Design and Analysis for Energy Storage Requirements of Modular Multilevel Converter Applying Per-unit Model

doi:10.12204/j.issn.1000-7229.2021.11.001

Abstract

Abstract:

The submodule capacitance is a key factor affecting the volume and cost of modular multilevel converter (MMC). The normalized value of the energy stored in an MMC with respect to the rated capacity is taken as the unified index for evaluating the capacitance usage of an MMC. A per-unit model that describes the relationship between energy storage requirement (ESR) value and the fluctuation rate of capacitor voltage is established. Thereby, the evaluation, calculation and analysis of the ESR values of MMCs with different rated parameters can be unified and simplified. Compared with the conventional methods of taking the capacitance value as the design and analysis index, the per-unit model for calculating the ESR value eliminates the influence of various parameters on the calculation results. It is revealed that the ESR value is mainly affected by the base modulation index and the per-unit active and reactive power output. Based on the proposed model, the variation of the ESR value with the modulation index and the power output range is analyzed in detail, which provides a clear and accurate basis for the design and optimization of MMC capacitance usage. The results of digital simulation and experimental results verify the proposed methods.

Key words: modular multilevel converter(MMC), capacitor voltage fluctuation, capacitance usage, rated energy-storage requirement

CLC Number:

TM721

ZHOU Yuebin, SONG Qiang, YANG Liu, ZHANG Nan, LI Zhengxuan, CAO Wanyu. Design and Analysis for Energy Storage Requirements of Modular Multilevel Converter Applying Per-unit Model[J]. ELECTRIC POWER CONSTRUCTION, 2021, 42(11): 1-12.

Figures/Tables 18

Fig.1 Diagram of the arm of MMC

Fig.2 Equivalent circuit for an MMC connected to AC grid

Fig.3 Flowchart for calculating the rated energy storage of MMC

Fig.4 PQ diagram of an MMC

Fig.5 Curve of the values of energy storage requirement of MMC varying with power factor angle

Fig.6 Curve of the capacitor voltage fluctuation rate varying with the power output.

Fig.7 PQ diagram of MMC when limiting the maximum reactive power

Fig.8 Curve of the values of energy storage requirement of MMC varying with power factor angle when limiting the maximum reactive pwoer

Fig.9 3-D Curve of the designed rated value of the energy storage requirement varying with the base modulation index and maximum allowable reactive power

Fig.10 Curves of the designed rated value of the energy storage requirement varying with the maximum allowable reactive power under several typical modulation indices

Table 1 Key parameters of the typical projects

Table 2 Key parameters of the simulation model

Fig.11 Comparison of the simulated and analytically calculated results for the capacitor voltage fluctuation rate

Fig.12 Simulated waveforms of the capacitor voltage for several typical operating conditions

Fig.13 Comparison of the simulated and analytically calculated results for the capacitor voltage fluctuation rate when limiting the maximum reactive power

Table 3 Key parameters of the experimental prototype

Fig.14 The experimental waveforms when φ=0

Fig.15 The experimental waveforms when φ=π/2

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