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Int. J. Engg. Res. & Sci. & Tech. 2015 xxxxxxxxxxxxxxxxxxxxxxxx, 2015

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ELIMINATION OF HARMONICS IN CASCADED

MULTILEVEL INVERTER USING SOFT

COMPUTING TECHNIQUE

S Saravanasankar1*, D Sivabalan1, P Elango1, A John Simon Raj1

This paper presents the cuckoo optimization method for harmonic elimination in a cascadedmultilevel inverter.The main objective is to eliminate 3rd, 5th, 7th, 13th harmonicsusing the SelectiveHarmonic Elimination Pulse Width Modulation strategy order by solving non linear equations,while the fundamental component is satisfied.In this paper,the cuckoo algorithm(CA) is appliedto a 7level inverter for solving the equations.The algorithm based on parasitic behavior of thecuckoo bird and levyflights used for random walks(or)Brownian motions also the it is starts withan initial population.This method has high efficiency when compared to other metaheuristicsalgorithm.MATLAB software is used for optimization.It will reduced the Total Harmonics Distortionand eliminate the harmonics.

Keywords: Metaheuristis algorithm cuckoo algorithm, Eliminate harmonics, Selective harmonicelimination(SHE-PWM), Parastics behavior, Total Harmonic Distortion

*Corresponding Author: S Saravanasankar � [email protected]

1 Department of EEE, Jay Shriram Group of Institutions, Tirupur.

Int. J. Engg. Res. & Sci. & Tech. 2015

Research Paper

INTRODUCTION

Using travelling salesman problem it is used to

solving two evolutionary, conventional and bio-

inspired algorithms. The performance are

comparing basis on the tour length. It is important

to save time and cost. Using TSP, Bio-inspired

algorithms like cuckoo and firefly will provide

feasible and can attain better performance than

other methods. This Bio-inspired algorithm could

be done by fine-tuning their self adaptive

parameters and by using hybrid coding. There

are several heuristic and metaheuristic algorithms

have been used to solve a wide range of NP-hard

problems. A large number of real-life optimization

problems in science, engineering, economics,

and business are complex and difficult to solve.

They can’t be solved in an exact manner within a

reasonable amount of time. Real-life optimization

problems have two main characteristics, which

make them difficult: they are usually large, and

they are not pure, i.e.; they involve a

heterogeneous set of side constraints .

Metaheuristic techniques are the basic alternative

solution for this class of problems. Recently,

many researchers have focused their attention

on a metaheuristics. A metaheuristic is a set of

algorithmic concepts that can be used to define

heuristic methods applicable to a wide set of

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Int. J. Engg. Res. & Sci. & Tech. 2015 S Saravanasankar et al., 2015


different problems. The use of metaheuristics hassignificantly increased the ability of findingsolutions practically relevant combinatorialoptimization problems in a reasonable time.Prominent examples of metaheuristics areEvolutionary Algorithms, Simulated Annealing,Tabu Search, Scatter Search, VariableNeighborhood Search, Memetic Algorithms, AntColony Optimization, Cuckoo Search, and others.Which successfully solved problems includescheduling, timetabling, network design,transportation and distribution problems, vehiclerouting, the traveling salesman problem andothers .

LITERATURE SURVEYTo reduce the harmonics content is to minimizethe THD for the output phase voltage of inverterup to 50 harmonics is considered to calculatethe THD. Using Switching angles specific lowerorder dominant harmonics are suppressed. Thismethod is known as selective harmonicelimination (SHE) or programmed PWMtechnique in technical literatures[1].THD ofmultilevel inverters with unequal dc sources is afirst method and Integrating waveform is secondmethod using this obtain the results are faster,simpler, and ideally accurate[2].SHE-PWM a five-level voltage source inverter with elimination of11 low-order harmonics is investigated. The SHE-PWM equations that can eliminate harmonicsfrom 5th to 35th with modulation index M varyingfrom 0 to 1.15 are formulated [3].DFCM converterused to doubling the voltage levels and improvethe output voltage frequency spectrum this willachieved by adding only two low-power switchesand one dc voltage source [4]. The unbalanceddegree of the compensation currents that the star-connected STATCOM can deal with is limited to

