Three-Axis Decoupling Controller for Modular Multilevel Converter

doi:10.12204/j.issn.1000-7229.2022.05.004

Abstract

Abstract:

Through the analysis of the modular multilevel converter (MMC) with the traditional d-q axes decoupling controller, this paper reveals the coupling effect between the DC voltage and the sub-module capacitor voltage, which also results in the equivalent capacitance on the DC port and the deviation of the capacitor voltage. Through considering the sum of the upper and lower arm voltages as the control variable, the DC current is introduced as the third inner-loop state variable, and a three-axis decoupling controller with three inner-loop state variables is proposed. Applying the introduced DC current state variable, the control of the DC voltage can be decoupled from the sub-module capacitor voltage. The direct control of the sub-module capacitor voltage and the fast and flexible control of the DC voltage/current can be realized simultaneously. On the basis of the proposed inner-loop three-axis decoupling controller, various outer-loop controllers are designed for different operating modes of MMC, which can flexibly realize the control of DC voltage, active power, reactive power, and capacitor voltages. The feasibility and effectiveness of the proposed three-axis decoupled controller have been verified by using simulation results for different operating modes.

Key words: modular multilevel, dq decoupling control, internal DC voltage, three-axis decoupling control

CLC Number:

TM721

WANG Kailun, SONG Qiang, ZHOU Yuebin, YANG Liu, ZHANG Nan. Three-Axis Decoupling Controller for Modular Multilevel Converter[J]. ELECTRIC POWER CONSTRUCTION, 2022, 43(5): 29-39.

Figures/Tables 15

Fig.1 Diagram of an MMC

Fig.2 Block diagram of the two-axis dq decoupling controller

Fig.3 DC-side dynamic equivalent circuit of MMC applying two-axis decoupling controller

Fig.4 DC-side dynamic equivalent circuit of MMC applying three-axis decoupling controller

Fig.5 Block diagram of three-axis controller

Fig.6 Block diagram of P/Q power control

Table 1 Model parameters of MMC

Fig.7 Simulation circuit of P/Q power control

图8 Simulation waveform of P/Q control mode

Fig.9 Block diagram of Udc/Q control

Fig.10 Simulation circuit of Udc/Q control

Fig.11 Simulation waveform of Udc/Q control mode

Fig.12 Simulation waveform of constant DC voltage control mode

Fig.13 Block diagram of constant DC current control

Fig.14 Simulation waveform of constant DC current control mode

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