High-frequency operation of a DC/AC/DC system for HVDC applications

Voltage ratings for HVdc point-to-point connections are not standardized and tend to depend on the latest available cable technology. DC/DC conversion at HV is required for interconnection of such HVdc schemes as well as to interface dc wind farms. Modular multilevel voltage source converters (VSCs), such as the modular multilevel converter (MMC) or the alternate arm converter (AAC), have been shown to incur significantly lower switching losses than previous two- or three-level VSCs. This paper presents a dc/ac/dc system using a transformer coupling two modular multilevel VSCs. In such a system, the capacitors occupy a large fraction of the volume of the cells but a significant reduction in volume can be achieved by raising the ac frequency. Using high frequency can also bring benefits to other passive components such as the transformer but also results in higher switching losses due to the higher number of waveform steps per second. This leads to a tradeoff between volume and losses which has been explored in this study and verified by simulation results with a transistor level model of 30-MW case study. The outcome of the study shows that a frequency of 350 Hz provides a significant improvement in volume but also a penalty in losses compared to 50 Hz

How to operate a liver tumour you cannot see

Backround As recent chemotherapy regimens for metastatic colorectal cancer become more and more effective in a neoadjuvant setting before liver surgery, a "complete" clinical response is sometimes documented on imaging. Without operation though, metastatic recurrence is likely to commence within 12 months. Surgeons now face the problem to resect non-visualizable and non-palpable lesions. Methods Computer-based virtual surgery planning can be used to fuse pre- and postchemotherapy computed tomography data to develop an operative strategy. This information is then intraoperatively transferred to the liver surface using an image-guided stereotactically navigated ultrasound dissector. This enables the surgeon to perform a resection that is otherwise not possible. Results During operation, detection of the lesion through palpation or ultrasound was impossible. After registering the virtual operation plan into the navigation system, the planned resection was performed without problems. Histopathologic workup showed vital tumor cells in the specimen. Conclusion The new image-guided stereotactic navigation technique combined with virtual surgery planning can solve the surgeon's dilemma and yield a successful operation

Multi-Robot Team Design for Real-World Applications

Real-world applications that are ideal for robotic solutions are very complex and challenging. Many of these applications are set in dynamic environments that require capabilities distributed in functionality, space, or time. These applications, therefore, often require teams of robots to work together cooperatively to successfully address the mission. While much research in recent years has addressed the issues of autonomous robots and multi-robot cooperation, current robotics technology is still far from achieving many of these real world applications. We believe that two primary reasons for this technology gap are that (1) previous work has not adequately addressed the issues of fault tolerance and adaptivity in multi-robot teams, and (2) existing robotics research is often geared at specific applications, and is not easily generalized to different, but related, applications. This paper addresses these issues by first describing the design issues of key importance in these real-worl..