The present invention relates to a method for providing a fast reactive power support to a power grid. In particular, the present invention relates to a method for providing fast reactive power support from a wind power facility already being operated at or near its rated power level.

BACKGROUND OF THE INVENTION

It is a well-established technique to provide reactive power support to a power grid from a wind turbine. Reactive power support is typically provided in case the grid voltage for some reasons decreases.

However, according to traditional methods fast reactive power support may be difficult, if not impossible, to provide if the wind turbine selected for this purpose is already being operated at or near its rated power level. The reason for this mainly being that the overall current limit of the selected wind turbine may be exceeded if the reactive power support is launched while the wind turbine is already being operated at or near its rated power level. Obviously, exceeding the current limit of the wind turbine might damage the wind turbine.

Traditional methods solve the current limit problem by slowly derating the selected wind turbine. However, this typically leads to unwanted grid instability as well as an increased rotor speed which has to be dealt with by pitching the rotor blades out of the wind in order to match the wind power input with the derated power level.

It may be seen as an object of embodiments of the present invention to provide a method for providing a fast reactive power support even though the wind turbine is operated at its rated power level.

DESCRIPTION OF THE INVENTION The above-mentioned object is complied with by providing, in a first aspect, a method for operating a wind power facility in order to provide reactive power support to a power grid, the method comprising the step of

- increasing an amount of reactive power injected into the power grid, - decreasing an amount of active power injected into the power grid by a certain amount, and

- dissipating and/or storing essentially said certain amount of active power in power dissipation and/or power storage means. In the present context the term wind power facility should be understood broadly. Thus, the term wind power facility may cover a single wind turbine or a wind power plant comprising a plurality of wind turbines.

It is an advantage of the method of the present invention that reactive power may be provided fast and swift so that for example grid instabilities may be avoided. Grid instabilities may occur if the grid voltage falls below a predefined value, such as below a predefined voltage deadband around the nominal voltage level.

The sum of the amount of active power injected into the power grid and the amount of active power dissipated and/or stored may be essentially constant over a given time period. Thus, the power generated by a wind turbine generator is remained essentially constant of a given period of time before rotor blade pitching, power reduction etc. is initiated. This time period may be in the range of 0-2 seconds.

In general, the method of the present invention ensures that current limits of the wind power facility are not exceeded even though the amount of reactive power to the power grid is increased while the wind power facility is being operated at or near its rated power level. This is ensured by decreasing the amount of active power provided to the power grid. The step of increasing the reactive power and decreasing the active power is preferably performed essentially simultaneously.

The power dissipation means may comprise a DC chopper comprising a number of dump load resistors, whereas the power storage means may comprise a number of batteries and/or capacitors. Combined power dissipation and power storage means may also be applicable in relation to the present invention.

The method according to the present invention may further comprise the step of pitching wind turbine rotor blades out of the wind over time so as to reduce the wind power input. The rotor blade pitching may be initiated after an optional delay period of 0-2 seconds. After the optional delay period the rotor blade pitching may be carried out during a period of 5- 10 seconds. Similarly, the dissipated and/or stored amount of active power may be decreased accordingly over time in order to match the decreased wind power input. Thus, the dissipated and/or stored amount of active power may be decreased over a period of 5-10 seconds after an optional delay period of 0-2 seconds.

In a second aspect the present invention relates to a wind turbine facility for providing reactive power support to a power grid, the wind turbine facility comprising - reactive power control means for increasing an amount of reactive power injected into the power grid, and

- active power control means for decreasing an amount of active power injected into the power grid by a certain amount, and for dissipating and/or storing essentially said certain amount of active power in power dissipation and/or power storage means. Thus, the second aspect of the present invention relates to means for carrying out the method according to the first aspect. Again, the wind power facility may comprise a wind turbine or a wind power plant.

As mentioned in the relation to the first aspect of the present invention the power dissipation means may comprise a DC chopper comprising a number of dump load resistors. Moreover, the power storage means may comprise a number of batteries and/or capacitors from where reactive power may be provided if required.

The operation of the wind turbine facility in case a fast reactive power support is required follows the route outlined in relation to the first aspect of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will now be explained in further details with reference to the accompanying figures, where

Fig. 1 shows a typical PC chart,

Fig. 2 illustrates how various parameters are changed,

Fig. 3 shows a flow chart illustrating aspects of the method of the present invention, and Fig. 4 shows a wind turbine facility being capable of carrying out the invention. While the invention is susceptible to various modifications and alternative forms specific embodiments have been shown by way of examples in the drawings and will be described in detail herein. It should be understood, however, that the invention is not intended to be limited to the particular forms disclosed. Rather, the invention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION OF THE INVENTION

In its most general aspect the present invention relates to a method and a wind turbine facility that facilitates a fast and swift reactive power support to a power grid even though the power source to deliver the reactive power is already being operated at or near its rated power level. The power source is typically a wind turbine facility, such as a single wind turbine or a wind power plant, or a part thereof, comprising a plurality of wind turbines.

The fast and swift behaviour of the method of the present invention is provided by promptly reducing the amount of active power injected into the power grid, and dissipated and/or storing this amount of active power in some sort of dump load and/or power reservoir to that the load on the wind turbine generator remains essentially unaffected. The essentially unaffected load on the wind turbine generator prevents that the rotor speed increases. As it will be explained later the dissipation and/or storing of power may be performed on a relative short term scale, i.e. from a few seconds to a few minutes. Referring now to Fig. 1 at typical PQ chart of a wind turbine generator is shown. The underlying idea of the present invention is to extend the PQ chart in order to provide a large and fast reactive power capability of the wind turbine.

