Voltage control of a direct current link

Abstract

A device (100, 200) for controlling a direct current link voltage (V_DC) of a direct current link (103, 203) connected between a generator side converter section (105, 205) and a utility grid side converter section (107, 207) of a wind turbine (260) is described, the device comprising: a generalized predictive control module (133) adapted to receive a current value (V_DC) of the direct current link voltage and a reference value (V*_DC) of the direct current link voltage as control module input values (135) and to derive a reference value (I*_DC) of a direct current current based on the control module input values (135), the reference value of the direct current current defining a direct current current to be injected into the direct current link, wherein the device is adapted to control the generator side converter section (105, 107) and / or the grid side converter section based on the reference value (I*_DC) of the direct current current.

Description

Technical Field

[0001] This invention relates to an arrangement for controlling the DC link voltage of a DC link connected between a generator-side converter section and a utility grid-side converter section of a wind turbine. The invention also relates to wind turbines and a method for controlling the DC link voltage of a wind turbine's DC link. Technical Background

[0002] A wind turbine includes a rotor with multiple rotor blades connected thereto, wherein the rotor is mechanically coupled to a generator, such as a synchronous generator that generates AC power as the rotor rotates. A conventional wind turbine also includes a generator-side converter section connected to the generator, a DC link connected to the DC output terminals of the generator-side converter section, and a grid-side converter section connected to the DC link via its DC terminals and optionally connected to, or potentially connected to, the public power grid via its AC output terminals and one or more transformers. The DC link includes a capacitor connected between the two DC buses. During wind turbine operation, the rotor speed may vary due to changing wind conditions. Furthermore, transient disturbances, such as those affecting voltage or frequency, may exist in the public power grid. These factors affect the voltage across the DC link.

[0003] Stable DC link voltage is important for the high-performance operation of wind power generation systems, as DC link voltage fluctuations can affect the lifespan of DC link capacitors and cause current harmonics on both the machine side and the grid side.

[0004] To achieve a stable DC-link voltage, good interference suppression capability is crucial for the DC-link voltage controller. When controlling the DC-link voltage on the motor side, the main disturbances are variations in generator speed and active power drawn from the grid. When controlling the DC-link voltage on the grid side, the main disturbances are variations in grid voltage conditions (amplitude, frequency, imbalance, harmonics, etc.) and active power injected from the motor side.

[0005] Traditionally, proportional-integral (PI) controllers are used for DC link voltage control. A PI controller can use the DC link voltage error as input and can generate a DC power reference. This DC power reference can traditionally be converted to a current reference and can also be converted to a voltage reference, which is supplied as a control signal to one of the converters.

[0006] However, it has been observed that conventional control schemes have relatively low disturbance rejection capability, i.e., a relatively low ability to handle or suppress disturbances such as those related to voltage on the grid side or speed on the generator side. Furthermore, changing the coefficients of the integral part of the PI controller may be inappropriate, as control errors can only be gradually eliminated through the integral process. Larger integral coefficients can accelerate transient processes and improve disturbance rejection capability, but they also increase the likelihood of overshoot when the DC link voltage reference changes, and reduce system stability.

[0007] Furthermore, other traditionally used methods (including finite control set model predictive control (FCS-MPC)) have some drawbacks, particularly regarding the generation of switching signals that lead to non-fixed switching frequencies of the converter, and thus the distortion of the AC current due to harmonics distributed across a wide frequency band. In addition, finite control set model predictive control suffers from static control errors due to parameter mismatch unless additional auxiliary controllers are used.

[0008] Furthermore, the traditionally used dead-beat predictive DC-link voltage control also has drawbacks because it requires fast and accurate DC load power estimation and feedforward, which is often difficult and undesirable in practical systems. In addition, dead-beat predictive control suffers from static control errors due to parameter mismatch. Moreover, the fixed-step approximation method makes DC-link voltage control stringent, as all different transient processes are forced to continue for the same period.

[0009] Therefore, there may be a need for: apparatus and corresponding methods for controlling the DC link voltage of the DC link connected between the generator-side converter section and the grid-side converter section of a wind turbine, wherein the aforementioned problems are at least partially overcome. There may be a particular need for: control methods and apparatus in which interference suppression capability is improved while reliably maintaining the DC link voltage at a desired value. Summary of the Invention

[0010] This need can be met by the subject matter of the independent claims. Advantageous embodiments of the invention are described in the dependent claims.

[0011] According to an embodiment of the present invention, an apparatus is provided for controlling the DC link voltage of a DC link connected between a generator-side converter section and a grid-side converter section of a wind turbine. The apparatus includes: a generalized predictive control module adapted to: receive a current value of the DC link voltage and a reference value of the DC link voltage as control module input values, and derive (e.g., in a static coordinate system) a reference value of a DC current (e.g., I*_DC) based on the control module input values, the reference value of the DC current defining the DC current to be injected into the DC link, wherein the apparatus is adapted to control the generator-side converter section or the grid-side converter section based on the reference value of the DC current (e.g., I*_DC).

[0012] The device can be implemented, for example, as a software and / or hardware module in the controller of a wind turbine. The DC link can include a first DC bus and a second DC bus, with a capacitor or capacitor system connected between the two DC buses. The DC link can include other components such as switches, which can be connected in parallel with the capacitor to release voltage, for example, in the event of an overvoltage. The switch can be configured as a controllable switch, such as a power transistor. Thus, energy can also be released under specific conditions, such as during a transient overvoltage.

