Abstract
In the medium voltage direct current (MVDC) transmission system, a small number of MMC sub modules will reduce the power quality. In this paper, based on the research background of Photovoltaic Medium Voltage Direct Current (PV-MVDC) system, a Hybrid Modulation Strategy based on the Decoupled Double Synchronous Reference Frame (DDSRF) control strategy is proposed. The dual arm complementary hybrid modulation combining nearest-level-SPWM (NL-SPWM) can keep the number of SMs in the ON state constant. Then, the corresponding voltage sharing control algorithm of sub module (SM) is introduced. Through theoretical calculation, the modulation strategy can be found to stabilize the DC voltage and reduce the harmonic content. A 32-level MMC system has been developed to verity that the proposed hybrid modulation strategy and its SM voltage sharing algorithm have the advantages of restraining circulating current and maintaining capacitor voltage balance.
Highlights
New energy power generation is one of the effective ways to solve the challenges of rapid growth of electricity demand and economic flexible operation of power grid
A 32 level modular multilevel converter (MMC) topology is built for application in PV-medium voltage direct current (MVDC) System
MMC control strategy based on Decoupled Double Synchronous Reference Frame (DDSRF) is established
Summary
New energy power generation is one of the effective ways to solve the challenges of rapid growth of electricity demand and economic flexible operation of power grid. A novel multilevel inverter is proposed in [2], which can reduce the number of switches and modular DC voltage sources, and ensure a higher level at the output. It is not suitable for medium and high voltage DC transmission. The large capacity distributed photovoltaic power station is connected to the medium and high voltage DC power grid, which helps to realize the economic operation of the system. If the common modulation strategy like nearest level modulation (NLM) is adopted, there will be obvious low-voltage harmonic and current distortion, which will reduce the power quality. The switching frequency IGBT in arms is high which results in high switching losses
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