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Selecting the Right Cable for Collector Systems
Consider the situation of a collector cable failure in the winter. The snow is three feet (1 m) deep and the ground is frozen. While the site operator is certain that his collector system has failed, it is going to be days or even weeks before the fault can be repaired. These failures could result in tens of thousands of dollars in lost revenue, damaged equipment from fault currents, and degradation of existing equipment.  Under these extreme environmental conditions, the emergency crews would have to wait for reasonable weather conditions and then attempt to get equipment in place to locate the fault, followed by several days to thaw the ground. Finally, excavation equipment has to be moved in to make repairs. On hillsides and in remote areas this is a costly task. Can failures be avoided? Yes, specify a premium cable system. Wind farm developers spend their time and investment on turbine-centric activity to assure the availability of their energy producing assets, but collector cable design is a much overlooked area. Adequate collector cable design can strengthen one of the weakest links in today’s wind farm reliability chain. Many factors play a role in the design of a reliable cable system. They are as follows:
 Neutral Shield Size The size of the copper neutral shield should be one third of the conductor cross section or greater. This makes certain that the thermal effects due to system harmonics are considered, faults will not burn a large section of the neutral shield, and nondestructive fault location and partial discharge (PD) diagnostics will be effective. Cable Length and Cross-bonding Cable section lengths should be limited to 8,000 feet (2,438 m) between sectionalizing cabinets. This allows faults to be sectionalized. These cabinets provide good accessibility for cross-bonding. Cross-bonding, however, should only be used where the interconnections are accessible above ground. Cross-bonding points are inherently unreliable, mainly because they are labour and skill intensive. Conductor Type and Connector Design The all-copper system, consisting of copper conductors and copper connectors, will ensure the most reliable system. Joints are the weak points in any cable system. The “all-copper” solution ensures a reliable service life. Thermal Properties of the Soil Thermal properties of the soil surrounding the cable should be tested and accounted for in the design. Ampacity calculations are only valid if the thermal properties of the soil are known. Quality of Cable Purchased Cable manufacturers should be specified and their quality control programs verified. Their factory acceptance tests should be studied, verified or witnessed. This will assure that quality products with good reliability are used. Joints Joint design should be simple, and application-specific training should be required. Training by the manufacturer on the specific joint design is recommended to assure that crews understand the installation process. |
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