and are widely used for power supplies, single-phase UPSs, and elaborate high-power topologies when used in multicell configurations. Three-phase VSIs are used in applications that require sinusoidal voltage waveforms, such as ASDs, UPSs, and some types of FACTS devices such as the STATCOM. They are also used in applications where arbitrary voltages are required as in the case of active filters and voltage compensators. Current source inverters are used to produce an AC output current from a DC current supply. This type of inverter is practical for three-phase applications in which high-quality voltage waveforms are required. A relatively new class of inverters, called multilevel inverters, has gained widespread interest. Normal operation of CSIs and VSIs can be classified as two-level inverters, due to the fact that power switches connect to either the positive or to the negative DC bus. If more than two voltage levels were available to the inverter output terminals, the AC output could better approximate a sine wave. It is for this reason that multilevel inverters, although more complex and costly, offer higher performance. Each inverter type differs in the DC links used, and in whether or not they require freewheeling diodes. Either can be made to operate in square-wave or pulse-width modulation (PWM) mode, depending on its intended usage. Square-wave mode offers simplicity, while PWM can be implemented several different ways and produces higher quality waveforms.[8] Voltage Source Inverters (VSI) feed the output inverter section from an approximately constant-voltage source. The desired quality of the current output waveform determines which modulation technique needs to be selected for a given application. The output of a VSI is composed of discrete values. In order to obtain a smooth current waveform, the loads need to be inductive

at the select harmonic frequencies. Without some sort of inductive filtering between the source and load, a capacitive load will cause the load to receive a choppy current waveform, with large and frequent current spikes.[8] There are three main types of VSIs: 1. Single-phase half-bridge inverter 2. Single-phase full-bridge inverter 3. Three-phase voltage source inverter 2.3 Single-phase half-bridge inverter

Fig 2.1 The AC input for an ASD.

Fig 2.2 Single-Phase Half-Bridge Voltage Source Inverter The single-phase voltage source half-bridge inverters, are meant for lower voltage applications and are commonly used in power supplies.[8] Figure 2 shows the circuit schematic of this inverter. Low-order current harmonics get injected back to the source voltage by the operation of the inverter. This means that two large capacitors are needed for filtering purposes in this design. As Figure 2.2 illustrates, only one switch can be on at time in each leg of the inverter. If both switches in a leg were on at the same time, the DC source will be shorted out. Inverters can use several modulation techniques to control their switching schemes. The carrier-based PWM technique compares the AC output waveform, vc, to a carrier voltage signal, vΔ. When vc is greater than vΔ, S+ is on, and when vc is less than vΔ, S- is on. When the AC output is at frequency fc with its amplitude at v c, and the triangular carrier signal is at frequency fΔ with its amplitude at vΔ, the PWM becomes a special sinusoidal case of the

carrier based PWM.[8] This case is dubbed sinusoidal pulse-width modulation (SPWM).For this, the modulation index, or amplitude-modulation ratio, is defined as ma = vc / v∆ . The normalized carrier frequency, or frequency-modulation ratio, is calculated using the equation mf = f∆ / fc . If the over-modulation region, ma, exceeds one, a higher fundamental AC output voltage will be observed, but at the cost of saturation. For SPWM, the harmonics of the output waveform are at well-defined frequencies and amplitudes. This simplifies the design of the filtering components needed for the low-order current harmonic injection from the operation of the inverter. The maximum output amplitude in this mode of operation is half of the source voltage. If the maximum output amplitude, m a, exceeds 3.24, the output waveform of the inverter becomes a square wave.[8] As was true for PWM, both switches in a leg for square wave modulation cannot be turned on at the same time, as this would cause a short across the voltage source. The switching scheme requires that both S+ and S- be on for a half cycle of the AC output period. [8] The fundamental AC output amplitude is equal to vo1 = vaN. Its harmonics have an amplitude ofvoh =vo1 / h.

Therefore, the AC output voltage is not controlled by the inverter, but rather by the magnitude of the DC input voltage of the inverter.[8] Using selective harmonic elimination (SHE) as a modulation technique allows the switching of the inverter to selectively eliminate intrinsic harmonics. The fundamental component of the AC output voltage can also be adjusted within a desirable range. Since the AC output voltage obtained from this modulation technique has odd half and odd quarter wave symmetry, even harmonics do not exist. [8] Any undesirable odd (N-1) intrinsic harmonics from the output waveform can be eliminated.

2.4 Single-phase full-bridge inverter

Fig 2.3: Single-Phase Voltage Source Full-Bridge Inverter

FIGURE 4: Carrier and Modulating Signals for the Bipolar Pulsewidth Modulation Technique The full-bridge inverter is similar to the half bridge-inverter, but it has an additional leg to connect the neutral point to the load. [8] Figure 3 shows the circuit schematic of the single-phase voltage source full-bridge inverter. To avoid shorting out the voltage source, S1+ and S1- cannot be on at the same time, and S2+ and S2- also cannot be on at the same time. Any modulating technique used for the full-bridge configuration should have either the top or the bottom switch of each leg on at any given time. Due to the extra leg, the maximum amplitude of the output waveform is Vi, and is twice as large as the maximum achievable output amplitude for the half-bridge configuration. States 1 and 2 from Table 2 are used to generate the AC output voltage with bipolar SPWM. The AC output voltage can take on only two values, either Vi or –Vi. To generate these same states using a half-bridge configuration, a carrier based technique can be used. S+ being on for the half-bridge corresponds to S1+ and S2- being on for the full-bridge. Similarly, S- being on for the half-bridge corresponds to S1- and S2+ being on for the full bridge. The output voltage for this modulation technique is more or less sinusoidal, with a fundamental component that has an amplitude in the linear region of ma less than or equal to one vo1 =vab1= vi • ma. Unlike the bipolar PWM technique, the unipolar approach uses states 1, 2, 3 and 4 from Table 2 to generate its AC output voltage. Therefore, the AC output voltage can take on the values Vi, 0 or –V [1]i. To generate these states, two sinusoidal modulating signals, Vc and –Vc, are needed, as seen in Figure 4. 2.5 AC/AC converters Converting AC power to AC power allows control of the voltage, frequency, and phase of the waveform applied to a load from a supplied AC system . [10] The two main categories that can be used to separate the types of converters are whether the frequency of the waveform is changed.[11] AC/AC converter that don't allow the user to modify the frequencies are known as AC Voltage Controllers, or AC Regulators. AC converters that allow the user to change the frequency are simply referred to as frequency converters for AC

to AC conversion. Under frequency converters there are three different types of converters that are typically used: cycloconverter, matrix converter, DC link converter (aka AC/DC/AC converter). 2.6 AC voltage controller The purpose of an AC Voltage Controller, or AC Regulator, is to vary the RMS voltage across the load while at a constant frequency.[10] Three control methods that are generally accepted are ON/OFF Control, Phase-Angle Control, and Pulse Width Modulation AC Chopper Control (PWM AC Chopper Control).[12] All three of these methods can be implemented not only in single-phase circuits, but three-phase circuits as well. 

