# Introduction Multilevel inverter is based on the fact that sine wave can be approximated to a stepped waveform having large number of steps. The steps being supplied from different DC levels supported by series connected batteries or capacitors. The unique structure of multilevel inverter allows them to reach high voltages and therefore lower voltage rating device can be used. As the number of levels increases, the synthesized output waveform has more steps, producing a very fine stair case wave and approaching very closely to the desired sine wave. It can be easily understood that as motor steps are included in the waveform the harmonic distortion of the output wave decrease, approaching zero as the number of levels approaches infinity. Hence Multi-level inverters inverters offer a better choice at the high power end because the high volt-ampere ratings are possible with these inverters without the problems of high dv/dt and the other associated ones [5]. Nowadays different topologies have been reported for multi-level inverters. They are named as neutral point clamped (diode-clamped) inverter, flying capacitor (capacitor-clamped) inverter and cascaded multi-cell with separated dc source inverter [6]. Among them, the FCMLI does not require isolated DC sides and additional clamping diodes, the snubberless operation is possible and it is easy to be expanded to the multilevel. The FCMLI offers a great advantage with respect to the availability of voltage redundancies. This inverter uses capacitors to limit the voltage of the power devices. They are defined as different combinations of capacitors allowing the charging or discharging of the individual flying capacitors in order to produce the same phase leg voltage. This advantage provides the special opportunity for controlling the individual voltage on flying capacitors [6]- [7]. Many studies have shown that under certain conditions, a simple open loop control guarantees natural balancing of the flying capacitor. n Recent years, there has been an increasing interest in electrical power generation from renewable-energy sources, such as photovoltaic (PV) or wind-power systems [1], [2]. The benefits of power generation from these sources are widely accepted. They are essentially inexhaustible and environmentally friendly. Among the different renewable-energy sources possible to obtain electricity, solar energy has been one of the most active research areas in the past decades, both for gridconnected and stand-alone applications [3]- [4]. clamped DC-DC boost converter have been proposed. This scheme maintains the output voltage stability, reducing the harmonic content in the output there by improving the overall system performance. This paper has been designed using a seven level flying capacitor multilevel inverter with the appropriate closed loop schemes there by THD can be significantly reduced which is confirmed through simulation investigation. For this proposed topology, DC input to the system is a solar energy which is harvested from the Sun based on the irradiation and insolation level. In this system a single stand-alone PV module of 12V capacity is used to obtain the DC voltage. Then this obtained DC voltage is given as the input to the Capacitor Clamped DC-DC Boost Converter. By the boost DC-DC converter topology, the 12V DC is boosted into 48V DC. In order to maintain the constant DC voltage from the boost converter, a closed loop block is designed in which the actual DC voltage is compared with the 48V DC set value. Based on the error signal Then based upon the specified switching frequency of MOSFET, switching pulses are given to the single switch in the boost converter. Then the boosted DC voltage is given as the input to the seven level FCMLI which is used for 48V DC to 48V AC conversion. For The overall Simulation diagram of the proposed closed loop system for FCMLI is shown in Figure 2. The difference between the MBC and the conventional one is that in the MBC, the output is Vc times N, where N+1 is the converter's number of levels taking into account the zero level as shown in Figure 3. This behavior is achieved, thanks to the voltage multiplier in the boost converter's output that is driven by the only switch in the converter. # IV. Flying Capacitor Multilevel Inverter (FCMLI) a) Basic Configuration The FCMLI requires a large number of capacitors to clamp the device (switch) voltage to one capacitor voltage level. Provided all the capacitors are of equal value, the size of the voltage increment between two consecutive legs of the clamping capacitors defines the size of voltage steps in the output waveform, if the voltage of the main dc-link capacitor is