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

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.

Compensation of Unbalanced Low-Voltage Grids Using a Photovoltaic Generation System with a Dual Four-Leg, Two-Level Inverter

“…One of the proposed topologies uses three three-phase, two-level VSIs and divides the PV panels into three groups [41]. Multilevel inverters composed of two-level VSIs based on the concept of using cascaded transformers have also been proposed [42][43][44][45][46][47]. In such configurations, the AC multilevel waveform is obtained by means of the transformer turn ratios.…”

Study and evaluation of distributed power electronic converters in photovoltaic generation applications

“…Module PV inverters are commonly used in low-power PV applications since they enable module-level monitoring and diagnostics [42]. As the module converter optimizes a single PV panel with an independent MPPT control, this topology minimizes the effects of partial shadowing and power mismatch.…”

“…In this configuration, each string of PV panels connected in series is connected to a separate DC-DC converter before being connected to the central DC/AC inverter, as can be seen in Figure 2.8. Since MPPT is implemented independently on each string of the string/multi-string PV inverter, this individual DC-DC converter connection will potentially boost the MPPT power, resulting in better overall energy output [42]; nowadays, they represent a market share of 52% [3]. This topology, however, needs more power electronic converters and reactors, resulting in higher costs and losses, and there are still PV panel mismatches in each string.…”

“…Figure 3.8 depicts the MMC single-phase connection and the submodules string connection streamlining across each arm, where the MMC normally has an upper and lower arm in each phase leg. The two-transistor in series and the compact in parallel with capacitor architecture represents one submodule that may be adjusted in various topologies to satisfy the voltage scale required by the application when used with a converter [42]. Furthermore, the number of conversion levels is determined by the submodules linked in each arm, with the number of level conversions equal to the number of submodules in each arm plus one in a non-interleaving situation.…”

“…They offer various advantages over other multilevel topologies, such as fault-tolerant operation, easy to achieve capacitor voltage balance, low harmonic distortion, and elimination of the bulky DC-link capacitor [17]. They have therefore seen rapid growth in a broad field of applications, including DC transmission, solid-state transformers, and renewable energy integration [42].…”