CHAPTER 1 INTRODUCTIONINTRODUCTION Multilevelinverter continues to receive more attention because of their high voltagecapability and efficiency, high EMI. Low switchinglosses. Nowadays multilevel inverters are becoming increasingly popular inpower applications , as multilevel inverter are becoming increasingly popularin power applications, as multilevel inverters have the ability to meet theincreasing demand of power rating and power quality with reduced harmonicdistortion and lower EMI Multilevel inverter has advantages overthe conventional two-levels inverters that uses several switches to achieve forhigh switching frequency Pulse Width Modulation(PWM).
Multilevelinverter uses number of power semiconductor switches, dcsources(batteries/capacitors) to synthesize staircase output voltage waveform.By increasing number of levels the output voltage waveform approaches near tosine wave improving its quality. Due to these advantages they found wideapplications in adjustable speed drives, HVDC, FACTS, wind farms, photovoltaicsystems, electric vehicles and so on Multilevel inverter has advantages over the conventional two levelsinverter that uses several switches to achieve for high switching frequency Pulse width modulation(PWM).
Multilevelinverter has some features are as follow: Lowdistortion and lower dv/dt can begenerated. Multilevelinverter can draw input current with very low distortion.Multilevel inverter generate smallcommon mode voltage.Multilevel inverters can be operate withlower switching frequency MLISTRUCTURES Multilevel inverters have anarrangement of power switching devices and capacitor voltage sources.Multilevel inverters are suitable for high-voltage applications. It haveability to synthesize output voltage waveforms with a better harmonic spectrumand it attain higher voltages with alimited maximum device rating. Thereare three main types of multilevel inverters: Diode-clamped (neutral clamped)Capacitor-clamped (flying capacitors)Cascaded H bridge inverter Fig.
1.1 Classification of multilevel inverters 1.2.1Diode Clamped The diode –clamped inverter isalso known as the neutral-point clamped inverter(NPC) which was introduced bynabe at al(1981).The diode-clamped inverter consist of two pairs of seriesswitches (upper and lower)in parallel with two series capacitors where theanode of the upper diode is connected to the midpoint(neutral) of thecapacitors and its cathode to the midpoint of the upper pair of switches; thecathode of the lower diode is connected to the midpoint of the capacitors anddivides the main DC voltage into smaller voltages. The middle point of the twocapacitors can be defined as the “neutral point”.
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The NPC uses single DC bus that is subdivided into anumber of voltage levels by series strings of capacitors. 1.2.1.
1Operationof Diode-clamped Threelevel diode-clamped converter in which the dc bus consist of two capacitor,. For dc-bus voltage , the voltage acrosseach capacitors is and each device voltage stress will be limitedto one capacitor voltage level through clamping diodes. To explain how thestaircase voltage is synthesized, the neutral point n is considered as the output phase voltage reference point.
There are three switch combination tosynthesize three-level voltages across a and n.Voltage level turn on the switches and Voltage level, turn on the switches and Voltage level turn on the switches For a three-level diode-clampedinverter if point 0 is taken as the ground reference, the output voltage hasthree states 0, + and Vdc . Theline-line voltages of two legs with the capacitors are ,, -. Threephases are necessary to generate a three-phases voltages. Fig.1.2 Diodeclamped MLI 1.
2.1.2Features of Diode-clampedHigh voltage rating required forblocking diodesUnequal device ratingCapacitor voltage unbalance1.2.1.3Diode- Clamped MLI ApplicationStatic var compensationVariable speed motor drivesHigh voltage system interconnectionsHigh voltage dc and ac transmission lines1.
2.1.4Advantageof Diode-Clamped All of the phases share a common dc bus,which minimize the capacitance requirements of the converter.
For this reason,a back-back topology is not possible but also practical for uses such as ahigh-voltage back-back inter- connection or an adjustable peed drive.The capacitors can be pre-charged as a group.Efficiency is high for fundamentalfrequency switching. When the number of levels is highenough, harmonic content will be low enough to avoid the need for filters .1.2.
