An answer to a previous question explained that a turbine in stop mode may deliver reactive power to the grid. It also takes active power from the grid to run its control system, lighting system and some other functions. I don't understand why it delivers reactive power to the grid when it is taking active power from it? What is the purpose for that?
Wind turbine generators follow specific rules for operation called "grid code". The transmission system operator (TSO) imposes some requirements through these grid codes that all grid-connected wind turbine generators (WTGs) should follow when they are connected to the grid.
In general, reactive power regulation required from wind turbine generators are based on wind farm (WF)/wind turbine capacity, grid voltage level and grid stiffness. In general, WTG reactive power control may follow one of following three modes.
1) Reactive power control mode: TSO asks WTG/WF operator to provide specific amount of reactive power.
2) Power factor control mode: TSO asks WTG/WF operator to provide reactive power with respect to active power generated by WTG/WF.
3) Voltage control mode: TSO asks WTG/WF operator to provide reactive power to the grid as a function of PCC (point of common connection) voltage.
Now if WTG/WF works in first or third mode, it should provide reactive power to grid if that is the requirement from TSO regardless active power generation (If that is the grid code requirement).
The purpose of this from TSO point of view is to regulate the grid voltage by supplying/consuming reactive power from WTG/WF as it is done by conventional power plants.