"SINGLE PHASE MULTILEVEL INVERTER FOR 11 LEVELOUTPUT USING CASCADED H-BRIDGES Multilevel inverters are mainly utilized to synthesis a desired single- or three-phase voltagewaveform. The desired multi-staircase output voltage is obtained by combining several dc voltagesources. Solar cells, fuel cells, batteries and ultra-capacitors are the most common independentsources used. Nowadays, there exist three commercial topologies of multilevel voltage-sourceinverters: diode clamped, cascaded H-bridge (CHB), and flying capacitors (FCs). Among theseinverter topologies, cascaded multilevel inverter reaches the higher output voltage and powerlevels (13.8 kV, 30 MVA) and the higher reliability.Advantages of Cascaded H-Bridge MLI:? Cascaded H-bridge MLI topology requires symmetric switch ratings. ? This topology is efficiently utilized in connecting different power sources to grid system?Cascaded H-bridge MLI’s have been applied where high power and power quality areessential, for example, reactive power compensation applications, photovoltaic powerconversion, uninterruptible power supplies, and magnetic resonance imaging.?Furthermore, one of the growing applications for multilevel motor drives is electric andhybrid power trains.Design and Simulations:To design MLI for 11 level output voltage, M = 2N + 1, results in N = 5 where M is levels ofoutput voltage and N is number of stages/H-bridges. The MATLAB/Simulink model of the 11level output 5 stage cascaded MLI is shown in fig 2 and internal circuitry of single stage is shownin fig. 1.Figure 1- Internal Circuitry of single H-bridgePage 1 of 105 stage cascaded H-bridge MLI for 11 level output voltage Figure 2- Circuit diagram of 5 stage cascaded MLI for 11 level output voltageOptimum Firing Angles/Time for Reduced THD:[1]Firing angles of different stages of cascaded H-bridges are optimized using genetic algorithm.These angles ensure minimum THD for 11 level output voltage ignoring triplen harmonics.Optimal angles and corresponding time for 50 Hz are given in table 1.The simulated output voltage waveform for 11 level voltage using five cascaded H-bridges andthe respective current wave form is shown in fig. 3 and fig. 4 respectively. Page 2 of 105 stage cascaded H-bridge MLI for 11 level output voltage Angle . Time(mS)Radian Degreea 0.0562 3.2201 0.1791 a 0.1687 9.6639 0.5372 a 0.2808 16.0866 0.8943 a 0.4738 27.1474 1.5084 a 0.668 38.2710 2.1265 Table 1 - Optimal firing angles/timingFigure 3- Output Voltage SignalFigure 4- Output current waveform vs pure sinusoidalPage 3 of 105 stage cascaded H-bridge MLI for 11 level output voltage Switches Rating:KVA Rating = 5 KVAOutput Voltage = V = 220 VOutput Current = I = 5K/220 = 22.727 A0 Output Peak Current = I = 32.14 AmStage 1 (S , S S S ):1 2, 3, 4 To calculate current through switches in worst case scenario we assume pure resistive load.3.0853 1 ( ) ???? 1 = ? ?? ??? ?? ???2? 0.0562 = 10.21 A1/2 3.0853 1 2 2 ???? 1 = ( ? ?? ??? (?? ) ??? )2? 0.0562 = 15.77 AStage 2 (S , S S S ):5 6, 7, 8 2.9729 1 ( ) ???? 2 = ? ?? ??? ?? ???2? 0.1686 = 10.08 A1/2 2.9729 1 2 2 ???? 2 = ( ? ?? ??? (?? ) ??? )2? 0.1686 = 15.17 AStage 3 (S , S S S ):9 10, 11, 12 2.8608 1 ( ) ???? 3 = ? ?? ??? ?? ???2? 0.2807 = 9.82 APage 4 of 105 stage cascaded H-bridge MLI for 11 level output voltage 1/2 2.8608 1 2 2 ???? 3 = ( ? ?? ??? (?? ) ??? )2? 0.2807 = 14.55 AStage 4 (S , S S S ):13 14, 15, 16 2.6677 1 ???? 4 = ? ?? ??? (?? ) ???2? 0.4738 = 9.10 A1/2 2.6677 1 2 2 ???? 4 = ( ? ?? ??? (?? ) ??? )2? 0.4738 = 13.4 AStage 5 (S , S S S20):17 18, 19,3.0853 1 ???? 5 = ? ?? ??? (?? ) ???2? 0.0562 = 8.03 A1/2 3.0853 1 2 2 ( ) ???? 5 = ( ? ?? ??? ?? ??? )2? 0.0562= 12.17 ACurrent I I avg rmsStage (A) (A)Stage-1 10.21 15.77Stage-2 10.08 15.17Stage-3 9.82 14.55Stage-4 9.10 13.4Stage-5 8.03 12.17Page 5 of 105 stage cascaded H-bridge MLI for 11 level output voltage Average Value 9.448 14.212Table 2- Current ratings of switchesDiode Ratings:To calculate current through diodes in worst case scenario we assume inductive load wherecurrents lags the voltage by p/2 radians. Calculations are tabulated below.Current I Iaverage rms DiodeD-1 6.873 11.762D-2 5.402 10.949D-3 4.829 13.86D-4 7.1615 12.836D-5 6.3025 11.76D-6 5.402 10.96D-7 4.8316 13.19D-8 6.5905 12.135D-9 5.744 11.76D-10 5.4025 10.996D-11 4.845 12.51D-12 6.03 10.95D-13 4.8278 10.95D-14 4.828 10.54D-15 4.4865 12.51D-16 6.03 9.87D-17 3.9931 10.13D-18 4.2737 10.76D-19 4.5687 11.36Page 6 of 105 stage cascaded H-bridge MLI for 11 level output voltage D-20 5.115 11.6657Table 3- Diode Current ratingsAverage I = 5.376 AD-avg Average I = 11.665 AD-rms FFT Analysis:FFT analysis is carried out fundamental frequency of 50 Hz, whereas maximum frequency forcalculating THD is 2500 Hz and therefore, total number of harmonics used for THD analysis willare 2500/50 = 50. In the fig. 5, THD is calculated considering first 209 harmonics which comesout to be 17.35 %. However, in accordance to the standard for calculating THD first 50 harmonicsgive 17.07 % THD and hence ignoring triplen harmonics:THD = 2.28 %Figure 5- FFT analysisPage 7 of 105 stage cascaded H-bridge MLI for 11 level output voltage "