Grid frequency control in offshore wind farms

Abstract

A method (200) of controlling a wind farm (10), the wind farm (10) comprising a plurality of wind turbines (11) installed in an AC sub-grid (12), the AC sub-grid (12) being connected to an energy conversion device (21) via a high voltage rectifier (22) and a DC line (20), the method comprising: controlling the power feed of each wind turbine to maintain a setpoint grid frequency (210) of the sub-grid; sensing a grid frequency (212) in the sub-grid; determining a reference quantity based on a difference between the sensed grid frequency and a setpoint grid frequency (214); and controlling the energy conversion device (216) according to the determined reference quantity.

Description

Technical Field

[0001] The present disclosure relates to the field of circuit arrangements for alternating current (AC) main power supplies or AC power distribution networks. In particular, the present disclosure proposes novel methods and devices for transmitting power between AC sub-grids via high voltage direct current (DC) links. Background Art

[0002] Offshore wind farms that use high voltage direct current (HVDC) to transmit electricity to onshore are now a well-established technology. The traditional principle is to use HVDC with voltage source converters (VSC) at sea and onshore. The offshore converter operates in grid-forming mode, maintaining a nominal constant voltage magnitude and frequency of the offshore grid. The onshore converter operates in DC bus voltage control mode, with reference to the DC link from the wind farm. This principle allows wind turbines to operate in grid-following mode in the same way as if they were connected to the onshore AC mains grid. HVDC transmission accepts any active power that a wind turbine can produce at any given time; no communication is required between the turbine and the HVDC terminals.

[0003] Recently, there has been a demand to replace offshore VSC HVDC terminals with high voltage diode rectifiers, with the main drivers being cost, space and weight. The desired configuration is Figure 1 , where the DC link and rectifier are labeled with reference numerals 20 and 22, respectively. However, this approach faces several challenges, including the following:

[0004] - The power transfer level through the diode rectifier is determined by the difference in voltage magnitude between the AC side and the DC side. To enable HVDC transmission to accept the active power generated by wind turbines, it is necessary to control the AC bus voltage magnitude at sea or the DC bus voltage on land.

[0005] - All or at least most wind turbines must be operated in a grid-type mode to maintain the size and frequency of the offshore grid.

[0006] These two challenges are intertwined because HVDC active power control interacts with offshore grid configuration control. Finding a robust design is challenging, especially considering that there may be hundreds of wind turbines in an offshore grid.

[0007] A solution is proposed in [1]. It is suggested to use a global positioning system (GPS) to provide synchronization signals to the wind turbines. In [2], a frequency control based on a distributed phase-locked loop (PLL) is proposed as a decentralized control, enabling each wind turbine to operate as a grid-type converter without the need for a communication link or a GPS signal. In [3], a variation is proposed, in which the active power change of each wind turbine is initiated by adjusting the voltage angle of the wind turbine converter. A change in the voltage angle without proper adjustment of the voltage magnitude results in a frequency deviation, which is detected by all wind turbine converters. This deviation is then converted into a voltage magnitude change via a proportional-integral regulator in each wind turbine converter. Therefore, unlike the solutions in [1] and [2], all wind turbine converters change their voltage magnitude synchronously.

[0008] The solution in [3] is probably the most promising proposed so far. However, it is somewhat complex because it mixes offshore frequency and voltage magnitude control. Furthermore, the magnitude of the offshore grid voltage variation may not always be ideal, especially if significant swings between zero and maximum power transfer are required.

[0009] If it is considered more attractive to change the onshore DC bus (or DC link) voltage, an offshore-onshore communication link is required. The signal sent over the link may be the total active power generated by all wind turbines. However, the active power injected into the onshore AC grid by the onshore HVDC converter can be adjusted to match this signal, which will indirectly adjust the onshore DC bus voltage. However, this solution is also a bit complicated because it requires communication links from all wind turbines. In addition, it does not take into account transmission losses, resulting in a difference between the required and actual transmitted power.

[0010] The application published as WO2015165517A1 discloses a system in which power is transmitted from several wind turbines over a common AC line, fed to DC via a modular rectifier and then reconverted to AC for use in the AC grid. The wind turbine receives reactive power Q from a regulator I,Ref In the wind turbine internal control, the active power P I,Ref The selection of the reference is delegated to the regulator.