ON/OFF Control: Typically used for heating loads or speed control of motors, this control method involves turning the switch on for n integral cycles and turning the switch off for m integral cycles. Because turning the switches on and off causes undesirable harmonics to be created, the switches are turned on and off during zero-voltage and zerocurrent conditions (zero-crossing), effectively reducing the distortion.[12]



Phase-Angle Control: Various circuits exist to implement a phase-angle control on different waveforms, such as half-wave or full-wave voltage control. The power electronic components that are typically used are diodes, SCRs, and Triacs. With the use of these components, the user can delay the firing angle in a wave which will only cause part of the wave to be outputted.[10]



PWM AC Chopper Control: The other two control methods often have poor harmonics, output current quality, and input power factor. In order to improve these values PWM can be used instead of the other methods. What PWM AC Chopper does is have switches that turn on and off several times within alternate half-cycles of input voltage.[12]

2.7 Matrix converters and cycloconverters Cycloconverters are widely used in industry for ac to ac conversion, because they are able to be used in high-power applications. They are commutated direct frequency converters that are synchronized by a supply line. The cycloconverters output voltage waveforms have complex harmonics with the higher order harmonics being filtered by the machine inductance. Causing the machine current to have fewer harmonics, while the remaining harmonics causes losses and torque pulsations. Note that in a cycloconverter, unlike other converters, there are no inductors or capacitors, i.e. no storage devices. For this reason, the instantaneous input power and the output power are equal.[13]



Single-Phase

to

Single-Phase Cycloconverters:

Single-Phase

to

Single-Phase

Cycloconverters started drawing more interest recently because of the decrease in both size and price of the power electronics switches. The single-phase high frequency ac voltage can be either sinusoidal or trapezoidal. These might be zero voltage intervals for control purpose or zero voltage commutation. 

Three-Phase to Single-Phase Cycloconverters: There are two kinds of three-phase to single-phase cycloconverters: 3φ to 1φ half wave cycloconverters and 3φ to 1φ bridge cycloconverters. Both positive and negative converters can generate voltage at either polarity, resulting in the positive converter only supplying positive current, and the negative converter only supplying negative current. With recent device advances, newer forms of cycloconverters are being developed,

such as matrix converters. The first change that is first noticed is that matrix converters utilize bi-directional, bipolar switches. A single phase to a single phase matrix converter consists of a matrix of 9 switches connecting the three input phases to the tree output phase. Any input phase and output phase can be connected together at any time without connecting any two switches from the same phase at the same time; otherwise this will cause a short circuit of the input phases. Matrix converters are lighter, more compact and versatile than other converter solutions. As a result, they are able to achieve higher levels of integration, higher temperature operation, broad output frequency and natural bi-directional power flow suitable to regenerate energy back to the utility. The matrix converters are subdivided into two types: direct and indirect converters. A direct matrix converter with three-phase input and three-phase output, the switches in a matrix converter must be bi-directional, that is, they must be able to block voltages of either polarity and to conduct current in either direction. This switching strategy permits the highest possible output voltage and reduces the reactive line-side current. Therefore the power flow through the converter is reversible. Because of its commutation problem and complex control keep it from being broadly utilized in industry. Unlike the direct matrix converters, the indirect matrix converters has the same functionality, but uses separate input and output sections that are connected through a dc link without storage elements. The design includes a four-quadrant current source rectifier and a voltage source inverter. The input section consists of bi-directional bipolar switches. The commutation strategy can be applied by changing the switching state of the input section while the output section is in a freewheeling mode. This commutation algorithm is

significantly less complexity and higher reliability as compared to a conventional direct matrix converter.

4. INVERTER An inverter is an electrical device that converts direct current (DC) to alternating current (AC); the converted AC can be at any required voltage and frequency with the use of appropriate transformers, switching, and control circuits. Static inverters have no moving parts and are used in a wide range of applications, from small switching power supplies in computers, to large electric utility high-voltage direct current applications that transport bulk power. Inverters are commonly used to supply AC power from DC sources such as solar panels or batteries. The electrical inverter is a high-power electronic oscillator. It is so named because early mechanical AC to DC converters were made to work in reverse, and thus were "inverted", to convert DC to AC.

3.1Cascaded H-Bridges inverter A single-phase structure of an m-level cascaded inverter is illustrated in Figure 4.1. Each separate dc source (SDCS) is connected to a single-phase full-bridge, or H-bridge,

inverter. Each inverter level can generate three different voltage outputs, +Vdc, 0, and –Vdc by connecting the dc source to the ac output by different combinations of the four switches, S 1, S2, S3, and S4. To obtain +Vdc, switches S1 and S4 are turned on, whereas –Vdc can be obtained by turning on switches S2 and S3. By turning on S1 and S2 or S3 and S4, the output voltage is 0. The ac outputs of each of the different full-bridge inverter levels are connected in series such that the synthesized voltage waveform is the sum of the inverter outputs. The number of output phase voltage levels m in a cascade inverter is defined by m = 2s+1, where s is the number of separate dc sources. An example phase voltage waveform for an 11-level cascaded H-bridge inverter with 5 SDCSs and 5 full bridges is shown in Figure 4.2. The phase voltage

+

…(4.1)

For a stepped waveform such as the one depicted in Figure 4.2 with s steps, the Fourier Transform for this waveform follows

…(4.2)

Fig.3.1Single-phase structure of a multilevel cascaded H-bridges inverter

Fig.3.2 Output phase voltage waveform of an 11-level cascade inverter with 5 separate dc sources. The magnitudes of the Fourier coefficients when normalized with respect to V dc are as follows:

…(4.3)

The conducting angles, θ1, θ2, ..., θs, can be chosen such that the voltage total harmonic distortion is a minimum. Generally, these angles are chosen so that predominant lower frequency harmonics, 5th, 7th, 11th, and 13th, harmonics are eliminated. More detail on harmonic elimination techniques will be presented in the next section. Multilevel cascaded inverters have been proposed for such applications as static var generation, an interface with renewable energy sources, and for battery-based applications. Three-phase cascaded inverters can be connected in wye, as shown in Figure 4.3, or in delta. Peng has demonstrated a prototype multilevel cascaded static var generator connected in parallel with the electrical system that could supply or draw reactive current from an electrical system. The inverter could be controlled to either regulate the power factor of the current drawn from the source or the bus voltage of the electrical system where the inverter was connected.