[0013] The generator-side converter section can be configured as an AC-DC converter capable of converting AC power to DC power. The generator may, for example, include a permanent magnet synchronous generator, comprising stator windings and a rotor, the rotor comprising permanent magnets. The rotor may be connected to multiple rotor blades driven by impinging wind.

[0014] The grid-side converter section can be configured as a DC-AC converter capable of converting DC power into AC power of a fixed frequency, which will ultimately be supplied to the public grid via one or more transformers. The AC power from the generator or grid-side converter section can specifically be implemented as three-phase AC power.

[0015] Generalized predictive control (GPC) generates a sequence of (future) control signals within each sampling interval to optimize the control of the controlled system. This is performed by minimizing a (complex) cost function. Due to the high computational power required for GPC, this control method has not yet been applied to the control of DC link voltages. GPC can belong to the group of "remote predictive controllers" and can generate a set of future control signals in each sampling interval, but only the first element of the control sequence is applied to the system input. GPC can be based on or can include a physical / mathematical model of the process to be controlled.

[0016] Compared to traditional PI controllers, the main characteristic of GPC is that it transforms the control problem into an optimization problem by predicting the future state of the controlled device, and performs prediction and optimization in each control interval. In contrast, PI controllers use past control errors to generate control signals without any prediction or optimization process.

[0017] The device can be configured to derive a reference value for a DC current from which a corresponding reference current (particularly an AC current) for the stator current or grid current can be derived. The stator current can be or relates to the current flowing through the stator windings of the generator, which then flows through the generator-side converter section and subsequently to the DC link. The grid current can be or relates to the current flowing through the power grid, which then flows through the grid-side converter section and from there to the DC link.

[0018] According to an embodiment of the present invention, the generalized predictive control module includes physical models of a generator-side converter section, a DC link, and a grid-side converter section. This model can derive the change in DC voltage as a function of the actual value of the DC current injected into the DC link and the interference current. Thus, for example, the model can be based on differential equations relating the change in voltage across the capacitor across the DC link to the injected current and the interference current. Conventional PI controllers do not include such a physical / mathematical model. The current value of the DC link voltage can be a measured value, for which the device can include a measuring sensor or measuring device. A reference value for the DC link voltage can be predefined. The reference value of the DC link voltage is predefined based on the generator's voltage level and the grid voltage to which the grid-side converter is connected.

[0019] The reference value (e.g., I*_DC) may, for example, involve the DC current in a static coordinate system (i.e., a non-rotating system). Internally, the generalized predictive control module can perform calculations to predict the increment and / or actual value of the voltage across the DC link in order to derive the reference value of the DC current. This prediction can be performed across a prediction range (e.g., N time steps). Furthermore, the generalized predictive control module may include the definition of a cost function that takes multiple predicted values ​​across the prediction range as input and is optimized or minimized by the generalized predictive control module. When minimizing, an optimal reference value for the DC current can be found that minimizes the cost function across the prediction range. When the prediction crosses the prediction range, control can be improved and the voltage can precisely match the reference DC voltage. The reference value of the DC current can be used as a signal from which a control signal can ultimately be derived, used to control one of the converter sections according to the specific application.

[0020] The choice between using a motor-side converter or a grid-side converter to control the DC link voltage is determined by the overall design of the turbine's control architecture, and both options are considered in product development and research. If the motor-side converter is used to control the DC link voltage, maximum power point tracking (MPPT) should be achieved by the grid-side converter by adjusting the active power injected into the unit grid based on the output of the MPPT module. Conversely, if the grid-side converter is used to control the DC link voltage, MPPT should be achieved by the motor-side converter by adjusting the rotor speed or the active power extracted from the turbine based on the MPPT module.

[0021] Depending on which converter section is controlled, the actual value of the DC current can flow from the corresponding converter section to the DC link, that is, to the capacitors connected between the DC buses of the DC link. When the generalized predictive control module is used to control the DC link voltage, transient suppression capability can be improved while achieving the desired DC voltage across the DC link.

[0022] According to an embodiment of the invention, the apparatus further includes an arithmetic element that receives at least the following values ​​as arithmetic element inputs: a reference value of DC current (e.g., I*_DC); a current value of DC link voltage (e.g., V_DC); the arithmetic element is adapted to calculate, based on the input values ​​of the arithmetic element, specifically by applying the power balance equation, the q component (e.g., I*_sq) of the reference value of the stator current or the d component (e.g., I*_gd) of the reference value of the grid current in a synchronous reference system (e.g., rotating synchronously with the electrical frequency of a generator connected to the generator-side converter section).

[0023] When the q-component or d-component of the reference value of the stator current or grid current is calculated by arithmetic elements, the calculation of the final control signal used for the corresponding converter section can be simplified. Thus, in particular, a current controller can be used that then receives the reference value of the stator current or grid current as a complex value or receives the difference between the complex reference value of the stator current or grid current and (e.g., transformed into a rotating coordinate system) the measured value of the stator current or grid current.

