With increasing penetration of the renewable energy, the grid-tied PWM inverters need to take corresponding responsibilities for the security and stability of future grid, behaving like conventional rotational synchronous generator (RSG). Therefore, recognizing the inherent relationship and intrinsic differences between inverters and RSGs is essential for such target. By modeling the typical electromechanical transient of grid-tied PWM inverters, this paper first proves that PWM inverters and RSGs are similar in physical mechanism and equivalent in mathematical model, and the concept of static synchronous generator (SSG) is thereby developed. Furthermore, the comprehensive comparison between RSG and SSG is carried out in detail, and their inherent relation is built. Based on these findings, the rationality and feasibility of migrating the concepts, tools, and methods of RSG stability analysis to investigate the dynamic behaviors and stability issues of SSG is therefore confirmed. Taking stability issues as an example, the criteria of small signal and transient stability of a typical grid-tied PWM inverter is put forward to demonstrate the significance of the developed SSG model (including synchronizing coefficient, damping coefficient, inertia constant, and power-angle curve), providing clear physical interpretation on the dynamic characteristics and stability issues. The developed SSG model promotes grid-friendly integration of renewable energy to future grid and stimulates interdisciplinary research between power electronics and power system.

To reduce the frequency deviation and the rate of change of frequency (RoCoF) in a low inertia power system, some converters are required to provide the frequency response (FR) power normally associated with the frequency deviation and/or the RoCoF, by droop/inertia/PD control. In this paper, a rapid power compensation (RPC) based FR strategy is developed to optimize the ability to compensate grid imbalance power, by fully exploiting the converter idle capacity. To this end, first, mathematical proof demonstrated the improved performance of the RPC strategy in terms of frequency deviation suppression versus droop control, and in terms of RoCoF suppression versus inertia control, with identical converter capacity limit. Moreover, it is proved that the RPC strategy can achieve consistent FR performance w.r.t. the optimal PD control, i.e. it can maximize the suppression of frequency deviation and RoCoF simultaneously, yet avoiding the limitations due to unknown grid parameters. Finally, by analyzing the operation modes and identifying the pertinent switching logic, detailed implementation of the proposed RPC strategy is developed. Its superb FR performance is verified by experiment results in a 2-converter low-inertia system, and simulation results in an IEEE 4-machine 2-area system.

Active power filters are conventionally utilized to compensate steady-state harmonic currents and reactive power in the utility, yet their capabilities are usually limited if the elimination of undesirable effects associated with decaying DC components during transients is the target, especially under weak grid condition. In this paper, active cancellation of the typical first-order decaying DC-mode transients is explored. To this end, a real-time detection algorithm of decaying DC-component is firstly developed for a generic single-phase distorted AC current signal. Furthermore, fast and robust elimination of both decaying DC-transient and harmonics can be performed simultaneously in 3-phase grids, by adoption of moving average filters in $d-q$ frame and subsequent calculations. Hardware-in-the-loop experiment results verified the effectiveness of the proposed technique.

Droop control, inertia control, and PD control are three typical frequency response (FR) strategies of grid-tied inverters interfaced renewable energy sources (RES) to restrain the power system frequency indicators, i.e., frequency deviation ( $\Delta \omega $ ) and the rate of change of frequency (RoCoF). Due to the highly limited inverter capacity, the FR performance in power systems with high RES penetration should be optimized. To this end, this paper first analyzes the impact of high-penetration renewable energy injection on power system frequency stability, and derives the system frequency expressions in the absence of an FR scheme, and in the presence of droop/inertia/PD controls that are representative for various virtual synchronous generator-based FR schemes. Under the same constraint of available FR capacity, control parameter ranges of these FR schemes are obtained, and their optimal conditions for restraining the maxima of $\Delta \omega $ and RoCoF are identified and compared analytically. Comparison conclusions are finally proved by the provided experiment and simulation results.

Inverter-based standalone power systems (ISPSs), with limited inertia/damping, are prone to significant changes in frequency indicators encompassing frequency deviation and rate of change of frequency (RoCoF), which can easily exceed the pertinent relay thresholds and cause power outages. Limited by estimating the disturbance and system inertia/damping deficiency, existing controls cannot ensure system frequency stability. To overcome this issue, a frequency trajectory planning (FTP) based strategy is developed in this article to improve frequency stability of droop-controlled ISPS. This frequency-indicator-oriented control relates system frequency with several pre-defined planning parameters, decoupling from system's disturbances and inertia/damping deficiency that are difficult to evaluate. During normal operation or when the disturbance level is insignificant, the inverter operates in standard droop mode since the system intrinsic inertia/damping features suffice to regulate the frequency. In the event of a large disturbance, an FTP block is triggered by detected frequency indicators, and a marginally safe frequency trajectory is planned according to the requirement of the grid code. In this case, by tracking the planned frequency trajectory, the inverter provides suitable inertia/damping support needed by the system, thereby guaranteeing frequency stability of the ISPS. Finally, simulation and experimental results proved the effectiveness and advancement of the FTP based strategy.

Droop-controlled inverters reduce transient and steady-state frequency deviations (FDs) by providing frequency regulation (FR) power proportional to the FD during primary FR. However, with the power–frequency droop mechanism, the gap between a disturbed load and an inverter's dispatch instruction can only be eliminated by increasing the FR power at the expense of increased FDs, which are thus prone to exceeding the pertinent relay thresholds. To address this issue, a preset power based droop control scheme is developed by rapidly presetting a constant power in the active power loop to modify the dispatch instruction at the beginning of a large disturbance event. With the preset power framework, the adjusted dispatch instruction significantly suppresses most of the imbalanced power; hence the traditional droop control can eliminate the remaining imbalanced power by providing a small FR power with small FDs. To this end, first, this article demonstrates that the principle of preset power is to reduce the droop power related to FDs. Based on the above principle, analysis shows that the adaptability and rapidity of the preset power are required for better FR performance. Then, the implementation of the proposed control is elaborated, including the fast calculation of the preset power and the mode switching method. Finally, the experiment and simulation results verify the superior FR performance of the proposed control.