Abstract
The sequence impedance of modular multilevel converters (MMC) is the premise to analyze the stability of MMC-based high voltage direct current (HVDC) system. However, few papers have modeled MMC impedance with comprehensive consideration of different control modes and voltage feedforward, the high-frequency impedance characteristics of the MMC have not been intensively studied, either. In fact, the voltage feedforward has an important influence on the high-frequency impedance characteristics of the MMC, which can not be ignored. This article proposes a sequence impedance for MMC considering complete control loops. Not only current inner-loop, circulating current control loop and phase-locked loop (PLL) are considered, but also different control modes including power control, DC voltage control and AC voltage control are considered. It is worth mentioning that especially the voltage feedforward and time delay are taken into account. Based on this impedance model, the characteristics of the MMC are thoroughly analyzed, and the analysis shows that the voltage feedforward and time delay will greatly affect the high-frequency characteristics, which will deeply affect the stability of the system. Meanwhile, the influences of PLL dynamics, outer-loop control, circulating current control and parameters of controller on the sequence impedance are also analyzed in detail.
Highlights
Modular multilevel converters (MMC) are very suitable for renewable energy system and have been widely applied in high-voltage direct current (HVDC) transmission system due to its advantages of modularity, low switching frequency and low distortion output voltage waveforms [1,2,3,4,5,6]
The correctness of derived analytical impedance model is verified by simulation. Based on this impedance model, the impedance characteristics considering different control modes are described, how the voltage feedforward and time delay affect the high-frequency characteristics of MMC impedance are analyzed in detail, and the resonance peak caused by voltage feedforward and time delay is pointed out and analyzed
This paper proposes an accurate sequence impedance model of MMC considering complete control loops and three control modes, especially time delay and voltage feedforward are considered
Summary
Modular multilevel converters (MMC) are very suitable for renewable energy system and have been widely applied in high-voltage direct current (HVDC) transmission system due to its advantages of modularity, low switching frequency and low distortion output voltage waveforms [1,2,3,4,5,6]. Based on the above analysis, a more accurate MMC impedance considering complete control loops including different control modes, time delay and voltage feedforward is necessary to be established, and the corresponding influence of control loops on high-frequency impedance characteristics should be further analyzed. The correctness of derived analytical impedance model is verified by simulation Based on this impedance model, the impedance characteristics considering different control modes are described, how the voltage feedforward and time delay affect the high-frequency characteristics of MMC impedance are analyzed in detail, and the resonance peak caused by voltage feedforward and time delay is pointed out and analyzed. The main contribution of this paper is summarized as: 1) an accurate impedance model of MMC is established with consideration of complete control loops including different control modes, time delay and PLL, and voltage feedforward is especially considered; 2) the high frequency characteristics of MMC impedance are analyzed in detail, and in particular, the resonance.
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