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Highlights • The system stability with grid current feedback active damping is analyzed. • The stability criterions of control parameters are established. • A systematic controller design method is proposed. Abstract An LCL filter is commonly used in grid-connected inverter to attenuate the switching harmonics; nevertheless, the stability problems will arise in the grid current control if the resonance of LCL filter is not properly damped. High pass filter-based grid current feedback active damping is an effective solution to solve the aforesaid problem. However, for systems with digital control, the system may exhibit non-minimum phase system characteristics due to the control delay effect, which will affect the system stability. Moreover, the frequency responses of current controller parameters and active damping parameters are coupled. Therefore, to ensure the system stability and good dynamic performance, the current controller parameters and active damping parameters should be designed together, considering the control delay effect. For this issue, a systematic controller design method is proposed for the digitally controlled LCL -type grid-connected inverter with grid-current-feedback active damping. With the proposed method, the satisfactory regions of the control parameters for satisfying the system specifications can be obtained, from which the proper control parameters can be easily and quickly picked out, without the trial and error. Finally, the proposed design method is verified by the experiment. [ABSTRACT FROM AUTHOR]

• The essential requirements for zero-error tracking of current loop are revealed. • The relationship between the post-fault current reference and EMF under various LVRT control strategies is obtained. • A disturbance feedforward control is proposed to accurately realize the transient control targets during LVRT. The existing low voltage ride-through (LVRT) control strategies usually modify the post-fault current reference to contain some transient and negative-sequence components, to achieve the controllable LVRT operating of type-3 wind turbine (WT) under severe grid fault. However, the modified current reference and post-fault electromotive force (EMF) disturbance behave as the multi-frequency (i.e., 0-, 50- and 100-Hz) components in the synchronous dq -frame, thus the classic PI controller-based current control cannot provide zero-error tracking of the aforesaid reference, and leading to the inaccurate implementation of transient control targets. To solve this problem, based on the relationship between the modified current reference and post-fault EMF, an improved disturbance feedforward control is proposed to simultaneously suppress EMF disturbance and improve the tracking capability of modified current reference. As a result, the transient control targets of the improved LVRT control strategies can be implemented accurately. Moreover, the method is easy to implement. Finally, the proposed method is verified by the simulations on a 1.5-MW type-3 WT. [ABSTRACT FROM AUTHOR]

The system inertia is gradually decreasing and frequency security issues are becoming more prominent with the increasing penetration of wind power. To ensure the safety and stability of power system, many countries have updated their grid codes to reinforce the frequency regulation requirements (FRRs) for wind power plants (WPPs). However, most review articles on grid codes cover all grid-integration metrics, while the review of FRRs is inadequate and incomplete; moreover, it shall be updated to accommodate the state-of-the-art grid codes. To cope with it, this study presents a comprehensive review of FRRs for WPPs in modern grid codes, covering 12 representative countries or organizations such as those with the highest development level, the fastest development speed, and the largest installations in wind power. The FRRs in their grid codes for wind power integration are compared and analyzed from three aspects: frequency operating range, primary frequency response and inertia response. Based on the comparative analysis, combined with relevant policy documents, some future trends related to FRRs are concluded. Furthermore, electricity generation sources in each country are analyzed to enrich the understanding of the reasons behind the differences in FRRs. The review helps the transmission system operator to compare and analyze international grid codes and optimize their own grid codes. Moreover, it enables wind turbine manufacturers to master the latest grid codes and grasp the future trends, and helps research focus on practical needs and solve the pain points of wind power development. [Display omitted] • Frequency regulation requirements for wind power integration are reinforced. • A comprehensive comparison of 12 representative grid codes is presented. • Three key aspects in frequency regulation requirements are compared and analyzed. • Future trends related to frequency regulation requirements are concluded. [ABSTRACT FROM AUTHOR]

When working in dynamic environment, traditional SLAM framework performs poorly due to interference from dynamic objects. By taking advantages of deep learning in object detection, a semantic simultaneous localization and mapping framework named Dynamic-SLAM is proposed, in order to solve the problem of SLAM in dynamic environment. First, based on the convolutional neural network, an SSD object detector which combines prior knowledge is constructed to detect dynamic objects in the newly detection thread at semantic level. Then, in view of low recall rate of the existing SSD object detection network, a missed detection compensation algorithm based on the speed invariance in adjacent frames is proposed, which greatly improves the recall rate of detection. Finally, a feature-based visual SLAM system is constructed, which processes the feature points of dynamic objects through a selective tracking algorithm in the tracking thread, to significantly reduce the error of pose estimation caused by incorrect matching. The recall rate of the system is increased from 82.3% to 99.8% compared with the original SSD network. Several experiments show that the localization accuracy of Dynamic-SLAM is higher than the state-of-the-art systems. The system successfully localizes and constructs an accurate environmental map in real-world dynamic environment by using a mobile robot. In sum, our experimental demonstrations verify that Dynamic-SLAM shows improved accuracy and robustness in robot localization and mapping comparing to the state-of-the-art SLAM system in dynamic environment. • An SSD object detector is constructed to detect dynamic objects with prior knowledge in the newly detection thread. • Missed detection compensation algorithm based on the speed invariance in adjacent frames is proposed. • Selection tracking algorithm is proposed to eliminate the dynamic objects and improve the robustness and accuracy of the system. • A feature-based Dynamic-SLAM system combined semantic information is constructed to realize the detection of dynamic environment in robot localization and mapping. [ABSTRACT FROM AUTHOR]

• A range-based multi-robot relative pose estimation system is proposed. • The graph optimization method is adopted to improve the performance. • The "single robot – multi tags" structure is designed for observability. • Centimeter-level relative positioning accuracy is achieved in experiments. The relative position and orientation estimation is one of the most fundamental components for multi-robot systems, while it still remains an open and challenging research topic in infrastructure-free environments. In this paper, a novel graph optimization and UWB range based relative positioning and orientation estimation framework is proposed, which improves the estimation performance with a sliding window for multi-robot system in anchor-free and GNSS-denied environment. Single shot estimation capability of relative position and orientation benefits from the "single robot – multi tags" structure, which greatly improves the robustness and adaptability of the system. Real-site experiments demonstrates that our system can achieve centimeter-level relative positioning performance, and the influence of distance between nodes and the number of nodes on system performance is fully discussed through simulation, which lays a comprehensive preparation for future practical application. [ABSTRACT FROM AUTHOR]

• A nano-second, wireless, non GNSS-aided and low-cost clock synchronization solution for multi-node distributed cooperative systems-Wicsync-is presented. • An optimized clock synchronization protocol is modelled and proposed. • A low-cost hardware demo based on the UWB is developed and equipped with Wicsync. • The details of hardware parameters are characterized and discussed in the simulation model. The clock synchronization accuracy directly determines the performance of wireless distributed cooperative systems. However, the mature high accuracy wireless clock synchronization schemes are generally based on GNSS or other auxiliary sources to complete the synchronization process, which cannot be applied to GNSS degraded and denied environments. In this paper, a wireless high-accuracy clock synchronization solution for multi-node distributed cooperative systems – Wicsync – is proposed, which contains an optimized wireless two-way clock synchronization and mutual calibration protocol to support wireless multi-node clock synchronization. Based on the UWB synchronization hardware architecture with local clock phase adjustment we designed, Wicsync implements a non GNSS-aided, high-accuracy, multi-node, low cost wireless clock synchronization performance through the protocol. Experiments shows that the Wicsync can support the instantaneous clock synchronization accuracy of less than 3 ns (@75 m at least) for 10 nodes (more than 100 nodes theoretically). [ABSTRACT FROM AUTHOR]