The present invention relates to a method for operating wind turbine generators during high- wind operation. In particular, the present invention relates to a method for safe operation of wind turbine generators in order not to damage the generator/converter system of such wind turbine generators.

BACKGROUND OF THE INVENTION

Wind turbine generators as well as wind power plants are configured to produce and deliver active power to an associated power grid. They are in addition configured to produce or absorb reactive power in order to assist stabilizing the associated power grid. Wind turbine generators and wind power plants are sold with a certain capacity of reactive power. In general, doubly-fed induction generators (DFIGs) have limited reactive power capacities and they are very sensitive to generator speed and active power production. Typically, the active power, P, and reactive power, Q, capabilities are illustrated using a so- called P-Q chart which depicts an envelope of the active/reactive power production. With reference to Fig. la the wind turbine generator is capable of providing a continuous production at any combination of active power, P, and reactive power, Q, inside the envelope being denoted "Normal operation". The maximum reactive capability (Qmax) is only available until a certain active power level. When the active power approaches maximum power (Pmax) the reactive capability decreases/increases to Qrp/Qrn, where Qrp is a positive reactive power level and Qrn is a negative reactive power level.

During normal operation of a wind turbine generator the active power, P, depends on the wind speed, and this dependency can be illustrated as the upper power curve in Fig. lb. For lower wind speeds the active power, P, is restricted by the energy available in the wind. When the rated power of the wind turbine generator has been reached, the power is saturated to protect the wind turbine generator components, until the wind turbine generator shuts down due to high wind. As illustrated by the lower power curve in Fig. lb the reactive power capability, Q, is highest when the active power, P, is slightly below the rated power level. The reactive power, Q, thus has a peak at lower wind speeds, i.e. before the active power, P, reaches its rated value. For higher wind speeds the reactive power is limited in accordance with a P-Q chart, i.e. by Qrp and Qrn, cf. Fig. la. Referring again to active power curve in Fig. lb a limited active power production for very high wind speeds is allowed, i.e. above the wind speed where the wind turbine generator normally shuts down. Generally, the generator speed must be reduced to protect the rotor at such wind speeds. To protect the drivetrain when reducing the generator speed, the power must also be reduced. This results in a decrease of the active power, P, during high-wind operation (HWO). In accordance with the P-Q chart, the reactive power capability, Q, will increase as the active power, P, decreases, cf. Fig. la and the reactive power curve Fig. lb. However, the P-Q chart is only valid for certain power generator-speed configurations and these do not necessarily comply with HWO where the generator speed is chosen from a rotor- protection perspective. Operation with high reactive power during HWO operation might thus lead to severe temperatures in the generator.

In conclusion, with the current technology the P-Q chart in combination with high-wind operation of a wind turbine generator imposes serious risks on the generator/converter system. It may therefore be seen as an object of embodiments of the present invention to provide a method for safe operation of a wind turbine generator during high-wind operation.

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 during high-wind operations, the method comprising the steps of

1) determining one or more operational parameters, the one or more operational

parameters comprising at least one of a determined wind speed or are being based on at least a measured active power value and a measured generator speed, and 2) limiting one or more reactive power capabilities of the wind power facility to one or more predetermined reactive power values if at least one determined operational parameter differs from a predetermined value or a predetermined range.

The term wind power facility is to be understood broadly thus including for example a single wind turbine generator or a plurality of wind turbine generators forming a wind power plant. The term one or more reactive power capabilities is to be understood as the available amounts of reactive power at a given time. As addressed below the available amounts of reactive power may be positive and/or negative. The term high-wind operation (HWO) refer to the operational mode above a certain wind speed, and may include wind speeds where the wind power facility is normally closed down for safety reasons. In general, HWO is operation at a reduced power output and/or a reduced generator speed above a wind speed threshold being higher than the rated wind speed so that the turbine is operated to output a power which is lower than the rated power and/or is operated at a generator speed which is lower than rated generator speed. In this regard it is mentioned that the wind power facility may be operated to deliver or absorb reactive power even though it does not produce active power.

The method of the present invention is advantageous in that it avoids imposing risks on the generator/converter system of the wind power facility due to the limitation imposed on the one or more reactive power capabilities.

Generally, the limitation is imposed on the one or more reactive power capabilities if the one or more determined operational parameters either differ from a predetermined value or a predetermined range. Thus, if for example the one or more determined operational parameters are larger than a predetermined value the limitation on the one or more reactive power capabilities may be imposed. Alternatively, if the one or more determined operational parameters fall outside a predetermined range the limitation on the one or more reactive power capability may be imposed.