Peng and Joos have also shown that a cascade inverter can be directly connected in series with the electrical system for static var compensation. Cascaded inverters are ideal for connecting renewable energy sources with an ac grid, because of the need for separate dc sources, which is the case in applications such as photovoltaic’s or fuel cells. Cascaded inverters have also been proposed for use as the main traction drive in electric vehicles, where several batteries or ultra capacitors are well suited to serve as SDCSs. The cascaded inverter could also serve as a rectifier/charger for the batteries of an electric vehicle while the vehicle was connected to an ac supply as shown in Figure 4.3. Additionally, the cascade inverter can act as a rectifier in a vehicle that uses regenerative braking.

Fig.3.3 Three-phase wye-connection structure for electric vehicle motor drive and battery charging. The main advantages and disadvantages of multilevel cascaded H-bridge converters are as follows

3.1.1 Advantages 

The number of possible output voltage levels is more than twice the number of dc



sources (m = 2s + 1). The series of H-bridges makes for modularized layout and packaging. This will enable the manufacturing process to be done more quickly and cheaply.

3.1.2 Disadvantages 

Separate dc sources are required for each of the H-bridges. This will limit its application to products that already have multiple SDCSs readily available.

3.2 Diode-Clamped multilevel inverter The neutral point converter proposed by Nabae, Takahashi, and Akagi in 1981 was essentially a three-level diode-clamped inverter. In the 1990s several researchers published articles that have reported experimental results for four-, five-, and six-level diode-clamped converters for such uses as static var compensation, variable speed motor drives, and highvoltage system interconnections. A three-phase six-level diode-clamped inverter is shown in Figure 3.5. Each of the three phases of the inverter shares a common dc bus, which has been subdivided by five capacitors into six levels. The voltage across each capacitor is V dc, and the voltage stress across each switching device is limited to V dc through the clamping diodes. Table 3.1 lists the output voltage levels possible for one phase of the inverter with the negative dc rail voltage V0 as a reference. State condition 1 means the switch is on, and 0 means the switch is off. Each phase has five complementary switch pairs such that turning on one of the switches of the pair requires that the other complementary switch be turned off. The complementary switch pairs for phase leg a are (Sa1, Sa’1), (Sa2, Sa’2), (Sa3, Sa’3), (Sa4, Sa’4), and (Sa5, Sa’5). Table 4.1 also shows that in a diode-clamped inverter, the switches that are on for a particular phase leg are always adjacent and in series. For a six-level inverter, a set of five switches is on at any given time.

Fig.3.4 Three-phase six-level structure of a diode-clamped inverter.

Diode-clamped six-level inverter voltage levels and corresponding switch states are as

Table 3.1 Output voltage levels of one phase inverter

3.2.1 Advantages 

All of the phases share a common dc bus, which minimizes the capacitance requirements of the converter. For this reason, a back-to-back topology is not only possible but also practical for uses such as a high-voltage back-to-back inter-

 

connection or an adjustable speed drive. The capacitors can be pre-charged as a group. Efficiency is high for fundamental frequency switching.

3.2.2 Disadvantages 

Real power flow is difficult for a single inverter because the intermediate dc levels



will tend to overcharge or discharge without precise monitoring and control. The number of clamping diodes required is quadratically related to the number of levels, which can be cumbersome for units with a high number of levels.

3.3 Flying Capacitor multilevel inverter Meynard and Foch introduced a flying-capacitor-based inverter in 1992. The structure of this inverter is similar to that of the diode-clamped inverter except that instead of using clamping diodes, the inverter uses capacitors in their place. The circuit topology of the flying capacitor multilevel inverter is shown in Figure 4.7. This topology has a ladder structure of dc side capacitors, where the voltage on each capacitor differs from that of the next capacitor.

The voltage increment between two adjacent capacitor legs gives the size of the voltage steps in the output waveform.

Fig.3.5 Three-phase six-level structure of a flying capacitor inverter. One advantage of the flying-capacitor-based inverter is that it has redundancies for inner voltage levels; in other words, two or more valid switch combinations can synthesize an output voltage. Table 3.2 shows a list of all the combinations of phase voltage levels that are possible for the six-level circuit shown in Figure 3.7. Unlike the diode-clamped inverter, the flying-capacitor inverter does not require all of the switches that are on (conducting) be in a consecutive series. Moreover, the flying-capacitor inverter has phase redundancies, whereas the diode-clamped inverter has only line-line redundancies. These redundancies allow a choice of charging/discharging specific capacitors and can be incorporated in the control system for balancing the voltages across the various levels. In addition to the (m-1) dc link capacitors, the m-level flying-capacitor multilevel inverter will require (m-1) × (m-2)/2 auxiliary capacitors per phase if the voltage rating of the capacitors is identical to that of the main switches. One application proposed in the literature for the multilevel flying capacitor is static var generation. The main advantages and disadvantages of multilevel flying capacitor converters are as follows.

3.3.1 Advantages 

Phase redundancies are available for balancing the voltage levels of the capacitors.



Real and reactive power flow can be controlled.



The large number of capacitors enables the inverter to ride through short duration outages and deep voltage sags.

3.3.2 Disadvantages 

Control is complicated to track the voltage levels for all of the capacitors. Also, precharging all of the capacitors to the same voltage level and startup are complex.



Switching utilization and efficiency are poor for real power transmission.



