Power-synchronization control (PSC) and vector current control (VCC), the latter including cascaded outer loops, are, respectively, considered to be grid-forming and grid-following schemes. They have significant structural differences. Despite that, it is here shown that the two schemes can be unified by making a series of minor modifications to PSC. This results in a universal controller, allowing various combinations of the two schemes to be explored. Fundamentally, it allows PSC to be used as guideline for a robust VCC design, permitting stable and well-performing operation irrespective of the grid strength.

In grid-forming control, the swing equation of a synchronous machine is emulated by a power controller. Thereby, frequency droop, power-oscillation damping, and/or virtual inertia can be obtained. In this letter it is shown that a phase-locked loop (PLL) - which is normally grid following - can be designed so as to, in turn, emulate a generic power controller, thus, becoming grid forming.

This paper deals with grid-voltage sensorless synchronization and control under unbalanced grid conditions. A three-phase grid-connected converter equipped with an LCL filter is considered, and no signals other than the converter currents and the dc-link voltage are measured for control. An augmented adaptive state observer is proposed for estimation of the positive- and negative-sequence components of the grid voltage. Dynamic performance limitations of the proposed method and effects of parameter errors are analyzed. The proposed observer is tested as part of a sensorless control system. Experimental results show that the proposed method works well even under highly unbalanced grid conditions.

The phase-locked loop (PLL) is a commonly used synchronization control method for grid-tied inverters. The PLL-synchronized inverters tend to have poor stability robustness with weak grid interconnections, especially when the PLL is designed with a high control bandwidth. To tackle this challenge, this article proposes an enhanced q -axis voltage-integral damping control, which not only stabilizes PLL-synchronized inverters in weak grids but also lifts the restriction on PLL bandwidth. This superior feature enables inverters to operate stably in ultraweak grids and with a superior transient response. The experimental tests confirm the performance of the method with 400-Hz PLL bandwidth under a short-circuit ratio of 1.28 of the grids.

The main purpose of this article is to elaborate on the limitations of using frequency-domain passivity theories in analyzing grid-inverter interactions within the low-frequency range. It primarily covers three levels of limitations: 1) the limitations and selection criteria of two kinds of passivity index, 2) potential conflicts between different passivity index tuning methods, and 3) the relationship between the frequency range of negative passivity index and system stability robustness. The findings suggest that caution should be exercised when applying passivity theory, particularly in the low-frequency range.

A multifunctional grid-forming control method based on a disturbance observer structure is proposed for grid converters. The proposed method has several different control modes, including the grid-forming, grid-following, and current control modes. The proposed method does not require any separate synchronization loop as it is incorporated in the disturbance observer. Links to existing control methods are shown and tuning recommendations are provided based on small-signal stability analysis. Furthermore, robustness against the inductance error in the grid-forming mode is analytically shown. Experiments with a 12.5-kVA converter are used to assess power-tracking performance. The method is shown to have excellent dynamic performance regardless of grid strength and to provide seamless switching between grid-forming and grid-following modes. In addition, the effect of grid voltage sags and grid-frequency deviations is studied.