[0011] Research paper XP006085028 (Lijun Xie et al. “Frequency regulation participation of offshore wind farm integrated by diode-rectifier HVDC system”. J. Eng.. Vol. 2019. No. 16. pp. 977–981. doi:10.1049 / joe.2018.8597) deals with large wind farms where power from wind turbines connected to an AC subgrid is transmitted to an onshore AC system via a diode high-voltage DC rectifier. The diode rectifier is required to recouple the AC subgrid and the onshore AC system in such a way that the offshore AC voltage varies with the voltage of the HVDC segment. The paper proposes to configure a supplementary control mechanism for the wind turbines – in addition to the main frequency and inertial response control – through which the reference DC voltage U is varied. dcref , to compensate for any frequency drop detected by the grid-side converter (relative to the reference grid frequency f s ). According to the authors, supplementary control mechanisms are suitable for "telecom-free" implementations.

[0012] In the context of offshore wind farms, the problem of replacing the offshore VSC HVDC terminals with HVDC rectifiers in a commercially acceptable manner remains unsolved. Summary of the invention

[0013] The object of the present disclosure is to propose a method and apparatus suitable for controlling a plurality of wind turbines installed in an AC subgrid connected to an energy conversion device via a DC link, thereby reducing or eliminating the need for a voltage source converter (VSC) on the subgrid side of the DC link. Another object is to use a reference quantity of the wind farm response as a first order system in such a method and apparatus.

[0014] At least some of these objects are achieved by the present invention, as defined in the independent claims. The dependent claims relate to advantageous embodiments.

[0015] In a first aspect of the present disclosure, a method for controlling a wind farm is provided, the wind farm comprising a plurality of wind turbines installed in an AC subgrid, the AC subgrid being connected to an energy conversion device via a high voltage rectifier (22) and a DC line (or DC link). The method comprises: controlling the power feed-in of each wind turbine to maintain a set point grid frequency of the subgrid; sensing the grid frequency in the subgrid; determining a reference quantity based on a difference between the sensed grid frequency and the set point grid frequency; and controlling the energy conversion device according to the determined reference quantity.

[0016] The inventors have realised that if the energy conversion device (e.g. an HVDC converter or some type of energy storage device) is controlled based on the difference between the sensed grid frequency and the set point grid frequency, the control of the wind farm will have favourable dynamics. In particular, the proposed solution operates without having to provide a synchronisation signal to the wind turbines. It will be appreciated that the regulation of the energy conversion device causes an indirect control action on the wind turbines through the intermediary of the DC link.

[0017] In various embodiments, the energy conversion device may be controlled directly according to the difference between the sensed grid frequency and the set point grid frequency (P regulator), or according to a quantity derived therefrom, such as the time integral of the difference (I regulator). The reference quantity may be applied as an active power reference or an active output current reference of the energy conversion device.

[0018] In a second aspect of the present disclosure, a controller is provided for controlling a wind farm, the wind farm comprising a plurality of wind turbines installed in an AC subgrid, the AC subgrid being connected to an energy conversion device via a high voltage rectifier and a DC line. The controller is configured to: control the power feed-in of each wind turbine to maintain a set point grid frequency of the subgrid; sense the grid frequency in the subgrid; determine a reference quantity based on a difference between the sensed grid frequency and the set point grid frequency; and control the energy conversion device according to the determined reference quantity.

[0019] Generally speaking, the controller according to the second aspect shares the effects and advantages of the method according to the first aspect, and it can be implemented with an equivalent degree of technical variations.

[0020] The present disclosure also relates to a computer program comprising instructions for causing a computer, in particular a controller, to perform the above method. The computer program may be stored or distributed on a data carrier. As used herein, a "data carrier" may be a transient data carrier (such as a modulated electromagnetic wave or light wave) or a non-transient data carrier. Non-transient data carriers include volatile and non-volatile memories, such as permanent storage media and non-permanent storage media of magnetic, optical or solid-state type. Still within the scope of a "data carrier", such a memory may be fixedly mounted or portable.

[0021] Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless otherwise explicitly defined. All references to "an / an / the element, device, component, member, step, etc." should be open to interpretation as referring to at least one instance of an element, device, component, member, step, etc., unless otherwise explicitly stated. Any method steps disclosed herein do not have to be performed in the exact order described, unless explicitly stated. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Aspects and embodiments will now be described by way of example with reference to the accompanying drawings, in which:

[0023] Figure 1 is a schematic diagram of a wind farm, wherein a plurality of wind turbines are installed in an AC subgrid, wherein power is transmitted from the subgrid via DC lines;

[0024] Figure 2 is a flow chart of a method for controlling a wind farm according to an embodiment herein; and

[0025] Figure 3 is a more detailed illustration of a wind farm with associated control equipment. DETAILED DESCRIPTION

[0026] Aspects of the present disclosure will be described more fully below with reference to the accompanying drawings, on which certain embodiments of the invention are shown. However, these aspects may be embodied in many different forms and should not be considered limiting; rather, these embodiments are provided by way of example so that the present disclosure is comprehensive and complete and fully conveys the scope of all aspects of the invention to those skilled in the art. Throughout the specification, like numbers refer to like elements.

