For doubly fed induction generator (DFIG)-based wind energy conversion systems (WECSs), large electromotive force will be induced in the rotor circuit during grid faults. Without proper protection ...scheme, the rotor side of DFIG will suffer from overcurrents, which may even destroy the rotor-side converter (RSC). To mitigate this problem, a new flux-linkage-tracking-based low-voltage ride-through (LVRT) control strategy is proposed to suppress the short-circuit rotor current. Under the proposed control strategy, the rotor flux linkage is controlled to track a reduced fraction of the changing stator flux linkage by switching the control algorithm of RSC during grid faults. To validate the proposed control strategy, a case study of a typical 1.5-MW DFIG-based WECS is carried out by simulation using the full-order model in SIMULINK/SimPowerSystems. In the case study, a comparison with a typical LVRT method based on RSC control is given, and the effect of the control parameter on the control performance is also investigated. Finally, the validity of the proposed method is further verified by means of laboratory experiments with a scaled-size DFIG system.
In this paper, the low-voltage ride-through (LVRT) capability of the doubly fed induction generator (DFIG)-based wind energy conversion system in the asymmetrical grid fault situation is analyzed, ...and the control scheme for the system is proposed to follow the requirements defined by the grid codes. As analyzed in the paper, the control efforts of the negative-sequence current are much higher than that of the positive-sequence current for the DFIG. As a result, the control capability of the DFIG restrained by the dc-link voltage will degenerate for the fault type with higher negative-sequence voltage component and 2φ fault turns out to be the most serious scenario for the LVRT problem. When the fault location is close to the grid connection point, the DFIG may be out of control resulting in non-ride-through zones. In the worst circumstance when LVRT can succeed, the maximal positive-sequence reactive current supplied by the DFIG is around 0.4 pu, which coordinates with the present grid code. Increasing the power rating of the rotor-side converter can improve the LVRT capability of the DFIG but induce additional costs. Based on the analysis, an LVRT scheme for the DFIG is also proposed by taking account of the code requirements and the control capability of the converters. As verified by the simulation and experimental results, the scheme can promise the DFIG to supply the defined positive-sequence reactive current to support the power grid and mitigate the oscillations in the generator torque and dc-link voltage, which improves the reliability of the wind farm and the power system.
Loss of synchronization between wind farm and power grid during severe grid faults would cause wind farm tripping. In this paper, the mechanism of grid-synchronization is uncovered, described as ...motion of an autonomous nonlinear differential equation with specific initial states. The revealed mechanism indicates that even though steady-state working point exists, improper initial states and poor system dynamic properties could lead to synchronization instability. In order to keep wind farm synchronous with the power grid during severe grid faults, special requirements on system dynamic properties are stated. Moreover, to satisfy all the requirements, a current injecting method is proposed. By adjusting active and reactive output currents of the wind farm, the proposed method could ensure system synchronization stability during severe grid faults. Implementation of the proposed method on PMSG- and DFIG-based wind farm is illustrated. Simulation results validate the analysis and the control method.
This paper proposes a fast and robust phase estimation algorithm (PEA), a counterpart of the phase-locked loop (PLL), for heavily distorted grid conditions. The PEA, named as the WLSE-PEA, consists ...of the moving average filter (MAF), the weighted least squares estimation (WLSE), the frequency-locked loop (FLL), and the zero crossing detection (ZCD). The MAF can eliminate all the odd-order harmonics in the distorted grid voltages. The WLSE is employed to estimate the fundamental positive-sequence component. The combination of the ZCD and the FLL enables the WLSE-PEA to be adaptive to frequency deviations. Compared with some advanced phase-locked loops, the proposed WLSE-PEA has a faster response. Also, it is robust to common grid disturbances including three-phase unbalances, dc offsets, harmonics, phase jumps, and voltage sags. Simulation and experimental results have verified the effectiveness and advantages of the WLSE-PEA.
Control of excited‐state dynamics is key in tuning room‐temperature phosphorescence (RTP) and thermally activated delayed fluorescence (TADF) emissions but is challenging for organic luminescent ...materials (OLMs). We show the regulation of TADF and RTP emissions of a boron difluoride β‐acetylnaphthalene chelate (βCBF2) by controlling the excited‐state dynamics via its J‐ and H‐aggregation states. Two crystalline polymorphs emitting green and red light have been controllably obtained. Although both monoclinic, the green and red crystals are dominated by J‐ and H‐aggregation, respectively, owing to different molecular packing arrangements. J‐aggregation significantly reduces the energy gap between the lowest singlet and triplet excited states for ultra‐fast reverse intersystem crossing (RISC) and enhances the radiative singlet decay, together leading to TADF. The H‐aggregation accelerates the ISC and suppresses the radiative singlet decay, helping to stabilize the triplet exciton for RTP.
J‐aggregates significantly reduce the energy gap between the lowest singlet (S1) and triplet (T1) excited states and contribute to the reverse intersystem crossing (RISC) process; while H‐aggregates will accelerate ISC and suppress radiation transition of S1 to realize phosphorescence. This provides crucial information for the deep undrstanding of the internal mechanism of the excited state dynamics controlled by different aggregates.
An active-damping strategy is proposed for the suppression of speed and torsional oscillations in permanent-magnet synchronous generator (PMSG)-based wind-energy conversion systems (WECSs). ...Direct-driven configuration with PMSG is an attractive choice for WECS because of the gearbox elimination and cost reduction due to small pole-pitch design. However, speed and torsional oscillations appear when the generator is directly connected to the wind turbine without any assistant damping device. Based on small-signal analysis, a low-bandwidth design for the power or generator torque controller of PMSG can help to reduce the oscillation amplitude, but the system dynamic performance is thus sacrificed. From the power-flow's point of view, the oscillation is reflected in the dc-link current. With the help of switch function modeling based on the space-vector-modulation scheme, the average dc-link current can be estimated and applied to the compensation strategy, which provides positive damping resulting in stability improvement. The simulation and experiment results verify the theoretical analysis and the validation of the proposed strategy.
In the robot-assisted puncture surgery, the posture of the needle tip may not be available directly owing to the small dimension of the needle tip and the existence of measurement noise. On the other ...hand, the existing high-gain observers are robust against model uncertainty but sensitive to measurement noise. The peaking phenomenon is also a typical problem when applying those high-gain observers. Aiming at these issues, an improved high-gain observer is devised to observe the pose of the needle tip in this brief. To reduce the effect of the measurement noise and the peaking phenomenon, the power of observer gain is designed as 2 by extending the observer degree and a correction term of measurement estimation is added to improve the property of the observer against the measurement noise. Finally, the performance of the proposed observer is verified via a simulation example.
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