Vdc the voltage of the innermost capacitor clamping the innermost two devices is Vdc/ (n-1). The voltage of the next innermost capacitor will be Vdc/(n-1) + Vdc/ (n-1) = 2 Vdc/(n-1) and so on. Each next clamping capacitor will have the voltage increment of Vdc/(n-1) from its immediate inner one. The voltage levels and the arrangements of the flying capacitors in the FCMLI structure assures that the voltage stress across each main device is same and is equal to Vdc/(n-1) for an n-level inverter. V. # Results and Analysis The proposed topology of closed loop system for Flying Capacitor Multilevel Inverter (FCMLI) consists of various models. The corresponding output voltage waveform of 12V PV Panel is shown in the Figure 6. Here Multicarrier PWM in turn PD-PWM technique is employed for generating appropriate PWM pulse for respective switches the MLI. The Figure 10 shows the Pulse Generation Model Using Multicarrier PWM -PDPWM technique for Seven Level FCMLI. III. # Model The Figure 12 shows the comparison of six number of carrier signals and the sinusoidal modulating signal. In which the sinusoidal signal is superimposed over the carrier signals for generating appropriate PWM pulses. The Figure 12 shows the comparison of six number of carrier signals and the sinusoidal modulating signal. In which the sinusoidal signal is superimposed over the carrier signals for generating appropriate PWM pulses. In order to reduce the THD level of MLI output a closed-loop model is developed and its corresponding output voltage is shown in Figure 17. The Figure 18 shows the corresponding output voltage waveform of a step-up transformer (48/230V AC). This AC voltage is fed to the Single Phase Lamp Load. The Total Harmonic Distortion (THD) is used to characterize the linearity of audio systems and the power quality of electric power systems. # S. No # Carrier The formula below shows the calculation for THD on a voltage signal. The end result is a percentage comparing the harmonic components to the fundamental component of a signal. The higher the percentage, the more distortion that is present on the mains signal. (1) From the Figure 19, it is observed that THD value of 37.15% is obtained for Open-Loop Multilevel Inverter. This value can be reduced by Closed-Loop design. The result of THD analysis performed for closed-loop systems is shown in Figure 20. # Conclusion Thus the investigation study for seven level FCMLI was performed with / without closed-loop feedback schemes. In the proposed method, a closed-loop system was implemented for a Seven Level FCMLI. The newly developed design will improve the system performance and maintains the voltage stability there by stable operation can be ensured without more disturbances. The closed loop controller is also designed for Capacitor Clamped DC-DC Boost Converter there by constant output voltage at the output of the boost converter can be achieved. The performance was verified through simulation models by using MATLAB software tool. From the simulation study, the important conclusions are, Investigation Report: The main inference from the proposed concept is, for the developed Seven Level FCMLI system the THD value of open-loop model is 37.15% and for the closed-loop model is 10.18%. Thus the percentage of THD value can be minimized by implementing the closed-loop control for FCMLI there by better performance in its output can be achieved. 1![Figure 1 : Overall Block Diagram of Proposed System](image-2.png "Figure 1 :") 2![Figure 2 : Overall Simulation Model of Proposed System III. Capacitor Clamped Dc-Dc Boost Converter a) Significance of Usage A Capacitor Clamped DC-DC Boost Converter with single switching element -MOSFET is proposed in this paper. Instead of using two or more than two number of switching devices as in the case of conventional half-bridge or full-bridge DC-DC converters, the proposed topology uses only one switching device there by fast switching with less switching loss is possible with this type of converter.](image-3.png "Figure 2 :") 3![Figure 3 : Basic Configuration of DC-DC MBC for Nx or N+1 levels](image-4.png "Figure 3 :") 4![Figure 4 : One Phase Leg of a Seven Level lying Capacitor Multilevel Inverter A one phase leg of seven-level inverter as shown in Figure 4. For a three phase inverter, two more legs of same construction are coupled to the same dclink battery Vdc. In this figure each switch SA1 to SA6 and S'A1 to S'A6 consists of a power semiconductor device (e.g. GTO, IGBT) and an anti-parallel diode. Voltages VC, Vc2, Vc3, Vc4, Vc5, and Vc6 are Vdc, 5/6 Vdc, 2/3 Vdc, Vdc/2, Vdc/3, and Vdc/6 respectively, as n= 7.b) Features of FCMLIThe FCMLI has the following