[0027] Figure 1 In its left-hand portion a wind farm 10 is shown, comprising a plurality of wind turbines 11, installed in an AC subgrid 12. In the terminology of the present disclosure, a "wind farm" (or wind power station or wind park) refers to a group of wind turbines installed in one place. Typically, the horizontal spacing of wind turbines in a wind farm is minimum or close to minimum, in the sense that closer spacing means significant energy losses due to blocking effects. An acceptable level of energy loss may be related to the applicable monetary and / or environmental costs of using the land occupied by the wind farm. In conventional installations of the present disclosure, the spacing of identical wind turbines may typically be between 3 and 4 times the rotor diameter. A wind farm 10 may include several dozen wind turbines 11; their total number may be, for example, N=50.

[0028] The present disclosure relates in particular to such wind farms, in which the power feed-in of each wind turbine 11 is controlled by means of a droop control algorithm, in particular by means of active power / grid frequency droop. That is, the setpoint grid frequency ω1 [rad / s] at the nth wind turbine 11 and the actual grid frequency ω g,n The difference between their respective output active power reference P ref,n And the actual output active power P n The contact is as follows:

[0029] ω1-ω g，n =-Kp (P ref,n -P n ), (1)

[0030] Where K p is the positive control gain. Power reference P ref,n To maintain a balance between input power and output power (and losses) in the HVDC converter, a variable power reference P can be set. ref,n As a function of wind speed; specifically, when the wind speed increases, the power reference P ref,n It may need to be set higher, otherwise the voltage on the DC link 20 will increase. The setpoint grid frequency may be the nominal grid frequency, such as exactly 50 Hz or another specified value. Instead of control law (1), a control law with a static gain K p A filter is provided by which the desired amount of virtual inertia and / or virtual damping can be introduced into the droop control algorithm.

[0031] One way to implement this control is through power synchronization control (PSC), as described in [4], where the control law (1) solves ω g,n , and is integrated into the voltage angle at the converter terminals.

[0032] Another way to implement (1) is by using a so-called meshed phase-locked loop (PLL), as described in [5]. Since all turbines must run synchronously, it is assumed that in the steady state ω g,n =ω g , where ω g can be described as the global subgrid frequency. Thus, the effect of a non-zero deviation in power, P ref,n –P n ≠0, it will be ω g Deviation from ω1. Specifically, with P n There is no corresponding matching P ref,n An increase in ω usually results in g,n increases, resulting in the global subgrid frequency ω g Increase.

[0033] The subgrid 12 is connected to the energy conversion device 21 via a DC line 20. As an interface between the subgrid 12 and the DC line 20, a HVDC rectifier 22 is installed. The HVDC rectifier 22 can be a passive device constructed using diodes connected in series. The DC line 20 can be a submarine cable, connecting the offshore wind farm 10 to the onshore energy conversion device 21, which can have a total length of one kilometer or more. Figure 1 The DC line 20 shown may also correspond to an onshore-onshore link or an offshore-offshore link.

[0034] exist Figure 1 In the example use case shown, the energy conversion device 21 is a HVDC converter that connects the DC line 20 to the AC transmission grid (or main power grid) 30. In an alternative use case, the energy conversion device 21 can be an energy storage device configured to convert the input DC energy into chemical energy during certain periods and regenerate electrical energy during other periods. For example, the regenerated electrical energy can be fed into the transmission grid. The energy storage device can include a battery. The energy storage device can also be an electrical energy to hydrogen converter, including an electrolytic cell, a gas tank, and a fuel cell for power generation.

[0035] As described in more detail below, the energy conversion device 21 and the HVDC rectifier 22 may be provided with respective controllers 41, 42 operable to exchange information via a communication link 50 (such as a radio link, an optical link or a wired data link). The wired data link may specifically comprise a modulated signal superimposed on the current flowing on the DC link 20 (power line communication).

[0036] The inventor proposes a method 200 for controlling a wind farm 10, comprising: Figure 2 . It should be understood that some of these steps are optional (i.e., not present in some embodiments of method 200). In addition, the steps do not have to be disjoint in time; rather, their execution may overlap in time.

[0037] The step 210 of controlling the power feed-in of each wind turbine to maintain a setpoint grid frequency, for example by a droop control algorithm, has been described above. It should be understood that this control may continue in parallel with the execution of further steps of the method 200 . Figure 2 The cycle in shows this graphically.

