Optimal Sizing of Grid Connection Transformers Overhead Lines for Onshore Wind Farms
Traditional power transmission comprises bulk transfer of electrical energy from power stations (generating plant) to substations located near demand centers. Conventional power plants use nuclear or fossil (coal, oil or gas) fuels or water to generate electricity. These primary sources of energy can be accumulated or stored and hence electricity can be generated and dispatched as required. Depending on the technology used and utility imperatives the capacity factor i.e. the ratio of the actual electrical energy produced in a given period to the hypothetical maximum possible energy that can be produced is typically in the range of 80-100%. The associated transmission infrastructure is usually designed to ensure that the full output of the power plant can be delivered at all times and usually to withstand a single contingency outage without restricting power delivery. In contrast to this, electricity production in wind farms is determined by the available wind velocity. Due to the availability of wind and the variability of the wind speed, the capacity factor of a wind farm is usually in the region 20 to 40 %. The utilisation of the transmission infrastructure is accordingly low and the capability to deliver the full rated output of a wind power facility at all times can seldom be cost justified. A large part of the connection cost for a wind farm establishment is comprised of grid connection transformers and overhead lines. If a grid connection transformer is rated for the maximum output of a wind facility it will be run at part load for the majority of the time due to the available wind speed and hence be overrated and overpriced when compared with the practical energy transport requirement. In contrast, if the transformer is rated to only carry part of the wind facility rated capacity there will be periods when it is overloaded if the output of the facility is not deliberately curtailed. Overloading can result in a reduction in the expected service life of a transformer. Oversized transformers and oversized transmission lines carry a higher capital cost and whilst oversized transformers increase parasitic losses increased line conductors can reduce parasitic losses. Increased capital costs and in service losses impacts the business case for a wind facility and hence the selection of transformer and line ratings is an important consideration at the outset. This paper presents basic guidelines for optimal sizing of the grid connection transformers and overhead lines for onshore wind farms.