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Abstract-This paper presents a novel modulation strategy for n-phase neutral-point-clamped (NPC) converters. The proposed modulation strategy is able to control and completely remove the low frequency neutral-point (NP) voltage oscillations for any operation point and load types. Even when unbalanced and/or nonlinear loads are considered, the NP voltage remains under total control. Consequently, the strategy is very attractive for nphase active filters. In addition, it enables the use of low capacity film capacitors in NPC converters.The proposed modulation takes the carrier-based modulation strategy as a basis. It is formulated following a generalized approach that makes it expandable to n-phase NPC converters. In addition, the NP voltage is controlled directly using a closedloop algorithm that does not rely on the use of the linear control regulators or the additional compensators used in other modulation algorithms. Therefore, no tuning of parameters is required and it performs optimally for any operating conditions and kind of loads, including unbalanced and nonlinear loads. Although the high frequency harmonic content of the output voltages may increase, the weighted total harmonic distortion generated by the proposed strategy is similar to that of a standard sinusoidal pulsewidth modulated strategy. The proposed modulation algorithm has been tested in a four-leg NPC converter prototype performing as a three-and four-phase system and operating with balanced and unbalanced loads.

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The droop control method is usually selected when several distributed generators (DGs) are connected in parallel forming an islanded microgrid. This is because of the advantages it offers such as flexibility, absence of critical communications etc. Besides, several studies add a fictitious impedance to improve the performance of the original droop method. However, only a few studies deal with the design of this fictitious impedance, which is necessary to ensure an improvement in the dynamics and stability of the microgrid. In addition, these studies do not consider load variations for the design of the fictitious impedance, which is a habitual event in these systems. On the other hand, some studies propose a restoration control to bring the frequency and voltage amplitude of the microgrid to their nominal values. However, these do not deal with the design of the dynamics of this control to maintain a good transient and to ensure the stable performance of the microgrid. This study proposes the design of a fictitious impedance that ensures the stable operation of an experimental microgrid without power oscillations during load jumps and throughout its entire load range. This study also proposes a new restoration control that allows to take into account the possible inertias, delays etc. of the DGs and reduces the bandwidth of the required communications. Moreover, the proposed restoration control is properly designed to guarantee a good transient and the satisfactory performance of the microgrid. Experimental results confirm the validity of the proposed controls.