A WIND TURBINE GENERATOR

Technical field

The invention relates to a method and a system for controlling a wind turbine generator to ensure safe operation of the wind turbine generator, particularly, although not necessarily, during high speed wind conditions. Background to the invention

A common type of wind turbine generator is the three-bladed upwind horizontal-axis wind turbine generator (HAWT), in which the turbine rotor is at the front of the nacelle and facing the wind upstream of its supporting turbine tower. The blades capture the energy of the wind which is converted to mechanical torque at the rotor which is then transferred through a drive train to a generator. The generator converts the mechanical power into electrical power which is then injected into the electrical grid, which may be by way of a power electronics frequency converter which takes into account grid requirements. Typically, the control characteristic of a wind turbine generator is expressed as a power curve, as shown in Figure 1 , which partitions the operation of the wind turbine generator into a number of regions. Region A covers operation of the wind turbine where the wind speed is too low to drive the blades to generate power. Region A extends to the 'cut-in' wind speed V _C i at which point the wind flow is energetic enough for the wind turbine generator to be activated in order to start generating power. The operation then moves into region 'B' in which the wind speed is above the cut-in wind speed V _C i but is too low for the generator to produce maximum or 'rated power'. Thus, in region 'B', which is also known as below-rated operation, the wind turbine is controlled to maximise the power captured from the wind. Usually, generator torque control regulates the rotor speed to optimise the blade tip speed ratio, whilst blade pitch is held substantially constant.

Where the wind speed increases into region 'C, it is considered to be at or above rated wind speed V _R , such that the wind turbine generator is able to produce its rated power P _R . As the skilled person will understand, the rated power is the maximum power output which the generator is designed to produce on a continuous basis. Therefore, in this above rated power region, the control objective is to maintain the power output so that it is substantially constant, and this is typically achieved by blade pitch control. During blade pitch control, the l generator torque is held substantially constant, although generator torque control may be used to regulate relatively high frequency speed variations of the rotor.

The use of blade pitch control and generator torque control can maintain rated power for a comparatively wide range of wind speeds. However, a point will be reached at which the wind speed is too high for continued safe operation of the wind turbine generator at which point it must be shut down to prevent damage to the generator and other components. This point is referred to as the cut-off wind speed V _∞ - As an alternative to shutting down the turbine at the cut-off wind speed V _∞ , or max-rated wind speed, it is also a known approach to ramp down or de-rate the operation of the wind speed more gradually rather than have a hard cut-off point. This ramp down approach is less aggressive in terms of loading on the wind turbine components and can reduce damage to the generator and drive train for example. The ramp down part of the curve is illustrated at region '.

Throughout the operational regions A-D, the control regime applies a supervisory control function to monitor the rotor speed to ensure that it does not exceed a predetermined rotor speed threshold. Since output power is generally proportional to the rotor speed, this rotor speed threshold is shown on Figure 1 as N LIMIT- If rotor speed does rise to unacceptable levels, perhaps due to a failure in speed control, the control regime can act to shut down the wind turbine to avoid the rotor spinning too fast which may cause excessive wear and damage to certain components of the wind turbine, such as the main rotor bearing and the gearbox. An opportunity exists for improving the sophistication of the supervisory speed limit function, and it is against this background that the invention has been devised.

Summary of the invention

In one aspect, the invention provides a method of controlling a wind turbine, the method comprising monitoring a speed reference parameter of the wind turbine, and setting an overspeed threshold in dependence on the speed reference parameter, so that the overspeed threshold varies with the operational speed of the wind turbine.

Advantageously, the invention enables the overspeed threshold to adapt to the prevailing rotor speed reference, so that a faster response to uncontrolled rotor speed rises is achieved. For example, during below rated operating conditions, the rotor speed is lower than at rated operated conditions. By virtue of the invention, the overspeed threshold during below rated conditions is set lower than at rated conditions so that any uncontrolled speed rises are caught early, and appropriate action taken, for example shutting down the wind turbine. This is important, particularly during high wind conditions, as shutting down the wind turbine at a relatively early point will prevent the rotor speeds rising too high and thus will reduce extreme loading on the blades, drive train and the nacelle/tower structure.

The invention has particular utility in an operational region of the wind turbine in which the wind speed exceeds a max rated wind speed threshold. In such a case, the overspeed threshold can be set as a function of the rotor speed as the rotor speed is reduced as a protective measure against high wind speeds.

