Local Carrier PWM for Modular Multilevel Converters with Distributed PV Cells and Circulating Current Reduction

Zaid A. Aljawary, Santiago de Pablo, Luis Carlos Herrero- de Lucas and Fernando Martinez-Rodrigo
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Zaid A. Aljawary: Information Technology Department, Faculty of Science and Technology, University of Human Development, Sulaymaniyah 46001, Iraq
Santiago de Pablo: Department of Electronics Technology, University of Valladolid, 47011 Valladolid, Spain
Luis Carlos Herrero- de Lucas: Department of Electronics Technology, University of Valladolid, 47011 Valladolid, Spain
Fernando Martinez-Rodrigo: Department of Electronics Technology, University of Valladolid, 47011 Valladolid, Spain

Energies, 2020, vol. 13, issue 21, 1-21

Abstract: A new topology has been recently proposed for grid-connected photovoltaic (PV) systems, using modular multilevel converters (MMCs) and distributing PV panels throughout the MMC cells. This topology has two main advantages: it reduces the power losses related to moving the energy into the MMC capacitors from an external source, and it removes the losses and costs related to the DC to DC converters used to track the maximum power point on string converters or central converters, because that task is delegated to MMC cells. However, traditional pulse width modulation (PWM) techniques have many problems when dealing with this application: the distortion at the output increases to unacceptable values when MMC cells target different voltages. This paper proposes a new modulation technique for MMCs with different cell voltages, taking into account the measured cell voltages to generate switching sequences with more accurate timing. It also adapts the modulator sampling period to improve the transitions from level to level, an important issue to reduce the internal circulating currents. The proposed modulation has been validated using simulations that show a consistent behavior in the output distortion throughout a wide operation range, and it also reduces the circulating currents and cuts the conduction losses by half. The behavior of this new topology and this new modulation has been compared to the mainstream topology with external PV panels and also to a fixed carrier modulation.

Keywords: pulse width modulation; modular multilevel converter; photovoltaic power systems; distributed PV panels; circulating current reduction (search for similar items in EconPapers)
JEL-codes: Q Q0 Q4 Q40 Q41 Q42 Q43 Q47 Q48 Q49 (search for similar items in EconPapers)
Date: 2020
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