New Modular Multilevel DC–DC Converter Derived from Modified Buck–Boost DC–DC Converter

Abstract

Raising the electrification ratio to 100% is still a formidable challenge in Indonesia, especially in the remote areas of the eastern part of the archipelago. A DC microgrid system is one of the most viable solutions to increase the electricity supply in remote areas, taking advantage of various renewable energy sources that are located near the rural load centers. A DC–DC power converter for a rural DC microgrid system needs to have a high voltage gain to facilitate the power conversion from low-voltage PV output to a high-voltage DC microgrid bus, a very low input ripple current to help maintain the PV or battery lifetime, and be highly modular for ease of transport and assembly. Many topologies have been proposed to obtain high voltage gain, very low ripple current, and modularity. However, they usually use either bulky and lossy magnetic components, are sensitive to component parameter variance and need special voltage-balancing techniques, or have different component ratings for their multilevel configuration which weakens the modularity aspect. This paper proposes a new modular multilevel DC–DC converter that is very suitable for rural DC microgrid applications based on a modified buck–boost topology. The proposed converter is easily stackable to achieve high voltage gain and does not require any voltage balancing techniques, thus enhancing the modularity characteristics and simplifying its control method. Moreover, the ripple current can be reduced by employing a multiphase configuration. This converter can also facilitate bidirectional power flow to serve as a battery charger/discharger. A comprehensive analysis of voltage gain and ripple current are presented to explain the inner workings of this converter. Finally, the performance of this converter is verified through simulation and experiment, showing the converter’s modularity, bidirectional power capability, and potential to achieve voltage gain and ripple-current requirements of the DC microgrid system.

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“…Instead of directly transferring energy to the DC bus, the energy of the PV system will pass through an MMC-Boost converter to the DC bus to stabilize the output voltage on the DC bus and to maximize the energy transfer from the PV panels to the output [23]. The MMC-Boost converter is stacked with modules to easily increase the output voltage without using voltage balancing algorithms on capacitors.…”

“…Specifically, in a multi-level structure, the current ripple will decrease, enhancing conversion quality. This structure has the advantage of easy construction with compact size, high energy density, and quick response [23]. However, the former still has some drawbacks, such as high starting current and difficulty in voltage adjustment.…”

“…In summary, only the first or last cell needs to be controlled to generate the output voltage of the converter, simplifying the control process. To reduce the current signal's ripple, the Boost-Buck converter topology is improved to a two-phase multilevel MMC-Boost converter with an LC filter at the input of the first cell, as exhibited in Figure 3 [23]. The MMC-Boost converter in Figure 3 operates by cell and sets the α value of the other cells to 0.5.…”

“…Assuming the input voltage source is constant and there is no current ripple, capacitors are considered ideal. Thus, the output parameters depend on the selected inductance, switching frequency, duty cycle, and input current [23][24]. The output voltage of the proposed converter in Figure 2 is described by:…”

“…Adjusting the duty cycle of the first cell will result in a wider voltage conversion ratio compared to adjusting the last cell [23]. Therefore, the control of the duty cycle in the first cell will be utilized.…”