Individual large wind plants in certain parts of the U.S. may be spread out over many tens of square miles to take advantage of local terrain and other geographic features for increasing energy production. The actual “footprint” of a large plant – the amount of land actually taken up by turbines, road, etc. -, is much smaller. This often-cited positive attribute of wind plants leads to a situation where residents in these rural areas physically live within the wind plant. A substantial network of medium voltage collector lines is required to connect each turbine to the transmission system delivery point. During times of high wind plant production, the collection feeder currents can reach hundreds of amperes, depending obviously on the feeder operating voltage and the number of turbines interconnected. Common structure framings for these voltage classes and the resultant conductor spacings can give rise to a fair amount of asymmetrical electromagnetic coupling to other conductors on the structure, especially neutral conductors in the collector circuits themselves or joint-use distribution circuits providing electrical service to customers living near or amongst the wind turbines. In the balanced three-phase collector lines, the longitudinal electric field resulting from the structure framing asymmetry induces a voltage into the collector circuit neutral conductor, even when the phase currents are perfectly balanced in magnitude and phase. The induced voltage is a function of the degree of asymmetry in the structure framing, the magnitude of the collector feeder line currents, and the length of the circuit. On collector feeders with multiple connections to ground, the induced voltage gives rise to circulating currents. In any case, the potential of the neutral conductor is raised well above what would be attributed to any unbalanced (zero sequence) current flow from operation of the turbines, which are by design symmetrical. While the elevated neutral-to-remote earth potentials are of little concern for win plant operation, they can become problematic in situations where a local distribution circuit shares right-of-way with the wind plant collector line. Resulting neutral-to-ground voltages at rural services can be raised above what is allowed by local utility guidelines. This paper provides a quantitative discussion of the electromagnetic phenomena responsible for elevated neutral potentials on rural distribution system neutrals on joint-use right-of-ways with high-current wind plant collection feeders. Detailed computer modeling illustrates the many factors that determine the profile of the neutral potentials along the length of the circuit, and suggests design improvements that can minimize the impact of high-current collector lines on neutral potentials in local services.
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