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.
Highlights
Photovoltaic installations are growing throughout the world, nearly according to Moore’s law, duplicating the cumulative installed PV capacity roughly every two years: about 2 GWp were installed in 2002, 40 GWp were available in 2010 and more than 480 GWp operated all over the world in2018 [1,2]
PhaseDue circulating current (i za) and the DC fault current within the phase-leg of the modular multilevel converters (MMCs)
In order to expand the application of MMCs to medium-voltage and high-voltage renewable energy applications, this paper proposes a new modulation method, named local carrier pulse width modulation (LC-PWM), to deal with different voltages in the MMC cells, and the higher precision of this technique reduces the resulting circulating currents and the output distortion throughout a wider operational range
Summary
Photovoltaic installations are growing throughout the world, nearly according to Moore’s law, duplicating the cumulative installed PV capacity roughly every two years: about 2 GWp were installed in 2002, 40 GWp were available in 2010 and more than 480 GWp operated all over the world in. Energies 2020, 13, 5585 grid-connected PV plants larger than tens of KWp [4,5,6,7], with a market share of about 44% [1,7] This technology generally achieves high productivity with a lower cost, yet it requires high-voltage DC cables [5] and uniform irradiance. PV inverters are generally used, allowing module-level balance is based on sorting capacitor values and the to be or deactivated monitoring and diagnostics [8] This voltage topology minimizes theSMs effects ofactivated partial shadowing and are selected using because the sign of arm current [25].
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