The speed reference parameter may be a rotor speed reference parameter. However, it is important to note that the speed reference parameter may be any parameter that gives a reasonably accurate indication of the machine speed of the wind turbine, whether or not it is a direct speed measurement, either of the rotor or of the other components of the generation equipment such as gearbox or generator.

In one embodiment, the overspeed threshold may be set in dependence on an overspeed margin parameter, which may be substantially constant, or which may be determined dynamically in dependence on one or more condition, such as wind speed, rotor speed, or wind turbulence factor, by way of example. In one embodiment the overspeed threshold may be determined by the expression:

Overspeed threshold T = [(N _M ARGIN / N _RAT ED)*100] * N _REF where N MARGIN is an absolute value of the over speed at rated power, NRATED is the rated rotor speed and where N REF is the prevailing speed reference.

The invention may also be expressed as a system for controlling a wind turbine, comprising a controller that monitors a wind speed reference parameter of the wind turbine, and determines an overspeed threshold in dependence on the speed reference parameter so that the overspeed threshold varies with the operational speed of the wind turbine.

Preferred and/or optional features of the first aspect of the invention may be combined with the second aspect of the invention. The invention also provides a computer program product downloadable from a communication network and/or stored on a machine readable medium, comprising program code instructions for implementing a method as described above, and also provides a a machine readable medium having stored thereon such a computer program product.

Brief description of the drawings

Embodiments of the invention will now be described by way of example only to the following drawings, in which:

Figure 1 is an illustration of a typical power curve for a wind turbine;

Figure 2 is a front view of a wind turbine; Figure 3 is a schematic view of a power generation system of the wind turbine in

Figure 2;

Figure 4 is a wind turbine power curve that illustrates an embodiment of the invention; and

Figure 5 is a schematic block diagram of the embodiment illustrated in Figure 4. Detailed description of embodiments of the invention With reference to Figure 2, a wind turbine installation 2 includes a wind turbine module 4 mounted on top of a tower 6 which is itself fixed into a foundation 8 in the usual manner. The wind turbine module 4 houses and supports the various power generating components of the wind turbine installation 2, one of which is a rotor 10 comprising a hub 12 and three blades 14 that define a rotor disc 16. The wind turbine installation shown in Figure 2 is a horizontal axis wind turbine (HAWT) which is a common type of system, although other types exist, to which the invention is also applicable.

As is known, the flow of wind acting on the blades 14 spins the rotor 10 which drives the power generation equipment housed in the wind turbine module 4. The power generation equipment is shown in more detail in Figure 3. Figure 3 illustrates an example of a power generation system architecture which gives context to the invention, as will become apparent. Represented schematically as a system diagram the wind turbine module or 'system' includes features that are significant for this discussion, but it should be appreciated that many other conventional features are not shown here for brevity, for example yaw control equipment, control network, local power distribution network and so on. However, the skilled person would understand that these features would be present in a practical implementation, and so their presence is implied. Also it should be noted that the specific architecture discussed here is used as an example to illustrate the technical functionality of the invention, and so the invention may be implemented by a system having a different specific architecture.

Returning to the figure, the rotor 10 drives a transmission 20 by way of an input drive shaft 22. Although the transmission 20 is shown here in the form of a gearbox, it is also known for wind turbines to have a direct-drive architecture which do not include a gearbox. The transmission 20 has an output shaft 24 which drives a generator 26 for generating electrical power. Three phase electrical power generation is usual in utility scale wind turbine systems, but this is not essential for the purpose of this discussion.