The large numbers of capacitors are both more expensive and bulky than clamping diodes in multilevel diode-clamped converters. Packaging is also more difficult in inverters with a high number of levels.

3.4 Other multilevel inverter structures Besides the three basic multilevel inverter topologies previously discussed, other multilevel converter topologies have been proposed; however, most of these are “hybrid” circuits that are combinations of two of the basic multilevel topologies or slight variations to them. Additionally, the combination of multilevel power converters can be designed to match with a specific application based on the basic topologies. In the interest of completeness, some of these will be identified and briefly described.

Table 3.2 Flying-capacitor six-level inverter redundant voltage levels and Corresponding switch states

3.4.1 Generalized multilevel topology Existing multilevel converters such as diode-clamped and capacitor-clamped multilevel converters can be derived from the generalized converter topology called P2 topology proposed by Peng as illustrated in Figure 4.8. The generalized multilevel converter topology can balance each voltage level by itself regardless of load characteristics, active or reactive power conversion and without any assistance from other circuits at any number of levels automatically. Thus, the topology provides a complete multilevel topology that embraces the existing multilevel converters in principle.

Figure 3.8 shows the P2 multilevel converter structure per phase leg. Each switching device, diode, or capacitor’s voltage is 1V dc, for instance, 1/ (m-1) of the DC-link voltage. Any converter with any number of levels, including the conventional bi-level converter can be obtained using this generalized topology.

Fig3.6 Generalized P2 multilevel converter topology for one phase leg.

3.4.2 Mixed-Level hybrid multilevel converter To reduce the number of separate DC sources for high-voltage, high-power applications with multilevel converters, diode-clamped or capacitor-clamped converters could be used to replace the full-bridge cell in a cascaded converter. An example is shown in Figure 3.9. The nine-level cascade converter incorporates a three-level diode-clamped converter as the cell. The original cascaded H-bridge multilevel converter requires four separate DC sources for one phase leg and twelve for a three-phase converter. If a five-level converter replaces the full-bridge cell, the voltage level is effectively doubled for each cell. Thus, to achieve the same nine voltage levels for each phase, only two separate DC sources are needed for one phase leg and six for a three-phase converter. The configuration has mixed-level hybrid multilevel units because it embeds multilevel cells as the building block of the cascade

converter. The advantage of the topology is it needs less separate DC sources. The disadvantage for the topology is its control will be complicated due to its hybrid structure.

Fig.3.7 Zero-voltage switching capacitor-clamped inverter circuit.

3.4.3 Soft-Switched multilevel converter Some soft-switching methods can be implemented for different multilevel converters to reduce the switching loss and to increase efficiency. For the cascaded converter, because each converter cell is a bi-level circuit, the implementation of soft switching is not at all different from that of conventional bi-level converters. For capacitor-clamped or diodeclamped converters, soft-switching circuits have been proposed with different circuit combinations. One of soft-switching circuits is a zero-voltage-switching type which includes auxiliary resonant commutated pole (ARCP), coupled inductor with zero-voltage transition (ZVT), and their combinations as shown in Figure 4.10.

3.4.4 Back-to-Back diode-clamped converter Two multilevel converters can be connected in a back-to-back arrangement and then the combination can be connected to the electrical system in a series-parallel arrangement as shown in Figure 4.11. Both the current demanded from the utility and the voltage delivered to the load can be controlled at the same time. This series-parallel active power filter has been referred to as a universal power conditioner when used on electrical distribution systems and as a universal power flow controller when applied at the transmission level. Previously, Lai and Peng proposed the back-to-back diode-clamped topology shown in Figure 4.12 for use as

a high-voltage dc inter connection between two asynchronous ac systems or as a rectifier/inverter for an adjustable speed drive for high-voltage motors. The diode-clamped inverter has been chosen over the other two basic multilevel circuit topologies for use in a universal power conditioner for the following reasons: 

All six phases (three on each inverter) can share a common dc link. Conversely, the cascade inverter requires that each dc level be separate, and this is not



conducive to a back-to-back arrangement. The multilevel flying-capacitor converter also shares a common dc link; however, each phase leg requires several additional auxiliary capacitors. These extra capacitors would add substantially to the cost and the size of the conditioner.

Because a diode-clamped converter acting as a universal power conditioner will be expected to compensate for harmonics and/or operate in low amplitude modulation index regions, a more sophisticated, higher-frequency switch control than the fundamental frequency switching method will be needed. For this reason, multilevel space vector and carrier-based PWM approaches are compared in the next section, as well as novel carrierbased PWM methodologies.

CHAPTER 4 DFC-ANPC TOPOLOGY AND OPERATION

Fig. 4.1. Five-level DFC-ANPC topology The five-level DFC-ANPC topology, as shown in Fig. 4.1, can be viewed as two three-level FCM converter units connected to the output through line frequency switches S5 and S6 . The top FCM unit uses high-frequency switches S1 ,S 1 ,S2 , and S 2 along with capacitor C1 to generate the positive half cycle of the pulsewidth-modulated (PWM) waveform. The generated voltage is transferred to the output through S5 during the positive half cycle. During the negative half cycle, the bottom FCM unit, consisting of S3 ,S 3 ,S4 ,S 4 , and C2 , generates the PWM waveform, which is transferred to the output through S6 . The voltage of the FCs, C1 and C2 , can be individually balanced through the existing redundant states of each FCM unit. Table I lists the switching states for the DFC-ANPC converter and the effect of each state on the FCs voltages. The redundant states for level +E, i.e., +EP and +E0, are used to balance C1 ’s voltage. Similarly, the redundant states for level −E, i.e., −E0 and −EN, are used to balance C2 ’s voltage. During the normal operation, the operating voltage of both FCs in the DFC-ANPC five-level converter is a quarter of the dc-link voltage, which means E. This results in clamping the voltage stress of high-frequency switches to E. For line frequency switches, however, voltage stress is half of the dc-link TAB voltage, i.e., 2E, which may be realized by two switches in series, as shown in Fig. 1. An important feature of this topology is the “soft cycle commutation” between the positive and negative half cycles. States 0P, 0N, and 00, as listed in Table I, can generate level 0 either through the top part switches S 1 ,S 2 ,S5 , the bottom part switches S3

,S4 ,S6 , or both. The inbound and outbound current paths in each case are shown in Fig. 2. At the transition from 0P to 00, S6 must turn on while the voltage across it is near zero. When switching back from 00 to 0P, S6 must turn off while the voltage across it is near zero. In a similar fashion, the voltage across S5 is near zero when switching between 00 and 0N. Therefore, if 00 is used as an intermediate state between 0P and 0N, line frequency switches S5 and S6 will hold zero-voltage switching operation at all times. So, when the phase voltage half cycle changes, the operating FCM unit can be softly detached from output, and the operation can be softly handed over to the other FCM unit. An advantage of this phenomenon is the elimination of switching loss on S5 and S6 . More importantly, no transient voltage balancing snubber is required when realizing S5 and S6 by seriesconnected switches. Note that the blocking mode voltage balancing resistors may still be required due to the switching devices’ cutoff current tolerance [16].

