International Journal of Emerging Technology and Advanced Engineering Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 3, Issue 2, February 2013) A Review on Mitigation of Harmonics in Cascaded H-Bridge Multilevel Inverter using optimization Techniques Amrita Tuteja 1, Dr. Amita Mahor2, Aarti Sirsat3 1 P.G. Scholar, NIIST, Bhopal (M.P.), India 2 H.O.D. Electrical & Electronics Department, NIIST, Bhopal (M.P.) 3 P.G.Scholar, SOI DAVV Indore (M.P.), The key issue in designing an effective multilevel inverter is to ensure that the total harmonic distortion (THD) of the output voltage waveform is within acceptable limits. To control the output voltage and to eliminate the undesired harmonics in multilevel converters various modulation methods such as sinusoidal pulse width modulation (PWM) and space-vector PWM techniques are suggested[6],[7]. However, PWM techniques are not able to eliminate lower order harmonics completely. Another approach is to choose the switching angles so that specific higher order harmonics such as the 5th, 7th, 11th, and 13th are suppressed in the output voltage of the inverter. This method is known as selective harmonic elimination (SHE) or programmed PWM techniques in technical literature [8]–[10]. A fundamental problem with such method is to obtain the arithmetic solution of nonlinear transcendental equations which contain trigonometric terms and naturally present multiple solutions. This set of nonlinear equations can be solved by iterative techniques such as the Newton–Raphson method [8]– [11].However, such techniques need a good initial guess which should be very close to the exact solution patterns. Therefore, the Newton–Raphson method is not feasible to solve the SHE problem for a large number of switching angles if good initial guesses are not available. To solve the SHE problem another approach based on mathematical theory of resultant is proposed[12],where the SHE problem are converted into an equivalent set of polynomial equations and then the mathematical theory of resultant is utilized to find all possible sets of solution. This approach, however, appears to be unattractive because as the number of inverter level increases, so does the degree of the polynomials of the mathematical model. This is likely to lead to numerical difficulty and substantial computational burden as well. The Particle Swarm Optimization (PSO) approach is developed to deal with the SHE problem with unequal Dc sources. Particle Swarm Optimization (PSO) is a relatively recent heuristic search method whose mechanics are inspired by the swarming or collaborative behavior of biological populations. Abstract - Multilevel inverters have been drawing growing attention in the recent years especially in the distributed energy resources area due to the fact that several batteries, fuel cells, solar cell, wind and micro turbines can be connected through multilevel inverter to feed a load or the ac grid. Multilevel inverter becomes an alternative in the area of highpower medium-voltage energy control. Therefore, Harmonic Reduction in multi level inverters are considered very important task. For eliminating the harmonics lot of researchers have suggested modifications in the circuit topology and optimal selection of firing angle using conventional and nonconventional optimization algorithms. This paper presents a review application by different optimization techniques for harmonic reduction in multilevel inverters. Keywords- Cascaded Multilevel Inverter, Genetic Algorithm (GA), H-bridge, Multilevel Inverter, Particle Swarm optimization (PSO), Total Harmonic Distortion (THD). I. INTRODUCTION A multilevel converter is a power electronic system that synthesizes a desired output voltage from several levels of dc voltages as inputs. With an increasing number of dc voltage sources, the converter output voltage waveform approaches a nearly sinusoidal waveform while using a fundamental frequency-switching scheme. The primary advantage of multi level inverter is their small output voltage, results in higher output quality, lower harmonic component, better electromagnetic computability, and lower switching losses. [1][2] Multilevel inverters also have interest in the field of high-voltage high-power applications such as laminators, mills, conveyors, compressors, UPS systems. Some of the fundamental multilevel topologies include the diodeclamped [3], flying capacitor [4], and cascaded H-bridge structures [5]. The most familiar power circuit topology for multilevel converters is based on the cascade connection of an s number of single-phase full-bridge inverters to generate a (2s + 1) number of levels. 