CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to European Patent Application No. EP 19156889.8, having a filing date of Feb. 13, 2019, the entire contents of which are hereby incorporated by reference.

FIELD OF THE TECHNOLOGY

The following relates to an arrangement and to a method for determining an operation parameter reference for controlling a generator side converter portion coupled to a generator, in particular of a wind turbine. Further, the following relates to an electric energy system comprising the arrangement and further relates to a wind turbine comprising the electric energy system. In particular, the following relates to a control method intended to improve the performance of a harmonic control in vector-controlled drives, namely, the control of permanent magnet synchronous generators used in wind turbine applications.

BACKGROUND

Harmonic control, in particular in permanent magnet generators, may be required in order to control noise and vibration as well as to increase system efficiency. Harmonic control may in particular be applied to direct drive generators which do not have a gearbox in the drive train. Conventionally, a fundamental current controller and a harmonic current controller may be utilized in a vector controlled drive.

However, the coexistence of a fundamental current controller (FCC) and a harmonic current controller (HCC) in a vector-controlled drive may bring some control issues such as high control effort, poor dynamic performance, and instabilities.

Concerning harmonic current control, additional controllers may conventionally be added in parallel with a fundamental current controller to provide the final control voltage to a converter drive. However, good performance is not achievable for lower harmonic orders (for example: 2f), showing very slow dynamics. This would have a negative impact to the turbine performance when it goes through the variation in speed and power as wind changes.

It should be noted that the presence of 2f harmonic is common in a large generator with segmented stator, and the 2f harmonic level can be significantly increased when the generator is operated in a fault-tolerant mode. A common attempt to improve transient performance of a controller is to add a feed-forward path, which based on a machine model determines the voltage demand to achieve a given current reference. This approach is difficult to realize for harmonic current control because a large number of parameters depending on load and rotor position need to be known accurately. Simulation results show that little improvement can be achieved.

Thus, there may be a need for an arrangement and for a method for determining an operation parameter reference for controlling a generator side converter portion coupled to a generator, wherein at least some of the above-mentioned issues are addressed, wherein in particular a simple solution having sufficient system stability is achieved. In particular improvements with focus on fast control of low harmonic frequencies may be desired.

SUMMARY

Embodiments of the present invention may in particular be applied for the harmonic control in vector-controlled drives, namely the control of permanent magnet synchronous generators used in for example wind turbine applications.

According to an embodiment of the present invention it is provided an arrangement for determining an operation parameter reference for controlling a generator side converter portion coupled to a generator, in particular of a wind turbine, the arrangement comprising: at least one arithmetic element configured to derive at least one harmonic current error by subtracting a generator output current from at least one harmonic current reference; at least one harmonic current controller configured to determine at least one harmonic reference current deviation based on the harmonic current error; another arithmetic element configured to derive a fundamental current error by subtracting the generator output current from a sum of a fundamental current reference and the at least one harmonic current reference; still another arithmetic element configured to determine a modified fundamental current error as a sum of the fundamental current error and the harmonic reference current deviation; a fundamental current controller adapted to determine the operation parameter reference based on the modified fundamental current error.

The arrangement may be part of a wind turbine controller, such as implemented in software and/or hardware. The operation parameter reference may be a reference value of an operation parameter of the generator. The operation parameter may at least partly define the operation of the generator, for example regarding its electrical output, its mechanical or electrical rotational speed, the generated torque or the like. In particular, the operation parameter reference may be a voltage reference which may be supplied to the generator side converter portion (of a for example AC-DC-AC converter) for actually controlling the generator side converter portion, in particular after having determined, from the operation parameter reference, pulse width modulation signals suitable for being supplied to gates of controllable switches in the converter portion.

The generator output current may have been measured. The fundamental current reference and also at least one harmonic current reference may be determined or defined in a number of ways. The fundamental current reference and the harmonic current reference may for example just be a constant or may be derived by looking up a look-up table or the like. In other embodiments, the fundamental current reference and/or harmonic current reference may depend on the operation parameter reference itself, may depend on the generator output current and may also depend on a DC voltage of a DC link between the generator side converter portion and a grid side converter portion.

The converter may for example be or comprise a AC-DC-AC converter which may be configured to convert a variable frequency output power provided by the generator to a substantially DC power at the DC link and from the DC power to a fixed frequency AC output power, for example having a frequency of 50 Hz or 60 Hz.

