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Keywords« Multilevel converters», « Reactive power», « Static Synchronous Compensator (STATCOM)». AbstractThe aim of this paper is to investigate the ability of star-and delta-connected H-bridge multilevel STATCOMs to exchange negative-sequence current with the grid. Zero-sequence injection is utilized for capacitor voltage balancing. It is shown that a singularity for these configurations exists, leading to limitations in the utilization of the compensator for asymmetry compensation purposes. IntroductionThe tendency in the market clearly indicates that Voltage Source Converter (VSC) based compensators will be more and more utilized in the power systems, especially in the dynamic range for industry applications. In shunt configuration (also named STATic COMpensator, STATCOM), the VSC has a smaller footprint than the equivalent thyristor-based solution (or Static Var Compensator, SVC) and smaller harmonic filters are required. The converter behaves as a current controlled voltage source instead of a controlled susceptance as is the case of the SVC and allows fast and flexible control of the voltage at the connection point. The H-bridge multilevel converter (also called chain-link converter) is considered one of the most attractive topologies available in the market for grid applications and it is today used by several manufacturers. Since the number of modules, or cells, is proportional to the AC voltage at the connection point, the system is scalable and it can be used without a step-up transformer for different applications [1]. This topology can be either delta or star connected. The main disadvantage with modular topologies for FACTS applications is the lack of a common dc link and therefore the difficulty in transferring energy between phase legs, in particular in case of unbalanced grids. To facilitate energy exchange, the converter must be significantly over-rated in terms of current (for the delta connection) or voltage (star connection). When star connection is utilized, a zero-sequence voltage must be introduced for the floating Y-point of the converter to exchange energy between the phases when compensating unbalanced loads [2][3][4][5]. This results in the need for an increased number of series connected cells in order to realize the needed phase voltage. On the other hand, the delta configuration allows negative-sequence compensation by letting a zero-sequence current circulate in the delta, which increases the required current rating for the compensator [5][6]. The needed overratings for the converter is a function of the negative-sequence current to be injected by the converter into the grid. The required zero-sequence voltage (or current, depending on the selected configuration) increases exponentially when the amount of positive-and negative-sequence compensation approaches the same value, which implies that there is a limit in the compensation that can be provided by the system. This singularity happens for both delta and start configuration; for the former, when equal amounts of

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