features.i. Voltage stresses across the devices are equal.ii. Switching combination chosen affects the current rating of the capacitor iii. Redundancy of switching combinations. iv. Single capacitor voltage regulating circuit is required.](image-5.png "Figure 4 :") ![They are i. Photovoltaic Generation ii. Capacitor Clamped DC-DC Boost Converter Model. iii. Open Loop FCMLI Model. iv. Closed Loop FCMLI Model. a) Photovoltaic Generation Model The Simulation diagram for the PV Generation by a single stand-alone 12V PV module is shown in Figure 5.](image-6.png "") 5![Figure 5 : MATLAB Model of Photovoltaic Generation It consists of Insolation block, Single PV module block for DC voltage generation and display & scope blocks for displaying the output in numerical and graphical form respectively. The technical specifications of the PV module used in the proposed system are listed in theTABLE II.](image-7.png "Figure 5 :") 6![Figure 6 : Output Voltage Waveform of PV Module](image-8.png "Figure 6 :") ![Clamped Dc-Dc Boost Converter Model From the PV module a 12V DC voltage is obtained. This unidirectional voltage is then given to the Capacitor Clamped DC-DC Boost Converter for boosting up the voltage up to 48V DC. The Simulation diagram of the Capacitor Clamped DC-DC Boost Converter is shown in Figure 7. Thus from the PV Panel, a 12V DC of Unidirectional Voltage is produced. Then this low value of DC voltage is given to the Boost converter for achieving desired DC voltage for giving input to the Multilevel Inverter.](image-9.png "") 7![Figure 7 : MATLAB Model of Capacitor Clamped Boost Converter](image-10.png "Figure 7 :") 8![Figure 8 : Output Voltage Waveform of Boost Converter Model](image-11.png "Figure 8 :") 9![Figure 9 : MATLAB Model of FCMLI The proposed Seven Level FCMLI will produce the seven levels of stepped AC output waveforms at its output. The switching sequence of various switches used in the Inverter model are determined by the PWM pulses from the pulse generation In this Pulse Generation Model, there are six number of carrier signals which are produced by Repeating sequence concept and these signals are compared with the Sinusoidal signal with the help of the relational operator (>=).Here Multicarrier PWM in turn PD-PWM technique is employed for generating appropriate PWM pulse for respective switches the MLI.The Figure10shows the Pulse Generation Model Using Multicarrier PWM -PDPWM technique for Seven Level FCMLI.](image-12.png "Figure 9 :") 10![Figure 10 : Pulse Generation Model for Multilevel Inverter Using Multicarrier (PDPWM) Technique The Figure 11 shows the six number of carrier signals in PD-PWM Pulse Generation Model.](image-13.png "Figure 10 :") 11![Figure 11 : Carrier Signals Used in Pulse Generation](image-14.png "Figure 11 :") ![[0 0.005/20 0.01/20 0.015/20 0.02/20] [-1.5 -2 -1.5 -2 -1.5]](image-15.png "") 12![Figure 12 : PDPWM Modulation Scheme The corresponding open loop output voltage waveform of proposed Multilevel Inverter is shown in Figure 13.](image-16.png "Figure 12 :") 13![Figure 13 : Open Loop AC Output Voltage Waveform of FCMLI From the Open loop output, it is observed that seven level stepped output waveform with the magnitude of nearly 48V is achieved but it is slightly distorted. Because of this the Total Harmonic Distortion (THD) of open-loop output is having more percentage values. Thus in order to reduce the THD level of Multilevel Inverter output and to improve the performance of the system, a closed-loop design is essential for Multilevel Inverter. By implementing the closed-loop system, the THD level can be reduced to certain extend.](image-17.png "Figure 13 :") 3![Carrier Signal Generation For Pulse Generation Model d) Closed Loop FCMLI Model i. Voltage Controller Model For designing closed-loop system for proposed MLI, various blocks such as Voltage Controller block, Sin Generator block are used. The various blocks used in the voltage controller model are Zero-order Hold, gain, Absabsolute, comparator, discrete-time integrator, saturation and rate transition blocks. The Figure 14 shows the voltage controller -PID controller model for closed-loop design of Seven Level FCMLI.](image-18.png "Table 3 :") 14![Figure 14 : Simulation Model of Voltage Controller in Closed-loop System](image-19.png "Figure 14 :") 16![Figure 16 : Single Phase Sinusoidal Output of Sin Generator For Pulse Generation Model Thus after the comparison of Sinusoidal modulating signal and generated carrier signals using PDPWM Technique in Pulse Generation Model, the appropriate PWM switching pulses are generated. These pulses are used to trigger the switching devices in proposed MLI in a proper sequence to produce the