The arrangement may be configured to control (in particular concurrently) at least one or two or three or four or even more harmonics of a fundamental frequency of the operation parameter by deriving multiple harmonic current errors (from generator current and respective harmonic references) and providing multiple harmonic current controllers. All respective harmonic reference current deviations (as derived by the respective harmonic current controllers) may be added to the fundamental current error to derive the modified fundamental current error which may then be supplied to the fundamental current controller.

All harmonic current references, regarding all different harmonics to be controlled, for example a second, a fourth, a sixth, an eighth, and/or a twelfth harmonics may be considered. For each of the harmonics to be controlled, a corresponding harmonic current controller may be comprised in the arrangement which is configured to handle the particular harmonic, such as e.g. a second, a fourth, a sixth, an eighth or a twelfth harmonic of the fundamental electrical frequency of the generator.

The generator output current (or the current error) may be bandpass-filtered (passing through the harmonics under consideration) before being subtracted from the respective harmonic current reference.

In general, the fundamental current controller (and the harmonic current controller) may comprise PI-controllers which may be configured such that they produce output signals which, when supplied as operational parameter reference to the converter portion, lead to a decrease of the errors (i.e. the fundamental current error and the harmonic current error, respectively). Thereby, the control may in particular be applied in the fundamental dq-frame within the fundamental current controller and may be applied in the respective harmonic dq-frame in the respective harmonic current controller. Embodiments of the present invention support a respective control of the converter generator portion in order to eliminate one or more of undesired harmonic(s) or may also enable to achieve particular levels of particular harmonics at the output of the generator which levels may not be zero. Thereby, an effective control which may be implemented in a simple manner and a reliable stable operation may be provided.

According to an embodiment of the present invention, the operation parameter reference is a voltage reference or a power reference or a rotational speed reference or a torque reference for controlling the generator side converter portion. Thereby, a voltage control, a power control, a speed control, or a torque control may be applied. Thus, conventional control methods may be supported by the arrangement.

According to an embodiment of the present invention, the harmonic current reference comprises an oscillation corresponding to N times a fundamental electrical frequency of the generator, wherein N is a natural number larger than one, in particular being 2 or 4 or 6 or 8 or 12, wherein the harmonic current reference is in particular different from zero.

The fundamental electrical frequency of the generator may be derived based on the rotational speed of the generator and taking into account the number of poles and/or number of permanent magnets. Several different harmonics may be controlled by the arrangement by providing for each harmonics to be controlled a respective harmonic current controller. Each of the harmonic current controllers may receive as input a difference between the respective harmonic current reference and the filtered (e.g. bandpass for the respective harmonic) version of the generator output current. All outputs, i.e. the harmonic reference current deviations for the different harmonics may later be summed and added to the fundamental current error to arrive at the modified fundamental current error. Thus, a flexible control may be provided.

According to an embodiment of the present invention, the operational parameter reference and/or fundamental current error and/or harmonic current reference and/or harmonic reference current deviation are given in a fundamental dq-frame rotating with the fundamental electrical frequency of the generator. The fundamental dq-frame rotates with the fundamental frequency of the generator. Thereby, processing the quantities may be performed in a simple manner.

According to an embodiment of the present invention, the fundamental current controller comprises two PI-controller, to determine the operation parameter reference, using for each of the d-component and the q-component of the modified fundamental current error in the fundamental dq-frame one PI-controller of the two PI-controllers.

The harmonic current controller may be comprised of four PI controllers, two for the positive sequence harmonic dq-frame (rotating with the harmonic frequency in forward direction) as already mentioned, and the other two for the negative sequence harmonic dq-frame (rotating with the harmonic frequency in backward direction).

The PI-controller(s) may each comprise a proportional branch and an integral branch each receiving the respective current error as input. The outputs of the two branches are respectively added together for each of the d-component and the q-component in the fundamental dq-frame. Thereby, conventionally available controllers may be utilized in embodiments according to the embodiment of the present invention.

According to an embodiment of the present invention, the harmonic current controller comprises: a transformation module configured to transform the harmonic current error in the fundamental dq-frame into a harmonic dq-frame (e.g. in two separate positive and negative directions), the harmonic dq-frame rotating with the respective multiple of the fundamental frequency; two PI-controllers configured to receive the components of the harmonic current error in the harmonic positive sequence dq-frame as inputs; two other PI-controllers configured to receive the components of the harmonic current error in the harmonic negative sequence dq-frame as inputs; a backtransformation module configured to backtransform the outputs of the PI-controllers and the outputs of the other PI-controllers into the fundamental dq-frame; and an addition element for adding the transformed outputs of the PI-controllers and the transformed outputs of the other PI-controllers to obtain the harmonic reference current deviation.

