Active harmonic elimination in multilevel converters using FPGA control

This paper presents an optimal total harmonic distortion (THD) control algorithm referred to as active harmonic elimination method for cascaded H-bridges multilevel converter control with unequal DC sources. First, the multilevel converter is decoupled into unipolar converters, the low order harmonics, such as the 5th, 7th, 11th and 13th are eliminated by using elimination theory, and the minimum THD combination of unipolar converters for multilevel converter control is found. Next, the magnitudes and phases of the residual higher harmonics are computed and subtracted from the original output voltage waveform to eliminate these higher harmonics. To validate the proposed algorithm, the method is simulated by Matlab first. After the simulation, an experimental 11-level H-bridge multilevel converter with a real-time controller based on Altera FLEX 10 K field programmable gate array (FPGA) is used to implement the algorithm with 8 μs control resolution. The experimental results show that the method can effectively eliminate the specific harmonics as expected, and the output voltage waveforms have low THD.