High Neutral-to-Earth Potentials on Rural DistributionFeeders Sharing Right-of-Way with Wind plantCollector Lines

摘要

Individual large wind plants in certain parts of the U.S. may be spread out over many tens of squaremiles 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. -, ismuch smaller. This often-cited positive attribute of wind plants leads to a situation where residents inthese rural areas physically live within the wind plant.A substantial network of medium voltage collector lines is required to connect each turbine to thetransmission system delivery point. During times of high wind plant production, the collection feedercurrents can reach hundreds of amperes, depending obviously on the feeder operating voltage and thenumber of turbines interconnected. Common structure framings for these voltage classes and theresultant conductor spacings can give rise to a fair amount of asymmetrical electromagnetic coupling toother conductors on the structure, especially neutral conductors in the collector circuits themselves orjoint-use distribution circuits providing electrical service to customers living near or amongst the windturbines. In the balanced three-phase collector lines, the longitudinal electric field resulting from thestructure framing asymmetry induces a voltage into the collector circuit neutral conductor, even whenthe phase currents are perfectly balanced in magnitude and phase. The induced voltage is a function ofthe degree of asymmetry in the structure framing, the magnitude of the collector feeder line currents, andthe length of the circuit. On collector feeders with multiple connections to ground, the induced voltagegives rise to circulating currents. In any case, the potential of the neutral conductor is raised well abovewhat 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 littleconcern for win plant operation, they can become problematic in situations where a local distributioncircuit shares right-of-way with the wind plant collector line. Resulting neutral-to-ground voltages atrural services can be raised above what is allowed by local utility guidelines. This paper provides aquantitative discussion of the electromagnetic phenomena responsible for elevated neutral potentials onrural distribution system neutrals on joint-use right-of-ways with high-current wind plant collectionfeeders. Detailed computer modeling illustrates the many factors that determine the profile of the neutralpotentials along the length of the circuit, and suggests design improvements that can minimize theimpact of high-current collector lines on neutral potentials in local services.