Hydrogen production using electrolyzers has gained attention in the course of the global energy transition. Even though the technology itself is well known, the upcoming large-scale integration of electrolyzers poses a challenge for the power system. In inverter-dominated grids, it is beneficial for an electrolyzer to not only serve as a load but also provide system services. However, the unique properties of an electrolyzer need to be considered. In this paper, we present a grid-forming controlled electrolyzer that supports the grid by providing low-voltage ride-through capability and asymmetric inertia during frequency events. The model includes a fault current limitation, prioritizes reactive current, and establishes a proper ramp-up after fault clearing. It respects the limitations of the electrolysis process through appropriate control. A dynamic cell model for proton exchange membrane electrolysis is used on the DC side. The effectiveness of the approach is demonstrated through RMS simulations conducted in PowerFactory.

The declining inertia due to fewer synchronous machines in modern power systems requires power electronic converters with grid-forming properties. Accurate dynamic models of such converters are required to study the influence of the grid-forming control on power system stability. Vendor-specific models are generally not disclosed. An open-source EMT model of an industrial grade controller called SelfSync, including a current limitation controller called SelfLim, is provided in the widely used simulation software DIgSILENT PowerFactory in this work. The controller is compared to a generic model in various study cases using the FNN grid-forming test system and the Nordic 72-bus system. It is shown that the controller is suitable for the analysis of complex contingencies.

The analysis and efficient dynamic simulation of power systems with a large number of distributed generators using grid-forming converter control requires reduced order models. The natural separation of time scales allows for the simplification of the inner control loops, i.e., voltage and current controller. This work provides a structured overview of model reduction approaches of the inner loops and introduces new reduced models based on transfer function approximation. The models are thoroughly tested using frequency response analysis, eigenvalues sensitivity analysis, time-domain simulation and stability regions for various scenarios, varying controller and electrical parameters. Furthermore, the influence of simplifications of the network lines is investigated. It becomes apparent that there is no single reduced model that is suitable for all scenarios. Models ought to be selected depending on the specific scenario. Guidelines for the model selection as well as the choice of controller parameters are provided.

Grid-forming inverter control is recently discussed for bulk power systems and is already in use for islanded microgrids. A common control type is the droop control. Numerous variants of the basic droop control have been proposed. However, there is lack of performance comparison of the droop variants in literature. Their superiority has only been demonstrated for some specific microgrid scenarios. This work composes benchmark scenarios to assess and compare the applicability of droop control variants and also their combination with virtual impedances under practical conditions. A number of microgrid topologies and the interaction with synchronous machines are considered to benchmark the performance. Static criteria, such as the steady-state power sharing, as well as dynamic stability criteria, are taken into account for modal analysis. To guarantee a meaningful comparison, a genetic algorithm tailored to the problem is used to optimise controller parameters for each controller type. Results indicate that the combination with virtual impedance has a more decisive effect on stability than the droop variant. The outcome is relevant for microgrid stability analysis in numerous contexts, such as optimal placement of inverters or topology optimisation, where the choice of the most suitable controller type with optimised parameter sets is key.

Islanded microgrids can enhance the resilience of power systems. Their stability is subject to ongoing research. This work introduces a unique way of sensitivity analysis, parameter tuning and optimization with respect to small-signal stability of grid-forming and grid-supporting droop controlled inverters in microgrids. A large selection of parameters is incorporated and simultaneously optimized based on a genetic algorithm. The sensitivity of parameters so far not considered in literature, such as the measurement filter time constant, is analysed. Simplifications often accepted as valid, e.g. the neglect of current and voltage controller or the line dynamics, are reviewed using the optimized parameter sets and providing new insights into the accuracy of model reduction techniques. Results indicate that the stability can be enhanced drastically by simultaneously optimizing a wide range of parameters. Model simplifications often seen as valid in literature must be carefully considered, as they can result in conservative stability assessment, especially for grid-forming droop control. The results of this work are relevant for the microgrid small-signal stability analysis in numerous contexts, such as reconfiguration, topology optimization or optimal placement of inverters.

Islanded microgrids allow for a continuous supply of customers even when there is an outage in the bulk power system. The frequency control and stability in microgrids is an ongoing field of research. This work investigates the influence of synthetic inertia control of doubly-fed induction generator wind turbines on the dynamics of inverter dominated microgrids with droop control. A special focus is placed on the impact of the phase-locked loop dynamics.

One of the most challenging aspects of controlling microgrids is islanding and resynchronization with the grid. During this process, unwanted disconnection of inverters due to voltage fluctuations or frequency deviations may occur. In order to investigate these phenomena in more detail, inverter models are required, which support the corresponding protective functions. This work describes a concept for modelling an inverter in accordance with the minimum technical requirements for connection and parallel operation of generating plants on the low-voltage grid in Germany for future investigation of the islanding and resynchronization process of microgrids.

The increasing number of distributed generators and storage makes it possible to operate parts of the distribution system as islanded microgrids. A critical aspect of microgrid operation is the resynchronization with the external grid. If the voltages of both systems are not exactly aligned, transients that burden the microgrid components occur. In this work, the stress of diesel powered synchronous generators in low voltage microgrids for various scenarios of voltage angle, amplitude and frequency deviations is investigated. Furthermore, the influence of the R/X-ratio of the electric grid as well as the active and reactive power and the inertia constant of the synchronous generators is examined. The results are important to anticipate generator loadings and to enhance the synchronization control in order limit the burden for the microgrid components.

This work investigates the power sharing of distributed energy resources, such as diesel synchronous generators and inverter-coupled batteries, in islanded microgrids after load transients. Firstly, the necessary accuracy of the diesel synchronous generator model for dynamic simulation and small signal stability studies is examined. Secondly, the controller parameters are optimized deploying a genetic algorithm to enhance the transient load sharing between synchronous generators and battery inverters.

This study is addressing the issues regarding reduction in the necessary network expansion by taking flexibility options into account. If a certain amount of the annual energy from renewable generation is curtailed, the expansion of the power grid can be reduced. This study presents a comparison of the necessary expansion of a high-voltage distribution grid (110 kV) under consideration of different curtailment approaches for renewable energy sources. Here, the necessary expansion is calculated, if a static or a dynamic curtailment is applied and then compared with the calculations without any curtailment mechanism. The curtailment methods are tested using a probabilistic expansion planning method based on the calculation of probabilistic load flow.

One of the most challenging aspects regarding the control of microgrids is the reconnection with the bulk power system. During this process, high stresses can occur for the microgrid components, depending on the alignment of the voltages of the two systems. How deviations in frequency, amplitude and angle of the voltages, as well as a time delay in the switching of the control of the grid-forming generators in the microgrid to grid-feeding control affects these stresses is investigated in this work. Simulations are carried out in two real MV networks with power electronic-based and synchronous generators. It is shown that an angle deviation can strongly affect the stresses of synchronous generators, while delays in the switching of the control mode result in high loads for inverter-based generators. The results can be used to determine the dimensioning of generators in microgrids as well as the layout of the protection system.