4.2I. MODULATION TECHNIQUES Various modulation techniques may be adapted for the DFCANPC topology. Carrier-based modulation with sinusoidal or modified reference as well as noncarrier-based techniques such as space vector modulation (SVM) and selective harmonic elimination (SHE) may be used to generate the gate signals [5], [19]. The choice of a modulation technique is mostly a tradeoff among the requirements of the application, complexity of the software, and relative cost of the control hardware. For the DFC-ANPC topology, the main requirement is to ensure the FC voltages are balanced and the neutral point voltage is maintained at half of the dc-link voltage

Fig 4.2 Soft cycle commutation concept. Inbound and outbound current paths for states (a) 0P, (b) 00, and (c) 0N.

A. Carrier-Based Modulation Carrier set’s arrangement and reference waveform’s shape are the main sources of varieties in carrier-based modulation techniques for multilevel converters. As for carrier set’s arrangement, level-shifted carriers (LSCs) and phase-shifted carriers (PSCs) are the two main categories that are suitable for diode-clamped and multicell structures. Two members in the LSC family, alternative phase opposition disposition (APOD) and phase disposition (PD), are known to generate the best results for singe-phase and three-phase converters, respectively [19]. PSC in its original form has been shown to generate a multilevel PWM waveform that matches with APOD [20]. Also a modified version of PSC with dynamic phase shift has been shown to match with PD [21]. The reference for single-phase applications is usually a simple sinusoidal waveform. For three-phase applications, a variety of reference waveforms are available due to the possibility of common-mode injection in three-phase structure. This flexibility has been used to serve different purposes such as increased dc-link utilization, lower THD, lower loss, and neutral point voltage control [22], [23]. For the DFC-ANPC converter, a hybrid modulation technique is required due to the hybrid structure of the topology. Fig. 3 illustrates the carrier-based modulation technique using PSC with sinusoidal reference for single-phase case. It is intuitive to separate the operation to positive and negative half cycles, since each one is generated with an independent three-level FCM unit. The gate signals for each FCM unit is then generated using PSC to provide natural voltage

balancing for the FCs [24]. Switches S5 and S6 must be on during the positive and negative half cycles, respectively, to connect the associated FCM unit to the output. Note that, soft cycle commutation, S5 and S6 can be achieved by a short duration of overlap at transition from positive half cycle to negative half cycle and vice versa. The PWM waveform generated at the output matches the APOD scheme. For three-phase cases, a similar approach may be adopted except that, to generate a PD scheme equivalent, the positive half cycle carriers should hold π/2 phase shift with respect to the negative half-cycle carriers. Also, the carriers incorporate a dynamic phase shift, which always adds up by π/2 at the carrier band transitions for sampled reference waveforms [21]. For the reference waveform, centered space vector PWM (CSVPWM) sampled at half PD carrier period can provide similar performance as SVM [22], [25]. Fig. 4 illustrates the modulation technique using sampled CSV-PWM along with modified PSC for the DFC-ANPC converter. It is important to choose a reference waveform with balanced common-mode injection to maintain the neutral point’s voltage balance. Also, higher frequency and lower amplitude of the injected common mode can decrease the neutral point’s voltage ripple. B. Non-carrier-based Modulation For non-carrier-based modulation techniques such as SVM and SHE, the five-level PWM waveform may be generated first and then decomposed to the required switching signals. In this procedure, as shown in Fig. 5, the five-level PWM waveform is first separated to positive and negative half-cycle three-level PWMs. Each half cycle is then decomposed to switching signals either through a state machine decoder or an active balancing algorithm. The state machine decoder ensures that transitions are uniformly distributed between the switches and, therefore, provides natural voltage balancing [26]. The active balancing algorithm compares the FCs voltages to their target value at each transition. Based on this comparison, it decides whether the capacitor needs to be charged or discharged. Then considering the direction of output current at the moment, it determines which redundant state to switch to, to provide the charging or discharging and, therefore, to balance the FCs voltages [27]. To provide soft cycle commutation, cycle separation unit needs to know the half cycle change before it actually occurs. This cannot be extracted from the five-level PWM signal itself, since the half-cycle change cannot be detected until the first transition in the next half cycle occurs when it is too late to generate the 00 state. Therefore, PWM modulator should notify the halfcycle separator block of half-cycle change in advance. It is important to note that this procedure is independent of the adopted modulation technique. Therefore, it can be used with

carrier-based modulation techniques or non-carrier-based ones. This should be a good alternative when the complexity of the carrier-based technique is relatively high, e.g., for PD scheme.

Fig 4.3 Carrier-based modulation using PSC with sinusoidal reference for single-phase converters. (a) Reference and carriers arrangement. (b) Gate signals. (c) Output waveform

CHAPTER 5 SIMULATION RESULTS 5.1 Introduction MATLAB is an interactive software system for numerical computations and graphics. As the name suggests, MATLAB is essentially designed for the matrix computations such as 1. Solving systems of linear equations. 2. Computing Eigen values and Eigen vectors. 3. Factoring matrices etc. MATLAB has a variety of graphical capabilities and can be extended through the programs written in its own programming language. A number of these extend MATLAB’s capabilities to nonlinear problems, such as the solution of initial value problems for ordinary differential equations. MATLAB is designed to solve problems numerically, that is, in finite precision arithmetic. Therefore it produces approximate rather than exact solutions, and should not be confused with a symbolic computation system such as Mathematical or Marple. It should be understood that this does not make MATLAB better or worse than an SCS, it is a tool designed for different tasks and it is therefore not directly comparable.

