Grid reinforcing wind generation

Licentiatavhandling, 2006

In this thesis, effects of some grid assisting possibilities that wind turbines could be equipped with, are investigated. The background is that concerns regarding the stability of the already existing power system in the presence of large wind farms have been risen. Variable speed wind turbines have power electronic based frequency converters already included in their design and the idea used in this thesis is to utilize the fast controllability of these converters to assist the grid. It is found that the transient stability of conventional synchronous generators located nearby a wind farm, can be increased if a suggested contingency operational mode is incorporated into the control of the wind farm. Another finding is that large wind farms integrated into the transmission level with their control properly modified, can increase the steady state power transfer limit of the transmission line as well as assist the grid to delay or even prevent a voltage collapse event. It is also shown that wind farms integrated into the grid in the distribution level together with different loads, have the possibility to increase the short term voltage stability of the network provided that their controls have been modified accordingly. Finally, the suggested grid assisting methods are incorporated into the control of two large planned wind farms in the southern part of the Swedish grid and tested in the CIGR\'{E} Nordic32 test grid, which is taken as a representation of the Swedish transmission network. It is concluded that care has to be taken if voltage control mode of operation is utilized in a wind farm located close to a conventional synchronous generator. The interaction between the operational mode of a wind farm and the overall synchronous generator control may interact with each other and could lead to a reduction of damping of power oscillations in nearby transmission lines instead of increasing the damping.