The harmonic dq-frame rotates with the respective multiple of the fundamental frequency corresponding to the harmonic to be controlled. After transforming the harmonic current error into the harmonic dq-frame, the corresponding error may substantially be a DC value. Thereby, calculations and the control may be simplified.

According to an embodiment of the present invention, the arrangement further comprises an electrical speed estimator configured and connected to estimate an electrical rotational speed and/or an electrical angle of the generator based on the generator output current and in particular further based on the operation parameter reference such as reference voltage.

Thus, measuring the mechanical speed/frequency may not be necessary, thereby simplifying the method.

According to an embodiment of the present invention, the fundamental current controller and/or the harmonic current controller receives as input the electrical rotational speed (or frequency) and/or an electrical angle estimated by the electrical speed estimator. The electrical rotational speed and the electrical angle may be required for performing the control and in particular to perform a transformation into the harmonic dq-frame.

According to an embodiment of the present invention, the arrangement further comprises at least one harmonic current reference calculation module configured to calculate the harmonic current reference based on at least one current reference input; and/or a fundamental current reference calculation module configured to calculate the fundamental current reference based on at least one of the current reference input, wherein the current reference input comprises: the operation parameter reference, in particular voltage reference, and/or the generator output current, and/or a DC-link voltage, and/or an electrical frequency of the generator, and/or an electrical angle of the generator, and/or a torque, and/or an acceleration and/or a sound pressure.

The reference calculation modules are optional and may be missing in other embodiments. The current references may be simply constants supplied to the arrangement. Depending on which quantity is controlled (for example voltage, power, torque), the respective current reference calculation modules may be configured differently, in particular regarding the configuration, such as look-up tables or the like. For each harmonic to be controlled, a respective harmonic current reference calculation module may be provided.

According to an embodiment of the present invention, the arrangement further comprises a dq-abc-transformation module configured to transform the operation parameter reference given in the fundamental dq-frame into a stator frame; a modulator configured to derive pulse width modulation signals based on the operation parameter reference given in the stator frame and connected to supply the pulse width modulation signals to gates of controllable switches comprised in the generator side converter portion. Thereby, conventional pulse width modulation control methods are supported.

According to an embodiment of the present invention, the arrangement further comprises an abc-dq-transformation module configured to transform the generator output current in the stator frame into the fundamental dq-frame. Thereby, the output current of the generator may conveniently be subtracted from the current references before supplying the difference to the respective harmonic current controller and the fundamental current controller.

According to an embodiment of the present invention it is provided an electric energy system, comprising: a generator; a converter comprising a generator side converter portion coupled to the generator; and an arrangement according to one of the preceding embodiments.

The converter may further comprise a grid side converter portion which may be connected to a utility grid to which electric energy is output from the electric energy system.

Furthermore, a wind turbine comprising the electric energy system is provided which may comprise a wind turbine tower, a nacelle provided on top of the wind turbine tower, wherein the generator and the converter and also the arrangement may be arranged e.g. within the nacelle or in another component of the wind turbine.

It should be understood that features, individually or in any combination, disclosed, explained, provided or applied for an arrangement for determining an operation parameter reference for controlling a generator side converter portion may also be applied, individually or in any combination, to a method for determining an operation parameter reference for controlling a generator side converter portion according to embodiments of the present invention and vice versa.

According to an embodiment of the present invention it is provided a method for determining an operation parameter reference for controlling a generator side converter portion coupled to a generator, in particular of a wind turbine, the method comprising: deriving at least one harmonic current error by subtracting a generator output current from at least one harmonic current reference; determining at least one harmonic reference current deviation based on the at least one harmonic current error; deriving a fundamental current error by subtracting the generator output current from a sum of a fundamental current reference and the at least one harmonic current reference; determining a modified fundamental current error as a sum of the fundamental current error and the harmonic reference current deviation; determining the operation parameter reference based on the modified fundamental current error.

Furthermore, it is provided a method for controlling a generator side converter portion coupled to a generator, comprising: performing a method for determining an operation parameter reference according to the preceding embodiment and controlling the generator side converter portion coupled to a generator based on the operation parameter reference.

Embodiments of the present invention are now described with reference to the accompanying drawings. The embodiment is not restricted to the illustrated or described embodiments.

The aspects defined above and further aspects of the embodiment 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 embodiment will be described in more detail hereinafter with reference to examples of embodiment but to which the embodiment is not limited.

