Method for operating an electric machine using a dynamic ca pacity curve

Field of invention

The present invention relates to the field of methods for op erating an electric machine having a generator, in particular wind turbines. Furthermore, the present invention relates to a wind turbine with an operating system that is configured to perform the described method.

Art Background

Electric machines that comprise a generator (having a rotor and a stator) are often operated using a power capacity curve (also termed power curve or power envelope) . Hereby, the pow er capacity curve may reflect the relation between the pro duced power (for example in Watt) and the speed of the rotor of the generator (for example in RPM) (or the wind speed) . In general, the higher the rotor speed is, the higher will be the produced power. Hence, the power capacity curve increases with higher rotor speed. In a similar manner, the electric machine may be operated using a torque capacity curve which may be used in order to reflect the relation between e.g. the rotor speed and the torque.

However, in many technical fields (e.g. energy production us ing wind turbines), the produced power (and/or the torque) has to be limited due to operational stability and material fatigue. In order to achieve this, active power and/or torque damping can be performed, for example by pitching the blades of a wind turbine. Hence, the power capacity curve has to de fine a maximum operation power limit depending on several pa rameters such as rotor speed, temperature, active power damp ing and material stability (e.g. of the generator) . According to the prior art, the power (or torque) capacity curve is a static curve that is based on the rotor speed on ly. This static power capacity curve limits a steady state power capacity curve (reference) from a power control system. However, this prior art technique causes several issues: at low rotor speeds, the static power capacity curve is very conservative, as a required power damping is only a fraction of what is assumed and is included in the calculation of the power capacity curve. Since the power capacity curve is stat ic, the unexploited part of the power damping is lost at (in particular low) speeds, wherein the required power damping is below a damping budget.

As a consequence of this circumstance, a resonance speed avoider (RSA) , which increases power to prevent a wind tur bine from operating too close to the resonance of the 3rd harmonic rotor frequency and the tower frequency, needs more power at low speeds, but is limited by the static power ca pacity curve.

There may be a need for operating an electric machine (in particular a wind turbine) in a robust and stable manner, while providing an optimal power production.

Summary of the Invention

This need may be met by the subject matter according to the independent claims. Advantageous embodiments of the present invention are described by the dependent claims.

According to a first aspect of the invention there is provid ed a method for operating an electric machine (in particular a wind turbine) having a generator with a rotor and a stator. The method comprising: i) evaluating an active damping

(power damping and/or torque damping) applied to the electric machine, ii) (dynamically/continuously) deriving (estimating) a damping criterion from the evaluated applied active damping (in other words: estimate or measure the active damping term sent to the power or torque reference and estimate the size of the damping), and iii) (dynamically/continuously) shifting a dynamic capacity curve towards a maximum allowed level (e.g. maximum allowed power level or maximum allowed torque level) . Hereby, the maximum allowed level is based on the damping criterion and a first operation criterion and/or a second operation criterion. Furthermore, the dynamic capacity curve is a dynamic power capacity curve and/or a dynamic torque capacity curve.

According to a second aspect of the invention, there is pro vided a wind turbine, comprising: i) a generator having a ro tor and a stator, and ii) an operating system, wherein the operating system is configured to perform the method de scribed above.

According to an exemplary embodiment, the invention may be based on the idea that an electric machine (in particular a wind turbine) can be operated in a robust and stable manner, while providing an optimal power production, when a dynamic capacity curve (a power capacity curve or a torque capacity curve) is applied that continuously takes into account an evaluation of applied active damping (to the electric ma chine) and continuously updates (estimates) a damping crite rion. Said damping criterion is then used to dynamically shift the capacity curve towards a maximum allowed level. The maximum allowed level is based on the damping criterion and on at least a first criterion that reflects the maximum pow er/torque capacity (e.g. a stability limit of the generator) and/or a second criterion that reflects the steady state ca pacity (e.g. fatigue limits of wind turbine components) .

In the described manner, a dynamic capacity curve is provided that may be optimized for medium and low rotor speeds, and may enable the most optimal possible capacity curve through out the entire operation region.

According to an embodiment of the invention, the first opera tion criterion comprises a maximum power capacity curve or a maximum torque capacity curve. This may provide the advantage that the maximum allowable power/torque can be directly taken into account, when dynamically shifting the dynamic capacity curve .

For example, the maximum power capacity curve (or peak power envelope) may be defined by converter and/or generator sta bility limits that ensure a healthy instantaneous operation.

