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Thursday, 26 April 2018

Modelling and analysis of modular multilevel converter for solar photovoltaic applications to improve power quality

The design of control circuit for a solar fed cascaded multilevel inverter to reduce the number of semiconductor switches is presented in this study. The design includes binary, trinaryand modified multilevel connection(MMC)-based topologies suitable for varying input sources from solar photovoltaics (PV). In binary mode, 2Ns +1 − 1 output voltage levels are obtained where Ns is the number of individual inverters. This is achieved by digital logic functions which includes counters, flip-flops and logic gates. In trinary mode, 3Ns levels are achieved by corresponding look-up table. MMC intends design in both control and power circuits to provide corresponding output voltage levels by appropriate switching sequences. Hence to obtain a 15-level inverter, the conventional method requires 28 switches and in binary mode 12 switches are needed. In trinary mode with the same 12 switches, 27 levels can be obtained whereas in MMC only 7 switches are employed to achieve 15 levels. The advantage of these three designs is in the reduction of total harmonic distortion by increasing the levels. Simulations are carried out in MATLAB/Simulink and comparisons were made. All the three topologies are experimentally investigated for a 3 kWp solar PV plant and power quality indices were measured.



Fig.1 Single stage 15-level inverter power circuit


Fig. 2 Solar PV with partial shaded condition for a 15-level CMLI
a Variation of panel irradiance
b 15-level output voltage waveform

Fig. 3 15-level output voltage waveform achieved from three stage inverter
a 15-level output voltage waveform
b FFT analysis for three stage 15-level CMLI

Fig. 4 Output voltage waveform and its corresponding harmonic spectrum
a 27-level output voltage waveform
b FFT analysis for three stage 27-level CMLI

Fig. 5 Output voltage waveform and its corresponding harmonic spectrum
a 15-level output voltage waveform
b FFT analysis for one stage 15-level CMLI

The power quality improvement for a solar fed CMLI with reduced number of semiconductor switches is investigated in this paper. The required 15-level output is achieved with only 12 switches in binary mode and 7 switches in MMC mode. In addition, 27-level output is obtained with 12 switches through trinary mode. The mathematical model for solar PV is carried out which is considered as the input to the inverter stages. A detailed simulation study is carried out for various levels and comparison has been made. A 3 kWp solar PV fed CMLI is implemented for all the three topologies and harmonics analysis was made. Based on the observations, the proposed method provides the multiple advantages which include reduced THD, less cost, simple design, minimum computational complexity and the absence of transformers, boost converters, detailed look-up table and filter circuit. Moreover, these methods are much suitable for standalone/grid interacted PV systems to improve power quality.
1 Rahim, N.A., Selvaraj, J.: Multistring five-level inverter with novel PWM control scheme for PV application, IEEE Trans. Ind. Electron., 2010, 57, (6), pp. 21112123
2 Selvaraj, J., Rahim, N.A.: Multilevel inverter for grid-connected PV system employing digital PI controller, IEEE Trans. Ind. Electron., 2009, 56, (1), pp. 149158
3 Rahim, N.A., Chaniago, K., Selvaraj, J.: Single-phase seven-level grid-connected inverter for photovoltaic system, IEEE Trans. Ind. Electron., 2011, 58, (6), pp. 24352443
4 Barbosa, P.G., Braga, H.A.C., do Carmo Barbosa Rodrigues, M., Teixeira, E.C.: Boost current multilevel inverter and its application on single-phase grid-connected photovoltaic systems, IEEE Trans. Power Electron., 2006, 21, (4), pp. 11161124
5 Villanueva, E., Correa, P., Rodríguez, J., Pacas, M.: Control of a single-phase cascaded H-bridge multilevel inverter for grid-connected photovoltaic systems, IEEE Trans. Ind. Electron., 2009, 56, (11), pp. 43994406