Perturbation observer based adaptive passive control for damping improvement of multi-terminal voltage source converter-based high voltage direct current systems

This paper proposes a Perturbation Observer-based adaptive passive control (POAPC) for damping improvement of multi-terminal voltage source converter-based high voltage direct current (VSC-MTDC) systems. The POAPC is designed for an N-terminal VSC-MTDC system, and the perturbation is defined as a lumped term including interactions between terminals, unmodelled dynamics and unknown external disturbances, which are estimated online via a perturbation observer. Then an extra system damping is injected for each terminal to improve the transient dynamics via passivation. The POAPC does not require an accurate system model and measurements of full states; only the DC voltage and active and reactive power need to be measured. Case studies are carried out on a four-terminal VSC-MTDC system under four scenarios such as active and reactive power reversal, faults at AC bus, offshore wind farm connection, and fast time-varying parameter uncertainties. Simulation results verify its effectiveness under various operating conditions, compared with that of conventional proportional-integral (PI) control and classical passive control (PC). </jats:p

Wide-Area Damping Controller of FACTS Devices for Inter-Area Oscillations Considering Communication Time Delays

The usage of remote signals obtained from a wide-area measurement system (WAMS) introduces time delays to a wide-area damping controller (WADC), which would degrade system damping and even cause system instability. The time-delay margin is defined as the maximum time delay under which a closed-loop system can remain stable. In this paper, the delay margin is introduced as an additional performance index for the synthesis of classical WADCs for flexible ac transmission systems (FACTS) devices to damp inter-area oscillations. The proposed approach includes three parts: a geometric measure approach for selecting feedback remote signals, a residue method for designing phase-compensation parameters, and a Lyapunov stability criterion and linear matrix inequalities (LMI) for calculating the delay margin and determining the gain of the WADC based on a tradeoff between damping performance and delay margin. Three case studies are undertaken based on a four-machine two-area power system for demonstrating the design principle of the proposed approach, a New England ten-machine 39-bus power system and a 16-machine 68-bus power system for verifying the feasibility on larger and more complex power systems. The simulation results verify the effectiveness of the proposed approach on providing a balance between the delay margin and the damping performance

Treatment of stiff thoracic scoliosis by thoracoscopic anterior release combined with posterior instrumentation and fusion

Background. Thoracoscopic anterior release has been shown that it can effectively improve spinal flexibility in animal and human cadaveric studies, and has been advocated for use in patients with scoliosis. This prospective case series aims to investigate the improvement of the spinal flexibility and the effectiveness in deformity correction by anterior thoracoscopic release and posterior spinal fusion. Methods. Eleven patients with stiff idiopathic thoracic scoliosis underwent anterior thoracoscopic release followed by posterior instrumentation. The average number of discs excised was five. Spinal flexibility was assessed by the fulcrum bending technique. Cobb angle before and after the anterior release was compared. Results. The patients were followed for an average of 5.6 years (range 2.2 to 8.1 years). Fulcrum bending flexibility was increased from 39% before the thoracoscopic anterior spinal release to 54% after the release. The average Cobb angle before anterior release was 74° on the standing radiograph and 45° with the fulcrum-bending radiograph. This reduced to 34° on the fulcrum-bending radiograph after the release, and highly corresponded to the 31° measured at the post-operative standing radiograph. Conclusion. It was demonstrated in patients with stiff idiopathic thoracic scoliosis that thoracoscopic anterior spinal release can effectively improve the spinal flexibility and increase the correction of the spinal deformity.published_or_final_versio

Simple metal oxides as efficient heterogeneous catalysts for epoxidation of alkenes by molecular oxygen

Magnetite iron oxide (Fe3O4) has been found to be an efficient heterogeneous catalyst for the epoxidation of alkenes by molecular oxygen in the absence of a sacrificial reductant among various transition metal oxides. The reaction probably proceeds via a radical mechanism