The generator 26 is connected to a frequency converter 30 by a suitable three-phase electrical connector such as a cable or bus 32. The frequency converter 30 is of conventional architecture and, as is known, converts the output frequency of the generator 26 to a voltage level and frequency that is suitable for supplying to an electrical grid 34 via a transformer 36 and filter 37. It will be appreciated that the specific architecture described here is a two-level back-to-back voltage source full scale frequency converter (FSC) system, which includes a generator-side converter 38 and a grid side converter 40 which are coupled via a DC link 42. The general architecture of such a system is conventional and will not be described in more detail. Furthermore, the skilled person will understand that other architectures are known, such as doubly-fed induction generator-based systems (DFIG). To put the invention into context, a brief discussion of the control strategy of the wind turbine will now be provided by way of example. As is known, variable-speed wind turbines typically operate under two main control strategies: below-rated power and above-rated power. As is known, the term 'rated power' is used here in its accepted sense to mean the power output at which the wind turbine system is rated or certified to produce under continuous operation. Similarly, the use of the term 'rated wind speed' should be understood to mean the lowest wind speed at which the rated power of a wind turbine is produced. Below rated power occurs at wind speeds between the cut-in speed and rated wind speed which, typically, is between 10 and 17m/s, but may be different depending on the size of the wind turbine. In this operating region, the wind turbine module 4 is operable to control the rotor speed so as to maximise the energy captured from the wind. This is achieved by controlling the rotor speed so that the tip speed ratio is at an optimum value, namely between 6 and 7. To control the rotor speed, the wind turbine module 4 controls the generator torque so as to track a power reference, as will be described.

Above-rated power occurs when the wind speed has increased to, or exceeds, the rated wind speed. In this operating condition, the objective of the wind turbine module 4 is to maintain a constant output power. This is achieved by controlling the generator torque to be substantially constant, so as to track a constant power reference, but varying the pitch angle of the blades which adjusts the resulting lift and drag force of the blade in the rotor plane. This will control the torque transferred to the rotor shaft so that the rotational speed, and also the generated power of the system, is kept constant below a set threshold.

In order to achieve the below-rated power and above-rated power control objectives, the wind turbine module 4 is equipped with a controller 50. The controller 50 is operable to control the frequency converter 30 via torque reference T _RE F to influence the torque exerted on the rotor 10 by the generator 26, and also to control the pitch of the blades via pitch reference P0 _REF , and thereby the speed of the rotor 10, through a blade pitch adjustment system 52.

The controller 50 receives a plurality of control inputs, but two control inputs are shown specifically here: a rotor speed reference parameter N _REF and a power reference parameter PREF which are provided by a higher level controller, such as an operational sequence controller 53 either directly to the controller 50 or through a data distribution network based on a suitable protocol, such as Ethernet. The controller 50 also receives monitoring inputs so that it can determine the correct operation of the components under its control. Specifically, the controller 50 receives a machine speed parameter N _s which may come from speed sensors 54 associated with the rotor, the transmission, or the generator, and a power output parameter P _S from the frequency converter 30.

During below-rated conditions, the control system 50 is operable primarily to control the generator torque, which is associated with, and calculated from, the power reference P _RE F, by outputting a demanded torque signal T _RE F to the frequency converter 30 in order to track the power reference P _REF . Similarly, at operating conditions above-rated power, the control system 50 is operable to hold the generator torque substantially constant (and, therefore, to track the constant power reference) but to provide control input P0 _RE F to the pitch control system 52 to modulate, collectively, the pitch angles of all three blades of the rotor 10. As will be appreciated from the above discussion, the wind turbine module 4 is provided with a facility to control the rotor speed during a wide range of wind speeds in order to optimise the power generation of the system. In addition to the power control functionality, the controller also provides supervisory control in the form of a dynamic overspeed limit function, as will now be described in more detail.

In contrast to the prior art, in which such supervisory rotor speed control is implemented as a fixed overspeed limit across the operational envelope of the wind turbine, in the invention the overspeed limit is variable and is set in dependence on a reference parameter that is indicative of the rotor speed reference for the wind turbine N _REF .

One benefit of this approach is that since the overspeed limit adapts to the prevailing rotor speed reference, it provides a faster response to rising machine speed, particularly at below rated and above cut-off wind speed operational conditions. During these operating conditions, the rotor speed, and therefore also the speed of the transmission and the generator, are significantly lower than when the wind turbine is running at rated power. So, appropriate action can be taken to shut down the wind turbine if it is detected that the rotor speed is rising to unacceptable levels, thereby helping to prevent damage occurring to the system. Particularly during high wind conditions, which causes high thrust forces on the blades, shutting down the wind turbine at an earlier point, before the rotor speeds rises too high, will reduce extreme loading of forces on the blades, drive train, the nacelle and the tower.

