Comparative Analysis of Flying Capacitor and Cascaded Multilevel Inverter Topologies using SPWM Akhila.A #1, Manju Ann Mathews *2, Dr.Nisha.G.K #3 # PG Scholar, Department of EEE, Kerala University, Trivandrum, Kerala, India. * Assistant Professor, Department of EEE, Kerala University, Trivandrum, Kerala, India # Associate Professor, Department of EEE, Kerala University, Trivandrum, Kerala, India Abstract Multilevel inverters have several advantages over the conventional two level inverters due to their lower switching frequency, reduced switching stresses, low EMI and EMC issues, ability to achieve high voltage levels, low THD etc. Multilevel inverters can synthesize the desired voltage from several levels of DC voltages. This paper presents the comparison of three level Flying capacitor and Cascaded multilevel inverters by analysing the total harmonic distortion. The PWM used here is Sinusoidal Pulse Width Modulation. By comparing the Total Harmonic Distortion the advantages of Cascaded multilevel inverter over the Flying capacitor is confirmed. The simulations were carried out using MATLAB/SIMULINK software. Keywords SPWM,THD,FC I. INTRODUCTION The main function of multilevel inverters is to synthesize a desired voltages from several levels of DC voltages [1].The major multilevel converter classification are[2]-[3]: 1. Diode clamped or Neutral clamped 2. Flying capacitor or Capacitor clamped 3. Cascaded multilevel inverters. The multilevel inverters draw input with low distortion. They can operate with lower switching frequency with lesser harmonics and reduced dv/dt stresses. In the multilevel inverters a sine wave can be approximated to a stepped waveform having larger number of steps so that the harmonic distortion reduces. That is as the number of levels increases the harmonic reduces. They can operate at both fundamental and high switching frequency PWM. The number of levels of an inverter is defined as the number of steps or constant voltage values that can be generated by the converter between the output terminal and an arbitrary reference node called the neutral. The flying capacitor is otherwise called as imprecated cell inverter. This is one of the alternative topology for the diode clamped multilevel inverter. The only difference is that in Flying Capacitor (FC) topology the clamping diodes are replaced by flying capacitors..the main disadvantage of this topology is that the output voltage is reduced to half of the input DC voltage. It requires more capacitance and also as the number of levels increases the control of the system become complex. The Cascaded multilevel inverters are formed by the series combination of single phase H- Bridge inverters. This topology can avoid an extra clamping diode and voltage balancing capacitors compared to others. The main feature is that it uses fewer components than the other types and also it has a simple control. The various applications of multilevel inverters are in industrial medium voltage motor drives, traction drive systems, FACTS, utility interface for renewable energy systems etc. Among the various multilevel inverter topologies mentioned above this paper presents the analysis and comparison of the Flying Capacitor and Cascaded multilevel inverters for resistive load. The research and analysis of the other topologies namely the Diode Clamped, Modular Multilevel Converter is going on. II. PULSE WIDTH MODULATION It is the most popular method of controlling the output voltage.in this method a fixed DC voltage is given to the inverter and a controlled AC output voltage is obtained by controlling the ON and OFF period of the inverter components. The modulation techniques used for high switching frequency PWM are: 1. Space vector modulation (SVM) 2. Sinusoidal PWM The PWM used here is Sinusoidal PWM. The multicarrier modulation used here is Phase Shifted PWM. In the all the (m-1) carrier waves having the same amplitude and frequency but the phase shift may vary according to the level of the inverter. Since the topologies discussed are for three level the phase shift between the two carriers is 180 0 which is obtained from the equation phase shift(ϕ s ) is given by: ϕ s ( 1) These are compared with three phase sine waves and the output pulses are fed to the corresponding switching devices. ISSN: 2231-5381 http://www.ijettjournal.org Page 6
III. REVIEW OF FLYING CAPACITOR AND CASCADED MULTILEVEL INVERTERS A. FLYING CAPACITOR MULTILEVEL INVERTER The topology of three phase three level flying capacitor multilevel inverter is shown in Fig.1.This topology is called so because the capacitors floats with respect to earth s potential [4]-[5]. The three level flying capacitor consists of twelve switches that is 2(m-1) switches, (m-1) DC bus capacitance and (m-1) (m-1)/2 balancing capacitors per phase. Considering A phase for obtaining positive voltage the switches S a1 and S a2 are ON and for obtaining negative voltage the switches S a3 and S a4 are ON. The zero level can be obtained in two ways that is either switches S a1 and S a3 are operated or switches S a2 and S a4 are operated. Fig.1Three level Flying capacitor multilevel inverter The PWM is in such a way that in each phase the first and the fourth switches are complementary and the second and third switches are complementary. The three phase reference voltages Va,Vb and Vc are displaced at 120 0 apart and are compared with (m-1) carrier waves having a frequency of 10kHz.The carriers for the first and the second switches are having a phase shift of 180 0 and