Method for operating an electric machine using a dynamic power curve

Abstract

A method for operating an electric machine using a dynamic capability curve. A method for operating an electric machine (particularly a wind turbine) having a generator, particularly a wind turbine, having a rotor and a stator, is described. The method includes: i) evaluating active damping applied to the electric machine; ii) estimating a damping criterion based on the evaluated active damping applied; and iii) shifting the dynamic capability curve toward a maximum permissible level. The maximum permissible level is based on the damping criterion and a first operating criterion and / or a second operating criterion. Furthermore, the dynamic capability curve is a dynamic power capability curve or a dynamic torque capability curve.

Description

Technical Field

[0001] The present invention relates to the field of methods for operating an electrical machine with a generator, in particular a wind turbine. Furthermore, the present invention relates to a wind turbine having an operating system configured to perform said method. Background Art

[0002] Electric machines, including generators (having a rotor and a stator), are typically operated using a power capability curve (also known as a power curve or power envelope). This power capability curve may reflect the relationship between generated power (e.g., in watts) and the speed (e.g., in RPM) of the generator's rotor (or wind speed). Generally speaking, higher rotor speeds result in higher power generation. Therefore, the power capability curve increases with higher rotor speeds. Similarly, electric machines may be operated using a torque capability curve, which may reflect, for example, the relationship between rotor speed and torque.

[0003] However, in many technical fields (for example, energy production using wind turbines), the generated power (and / or torque) must be limited due to operational stability and material fatigue. To achieve this, active power and / or torque damping can be implemented, for example by pitching the wind turbine blades. Consequently, the power capability curve must define a maximum operating power limit based on several parameters, such as rotor speed (for example, of the generator), temperature, active power damping, and material stability.

[0004] According to the prior art, the power (or torque) capability curve is a static curve based solely on rotor speed. This static power capability curve limits the steady-state power capability curve (reference) from the power control system. However, this prior art technique creates several problems: at low rotor speeds, the static power capability curve is very conservative because the required power damping is only a small fraction assumed and included in the calculation of the power capability curve. Because the power capability curve is static, the unused portion of the power damping is lost at (particularly low) speeds, where the required power damping is below the damping budget.

[0005] As a result of this situation, the Resonant Speed Avoider (RSA), which adds power to prevent the wind turbine from operating too close to the resonance of the 3rd harmonic rotor and tower frequencies, requires more power at low speeds but is limited by the static power capability curve.

[0006] It may be necessary to operate electrical machines, particularly wind turbines, in a robust and stable manner while providing optimal power production. Summary of the Invention

[0007] This need is met by the subject matter according to the independent claim. Advantageous embodiments of the invention are described by the dependent claims.

[0008] According to a first aspect of the present invention, a method for operating an electrical machine (in particular a wind turbine) having a generator with a rotor and a stator is provided. The method comprises: i) evaluating active damping (power damping and / or torque damping) applied to the electrical machine; ii) (dynamically / continuously) deriving (estimating) a damping criterion based on the evaluated applied active damping (in other words, estimating or measuring the active damping term sent to a power or torque reference and estimating the magnitude of the damping); and iii) (dynamically / continuously) shifting a dynamic capability curve toward a maximum permissible level (e.g., a maximum permissible power level or a maximum permissible torque level). The maximum permissible level is based on the damping criterion and a first operating criterion and / or a second operating criterion. Furthermore, the dynamic capability curve is a dynamic power capability curve and / or a dynamic torque capability curve.

[0009] According to a second aspect of the present invention, there is provided a wind turbine comprising: i) a generator having a rotor and a stator; and ii) an operating system, wherein the operating system is configured to perform the above method.

[0010] According to exemplary embodiments, the present invention may be based on the concept that an electrical machine (particularly a wind turbine) can be operated in a robust and stable manner while providing optimal power production by applying a dynamic capability curve (power capability curve or torque capability curve) that continuously takes into account an assessment of the active damping applied (to the electrical machine) and continuously updates (estimates) the damping criterion. The damping criterion is then used to dynamically shift the capability curve toward a maximum permissible level. This maximum permissible level is based on the damping criterion and at least on a first criterion reflecting the maximum power / torque capability (e.g., the stability limit of the generator) and / or a second criterion reflecting the steady-state capability (e.g., the fatigue limit of a wind turbine component).

