Hybrid Control Strategy For BCD Topology Based Modular .

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Circuits and Systems, 2016, 7, 1441-1454Published Online June 2016 in SciRes. 0.4236/cs.2016.78126Hybrid Control Strategy for BCD TopologyBased Modular Multilevel InverterVasudevan Karthikeyan1, Venugopal Jamuna21Electrical and Electronics Engineering Department, Dhanalakshmi Srinivasan College of Engineering andTechnology, Chennai, India2Electrical and Electronics Engineering Department, Jerusalem College of Engineering, Chennai, IndiaReceived 28 March 2016; accepted 20 April 2016; published 9 June 2016Copyright 2016 by authors and Scientific Research Publishing Inc.This work is licensed under the Creative Commons Attribution International License (CC tractIn this paper, a Binary Coded Decimal (BCD) topology of modular multilevel inverter with reducedcomponent count is proposed. For the control of this inverter, hybrid control strategy is used. Theproposed modular multilevel inverter uses asymmetrical dc sources and reduced number ofswitches topology. This hybrid modulation technique uses the multicarrier based Pulse WidthModulation (PWM) and the fundamental frequency modulation strategy. The hybrid modulationalgorithm is implemented with “NUC140” micro-controller. In comparison with the conventionaland some of the recently reported inverter topologies, the proposed inverter topology is able togenerate high number of voltage levels in the output by using minimum number of componentssuch as dc sources, power switches and driver circuits. This inverter offers significant performance with less number of components. The feasibility of the proposed topology is confirmed bysimulation and experimental results.KeywordsMultilevel Inverter, Reduced Components, Hybrid PWM, Asymmetrical dc Sources, HarmonicDistortion1. IntroductionA Multilevel Inverter (MLI) is an array of power semiconductor switches. It has gained increasing attention inindustry and research since it was introduced in the 1980s [1]. The advantages of this method over the conventional two-level inverter approach are: improved output power quality, lower output harmonics, lower voltagestress on the switches and load, improved amplitude of fundamental components and lower electromagnetic inHow to cite this paper: Karthikeyan, V. and Jamuna, V. (2016) Hybrid Control Strategy for BCD Topology Based ModularMultilevel Inverter. Circuits and Systems, 7, 1441-1454. http://dx.doi.org/10.4236/cs.2016.78126