[0024] According to an embodiment of the present invention, if control of the generator-side converter section is performed, the arithmetic element input values ​​further include: the number of pole pairs (e.g., N_p); the mechanical speed of the generator (e.g., ωr); the permanent magnet linkage of the generator's permanent magnets; wherein the arithmetic element is adapted to calculate the q component (e.g., I*_sq) of a reference value of the stator current or grid current in the synchronous reference system, wherein the device is adapted by further using a predetermined d component (e.g., I*_sd) of the reference value of the stator current or grid current in the synchronous reference system to obtain a complex reference value (e.g., I*_sdq) of the stator current or grid current in the synchronous reference system.

[0025] Therefore, appropriate control of the generator-side converter section can be achieved. The number of pole pairs can relate to the number of pole pairs on the generator rotor. The permanent magnet flux linkage of the permanent magnets can relate to the magnetic flux generated by the permanent magnets mounted on the generator rotor. The q-component of the stator current reference value can relate to achieving proper power balance and can be associated with controlling the voltage across the DC link. The predetermined d-component of the reference value can be, for example, a fixed value, such as being set to zero. The d-component can only relate to reactive power that may not be necessary for controlling the DC link voltage. Therefore, control of the DC link voltage can be achieved by controlling the generator-side converter section.

[0026] According to an embodiment of the present invention, if the control of the grid-side converter section is performed, the arithmetic element input value further includes: the magnitude of the grid voltage, wherein the arithmetic element is adapted to calculate the d component (e.g., I*_gd) of the reference value of the grid current in the synchronous reference frame, wherein the device is adapted by: additionally using a predetermined q component (e.g., I*_gq) of the reference value of the grid current in the synchronous reference frame to obtain a complex reference value (e.g., I*_gdq) of the grid current in the synchronous reference frame.

[0027] In this embodiment, the d-component or q-component of the reference value of the stator current or grid current can be a relevant quantity to control the DC link voltage. The predetermined q-component or d-component of the reference value of the stator current or grid current can be a fixed value, such as zero. Thus, depending on the application, the DC link voltage can be controlled by controlling the grid-side converter section.

[0028] According to an embodiment of the present invention, both the grid-side converter section and the generator-side converter section are controlled simultaneously.

[0029] According to an embodiment of the invention, the device further includes a current controller adapted to: receive a current controller input as a difference between a complex reference value (e.g., I*_sdq) of the stator current or grid current in a synchronous reference system and a complex measured value (e.g., I_sdq) of the stator current or grid current in a synchronous reference system, and calculate a reference value of voltage (e.g., V*_sdq) as a control signal for a generator-side converter section or a grid-side converter section.

[0030] The current controller can be implemented, for example, as a PI controller, which outputs a reference value of the voltage (e.g., in a synchronous reference frame) as a control signal for the generator-side converter section or the grid-side converter section to minimize the difference between the complex reference value of the stator current or grid current and the (actual) value of the stator current or grid current in the corresponding synchronous reference frame. The voltage reference value can be supplied to the corresponding converter section, which can cause the corresponding converter section to switch its power transistors so that the output voltage conforms to the voltage reference value. Thus, a specific value of the DC current is injected into the DC link according to the power balance equation and can be, for example, referred to as I_sdq (in the synchronous reference frame). The difference between the complex reference value, such as I*_sdq, and the actual value of the stator current or grid current converted to the synchronous reference frame, i.e., I_sdq, can be supplied to the current controller. The current controller can derive its voltage reference value such that this difference is minimized. Internally, the current controller may include other control elements different from the PI controller.

[0031] According to an embodiment of the invention, the generator-side converter section or the public grid-side converter section is adapted to receive a reference value of voltage (e.g., V*_sdq if controlling the generator-side converter) as a control signal, and operate a plurality of controllable switches based on the control signal to inject a DC current (the current value) corresponding to a complex reference value (e.g., I*_sdq) of the stator current or grid current in the synchronous reference system into the DC link.

[0032] Depending on which converter section is being controlled, different voltage references can be provided to the grid-side converter and the generator-side converter. DC link voltage control can be used for either the grid-side converter or the generator-side converter, but not simultaneously for both.

[0033] In order to control the two converter sides, the following requirements exist: to generate two sets of reference voltages: (1) V*sdq for the generator-side converter; (2) V*gdq for the grid-side converter.

[0034] If DC link voltage control is performed on the generator-side converter section, only the generation of V*sdq will be described in detail, while the second block diagram of the control method suitable for generating V*gdq will be as conventionally known.

[0035] If DC link voltage control is performed on the grid-side converter section, only the generation of V*gdq will be described in detail, while the second block diagram of the control method suitable for generating V*sdq will be as conventionally known.

[0036] The controllable switches in the converter section can be configured, for example, as IGBT transistors. Pulse width modulation methodologies can be used to control the corresponding gates of the power transistors. For example, space vector pulse width modulation (SVPWM) can be employed.

[0037] According to an embodiment of the present invention, the generalized predictive control module is adapted to apply a discrete predictive equation of DC voltage as a function of the current value of the DC current injected into the DC link (e.g., I_DC), the interference current (e.g., I_L), and the capacitance of the capacitor of the DC link.

[0038] Specifically, the value and increment of the DC link voltage can be predicted across a prediction range (e.g., N time steps). The discrete prediction equations can employ a physical model or be based on physical laws relating to the voltage and current across the DC link capacitor. When, for example, controlling the generator-side converter section, the current value of the DC current can be the current output by the generator-side converter section at its DC output terminal and flowing to the capacitor of the DC link. The interference current can be the current flowing to the grid-side converter section (as part of the current flowing out of the generator-side converter section). A portion of the current value of the DC current flowing from the generator-side converter section can be the charging current that eventually flows to the DC link capacitor.

