Abstract
A modular multilevel converter’s (MMC’s) submodule (SM)-capacitor voltage will increase under unbalanced grid conditions. Depending on the imbalance level, the voltage ripple can be considerably high, and it can exceed the pre-defined safe limits. If this occurs, the converter will trip, which can lead to serious stability problems for the grid. This paper first proposes an analytical solution for deriving the three-phase imbalanced SM ripple of an MMC under an unbalanced grid. With this analytical tool, the imbalance mechanism of the SM voltage ripple can be easily understood. What is more, the symmetrical component method is first applied to analyze the three-phase SM capacitor ripple, and the positive-/negative-/zero-sequence components of the three-phase SM voltage ripple are easily identified by the proposed analytical method. Then, based on this powerful analytical tool, the proper circulating-current profile to be injected can be obtained, allowing for the right compensation of the voltage ripple. Based on this approach, two new voltage ripple compensation methods are proposed in this paper. Simulations were carried out to validate the analytical description of the submodule-capacitor voltage ripple proposed in this paper. Moreover, simulation and experimental results are provided to validate the new compensation techniques introduced in this paper.
Highlights
The modular multilevel converter (MMC), illustrated in Figure 1, is the standard power electronics solution for high-power applications, such as in high-voltage direct-current (HVDC) transmission systems that operate as voltage sources [1,2,3]
The circulating-current control is able to properly track the AC components, since it is based on a proportional resonance (PR) controller
The experimental setup worked as a three-phase MMC inverter
Summary
Received: 21 December 2020The modular multilevel converter (MMC), illustrated in Figure 1, is the standard power electronics solution for high-power applications, such as in high-voltage direct-current (HVDC) transmission systems that operate as voltage sources [1,2,3]. The modular multilevel converter (MMC), illustrated, is the standard power electronics solution for high-power applications, such as in high-voltage direct-current (HVDC) transmission systems that operate as voltage sources [1,2,3]. The MMC presents a large number of components, including semiconductor devices and submodule capacitors These submodule capacitors are quite bulky and heavy, since they need to be designed with a considerably high capacitance in such a way as to keep the submodule-capacitor voltage ripple within safe limits. As a natural consequence of the MMC’s topology and operation, a voltage ripple exists in the submodule capacitor under normal operation conditions [8,9] Different grid phenomena, such as faults and imbalances, will affect the profile and the amplitude of the submodule-capacitor voltage ripple, and some dangerous situations can eventually occur. It is important to analyze the submodulecapacitor voltage ripple of an MMCunder different grid conditions
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