V. Karthikeyan, V. Jamunaterference [2] [3]. The concept of MLI is to add several small dc sources with appropriate switching sequence tothe array of switches so as to obtain a stepped waveform which resembles the ac sine waveform. Hence inverteroperation is obtained. As the number of steps in the output waveform increases, these merits will be enhanced.There are three types of commonly used MLI topologies: Neutral point or diode clamped (NPC) [4], flyingcapacitor (FC) [5] and cascaded H-bridge (CHB) [6] multilevel inverter. Among these MLI topologies CHBmultilevel inverter (CHBMLI) becomes more popular, because of its superior trustworthiness arising from itsmodularity and lesser number of hardware components. Series connection of several H-bridge inverters forms aCHBMLI. Each H-bridge can generate three voltage levels in the output. These inverters are classified as:Symmetric (each H-bridge is fed by equal dc sources) and Asymmetric (each H-bridge is fed by unequal dcsources) MLIs. An asymmetric CHBMLI configuration is preferred to produce more number of output levelswith the same number of power switches. Basically, there are two asymmetrical configurations: binary and ternary. There are various other asymmetrical MLI topologies proposed by many researchers [7]. Since the MLItopologies require reduced voltage stress on the switching devices, we can realize the high power inverters withlow power matured semiconductor technology [8]. MLIs have been used in many applications, such as adjustable speed drives, liquefied energy gas (LNG) plants, power quality devices and renewable energy generationsuch as photovoltaic, wind and fuel cells [8]-[11].The power quality of the MLI depends on the number of levels in the output. The main disadvantage of MLIis the requirement of more number of power switches and associated driver circuits with the increase in thenumber of levels at the output. This increases the circuit complexity and the overall cost and size of the system.Creation of asymmetrical dc sources is another challenge of MLI. With hybrid renewable energy generation,various voltage levels produced by different energy sources can be used as the dc sources of asymmetrical MLI.It avoids the use of flying capacitors or boost converters stage of the conventional hybrid energy generation system. To conquer these disadvantages, many topologies are introduced with reduced number of switches [7][12]-[21]. Major disadvantage with the conventional MLI is the requirement of small isolated dc voltage sourcesor series bank capacitors. This disadvantage is overcome by using the renewable energy generation sources.The modulation technique used to generate the gating signals is very vital to attain high performance controlin the MLIs. Various modulation techniques have been introduced to improve the performance of the MLIs [2][11] [22]-[27]. The commonly used modulation techniques are selective harmonic elimination (SHE) [22]-[24]carrier-based PWM (CBPWM) [25] and space vector PWM (SVPWM) [26] [27]. In [28], the MLI performancesof a SVPWM and a carrier based PWM are compared, and a carrier based PWM method is proposed to obtainan optimal output voltage in the MLI.In this paper, a BCD topology of modular multilevel inverter with unequal dc sources and reduced number ofswitches is proposed. The performance of the proposed inverter is controlled by the hybrid control strategy. Thistopology requires lesser number of power switches, power diodes and associated driver circuits than the conventional topologies. These advantages are proved by comparing the proposed topology with the conventionalsymmetric and various asymmetric optimal topologies reported in the literature. The power loss and conversionefficiency are estimated from the simulation results. Finally, the experimental results of a 9-level inverter arepresented and these validate all the theoretically obtained results.2. Structure of BCD Topology InverterFigure 1(a) shows the generalized circuit diagram of proposed 9-level inverter with reduced number of switches.The functional block diagrams of hybrid energy generation system with symmetrical MLI and Hybrid energygeneration with proposed asymmetrical MLI are given in Figure 1(b) and Figure 1(c). The proposed topologyof inverter is built by the cascaded connection of a number of sub-modules. Each sub-module consists of twounidirectional switches and one dc source. The switches in one sub-module should not be turned-on simultaneously. The dc sources are considered to follow the sequence of values like 1:20:21: . :2n (BCD numbers).These asymmetrical sources are derived from the various renewable energy sources (RES). It avoids the intermittent stage of boost converters (BC). One of the issues in MLIs is controlling the input dc source voltages.Several techniques were used to control the input dc voltages of sources [22]. A MLI with unequal dc sourcesallows lesser freedom of choice in controlling the input voltages based on the switching states. The operation ofthis topology divides the circuits into two parts: Magnitude or Level generator and Polarity generator. For9-level inverter, this topology uses two sub-modules (SM1, SM2) and three asymmetrical dc sources (Vdc, 20Vdc,21Vdc) in the magnitude or level generator part. For higher levels of output, the number of sub-modules can be1442

V. Karthikeyan, V. Jamunalcascaded in the level generator part. The level generator part is responsible for generating all the possible positive voltage levels of the output by proper switching function of switches in the level generator part. Table 1summarizes the parameters used in the present work.The polarity generator is a simple full H-bridge (HBM) which provides the positive and negative polarities tothe output of the level generator alternatively, so that the alternating output voltage is obtained. Switches SH1and SH2 are turned ON and SH3 and SH4 are turned OFF during the positive half cycle while switches SH3 andSH4 are turned ON and SH1 and SH2 are turned OFF during the negative half cycle. The switches in this polaritygeneration part are operated at the fundamental (line) frequency. Table 2 shows the switching patterns used togenerate various levels of output voltage. Switching status ‘0’ and ‘1’ represents the switch OFF and ON conditions. ‘x’ represents don’t care condition (either “0” or “1”).Each sub-module in the level generation part can generate one positive level and zero level output voltage.“m” numbers of dc sources are required for “N” levels in the output of the inverter,N 2m 1(1)n m 1(2)Number of sub-modules (n) required is given by,Figure 1. Proposed 9-level inverter (a) Circuit diagram (b) Hybrid energy generation with symmetrical MLI (c) Hybridenergy generation with proposed asymmetrical MLI.Table 1. Parameters of proposed inverter.ParameterValueNo. of Levels9No. of Sub-Modules2Input dc Sources3No. of Switches8Load50Ω1443