the maximum attainable output voltage of theconverter leg. Also the dc-link voltages of the star-connected cascade converter were effectivelycontrolled during the unsymmetrical fault [1].Reference rotation method rotates the modulationwaveform through different bands of carrierwaveforms by using line-line redundant voltagestates. This makes use of all the levels in theinverter even at low modulation indexes. Thecarrier common-mode dc-offset method toincrease the voltage levels and reduce harmonic-current distortion at low modulation indexes theseries-connected IGBTs in the three-levelunidirectional buck and boost choppers resultedin good voltage sharing. Also reducing theswitching frequencies of the IGBT’s[4].The natureof a low-voltage step in the inverter output makesa significant contribution to reducing motor currentripple and the resultant motor torque ripple, aswell as to mitigating undesirable effects ofcommon-mode voltage on ground leakagecurrent and/or bearing current[3]. Based ondifferent switching states, it is possible to achievemore voltage levels on output voltage by addingand subtracting dc link voltages compared withconventional multilevel inverters with the samenumber of components.Bydoing so, outputvoltage with superior quality can be obtained withless circuit and control complexity. Also,increasing the harmonic characteristic of theoutput voltage can decrease the size of the filter.The main advantage of this arrangement is thesimplicity to cascade several H-bridge cells forimprovement of the output voltage resolution withreduced number of components[6].SHE-PWMtechniques offer several advantages over othermodulation methods, including acceptableperformance with low switching frequency tofundamental frequency ratios, direct control over

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output waveform harmonics, and the ability toleave triple harmonics uncontrolled to takeadvantage of circuit topology in three phasesystems, and therefore have drawn greatattention in recent years. SPWM method and alsoreduces switching frequency with the similarwaveform quality in the full range of modulationindex. To eliminate higher order of harmonics bysimply generating the opposite of the harmonicsto cancel them[8].Comparison between threetypes of balance control strategy, namely, theactive voltage vector superposition, modulationindex regulation, and phase shift angle regulationhas been reported in for delta-connected twelvelevel CHI topology controlled by CB-PWM. It wasconcluded that the former offers good controlperformance with strong regulation capabilityfollowed by the phase shift angle regulationmethod and modulation index regulation method.A nineteen-level CHI-based STATCOM tosimultaneously balance the dc-link capacitorvoltages, minimize the inverter switching losses,and provide good current reference tracking [10].Elimination theory has been employed todetermine the switching angles to eliminatespecific harmonics, such as the fifth, seventh,11th, and the 13th. The benefit of the fundamentalfrequency switching method is its low switchingfrequency compared to other control method [12].Switching losses in these high-power high-voltageconverters represent an issue and any switchingtransitions that can be eliminated withoutcompromising the harmonic content of the finalwaveform is considered advantageous. The termmultilevelstarts with the introduction of the three-level converter. By increasing the number of levelsin a given topology, the output voltages have moresteps generating a staircase waveform, whichapproaches closely the desired sinusoidal

waveform and also offers reduced harmonicdistortion.An optimization technique assisted witha hybrid genetic algorithm was successfullyapplied to find the switching transitions of theSHE-PWM ac/ac converter [15].The cascadedinverters is producing the total output activepower and the other inverters are used as asupport to improve the output voltage waveformThe main features of this topology are the inherentgalvanic isolation, bidirectional power flow,reduction of the transformer size and weight, andhigh efficiency[18].

EXISTING SYSTEMDRAWBACKSElimination of low-order harmonics usingSHEPWM strategy is investigated.BA is appliedto solve the equations. Using the Newton -Rapson method does not suggest any optimumsolution for Infeasible. Also the Gauss Newtonmethod only used in evolutionary algorithmscannot find solutions only eliminate low orderharmonics. In this also optimum solution isinfeasible.BA codes have more complexity andgreater running time.

PROPOSED SYSTEMCONCEPTUsing Metaheuristics algorithm cuckoo algorithmto implement cascaded multilevel inverter reducethe harmonics. Giving various n input values tothe Natural Algorithm like cuckoo algorithm in thisonly best optimized value will taken and given tothe multilevel inverter. Optimization means findingout the firing angles for minimizing losses. In thisharmonics will reduce. It was observed that thebio-inspired algorithms provide feasible methodsfor TSPs and can attain better performance thanother methods. Firefly and cuckoo algorithm have

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been used with success to compute optimalswitching angles for multilevel inverters with manydc sources while minimizing several harmonics.In firefly algorithm, a firefly population is placed inrandom locations in the search space where thefireflies represent a candidate solution. In CS Infirefly algorithm, a firefly population is placed inrandom locations in the search space where thefireflies represent a candidate solution. Usingfirefly algorithm optimized result is obtained andusing the cuckoo algorithm it is efficient.

