This paper focuses the analysis around a gridconnected converter that operates under unbalanced voltage and current conditions. Two different scenarios have been selected to validate the proposed algorithms. In the first scenario, the converter operates as a reactive power compensator, exchanging reactive power with the grid, and minimizing the dc bus voltage oscillations. The second scenario is more challenging, since it operates as a load balancer. In this application, the unbalanced current consumption of the load is compensated by the converter, exchanging positive and negative sequence currents with the grid. Another objective in this second scenario is to compensate the reactive power of the unbalanced load. It is important to highlight that in both scenarios, any maximum limit of the converter can be exceeded due to the specific unbalanced voltage and current conditions. Three variables are considered critical for the converter: 1) output ac current limit; 2) output ac voltage limit; and 3) dc bus voltage oscillation limit. Thus, this paper proposes a control method, which limits online the reactive power reference in the first scenario and the exchanged current references in the second scenario in order not to exceed any of the mentioned critical variables. The corresponding experimental results are shown in Part II of this paper so as to validate the limitation algorithm obtained by means of the mathematical analysis carried out in Part I.

This paper is the continuation of Part I in which an online reference limitation strategy for shunt-connected converters is presented. This limitation strategy is successfully validated in this Part II of the paper by means of experimental results obtained in a laboratory setup. Two different experiment sets are carried out in order to check the two application scenarios described in Part I. On the one hand, the limitation strategy is implemented in the STATCOM application reducing the reactive power reference. On the other hand, the grid balancer functionality is implemented together with the new limitation strategy reducing the current references. In this last scenario, two different experiments are shown: 1) a switching-on of the converter; and 2) a voltage variation at the point of common coupling. In both functionalities, the corresponding reference limitations are activated due to overcurrent and overvoltage cases and the limitation due to the dc bus voltage oscillations is validated in the grid balancer functionality.

Purpose -The purpose of this paper is to consider both sides of a back-to-back AC-DC-AC interface. Design/methodology/approach -The paper presents a mathematical analysis, simulation, laboratory test in scaled model. Findings -The two main findings comprised concept of control methods for grid AC-DC-AC converter applied in renewable energy sources with variable speed operation under distorted grid. Active filtering functionality in case of non-linear current of a parallel load. Second, a control algorithm dedicated for two-level AC-DC converter applied in industrial networks with high-order harmonics compensation working under hard conditions -balanced and unbalanced voltage dips.Research limitations/implications -The paper shows preliminary results for AC-DC-AC converter and active filter (AF) during voltage dips and for harmonics compensation. Control methods and/or topology should be improved and tested in scale and after at high-power system. Practical implications -Power quality supplied/received to/from the grid can be increased. In case of low-cost system only AF can be applied to existing non-linear receivers. Moreover, in case of full AC-DC-AC converter energy saving and production is possible. Originality/value -Presented control methods give satisfactory results. Paper presents laboratory results for grid and machine side two different power circuits during steady states and transients. Moreover, active filtering operation during voltage dips is presented.

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