According to a further embodiment of the invention, the defi nition of the first operation criterion comprises a stability limit (in particular of the generator and/or a converter) . This may provide the advantage that the stability limit of the generator (and/or a converter), which actually limits the maximum power/torque, is taken into account.

The stability limit may for example comprise a temperature, a critical temperature, or a current limit.

According to a further embodiment of the invention, the sec ond operation criterion comprises a steady state capacity curve. This may provide the advantage that the healthy sus tained operation of one or more components of the electric machine (or wind turbine) can be directly taken into account, when dynamically shifting the capacity curve.

The steady state capacity curve may be, among others, defined by component temperature limits that ensures healthy sus tained operation. The steady state capacity curve may consid er material fatigue, especially at high temperatures. As a consequence, the second operation criterion may only be rele vant at higher rotor speeds, when e.g. the increase in torque elevates the temperature of components. According to a further embodiment of the invention, the defi nition of the steady state capacity curve comprises a (compo nent) fatigue limit (in particular one or more (electric ma chine) component fatigue limits) .

The steady state capacity curve may comprise at least one of the group consisting of: a temperature limit, a vibration limit, a current limit (of a component of the electric ma chine, respectively) . Hence, the fatigue limits of these com ponents can be considered, especially at higher power and/or torque levels.

According to a further embodiment of the invention, the damp ing criterion comprises an allowed damping budget of active power damping or active torque damping. In particular, the damping criterion comprises a damping feature that uses a ripple on power or torque. More in particular, active damping comprises active tower damping and/or active drive train damping. This may provide the advantage that the (continuous ly updated) active damping can be directly taken into ac count, when dynamically shifting the capacity curve. Hereby, for example, established and robust methods such as the ac tive tower damping and active drive train damping can be im plemented in a straightforward manner.

According to a further embodiment of the invention, the maxi mum allowed level comprises at least one of the group con sisting of: a rotor speed, a temperature limit, a damping budget, a stability limit of the generator and/or the con verter. In particular, the maximum allowed level is based on a maximum allowed power level and/or a maximum allowed torque level. This may provide the advantage that the maximum al lowed level can be adapted in a flexible manner to important aspects and current issues.

According to a further embodiment of the invention, the meth od further comprises, at low rotor speeds, shifting the dy- namic capacity curve towards the first operation criterion, thereby using the damping criterion as a limitation. In par ticular the damping criterion is used as the exclusive (only) limitation. In other words: shifting the dynamic capacity curve towards the first operation criterion minus the damping criterion. This may provide the advantage that, at low rotor speeds, only the first criterion (next to the damping crite rion) has to be taken into account and the method can be per formed more efficient.

According to a further embodiment of the invention, the meth od further comprises, at high rotor speeds, shifting the dy namic capacity curve towards the first operation criterion, thereby using the damping criterion and the second operation criterion as a limitation. In other words: shifting the dy namic capacity curve to the first operation criterion minus the damping criterion and the second operation criterion.

This may provide the advantage that, at high rotor speeds, where the temperature of components may increase, the temper ature stability (in form of the second criterion) is consid ered specifically.

According to a further embodiment of the invention, the first operation criterion and the second operation criterion are essentially the same at low rotor speeds.

According to a further embodiment of the invention, the first operation criterion and the second operation criterion are essentially not the same at high rotor speeds.

At low rotor speeds, the steady state capacity curve and the maximum power/torque capacity curve may be more or less the same, whereas at high rotor speeds (nominal power) the steady state capacity curve may be lower than the maximum pow er/torque capacity curve.

The terms "low rotor speed" and "high rotor speed" depend on the applied generator and its environment. In an exemplary embodiment, wherein the generator is part of a wind turbine, a speed range between 1 and 7,5 RPM may be considered as low (up to medium) speed, while a speed range between 7,5 and 12 RPM may be considered as (medium up to) high speed.

According to a further embodiment of the invention, the meth od further comprises applying a resonance speed avoidance (RSA) feature to the electric machine. This may provide the advantage that an established and robust wind turbine feature may be implemented in an efficient manner, while its perfor mance can be increased.

The RSA increases power to prevent a wind turbine from oper ating too close to resonance of the 3rd harmonic rotor fre quency and the tower frequency and requires power especially at low speeds. Using the method described above, the RSA may have an expanded range of operation.

According to a further embodiment, the wind turbine is a di rect drive wind turbine or a gearbox wind turbine. This may provide the advantage that the described operating method can be directly implemented into established and robust systems.

It has to be noted that embodiments of the invention have been described with reference to different subject matters.

