High voltage direct current (HVDC) technology has begun to gather a high degree of interest in the last few decades, showing a fast evolution of achievable voltage levels, transfer capacities, and transmission lengths. All these changes occurred in a context in which power system applications are highly dependent on HVDC technologies such as energy generation from renewable sources (e.g., energy generated in offshore wind power plants), power exchanges between asynchronous networks, submarine cables, and long-length transmission overhead lines have become more common worldwide. This paper tries to summarize the current state of HVDC technologies, both voltage-source converters and current-source converters, the main components of converter substations, control strategies, key challenges arising from their use, as well as the future prospects and trends of HVDC applications. This paper represents the first step in setting the background information for analyzing the impact of a VSC-HVDC connection on the stability of the Romanian transmission network during steady-state and dynamic operation.

The doubly fed induction generators (DFIGs) are used in almost 50% of the offshore and onshore wind power plants. The DFIGs offer advantages compared to fix speed induction generators. The DFIGs are sensitive to grid voltage disturbances and require additional protection for the rotor side power electronic converter, like crowbar protection. In the present paper two active crowbar topologies are compared, IGBT with bypass resistor and rectifier with IGBT, which were the most studied in the scientific literature. A DFIG model with analyzed crowbar protections was implemented in Matlab/Simulink. The simulation results reveal that both solutions can limit the rotor current and the dc link voltage values. However, the solution with rectifier offers better results.

This paper proposes a control system, for crowbar protection and voltage support, for wind turbines equipped with doubly fed induction generator. The control system based on PI controller enables the low voltage ride through capability of the wind turbines. The reactive power control of doubly fed induction generators is mostly achieved by controlling its rotor side converter. The proposed control system allows regulating the reactive power using the rotor side converter as well as the grid side converter. During crowbar activation, the rotor side converter can no more control the reactive power. The proposed system controls the voltage at the point of common coupling to be above the imposed low voltage ride through curve, until the grid side converter returns to normal operation. The proposed models were implemented in Matlab/Simulink. The case studies refer to a control system with hysteresis band for the crowbar protection of a 3MW doubly fed induction generator based wind turbine. The simulation results highlights that the proposed control improves the coordination between grid side converter, rotor side converter, and crowbar protection.

This paper presents a complete simulator developed in Matlab™ for a photovoltaic (PV) module. The most important contributions of the paper is the use of single and double diode model to simulate a PV module. In the paper are also implemented numerical methods to provide more characteristic parameters that are not provided by the manufacturers such as R_s, R_p, I_pv, I₀₁ and I₀₂. The numerical methods are suitably implemented to face the nonlinear nature of I-V and P-V curves that are characteristics to PV output. The accuracy of the simulator using single and double diode models is verified by applying the data-sheet from manufacturers. Furthermore, the double-diode model simulation results performed in Matlab™ are analyzed in comparison with the experimental data of a real 310 Wp photovoltaic module unit. The paper also highlights the effects of different shunt and series resistances and how it operates in different temperature conditions and irradiation levels.

The paper analyzes the capability of a wind power plant, based on doubly-fed induction generators (DFIGs) with an improved control system, to fulfill low voltage ride-through (LVRT) capability of the wind turbines providing the reactive power support. According to grid code requirements, tripping of wind generators under grid faults is not allowed. The sensitivity of DFIGs to grid faults requires the use of crowbar protection for not damaging the electronic equipment. In this paper, the model of a wind power plant based on DFIG using active crowbar protection is developed in Matlab/Simulink. Nevertheless, the results indicate that DFIGs equipped with crowbar protection can protect the electronic equipment, but the rotor side converter is blocked due to crowbar activation and the reactive power control is lost. Therefore, for improving the LVRT, a coordinated control system of the grid side converter, active crowbar protection, and rotor side converter is implemented and proposed, The grid side converter of DFIG is controlled similar to a STATCOM, considering also the control parameters of the crowbar protection. The simulation results show that the proposed coordinated control grid side converter'active crowbar-rotor side converter can supply the reactive power in order to answer the grid codes requirement for LVRT. Therefore, this control system can be useful to improve the LVRT capability of wind power plants based on DFIG.