[0039] The interference current can be very large, potentially constituting a major portion of the total current from the generator side. GPC does not require this interference current to be small because the integrator embedded within the GPC automatically removes its effects; therefore, interference current (estimated or measured) is not needed in the GPC implementation.

[0040] This enables the implementation of a generalized predictive control module. Discrete prediction equations can be given in incremental, non-incremental, or both. Using the incremental form of the discrete prediction equations eliminates the DC voltage bias. The DC voltage can be predicted based on a previously unknown but ultimately optimized DC current. This improves the control of the DC link voltage.

[0041] According to an embodiment of the invention, the generalized predictive control module is adapted to solve an optimization problem involving the minimization of the error of the DC link voltage using a cost function, wherein the solution is obtained by applying quadratic programming theory using matrix equations, wherein a vector (e.g., ΔU) of the increments (at different time steps) of the reference value of the DC current is obtained as the solution.

[0042] The cost function can be defined according to the application. (For example, a reference tracking error (e.g., Y*-Y) and incremental control operation (e.g., ΔU) are recommended to form the cost function by giving them different weighting factors. Other terms (e.g., the difference between stator current or grid current and the corresponding stator current or grid current limit) can also be added to the cost function, but the implementation of generalized predictive control can be more complex.)

[0043] Quadratic programming is a traditionally known technique for solving optimization problems. This simplifies implementation. The solution vector can include the increments of the reference value of the DC current at different time steps.

[0044] According to an embodiment of the invention, the generalized predictive control module includes an accumulator adapted to accumulate elements, particularly the first element of the incremental solution vector, to obtain a reference value (I*_DC) for the stator current or grid current. In a discrete implementation, the summation can be performed, for example, as 1 / (1-Z). -1 ).

[0045] According to an embodiment of the invention, the accumulator is adapted to perform a limit such that a reference value of the DC current (e.g., I*_DC) is limited to a predefined range in order to limit the AC current to a predefined AC current limit (of the generator stator current or the grid current).

[0046] This limitation can then advantageously also limit the amount of alternating current subsequently calculated, which relates to the flow of alternating current in the corresponding converter or generator stator. This, in turn, prevents damage to components of the wind turbine or the drivetrain to the grid.

[0047] According to an embodiment of the present invention, a wind turbine is provided, comprising: a synchronous generator, particularly a permanent magnet synchronous generator, mechanically coupled to a rotor, wherein a plurality of rotor blades are mounted on the rotor; a generator-side converter section, particularly an AC-DC converter, electrically connected to an AC output terminal of the generator; a DC link, electrically connected to a DC output terminal of the generator-side converter section; a grid-side converter section, particularly a DC-AC converter, having a DC terminal electrically connected to the DC link and an AC terminal electrically connectable to a public power grid; and an apparatus according to one of the foregoing embodiments, connected to control the generator-side converter section or the grid-side converter section.

[0048] In other embodiments, the device can be used in another energy-generating facility that also includes a DC link.

[0049] It should be understood that, according to embodiments of the present invention, features of means disclosed, described, explained or applied individually or in any combination to control the DC link voltage of a DC link can also be applied individually or in any combination to a method for controlling the DC link voltage of a DC link connected between a generator-side converter section and a utility grid-side converter section of a wind turbine, and vice versa.

[0050] According to an embodiment of the present invention, a method is provided for controlling the DC link voltage of a DC link connected between a generator-side converter section and a grid-side converter section of a wind turbine. The method includes: using a generalized predictive control module; receiving a current value of the DC link voltage and a reference value of the DC link voltage as control module input values ​​by the generalized predictive control module; deriving (e.g., in a static coordinate system) a reference value of a DC current (e.g., I*_DC) based on the control module input values, the reference value of the DC current defining a DC current to be injected into the DC link; and controlling the generator-side converter section or the grid-side converter section based on the reference value of the DC current (e.g., I*_DC).

[0051] The aspects and other aspects of the invention defined above will be apparent from the examples of embodiments described below, and will be explained with reference to these examples. The invention will be described in more detail below with reference to examples of embodiments, but the invention is not limited to these examples. Attached Figure Description

[0052] Embodiments of the invention will now be described with reference to the accompanying drawings. The invention is not limited to the embodiments shown or described.

[0053] Figure 1 A control signal flowchart of a system including means for controlling the DC link voltage of a DC link according to an embodiment of the present invention is shown schematically.

[0054] Figure 2 A wind turbine according to an embodiment of the present invention is schematically illustrated; and

[0055] Figures 3 to 14 Experimental results of embodiments of the present invention are shown compared to existing technologies. Detailed Implementation

[0056] The illustrations in the accompanying drawings are schematic. Note that in different drawings, elements that are similar or identical in structure and / or function are given the same reference numerals or are given reference numerals that differ only in the first numeral. Description of an element not described in one embodiment may be obtained from the description of that element with respect to another embodiment.

[0057] Figure 1 The system schematically shown (depicted as a signal flow scheme) includes a device 100 for controlling the DC link voltage of a DC link 103 connected between the generator-side converter section and the grid-side converter section of the wind turbine.

