The properties stiff and weak are more difficult to handle in a short and precise definition. A stiff or a weak grid is not just a stiff or a weak grid, independently, since a grid could be characterised e.g. as frequency stiff and voltage weak. A frequency weak grid would influence the energy production of a fixed speed wind turbine in a less desirable manner. This has been studied in e.g. (S0rensen et al., 2000). Another problem related to the stiff/weak property is that, while a grid with a few connected wind turbines is stiff, the same grid might be characterised as weak at a higher number of connected wind turbines.
In order to assure a proper operating condition for the wind turbine and acceptable reliability and power quality for the grid, recommendations and standards have been developed for connection of wind turbine to the distribution grid. In Denmark, (KR 111, 1998) specifies the requirements for grid connection of wind turbines to the distribution system, based on the characteristics and methods specified in (IEC 61400-21, 2000).
Power quality and other issues related to grid-connected wind turbines are highlighted in a simulation study contained in three reports:
• (J0rgensen et al., 1996) with the focus on steady state voltage conditions. The influence of wind turbines on three different grids is analysed.
• (S0rensen & Kledal, 1996) with the focus on flicker. The flicker impact from both stall regulated and pitch controlled wind turbines is analysed on two different grids.
• (Vikkels0 et al., 1996) with the focus on harmonics and operating conditions of variable speed wind turbines. The distribution of harmonics is analysed for six different configurations of wind turbines on one grid.
If a project aims at installation of a wind power (or any other renewable source) on a weak grid, three options should be considered:
1. Appropriate reactive power compensation etc. of fixed speed wind turbines.
2. Use of wind turbines with controllable power output.
3. Reinforcement of the grid.
Which of these options is the right choice depends on the economics of the three options and the local planning.
An isolated grid is a very ambiguous term. There are no well-defined upper or lower power limits. The term covers a variety of system types, e.g.: wind-diesel, hybrid power, stand-alone etc. The characteristics of an isolated grid are typically equivalent to those of a weak grid. (EN 50160, 1999) is the only identified standard with recommendations for isolated grids.
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