1.5Disadvantage of Diode-clampedReal power flow is difficult for asingle inverter because the intermediate dc levels will tend to overcharge ordischarge without precise monitoring and control.The number of clamping diodes isquadratically related to the number of levels, which can be cumbersome forunits with a high number of levels. 1.2.
2Capacitor-clamped The capacitor clamped inverteralternatively known as flying capacitor was proposed by meynard and foch in1992. The structure of this inverter is similar to that of diode clampedinverter except that instead of using clamping diodes, the inverter usescapacitor involves series connection of capacitor clamped switching cells. Thistopology has a ladder structure of dc side capacitor, where the voltage on eachcapacitor differs from that instead of next capacitor. The voltage incrementbetween two adjacent capacitor legs gives the size of the voltages steps in theoutput waveform.1.2.2.1Operation of FCMLI In the operation of flying capacitor multi-levelinverter, each phase node( can beconnected to any node in the capacitor bank, .
Connection of thea-phase to positive node occurs when and are turned on and to the neutral point voltagewhen and are turned on. The negative node is connected when and are turned on. The clamped capacitor c1 ischarged when , ans are turned on and is discharged when and are turned on the charge capacitor can bebalanced by proper selection of the zero states. In comparisons to three-leveldiode-clamped inverter, an extra switching state is possible. In particular,these are two transistor states which make up the level. Considering thedirection of the a phase flying capacitor current for the redundant states, a decision can bemade to charge or discharge the capacitor and therefore , the capacitor voltagecan be regulated to its desired value by switching within the phase. As withthe three-level flying capacitor and therefore, the capacitor voltage can beregulated to its desired value by switching within the phase.
as with thethree-level flying capacitor inverter, the highest and lowest switching statesdo not change the charge of the capacitors. The two intermediate voltage levelcontain enough redundant states so that both capacitors. The two intermediatevoltage levels contain redundant states so that both capacitors can beregulated to their ideal voltages. Similarto the diode clamped inverter clamping requires a large number of bulkcapacitor to clamp the voltage. Provided that the voltage rating of each capacitor used is the same as that of themain power switch, an N level converter will require a total of clamping the flying-capacitor inverter does not require all of the switches thatare on (conducting) in a consecutive series. Moreover, the flying-capacitor inverterhas phase redundancies 1, 3. These redundancies allow a choice of charging /discharging specificcapacitors and can be incorporated inthe control system for balancing the voltages across the various levels.
Thevoltage synthesis in a five level capacitor-clamped converter has more flexibilitythan a diode-clamped converter. Capacitor-clamped multilevel invertertopologies are relatively new compared to the diode-clamped or the cascadedH-bridge cell inverter topologies. Redundancy in the switching states is available by using flying capacitorsinstead of clamping diodes. This redundancy can be used to regulate thecapacitor voltages and obtain the same desired level of voltage at the output.Figure 1.
3 shows a single-phase five-level capacitor-clamped multilevelinverter topology. The voltage across the capacitors is considered to be halfof DC source voltage Vdc . The output voltage consists of fivedifferent voltage level Vdc,-Vdc,0,Vdc, Vdc . Fig.1.
3 Topology ofcapacitor clamped MLI1.2.2.2Features of FCMLI The majorproblem in this inverter is the requirement of a large number of storagecapacitors. Provided that the voltage rating of each capacitor used is the sameas that of the main power switch, an m-level converter will require a total of auxiliary capacitors per phase leg in additionto ) main dc buscapacitors. With the assumption that all capacitors have the same voltagerating, an m-level diode –clamp inverter only requires capacitors. In order to balance the capacitorcharge and discharge, one may employ two or more switch combinations for middlevoltage levels(i.
e., and ) in one or severalfundamental cycles. Thus, by proper selection of switch combinations, theflying-capacitor multilevel converter may be used in real power conversions.However, it involves real power conversions, the selection of a switchcombination becomes very complicated, band the switching frequency needs to behigher than the fundamental frequency.1.
