Abstract
This paper investigates the dynamic behavior of a modular multi-level converter (MMC)-based HVDC link. An overall state-space model is developed to identify the system critical modes, considering the dynamics of the master MMC and slave MMC, their control systems, and the HVDC cable. Complementary to the state-space model, an impedance-based model is also derived to obtain the minimum phase margin (PM) of the system. In addition, a relative gain array (RGA) analysis is conducted to quantify the level of interactions among the control systems of master and slave MMCs and their impacts on stability. Finally, with the help of the results obtained from the system analysis (eigenvalue, phase margin, sensitivity, and RGA), the system dynamic performance is improved.
Highlights
Modular multi-level converter (MMC) has emerged as the preferred choice for voltage source converter (VSC)-based HVDC systems mainly due to low losses, low harmonic distortion, scalability, and redundancy [1,2]
The control design for an MMC is more challenging as compared to a conventional two-level VSC, which is due to the extra control actions required for the regulation of the internal energy balances
Various small-signal analyses were conducted on an MMC-based HVDC link
Summary
Modular multi-level converter (MMC) has emerged as the preferred choice for voltage source converter (VSC)-based HVDC systems mainly due to low losses, low harmonic distortion, scalability, and redundancy [1,2]. In order to evaluate the system stability and the level of the interactions among control loops, a linear model of the MMC-based HVDC system is needed, which can be derived using one of the following modeling approaches:. These equations are interconnected to the state-space equations of the control loops based on the similar input/output signals in order to formulate an overall linear model of an MMC This modeling approach offers a high level of modularity, the inclusion of the circulating current harmonics in the modeling is challenging [11]. In [14], a linear state-space model of an HVDC system is derived to analyze the system transient response, suggesting alternative controllers to improve the converter dynamic behavior It does not provide information on the system phase margin (PM) and the interactions among the control loops are neither addressed.
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