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Virtual synchronous generators (VSGs) are widely introduced to the renewable power generation, the variable-speed pumped storage units, and so on, as a promising grid-forming solution. It is noted that VSGs can provide virtual inertia for frequency support, but the larger inertia would worsen the synchronization stability, referring to keeping synchronization with the grid during voltage dips. Thus, this paper presents a transient damping method of VSGs for enhancing the synchronization stability during voltage dips. It is revealed that the loss of synchronization (LOS) of VSGs always accompanies with the positive frequency deviation and the damping is the key factor to remove LOS when the equilibrium point exists. In order to enhance synchronization stability during voltage dips, the transient damping is proposed, which is generated by the frequency deviation in active power loop. Additionally, the proposed method can realize seamless switching between normal state and grid fault. Moreover, detailed control design for transient damping gain is given to ensure the synchronization stability under different inertia requirements during voltage dips. Finally, the experimental results are presented to validate the analysis and the effectiveness of the improved transient damping method.

Introduction: Grid-forming control has received increasing attention for being an effective solution to cope with low-inertia and weak damping systems. Owing to the basic characteristics of transient voltage regulation, inertia support and primary frequency regulation (PFR), virtual synchronous generator (VSG) is the most promising candidate of grid-forming control scheme. The damping characteristic plays a significant role in stabilizing when the system is disturbed. However, the traditional approaches for damping emulation pose a number of problems, such as the introduction of phase-locked loop (PLL) that may lead to stability issues, or the blurring of the functional distinction between damping characteristic and primary frequency regulation. Moreover, the grid strength affects the operational characteristics of the converters. Methods: Based on the background of these issues, firstly, an effective transient damping power strategy is proposed in this paper. In contrast to conventional damping approaches, the proposed scheme provides a positive damping during transient period that suppresses the fluctuation of active power, and has no impact on the steady-state frequency droop characteristic. Ulteriorly, based on small-signal models and classical control theory, an parameters adjustment strategy for both active and reactive power control is proposed to considerably enhance the adaptability of the converter to the variations in grid strength. Results and Discussion: The results obtained from PSCAD/EMTDC and hardware-in-the-loop (HIL) platform verify that proposed control strategy exhibits excellent transient damping effect, the decoupling characteristic between fixed damping coefficient and PFR coefficient, and performs well across a broad spectrum of grid strengths. [ABSTRACT FROM AUTHOR]

A virtual synchronous generator (VSG) has a good adaptability to the weak grid but its grid-connected active power (GCAP) has the problem of a slow dynamic response under the active power command step. An optimization strategy of the GCAP dynamic response for the VSG based on the virtual negative impedance combined with the active power transient damping control algorithm is proposed in this paper. The optimization strategy first uses the virtual negative impedance control method to reduce the VSG equivalent output impedance and the GCAP dynamic response time of the VSG. Then, the transient damping as well as the inhibition ability of the GCAP dynamic oscillation for the VSG are enhanced by the active power transient damping control algorithm. The Matlab/Simulink simulation software is used to study the GCAP dynamic response performances of the VSG in the condition of the active power command step, and the experimental test platform of a VSG grid-connected system is established. The simulation and experimental results jointly verify the feasibility and superiority of the proposed strategy in improving the GCAP dynamic response characteristics of the VSG under a weak grid. [ABSTRACT FROM AUTHOR]

A virtual synchronous generator (VSG) has a good adaptability to the weak grid but its grid-connected active power (GCAP) has the problem of a slow dynamic response under the active power command step. An optimization strategy of the GCAP dynamic response for the VSG based on the virtual negative impedance combined with the active power transient damping control algorithm is proposed in this paper. The optimization strategy first uses the virtual negative impedance control method to reduce the VSG equivalent output impedance and the GCAP dynamic response time of the VSG. Then, the transient damping as well as the inhibition ability of the GCAP dynamic oscillation for the VSG are enhanced by the active power transient damping control algorithm. The Matlab/Simulink simulation software is used to study the GCAP dynamic response performances of the VSG in the condition of the active power command step, and the experimental test platform of a VSG grid-connected system is established. The simulation and experimental results jointly verify the feasibility and superiority of the proposed strategy in improving the GCAP dynamic response characteristics of the VSG under a weak grid.