2.2.3Advantages of FCMLILarge ‘n’ allows the capacitors extraenergy during long discharge transient.Phase redundancies are available forbalancing the voltages levels of the capacitors Lower total harmonic distortion when thenumber of levels ‘n’ is highActive and reactive power flow can becontrolled.Added clamping diode are not needed.The required number of voltage level canbe achieved without the use of the transformer. The assists in reducing thecost of the converter and again reduces power loses Series string of capacitor clamped sharethe same voltage.
The capacitor within the phase leg arecharged to different levels. The large number capacitors enables theinverter for deep voltage sag. 1.2.2.4Disadvantage of FCMLILarge numbers of capacitors are bulkyand more expensive than the clamping diodes used in the diode-clampedmultilevel inverter.Complex control is required to maintainthe capacitors voltage balanceSwitching utilization and efficiency arepoor for real power transmissionPre-charging all of the capacitors tothe same voltage level and start-up are complex.The capacitor have large fraction of thedc bus voltage,The large number of capacitor are usedin FCMLI so it is more expensivePackaging the inverter with number of levels are more difficult.
1.2.3Cascaded multilevel inverter The cascaded H-bridge inverter hastremendous interest due to the greater demand of medium–voltage high powerinverter.
The high power inverter to construct multilevel phase legs withseparate dc sources. One more alternatives for a multilevel inverter is thecascaded multilevel inverter or series H-bridge inverter. The series H- bridgeinverter appeared in 1975. Cascaded multilevel inverter was not fully realizeduntil two researchers, lai and peng.
They patented it and presented its variousadvantage in 1997. Since then ,CMI synthesis its output nearly sinusoidal voltage waveforms by combiningmany isolated voltage levels. By adding more H-bridge converter, the amount ofvar can simply increase without redesignthe power stage, and build-in redundancy against individual H-bridge converterfailure an be realized. A series of single-phase full bridge make up a phasefor the inverter. A three phase CMI topology is essentially composed of threeidentical phase legs of the series-chain of H bridge converters, which canpossibly generate different output voltage waveform and offers the potentialfor AC system phase-balancing.
This features is impossible in other VSCtopologies utilizing a common dc link. Since this topology consists of seriespower conversion cells, the voltage and power level may be easily scaled. Thedc link supply for each full bridge converter is provided separately, and thisis typically achieved using diode rectifier fed from isolated secondarywindings of a three-phase transformer. Phase-shifted transformer can supply thecell in medium –voltage system in order to provided high power quality at theutility connection.1.2.
3.1Operation of cascaded H bridge inverter Theoutput of each h-bridge can have three discrete level, result in a staircasewaveform that is nearly sinusoidal even without filtering. A singleH-bridge a three-level inverter, eachsingle phase full bridge inverter generates three level inverter, eachsingle-phase full-bridge inverter generates three voltages at the output – ,0 and Fig.
1.4 single H- bridgetopologyThefour switch S1,S2,S3 and S4 arecontrolled to generate three discrete output with levels,-, ,0 and . When S1 and S2 are on,the outputs is : when either pair S1and S3 or S2 and S4 are on, the output is 0 figure1.5 Shows a single-phase, five-level cascaded H-bridge cell inverter realizedby connecting two three a level conventional full bridge inverters in serieswas presented in Tolbert et al (1999).
Switch pairs S1and S3and S2 and S4 are complementary to each other. Thedifferent voltage levels that can beobtained at the output terminals are,-, 0, ,.If the dc voltage sources in both the inverter circuits connected in series arenot equal to each other, then nine levels can be obtained at the outputterminals. The number of levels in the output voltage can be increased by twoby adding an identical inverter in series. The n number of output phase voltagelevels in a cascaded inverter withseparate dc sources is 2(n-1)s possiblelevels. Fig.
1.5 five levelcascaded H-bridge inverter1.2.
3.2Advantage of cascaded H-Bridge inverterThe series structure allows a scalable,modularized circuit layout and packaging due to the identical structure of eachH-bridge. No extra clamping diodes or voltagebalancing capacitors are necessary. Switching redundancy for inner voltagelevels is possible because the phase voltage is the sum of the output of eachbridge.