[0011] In the manner described, a dynamic capacity curve is provided which can be optimized for medium and low rotor speeds and which can enable the most optimized possible capacity curve to be achieved in the entire operating region.

[0012] According to one embodiment of the invention, the first operating criterion comprises a maximum power capability curve or a maximum torque capability curve. This can provide the advantage that the maximum permissible power / torque can be directly taken into account when dynamically shifting the dynamic capability curve.

[0013] For example, the maximum power capability curve (or peak power envelope) may be defined by converter and / or generator stability limits that ensure healthy transient operation.

[0014] According to another embodiment of the invention, the definition of the first operating criterion includes a stability limit (in particular of the generator and / or converter). This offers the advantage that the stability limit of the generator (and / or converter) which actually limits the maximum power / torque is taken into account.

[0015] The stability limit may include, for example, a temperature, a critical temperature, or a current limit.

[0016] According to another embodiment of the invention, the second operating criterion comprises a steady-state capability curve. This may provide the advantage that healthy continued operation of one or more components of the electrical machine (or wind turbine) may be directly taken into account when dynamically shifting the capability curve.

[0017] The steady-state capability curve may be defined, in particular, by component temperature limits that ensure healthy, sustained operation. It may also take into account material fatigue, particularly at high temperatures. Consequently, the second operating criterion may only be relevant at higher rotor speeds, for example when an increase in torque raises component temperatures.

[0018] According to another embodiment of the invention, the definition of the steady-state capability curve comprises a (component) fatigue limit (in particular a fatigue limit of one or more (electric machine) components).

[0019] The steady-state capability curve may comprise at least one of the group consisting of: temperature limit, vibration limit, current limit (of a component of the electric machine, respectively). Thus, fatigue limits of these components may be taken into account, in particular at higher power and / or torque levels.

[0020] According to another embodiment of the present invention, the damping criterion includes an allowable damping budget for active power damping or active torque damping. In particular, the damping criterion includes a damping characteristic that uses ripple in power or torque. More specifically, the active damping includes active tower damping and / or active driveline damping. This offers the advantage that the (continuously updated) active damping can be directly taken into account when dynamically shifting the capacity curve. For example, established and robust methods, such as active tower damping and active driveline damping, can be implemented directly.

[0021] According to another embodiment of the present invention, the maximum permissible level comprises at least one of the following: a rotor speed of the generator and / or converter, a temperature limit, a damping budget, and a stability limit. In particular, the maximum permissible level is based on a maximum permissible power level and / or a maximum permissible torque level. This offers the advantage that the maximum permissible level can be flexibly adapted to important aspects and current issues.

[0022] According to another embodiment of the present invention, the method further comprises shifting the dynamic capability curve towards the first operating criterion at low rotor speeds, thereby using the damping criterion as a limitation. In particular, the damping criterion is used as the sole limitation. In other words, the dynamic capability curve is shifted towards the first operating criterion minus the damping criterion. This offers the advantage that, at low rotor speeds, only the first criterion (immediately adjacent to the damping criterion) needs to be considered, and the method can be performed more efficiently.

[0023] According to another embodiment of the present invention, the method further comprises shifting the dynamic capability curve towards the first operating criterion at high rotor speeds, thereby using the damping criterion and the second operating criterion as limits. In other words, the dynamic capability curve is shifted to the first operating criterion minus the damping criterion and the second operating criterion. This offers the advantage of specifically taking temperature stability (in the form of the second criterion) into account at high rotor speeds, where component temperatures may rise.

[0024] According to another embodiment of the invention, the first operating criterion and the second operating criterion are substantially the same at low rotor speeds.

[0025] According to another embodiment of the present invention, the first operating criterion and the second operating criterion are substantially different at high rotor speeds. At low rotor speeds, the steady-state capability curve and the maximum power / torque capability curve may be more or less the same, whereas at high rotor speeds (nominal power), the steady-state capability curve may be lower than the maximum power / torque capability curve.