[0038] In step 212, the grid frequency ω in the subgrid is sensed. g Grid frequency ω g The sensing or measuring may be performed. For example, the sensing or measuring may take place near the HVDC rectifier 22, such as at a point in the subgrid separated from the HVDC rectifier 22 by a low or negligible impedance.

[0039] In the next step 214, a reference quantity based on the difference between the sensed grid frequency and the set point grid frequency ω1 is determined. The reference quantity may be an active output power reference p of the onshore HVDC converter 21. ref Or active output current reference i p,ref . A pure integrator can be used, which can be expressed as:

[0040] p ref =-Ki f(ω1-ω g )dt (2a) or according to the operator corresponding to the Laplace transform variable s=d / dt, it can be expressed as

[0041]

[0042] In certain embodiments of the control method 200, the reference quantity also includes a term proportional to the difference between the sensed grid frequencies (P regulator) and / or the time derivative of the difference in the sensed grid frequencies (D regulator). This can be used to fine-tune the behavior of the control loop. Instead of using a constant set point frequency ω1 in (2a) to (2b), such as the nominal grid frequency, a reference frequency can be applied, deviating instantaneously from the nominal grid frequency. This enables an instantaneous response in the active power from the wind turbine 11 to frequency changes in the main grid (so-called frequency response) and similar disturbances.

[0043] In step 216, the energy conversion device 21 is controlled according to the reference variable. Likewise, step 216 may continue for the remaining duration of the method 200.

[0044] Now, if ω g Because P ref,n The value of p increases with the increase of one or more of the values ​​(step 218), then p ref Also starts to increase. Summing for N wind turbines gives:

[0045]

[0046] Where P = ∑ n P n is the total active power from the wind farm, and P ref =∑ n P ref，n Ignoring any communication delays, dynamic effects (such as inertia, rotating mass, and other energy storage in the operating system), and energy dissipation, in order to establish the instantaneous power balance P = p ref , and substituting (3) into (2b), we obtain:

[0047]

[0048] In other words, P in the controlled system is expected to be a time constant T = N / K i K p The first-order system response P ref .

[0049] In one set of embodiments, the method 200 is implemented in a first controller 41, which is arranged near the energy conversion device 21 (for example, the first controller 41 has a communication link to the energy conversion device 21, with acceptable reliability and capacity, preferably a wired communication link). Specifically, the first controller 41 can be arranged at the energy conversion device 21 or can be an essential component thereof. In this case, sensing the grid frequency in the subgrid (step 212) includes a step 212.1, that is, transmitting the sensed grid frequency to the first controller 41. Alternatively, a quantity derived from the sensed grid frequency, such as the difference ω1-ω g or its time integral, to the first controller 41. For this purpose, a communication link 50 may be utilized.

[0050] Figure 3 It concerns an embodiment in which the method 200 is implemented in a first controller near the energy conversion device 21. In detail, the PLL 14 is configured to obtain a voltage from the grid voltage v g Time evolution of estimated subgrid frequency ω g . Subgrid frequency ω g The forwarding is done via a communication link 50 which, in this example, is maintained by two wireless interfaces 51, 52. The subtractor component 41.1 of the first controller calculates the difference ω1-ω g , and feeds it to the integral controller 41.2. The output of the integral controller 41.2, i.e. the reference quantity p ref or p,ref , used as a control signal of the energy conversion device 21.

[0051] Figure 3 An example structure of an HVDC converter is also illustrated in detail, converting a three-phase AC voltage into a DC voltage through a YYD connection and two diodes. Figure 3 Unit 13 in represents a harmonic filter and / or a reactive power compensation unit. As suggested, unit 13 is an optional placement of a second controller 42 if no first controller 41 is provided (see next paragraph).

[0052] In another set of embodiments, the method 200 is implemented in the second controller 42, which is arranged near the wind farm 10, for example, where it is convenient to sense the subgrid frequency ω g Specifically, the second controller 42 may be installed at or in the HVDC rectifier 22 or at the unit 13. In this set of embodiments, controlling the energy conversion device 216 (step 216) includes a sub-step 216.1 of transmitting the determined reference quantity to the energy conversion device 21. The sub-step 216.1 may be performed by Figure 1 The communication link 50 shown performs.

[0053] Furthermore, the method 200 may be implemented in a distributed manner, for example with functionality in the first and second controllers 41 , 41 , and optionally with support from network (“cloud”) processing resources.

[0054] Aspects of the present disclosure have mainly been described above with reference to a few embodiments. However, as readily appreciated by a person skilled in the art, other embodiments than the embodiments disclosed above are equally possible within the scope of the invention, as defined by the appended patent claims.

