Distributed power generation plays a critical role in the stability and reliability of modern power systems. Due to the rapid growth of renewable energy generation, the requirements of the transmission and distribution system operators are becoming more stringent. Among these requirements, one of the most important is the Low-Voltage Ride-Through (LVRT) capability, which demands that the inverters remain connected to the grid and provide support during voltage sags. For this purpose, flexible current control algorithms stand out because they can manage unbalanced voltages and simultaneously achieve other control objectives. With the aim of presenting a concrete document focused on describing fundamental principles and discussing common design guidelines, this paper presents an overview of flexible current control strategies applied to LVRT capability. The operation features and design aspects of experiences reported in the literature are reviewed. Moreover, the paper proposes a general methodology to design LVRT flexible current control algorithms. Finally, current and future trends are discussed.

A method to model a conductor with undefined potential (commonly known as floating conductor), is sometimes required in the electric field analysis. This paper presentsand compares the main methods to deal with such issue, based on the traditional finite element formulation. The purpose is to guide the reader in the selection of a method under the following criteria: Accuracy, implementation and simplicity. The accuracy of each method was evaluated against the analytic solution of the capacitance matrix for a system of parallel cylindrical conductors. Based on the results of the simulations, the characteristics of the methods and this paper’s criteria, a qualification of each method performance is done. Additionally, particular cases in which a specific method could be the most suitable option to deal with floating conductors, are analyzed. In general, the Virtual Permittivity method exhibits the best performance. However, it is shown that the method’s accuracy is influenced by the rounding errors. This paper proposes an additional consideration on the method formulation in order to verify the accuracy of the results.

This paper presents two control algorithms for grid-tied three-phase inverters for renewable energy systems under unbalanced grid voltages. The algorithms are focused on the quality of the injected currents, so they are used to inject balanced currents despite the voltage unbalances. Each algorithm is composed of three control loops: the reference signal generation, the reference signal synchronization and the curent controller.  The first control loop generates the reference currents in order to regulate the active and the reactive powers delivered to the grid by the renewable energy system. The second loop is used to estimate the positive sequence component of the grid voltage. The last control loop generates the power inverter firing pulses in order to follow the reference current. In this paper are considered two algorithms, one algorithm in the abc coordinates and the other in the dq0 coordinates. In both cases, it is considered a Kalman filter as the synchronization algorithm. The algorithms performance is evaluated by some simulations in PSIM and a comparative analysis is made between them. The results showed that the two algorithms present an outstanding performance in the injection of balanced currents with low total harmonic distortion despite the voltage unbalance. Also, the Kalman filter gets the desired estimation in less than a quarter of the grid period.