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 recent years, active power filters have been widely studied. The research has mainly concentrated on voltage-source active filters, but some attention has also been paid to an alternative-to current-source active filters. Since voltage-source pulsewidth modulation (PWM) technology is widely used in industrial applications, this has also been more common in active filter use. In addition, current-source technology has been said to have drawbacks compared to voltage-source systems, such as high on-state losses in the PWM bridge and inefficient inductive energy storage element on the dc side of the bridge. In the paper, the two active filter topologies are compared. First, the main circuits and space-vector modulation techniques used are studied and the digital control systems are presented. The filtering performances of the systems with different kinds of nonlinear load are examined and finally the power losses of the active filters are studied. The comparison is based on measurements with the prototypes built. The results clearly show the properties of the active filters examined. In addition, the results prove that current-source active filters also offer a considerable choice for harmonic filtering

In this letter, an enhancement of power-synchronization control is proposed, whereby pole-zero cancellation in the closed-loop system is achieved. An effect thereof is that step-response ringing and overshoot are eliminated. For strong grids, the closed-loop bandwidth increases, allowing a shorter step-response rise time.

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.

The properties of complex space-vector models for asymmetric three-phase systems are investigated in this article. Most importantly, three alternative methods for the stability analysis of the asymmetric closed-loop systems are presented. The end results avoid the usage of matrix manipulations. It is shown how the theory can be applied to modeling and stability analysis of a grid-connected voltage-source converter (VSC). The methods are compared using numerical examples.

First-order L type supply Alters do not always offer sufficient attenuation of switching ripple currents in voltage- source active power filter applications. By using third-order LCL filters the level of the switching frequency current components can be reduced more effectively. However, because of the resonance phenomenon of the supply filter and the active filter wide operating bandwidth, the use of the LCL filter in the active filter applications is challenging. This paper compares an active and a passive method for LCL filter resonance damping, assesses their suitability for the active power filter application and presents their benefits and drawbacks. The passively damped LCL filter utilizes a control system designed for an active filter with a first-order L type filter, while the active damping method examined is based on the cascaded control of the mains side inductor and the capacitor currents. First, the frequency responses of the transfer function models are examined and second, the topologies are studied through experimental tests. The results presented show that both topologies filter the harmonics effectively and the switching ripple is attenuated. The active method requires more current sensors, while additional damping resistors are needed in the passively damped system. However, there are only slight differences both in the filtering results and the power losses.