5.1.1 Uses of MATLAB     

Computation and Math Development of Algorithm Prototyping, Modeling, and simulation Visualization, Data analysis, and exploration Engineering and Scientific graphics

Application development in that including Graphical User Interface building MATLAB is an interactive system whose basic data element is an array that does not require any dimensioning. It allows us to solve many technical computing problems, especially those with matrix and vector formulations. The name MATLAB stands for matrix

laboratory. MATLAB provides easy access to matrix software developed by the LINPACK and EISPACK projects, which together represent the art in software for matrix computation. MATLAB has evolved over a period of years. It is the standard instructional tool for introductory and advanced courses in mathematics, engineering, and science in university environment. MATLAB is the tool for high-productivity research, development, and analysis in industries. MATLAB features a family of application-specific solutions called toolboxes. Very important to most users of MATLAB, toolboxes allow you to learn and apply specialized technology. Comprehensive collections of MATLAB functions (M-files) are called toolbox. Toolboxes extend the MATLAB environment to solve particular type of problems. signal processing, control systems, neural networks, fuzzy logic, wavelets, simulation, and many other type of toolboxes are present.

5.1.2 The MATLAB language This is a high-level matrix/array language with control flow statements, functions, data structures, input/output, and object-oriented programming features. It allows both "programming in the small" to rapidly create quick and dirty throw-away programs, and "programming in the large" to create complete large and complex application programs.

5.1.3 The MATLAB working environment This is the set of tools and facilities that you work with as the MATLAB user or programmer. It includes facilities to manage the variables in your workspace and importing and exporting data. It also has tools for developing, managing, debugging, and profiling Mfiles, MATLAB's applications. Simulink is a software package for modeling, simulating and analyzing dynamical systems. It supports linear and non-linear systems, modeled in continuous time, sampled time or a hybrid of the two. Systems can also be a multi rate, i.e. have different parts that are sampled or updated at different rates. For modeling, SIMULINK provides a graphical user interface. Simulink includes comprehensive block libraries of sinks, linear, non linear components and connectors. We can

create our own blocks. Models are hierarchical with increasing levels of model details. This approach provides insight into how a model is organized and how its parts interact. After a model is defined, we can simulate it using a choice of integration methods, either from the SIMULINK menus or by entering command in the MATLAB’s command window. The simulation results can be seen in the scopes and display block while simulating. MATLAB analysis tools include linearization and trimming tools which can be accessed from command line, plus many tools in MATLAB and its application tool boxes. And because MATLAB and SIMULINK are integrated, we can simulate, analyze and revise our models in either environment at any point.

5.1.4 SIMULINK Simulink, developed by MathWorks, is a commercial tool for modeling, simulating and analyzing multi-domain dynamic systems. Its primary interface is a graphical block diagramming tool and a customizable set of block libraries. It offers tight integration with the rest of the MATLAB environment and can either drive MATLAB or be scripted from it. Simulink is widely used in control theory and digital signal processing for multi-domain simulation and Model-Based Design Simulink is a block diagram environment for multi-domain simulation and Model-Based Design. It supports system-level design, simulation, automatic code generation, and continuous test and verification of embedded systems. Simulink provides a graphical editor, customizable block libraries, and solvers for modeling and simulating dynamic systems. It is integrated with MATLAB, enabling you to incorporate MATLAB algorithms into models and export simulation results to MATLAB for further analysis.

5.1.4.2

Building the Model

Simulink provides a set of predefined blocks that you can combine to create a detailed block diagram of your system. Tools for hierarchical modeling, data management, and subsystem customization enable you to represent even the most complex system concisely and accurately. 5.4.1.2 electing Blocks

The Simulink Library Browser contains a library of blocks commonly used to model a system. As shown in Fig.3.2, these include:

Fig.3.1. Building a new model 

Continuous and discrete dynamics blocks, such as Integration and Unit Delay



Algorithmic blocks, such as Sum, Product, and Lookup Table



Structural blocks, such as Mux, Switch, and Bus Selector

We can build customized functions by using these blocks or by incorporating hand-written MATLAB, C, Fortran, or Ada code into the model. The custom blocks can be stored in their own libraries within the Simulink Library Browser.

Fig.3.2. Commonly used blocks

Simulink add-on products let you incorporate specialized components for aerospace, communications, PID control, control logic, signal processing, video and image processing, and other applications. Add-on products are also available for modeling physical systems with mechanical, electrical, and hydraulic components. To build a model as shown in Fig.3.1 by dragging blocks from the Simulink Library Browser into the Simulink Editor, we then connect these blocks with signal lines to establish mathematical relationships between system components. Graphical formatting tools, such as smart guides and smart signal routing, help we control the appearance of the model as we build it. We can add hierarchy by encapsulating a group of blocks and signals as a subsystem in a single block. The Simulink Editor gives a complete control over what we see and use within the model. For example, we can add commands and submenus to the editor and context menus. We can also add a custom interface to a subsystem or model by using a mask that hides the subsystem's contents and provides the subsystem with its own icon and parameter dialog box. 5.1 .4.2

Navigating Through the Model Hierarchy

The Explorer bar and Model Browser in Simulink helps to navigate the model. The Explorer bar indicates the level of hierarchy that we are currently viewing and lets we can move up and down the hierarchy. The Model Browser provides a complete hierarchical tree view of your model, and like the Explorer bar, can be used to move through the levels of hierarchy. 5.1.4.3 Managing Signals and Parameters Simulink models contain both signals and parameters. Signals are time-varying data represented by the lines connecting blocks. Parameters are coefficients that define system dynamics and behavior. Simulink helps to determine the following signal and parameter attributes as shown in Fig.3.3: 

Data type—single, double, signed, or unsigned 8-, 16- or 32-bit integers; Boolean; enumeration; or fixed point



Dimensions—scalar, vector, matrix, N-D, or variable-sized arrays



Complexity—real or complex values



Minimum and maximum range, initial value, and engineering units

If we choose not to specify data attributes, Simulink determines them automatically via propagation algorithms, and conducts consistency checking to ensure data integrity. These signal and parameter attributes can be specified either within the model or in a separate data dictionary. We can then use the Model Explorer to organize, view, modify, and add data without navigating through the entire model as shown in Fig.3.4.