METAHEURISTICSALGORITHMCUCKOO ALGORITHM

Start with ζ0 ∈ S, at t = 0

While (criterion)

Propose a new solution YT+1;

Generate a random number 0 ≤ Pt ≤ 1;

Zt+1 =

end

YT+1

At

With probability Pt

With probability 1 – Pt

end

DESIGN OF INVERTERAn inverter is a device that changes or invertsdirect current (DC) input to alternating current (AC)output. It doesn’t “create” or “make” electricity,just changes it from one form to another. DC in ischanged to AC out. Output is usually 120 or 240volts at 60-cycle alternating current to match linepower. Inverters are often a good choice for

applications that require the main engine tooperate at a job site. Powering hydraulic systemsor air compressors.Since, inverters are electronicdevices, we don’t have the noise from a separateengine.An inverter requires no fuel and virtuallyno maintenance. Inverter output is fully voltageand frequency regulated and functionsindependently from the speed of the engine. Inthis paper 17-level inverter is used to solving theequations using the two algorithms. Formula tocalculate level 2S+1 Where S indicates thenumber of voltage sources.

CHARACTERISTICS OFGOOD INVERTERIts output voltage waveform should besinusoidal.Its gain should be high.Its output voltageand frequency should be controllable in the desiredvoltage.The power required by its controllingcircuit should be minimum.Its overall cost mustbe minimum.Its working life must be long.Thesemi conductor device used in the inverter shouldbe minimum switching and conduction losses.

BLOCK FOR DIFFERENTTYPES OF ALGORITHMS

Figure 1: Block Diagram

Metaheuristics Algorithm

Tabu-search Simulated Annealing

Genetic algorithm

Memetic algorithm

Anticolony algorithm Bee algorithm Firefly algorithm Cuckoo algorithm

Two classical Algorithm

Two evolutionary

Algorithm

Four nature-inspired algorithm

Natural Algorithms

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OPERATIONMultilevel InverterMultilevel inverter is used to extract power fromsolar cells. It synthesizes the desired ac outputwaveform from several dc sources. Multilevelconverters are mainly utilized to synthesis adesired single- or three-phase voltage waveform.The desired multi-staircase output voltage isobtained by combining several dc voltagesources. Solar cells, fuel cells, batteries and ultra-capacitors are the most common independentsources used. One important application ofmultilevel converters is focused on medium andhigh-power conversion. Nowadays, there existthree commercial topologies of multilevel voltage-source inverters: neutral point clamped (NPC),cascaded H-bridge (CHB), and flying capacitors(FCs). Among these inverter topologies,cascaded multilevel inverter reaches the higheroutput voltage and power levels (13.8 kV, 30 MVA)and the higher reliability due to its modulartopology. There are three main types ofmultilevelconverters: diode-clamped, capacitor-clamped, andcascaded H-bridges. The detailedadvantages anddisadvantages of the threemultilevel converters willbe discussed in thischapter. Modulation techniquesthat have beenproposed include traditional PWMmethod, spacevector PWM method, space vectorcontrol

method, and selective harmonics eliminationmethod. Applications that have been proposedincludemultilevel rectifiers, DC/DC converters,large motor drives, distributed energy applications(such as staticvolt-ampere reactive (VAR)compensation), back-to-backhigh-voltage intertie,and adjustable speed drives.

CASCADED H-BRIDGEMULTILEVEL INVERTERThe cascaded H-bridge inverter has drawntremendous interest due to thegreater demandof medium-voltage high-power inverters. Itiscomposed of multiple units of single-phase H-bridge power cells. The Bridgecells are normallyconnected in cascade on their ac side toachievemedium voltage operation and lowharmonic distortion. The cascadedBridgemultilevel inverter requires a number ofisolated dc supplies, each ofwhich feeds an H-bridge power cell.More generally, a cascaded H-bridges multilevel inverter using s separate dcsources can produce a maximum of 2s + 1distinct levels in the output phase voltage.