An embodiment of the overspeed protection function is illustrated in Figures 4 and 5. Here, it can be seen that the overspeed limit or 'threshold T increases in dependence on the rotor speed reference N REF 3s the wind speed rises from the cut-in wind speed Vci to rated wind speed V _R . From rated wind speed V _R to the max rated wind speed V _M R, the speed reference NREF is substantially constant and, therefore, so too is the overspeed threshold T. Although in some embodiments, the max rated wind speed V _M R marks the point at which the turbine will be shut down, in this embodiment, as illustrated in Figure 4, the overspeed threshold T is seen to decrease in line with the reduction in speed reference N REF, from the max rated wind speed V _M R to an absolute wind speed limit V _A . The dynamic overspeed limit function is implemented in the controller 50 by an overspeed limit module 60, as shown in Figure 5. It is envisaged that the overspeed limit module 60 will be implemented as a piece of software residing on the hardware environment of the controller 50, although it may of course be a separate processing module that is configured to provide a suitable data input to the controller 50 so that the controller 50 is able to implement the overspeed limit function and, as such, take appropriate action to execute a controlled shut down of the wind turbine module 4 in the event that the overspeed threshold is exceeded. Still further, the overspeed limit module 60 may be implemented in a processing module that is separate to the controller 50 and which is configured to override the control provided by the controller 50 to trigger a shut down even in circumstances where the overspeed threshold is exceeded.

As shown in Figure 5, the overspeed limit module 60 receives as inputs the rotor speed reference parameter N _REF and an overspeed margin parameter N MARGIN and is operable to output an overspeed threshold T based on the two inputs.

In this specific embodiment, the overspeed margin parameter N MARGIN is a constant that is stored in a suitable memory area 62 and which may be configured during commissioning and testing of the wind turbine. It is envisaged that the overspeed margin parameter N _MARG IN will be an absolute value representing the difference between the rotor speed at rated power, or 'N RATED', and the maximum safe speed of the rotor. For example, if the rotor speed at rated power is 20rpm, then the maximum safe speed of the rotor may be considered to be 22rpm, which is a 10% increase on the rotor speed at rated power. Of course, it should be understood that this specific margin value is provided here as an example, and is not intended to be limiting.

The overspeed limit module 60 uses the absolute overspeed margin N MARGIN, determines an equivalent percentage overspeed margin value corresponding to N MARGIN and outputs an overspeed threshold T that is the product of the rotor speed reference N REF and the percentage overspeed margin value. Expressed another way, the overspeed limit module 60 implements the following equation:

Overspeed threshold T = [(N _M ARGIN / N _RAT ED)*100] * N _REF It will be appreciated, therefore, that the overspeed limit module determines an overspeed threshold that is a predetermined percentage of the rated rotor speed of the wind turbine across substantially all of the operating speed range of the wind turbine. This provides a more responsive speed limit function for the wind turbine.

In the above embodiment, the rotor speed reference is used as the speed reference parameter. However, it should be noted that this is just an example, and that other metrics indicative of rotor speed could be used, for example rated generator speed or rated gearbox speed.

In the above implementation, the absolute margin value N MARGIN is fixed across the operating speed range of the wind turbine, and this provides an effective dynamic method for adjusting the overspeed threshold T in all of the different operating conditions of the wind turbine. However, an alternative embodiment is envisaged in which the percentage value corresponding to N MARGIN could be modified dynamically in dependence on the current operating condition of the wind turbine.

For example the absolute margin value N MARGIN or the percentage value could be increased when operating below rated speed, in order to provide a looser speed threshold, as required. So, when the turbine is being ramped up, it is more likely that the rotor speed will vary, so a looser margin (e.g. 15% rather than 10%) will ensure that the system is not triggered unnecessarily. Conversely, when the wind turbine speed is being ramped down a tighter margin (e.g. 5% rather than 10%) would be more appropriate to ensure any loss of speed control is captured at an early stage. Furthermore, the speed margin could be varied based on climatic conditions. For example, a tighter (narrower) margin would be more appropriate for calm conditions where rotor speed fluctuations would be relatively unusual, whereas a looser margin would be appropriate for more blustery wind conditions where rotor speed fluctuations would be more common. It will be understood that information on such wind conditions could be gained from a local wind sensor or metrological centre so that a determination of wind turbulence could be made. In Figure 5, this is illustrated by the operating condition input, labelled as 64 and highlighted in dashed lines.

The skilled person will appreciate that the various embodiments described above may be modified without departing from the scope of the invention, as defined by the claims.