[0058] Figure 2 For example (using a physical connection diagram), a DC link 203 is shown connecting the generator-side converter section 205 and the utility grid-side converter section 207. The generator-side converter section 205 is connected to a generator 209 having a rotor 211 with multiple rotor blades 213 mounted thereon. As the rotor 211 rotates, the generator 209 outputs an AC power flow 215, which is supplied to the AC input terminals of the generator-side converter section 205. The generator-side converter section 205 is configured as an AC-to-DC converter that converts the AC power flow 215 into DC power output at a first DC terminal 217 and a second DC terminal 219. The first DC terminal 217 is connected to a first DC bus 221, and the second DC terminal 219 is connected to a second DC bus 223. A capacitor 225 is connected between the two DC buses 221 and 223. The DC buses 221 and 223 are also connected to DC terminals 227 and 229 of the utility grid-side converter section 207. The grid-side converter section 207 is configured as a DC-AC converter that converts DC power received at its DC input terminals 227, 229 into AC power, which is delivered to the grid 231 via a wind turbine transformer or other wind park transformer (not shown).

[0059] Refer again Figure 1 Used to control the DC link voltage V_DC (see...) Figure 2 The device 100 includes: a generalized predictive control module 133, which is adapted to receive the current value V_DC of the DC link voltage and a reference value V*_DC of the DC link voltage as control module input values ​​135 and derive a reference value (I*_DC) of the DC current (in a static coordinate system) based on the control module input values ​​(collectively referred to as 135).

[0060] Device 100 is configured to control: a converter section, namely generator-side converter section 105 (or, in an embodiment not shown, public grid-side converter section 107), which can be connected to... Figure 2 The converter sections 205 and 207 shown are similarly configured and connected. Figure 1The scheme shown represents a signal control scheme and not (necessarily) a physical connection scheme. Converters 105 / 107 are connected to generator 109 and ultimately cause a DC current (caused by the current through the stator windings of generator 109) (actually, for example, I_DC) to flow to DC bus 103. Thus, a DC link voltage V_DC is derived.

[0061] The device 100 also includes an arithmetic element 136, which receives at least a reference value of DC current I*_DC and a current value of DC link voltage V_DC as arithmetic element input values ​​139. Alternative methods can be used to derive I*_sq from I*_DC; for example, 136 could be a processing block. The arithmetic element (or typically a processing block) 136 is adapted to calculate, based on the arithmetic element input value 139, specifically by applying the power balance equation, the q component (I*_sq) of the stator current in a synchronous reference frame that rotates synchronously with the electrical frequency of the generator 109 connected to the generator-side converter section.

[0062] Figure 1 The converter section on the generator side is shown (e.g.) Figure 2 The signal flow diagram for case 205 is shown in the figure. In the case of controlling the grid-side converter section, the d-component (I*_gd) of the reference value of the grid current (index 'g' in I*_gd represents the grid) in the synchronous reference system is derived based on the arithmetic element input value (collectively referred to as 139). Furthermore, V*_gdq is calculated on the voltage reference and used to control the grid-side converter section that causes the grid current I_gdq.

[0063] When controlling the generator-side converter section 205 / 105, the arithmetic element input value 139 also includes the pole pair number N_p, the mechanical speed ωr of the generator 209 / 109, and the permanent magnet linkage of the generator's permanent magnets, named Ψm. The device is further adapted by using a predetermined d-component (I*_sd) of the stator current reference value in the synchronous reference system to obtain the complex reference value I*_sdq of the stator current in the synchronous reference system. In this document, element 141 multiplies the output of arithmetic element 136 by 2 / 3 to obtain the q-component of the stator current reference value I*_sq.

[0064] The complex reference value I*_sdq of the stator current, together with the actual value (I_sdq) of the stator current in the synchronous reference frame, is supplied to the differential element 143 that determines the current error 147.

[0065] The device 100 also includes a current controller 145, which is adapted to receive an error value 147 of the stator current output by the differential element 143 and to calculate a reference value V*_sdq of the voltage as a control signal for the generator-side converter section 105 or the grid-side converter section 107, such as... Figure 1 As shown in the figure, the generator-side converter section 105 and the public grid-side converter section 107 are adapted to receive a reference value V*_sdq as a control signal, and operate a plurality of controllable switches (e.g., IGBTs) based on the control signal to inject a DC current corresponding to a reference value I*_DC of voltage into the DC link 103.

[0066] exist Figure 2 In the example 200, including in a wind turbine 260, an embodiment 200 is shown of a device for controlling the DC link voltage V_DC of the DC link 203, which can be related to the control signal and Figure 1 The device 100 shown is configured similarly or identically. Device 200 is communicatively connected to generator-side converter section 205 and optionally connected to utility grid-side converter section 207. Depending on which converter section is controlled, device 200 supplies a corresponding reference value V*_sdq to the respective converter section.

[0067] Embodiments of the present invention implement DC-link voltage control based on generalized predictive control (GPC) for wind power generation systems based on permanent magnet synchronous generators (PMSG). Typically, a GPC controller, such as... Figure 1 The GPC module 133 is used for DC link voltage control in the wind power generation system. The input 135 of the GPC controller is the DC link voltage and its reference, and the output of the GPC controller is the reference value of the DC current to be injected into the DC link bus, i.e., I*_DC.

[0068] The current reference in the synchronous reference frame is calculated based on the output of the GPC controller.