30 International Journal of Emerging Technology and Advanced Engineering Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 3, Issue 2, February 2013) As an optimization technique, PSO is much less dependent on the start values of the variables in the optimization problem when compared with the widely used Newton-Raphson Another approach i.e. The Genetic Algorithm (GA) is stochastic optimization techniques. Genetic Algorithm is applied to compute the switching angles in a cascaded multilevel inverter to produce the required fundamental voltage. The problem of harmonic elimination is converted into an optimization task using binary coded task. It deals with all the problems that usually considered very hard such as integer variables, non-convex functions, no differentiable functions, domains not connected, badly behaved functions, multiple local optima, and multiple objectives. For these reasons, GA has been adopted in this study. II. C ASCADED H-B RIDGE MULTILEVEL INVERTER The cascaded multilevel inverter consists of a series of H bridge (single-phase full-bridge) inverter units. Each fullbridge can generate three different voltage outputs: +Vdc, 0, and −Vdc. However, all three multilevel inverters can produce staircase waveform as shown in Fig. 2. The number of output phase voltage levels in a cascaded multilevel inverter is 2S + 1, where S is the number of dc sources. For example, phase voltage waveform for a 7-level cascaded multilevel inverter with three isolated dc sources (S = 3) is shown in Fig. 2. Each H-bridge unit generates a quasi-square waveform by phase-shifting the switching timings of its positive and negative phase legs. III. Fig. 1. Cascaded multilevel inverter with separate dc sources. SELECTIVE H ARMONIC E LIMINATION A 7-level inverter waveform shown in fig.2 has three variables θ1, θ2, and θ3, where Vdc1, Vdc2, and Vdc3 are assumed to be equal. Considering equal amplitude of all dc sources, the Fourier series expansion of the output voltage waveform, shown in Fig.2, will be written: Fig. 2. The output voltage waveform of a 7-level inverter. 31 International Journal of Emerging Technology and Advanced Engineering Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 3, Issue 2, February 2013) ∞ V ( t) = ∑ Vn sin (n t) n=1 M. Sabahi et al. – This paper developed an optimal solution to the harmonic reduction problem in a cascade multilevel inverter with non equal DC sources using genetic algorithm. In this paper switching angles are generated for different values of Modulation index, considering minimum voltage total harmonic distortion (THD), where as selected harmonics are controlled within the allowable limits at all desired modulation indices. The computed angles are used in simulated circuit in MATLAB/Simulink to validate the results. N. Yousefpoor et al. proposed the optimal minimization of total harmonic distortion (OMTHD) technique, which is an efficient method for determining the switching angels to minimize the total harmonic distortion (THD). This paper compares OMTHD technique in cascaded multilevel inverter with constant DC sources & adjustable DC sources which can be controlled. Comparison results show that employing adjustable DC sources which can be controlled results in substantial improvement in the harmonic minimization. Burak Ozpineci & Leon M-Tolbert et al. developed p a genetic algorithm (GA) optimization technique which is applied to multilevel inverts for eliminating some higher order harmonics while maintaining the required fundamental voltage. This technique can be applied to multilevel inverters & the optimum switching angels are calculated to eliminate the harmonics. Then these angles are used in an experimental setup to validate the results. K. Haghdar et al.focuses an elimination of harmonics in a H- bridge multi level voltage source inverter. The basic concept is to eliminate specific harmonics by the proper selection of switching angle. In this paper generalized pattern search (GPS), Simulated Annealing (SA) & genetic algorithm (GA)are used to determine proper switching angles to eliminate harmonics in 13 level inverter. The proposed methods can be applied to all of the multilevel inverter. The simulation results showed that GPS & SA methods are more efficient than GA. S.H.Fathi & N.Yousefpoor et al. proposed an optimized harmonic stepped waveform technique (OHSW) for reducing harmonics in multilevel inverter. This technique is very efficient for improving the quality of multilevel inverter output voltage. In this paper three phase & single phase OHSW techniques are applied to the cascaded multilevel inverter with adjustable DC sources and constant DC sources .Comparison show that employing adjustable DC sources which can be controlled results in substantial improvement in the total harmonic distortion. John N. Ehissan et al.describes a method to compute the switching angles in a multilevel converter to produce the required fundamental voltage. (1) Where, Vn is the amplitude of nth harmonic. Switching angles are limited between zeros and (0 Because of odd quarter- wave Symmetric characteristics, harmonics with even order become zero. Consequently, Vn becomes: { ∑ ( } For odd ns 0 for even ns (2) The Fourier expansion for generated voltage waveform using the SHE-PWM method is given be V( ∑ (cos(n ) + cos(n ) + cos (n )+….cos(n ( The switching angles condition: - 0 (3) must satisfy the following (4) The number of harmonics which can be eliminated from the output voltage of the inverter is (s-1). For example, to eliminate the fifth –order harmonics for a five level inverter, equation set (5) must be satisfied. Note that the elimination of triple harmonics for the three –phase power system applications is not necessary, because these harmonics are automatically eliminated from the linevoltage cos + cos = ( cos( + cos( =0 (5) In (5), modulation index M is defined as M= and is the fundamental of the required voltage. IV. REVIEW ON THE APPLICATION OF D IFFERENT T ECHNIQUES IN MULTILEVEL INVERTER Said Barkati et al. – This paper presents two evolutionary algorithms for the optimized harmonic stepped waveform. Genetic Algorithm & Particle Swarm Optimization are applied to compute the switching angels in a three phase seven level inverter to produce the required fundamental voltage. In addition to this, these algorithms are also used to solve the starting point problem of the Newton Raphson Method. 