The virtual synchronous generator (VSG) was proposed to emulate a synchronous machine's dynamics when integrating power electronic converter-based distributed energy resources to the power grid. However, the VSG's synchronization stability during grid faults is not fully explored. The underlying mechanism of loss of synchronization still needs to be further revealed due to VSG's nonlinear characteristics. In this article, a step-by-step analytical method based on combining the linear and nonlinear models is proposed to analyze VSG's dynamic behaviors during a large disturbance. The relationship between the linear and nonlinear models is first brought to light, showing that the linear model is suitable for qualitative analysis to give an intuitive physical insight. Simultaneously, the latter is adopted for quantitative analysis to assess stability after a grid fault. Moreover, to avoid the conflict of the synchronization stability and the inertia response, a transient damping method is added in the active power control loop, which can simultaneously improve the synchronization stability and frequency stability. Design guidelines are also proposed to identify the parameter of the transient damping with different inertia requirements. Finally, the experimental results verify the analytical method and the theoretical analysis. [ABSTRACT FROM AUTHOR]

Anecessary requirement for automatic excitation regulation systems is ensuring the stability of the electric energy systems (EES) and the required quality of the transients (TP) of changes in operating parameters under the effect of various disturbances. In order to improve the damping of TEES, various stabilization channels of the automatic excitation regulator (AER) are used, as a rule. In the microprocessor regulator (MR) of the AER are stabilization channels, both internal (from the derivatives of the rotor current and stator voltage) and external (from the deviation and derivative of the frequency of stator voltage). The article compares the quality of stabilization of the operating parameters when using the AER signal in the frequency stabilization channels of the voltage on the generator buses (regular signal), and the differences between the deviation of the frequency of rotation of the generator shaft and the frequency of voltage on the station buses in EES mode and in certification mode. The effect of the parameters of the synchronous generator and of the conditions of its operation on the areas of the stability of the regulation system and the quality of damping is examined. It is shown that changing the parameters of the stabilization channel filters can increase the regions of stability and enhance the quality of damping of transient fluctuations. [ABSTRACT FROM AUTHOR]

Many transient-based techniques have been developed for leak detection in piping fluid systems, and they are thought to use leak-induced damping and reflection effects to detect and locate the problem. This work investigates the relative importance of the damping and reflective effects from the leak to the leak detection methods with the importance of both effects tested using four representative methods, which vary significantly in their modes of operation and provide the coverage of the major developments in the transient leak detection field. It was found that all transient-based methods are unable to detect and locate the leak in the absence of the leak-reflected signal, whereas the leak is detected and located even in the absence of leak-induced damping. The results indicate that the information previously thought to be contained in the damping of slow transients was found to be low frequency reflections from the leak. [ABSTRACT FROM AUTHOR]

Abstract: Mechanical spectroscopy has been used to study the thermal stress relaxation at the interfaces in magnesium matrix composites. The mechanical loss spectra measured as a function of temperature show an intrinsic component superimposed on transient damping. A model of thermal stress relaxation by dislocation motion controlled by a solid friction mechanism, where the transient damping depends non linearly on is developed. A systematic method is developed to identify the correct level of transient damping. [Copyright &y& Elsevier]

Mechanical spectroscopy has been used in order to investigate thermal stress relaxation in metal matrix composites. Mg- and Al-based composites reinforced with SiC long fibers have been processed by a gas-pressure infiltration technique. Fiber orientations either parallel or perpendicular to the composite axis have been obtained. Specimens have undergone thermal cycling between 150 and 450 K with different temperature rate &Tdot; and excitation frequency ω. A model where thermal stresses are relaxed by dislocation motion controlled by a solid friction mechanism has been used to interpret the experimental results. The model is in good agreement with experimental data and two fitting parameters can be calculated. The first parameter is sensitive to the mobile dislocation density during thermal cycling. The second one is sensitive to the interface strength. [Copyright &y& Elsevier]

Mechanical spectroscopy has been performed in magnesium matrix composites by means of an inverted forced torsion pendulum in order to study thermal stress relaxation occurring at the interface of such materials. It is well known that, because of the difference of thermal expansion between matrix and reinforcements, the thermal stresses can be built up at the interface when temperature changes. Composites were submitted to the thermal cycling in the range of 100–400 K and the mechanical loss was measured as a function of temperature for different temperature rates, T˙ and different excitation circular frequencies, ω. Composites with different reinforcement geometries have been investigated in order to determine the influence of reinforcement orientation in the matrix on the thermal stress relaxation. The main factor influencing the thermal stress relaxation is the coupling between these stresses and the applied stress. [Copyright &y& Elsevier]