stepped AC waveform from the output of MLI.In order to reduce the THD level of MLI output a closed-loop model is developed and its corresponding output voltage is shown in Figure17.](image-20.png "Figure 16 :") 17![Figure 17 : Closed Loop AC Output Voltage Waveform of FCMLI e) Load Voltage Waveform For connecting a Single Phase resistive lamp load, the 48V AC stepped output waveform from the MLI is given to the 48/230V AC step-up transformer.The Figure18shows the corresponding output voltage waveform of a step-up transformer (48/230V AC). This AC voltage is fed to the Single Phase Lamp Load.](image-21.png "Figure 17 :") ![Figure 18 : Output Load Voltage Waveform](image-22.png "Figure") voltage AC signal of 48V AC is given to the step-up transformer for stepping up the voltage level to 230V AC. Finally a resistive load -single phase lamp load is connected across the 230V AC output terminals. b) System Composition The described system in this paper is a standalone system which consisting of the following individual systems : -i. For generating the DC voltage signal, a suitable stand-alone PV module is used. ii. For boosting up the generated DC voltage, an appropriate Capacitor Clamped DC-DC boost converter is used. iii. For maintaining the constant DC output from boost converter, a closed loop controller is used for DC-DC converter. An Investigation Study of Total Harmonic Distortion in a Flying Capacitor Multilevel Inverter With / from the comparator, the PID controller produces the II. Proposed Topology of Closed Loop System a) Architecture of Proposed System In this paper, Proportional-Integral-Derivative (PID) controllers based closed loop system for seven level flying capacitor multilevel inverter and for capacitor iv.control signal to the PWM pulse generator block.controlling the FCMLI, a closed loop blockis designed which consists of voltage measurement,voltage controller -PID controller and Sin generatorblock. With the help of these blocks, a closed loopdesign is achieved for proposed FCMLI. Then this lowa suitable step-up transformer is used. vii. For connecting the developed system to the resistive load, a single phase lamp load is used.Table 1 : Technical Specification of a Proposed d) Simulation Model of Proposed System 2ModuleS. NoParametersSpecificationsUnits1Short Circuit Current (Isc)5.45AAmperes2Open Circuit Voltage (Voc)12VVoltage3Current at Pmax4.95AAmperes4Voltage at Pmax17.2VVoltage5Insolation1000 W/m 2 (Constant)Watts / metre 2 Seven Level Flying Capacitor Multilevel InverterFeedback SchemesWith Closed-Loop SchemeWithout Closed-Loop SchemeTotal HarmonicDistortion (THD) in10.18%37.15%percentage F © 2013 Global Journals Inc. (US) F © 2013 Global Journals Inc. (US) F © 2013 Global Journals Inc. (US) F © 2013 Global Journals Inc. (US) F © 2013 Global Journals Inc. (US) F © 2013 Global Journals Inc. (US) F © 2013 Global Journals Inc. (US) * Power-electronic systems for the grid integration of renewable energy sources: A survey JMCarrasco LGFranquelo JTBialasiewicz EGalvan RCPortillo Guisado MA MPrats JILeon NMoreno-Alfonso IEEE Trans. Ind. Electron 53 4 Jun. 2006 * Power electronics as efficient interface in dispersed power generation systems FBlaabjerg ZChen SBKjaer IEEE Trans. Power Electron 19 5 Sep. 2004 * ERoman RAlonso PIbanez S * Intelligent PV module for grid-connected PV systems DElorduizapatarietxe Goitia IEEE Trans. Ind. Electron 53 4 Jun. 2006 * Highperformance stand-alone photovoltaic generation system R.-JWai W.-HWang C.-Y.Lin IEEE Trans. Ind. Electron 55 1 Jan. 2008 * Matlabbased Simulation & Analysis of Three -level SPWM Inverter KapilJain PradyumnChaturvedi International Journal of Soft Computing and Engineering (IJSCE) 2 1 March 2012 * High performance Flying Capacitor based Multilevel Inverter fed Induction Motor KRamani DrAKrishnan International Journal of Recent Trends in Engineering 2 2 Nov 2009. June 2005 IEEE Trans. Ind. Electronics * High-Performance Stand-Alone Photovoltaic Generation System Rong-JongWai Wen-HungWang Chung-YouLin IEEE Trans. on Ind. Electronics 55 1 Jan 2008 * Grid-Connected Boost-Half-Bridge Photovoltaic Microinverter System Using Repetitive Current Control and Maximum Power Point Tracking ShuaiJiang DongCao YuanLi Fang ZhengPeng IEEE Trans. Power Electronics 27 11 Nov 2012 * Multicell converters: basic concepts and industry applications TMeynard HFoch PThomas JCourault RJakob MNahrstaedt IEEE Trans. Ind. Electronics 49 5 Oct. 2002 * Hybrid Multicell Converter : Topology and Modulation PabloLezana RobertoAceiton IEEE Trans. on Ind. Electronics 58 9 Sep 2011 * MHRashid 2001 Academic Press London * DWKang BKLee JHJeon TJKim DSHyun Asymmetrical carrier technique of