State‐of‐the‐art time‐domain models of power cables account for the frequency dependency of physical parameters to enable accurate transient simulations of high‐voltage transmission schemes. Owing to their formulation, these models cannot be directly converted into a state‐space form as required for small‐signal eigenvalue analysis. Thus, dc cables are commonly represented in high‐voltage direct current (HVDC) power system stability studies by cascaded pi‐section equivalents that neglect the frequency‐dependent effects. This study demonstrates how the conventional cascaded pi‐section model is unable to accurately represent the damping characteristic of the cable and how this can lead to incorrect stability assessments. Furthermore, an alternative model consisting of cascaded pi‐sections with multiple parallel branches is explored, which allows for a state‐space representation while accounting for the frequency dependency of the cable parameters. The performance of the proposed model is benchmarked against state‐of‐the‐art cable models both in the frequency domain and in the time domain. Finally, the study provides a comparative example of the impact of the cable modelling on the small‐signal dynamics of a point‐to‐point voltage‐source converter (VSC) HVDC transmission scheme.

The increasing diffusion of renewable energy sources in the power systems is likely to extend in the near future to power supply of railways. This paper compares the technical and economic benefits of several configurations with power electronics converters for the integration of photovoltaic sources into the railway power supply systems. For each of these configurations, a design methodology is proposed for selecting the ratings of the railway power supply components. The requirements for the phase imbalance on the feeding transmission line are assumed in accordance with current regulatory standards and, wherever necessary, phase balancers are added. The design methodology is applied to the power supply of a high-speed railway and the configurations under study are numerically compared based on their technical feasibility and economic cost, using the generation capacity of the photovoltaic source as independent variable. The analysis demonstrates that with a progressive integration of PV sources into railway systems, the configurations with power converters supplying the overhead lines could become more beneficial than more classical solutions where the overhead line is supplied via a transformer.

The single-stage grid-connected photovoltaic (PV) topology has recently drawn attention as it can reduce overall losses and installation costs. This paper presents a new control approach for single-stage grid-connected PV systems. The proposed controller is a combination of a finite control set model predictive control (FCS-MPC) and a maximum power point tracking (MPPT) algorithm, which ensures the extraction of maximum power from the PV panels and good transient performance for the output voltage and current. The disadvantages of classical MPPT algorithms in tracking the global maximum power point under fluctuating environmental conditions are avoided by including additional constraints in the cost function of the FCS-MPC. Further, the controller is tested for partial shading in PV. The performance of the proposed controller is compared with the two-stage and single-stage PV configuration with different controls and MPPT algorithms. The simulation results show that the single-stage PV system with the proposed control can effectively extract the maximum power from the PV system and maintain a stable output signal for the transient condition. Finally, experimental results according to a control hardware-in-the-loop (C-HIL) approach are presented to validate the effectiveness of the proposed algorithm.

This paper analyses the requirements for load balancing of a new transformer-less grid interface topology for large-scale electric vehicle (EV) charging infrastructures. The proposed configuration utilizes a Modular Multilevel Converter (MMC) to supply wireless EV chargers from each module. The inherent galvanic isolation provided by wireless inductive power transfer and the scalability of the MMC topology enable transformer-less connection to medium voltage (MV) distribution grids. This can reduce the footprint and copper volume of the internal power distribution for the parking infrastructure. The load distribution within the MMC topology depends on the location and power requirements of each EV to be charged. Requirements for load balancing by controlling the internal circulating currents of the proposed topology when supplying unevenly distributed loads are derived. It is also demonstrated how a second harmonic component of the circulating currents can be utilized to ensure balancing capability within each MMC arm, and how its required amplitude depends on the load distribution. The theoretical analysis and the performance of a corresponding control strategy are first verified by time-domain simulations of a large-scale infrastructure. Experimental results from a small-scale prototype based on an MMC where each arm has 12 modules with individual controllable loads are presented.

Eigenvalue-based analysis of small-signal dynamics in High Voltage Direct Current (HVDC) transmission systems requires cable models that are compatible with a state-space representation. While distributed parameter models accounting for frequency-dependent effects are inherently incompatible with a state-space representation, a conventional π-model can only represent the cable behavior accurately at a single frequency. Instead, a Frequency-Dependent π-model (FD-π), consisting of a lumped circuit representation with multiple parallel RL-branches in each π-section can be utilized to reproduce the frequency-dependency of the cable characteristics in a specified frequency range. Based on an evaluation of relevant error metrics for FD-π models, this paper demonstrates how the number of sections and the number of parallel branches in each section will influence the accuracy. From this starting point, an optimization algorithm for identifying the configuration that fulfills specified accuracy requirements with the lowest possible model order is introduced. A similar algorithm for identifying the most accurate FD-π model within a specified maximum model order is also proposed. Numerical results for different cable lengths and cross sections are presented to highlight their effect on the model, and it is demonstrated how the cable model configuration can influence the results from small-signal eigenvalue analysis of an HVDC transmission system.

In this article, five strategies for controlling the negative sequence currents of a virtual synchronous machine (VSM) under unbalanced conditions are comparatively assessed, considering both grid-connected and islanded operations. While these strategies have been individually proposed in recent literature, their characteristics and performance have not been systematically evaluated. Thus, a VSM structure serving as a basis for comparing the different strategies is presented in detail, with modifications for preventing double frequency power oscillations from influencing the virtual swing equation and the inertial dynamics. The active and reactive power oscillations, and the power transfer capability within the converter current limitation, are theoretically derived as a function of the unbalances in the local voltage. Simulation results and experimental validation on a 50-kVA modular multilevel converter (MMC) prototype with 12 sub-modules per arm demonstrate the validity of the theoretical analysis. The presented results serve as the basis for evaluating the applicability of the studied control strategies under different operating conditions.