Anticipative Sorting Control of Modular Multilevel Converters

Abstract

This paper describes a new sorting and modulation strategy for Modular Multilevel Converters (MMC), which has the ability to control the switching frequency of the power devices. In order to control the capacitor voltages of individual submodule (SM), the new scheme uses anticipative inserting and bypassing of the MMC submodules based on the arm or load current levels. The MMC arm currents provide an accurate early indication about the capacitor voltage changes, and allow anticipative control action of voltage gradients. In this way, SMs with large voltages may be switched off in anticipation when subjected to fast voltage transients, avoiding protection trips. To this end, the arm or load current is quantized into several levels and the actual current level defines the number of switching events in real time. Next, the list of SMs is voltage-based sorted and several SMs are switched on or off in anticipation, before their voltage reaches the global limit. In this way, more SMs are switched when the arm current is large, i.e. when voltages change fast. This algorithm tends to group together the SM voltages when the voltage increases, while still maintaining an overall low switching frequency. The paper presents a detailed description of this current level anticipative sorting (CLAS) and analyses its performance with respect to the conventional sort-and-switch algorithm. Experimental results on a low power MMC setup with 24 SMs per phase validate the proposed CLAS algorithm.