The one or more operational parameters to be determined may in principle be any parameter or a number of parameters that somehow relate to the operation of the wind power facility, however the one or more determined operational parameters comprises a determined wind speed or are being based on at least a measured active power value and a measured generator speed. The step of determining the wind speed may comprise one or more wind speed measurements. Alternatively or in combination therewith, the step of determining the wind speed may comprise one or more wind speed estimations. The step of determining the wind speed may further comprise the step of low-pass filtering the one or more wind speed measurements and/or the one or more wind speed estimations in order to avoid or at least reduce reactions to sudden wind speed fluctuations.

As an alternative to, or in combination with, a wind speed measurement and/or wind speed estimations the one or more determined operational parameters may be based on at least a measured active power value and a measured generator speed. This implementation may allow a higher reactive power capability. By properly processing a measured active power value and a measured generator speed a look-up table of imposable predetermined reactive power values is provided. According to the present invention the one or more reactive power capabilities may comprise a positive and/or a negative reactive power capability. In case of a positive reactive power capability, i.e. the wind power facility is capable of delivering reactive power, this may be limited to a positive predetermined reactive power value if at least one determined operational parameter differs from the predetermined value or a predetermined range. In case of a negative reactive power capability, i.e. the wind power facility is capable of absorbing reactive power, this may be limited to a negative predetermined reactive power value if at least one determined operational parameter differs from the predetermined value or a predetermined range. The values of the positive and negative predetermined reactive power values may be set as a percentage of the rated reactive power capability of the wind power facility. Thus, the positive and negative predetermined reactive power values may be set at for example 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60% or even more of the rated reactive power capability of the wind power facility. Also, the value of the positive predetermined reactive power value may be different from the numeric value the negative predetermined reactive power value. Thus, the value of the positive predetermined reactive power value may be smaller than the numeric value the negative predetermined reactive power value. As an example the positive predetermined reactive power value may be around 400 kVAr whereas the negative predetermined reactive power value may be around 600 kVAr for a 2 MW wind turbine generator.

The one or more predetermined reactive power values, including the positive and negative predetermined reactive power values, may be substantially constant values in a P-Q chart for a given wind speed in the HWO region. For other wind speeds in the HWO region the one or more predetermined reactive power values may take other substantially constant values. In order to avoid undesired reactive power transients the method of the present invention may further comprise the step of ensuring a smooth transaction between one or more reactive power capabilities and one or more predetermined reactive power values. Here, the one or more reactive power capabilities may be positive and/or negative reactive power capabilities, and the one or more predetermined reactive power values may be positive and/or negative predetermined reactive power values. The smooth transaction may be ensured by limiting one or more reactive power capabilities in accordance with one or more ramp rates until one or more predetermined reactive power values have been reached. One or more ramp rate limiter circuits may be applied for this purpose.

In addition to imposing a limit to the one or more reactive power capabilities the method of the present invention may further comprise the step of reducing an active power value from the wind power facility if at least one determined operational parameter differs from a predetermined value or a predetermined range.

In a second aspect, the present invention relates to a wind power control module for carrying out the method according to the first aspect. The wind power control module may comprise a suitable arrangement of hardware and/or software for carrying out the method.

In a third aspect, the present invention relates to a wind power facility comprising a control module for carrying out the method according to the first aspect. The control module may comprise a suitable arrangement of hardware and/or software for carrying out the method. Again, the term wind power facility is to be understood broadly thus including for example a single wind turbine generator or a plurality of wind turbine generators forming a wind power plant.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described in further details with reference to the accompanying figures, wherein Fig. la shows a prior art P-Q chart with an envelope curve illustrating the active/reactive production, and Fig. lb shows the active and reactive powers a function of wind speed according to prior art,

Fig. 2 shows a P-Q chart according to the present invention,

Fig. 3 shows the active and reactive powers a function of wind speed according to the present invention,

Fig. 4 shows a block diagram of a first embodiment of the present invention, and

Fig. 5 shows a block diagram of a second embodiment of the present 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 details 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

Aspects of the present invention relates to a method for operating a wind power facility, such as a wind turbine generator, during high-wind operations in order to avoid damage of for example the generator/converter system of the wind power facility. The wind turbine generator may in principle be any kind of wind turbine generator, including DFIGs and full- scale systems. The method according to the present invention involves a determination of one or more operational parameters, including at least one of the wind speed, the power and/or the generator speed. If at least one determined operational parameter differs from a predetermined value or a predetermined range at least one reactive power capability of the wind power facility is limited to a predetermined reactive power value which may be either positive or negative.

