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In recent years, grid forming (GFM) inverters have become an increasingly active research interest, due to their ability to form autonomous grids and to maintain synchronism also in purely power electronics-based environments. The main building blocks of these machines are several pulsewidth modulation (PWM) controlled switches, receiving energy from the dc side, followed by $LC$ or $LCL$ output filters to produce nearly sinusoidal currents injected into the power grid. The filter inductances are usually small, to avoid high costs, such that tiny voltage measurement-, delay-, or PWM timing-errors can cause large deviations in the output currents. This is mitigated by employing fast output current controllers (OCCs), which allow us to correct the effect of measurement errors and delays and accurately track the reference currents. These controllers can be implemented as proportional integral (PI) controllers in a rotating reference frame or proportional resonant controllers or even more complex structures. GFM inverters usually employ virtual impedances to obtain the reference currents for the OCCs. While very efficient in strong grids, such control architectures face stability problems if the grid short-circuit ratio becomes low. While for grid following (GFL) inverters, this problem has been well researched in the past years, and various solutions have been proposed, for GFM inverters, the adequate tuning of the virtual impedances and the current controller parameters has yet to be fully understood. In this paper we investigate the influence of the grid impedance, and various control parameters of a GFM inverter with PI current controllers and virtual impedances, and give recommendations for the tuning of all the parameters to achieve stability in a wide range of grid conditions. We further suggest an updated procedure to select $LCL$ filter values and demonstrate the suggested tuning in an experimental setup. To enable automatic tuning online, we employ a method for measuring the grid impedance by injecting a small probing current into the grid, at a frequency that is close but different from the grid frequency.

The synchronverter algorithm is a way to control a switched mode power converter that connects a DC energy source to the AC power grid. The main features of this algorithm are frequency and voltage droops as well as synthetic inertia, so that the inverter resembles a synchronous generator (SG). Many versions of this algorithm have been proposed and tested, but all share the same “basic control algorithm”, which is based on the equations of a SG. We analyze the sensitivity of the output currents of a synchronverter, with respect to the measurement errors. We show that some of the sensitivity functions exhibit high gains at the relevant frequencies, leading to distorted grid currents, which makes the use of this inverter control algorithm problematic. We then do a similar analysis assuming that we have controlled current sources available at the grid output of the converter, that we control using virtual currents generated in the algorithm. The virtual currents are flowing through virtual output inductors, that we can choose to be significantly larger than the actual output inductors. We show that using the current sources reduces the sensitivity considerably, thus indicating a better approach to synchronverter design.

The presence of distributed generation (DG) in a power distribution network can cause difficulties in managing the voltage, power flow and fault level as the network was designed for supplying passive loads. This study focuses on the control of a back-to-back (B2B) voltage source converter operated as a synchronverter and a soft open point (SOP) between adjacent feeders to mitigate the voltage fluctuation caused by DG and to increase the reliability of the supply without increasing the fault level. Better reliability is achieved as the B2B converter can continue to supply the loads when a loss of mains occurs on one side. The proposed control of the two DC–AC converters, which also needs to regulate the DC link voltage and adapts the synchronverter algorithm to the new application requirements, so that events will cause minimum disruption. Simulation shows that the B2B-synchronverter is capable of regulating the load voltage under normal mains connected condition, and also limiting the fault current and supplying the loads within its rating during islanding, providing seamless transition to the islanded mode.

We propose the parallel virtual infinite capacitor (PVIC) concept, which refers to two virtual infinite capacitors (VIC) connected to the same DC link and sharing one capacitor, one tuned to low frequencies (LF) and one tuned to high frequencies (HF). A PVIC can suppress voltage variations (ripple) in a wider frequency range than a usual VIC. The LF-VIC is controlled by a sliding mode controller to regulate the low-frequency component of the voltage to its reference value, and by a proportional-integral (PI) controller to maintain its state of charge within the desired operating range and achieve the ‘plug-and-play’ pattern of PVIC. The HF-VIC is controlled by another sliding mode controller to limit the high-frequency ripples and also to keep its state of charge within a reasonable operating interval. As our main application, we use the PVIC to replace a DC-link capacitor for voltage filtering on the DC link of a doubly fed induction generator (DFIG), which is driven by a WindPact (WP) 1.5-MW wind turbine under different grid conditions with turbulent wind input. The simulation study indicates that the PVIC provides much better voltage stabilisation than a DC-link capacitor with the same capacitance, especially in the low frequency range.

This article describes an interdisciplinary and theory-based radio campaign that has been developed to counteract, and sensitize citizens to hate speech in the Democratic Republic of the Congo (DRC). The article provides a brief overview of the instrumentalization of hate speech and the violent effects it has had in the Great Lakes region of Africa. A summary of the most recent events in the DRC is given. Here, hate speech was used in the presidential election campaigns in 2006, contributing to a polarization of the country and giving the campaign an ethnic underpinning. A radio program developed to counteract hate speech during the election campaigns is described. Its theoretical basis, the application of Staubs (1989) theory of the evolution of mass violence to hate speech, is presented. Based on this and other relevant psychological concepts, characteristics and psychological aspects of hate speech are summarized, and markers and guidelines are provided that allow listeners to detect and counteract hate speech.