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SHE-PWM with overmodulation mode in a three-level NPC inverter

The well known Selective Harmonic Elimination pulse width modulation (SHE-PWM) is widely used by medium voltage drive applications due its symmetry and control of the low order harmonics that affect output torque, even with low switching frequency. However, the complete harmonic elimination limits the maximum output voltage available, a critical problem for applications where the dc bus voltage level directly affects system cost. This paper proposes a method that extends SHE-PWM maps by calculating switching angles for higher modulation indexes, keeping control of the low-order harmonics level and total harmonic distortion (THD). The proposed method consists in a modification of the non-linear equations by adding a term that relax the harmonic elimination rule, allowing a defined limit level for each low order harmonic and increasing the fundamental output voltage. The obtained results are compared with the classic method Space Vector pulse-width modulation (SV-PWM) that provides higher output voltage but with no control of the low order harmonics [1]. Results showed that the method is able to provide more than 2% higher voltage in the output, keeping low order harmonics smaller than SV-PWM. Real world results are presented using Digital Signal Processor (DSP) to verify the real gain of the proposed method.