[0069] If the DC link voltage is controlled using the motor side (generator-side converter section), the q-axis current reference is calculated as follows: , where N p ω r and Ψ m The number of pole pairs, mechanical angular rotor speed, and permanent magnet flux linkage of the PMSG (e.g., fixed control parameters or calculated online using an estimator / observer); if the DC link voltage is controlled using the grid side, the d-axis current reference is calculated as: I * gd = , where V g It is the voltage amplitude of the power grid (e.g., measured).

[0070] The threshold of the GPC accumulator is adaptively adjusted to limit the current amplitude. If the DC link voltage is controlled from the motor side, the threshold is adjusted according to the rotor speed; if the DC link voltage is controlled from the grid side, the threshold is adjusted according to the grid voltage amplitude.

[0071] Other details are given below with respect to specific embodiments; however, the invention is not limited to the specific embodiments described herein.

[0072] A. GPC Implementation

[0073] The DC link bus of the PMSG wind power system is in Figure 2 As shown in the diagram, when using the motor side to control the DC link voltage, the DC load current, IL, is treated as interference. The dynamics of the DC link voltage can be described as follows:

[0074] (1). From (1), the discrete prediction equation in incremental form can be expressed as:

[0075]

[0076] Where Δ represents the incremental variable. Ts is the sampling period of the controller. C is the capacitance of the DC capacitor. k represents the current moment in the discrete time series. Here, since the DC load can be considered constant when designing the GPC controller, the incremental DC load current can be ignored. Therefore, (2) can be rewritten in the following standard form:

[0077]

[0078] Where xm is the extended state variable, and y and u are the output and input variables, respectively. Am, Bm, and Cm are coefficient matrices.

[0079] From (3), the prediction equation for the entire prediction range (N steps) can be expressed in a compact form:

[0080]

[0081] in,

[0082] .

[0083] For GPC, the cost function is typically designed in quadratic form, so the optimization problem can be expressed as:

[0084]

[0085] in The reference vector is Q = diag[Ql Q2...QN]T and R = diag[RlR2...RN]T, which are weighted matrices of the output and input variables, respectively. They are related to the control performance of the GPC and should both be positive semi-definite.

[0086] According to the theory of quadratic programming, the optimal solution of (5) is:

[0087]

[0088] Finally, the first element of the optimized solution is summed to obtain the DC current reference:

[0089]

[0090] in, .

[0091] B. Calculation of current reference in synchronous reference frame

[0092] The DC current reference obtained in (7) should be converted into a stator or grid current reference and then tracked by the internal current control loop. The stator or grid current reference calculation is based on the instantaneous power balance between the AC and DC sides of the converter.

[0093] When the DC link voltage is controlled using the motor side, the instantaneous power balance equation is:

[0094]

[0095] When the d-axis is oriented on the permanent magnet flux linkage and the voltage drop across the stator winding is ignored, (8) can be approximately written as:

[0096]

[0097] Then, the q-axis current reference can be obtained as follows:

[0098]

[0099] Finally, the q-axis current reference obtained in (10) is tracked by an internal current control loop, which can be a PI controller or any other control method (e.g., a GPC-based current controller). The d-axis current is independent of the DC link voltage control but can be controlled to achieve other objectives. It should be noted that the approximate equality in (9) will not introduce any static error due to the presence of the accumulator in (7).

[0100] Similarly, when the grid side is used to control the DC link voltage, the instantaneous power balance equation is:

[0101]

[0102] When the d-axis is oriented on the grid voltage, the d-axis current reference can be obtained as follows:

[0103]

[0104] Where Vg is the grid voltage amplitude.

[0105] C. Adaptive threshold for AC current limiting

[0106] In a practical system, the AC current should be limited. If a limiter is simply added to the d-axis or q-axis current reference, the GPC will suffer overshoot during transients. To address this, the accumulator in (7) should be responsible for AC current limiting, and it should function similarly to an integrator with anti-windup. To accurately limit the AC current, the accumulator threshold should be adaptively adjusted due to varying DC link voltage and rotor speed (when the DC link voltage is controlled by the motor side) or grid voltage (when the DC link voltage is controlled by the grid side). The adaptive threshold is also based on instantaneous power balance calculations, as shown in (8) and (11).

[0107] If the DC link voltage is controlled using the motor side, and the q-axis current constraint is defined as Then the threshold of the accumulator should be set to:

[0108]

[0109] If the grid side is used to control the DC link voltage, and the d-axis current constraint is defined as Then the threshold of the accumulator should be set to:

[0110]

[0111] The threshold is Figure 1 It is indicated by reference numeral 134 in the figure, and it is input into GPC module 133.

[0112] Figures 3 to 14 The coordinate system is shown with time as the horizontal axis (350) and voltage as the vertical axis (351). Figure 3 , 6 Experimental results according to embodiments of the present invention in figures 9 and 12, as well as results conventionally obtained in other figures (PI controller). In all Figures 3 to 14 In the diagram, line 353 represents the reference voltage, while line 355 represents the actual DC link voltage obtained.

[0113] It can be recognized that: in illustrating the results according to embodiments of the present invention Figure 3, 6 In cases 9 and 12, the actual voltage 355 obtained is very similar to the reference voltage 353, while for the conventional method, the actual voltage 355 obtained deviates from the reference value 355.

