Power System Voltage Stability Related to Wind Power Generation

As is well known the amount of installed wind power increases every day. In the past single wind turbines were installed separately, today wind turbines often are connected to the grid as a group of many turbines, i.e. wind parks. Grid integration considerations have moved from flicker phenomena in the low voltage grid to voltage stability analyses in the transmission grid. This paper starts with an overview of the voltage stability phenomena in a transmission system. Suitable models of wind power generation for power system stability studies - both long and short-term stability - are presented. Different levels of detail of the wind generation models are investigated carefully considering the type of stability study, result accuracy and simulation efficiency. Important aspects such as protection, fault ride-through capability and the capability of producing reactive power are taken into account. The models are then validated against measurements.Three types of wind turbines are considered in this study; fixed speed with induction generator, variable speed operation with doubly fed induction generator and variable speed turbine with full power converter.The main phenomenon that determines the behaviour of a system where fixed speed wind turbines are connected is the large reactive power consumed by the induction machines immediately after the fault has been cleared. This mechanism leads to a voltage stability problem with a risk of voltage collapse as a final consequence. Investigations in short and long term voltage stability have examined the factors affecting the stability limit such as: generated power, fault types, fault duration, fault clearing time, fault voltage and remaining voltage. Variable speed wind turbines with power electronic converter are capable of producing active power at unity power factor. Possibility lies to control reactive power during grid disturbance with the help of an over-rated grid side converter. In this paper the long- and short-term voltage stability aspects of variable speed wind turbines with power electronic converters are illustrated by case studies. Large scale wind power integration at the transmission level has the possibility to improve long term voltage stability of the grid by injecting reactive power. The possibility to produce reactive power increases with a decreased active power production. As well small scale integration at the distribution level can increase short-term voltage stability of the local grid by utilizing its fast reactive power control capability.Finally, several issues related to the modelling and the compatibility of wind power generation in typical power system simulation tools is discussed