Increasing wind farm capacity

The issue of wind farm capacity is addressed through study of a simple but generic system. Wind-farm size is defined in terms of its rating relative to system fault level at the wind-farm terminals. The voltage rise for a wind farm whose capacity is 20% of system fault rating is determined for various utility network reactance-to-resistance ratios (X/R). The focus is on a network with an X/R ratio of unity, typical of 33 kV networks in the UK. It is shown that capacitive compensation for a fixed-speed wind turbine generator can contribute to the voltage rise problem. A typical compensation scheme, with extra capacitance being switched in as active power increases, results in the maximum allowed voltage rise for a wind-farm capacity of 13% of system fault rating. A modified scheme, with capacitive compensation being reduced at greater active power outputs, enables the capacity to be increased to 20% of system fault rating. The modified scheme leads to slightly increased network losses. It has the advantage that the reduced capacitive compensation is below the level required for self excitation. An analogous study for a wind farm based on doubly-fed induction generators results in the same capacity limitation, but with slightly reduced losses.