The advantages for cascaded multilevel H-bridgeConverter is the following:

Figure 3: Single Phase H-Bridge Inverter

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(1) The series structure allows a scalable,modularized circuit layout and packaging.Due to the identical structure of each HBridge.

(2) No extra clamping diodes or voltagebalancing capacitors is necessary.

(3) Switching redundancy for inner voltageLevels are possible because the phasevoltageis the sum of each bridge’s output.

LEVEL OF 3RD, 5TH, 7TH, 13TH

HARMONIC ELIMINATIONImproving the efficiency of the multilevel inverterand quality of output voltage waveform. Sevenlevel reduced switches topology has beenimplemented with only seven switches.Fundamental Switching scheme and SelectiveHarmonics Elimination were implemented toreduce the Total Harmonics Distortion (THD)value. Selective Harmonics Elimination SteppedWaveform (SHESW) method is implementedtoeliminate the lower order harmonics.Fundamental switching scheme is used to controlthe power electronics switches in the inverter. Theproposed topology is suitable for any number oflevels. The harmonic reduction is achieved byselecting appropriate switching angles.It showshope to reduce initial cost and complexity henceit is apt for industrial applications. In this third andfifth level harmonicshave been eliminated. Inmultilevel inverter using five equal dc sources,the five switching angles weredetermined suchthat the 5th, 7th, 11th, and 13th orderharmonicswere eliminated while at the same time controllingthe value ofthe fundamental. The idea ofSymmetricPolynomials was used in conjunctionwith Resultant theory to solve the correspondingset of polynomial harmonic equations for allpossible switching angles.

HARMONIC ELIMINATIONSWITCHING SCHEMESThis section presents switching schemesinvolving harmonic elimination besides variousmultilevel switching schemes which wasdiscussed in previous chapter. More specifically,Bipolar Programmed PWM, UnipolarProgrammed PWM, and Virtual Stage PWM arediscussed. Bipolar Programmed PWM is one ofthe switching schemes involving harmonicelimination. In Bipolar Programmed PWM, theoutput voltage is either Vicor Vic. Figure 3.8illustrates the Bipolar Programmed PWMswitching scheme using three switching anglesand an Vdcequal to 12 V.

Bipolar Programmed PWMAnother advantage of Bipolar Programmed PWMis that control is not as complicated as someother switching schemes. Neglecting blankingtime, switches S11 and S13 are switched “on”and “off” together. Similarly, switches S12 andS14 are switched “on” and “off” together.

Bipolar Programmed PWM Using ThreeSwitching AnglesThat Bipolar Programmed PWM usespredetermined switching angles to cut notchesinto a square-wave output. These notches takethe voltage either from Victo Vicor from Victo Vic.The number of notches cut per fundamental cycleis equal to twice the number of switching anglesused, which was addressed in Mohan et al (1995).By using Fourier series theory, these switchingangles can be used to eliminate certainharmonics. For example, three switching anglescan be used to eliminate the fifth and seventhorder harmonics while at the same time controllingthe value of the fundamental. One of the mainadvantages of using Bipolar Programmed PWM

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concerns its applicability when low modulationindices are used. When low modulation indicesare used, the fundamental multilevel switchingscheme is unable to perform the desiredharmonic elimination process. The threeswitching angles are considered again, when thefundamental multilevel switching schemes areused, it is not able to eliminate both the fifth andseventh order harmonics and control the valueof the fundamental. However, BipolarProgrammed PWM can be used with lowmodulation indices were discussed in Chansonet al (2004). When a multilevel inverter utilizesBipolar Programmed PWM for a low modulationindex, typically one H-bridge is used. Anotheradvantage of Bipolar Programmed PWM isredundancy. If one H-bridge fails, another Hbridgecan be used to compensate the necessaryvoltage. Also, the desired voltage can be achievedby using a sequence of switching’s for each H-bridge inverter within short periods of time. BipolarProgrammed PWM can be used for highermodulation indices in addition to low modulationindices. If a multilevel inverter needs to use twoor more H-bridges in order to produce a desiredvoltage, one can choose a lower modulation indexand use Bipolar Programmed PWM on multipleH-bridges.

