SMART Cables for Observing the Global Ocean: Science and Implementation

The ocean is key to understanding societal threats including climate change, sea level rise, ocean warming, tsunamis, and earthquakes. Because the ocean is difficult and costly to monitor, we lack fundamental data needed to adequately model, understand, and address these threats. One solution is to integrate sensors into future undersea telecommunications cables. This is the mission of the SMART subsea cables initiative (Science Monitoring And Reliable Telecommunications). SMART sensors would “piggyback” on the power and communications infrastructure of a million kilometers of undersea fiber optic cable and thousands of repeaters, creating the potential for seafloor-based global ocean observing at a modest incremental cost. Initial sensors would measure temperature, pressure, and seismic acceleration. The resulting data would address two critical scientific and societal issues: the long-term need for sustained climate-quality data from the under-sampled ocean (e.g., deep ocean temperature, sea level, and circulation), and the near-term need for improvements to global tsunami warning networks. A Joint Task Force (JTF) led by three UN agencies (ITU/WMO/UNESCO-IOC) is working to bring this initiative to fruition. This paper explores the ocean science and early warning improvements available from SMART cable data, and the societal, technological, and financial elements of realizing such a global network. Simulations show that deep ocean temperature and pressure measurements can improve estimates of ocean circulation and heat content, and cable-based pressure and seismic-acceleration sensors can improve tsunami warning times and earthquake parameters. The technology of integrating these sensors into fiber optic cables is discussed, addressing sea and land-based elements plus delivery of real-time open data products to end users. The science and business case for SMART cables is evaluated. SMART cables have been endorsed by major ocean science organizations, and JTF is working with cable suppliers and sponsors, multilateral development banks and end users to incorporate SMART capabilities into future cable projects. By investing now, we can build up a global ocean network of long-lived SMART cable sensors, creating a transformative addition to the Global Ocean Observing System

Oceanus.

v. 42, no. 1 (2000

Offshore Electrical Networks and Grid Integration of Wave Energy Converter Arrays - Techno-economic Optimisation of Array Electrical Networks, Power Quality Assessment, and Irish Market Perspectives

Wave energy is an emerging industry and faces many challenges before commercial wave energy converter (WEC) arrays are installed. One of these challenges is the grid integration of WEC arrays. This includes offshore electrical networks, grid compliance, and access to electrical markets. This must be achieved in a technically viable manner and also at an acceptable cost. As electrical networks are expected to make up a large proportion of the overall WEC array CAPEX, perhaps up to 25%, this area is critical to the long term competitiveness of wave energy. The objectives of this thesis are to develop technically and economically acceptable electrical network designs for WEC arrays, evaluate voltage flicker issues for WEC arrays and develop design tools to analyse same, and evaluate the market scale for wave energy in Ireland, considering electrical integration issues in both the domestic and export markets. This thesis presents the optimum design for WEC array electrical networks. By building from the industry state of the art, including offshore wind experience, a comprehensive techno-economic optimisation process is undertaken. This includes optimising the key electrical interfaces between the WEC and the array electrical network, optimising the array network configuration, assessing efficiency of the network, and demonstrating that the network can be achieved at a cost which will allow competitiveness. Some challenges to the economics of WEC array electrical networks and some strategies for improving the economics are presented in this research also. The results provide timely guidance to WEC and WEC array developers. This research also demonstrates the critical link between voltage flicker emissions from WECs and the primary resource, i.e. ocean waves. Some practical assessment tools for the evaluation of this power quality issue are shown to assist in quantifying the problem. Also the full flicker performance of a candidate WEC is assessed helping characterise this link further. In this thesis both the domestic and export markets for Ireland’s wave energy resource are assessed as, although Ireland has an enviable wave energy resource, it is unclear where the market for this resource lies. This analysis shows that the medium term market for wave energy in Ireland is an export market. Also, although technically feasible, there is an additional cost for export transmission which must be considered in evaluating export markets. Some of the critical grid integration issues have been evaluated and addressed in this thesis. Future work is recommended in the areas of weather risk to cable installation at high energy wave sites, evaluating the benefits of shared electrical infrastructure across a range of renewable projects, designing offshore substations for WEC arrays, and quantifying the benefits of the addition of wave energy to the Irish renewable energy mix

