Abstract
This paper presents two innovative Fault-Ride-Through (FRT) strategies suited for Smart-Transformers (ST) supplying hybrid AC/DC distribution grids within a microgrid environment. The first strategy is suited for ST without a local energy storage, where its Medium Voltage (MV) inverter is operated in grid-tied mode. The proposed approach relies on the voltage sensitivity of resources connected to the ST fed distribution networks aiming to limit the MV inverter current. The second strategy is suited for ST incorporating local energy storage and operating its MV inverter in grid-forming mode, thus enabling islanding operation of a MV grid section. The proposed FRT strategy aims to regulate ST's output voltage by calculating the maximum voltage drop in the coupling filter in order to control the output current. The proposed strategies are evaluated exploiting appropriated simulation models and extensive operating conditions.

Abstract
The Smart Transformer (ST) is being envisioned as the possible backbone of future distribution grids given the enhanced controllability it provides. Moreover, the ST offers DC-link connectivity, making it an attractive solution for the deployment of hybrid AC/DC distribution grids which offer important advantages for the deployment of Renewable Energy Sources, Energy Storage Systems (ESSs) and Electric Vehicles. However, compared to traditional low-frequency magnetic transformers, the ST is inherently more vulnerable to fault disturbances which may force the ST to disconnect in order to protect its power electronic converters, posing important challenges to the hybrid AC/DC grid connected to it. This paper proposes a Fault-Ride-Through (FRT) strategy suited for grid-tied ST with no locally available ESS, which exploits a dump-load and the sensitivity of the hybrid AC/DC distribution grid's power to voltage and frequency to provide enhanced control to the ST in order to handle AC-side voltage sags. The proposed FRT strategy can exploit all the hybrid AC/DC distribution grid (including the MV DC sub-network) and existing controllable DER resources, providing FRT against balanced and unbalanced faults in the upstream AC grid. The proposed strategy is demonstrated in this paper through computational simulation.

Abstract

Abstract
This paper describes technical solutions based on advanced control functionalities for photovoltaic systems aiming to prevent voltage rise above technical limits in low voltage MicroGrids by limiting the injected active power. Due to the action of Maximum Power Point Tracking (MPPT) systems, it is expected that the output power of photovoltaic systems tracks the maximum value according to both solar and temperature conditions. Hence, limiting the active power to be injected into the low voltage network requires the accommodation of the generation surplus. An innovative approach is proposed for this purpose, exploiting a modified MPPT algorithm that finds a proper operation point considering also the grid operating conditions. The technical feasibility of this approach is evaluated through numerical simulations performed in the Matlab (R)/Simulink (R) simulation tool using the detailed models of the power electronic converters.

Abstract
This paper proposes technical solutions that can be implemented in variable speed permanent magnet synchronous generators driven wind turbine systems aiming to mitigate high voltage problems in low voltage MicroGrids by controlling the active power output. Due to the limited control capability of these systems, controlling the output power to prevent voltage rise will require the local accommodation of the generation surplus. For this purpose, additional control functionalities are developed to be integrated in the control systems of the power electronic based interfaces. Their performance is evaluated through numerical simulations performed in Matlab (R)/Simulink (R) environment and considering the detailed models of the power electronic converters. The results obtained demonstrate the effectiveness of the proposed control functionalities.

Abstract
Grid-forming (GF) converters based on renewable energy sources are a fundamental piece of future power systems. In particular, the design specifications of GF converters in photovoltaic (PV) applications are difficult to meet because PV inverters lack energy storage. The operation of GF-PV inverters under normal conditions has already been addressed in the existing literature. However, the operation in case of large disturbances, such as faults, has rarely been explored. In this paper, a GF controller for a two-stage PV inverter that is robust against faults is presented. This control system includes several improvements compared to the traditional GF controller. Power feedforwards and saturations are applied to improve the transient performance. Also, a method to keep the virtual swing equation synchronised when the current saturates is presented. Remarkably, there is no need to change the controller structure during faults. Simulations of a PV inverter connected to a simple power system based on a diesel generator and loads are conducted. The results show that the proposed countermeasures improve the performance of GF-PV inverters in case of faults. In addition, it is shown that keeping the phase of the virtual swing equation and the grid voltage space vector synchronised is important to avoid the collapse of the dc-link voltage. Suggestions for further research are presented in the last part of the work.