An On-Line Selective Harmonic Elimination Modulation Scheme for High-Power Medium-Voltage Current Source Converters

In high-power, medium-voltage (MV) pulse width modulated (PWM) current source converter (CSC) based drives, selective harmonic elimination (SHE) configured with amplitude modulation index control is commonly used to modulate the front-end PWM current source rectifier (CSR), as it possesses the ability to eliminate problematic low-order harmonics, while also offering full control over the DC-link current. To practically implement this variant of SHE, the switching angles solved for at discrete values of the amplitude modulation index, across its full operating range, must be pre-computed off-line, and then stored within a look-up table on the digital controller. As a result of this off-line implementation, conventional SHE configured with amplitude modulation index control features poor dynamic performance and requires the use of a large, memory-exhaustive look-up table. This paper presents an on-line SHE scheme that models each of the necessary independent switching angles as polynomial functions of the amplitude modulation index using curve-fitting techniques. This method of implementation allows for the switching angles to be accurately recomputed in real-time, thus offering improved dynamic performance and eliminating the need for a large look-up table. Additionally, the inherent benefits of conventional SHE are retained. Experimental verification proving the effectiveness of the proposed on-line SHE scheme is provided.