[0026] The terms "low rotor speed" and "high rotor speed" depend on the generator being used and its environment. In an exemplary embodiment in which the generator is part of a wind turbine, a speed range between 1 and 7.5 RPM may be considered low (to medium) speed, while a speed range between 7.5 and 12 RPM may be considered (to medium) high speed.

[0027] According to another embodiment of the invention, the method further comprises applying a Resonant Speed Avoidance (RSA) feature to the motor. This may provide the advantage that established and robust wind turbine features may be implemented in an efficient manner while improving their performance.

[0028] The RSA increases power to prevent the wind turbine from operating too close to the resonance of the 3rd harmonic rotor frequency with the tower frequency, and power is needed especially at low speeds.Using the above method, the RSA can have an extended operating range.

[0029] According to another embodiment, the wind turbine is a direct drive wind turbine or a gearbox wind turbine. This may provide the advantage that the described operating method may be directly implemented into already established and robust systems.

[0030] It should be noted that embodiments of the present invention have been described with reference to different subject matters. In particular, some embodiments have been described with reference to method-type claims, while other embodiments have been described with reference to apparatus-type claims. However, those skilled in the art will appreciate from the above and following description that, unless otherwise stated, any combination of features relating to different subject matters, in addition to any combination of features belonging to one type of subject matter, and in particular any combination of features from method-type claims and features from apparatus-type claims, are also considered to be disclosed by this document.

[0031] The above-defined aspects of the present invention as well as further aspects will be apparent from the examples of embodiment described hereinafter and will be explained with reference to these examples of embodiment. The invention will be described in more detail hereinafter with reference to examples of embodiment, but the invention is not limited to these examples of embodiment. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] FIG. 1 shows a diagram with a dynamic power capability curve according to an exemplary embodiment of the present invention. DETAILED DESCRIPTION

[0033] According to an exemplary embodiment, the method includes dynamically estimating a required damping budget (damping criterion) based on a steady-state capability curve and a peak (maximum) power capability curve, and dynamically shifting the power capability curve (power envelope) upward to a maximum allowed power production (maximum allowed level).

[0034] According to another exemplary embodiment, there is increased available power for certain features such as RSA (Resonance Speed Avoider), making those features more effective.

[0035] According to another exemplary embodiment, a wind turbine is enabled to achieve an annual energy production (AEP) optimized power level for low rotor speeds.

[0036] The illustrations in the accompanying drawings are schematic. In addition, spatially relative terms such as "front" and "rear," "upper" and "lower," "left" and "right" are used to describe the relationship of an element to another element as shown in the figures. Therefore, these spatially relative terms may apply to orientations in use that differ from the orientations depicted in the figures. However, it should be understood that all of these spatially relative terms refer to the orientations shown in the figures for ease of description and are not necessarily limiting, as devices according to embodiments of the present invention may adopt orientations different from those shown in the figures when in use.

[0037] Figure 1 shows a diagram of the power capability of a wind turbine. The rotor speed is plotted in RPM (revolutions per minute) on the abscissa. The power yield (in kilowatts) is shown on the ordinate. It goes without saying that the power yield increases depending on the rotor speed. However, active power damping is necessary with respect to the stability limits of the generator and also with respect to the temperature of the wind turbine components, which increases with higher power yield. The capability curve MPCC represents the maximum power capability curve. This maximum power capability curve (or first criterion) includes the stability limits of the generator (and / or the wind turbine's converter (e.g., AC-DC-AC converter)). Therefore, the curve MPCC represents the absolute power limit. The capability curve SSCC represents the steady-state capability curve (or second criterion) that includes the temperature (material fatigue) limits of the wind turbine components. At low rotor speeds, the curves MPCC and SSCC are essentially identical, since, at these low temperatures, temperature issues generally do not need to be considered. At higher rotor speeds, when the material temperature increases, the curves MPCC and SSCC are no longer identical (around 8 RPM in the example), wherein the MPCC is then higher than the SSCC.