[0114] In leading to Figure 3 , 6 In the experiments with results in 9 and 12, the DC link voltage was controlled by the motor side, and the proposed GPC and conventional PI were designed for two typical disturbances ( Figure 3 , 6 That is, the change in rotor speed and the change in DC load. Figure 9 The interference suppression capabilities of the proposed GPC and the conventional PI-based DC link voltage reference for tracking variations were compared. Simultaneously, the dynamic performance of the proposed GPC and the conventional PI-based DC link voltage reference for tracking variations was also compared. Figures 1 to 14 It can be observed that the interference suppression capability of traditional PI can be improved by using larger parameters, but it is still not as good as the proposed GPC. Regarding dynamic performance, the proposed GPC provides smooth and fast transient processes for both dynamic tests. However, PI with smaller parameters suffers from slow and unsmooth transient processes, while PI with larger parameters faces overshoot problems when the DC link voltage drops.

[0115] Therefore, it can be observed that the proposed GPC-based DC link voltage control strategy has better interference suppression capability than the traditional PI control (even when using a fairly large PI parameter). It should be noted that if the PI parameter is further increased, the overshoot in dynamic testing will be too large to accurately track changes in the DC link voltage reference, which should be avoided in practical systems.

[0116] Several advantages listed below can be achieved using embodiments of the present invention. However, it should be noted that not all advantages are necessarily obtained through embodiments of the present invention:

[0117] 1) Compared with PI controllers, embodiments of the present invention can have better interference suppression capabilities, which leads to a more stable DC link voltage and is beneficial to DC capacitor life and AC current quality.

[0118] 2) Compared to FCS-MPC, embodiments of the present invention can have a fixed switching frequency, thus reducing current harmonics and losses. Simultaneously, embodiments of the present invention inherently avoid static control errors caused by parameter mismatch.

[0119] 3) Compared to deadbeat predictive control, embodiments of the present invention can avoid DC load power estimation, thus simplifying the control system. Simultaneously, embodiments of the present invention inherently avoid static control errors caused by parameter mismatch. Furthermore, embodiments of the present invention can avoid the fixed-step gradual approximation method, thus improving dynamic performance.

[0120] Embodiments of the present invention may exhibit the following properties:

[0121] 1) Embodiments of the present invention can be based on generalized predictive control (GPC), which predicts the future state of the DC link voltage over a long prediction range. Therefore, an optimized DC current reference can be obtained by solving the optimization problem shown in (5). Compared to PI controllers that rely on the integral process of past control errors, GPC-based DC link voltage control inherently has advantages in terms of disturbance rejection capability.

[0122] 2) Embodiments of the present invention can utilize a cascaded control structure, and only the external loop, i.e., the DC link voltage control loop, is given in the present invention. Therefore, any existing current controller can be used for the internal loop, including a conventional PI controller with a PWM module, which results in a fixed switching frequency and low current harmonics. Meanwhile, as part of the GPC controller, the accumulator shown in (7) can inherently remove static control errors caused by parameter mismatch without requiring an auxiliary controller.

[0123] 3) The accumulator of GPC can remove static control error, so DC load power estimation is not required in the implementation of GPC. At the same time, the weighting matrices Q and R in (5) can automatically balance DC link voltage tracking and DC current limiting, so a fixed step size approximation method is not required.

[0124] Embodiments of this invention can provide high-performance DC link voltage control for PMSG-based wind power generation systems. Improved DC link voltage (with less fluctuation in the face of disturbances) can extend the lifespan of DC capacitors. This method offers increased flexibility to meet the requirements of DC link voltage control in various operating modes of wind turbines, such as active and / or reactive power generation and grid fault ride-through. Considering that DC link voltage ripple can cause harmonic distortion in the AC current, this invention can also reduce motor losses and AC current harmonics.

[0125] Furthermore, this invention can also be applied to other power electronic devices connected to the power grid, such as PV systems, PWM rectifiers, high-voltage direct current transmission (HVDC), etc.

[0126] It should be noted that the term "comprising" does not exclude other elements or steps, and "a" or "an" does not exclude a plurality. Furthermore, elements described in association with different embodiments may be combined. It should also be noted that reference numerals in the claims should not be construed as limiting the scope of the claims.

Claims

1. A device (100, 200) for controlling the DC link voltage (V_DC) of a DC link (103, 203) connected between a generator-side converter section (105, 205) and a public grid-side converter section (107, 207) of a wind turbine (260), the device comprising: Generalized predictive control module (133), suitable for: The measured value (V_DC) and the reference value (V*_DC) of the DC link voltage are received as input values ​​(135) to the control module. Based on the input value (135) of the control module, a reference value (I*_DC) for the DC current is derived. This reference value defines the DC current to be injected into the DC link. The device is adapted to control the generator-side converter section (105, 107) or the public grid-side converter section based on a reference value (I*_DC) of the direct current, and The device further includes an arithmetic element (136) that receives at least the following values ​​as arithmetic element input values ​​(139): The reference value of the DC current (I*_DC); The measured value of the DC link voltage (V_DC); The arithmetic element is adapted to calculate the q component (I*_sq) of the reference value of the stator current or the d component (I*_gd) of the reference value of the grid current in the synchronous reference system based on the arithmetic element input value (139). If the control of the generator-side converter section (105) is executed, the arithmetic element input value (139) further includes: The number of pole pairs (N_p); The mechanical speed (ωr) of the generator; The permanent magnet chain (psi_m) of the permanent magnet of the generator (109, 209). The arithmetic element is adapted to calculate the q component (I*_sq) of the reference value of the stator current in the synchronous reference frame, wherein the device is adapted by further using a predetermined d component (I*_sd) of the reference value of the stator current in the synchronous reference frame to obtain the complex reference value (I*_sdq) of the stator current in the synchronous reference frame, and / or If the control of the grid-side converter section (107) is executed, the arithmetic element input value further includes: The amplitude of the grid voltage (V_g). The arithmetic element is adapted to calculate the d component (I*_gd) of the reference value of the grid current in the synchronous reference frame, wherein the device is adapted by further using a predetermined q component (I*_gq) of the reference value of the grid current in the synchronous reference frame to obtain the complex reference value (I*_gdq) of the grid current in the synchronous reference frame.