Fig.3.3 Signal Attributes tab

Fig.3.4 Model Explorer Window

5.1.4.4Simulating the Model We can simulate the dynamic behavior of the system and view the results as the simulation runs. To ensure simulation speed and accuracy, Simulink provides fixed-step and variablestep ODE solvers, a graphical debugger, and a model profiler. 5.1.4.5 Choosing a Solver Solvers as shown in Fig.3.5 are numerical integration algorithms that compute the system dynamics over time using information contained in the model. Simulink provides solvers to support the simulation of a broad range of systems, including continuous-time (analog), discrete-time (digital), hybrid (mixed-signal), and multirate systems of any size.

Fig.3.5. Configuration Parameters dialog box showing the Solver pane.

These solvers can simulate stiff systems and systems with discontinuities. We can specify simulation options, including the type and properties of the solver, simulation start and stop times, and whether to load or save simulation data. We can also set optimization and diagnostic information. Different combinations of options can be saved with the model. 5.1.4.6 Running the Simulation

We can run your simulation interactively from the Simulink Editor or systematically from the MATLAB command line. The following simulation modes are available: 

Normal (the default), which interpretively simulates the model



Accelerator, which increases simulation performance by creating and executing compiled target code but still provides the flexibility to change model parameters during simulation



Rapid Accelerator, which can simulate models faster than Accelerator mode by creating an executable that can run outside Simulink on a second processing core

To reduce the time required to run multiple simulations, we can run those simulations in parallel on a multi-core computer or computer cluster. 5.1.4.7Analyzing Simulation Results After running a simulation, we can analyze the simulation results in MATLAB and Simulink. Simulink includes debugging tools to help to understand the simulation behavior. 5.1.4.8 Viewing Simulation Results We can visualize the simulation behavior by viewing signals with the displays and scopes provided in Simulink. We can also view simulation data within the Simulation Data Inspector, where we can compare multiple signals from different simulation runs. Scope is the block in Simulink by which we can measure and view the voltage, current, and power in electrical domain. Fig.3.6 shows the output of a multilevel converter through scope. Alternatively, we can build custom HMI displays using MATLAB, or log signals to the MATLAB workspace to view and analyze the data using MATLAB algorithms and visualization tools.

Fig.3.6. Multi-step waveform

5.1.4.9 Debugging the Simulation Simulink supports debugging with the Simulation Stepper, which lets we step back and forth through your simulation viewing data on scopes or inspecting how and when the system changes states. With the Simulink debugger we can step through a simulation one method at a time and examine the results of executing that method. As the model simulates, you can display information on block states, block inputs and outputs, and block method execution within the Simulink Editor.

5.1.5 SIM POWER SYSTEMS SimPowerSystems™ provides component libraries and analysis tools for modeling and simulating electrical power systems. The libraries include models of electrical power components, including three-phase machines, electric drives, and components for applications such as flexible AC transmission systems (FACTS) and renewable energy systems. Harmonic analysis, calculation of total harmonic distortion (THD), load flow, and other key electrical power system analyses are automated. SimPowerSystems was developed by Hydro-Québec of Montreal. SimPowerSystems models as shown in Fig.3.7 can be used to develop control systems and test system-level performance. We can parameterize the models using MATLAB® variables and expressions, and design control systems for the electrical power system in Simulink®. We can add mechanical, hydraulic, pneumatic, and other components to the model using Simscape™ and test them all in a single simulation environment. To deploy models to other

simulation environments, including hardware-in-the-loop (HIL) systems, SimPowerSystems supports C-code generation. Starting with MathWorks Release 13, SimPowerSystems and SimMechanics of the Physical Modeling product family work together with Simulink® to model electrical, mechanical, and control systems. Electrical power systems are combinations of electrical circuits and electromechanical devices like motors and generators. Engineers working in this discipline are constantly improving the performance of the systems. Requirements for drastically increased efficiency have forced power system designers to use power electronic devices and sophisticated control system concepts that tax traditional analysis tools and techniques. Further complicating the analyst’s role is the fact that the system is often so nonlinear that the only way to understand it is through simulation. Land-based power generation from hydroelectric, steam, or other devices is not the only use of power systems. A common attribute of these systems is their use of power electronics and control systems to achieve their performance objectives. SimPowerSystems was designed to provide a modern design tool that allows scientists and engineers to rapidly and easily build models that simulate powersystems. SimPowerSystems uses the Simulink environment, allowing you to build a model using simple click and drag procedures. Not only can you draw the circuit topology rapidly, but your analysis of the circuit can include its interactions with mechanical, thermal, control, and other disciplines. This is possible because all the electrical parts of the simulation interact with the extensive Simulink modeling library. Since Simulink uses MATLAB® as its computational engine, designers can also use MATLAB toolboxes and Simulink blocksets. SimPowerSystems and SimMechanics share a special Physical Modeling block and connection line interface. Users can rapidly put SimPowerSystems to work. The libraries contain models of typical power equipment such as transformers, lines, machines, and power electronics. These models are proven ones coming from textbooks, and their validity is based on the experience of the Power Systems Testing and Simulation Laboratory of Hydro-Québec, a large North American utility located in Canada. The capabilities of SimPowerSystems for modeling a typical electrical grid are illustrated in demonstration files. And for users who want to refresh their knowledge of power system theory, there are also self-learning case studies.

Fig.3.7. SimPowerSystems pane

5.1.6 Modeling Electrical Power Systems With SimPowerSystems, we build a model of a system just as we would assemble a physical system. The components in the model are connected by physical connections that represent ideal conduction paths. This approach describes the physical structure of the system rather than deriving and implementing the equations for the system. From the model, which closely resembles a schematic, SimPowerSystems automatically constructs the differential algebraic equations (DAEs) that characterize the behavior of the system. These equations are integrated with the rest of the Simulink model. We can use the sensor blocks in SimPowerSystems to measure current and voltage in your power network, and then pass these signals into standard Simulink blocks. Source blocks enable Simulink signals to assign values to the electrical variables current and voltage. Sensor and source blocks connects a control algorithm developed in Simulink to a SimPowerSystems network.

5.1.7 Modeling Custom Components SimPowerSystems enables to model custom components by using the fundamental elements included in its libraries and by combining these elements with Simulink blocks.

