When multiple virtual synchronous generators (VSG)s operate in parallel in an islanded microgrid, the sudden load power change could lead to significant transient circulating (and oscillating) power. In this paper, the transient circulating power is first analyzed with the two-VSGs small signal model. On this basis, the paper defines parameter design principles that theoretically eliminate all transient circulating power. Due to the line impedances uncertainty in practice, the ideal parameter design is not always available. Then, to suppress the remaining transient circulating power, an enhanced VSG (EVSG) control strategy is further proposed, by deriving and revising a two-VSGs transfer function from load disturbance to power angles difference. The proposed parameter design principles and the EVSG can be applied sequentially, so that the transient circulating power can be systematically suppressed without the side-effects as in existing methods. An experimental platform with 3 VSGs is established. The parameter design principles and the EVSG are thoroughly validated by experimental results. This is also the first time the 3-VSGs experimental results are reported, while all the existing literature has 2-VSGs data. It confirms that the two-VSGs analytical results can be extended to systems with more VSGs.

Grid-forming inverters (GFI) are used to retain the grid inertia and frequency stability with increasing inverter-based sources in the grid. However, as GFI is essentially a fundamental frequency voltage source, grid voltage distortion would induce significant grid current harmonics. In this article, the limitations of the existing types of GFI harmonic current suppression methods are analyzed from a perspective of the equivalent circuit model at harmonic frequencies. Based on this perspective, an intuitive fundamental and harmonic frequencies decoupled control (FHDC) scheme is proposed. At the fundamental frequency, FHDC tracks the voltage command from the GFI power control loop. At harmonic frequencies, the FHDC eliminates the grid-side current harmonics, by canceling out the capacitor current harmonics and eliminating the harmonics otherwise introduced into the inverter-side current command. Compared with existing methods, FHDC is more effective, and also adaptive to wide ranges of grid impedances; meanwhile, it needs no additional sensors and hardware, making it applicable to existing GFI hardware. Finally, in the experiment, the proposed FHDC is not only compared with the conventional GFI, the existing GFI harmonic suppression methods are also implemented in lab test to comparatively verify the superiority of the FHDC.

When multiple Virtual Synchronous Generators (VSGs) operate in parallel in an islanded grid, power and frequency oscillations will occur when one VSG goes offline. However, the existing literature does not cover the related analysis and transient suppression schemes for this scenario. To analyze these complex high-order system dynamics, this paper first establishes an intuitive equivalent circuit model for multiple VSGs, and the frequency domain expressions of the multi-machine VSG system during the VSG offline transient simulation are then derived. Based on the multi-VSG model and its transient oscillations analysis, this paper further proposes a configuration scheme for the equivalent circuit parameters. Equivalently, the virtual inertia, damping coefficient and virtual impedance of the VSGs can be configured. With the proposed parameter configuration scheme, the power oscillation during the VSG offline transient can be eliminated, as verified by experiments with a microgrid lab platform using three VSGs. Compared with the existing multi-VSG studies, the proposed scheme is not only the first attempt to study the transient suppression when a VSG goes offline, but also is more intuitive in analysis and less complicated in the controller parameter tuning.

Virtual synchronous generator (VSG) control enables grid-connected power electronic converters to retain the inertia support and frequency stability features of traditional synchronous generators in power grids. The power coupling of VSG remains a major issue, where reactive power deviates from the commands. Numerous existing VSG decoupling methods essentially focus on applying virtual impedances in different ways to alter the equivalent line impedance ratio. These methods are less effective under varying operating points. To address the problem, the voltage compensation term from virtual resistance and inductance is first made adaptive to varying operating points through the proposed online parameters’ adjustment. Then, it is further discovered that it is still not possible to achieve total power decoupling for the full operating range. Therefore, an additional voltage compensation term, in terms of the power angle variations, is proposed to eliminate the power coupling at high power ranges. The two proposed voltage compensation schemes are seamlessly integrated so that total VSG power decoupling can be achieved. Through comparative lab tests with existing methods, it is validated that the proposed method is more effective in eliminating reactive power coupling under varying and high-power operating points during both steady and transient states.

The stochastic fluctuations of AI computational loads inject harmonic currents into the DC bus, amplifying bus voltage ripples and weakening the power quality. Existing strategies typically rely on high-gain control strategies to minimize harmonic output impedance, aiming at full absorption of harmonic currents. However, such designs rarely consider engineering constraints such as capacity and current boundaries, which impose inherent limits on harmonic absorption. To address these issues, this paper proposes an adaptive harmonic impedance control and flexible compensation method for AI data centers. By integrating DC bus voltage feedforward with output current feedback, a virtual harmonic impedance control channel is constructed to enable real-time impedance shaping. Then, an adaptive gain regulation mechanism is developed to adjust harmonic impedance according to the available capacity and current margin. Compared with traditional strategies relying on fixed high gains or resonant links, the proposed method allows for the continuous regulation of harmonic impedance over a wide range. This enables the dynamic matching of harmonic absorption capability with the available capacity, effectively suppressing the risks of overcurrent, saturation, and stability degradation. Simulation and 8 kW experimental results verify the correctness and effectiveness of the proposed analysis and control strategy.