pulses obtained are fed to the corresponding switching device. B. CASCADED MULTILEVEL INVERTER The topology of three phase three level cascaded multilevel inverter is shown in Fig.2.It consists of twelve switches that is 2(m-1) switches, (m-1)/2 DC bus capacitance. Here clamping diodes and voltage balancing capacitors are absent. Considering A phase for obtaining positive voltage the switches S a1 and S a4 are ON and for obtaining negative voltage the switches S a2 and S a3 are ON. The zero level can be obtained in two ways that is either switches S a1 and S a2 are operated or switches S a3 and S a4 are operated. Fig.2Three level Cascaded multilevel inverter Let Va, Vb and Vc be the three phase reference voltages which are displaced at 120 0 apart. V a =V m Sin (ωt) (2) V b =V m Sin( ωt- V b =V m Sin( ωt+.. (3). (4) The frequency of carrier is 10 khz. The carriers for the switches S a1 and S a2 are 180 0 out of phase. Considering A phase the complementary pairs are switches(s a1,s a2 ) and (S a3,s a4 ). III RESULTS AND DISCUSSIONS The comparison of three level flying capacitor and cascaded multilevel inverters are conducted using MATLAB/SIMULINK. Here the input and switching frequency for both the converters are 800V and 10kHz respectively. The multi carrier phase shifted PWM is used as the basic modulation for both the topologies. A.FLYING CAPACITOR MULTILEVEL INVERTER Fig.3 shows the simulation circuit of three level flying capacitor multilevel inverter. Fig.3Simulation circuit of flying capacitor multilevel inverter Fig.4 shows the Sinusoidal Pulse Width Modulation. Fig.5 shows the PWM input and switching pulses for ISSN: 2231-5381 http://www.ijettjournal.org Page 7
Fig.4 Sinusoidal PWM Fig.7.Line voltage Fig.5 PWM input and switching pulses for A phase Fig.8.Line voltage THD Fig.9.Phase voltage THD Fig.6.Phase voltage B.CASCADED MULTILEVEL INVERTER A phase. Fig.6 shows the phase voltage and Fig.7 shows the line voltage respectively.fig.8 and Fig.9 shows the line voltage and phase voltage THDs respectively. The line and phase voltage THDs are 1.02% and 1.33% respectively. Fig. 10 Simulation circuit of cascaded multilevel inverter ISSN: 2231-5381 http://www.ijettjournal.org Page 8
Fig.10 shows the simulation circuit of three level cascaded multilevel inverter.fig.11 shows the SPWM. Fig.14Phase voltage Fig.11.Sinusoidal PWM Fig.12PWM input & switching pulses for A phase Fig.15.Line voltage THD Fig.16.Phase voltage THD Fig.13Line voltage Fig.12 shows the PWM input and switching pulses for A phase.fig.13 and Fig.14 shows the line voltage and phase voltages respectively.fig.15 and Fig.16 shows the line and phase voltagethds and their values are 1.02% and 1.02% respectively. IV TABLE 1 Comparison of voltage THD for various multilevel converter topologies Sl N o. Topology 1 Three level Flying capacitor Multilevel inverter Voltage THD(%) Line voltage Phase voltag e 1.08 1.33 2 Three level Cascaded Multilevel inverter 1.02 1.02 ISSN: 2231-5381 http://www.ijettjournal.org Page 9
Table 1 shows the comparison of THD values of Flying capacitor and Cascaded multilevel inverter topologies and from the table given above it is clear that the Total Harmonic Distortion decreases when moving from Flying capacitor to Cascaded multilevel inverter. V.CONCLUSION The simulation of three level Flying capacitor or Capacitor Clamped and Cascaded multilevel inverters was carried out using Phase Shifted PWM.The performance of the inverters was analysed using MATLAB/SIMULINK. From the above results obtained the advantages of Cascaded multilevel inverter over the Flying capacitor multilevel inverter is confirmed. REFERENCES [1] S.Salini, Voltage Clamping in Diode Clamped Multilevel Inverter Using Sinusoidal PWM, International Journal of Engineering Trends and Technology, vol. 6, no. 2, pp. 97-103, Nov.2013. [2] Tolbert. L. M and Pend. F. Z, Multilevel Converter as a Utility Interface for Renewable Energy Systems, IEEE Power Engineering Society Meeting, Vol. 2, pp. 1271-1274,2000. [3] Bose BK. Power electronics an emerging technology. IEEE Transactions on Industrial Electronics, vol.36, no.3, pp. 403 12, Aug 1989.. [4] Akhila.A, Lekshmi.K.R, Dr. Nisha.G.K, Analysis and Comparison of Flying Capacitor and Modular Multilevel Converters Using SPWM, International Journal of Innovative Research and Advanced Studies,vol.3,no.8,pp.352-3571,July. 2016 [5] Yasmeena,Dr.G.Thulasi Ram Das, Simulation Study of the Three-Phase Flying Capacitor Inverters:Modulation Strategies and Applications, International Journal of Electrical Electronics and Telecommunication Engineering, vol.44, no. 2, pp.631-640, Jun 2013. [6] Zedong Zheng,Kui Wang,Lie Xu,Yongdong Li, A Hybrid Cascaded Multilevel Converter for Battery Energy Management Applied in Electric Vehicles, IEEE Trans. Power Electron., vol. 29, no. 7, pp. 3537 3545, Jul 2014. [7] JoseRodriguez,Leopoldo.G.Franquelo,SamirKouro,Jose.I.L eon,ramon.c.portil Multilevel Converters an Enabling Technology for High power Applications, IEEE Trans. Ind. Applicat., vol. 97, no. 11, pp. 1786-1816,Nov 2009 [8] Fei Wang, Sine-Triangle versus Space-Vector Modulation for Three-Level PWM Voltage-Source Inverters, IEEE Trans. Ind. Applicat., vol. 38, no. 2, pp. 500-506, Mar/Apr2009 [9] Akhila. A, Manju Ann Mathews, Nisha. G.K, Total Harmonic Distortion Analysis of Diode Clamped Multilevel Inverter with Resistive and Inductive Load, International Journal for Scientific Research and Development,vol.4,no.4,pp.1279-1283,June. 2016. ISSN: 2231-5381 http://www.ijettjournal.org Page 10