A novel small-scale self-focusing suppression method for ultrahigh peak power lasers

We proposed a novel method, using an asymmetric four grating compressor (AFGC) to improve the spatial uniformity of laser beams, to suppress the small-scale self-focusing (SSSF) during the post-compression of ultrahigh peak power lasers. The spatial uniformity is an important factor in performing post-compression, due to the spatial intensity nonuniformity will be enhanced while going through a nonlinear process. And what's more, the strong intensity spikes induced during nonlinear process can seriously damage the subsequent optical components. Moreover, the three-dimensional numerical simulations of the post-compression are implemented based on a petawatt (PW) class laser with a standard compressor and an AFGC. The results show that the post-compression with AFGC can shorten the laser pulses from 30fs to sub-10fs and meanwhile efficiently suppress SSSF. This work provides a promising scheme for the post-compression scaling to PW and even 10PW lasers

Guest editorial : Dynamic analysis, control, and situation awareness of power systems with high penetrations of power electronic converters

In recent decades, global power grids have evolved with a rapid and extensive development of power electronic converters (PEC), including renewable energy systems (RES), high-voltage DC (HVDC) transmission, flexible AC transmission system (FACTS), energy storages, and microgrids. The distinct characteristics of power electronic devices traditional synchronous generators, especially their rapid control speed, wide-band performance and lack of inertia response and spinning reserve, are altering grid dynamics, and inducing new stability challenges. Continuation of such trends could further exacerbate the risk to the stability of power grids because of factors such as low inertias, lack of spinning reserve to quickly nullify active power mismatch between demand and supply. Therefore, scientific investigations on novel dynamic modelling and stability analysis methods, data-driven monitoring and situation awareness on grid inertia-power-frequency evolution, grid dynamic frequency forecast methodologies in consideration of novel PEC control schemes, and advanced PEC grid integration control schemes to minimise frequency management risks become increasingly crucial for the secured operations of power systems with high PEC penetrations. In this Special Issue, namely ‘Dynamic Analysis, Control, and Situation Awareness of Power Systems with High Penetrations of Power Electronic Converters’, we have presented eight original papers of sufficient quality and innovation. The 10 eventually accepted papers can be clustered into three two categories, namely novel control design, stability and fault analysis

Stationary-Frame Modeling of VSC Based on Current-Balancing Driven Internal Voltage Motion for Current Control Timescale Dynamic Analysis

High-frequency oscillations caused by voltage source converters (VSCs) are constantly emerging in power systems with the increasing penetration of renewable energies. VSC models in current control timescale play a pivotal role in the analysis of these oscillation issues. Conventional VSC models show few physical mechanisms of VSC dynamics during system oscillations. Hence, this paper proposes a VSC model from the viewpoint of its internal voltage (namely, VSC output voltage), which is driven by current balancing between the current reference and the feedback in a stationary frame. The proposed model can be used to study VSC dynamic characteristics in an intuitionistic physical way. Based on the proposed model, it is found that VSC current reference in a stationary frame varies with internal voltage dynamic due to the essence of active and reactive current control in a current control timescale. Additionally, in a stationary frame, the dynamic relation between internal voltage and error current embodies a generalized integrating characteristic with adaptable center frequency determined by a phase-locked loop, which guarantees the zero steady state error of VSC current control. Comparisons of simulations between the proposed model and a switch-based model validates the effectiveness of the proposed model

Improved rotor current control of wind turbine driven doubly-fed induction generators during network voltage unbalance

This paper investigates an improved control and operation of a doubly-fed induction generator (DFIG) system under unbalanced network conditions. A new rotor current control scheme is presented, which consists of a main controller and an auxiliary compensator. The main controller is constructed in the same way as the conventional vector control design without involving sequential-component decomposition in order to guarantee system stability and high transient response. While the auxiliary controller is specially designed to control the negative sequence current taking into account the impact of the main controller on negative sequence components. Simulated results on a commercial 1.5-MW DFIG system and experimental tests on a 1.5-kW DFIG prototype are provided and compared with those of conventional vector control and dual PI current control schemes to demonstrate the effectiveness of the proposed control strategy during steady-state and transient conditions when the network voltage is unbalanced