Wind Power - Added Value for Network Operation

Doctoral thesis, 2008

This dissertation deals with the investigation on different value added properties of variable speed wind turbines (VSWT) that stem from the flexible controllability of converter interfaced wind turbines (WT). Improvements in voltage and transient stability of a nearby grid, small-signal stability improvement on the power system, frequency control support for the network operation, as well as the technical and the economic issues related to the reactive power ancillary service provision, are among the issues covered. To demonstrate that additional control functions can be incorporated in real installations, a real situation is presented where the short-term voltage stability is improved as an additional feature of an existing voltage source converter (VSC) high voltage direct current (HVDC) installation. A finding is that the voltage and the transient stability performance of the used Cigre Nordic 32-bus test system during disturbances is improved when the wind farm (WF) is complying with the E.ON code compared to the traditional unity power factor operation. Further improvements are noticed when the example grid code is modified (increased slope of the reactive current support line and extended reactive current support). The damping of the inter-area mode and the local mode (where the WF is connected) of oscillation of the studied two-area power system are increased in the presence of the WF. It is also noticed that, the damping associated with the inter-area mode is slightly better when the constant reactive power mode is applied compared to the voltage control mode of operation. Another finding is that the example WT exhibits a slow well damped system mode which depends on the WT controllers (torque and pitch controllers, and pitch compensator). Due to the non-minimum phase characteristic of hydro dominated systems, a temporary active power support from WFs, utilizing the stored rotational energy in the moving turbine blades, could be helpful in reducing the network frequency fall. In this regard, it is found that an example WT system can provide 0.1pu extra active power support for 10s without any larger effect on the WT operation. When this arrangement is used, both the temporary droop and the reset time of the existing speed governing system need to be reduced to maintain certain benchmark stability properties. From the case investigated, it is found that grid-side converters (GSC), designed to handle only rated active power, cost around 1.5% of the total investment of the WF. However, a 50% over-rated GSC would cost around 2.25% of the total investment of the WF, and would be capable of providing 0.65~pu reactive power at the grid connection point under nominal conditions. Another finding is that higher wind speed prediction errors, i.e.~a WF site with a high degree of wind variations, may result in higher payments to the WF for the reactive power service, mainly due to the increased lost opportunity cost (LOC) component.