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Abstract-This paper studies different circulating current references for the modular multilevel converter (MMC). The circulating current references are obtained from the instantaneous values of the output current and modulation signal of the phase-leg. Therefore, determination of the amplitude and phase of the output current is not needed, which is a significant improvement compared to other methods such as those based on injecting specific harmonics in the circulating currents. Among the different methods studied in this paper, a new method is introduced, which is able to reduce the capacitor voltage ripples compared to the other methods. A closed-loop control is also proposed which is able to track the circulating current references. With the discussed methods the average values of the capacitor voltages are maintained at their reference while the voltage ripples are kept low. Experimental results are presented to demonstrate the effectiveness of the proposed and discussed methods.

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The increasing penetration of renewable energy sources (RES) poses a major challenge to the operation of the electricity grid owing to the intermittent nature of their power output. The ability of utility-scale battery energy storage systems (BESS) to provide grid support and smooth the output of RES in combination with their decrease in cost has fueled research interest in this technology over the last couple of years. Power electronics (PE) is the key enabling technology for connecting utility-scale BESS to the medium voltage grid. PE ensure energy is delivered while complying with grid codes and dispatch orders. Simultaneously, the PE must regulate the operating point of the batteries, thus for instance preventing overcharge of batteries. This paper presents a comprehensive review of PE topologies for utility BESS that have been proposed either within industry or the academic literature. Moreover, a comparison of the presently most commercially viable topologies is conducted in terms of estimated power conversion efficiency and relative cost.Index Terms-Battery energy storage system, dc-ac converter, dc-dc converter, power conversion, power electronics I. INTRODUCTIONRenewable energy sources (RES), including wind turbines and solar PV systems, have been installed at a fast pace globally in recent years [1], [2]. The intermittent nature of output power from RES becomes a serious concern for the stability of the grid, particularly with increased RES penetration and at times when a high percentage of instantaneous demand is supplied by RES. In the case of Germany where 80% of instantaneous demand was supplied by RES on the 23 rd August 2015 [3], significant operating reserves were required to meet the demand in case of a sudden decrease in the output of RES, thus causing an increase in the operational cost of the electricity network. Utility-scale battery energy storage systems (BESS) featuring fast response characteristics can provide an economic and promising alternative to smooth the output power of RES [4] and provide operating reserves [5], as there is virtually no cost to the system when BESS are in reserve state i.e. not providing power [6].

The modular multilevel converter (MMC) is the state-of-the-art for multilevel converter topologies. This study presents the operation of the MMC using the multilevel selective harmonic elimination pulse-width modulation (MSHE-PWM) technique. MSHE-PWM offers tight control of the low-order harmonics and the lowest switching frequency for the power semiconductors among all modulation techniques. A comprehensive analysis of the modulation methods for the MMC leads to two different modulation patterns for MSHE-PWM. A method for selecting the number of sub-modules in the phase-legs of the converter is also proposed in this study. Simulation results for both patterns are provided and verified through matching experimental results from a single phase 11-level laboratory prototype.

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This paper introduces a low complexity implementation of the voltage balancing algorithm aiming to reduce the switching frequency of the power devices in modular multilevel converters (MMCs). The proposed algorithm features a relatively simple implementation without any conditional execution requirements and is easily expandable regardless of the number of submodules (SMs). Two modulation techniques are evaluated, namely the staircase modulation and the phase-disposition pulse width modulation (PD-PWM) under the conventional and the proposed algorithm. Using a circulating current controller in an MMC with 12 SMs per arm, PD-PWM yields better results compared to the staircase modulation technique. The test condition for this comparison is such that the power devices operate at a similar switching frequency and produce similar amplitudes to the capacitor voltage ripples in both modulation techniques. The results are verified through extensive simulations and experiments on a low power phase-leg MMC laboratory prototype.

Harmonic resonances in traction power supply systems (TPSSs) have attracted great attention due to their potentially destructive impact on the safe and stable operation of railways. Based on operation data acquired from Chinese railways, this paper presents a comprehensive review of this issue considering both academic and engineering requirements. Analysing actual incidents, the general patterns and effects of TPSS resonances are derived. The relation between the TPSS and the locomotive is illustrated by circuit models. Relevant methods for modeling locomotives and TPSSs are discussed. Both advanced resonance analysis methods giving general influence factors and a simplified resonance analysis explaining resonance features in a practical manner are discussed. Multiple groundbased and on-board solutions for resonance elimination are presented. At last, pre-identifying resonances in actual systems is investigated for addressing the open topic of resonance prevention.