CONTRIBUTION OF WIND FARMS TO ANCILLARY SERVICES

In the last decade, wind energy has expericnced a substantial growth in Europe with an increase in generation capaciiy from 2,5 GW in 1995 to 34 GW at (lie end of 2004 This incTease has raised new probiems and constraints which led System opera tors, electric utilities, governments or regulatory boards to define lechnical requirements for the grid connection of wind farms and more generaily of distributed generation (DG) units. At flrst, the requirements for wind fanns were rather "soft" (at least softer than for other DG units) and were mainiy intended to limit the "disturbances" caused by wind energy on power quality and grid operation. But with the ever increasing developmenl of wind power, the impacts on the grids become more and more significant leading to the definition of more and more "scvcrc" requiremcnts, In parltcular, wind farms (WF) arc now more and more often askcd to provide some son of anciilary services such as contribution to voltage/reactive power control and rrequency/active power control. This paper focuscs on the possible provision of such anciilary services by wind faims. Regarcling reactive power and voltage contTOl: -Doubly-fed Induction GEaerarors (DFIG) and Synchronous or Induction GeneratoTS with full power electron ics interfaces (SIG) can liave reactive power control capabilities (both in production and absorption) depending on the rating of their power electronics converters. These capabilities {characterized by a fast dynamie respon.se) can be used to perform voltage control. Moreover, if required, extemal reactive power compensation device.s niay also be installed. -Classical Induction G encrât ors (OG) do not hâve such reactive power control capahilities and therefore require externat devices for reactive power and voltage control. -Simulations carried oui show that WFs with voltage control capabilities can significantly sustain the network voltage in case of grid events and mus may efficiently support the power System stability. Regard ing frequency control: the resuits show that when the Wind Turbine Generator (WTG) is full y loaded (maximum active power generation], appre-priate use or the pitch control may enabic the WTG (DFIG, SIG or CIG) to contribute to frequency control. In case of partial toad. the contribution to frequency conlrol can be achieved either by "disoplimizing'" the wind energy conversion by mcans of aie pitch control. or by setting a non-optimal rotor speed for DFIG or SIG. However, a judicious way to use variable speed ge&eratCffS may also be looperate them as mertial flywheels,