A Full-Level Modulation Strategy for Half Bridge-Full Bridge Hybrid MMC in DC Distribution Grids

doi:10.12204/j.issn.1000-7229.2022.06.008

Abstract

Abstract:

In order to improve the operating performance of half bridge - full bridge hybrid modular multi-level converter (MMC) with small quantities of submodules and promote its application in medium and low voltage fields such as DC distribution grids, a novel full-level modulation (FLM) strategy is proposed in this paper. The full-level output capability of full-bridge submodule (FBSM) is utilized in the proposed FLM, where a selected FBSM in arm operates in the “PWM mode”. The obtained full-level PWM wave is superimposed with the original step wave to further approximate the sine modulation wave. The operating mode of submodules is determined by the voltage sorting result, and at any time, there are two FBSMs in the phase unit operating in the “PWM mode”, outputting complementary full-level PWM waves. The results of simulation based on Matlab/Simulink and physical experiment reveal that the quality of voltage and current on AC side under FLM is much higher than that of the conventional nearest level modulation strategy. Compared with the carrier phase-shifted pulse width modulation, FLM performs better in terms of operating loss, control complexity and circulating current, which can satisfy the modulation requirements of medium and low voltage fields.

Key words: half bridge - full bridge hybrid MMC, small quantities of submodules, DC distribution grids, full-level modulation strategy

CLC Number:

TM72

ZHANG Guihong, WANG Chen, ZHANG Xiangcheng, WANG Shibin, BAI Zuoxia, ZHANG Zhongfeng. A Full-Level Modulation Strategy for Half Bridge-Full Bridge Hybrid MMC in DC Distribution Grids[J]. ELECTRIC POWER CONSTRUCTION, 2022, 43(6): 75-83.

Figures/Tables 14

Fig.1 Topology of half bridge - full bridge hybrid MMC where 50% of the submodules are FBSM

Fig.2 Modulation principle of NLM strategy

Fig.3 Modulation principle of CPS-PWM strategy

Fig.4 Schematic diagram of full-level modulation strategy

Fig.5 Output voltage waveforms of the two FBSMs under the “PWM mode” in the phase unit

Fig.6 Flow chart for determining the mode of SMs

Fig.7 Simulation model of single-terminal MMC

Table 1 Parameters of simulation model

Fig.8 Simulation results of FLM strategy

Fig.9 Comparison of operating characteristics among three modulation strategies

Fig.10 Comparison of voltage quality, current quality and switching loss among three modulation strategies

Fig.11 Experiment platform of single-terminal MMC

Table 2 Parameters of experiment platform

Fig.12 Experiment results of FLM strategy

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