[0038] Conventionally, wind turbines operate using a static power capability curve, shown as a prior art SPCC. As can be seen from the figure, at low rotor speeds, the SPCC is significantly lower than both the MPCC and SSCC, resulting in a significant loss of power production. At high rotor speeds, the SPCC is essentially the same as the SSCC. Increasing the static power capability curve at low rotor speeds increases the risk of: i) driving the wind turbine into unstable operation, which could result in shutdown; and / or ii) operating the wind turbine at excessively high temperatures (particularly for wind turbine components), resulting in shorter component life and / or reduced power production.

[0039] In contrast to this prior art example, the aforementioned dynamic power capability curve (DPCC) is very close to both the MPCC and SSCC, even at low speeds. This saves power output and enables the aforementioned advantages to be provided in an efficient and robust manner. This difference from the prior art is particularly possible because the DPCC is dynamically shifted towards the MPCC, taking into account the continuously (dynamically) updated power damping criteria (based on the active power damping applied).

[0040] In the example of FIG1 , the dynamic capability curve is described as a dynamic power capability curve. However, the described example is also applicable when the dynamic capability curve is a dynamic torque capability curve.

[0041] It should be noted that the term "comprising" does not exclude other elements or steps, and the use of the word "a", "an" or "an" does not exclude a plurality. Furthermore, elements described in connection with different embodiments may also be combined. It should also be noted that reference signs in the claims should not be construed as limiting the scope of the claims.

[0042] It is noted that, in further developments of the invention, features from different illustrative embodiments described herein can also be combined.It is also noted that reference signs in the claims should not be construed as limiting the scope of the claims.

Claims

1. A method for operating an electric machine having a generator, the generator having a rotor and a stator, the method comprising: evaluating active damping applied to the motor; The damping criteria are obtained based on the evaluated applied active damping; as well as Shift the dynamic capability curve towards the maximum permissible level, wherein the maximum permissible level is based on the damping criterion and a first operating criterion and / or a second operating criterion; Wherein, the dynamic capability curve is a dynamic power capability curve (DPCC) or a dynamic torque capability curve, wherein the first operating criterion comprises a maximum power capability curve (MPCC) or a maximum torque capability curve, and Wherein, the second operating criterion includes a steady state capability curve (SSCC).

2. The method according to claim 1, wherein The electrical machine is a wind turbine.

3. The method according to claim 1, wherein The first operating standard is defined as follows: Stability limit.

4. The method according to claim 3, wherein: The definition of the first operating standard includes: The stability limit of the converter of the generator and / or the electric machine.

5. The method according to claim 1, wherein The definition of the steady state capability curve (SSCC) includes: Fatigue limit.

6. The method according to claim 5, wherein: The definition of the steady state capability curve (SSCC) includes: At least one of the group consisting of: temperature limit, vibration limit, current limit.

7. The method according to claim 1, wherein The damping criteria include: Permissible damping budget for active power damping or active torque damping.

8. The method according to claim 7, wherein: The damping criteria include: Use the damping characteristics of the ripple in power or torque.

9. The method according to claim 1, wherein The maximum allowed level comprises a maximum allowed power level and / or a maximum allowed torque level.

10. The method according to claim 9, wherein: The maximum allowed level comprises at least one of the group consisting of: a rotor speed of the generator and / or converter, a temperature limit, a damping budget, a stability limit.

11. The method according to claim 1, wherein The method further comprises: At low rotor speeds, the dynamic capability curve is shifted towards the first operating criterion, thereby using the damping criterion as a limit.

12. The method according to claim 11, wherein The method further comprises: Only the damping criterion is used as the limitation.

13. The method according to claim 1, wherein The method further comprises: At high rotor speeds, the dynamic capability curve is shifted towards the first operating criterion, using the damping criterion and the second operating criterion as limits.

14. The method according to claim 1, wherein The first operating criterion and the second operating criterion are the same at low rotor speeds.

15. The method according to claim 1, wherein The first operating criterion and the second operating criterion are different at high rotor speeds.

16. The method according to claim 1, wherein The method further comprises: A resonant speed avoidance (RSA) feature is applied in the electric machine.

17. A wind turbine comprising: a generator having a rotor and a stator; as well as An operating system, wherein the operating system is configured to execute the method according to any one of the preceding claims 1-16.

18. The wind turbine according to claim 17, in, The wind turbine is a direct drive wind turbine or a gearbox wind turbine.

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