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UT Power Engineering Laboratory - "Powering the Future "

Cascaded Multilevel Inverters: off grid and grid-tied applications


Faete Filho
Dr. Leon M. Tolbert
Burak Ozpineci
Tiago H. A. Mateus
Helder Z. Maia
Joao O. P. Pinto


In the off grid application a new approach for selective harmonic elimination in a 11-level cascaded multilevel inverter with separate DC sources are being studied. The DC sources feeding the multilevel inverter are considered to be varying in time. This method uses genetic algorithms to obtain switching angles offline for different DC source values and uses neural networks to determine the switching angles that correspond to the real-time values of the DC sources for each phase. This implies that each one of the DC sources of this topology can have different values at any time but the output fundamental voltage will stay constant and the harmonic content will still meet the specifications. The switching angles are updated at each cycle of the output fundamental voltage.

A grid tied application is being studied with focus on a single-phase 11-level (5 H-bridges) cascade multilevel DC-AC grid-tied inverter. Each inverter bridge is connected to a 200W solar panel. OPAL-RT lab was used as the real-time control system platform where a Maximum Power Point Tracking (MPPT) algorithm was implemented based on the inverter output power to assure optimal operation of the inverter when connected to the power grid as well as a Phase Locked Loop (PLL) for phase and frequency match. A novel SPWM scheme is proposed to be used with the solar panels that can account for voltage profile fluctuations among the panels during the day. Simulation and experimental results are shown for voltage and current during synchronization mode and power transferring mode to validate the methodology for grid connection of renewable resources.

Cascaded Multilevel Inverter

Fig. 1. 11-Level Cascade multilevel inverter: (a) topology and (b) voltage output waveform generation

Cascaded Multilevel Inverter

Fig. 2. 11 Level cascade multilevel Inverter and Control System

Related publications and presentations

1. Filho, F.J.T.; Mateus, T.H.A.; Maia, H.Z.; Ozpineci, B.; Pinto, J.O.P.; Tolbert, L.M., "Real-time selective harmonic minimization in cascaded multilevel inverters with varying DC sources," Power Electronics Specialists Conference, 2008. PESC 2008. IEEE , vol., no., pp.4302-4306, 15-19 June 2008.

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