V. Karthikeyan, V. JamunaTable 2. Switching functions of proposed inverter.LevelOutput VoltageMagnitude Generator Sa1 Sb1 Sa2 Sb2Polarity Generator SH1 SH2 SH3 110001194Vdc10100011Maximum output voltage obtainable is,Vo max N 1( 2 ) V ( 2 ) Vm 1dcdc(3)where, Vdc is the dc source voltage.Number of power switches (Nsw) required for a single phase inverter is, log ( N 1) 0.301 N sw 2 ( m 1) 4 40.1505 (4)Number of driver circuits required is,N driver N sw(5)In the proposed 9-level inverter, four levels of positive output voltage are generated by the level generatorpart. As the switch Sb of each sub modules (SM1 & SM2) conducts, the output voltage level of Vdc is obtained.As the switches Sa of SM1 and Sb of SM2 are operated, the output voltage level becomes 2Vdc. For the outputvoltage level of 3Vdc, the switches Sb of SM1 and Sa of SM2 are triggered and for the output voltage level of 4Vdc,the switches Sa of each sub modules are triggered. By properly controlling the ON and OFF states of the switches in the H-bridge module, the four-level unidirectional output voltage waveform is converted into bidirectionaleight-level output voltage waveform. The zero level output voltage is achieved by turning ON the switches SH1and SH3 or SH2 and SH4.3. Hybrid Control StrategyTo generate high quality output with MLI, various modulation techniques are used. Out of these techniques,SHE or CBPWM techniques are commonly used because of ease of controllability. With fundamental frequencymodulation, each switch has to be turned ON and OFF once per cycle of the fundamental frequency output. Thisfundamental frequency modulation provides minimal switching loss, but the harmonic contents at the outputvoltage become high. The quality of the output is enhanced in the CBPWM techniques, but the switching lossesand the circuit complexity are increased.The idea behind this hybrid control strategy is to combine the advantages of both the fundamental frequencymodulation and the CBPWM. In fundamental frequency modulation, one carrier signal and one reference signalare used. These signals are compared and produce the switching pulses at fundamental frequency. In CBPWM,phase disposition PWM (PDPWM) is more popular. THD content is much lesser with PDPWM control strategybased MLI. In this paper, a unipolar phase disposition PWM (UPDPWM) is considered as the number of carriersrequired in the UPDPWM is half of the carriers required by the PDPWM. In this modulation technique, theswitches in the polarity generation part are operated at the fundamental frequency and the switches in the levelgeneration part are operated by the UPDPWM. Figure 2 shows the functional block diagram of the proposedhybrid modulation method. It consists of fundamental frequency PWM generator and UPDPWM generator togenerate the modulated control pulses. These pulses are combined by the combinational logic circuit and produce the sequence of switching pulses required for the modular multilevel inverter.1444

V. Karthikeyan, V. JamunalFigure 2. Block diagram of hybrid modulation for a 9-level inverter.3.1. Fundamental Frequency PWM GenerationA basic comparator is used for the fundamental frequency PWM generation. The modulating sine waveform atfundamental frequency is compared with the zero reference and produces the pulses at the fundamental frequency.These pulses are processed by the combinational logic formed by (6)-(9). The pulses produced by the combinational logic circuit are used to control the switches in the polarity generation part of the proposed inverter.G H1 ( P1 P2 P3 P4 ) S1(6)G H2 ( P1 P2 P3 P4 ) S2(7)G H3 ( P1 P2 P3 P4 ) S3(8)G H4 ( P1 P2 P3 P4 ) S4(9)where, GH1, GH2, GH3, GH4 are the gating pulses applied to the switches H1, H2, H3 and H4 in the polarity generation part. S1, S2, S3, S4 are the pulses produced by the fundamental frequency PWM generation part. P1, P2, P3and P4 are the pulses produced by the UPDPWM generation part.3.2. UPDPWM GenerationThe modulating and high frequency carrier signals used for UPDPWM are shown in Figure 3. In UPDPWM,four triangular carrier signals are compared with the unipolar modulating signal. All the carrier signals are inphase, but they are level shifted. The pulses (P1, P2, P3 and P4) produced by the comparators give the level transition point of the output waveforms and these signals are processed by