This paper addresses an advanced vector current control for a voltage source converter (VSC) connected to a weak grid. The proposed control methodology permits high-performance regulation of the active power and the voltage for the feasible VSC range of operation. First, the steady state characteristics for a power converter connected to a very weak system with a short circuit ratio (SCR) of 1 are discussed in order to identify the system limits. Then, the conventional vector control (inner loop) and the conventional power/voltage control (outer loop) stability and frequency responses are analyzed. From the analysis of the classic structure, an enhanced outer loop based on a decoupled and gain-scheduling controller is presented and its stability is analyzed. The proposed control is validated by means of dynamic simulations and it is compared with classic vector current control. Simulation results illustrate that the proposed control system could provide a promising approach to tackle the challenging problem of VSC in connection with weak AC grids.

This paper deals with multiterminal voltage source converter (VSC) HVDC transmission systems for the connection of offshore wind power plants to the main land ac grid. A droop-based control scheme is considered. The droop controllers have been designed base on a mixed sensitivity criterion by solving a convex optimization problem with linear matrix inequalities. The system is analyzed by means of simulations and experimentally in a scaled platform. Simulations show the control performance during a wind speed change and a voltage sag in the main ac grid. Experimental results include wind power changes (increase and decrease) and an eventual VSC loss (both considering grid-side and wind farm VSC loss). In all the cases, the simulation and the experimental results have shown a good system performance.

This paper focuses on the droop-based dc voltage control design for multiterminal VSC-HVDC grid systems, considering the ac and the dc system dynamics. The droop control design relies on detailed linearized models of the complete multiterminal grid, including the different system dynamics, such as the dc grid, the ac grid, the ac connection filters, and the converter inner controllers. Based on the derived linear models, classical and modern control techniques are applied to design the different controllers, including a multivariable frequency analysis to design the grid voltage droop control. In combination with the droop control, a dc oscillation damping scheme is proposed in order to improve system performance. The control design is validated through simulations of a three-terminal system.

Short circuit ratio is an outdated measure of system strength in converter dominated systems as the useful behaviour of inverter-based resources is not represented. While still useful for determining fault current provision, converter connected generation contributes to short circuit level and system strength in a different way to traditional synchronous generation. Additionally, traditional measures only account for system strength at the fundamental frequency while converters possess complex interactions across a frequency range. A novel method of determining system strength across this frequency range from the network impedances is proposed in this paper. The approach known as the Grid Strength Impedance Metric, calculates the system strength independently from short circuit level. The unique contribution of grid-following and grid-forming converters is represented by including the multiple-input multiple-output converter output impedances in the calculation. Small-signal impedance models are employed, and the metric is largely concerned with short-term voltage stability, but some inferences can be made for longer timescales. The approach is validated using time domain simulations to check for any identified areas of weakness in the frequency range. The Grid Strength Impedance Metric successfully represents the useful behaviour of grid-forming converters, indicating an enhancement in voltage stiffness in locations of deployment.

This paper presents a complete review of MVDC applications and their required technologies. Four main MVDC applications were investigated: rail, shipboard systems, distribution grids, and offshore collection systems. For each application, the voltage and power levels, grid structures, converter topologies, and protection and control structure were reviewed. Case studies of the varying applications as well as the literature were analyzed to ascertain the common trends and to review suggested future topologies. For rail, ship, and distribution systems, the technology and ability to implement MVDC grids is available, and there are already a number of case studies. Offshore wind collection systems, however, are yet able to be implemented. Across the four applications, the MVDC voltages ranged from 5–50 kV DC and tens of MW, with some papers suggesting an upper limit of 100 kV DC and hundreds of MV for distribution networks and offshore wind farm applications. This enables the use of varying technologies at both the lower and high voltage ranges, giving flexibility in the choice of topology that is required required.