As previously disclosed Fig. la shows a prior art P-Q chart depicting the envelope of an active/reactive power production. The wind turbine generator is capable of continuous power production at any combination of active power, P, and reactive power, Q, inside the envelope being denoted "Normal operation". Fig. lb shows the result of a traditional regulation scheme with active power (upper curve) and reactive power (lower curve) as function of wind speed. The region for HWO, i.e. very high wind speeds, is depicted as well.

As depicted in Fig. lb the active power is reduced in the HWO region in order to protect for example the rotor and the gear box. According to the P-Q chart the reactive power capability will then increase in the HWO region, i.e. at very high wind speeds, as depicted by the reactive power curve in Fig. lb. In the HWO region the generator speed is driven by the rotor loads and not from a generator/converter perspective. This is highly disadvantageous in that the generator and/or converter systems risk very high temperatures which could be damaging. It should be noted that Fig. lb is plotted for positive reactive powers. However, a similar plot could be drawn for negative reactive powers.

According to the present invention a pre-determined limitation on the reactive power capability (positive or negative) is imposed when an operational parameter differs from a predetermined value or a predetermined range. The operational parameter being the wind speed, the active power and/or the generator speed. In one embodiment of the present invention the operational parameter is the wind speed which may be measured and/or estimated. In this embodiment the envelope "Normal operation" in the P-Q chart will still be valid for wind speeds below a predetermined value, cf. Fig. 2. However, for wind speeds above the predetermined value, i.e. in the HWO region, the reactive power capability will be limited by Qrp and Qrn as indicated by the two horizontal lines in Fig. 2.

The predetermined value of the wind speed may be set taking the specifications of the wind turbine generator into account. For example, the predetermined value of the wind speed may be larger than 12 m/s, such as larger than 15 m/s, such as larger than 18 m/s, such as larger than 20 m/s, such as larger than 25 m/s.

Similarly, the values of Qrp and Qrn may be selected in accordance with the specifications of the wind turbine generator. As previously addressed the values of Qrp and Qrn may numerically be identical or they may be different. For example, the numeric value of Qrn may be larger than Qrp in that Qrp may be around 400 kVAr and Qrn maybe around 600 kVAr for a 2 MW wind turbine.

Turning now to Fig. 3 a power regulation scheme according to the present invention is depicted. In Fig. 3 the active power as a function of wind speed is shown in the upper curve, whereas the lower curve shows the reactive power as function of wind speed. The region for HWO is depicted as well. With the proposed solution the reactive power capability will remain unchanged and constant during the HWO region despite the fact that the active power is decreased in the HWO region. Fig. 3 has been plotted for positive reactive powers. However, a similar wind speed dependency may be plotted for negative active powers.

An example implementation of this embodiment is illustrated in Fig. 4 in the form of a block diagram. As seen in Fig. 4 a measured or estimated wind speed is initially low-pass filtered in order to avoid or at least limit reactions to fast wind fluctuations. The filtered wind speed is then compared to the predetermined wind speed value which typically corresponds to the wind speed where the power generation starts to be reduced. If the measured or estimated wind speed is below the predetermined wind speed value (TH) flexible power limits for the positive and negative reactive powers are provided. These power limits may be Qmax, Qmin or even higher power limits. If the filtered wind speed is above predetermined wind speed value (TH) more restrictive reactive power limits Qrp, Qrn are imposed. In Fig. 4 the upper signal path of the block diagram handles the negative reactive power, whereas the lower signal path handles the positive reactive power. In order to ensure a smooth transaction between an actual power level and the limitations Qrp, Qrn to be imposed, i.e. to avoid power transients and thereby fast power jumps, the reactive power capability is reduced via a ramp rate limiter before the limitations Qrp, Qrn are imposed on the existing reactive power logic. As seen in Fig. 4 the reactive power limits Qrp, Qrn are imposed via a step function at the predetermined wind speed value (TH). It should be noted that other types of transitions, such as a graduated transition or a step-wise transition, may be applied instead.

The embodiment depicted in Fig. 4 may be implemented in various ways, such as via software or hardware or a combination thereof.

An implementation of another embodiment of the present invention is illustrated in Fig. 5 in the form of a block diagram. In this embodiment a region of safe operation and a HWO region, i.e. a region requiring power limits Qrp, Qrn, could be identified in an active power vs generator speed map. From measurements of active power and the generator speed a look- up table setting the reactive power limits Qrp, Qrn to be applied may be generated. This embodiment potentially allows more reactive power capability.