Electric grid codes are expected to change in near future to accommodate an increased number of distributed generation units in the distribution power system without impairing its power quality. It is desired that the generators remain connected during voltage sags and provide ancillary services, such as voltage and reactive power control, ensuring the operational stability of the power system. This paper explores how the existing strategies for active and reactive power injection impact the operation of grid-tied inverters in terms of required power, current flowing and reduction of active power delivery during the voltage sags. Such inputs are relevant to properly size converters for operation under fault events. In addition, this paper contributes to devise ${1}$ ) constant peak current control, ${2}$ ) constant active current control and ${3}$ ) constant average active power control strategies for three-phase four-wire grid-tied inverters considering the natural (abc) reference frame. The design and implementation of the investigated power injection strategies are discussed, and their effectiveness and technical viability are analyzed through dynamic computational simulations under symmetrical/asymmetrical voltage sags, and variation of the short-circuit ratio.

Most of the distributed control strategies for grid-connected power converters are droop-based approaches composed of converters driven in voltage-control mode, based on local and shared data with adjacent units. They are usually combined with consensus protocols to deal with the trade-off between power sharing accuracy and voltage/frequency regulation. To achieve the desired results these control systems usually incorporate other techniques and need to take into account details of primary control dynamic. Additionally, power flow control and current unbalance compensation at the PCC are rarely addressed in such approaches. Contrariwise, the centralized control strategy power-based control has been successful in achieving these functionalities. It is oriented to a set point selection to the whole system, considering converters driven in current-control mode and a central converter in voltage-control mode. However, the dependence on centralized communication network in this method still requires improvement. Thereby, the complementary features of both strategies are combined herein in the consensus power-based control, based on a master/slave peer-to-peer integration using sparse communication. This model-free approach provides all aforementioned benefits to the grid without any other technique. Implementation complexity and costs are decreased, while the flexibility and reliability are enhanced. All these achievements are demonstrated by simulation results under different operational conditions and compared to previous works.

Different control strategies for microgrid applications have been developed in the last decade. In order to enhance flexibility, scalability and reliability, special attention has been given to control organisations based on distributed communication infrastructures. Among these strategies, the implementation of consensus protocol stands out to cooperatively steer multi-agent systems (i.e., distributed generators), which is justified by its benefits, such as plug and play capability and enhanced resilience against communication failures. However, as the consensus protocol has a long trajectory of development in different areas of knowledge including multidisciplinary subjects, it may be a challenge to collect all the relevant information for its application in an emerging field. Therefore, the main goal of this paper is to provide the fundamentals of multi-agent systems and consensus protocol to the electrical engineering community, and an overview of its application to control systems for microgrids. The fundamentals of consensus protocol herein cover the concepts, formulations, steady-state and stability analysis for leaderless and leader-following consensus problems, in both continuous- and discrete-time. The overview of the applications summarises the main contributions achieved with this technique in the literature concerning microgrids, as well as the associated challenges and trends.

This paper presents two hybrid filter topologies applied to a hot strip mill electrical system with cycloconverters, to evaluate the harmonic compensation and damping of harmonic resonance. In addition, an evaluation of the performance of hybrid filters in reducing the current rating of the passive filter is also presented. The most relevant waveforms of the system and its harmonic spectrum are analyzed, to assess the performance of each filtering structure in improving the power quality. The results obtained through digital simulation show that a series hybrid filter provides harmonic damping and allows enhancing the harmonic compensation characteristics of the passive filtering system, using an active filter of lower rated power compared to a parallel hybrid filter. However, the series hybrid filter does not present good filtering characteristics in the high frequency range, what can be performed by a parallel hybrid filter with a higher rating.

The use of variable frequency drives (VFD) in floating, production, storage, and offloading (FPSO) units provides well-known benefits to the power system, such as: soft-starting of motors, increase of energy-efficiency, reduction of short-circuit currents, and enhancement of the load shedding capability. The VFDs with passive front-end rectifiers are the most commonly used for these applications due to their wider commercial availability and lower cost compared to VFDs with active front-end rectifiers. However, the latter can control the exchange of reactive power with the grid, which may contribute to voltage support and power factor regulation at the electrical system. This paper presents a case study considering the Production operating mode of an FPSO unit with two VFDs connected to the largest motors (i.e., water injection pumps). Simulations of the FPSO power system are evaluated using both active and passive VFDs, considering scenarios with analysis of steady-state, start of largest motors and symmetrical three-phase short-circuit.