TRAVELLING SALESMANPROBLEMThe Travelling Salesman Problem (TSP)represents a large class of problems known ascombinatorialoptimization problems. These aredifficult to solve using traditional methods andifsolvable, computations tend to be very timeconsuming.Hence we settle for approximatedresults which in less time, end up giving aresultthat isn’t necessarily the best tour, but

instead a tour that is close to the best tour .Aformof the TSP was introduced by Euler in 1759 andlater in 1948 Rand Corporation formally namedand introduced it . It defined the ClassicalTravelling Salesman Problem (CTSP) as aproblem where starting from one city it is requiredby the salesman to visit every other city only oncein a way that the total distance covered isminimized. As the number of cities increases, thecomplexity of the problem increases exponentiallydue to the number of possible solutions increasedvery heavily.CTSP and a variant of TSP namedas Random Travelling Salesman Problem(RTSP) are solved using conventional,evolutionary and bio-inspired algorithms. Theperformances are compared on the basis of tourlength. For CTSP, six available datasets onTSPLIB are used and for RTSP, four datasetsare randomly generated having all the citycoordinates generated in a pre-defined range from0 to 100.

CUCKOO-ALGORITHM (CS)CS is a population based optimization algorithminfluenced by the parasitic behavior of the cuckoobird whose female lays their eggs in the nests ofother birds.In CS algorithm, a set of nests withone egg each are placed in random locations inthe search space where the eggs represent acandidate solution. A search pattern called LevyFlights is used which is considered more efficientthan random walks or Brownian motions.

Pseudo Code for CS-TSP1. Generate a random dataset or add a standard

dataset from TSPLIB (as a part of initializingpopulation of cuckoo nests);

2. While (termination criteria is not met)

3. Get a cuckoo randomly (say, i) and replace its

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solution by performing Levy flights;

4. Evaluate its quality/fitness;

5. Choose a nest among n (say, j) randomly;

6. Replace j by the new solution if fitness valueof i is more than fitness of j;

7. Abandon worse nests build new ones;

8. Keep the best solutions/nests;

9. Select the best nests representing the solution;

SEVENLEVEL MULTILEVELIVERTER

Seven-level cascaded multilevel inverterwaveform generationSimilarly, the ac output voltage level can beobtained in the same manner. If NS is the numberof dc sources, the output phase level is m=NS+1.Controlling the conducting angles at differentinverter levels can minimize the harmonicdistortion of the output voltage.

Figure 4: Sevenlevel Multilevel Inverter

The phase output voltage is synthesized by thesum of three inverter outputs,

Van = Va1+Va2+Va3 ...(1)

Each inverter can be generate three differentoutputs, +Vdc, 0 and –Vdc, by connecting dc sourceto that ac output side by different outputcombinations of four switches S1,S2,S3 and S4.Using the top level as the example, turning on S1and S4 yields Va4 = +Vdc. The turning on S2 andS3 yields Va4 = -Vdc. Turning off all switches yieldsVa4 = 0.

The output voltage of the inverter is almostsinusoidal, and it has less than 5% total harmonicdistribution (THD) with each of the H-bridgesswitching only at fundamental frequency. If thephase current ia, as shown in Figure 9.8b, issinusoidal and leads or lags the phase voltageVan by 90°, the average charge to each dccapacitor is equal to zero over one cycle.Therefore, allSDCS capacitor voltages can bebalanced.

CONCLUSIONUsing Metaheuristicsalgorithm of cuckooalgorithm to implement cascaded multilevelinverter reduce the harmonics. Giving various ninput values to the Natural Algorithm of cuckooalgorithm in this only best optimized value willtaken and given to the multilevel inverter. Using7level multilevel inverter to reduce the switchingangles and getting optimized value.Optimizationmeans finding out the firing angles for minimizing

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losses. In this harmonics will reduced. It wasobserved that the bio-inspired algorithms providefeasible methods for TSPs and can attain betterperformance than other methods. CuckooAlgorithm, have been used with success tocompute optimal switching angles for multilevelinverters with many dc sources while minimizingseveral harmonics.

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8. Fei W, Du X, and Wu B (2010), “Ageneralized half-wave symmetry SHE-PWM formulation for multilevel voltageinverters,” IEEE Trans. Ind. Electron., Vol.57, No. 9, pp. 3030–3038.

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