2. The apparatus according to claim 1, further comprising a current controller (145), said current controller (145) being adapted to: Receives a current controller input as a complex reference value (I*_sdq) of the stator current in the synchronous reference frame, and The reference value of the calculated voltage (V*_sdq) is used as the control signal for the generator-side converter section.

3. The apparatus according to any one of claims 1-2, wherein, The generator-side converter section or the public grid-side converter section is adapted to receive the reference values ​​of the voltage (V*_sdq, V*_gdq) as control signals, operate multiple controllable switches based on the control signals to obtain the desired stator or grid current corresponding to the complex reference values ​​(I*_sdq, I*_gdq), and finally inject the desired DC current into the DC link bus.

4. The apparatus according to any one of claims 1-2, wherein, The generalized predictive control module (133) is adapted to apply a discrete predictive equation for the DC link voltage as a function of the current value (I_DC) of the DC current injected into the DC link, the interference current (I_L), and the capacitance of the capacitor of the DC link.

5. The apparatus according to any one of claims 1-2, in, The discrete prediction equation is given in incremental form. The DC link voltage is predicted over multiple discrete time steps (N) via matrix equations.

6. The apparatus according to any one of claims 1-2, in, The generalized predictive control module (133) is adapted to use the cost function (J) to solve the optimization problem. The minimization of the error of the DC link voltage is involved, wherein the vector (ΔU) of the increment (ΔI_DC) of the reference value (I_DC) of the DC current is obtained as a solution, wherein the solution (ΔU) is obtained by applying quadratic programming theory using matrix equations.

7. The apparatus according to any one of claims 1-2, wherein, The generalized predictive control module (133) includes an accumulator adapted to accumulate elements of the incremental vector (ΔU) to obtain the reference value (I*_DC) of the DC current.

8. The apparatus according to claim 7, wherein, The accumulator is adapted to perform a limit such that the reference value of the DC current (I*_DC) is limited to a predefined range, so as to limit the AC current to a predefined AC current limit.

9. A wind turbine (260), comprising: A permanent magnet synchronous generator (209) is mechanically coupled to a rotor (211), with multiple rotor blades (213) mounted on the rotor (211); The generator-side converter section (205) has an AC-DC converter that is electrically connected to the AC output terminal of the generator (209); A DC link (203) is electrically connected to the DC output terminals (217, 219) of the generator-side converter section (205). The grid-side converter section (207) has a DC-AC converter, has DC terminals (229, 227) electrically connected to the DC link (203) and has AC terminals electrically connected to the public power grid (231); as well as The device (200) according to any one of claims 1-8 is connected to control the generator-side converter section or the grid-side converter section.

10. A method for controlling the DC link voltage (V_DC) of a DC link (103, 203) connected between a generator-side converter section (105, 205) and a public grid-side converter section (107, 207) of a wind turbine (260), the method comprising: Use the generalized predictive control module (133); The generalized predictive control module receives the measured value (V_DC) of the DC link voltage and the reference value (V*_DC) of the DC link voltage as input values ​​(135) to the control module. The generalized predictive control module derives a reference value (I*_DC) for the DC current based on the input value (135) of the control module. This reference value defines the DC current to be injected into the DC link. The generator-side converter section and / or the utility grid-side converter section are controlled based on the reference value (I*_DC) of the DC current. The method further includes receiving at least the following values ​​as arithmetic element input values ​​(135): The reference value of the DC current (I*_DC); The measured value of the DC link voltage (V_DC); Based on the arithmetic element input value (139), calculate the q component (I*_sq) of the stator current reference value or the d component (I*_gd) of the grid current reference value in the synchronous reference system. If the control of the generator-side converter section (105) is executed, the arithmetic element input value (139) further includes: The number of pole pairs (N_p); The mechanical speed (ωr) of the generator; The permanent magnet chain (psi_m) of the permanent magnet of the generator (109, 209). Specifically, the q-component (I*_sq) of the reference value of the stator current in the synchronous reference frame is calculated, wherein a predetermined d-component (I*_sd) of the reference value of the stator current in the synchronous reference frame is used to obtain the complex reference value (I*_sdq) of the stator current in the synchronous reference frame, and / or If the control of the grid-side converter section (107) is executed, the arithmetic element input value further includes: The amplitude of the grid voltage (V_g). Specifically, the d-component (I*_gd) of the reference value of the grid current in the synchronous reference system is calculated, and the complex reference value (I*_gdq) of the grid current in the synchronous reference system is obtained by using a predetermined q-component (I*_gq) of the reference value of the grid current in the synchronous reference system.

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