Exploring the capabilities of active grids

Active grids are commonly used in wind tunnels to generate turbulence with different characteristic features. In contrast to the common objective to generate turbulence with a very high Reynolds number, this work focuses on a method of blockage induced flow design for the generation of special flow structures. Particularly, we aim to investigate the underlying constraints of this excitation method. For this purpose, the scale dependency of the excitation is studied by clearly defined structures such as periodic sinusoidal velocity variations, velocity steps, and single gusts. It is shown that the generation process is limited by the reduced frequency of the active grid motion. For low values of reduced frequencies the imprinted flow structures remain undamped, whereas for higher reduced frequencies they are damped. This insight leads to the constraint that the active grid motion needs to be modified to compensate for the underlying dynamic damping effects. Thus, the inserted energy has to be increased for the corresponding reduced frequencies. This finding can be transferred to the generation of turbulent flows, for which an exemplary adaption is shown . Graphic abstract

The Strategy of Active Grid Frequency Support for Virtual Synchronous Generator

Virtual synchronous generator (VSG) control is a promising control approach for voltage source converters as an interface between new energy sources and the power grid. VSG is a grid-friendly control scheme, which can imitate the mechanical inertia of the synchronous generator (SG) and the power droop characteristics. Yet, the droop characteristics imitation of SG induces the frequency variation of the grid-connected inverter along with the droop characteristic curve, which will deteriorate the performance of the grid frequency support during the transient process. In this paper, a control scheme, which shapes the droop curve during the disturbance, is proposed for active grid frequency support. First, a load disturbance extraction strategy with a high-pass filter is applied in the proposed method, and the disturbance component is effectively extracted to compensate for the frequency reference variation in traditional VSG control. The grid frequency is actively supported by shaping the droop curve of active power to the frequency of VSG during the disturbance. Afterward, H∞ and H2 norms are used as the objective function to quantify the control performance of the proposed method, and the particle swarm optimization (PSO) algorithm is applied to optimize the control parameters of the proposed method. With a well-optimized high-pass filter, the active support performance is further improved. Finally, the simulation results and hardware in the loop (HIL) tests verify the effectiveness of the proposed method.