Fig.3.8. Simpower system Libraries

Components provided in SimPowerSystems as shown in Fig.3.8 include: Electrical elements: Linear and saturable transformers; arrestors and breakers; and transmission line models. Electric machinery: Models of synchronous, permanent magnet synchronous, and DC machines; excitation systems; and models of hydraulic and steam turbine-governor systems Power electronics: Diodes, simplified and complex thyristors, GTOs, switches, IGBT models, and universal bridges that allow selection of standard bridge topologies Control and measurement: Voltage, current, and impedance measurements; RMS measurements; active and reactive power calculations; timers, multimeters, and Fourier analysis; HVDC control; total harmonic distortion; and abc-to-dq0 and dq0-to-abc transformations Electrical sources: To implement sinusoidal current source, sinusoidal voltage source, generic battery model, Controlled AC Current and Voltage sources, DC Voltage Source. To implement three-phase voltage source with programmable time variation of amplitude, phase, frequency, and harmonics, and to implement three-phase source with internal R-L impedance. The entire blocksets is shown in Fig.3.9.

Fig.5.9.Blocksets of electrical sources used in SimPowerSystems

Three-phase components: RLC loads and branches; breakers and faults; pi-section lines; voltage sources; transformers; synchronous and asynchronous generators; and motors, analyzers, and measurements Electric Drives and Other Application Libraries SimPowerSystems provides the following specialized application libraries: Flexible AC Transmission Systems (FACTS): Phasor models of flexible AC transmission systems Distributed Resources: Phasor models of wind turbines Electric Drives: Editable models of electric drives that include detailed descriptions of the motor, converter, and controller for each drive. The Electric Drives library includes permanent magnet, synchronous, and asynchronous (induction) motors. The converters and controllers implement the most common strategies for controlling the speed and torque for these motors, such as direct-torque control and field-oriented control. SimPowerSystems supports the development of complex, self-contained power systems, such as those in automobiles, aircraft, manufacturing plants, and power utility applications. You can combine SimPowerSystems with other MathWorks physical modeling products to model complex interactions in multi-domain physical systems. The block libraries and simulation methods in SimPowerSystems were developed by Hydro-Québec of Montreal.

Fig.5.10. Circuit of a transmission line

Fig.5.11. Same circuit designed in Simulink window

Thus users can rapidly put SimPowerSystems to work. The libraries that containing models of typical power equipment such as transformers, lines, machines, and power electronics are used to construct a electrical circuit shown in Fig.3.10 and the completely designed circuit of the same in Simulink window as shown in Fig.3.11.

5.1.8 Connecting to Hardware We can connect the Simulink model to hardware for rapid prototyping, hardware-in the-loop (HIL) simulation, and deployment on an embedded system. 5.1.8.1 Running Simulations on Hardware

Simulink provides built-in support for prototyping, testing, and running models on low-cost target hardware, including Arduino®, LEGO® MINDSTORMS® NXT, PandaBoard, and BeagleBoard. We can design algorithms in Simulink for control systems, robotics, audio processing, and computer vision applications and see them perform in real time.

Fig.3.12. Hardware Interface to simulink

Simulink provides built-in support for prototyping, testing, and running models on low-cost target hardware, including Arduino®, LEGO® MINDSTORMS® NXT, and BeagleBoard as shown in Fig.3.13.

Fig.3.13. low-cost target hardware

With Real-Time Windows Target™, we can run Simulink models in real time on Microsoft ® Windows® PCs and connect to a range of I/O boards to create and control a real-time system as shown in Fig.3.12. To run the model in real time on a target computer, we can use xPC

Target™ for HIL simulation, rapid control prototyping, and other real-time testing applications. See xPC Target Turnkey for available target computer hardware. Simulink models can be configured and made ready for code generation. By using Simulink with addon code generation products, you can generate C and C++, HDL, or PLC code directly from your model.

5.1.9 APPLICATIONS A number of MathWorks and third-party hardware and software products are available for use with Simulink. For example, Stateflow extends Simulink with a design environment for developing state machines and flow charts. Coupled with Simulink Coder, another product from MathWorks, Simulink can automatically generate C source code for real-time implementation of systems. As the efficiency and flexibility of the code improves, this is becoming more widely adopted for production systems, in addition to being a popular tool for embedded system design work because of its flexibility and capacity for quick iteration. Embedded Coder creates code efficient enough for use in embedded systems. xPC Target together with x86-based real-time systems provides an environment to simulate and test Simulink and Stateflow models in real-time on the physical system. Embedded Coder also supports specific embedded targets, including Infineon C166, Motorola68HC12, Motorola MPC 555, TI C2000, TI C6000, RenesasV850 and Renesas SuperH. With HDL Coder, also from MathWorks, Simulink and Stateflow can automatically generate synthesizable VHDL and Verilog. Simulink Verification and Validation enables systematic verification and validation of models through modeling style checking, requirements traceability and model coverage analysis. Simulink Design Verifier uses formal methods to identify design errors like integer overflow, division by zero and dead logic, and generates test case scenarios for model checking within the Simulink environment. The systematic testing tool TPT offers one way to perform formal test- verification and validation process to stimulate Simulink models but also during the development phase where the developer generates inputs to test the system. By the substitution of the Constant and Signal generator blocks of Simulink the stimulation becomes reproducible.

SimEvents adds a library of graphical building blocks for modeling queuing systems to the Simulink environment. It also adds an event-based simulation engine to the time-based simulation engine in Simulink

5.2 Proposed simulation diagrams

CHAPTER 6 CONCLUSION In this paper, the operation of the DFC-ANPC topology has been investigated and the associated modulation techniques have been presented and verified by simulation and experimental results. Compared to the commercialized five-level FC-ANPC converter, the DFC-ANPC converter under the proposed modulation method has the following features: 1) provides more even loss distribution among semiconductor switches and thus higher power rating is expected; 2) eliminates the transient voltage balancing problem of series-connected switches; 3) decreases the switching loss and thus slight improvement in efficiency is expected; 4) can be extended to higher levels without transient voltage balancing problem. The comparison presented in this paper is mostly based on abductive reasoning and not quantified. For future work, a comparative study of the FC-ANPC and DFC-ANPC thermal

models will verify the thermal performance superiority and provide an estimation of the amount of extra power processing capability