Design and Simulation of Simplified Five-Level and Seven-Level Inverters Using Modified PWM For PV Applications Bhavani Gandarapu PG Student, Dept.of EEE Andhra University College of Engg Vishakapatnam, India bhavanigandarapu@gmail.com N.Prema Kumar Assoc.Prof, Dept.of EEE Andhra University College of Engg Vishakapatnam, India Abstract In this paper, Single phase simplified Voltage source multilevel inverter topologies using a modified Sinusoidal Pulse Width Modulation (SPWM) for photovoltaic applications are designed and simulated to overcome the limitations of multilevel inverters( complex, requiring more number of power electronic switches and passive components).a modified PWM control scheme with multiple reference signals and a carrier signal has been used to generate the PWM switching signals. The validity of the proposed inverter topologies based PV system is verified through simulation using MATLAB Simulink. To assess the single-phase simplified inverter topologies, they are compared with the conventional single-phase three-level inverter. The simplified topologies produce improved output with significant reduction in the number of power devices. technique that uses multiple reference signals and single carrier. it uses solar energy system consists of PV array, boost converter and modified PWM Inverter based H-bridge which are discussed in the following sections. II. PHOTOVOLTAIC POWER CONVERSION YSTEM The conventional single- phase three-level inverter based PV System shown in Fig.1. It consists of PV array, a dc-dc boost converter, load/grid and H-bridge as a power stage Keywords PhotoVoltaic system; Cascaded H-bridge Inverter; Sinusoidal Pulse WidthModulation I. INTRODUCTION PV Array DC-DC Boost Converter Single H-Bridge Inverter LOAD The demand for renewable energy has increased significantly over the years because of energy crisis. Solar energy and wind energy are promising among them due to advancement in power electronics semiconductor technology. Photovoltaic energy is clean, non polluting, noiseless, abundant in availability.a single power semiconductor device cannot handle large power, high voltage. It encouraged the usage of multilevel inverters over conventional topologies particularly for the renewable energy applications like Wind, PV systems. Multilevel converters produce high output with minimum losses [1]-[4]. In photovoltaic power conversion system, the dc output power of the photovoltaic array will be converted into ac output using an inverter. Various topologies and modulation techniques for the multilevel inverters are reported in literature [5]-[12]. This paper aim is to design and simulate Single-phase simplified five-level and seven-level multilevel converters with modified PWM control scheme for photovoltaic system. The switching pattern for the H-bridge switches and bidirectional switches is generated using Modified PWM Fig. 1 Conventional H-bridge converter based PV System. The Photo Voltaic (PV) cell converts the light energy into electrical energy and relation between the output current and output voltage is not linear as given in (1) and its output changes with climate conditions. PV cell modeled using single diode model as shown in Fig. 2[1],[16]. V I R s V I. R I I s ph Is exp 1 (1) m VT Rp Where Iph is Photo current, I D is Diode current, m is Diode ideality factor, v T is Thermal Voltage, T is absolute temperature, k is Boltzmann constant, E is charge of an electron, R s is series resistor, R p is parallel resistor. As each PV cell produces very low output voltage we have use PV modules in series/parallel to get required output and IGBT/MOFET based boost converter increases its output. Sinusoidal Pulse Width Modulation (PWM) based H-bridge inverter is used to convert it into desired AC and is fed to the load.it generates the output voltage having three values/levels Fig.3 : zero, positive (+Vdc), and negative ( Vdc) (assuming that Vdc is the supply voltage) which is poor ac output[12]- 1288 www.ijaegt.com
[16]. Iph ID IP I Rs capacitor voltage divider formed by C1, C2, and C3 (three bulk capacitor banks), PV array, dc-dc converter and load. That is, one additional bidirectional IGBT switch to Simplified single-phase five-level inverter topology produces the sevenlevel ouput[13]-[14]. DC Diode RP V Fig. 2 PV single diode model To overcome this limitation, a simplified inverter topology having an H-bridge stage with an auxiliary bidirectional switch in Fig. 4 used as a power stage and Dual reference modulation technique (uses two reference signals instead of one) is used to produce PWM switching signals for the fullbridge inverter switches and bi-directional switch to produce five level output voltage levels: zero, +1/2Vdc, Vdc, -1/2Vdc and Vdc. The bi-directional switch consists of four diodes and a IGBT switch is used between the dc bus capacitors and the full-bridge inverter Fig.7. This topology produce better output compared to 3-level.The modified power circuit and modulation techniques are reported in [13]-[14]. Fig 3.Simulated output voltage of 3-level inverter Fig.5 Single-phase seven level inverter topology The modified Sinusoidal PWM modulation technique uses three reference signals and one carrier signal to generate the PWM switching signals as shown in Fig.6.The reference voltages are compared the carrier wave and firing pulses for the 6 switches are generated. Switches S1, S3, S5, and S6 would be switching at the rate of the carrier signal frequency, whereas S2 and S4 would operate at a frequency that was equivalent to the fundamental frequency. Table 1 shows the switching combinations to generate seven-level output. Output Voltage S1 S2 S3 S4 S5 S6 Vdc ON OFF OFF ON OFF OFF PV Array DC-DC Boost Converter Modified PWM Inverter LOAD 2Vdc/3 OFF OFF OFF ON ON OFF Vdc/3 OFF OFF OFF ON OFF ON 0 OFF OFF ON ON OFF OFF Fig. 4 PV System using Modified converter topology -Vdc/3 OFF ON OFF OFF ON OFF In general, to generate a seven level output waveform we require minimum of eight power electronic switches in case of H-bridges with unequal dc sources and twelve power electronic switches H-bridges with equal dc sources. Simplified single-phase five-level inverter topology discussed above uses one conventional H-bridge inverter and one bidirectional switch to produce five-level output. Whereas the simplified single-phase seven-level inverter topology[14] shown in Fig. 5 comprises a conventional H-bridge inverter and two bidirectional switches as a power stage and a -2Vdc/3 OFF ON OFF OFF OFF ON -Vdc OFF ON ON OFF OFF OFF Table 1 Switch combinations to generate seven-level output 1289 www.ijaegt.com