In case the wind turbine is operating at its rated active power level the reactive power control range is typically a base region corresponding to the hatched region ranging from Q _base ,ind to Qbase,ca _P . The indexes "ind" and "cap" here refer to induction and capacitive, respectively. In order to increase reactive power capability beyond this hatched region the wind turbine has to reduce its active power production. Otherwise the current limit (dashed circle) of the wind turbine is violated.

In order to be able to provide a fast and swift reactive power response it is the aim of the present invention to maintain the same reactive power ramp rate in the whole extended range, i.e. inside and outside the hatched region. The reactive power ramp rate may vary within the range 1-50 per unit per second, which gives a certain MVar/s depended on the wind turbine size. As an example the reactive power ramp rate may be 20 per unit per second. For a 3 MW wind turbine this means that the reactive power ramp rate is 60 MVar/s, whereas in the case of a 1 MW wind turbine the reactive power ramp rate is 20 MVar/s.

The fast reactive power increase is provided by swiftly changing the working point of the wind turbine from working point (1) to working point (2). The active power provided to the power grid should be decreased with essentially the same speed. For wind turbines with DC choppers the otherwise unavoidable rotor speed increase can be counteracted by dissipating and/or storing active power into for example dump load resistors.

By implementing the above-mentioned type of control the need for expensive standby StatComs is avoided, because the full extended capability (Q _e xtended,md to Q _e xtended,ca _P ) of the wind turbine can be utilized with high reactive power ramp rates.

Referring now to Fig. 2 schematic illustrations of selected parameters are depicted. The reactive power increase is initiated at T=0. The parameters shown in Fig. 2 are as follows: a) Reactive power injected into power grid vs. time b) Active power injected into power grid vs. time c) Active power dissipated in DC chopper vs. time d) Active power at wind turbine generator terminals vs. time e) Rotor speed vs. time f) Rotor pitch vs. time

As shown in a) the increase of reactive power into the power grid is initiated at T=0. The reason for this may be many, such as a falling grid voltage. To avoid exceeding the current limit of the wind turbine the active power provided to the power grid is reduced at essentially the same time, see b), and with essentially the same rate. When the increased amount of reactive power is no longer required (not shown) the reactive and active power levels return to their respective levels prior to T=0.

The amount of active power not being injected into the power grid is dissipated in a DC chopper c) so that the load on the generator d) as well as the rotor speed e) remain essentially unchanged on the short term. The e) the essential constant rotor speed is shown as the solid line. For comparison e) also shows (dashed line) how the rotor speed will increase if the DC chopper is inactive or not present.

As depicted in f) the rotor blades are pitched out of the wind on the longer time scale. The pitching can be initiated immediately or after a time delay of typically 0-2 seconds. This also affects the generated power d) as well as the amount of power dissipated in the DC chopper c). As illustrated in d) the generator power decreases as the rotor blades are pitched out of the wind. Typically, the pitching process takes 5-10 seconds. As a consequence of the decreasing generator power the power dissipated in the DC chopper also decreases over the same time period. The decrease in power dissipation and/or storage c), generated power d) and the rotor blade pitch f) are all depicted as linear changes over time. However, non-linear dependencies may also be applicable as well. This also applies to the reactive power a) and active power b) changes which may be of a non-linear nature as well.

Thus, when a demand for reactive power support is detected, the amount of reactive power to the power grid is increased while the amount of active power is decreased. A DC chopper (or power storage means) is activated to maintain the load on the generator. On a longer time scale the rotor blades are pitched out of the wind so that the generator power, and thereby also the dissipated/stored amount of power, is decreased.

The demand for increasing the amount of reactive power may be triggered by for example a falling grid voltage. Thus, if the grid voltage falls below a certain limit, for example below a predefined deadband, the power distributor may require that additional reactive power is injected into the grid in order to increase or at least stabilize the grid voltage. In this context the power distributor may also require that a certain amount of reactive power reserve is available for support purposes. Fig. 3 shows a simplified flow-chart illustrating aspects of the method according to the present invention. A request for an increased amount of reactive power is received. In order to comply with this request, and at the same time, not exceeding any current limits of the wind power facility the reactive power to the power grid is increased, and the active power to the same power grid is decreased with a certain amount. The amount of active power no longer being provided to the power grid is instead being dissipated and/or stored in appropriate power dissipation/power storage means. After an optional delay period between 0 and 2 seconds pitching of the rotor blades out of the wind is initiated and the amount of dissipated and/or stored active power is decreased accordingly. At a later time the working conditions are normalised and a request for a decreased amount of reactive power is provided. In response to this request the reactive power and the active power may be brought back to their original power levels.

Fig. 4 depicts a wind turbine facility comprising a set of rotatably mounted rotor blades being mechanically coupled to a generator via a gear box. The generated power is provided to an associated power grid via a power converter. The power converter typically comprises a generator side AC/DC converter and a grid side DC/ AC converter. The AC/DC converter converts the three phase AC power from the generator into DC power. Similarly, the DC/ AC converts the DC power into AC power having a frequency that matches the nominal frequency of the power grid. The AC/DC converter and the DC/ AC converter are separated by an intermediate DC link to which the power dissipation or power storage arrangement is coupled. The power dissipation arrangement typically comprises a DC-chopper and a number of resistive dump loads. For power storage batteries and/or capacitors may be applicable. A wind turbine controller controls at least the power converter and the rotor blade pitching mechanism in response to control or sensor signals from at least the power grid and the generator.