32 International Journal of Emerging Technology and Advanced Engineering Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 3, Issue 2, February 2013) In these a complete analysis is done for a seven level converter, Where for a change of modulation index M1,the switching angles can be chosen to produce the desired fundamental voltage V1=M1(4sVdc/ while making the fifth & seventh harmonics identical zero. Ayoub Kavousi et al. present the Bee optimization method for harmonic elimination in a cascaded multilevel inverter. The main objective in selective harmonic elimination pulse width modulation is eliminating low order harmonics by solving non linear equations. In this paper, the Bee Algorithm (BA) is applied to a seven level inverter for solving the equations. The algorithm is based on food foraging behavior of a swarm of a honeybees. This method has higher precision & probability of convergence than genetic algorithm (GA). MATLAB Software is used for optimization comparisons of GA & BA. H. Taghizadeh and M. Tarafdar Hagh present the novel particle swarm optimization technique to determine the optimum switching angles of multilevel converters to produce the required fundamental voltage while at the same time not generate lower order harmonics. This optimization method is applied to transcendental equations characterizing the harmonic content to minimize low order harmonics. Comparing the results of PSO with mathematical. Methods for seven and eleven level inverter. It finds that PSO can simply find the optimum switching angles and also with comparing with Genetic Algorithm and it is clear that PSO has faster convergence with better quality solutions than GA approach to solve this problem. Mehrdad Tarafdar Hagh et al. developed an algorithm based on species-based PSO (SPSO) to deal with the problem where the number of switching angles is increased and their determination using conventional iterative methods in addition to GA and simple PSO techniques is not possible. So an MSPSO algorithm with adaptive adjustment of niche radius has been proposed to determine the optimum switching angles of multilevel inverters. Simulation and experimental results are provided for an 11level cascaded H-bridge inverter to validate the accuracy of computational results. Results show that all undesired harmonics up to 50th order have been effectively minimized at the output voltage waveform of inverter. Comparison of results with active harmonic elimination technique shows that the THD and the switching frequency of output voltage decreased dramatically. M. Sarvi M. R. Salimian used two algorithms: 1-genetic algorithm and 2- PSO is used to optimize THD in Multilevel inverters. Theoretical and simulated results are used to compare these techniques. Also in this paper proposes a method for optimization of specific harmonics and improving the characteristics of switching in multilevel inverters. In this method, the switching angle of each levels and the output voltage of them is determined and is used for optimization. Then the effect of changing in the output voltage of each inverter on reduction of one or more specific harmonic is analyzed. In this paper result of GA and PSO is compared and shows that GA is better for optimizing THD in multilevel converters. The amplitude of specific harmonics can be decreased better by changing the amplitude of each level in comparison of assuming constant amplitude. F.J.T Filho & T.H.A. Mateus et al. presents a new approach for real time computation of switching angles using artificial neural networks. The solutions were found off line using Genetic Algorithm (GA) to obtain the data set for use during the training process of the neural network & to explore the advantage of approximate solutions obtained by the GA. The trained neural network is used then for on-line real time determination of the angles. The paper gives details on the approach used together with the simulation & experimental results. S. Sangeetha & S. Jeevananthan proposed a programming tool to calculate the switching angles for eliminating the selected harmonics in a multilevel inverter (MLI).This paper presents the systematic analysis & algorithm based off-line precalculation of switching angle for a range of modulation index using resultant theory. In this the problem of selective harmonic elimination together with the voltage control is formulated as Fourier analysis ,simplified using resultant theory, reduced using Mathematica 6 and then solved by visual C++ programme by calulation of all possible triplent roots . Chuang Liu et al. presents a Stochastic Newton method to compute the switching angle in multilevel converters so as to produce the required fundamental voltage while at the same time not generate higher order harmonics. In this paper stochastic techniques are used to find the possible initial values, such as beginning process of Particle Swarm Optimization (PSO).This synthetic method is called Stochastic Newton Method (SNM), which is applied to solve the selective harmonic elimination problem for multilevel inverters.All sets of solutions are found for found for elevan & nineteen levels converters to show the simplicity,effictiveness & easy implementation of SNM. Zhi Zeying Liu Hui et al. presents the curve fitting techniques (CFT) & MATLAB programmes to determine the values of switching angles. All the initial switching angles are obtained by using MATLAB’S fsolves. 33 International Journal of Emerging Technology and Advanced Engineering Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 3, Issue 2, February 2013) [4 ] T.A. Meynard and H. Foch, "Multi-level choppers for high voltage applications," in Proc. Eur. Con! Power Electron. Appl., 1992, vol. 2, pp. 45-50. 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Wu, ―A generalized half-wave symmetry SHE-PWM formulation for multilevel voltage inverters,‖ IEEE Trans Ind. Electron., vol. 57, no. 9, pp. 3030–3038, Sep. 2010. [11 ] P. N. Enjeti, P. D. Ziogas, and J. F. Lindsay, ―Programmed PWM techniques to eliminate harmonics: A critical evaluation,‖ IEEE Trans. Ind. Appl., vol. 26, no. 2, pp. 302–316, Mar./Apr. 1990. [12 ] J. N. Chiasson, L. M. Tolbert, K. J. McKenzie, and Z. Du, ―Realtime computer control of a multilevel converter using the mathematical theory of resultant,‖ Math. Comput. Simul, vol. 63, no. 3–5, pp. 197–208, 2003. [13 ] Barkati S, Baghli L, Berkouk E.M, Boucherit M.S. "Harmonic elimination in diode-clamped multilevel inverter using evolutionary algorithms". Electric Power Systems Research 78 (2008) 1736– 1746. [14 ] Chuang Liu; Jianze Wang; Dan Luo; Fei Sha; , "Elimination of Harmonics in Multilevel Converters Using Stochastic Newton Method," Power and Energy Engineering Conference (APPEEC), 2010 Asia-Pacific , vol., no., pp.1-4, 28-31 March 2010 [15 ] Taghizadeh, H.; Hagh, M.T.; , "Harmonic Elimination of Cascade Multilevel Inverters with Nonequal DC Sources Using Particle Swarm Optimization," Industrial Electronics, IEEE Transactions on , vol.57, no.11, pp.3678-3684,Nov.2010 [16 ] Hagh, M.T.; Taghizadeh, H.; Razi, K.; , "Harmonic Minimization in Multilevel Inverters Using Modified Species- Based Particle Swarm Optimization," Power Electronics, IEEE Transactions on, vol.24, no.10, pp.2259-2267, Oct. 2009 [17 ] Sarvi, M.; Salimian, M.R.; , "Optimization of specific harmonics in multilevel converters by GA & PSO," Universities Power Engineering Conference (UPEC), 2010 45th International , vol., no., pp.1-4, Aug. 31 2010- Sept. 3 2010 In this paper a method to selected harmonic elimination (SHE) choosing initial values was presented in a singlephase full bridge voltage source inverter.The on-line real time control of SHE was realized. The simulation & the experimental results were separately done by MATLAB. Akira Nabae et al. proposed a new neutral point clamped pulse width modulation inverter composed of main switching devices to clamp the output. In these paper two inveter are compared .The neutral point clamped pulse width modulation (NPC PWM) inverter shows improved result and is appropiate for a wide range variable speed drive.Two inverter are compared analytically & experimentally .The neutral point clamped pulse width modulation inveter adopting new PWM techniques was used to eliminate higher order harmonics . Jagdish Kumar et al. implement the selective harmonic elimination method at fundamental switching frequency using Newton Raphson method that produces all possible solution sets. The proposed technique has many advantages such as it can produce all possible solution sets for any numbers of multilevel inverter without much computational burden, speed of convergence is fast. The proposed technique was successfully implemented for computing the switching angles for a seven level & eleven level cascade multilevel inverter. V. CONCLUSIONS The different techniques that is GA,PSO OSHW etc has been proposed to solve SHE problem with equal and non equal dc source in H-bridge cascaded multilevel inveter.The proposed methods is able to find the optimum switching angles in a simple manner. They also reduce the computational burden and running time and ensure the accuracy and quality of the calculated angles. REFERENCES [1 ] John N. Chiasson, Leon M. Tolbert, Keith J. McKenzie, Zhong Du, ― A Complete solution to the harmonic elimination problem‖, IEEE transactions on power electronics, Vol. 19, No.2, pp. 491-498, March 2004. [2 ] Jose Rodriguez, Jin-Sheng Lai and Fang Zheng, ―Multilevel Inverters: A survey of topologies, Control applications,‖ IEEE transactions on Industrial Electronics, Vol.49, No. 4, pp. 724738,August 2002. [3 ] M. Marchesoni, M. 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