A comparison of AC and HVDC options for the connection of offshore wind generation in Great Britain

This paper presents a comparison of two forms of cable connection of a distant offshore wind farm to a transmission system: AC and HVDC. The requirements of relevant industry standards in Great Britain (GB) that drive a connection design and, hence, its cost are highlighted along with an analysis of the ways in which AC cable connections might be made to comply while facilitating export of active power. Dynamic studies investigating responses to grid-side short circuit faults show that, in the particular scenarios studied, an AC connection of a wind farm in the place of a large synchronous generator is marginally detrimental while an HVDC connection is beneficial. A comparison of costs shows that the cross-over distance at which HVDC is cheaper than AC for wind farms of different sizes occurs at longer distances than have hitherto commonly been assumed, and AC connections benefit from reactive compensation not only at the point of common coupling and wind farm end but also at the connection mid-point

Water-Tree Modelling and Detection for Underground Cables

In recent years, aging infrastructure has become a major concern for the power industry. Since its inception in early 20th century, the electrical system has been the cornerstone of an industrial society. Stable and uninterrupted delivery of electrical power is now a base necessity for the modern world. As the times march-on, however, the electrical infrastructure ages and there is the inevitable need to renew and replace the existing system. Unfortunately, due to time and financial constraints, many electrical systems today are forced to operate beyond their original design and power utilities must find ways to prolong the lifespan of older equipment. Thus, the concept of preventative maintenance arises. Preventative maintenance allows old equipment to operate longer and at better efficiency, but in order to implement preventative maintenance, the operators must know minute details of the electrical system, especially some of the harder to assess issues such water-tree. Water-tree induced insulation degradation is a problem typically associated with older cable systems. It is a very high impedance phenomenon and it is difficult to detect using traditional methods such as Tan-Delta or Partial Discharge. The proposed dissertation studies water-tree development in underground cables, potential methods to detect water-tree location and water-tree severity estimation. The dissertation begins by developing mathematical models of water-tree using finite element analysis. The method focuses on surface-originated vented tree, the most prominent type of water-tree fault in the field. Using the standard operation parameters of North American electrical systems, the water-tree boundary conditions are defined. By applying finite element analysis technique, the complex water-tree structure is broken down to homogeneous components. The result is a generalized representation of water-tree capacitance at different stages of development. The result from the finite element analysis is used to model water-tree in large system. Both empirical measurements and the mathematical model show that the impedance of early-stage water-tree is extremely large. As the result, traditional detection methods such Tan-Delta or Partial Discharge are not effective due to the excessively high accuracy requirement. A high-frequency pulse detection method is developed instead. The water-tree impedance is capacitive in nature and it can be reduced to manageable level by high-frequency inputs. The method is able to determine the location of early-stage water-tree in long-distance cables using economically feasible equipment. A pattern recognition method is developed to estimate the severity of water-tree using its pulse response from the high-frequency test method. The early-warning system for water-tree appearance is a tool developed to assist the practical implementation of the high-frequency pulse detection method. Although the equipment used by the detection method is economically feasible, it is still a specialized test and not designed for constant monitoring of the system. The test also place heavy stress on the cable and it is most effective when the cable is taken offline. As the result, utilities need a method to estimate the likelihood of water-tree presence before subjecting the cable to the specialized test. The early-warning system takes advantage of naturally occurring high-frequency events in the system and uses a deviation-comparison method to estimate the probability of water-tree presence on the cable. If the likelihood is high, then the utility can use the high-frequency pulse detection method to obtain accurate results. Specific pulse response patterns can be used to calculate the capacitance of water-tree. The calculated result, however, is subjected to margins of error due to limitations from the real system. There are both long-term and short-term methods to improve the accuracy. Computation algorithm improvement allows immediate improvement on accuracy of the capacitance estimation. The probability distribution of the calculation solution showed that improvements in waveform time-step measurement allow fundamental improves to the overall result

TECTONIC INFLUENCES ON SURFICIAL PROCESSES AND DEFORMATION ALONG THE NANKAI ACCRETIONARY PRISM, SOUTHWEST JAPAN

Ph.D.Ph.D. Thesis. University of Hawaiʻi at Mānoa 201