Modified predictive sorting algorithm for full-bridge MMC in HVDC application

Abstract

High-voltage direct current (HVDC) transmission application necessitates the use of high voltage rating IGBTs in modular multi-level converters (MMC). This makes the use of pulse width modulation (PWM) infeasible, since the frequency rating of the high voltage switches available in the market is very low. The natural alternative is the use ofnearest level control (NLC) methods, where the submodules (SM) are switched in/out in the MMC arms depending on a sorting algorithm based on the SM capacitor voltages and arm currents. However, the conventional sorting algorithm either fails to provide a tight grip on the capacitor voltages or does that at the cost of a switching frequency much higher than the fundamental. The predictive sorting algorithm (PSA), used for half-bridge MMCs, is one method which helps to achieve a compromise between the two. For HVDC application, though, the MMC may consist of full bridges (FB) in order to handle DC faults. Hence this paper proposes an extension of the PSA for its use in full-bridge MMC (FB-MMC). The modified algorithm is tested on a FB-MMC based HVDC test system and a comparison of the switching frequency patterns obtained with this method with that of the conventional sorting method is presented. The effectiveness of the proposed modified algorithm is tested by subjecting the MMC to changes in power flow directions and reduction in DC voltage. The simulations are performed in PSCAD/EMTDC platform.