A large number of non-linear hardware and control units exists in power electronic system used in grid connected devices. The analytical transient stability analysis of grid-connected converters presents numerous difficulties. A common method to tackle this problem is the stability analysis using Lyapunovs method. By applying this method, difficulties arise not only from finding a suitable Lyapunov function, but also from checking the constraint of Lyapunov stability. If the appropriate Lyapunov function is a high-order polynomial, it is very challenging to test if it meets the constraints of Lyapunov stability in certain regions. In this paper, the sum-of-squares programming method is used to obtain the estimation of a converters domain of attraction with a relatively small number of iterations compared to classically applied methods, such as the Monte Carlo method. The estimation of the domain of attraction are verified by time-domain simulations and StarSims controller hardware-in-the-loop tests in this paper.

The precise control of output power by grid-connected converters relies on the correct identification and tracking of a grid voltage’s phase at the converter terminal. During severe grid faults, large disturbances cause the converter’s operating point to move away from the stable equilibrium point during normal operation. This leads to oscillations of both the active and reactive power fed into the grid. Using large-signal modelling, this study investigated the converter’s dynamic processes during and after such fault situations. The investigation considered the influence of the converter’s phase-locked loop (PLL), responsible for phase tracking, as well as that of the DC link on the converter-grid system, which has a major influence on the active power exchange with the grid. On this basis, this study also focused on the reactive current reference’s influence during and after fault clearing. Furthermore, an easily implementable strategy for reactive current injection, leading to minimum power oscillations, was presented. The results and the optimized strategies were validated via controller hardware-in-the-loop tests.

This paper presents an examination of grid-forming converters (GFM) under low-voltage-ride-through (LVRT) conditions. It emphasizes the influence of inner loop control strategies, and grid topologies on GFM performance. The study introduces a versatile equivalent modeling methodology suitable for different inner loop control strategies. Additionally, it evaluates three LVRT testing devices: the shunt impedance based voltage sag generator (SIVSG), programmable voltage source with impedance (PVS), and Hardware-in-the-Loop (HIL) system, highlighting their differences in simulating grid fault characteristics. The results indicate that SIVSG and PVS have limitations in mimicking actual grid fault scenarios, potentially leading to GFMs erroneously passing LVRT tests. Conversely, HIL-based testing methods more accurately replicate grid faults, offering a more reliable assessment of GFM performance.

With the increasing penetration of converter-based power sources into the power system, the performance of the converter has become a key factor for enhancing grid reliability, especially during asymmetrical faults. To meet the low voltage ride-through requirements, the converter should feed the reactive power to the grid for voltage support while ensuring the maximum current limitation for the converter’s safety. For such injections, the grid codes are defined. This paper presents a novel and simplified reference current generation scheme to fulfill the requirement of recent grid codes, ensure the current limit of the converter and confirm better utilization of the converter’s current capacity during asymmetrical faults. Moreover, it also discusses the new sequence extraction scheme based on the delay sample method in the stationary reference frame and the control modifications for the negative sequence current injection. The proposed scheme was tested for different priority injection schemes. Its performance was also compared with other control schemes. Detailed simulation studies, in MATLAB/Simulink, were presented to confirm the performance of the proposed scheme under different faulty conditions. The results confirmed the supremacy of the proposed scheme over the available schemes for better utilization of the converter’s current capacity during asymmetrical faults. It also ensured the peak current limitation of the converter while fulfilling the recent grid code requirements. Moreover, the results showed that the new scheme has 10% more current capacity compared to the other schemes due to better incorporation of the angle between the positive and negative phase sequences of the voltage.

Grid-forming converters are attracting attention for their significant advantages in terms of stability in a weak grid and simulated inertia. However, while they offer great flexibility due to the use of power semiconductors, they are also affected by their low current-carrying capacity. This means that during a fault, instead of the usual voltage control, a current limiting control is active, which changes the dynamic performance of the converter and influences transient stability. This manuscript focuses on the dynamic performance of grid-forming converters during the restart phase at the post-fault period, and proposes an initial phase threshold to prevent the converter from going into current saturation. Based on this, the manuscript proposes several restart strategies during the post-fault period, by using some fast resynchronization methods in order to meet the requirements of the converter’s stable operation and fast active power restoration. Finally, the above findings and the proposed strategies are validated by a joint control hardware-in-the-loop system.

Zusammenfassung Motiviert durch die Energiewende steuert die elektrische Energieversorgung in vielen Ländern auf eine Erzeugungsstruktur zu, welche von leistungselektronisch gekoppelten Erzeugungseinheiten, allen voran Windkraftwerken (WKW) und Photovoltaikanlagen, dominiert wird. Dies hat zur Folge, dass sich die Auslegung der elektrischen Betriebsmittel sowie die Planung und der Betrieb der Netze zunehmend an den Eigenschaften solcher Anlagen ausrichtet. Wegen ihrer technischen Komplexität stellen dabei Windkraftwerke (WKW) mit doppelt speisenden Generatoren (double-feed induction generator bzw. doubly-fed induction generator, DFIG) eine besondere Herausforderung dar, da bei ihnen sowohl die durch die Physik bestimmten Eigenschaften der rotierenden elektrischen Maschine als auch das maßgeblich durch Steuerung und Regelung beeinflusste Verhalten des Umrichters zum Tragen kommt. In diesem Artikel wird das Verhalten der aktuellen Technologie von DFIG bei Kurzschlüssen und anderen Netzfehlern beschrieben, der Stand der Technik in der für Anlagenauslegung wichtigen Kurzschlussnorm erläutert sowie ein neuer Vorschlag für die Nachbildung der WKW mit DFIG in dieser Norm unterbreitet. Weiters wird auf spezielle Anforderungen an den Netzschutz eingegangen.