Fig. 6 PWM Switching signal generation Simplified 5-Level inverter based PV system consists of PV array, dc-dc boost converter, power stage (conventional fullbridge with bidirectional switch) followed by load/grid. Each section is designed and simulated. The firing signals/pulses required for the IGBT switches in inverter are generated using modified PWM technique discussed in the previous section. Fig.8 shows the Model of 5-level multilevel inverter based PV system that uses an H-bridge stage with an auxiliary bidirectional switch is used as a power stage. An auxiliary circuit consists of four diodes and a IGBT switch is used between the dc bus capacitors and the full-bridge inverter. III. IMPLEMENTATION IN MATLAB In order to verify that the simplified inverter topologies with modified PWM control scheme can be practically implemented in a PV system, simulations were performed by using MATLAB SIMULINK. It also helps to confirm the PWM switching strategy.the complete PV system discussed in previous sections consists of PV array, dc-dc boost converter, power stage ( conventional full-bridge with bidirectional switches ) followed by load/grid. Each section is designed and simulated. The firing signals/pulses required for the IGBT switches in inverter are generated using modified PWM technique. PV cell is modeled using single diode model where a constant current source will be in parallel with both diode, Rp and is in series with Rs.The resultant PV output voltage is 160V, for the parameters of I=1000, Rp=10k and Rs = 0.001. Fig. 7 depicts the output of the boost converter as the output of the PV cell is very low and using the boost converter the output can be increase to feed the load. In boost converter MOSFET is used as switch with 10kHz switching frequency and 50% duty cycle. The designed boost converter component values are L= 6.25mH, Rs= 0.01 and C=320 F and increases Vpv= 160V to 300V which will be the dc-link voltage for the simplified multilevel inverter. Fig. 8 Model of 5-level multilevel inverter based PV system Fig 9 depicts Simulation results of PWM and PV system output voltage and also we can observe that Dual reference modulation technique (uses two reference signals instead of one) is used to produce PWM switching signals for the fullbridge inverter switches and auxiliary switch. The reference signals are Vref1=1V and Vref2=1V-0.5V=0.5V and Vcarrier=0.5V and R L=1k.The modified voltage source inverter produces 5-level output voltage consists of five voltage levels(+300v, -300V, 0V, +150V, -150V ). Fig. 7 Simulated PV followed by Boost converter output voltage Fig. 9 Simulation results of PWM and PV system output voltage 1290 www.ijaegt.com
Fig 10 shows the Model of 7-level multilevel inverter based PV system that uses conventional full-bridge stage with two auxiliary bidirectional switch is used as a power stage. An auxiliary circuit consists of four diodes and a IGBT switch is used between the dc bus capacitors and the full-bridge inverter. level output voltage consists of seven voltage levels (+300V,+200V, +100V, 0V, -100V, -200V, -300V). The modified H-bridge topology is significantly advantageous over the other inverter topologies of the same number of levels: less number of power switches, less anti parallel diodes and less capacitors. Fig. 10 Model of 7-level multilevel inverter based PV system Fig 12 Simulated output voltages of PV, Boost converter and inverter section IV. CONCLUSIONS Single diode model of PV cell is modeled and simulations are obtained. DC-DC Boost converter is designed and simulations are obtained for the same. Designed and simulated Single-phase simplified five-level and seven-level multilevel converters with modified PWM control scheme for photovoltaic system. It produces seven-level and five-level output with less number of power devices that results in improved output/performance, decrease in cost and switching losses. Fig.11 PWM switching signals for 7-level multilevel inverter. In Fig. 11 we can observe that three reference signals are compared with one carrier signal and the six firing signals/pulses required for the IGBT switches in inverter are generated using this modified PWM technique. Fig.12 depicts the modified PWM inverter output voltage across the load R L=1k which is a seven level output voltage i.e improved output waveform compared to conventional single phase three-level inverter output and cascaded fivelevel inverter outputt. PV output is increased to 300V by dc-dc boost converter and it will be input for the modified inverter topology. The modified voltage source inverter produces 7- V. REFERENCES [1] Integration of Renewable Energy Sources: A Survey IEEE Transactions On Industrial Electronics, Vol. 53, No. 4, August 2006. [2] Rodrigo Guido Araujo, Dr. Stefan Krauter, Kim Onneken and Dr. Volker Quaschning, Photovoltaic Energy Systems, Solar- Modules. [3] J.Gow and C.Manning, Controller arrangement for boost converter systems sourced from solar photovoltaic arrays or other maximum power sources, IEEE Proc. Electr. Power Appl., vol. 147, no.1, pp. 15-20, Jan.2000 [4] J. S. Lai and F. Z. Peng, Multilevel converters A new breed of power converters, IEEE Trans. Ind Applications. vol. 32, pp. 509 517, May/June 1996. [5] J. Rodriguez, J.-S. Lai, and F. Z. Peng, Multilevel inverters: A survey of topologies, controls, and applications, IEEE Trans. Ind. Electron., vol. 49, no. 4, pp. 724 738, Aug. 2002. [6] Leopoldo G. Franquelo,Jose Rodríguez, Jose I. Leon,Samir Kouro, Ramon Portillo,And Maria A.M. Prats The Age of 1291 www.ijaegt.com
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