In particular, some embodiments have been described with ref erence to method type claims whereas other embodiments have been described with reference to apparatus type claims. How ever, a person skilled in the art will gather from the above and the following description that, unless other notified, in addition to any combination of features belonging to one type of subject matter also any combination between features re lating to different subject matters, in particular between features of the method type claims and features of the appa ratus type claims is considered as to be disclosed with this document .

The aspects defined above and further aspects of the present invention are apparent from the examples of embodiment to be described hereinafter and are explained with reference to the examples of embodiment. The invention will be described in more detail hereinafter with reference to examples of embodi ment but to which the invention is not limited.

Brief Description of the Drawing

Figure 1 shows a diagram with a dynamic power capacity curve according to an exemplary embodiment of the invention.

Detailed Description

According to an exemplary embodiment, the method comprises: dynamically estimating the required damping budget (damping criterion) and dynamically shifting the power capacity curve (power envelope) up to the maximum allowed power production (maximum allowed level) based on a steady state capacity curve and a peak (maximum) power capacity curve.

According to a further exemplary embodiment, there is in creased power available for specific features such as RSA (resonance speed avoider) , thereby making those features more effective .

According to a further exemplary embodiment, a wind turbine is enabled to achieve annual energy production (AEP) opti mized power levels for low rotor speeds.

The illustration in the drawing is schematically.

Further, spatially relative terms, such as "front" and

"back", "above" and "below", "left" and "right", et cetera are used to describe an element's relationship to another el ement (s) as illustrated in the figures. Thus, the spatially relative terms may apply to orientations in use which differ from the orientation depicted in the figures. Obviously, though, all such spatially relative terms refer to the orien tation shown in the figures for ease of description and are not necessarily limiting as an apparatus according to an em- bodiment of the invention can assume orientations different than those illustrated in the figures when in use.

Figure 1 shows a power capacity diagram of a wind tur bine. On the abscissa, the rotor speed is shown in RPM (revo lutions per minute) . On the ordinate, the power production (in kilo Watts) is indicated. It goes without saying that the power production increases in dependence of the rotor speed. However, an active power damping is necessary with respect to stability limits of the generator and also regarding the tem perature (which increases with higher power production) of components of the wind turbine. The capacity curve MPCC rep resents the maximum power capacity curve. The maximum power capacity curve (or first criterion) comprises the stability limit of the generator (and/or a converter (e.g. AC-DC-AC converter) of the wind turbine) . This curve MPCC hence repre sents the absolute power limit. The capacity curve SSCC rep resents the steady state capacity curve (or second criterion) which comprises temperature (material fatigue) limits for components of the wind turbine. At low rotor speeds, the curves MPCC and SSCC are essentially the same, because at these low temperatures, in principle no temperature issues have to be taken into account. At higher rotor speeds, when the material temperatures increase, the curves MPCC and SSCC are not the same anymore (in the example around 8 RPM), wherein MPCC is then higher than SSCC.

Conventionally, a wind turbine is operated using a static power capacity curve which is shown as prior art SPCC. It can be seen from the diagram that, at low rotor speeds, the SPCC is much lower than MPCC and SSCC, thereby losing a high amount of power production. At high rotor speeds, the SPCC is essentially the same as SSCC. When increasing the static pow er capacity curve at low rotor speeds, this increases the risk for: i) driving the wind turbine into unstable operation which may result into shut-down, and/or ii) running the wind turbine at too high temperatures (in particular with respect to the components of the wind turbine) , which results in shorter component lifetime and/or less power production.

In contrast to this prior art example, the above described dynamic power capacity curve DPCC is very close to MPCC and SSCC even at low speeds. Thereby, power production is saved and the advantages described above can be provided in an ef ficient and robust manner. This difference to the prior art is in particular possible, because the DPCC is dynamically shifted towards the MPCC, taking into account a continuously (dynamically) updated power damping criterion (which is based on applied active power damping) .

In the example of Figure 1, the dynamic capacity curve has been described as a dynamic power capacity curve. However, the described example holds also true, when the dynamic ca pacity curve is a dynamic torque capacity curve.

It should be noted that the term "comprising" does not ex clude other elements or steps and the use of articles "a" or "an" does not exclude a plurality. Also elements described in association with different embodiments may be combined. It should also be noted that reference signs in the claims should not be construed as limiting the scope of the claims.

It is noted that it may also be possible in further refine ments of the invention to combine features from different il lustrative embodiments described herein. It should also be noted that reference signs in the claims should not be con strued as limiting the scope of the claims.