[0055] References

[0056] 1.C.Prignitz,H.-G.Eckel,S.Achenbach,F.Augsburger and A. “FixReF: A control strategy for offshore wind farms with different wind turbine types and diode rectifier HVDC transmission”. 2016 IEEE 7th International Symposium on Power Electronics for Distributed Generation Systems (PEDG).

[0057] 2. L. Yu, R. Li, and L. Xu. “Distributed PLL-based control of offshore wind turbines connected with diode-rectifier-based HVDC systems”. IEEE Trans. Power Del., Vol. 33. No. 3. pp. 1328-1336. June 2018.

[0058] 3. A. Bidadfar, O. Saborio-Romano, NA Cutululis and PE “Control of offshore wind turbines connected to diode-rectifier-based HVDC systems”. IEEE Trans. Sust. Energy. Vol. 12. No. 1. pp. 514-523. January 2021.

[0059] 4. L. Zhang, L. Harnefors, and H.-P. Nee. “Power-synchronization control of grid-connected voltage-source converters.” IEEE Trans. Power Syst., Vol. 25, No. 2, pp. 809-920. May 2010.

[0060] 5. L. Harnefors, M. Schweizer, J. Kukkola, M. Routimo, M. Hinkkanen, and X. Wang. “Generic PLL-based grid-forming control.” IEEE Trans. Power Electron., Vol. 37. No. 2. pp. 1201-1204. February 2022.

Claims

1. A method (200) of controlling a wind farm (10), the wind farm comprising a plurality of wind turbines (11), the plurality of wind turbines being installed in an AC subgrid (12), the AC subgrid being connected to an energy conversion device (21) via a high voltage rectifier (22) and a DC line (20), the method comprising: controlling (210) the power feed-in of each wind turbine to maintain a set point grid frequency (ω1) of the subgrid; Sense (212) the grid frequency (ω) in the subgrid g ); determining (214) a reference quantity based on a difference between the sensed grid frequency and the set point grid frequency; as well as The energy conversion device is controlled (216) according to the determined reference quantity.

2. The method (200) according to claim 1, wherein the reference quantity comprises a time integral of the difference in the sensed grid frequency.

3. The method (200) according to claim 1 or 2, wherein the reference quantity further comprises a term proportional to the difference in the sensed grid frequencies and / or a time derivative of the difference in the sensed grid frequencies.

4. The method (200) according to any one of the preceding claims, wherein the reference quantity is an active output power reference or an active output current reference of the energy conversion device (21).

5. The method (200) according to any one of claims 1 to 4, wherein the energy conversion device (21) is a high voltage DC converter, which connects the DC line (20) to an AC power transmission grid (30).

6. The method (200) according to any one of claims 1 to 4, wherein the energy conversion device (21) is an energy storage device, such as an electricity to hydrogen converter or a battery.

7. The method (200) according to any of the preceding claims, wherein the power feed-in of each wind turbine (11) is controlled (210) by means of a droop control algorithm, in particular by means of active power / grid frequency droop.

8. The method (200) according to any of the preceding claims, wherein the power feed-in of each wind turbine (11) is controlled (210) according to a variable active power reference.

9. The method (200) according to any of the preceding claims, wherein the grid frequency in the subgrid is sensed (212) near a high-voltage DC rectifier (22), the high-voltage DC rectifier (22) being arranged on the subgrid side of the DC link (20).

10. The method (200) according to any one of claims 1 to 9, implemented in a first controller (41), the first controller (41) being arranged near the energy conversion device (21), Wherein sensing (212) the grid frequency in the subgrid comprises transmitting (212.1) the sensed grid frequency or a quantity derived therefrom to the first controller.

11. The method (200) according to claims 1 to 9, implemented in a second controller (42), the second controller (42) being arranged near the wind farm (10), The controlling (216) of the energy conversion device (21) comprises transmitting (216.1) the determined reference quantity to the energy conversion device.

12. The method (200) according to claim 10 or 11, wherein the communicating step (212.1, 216.1) is performed using a radio link, an optical link or a wired data link (50).

13. The method (200) according to any of the preceding claims, wherein the subgrid (12) is an offshore grid.

14. A controller (41, 42) for controlling a wind farm (10), the wind farm comprising a plurality of wind turbines (11), the plurality of wind turbines being installed in an AC subgrid (12), the AC subgrid being connected to an energy conversion device (21) via a high voltage rectifier (22) and a DC line (20), the controller being configured to: controlling the power feed-in of each wind turbine to maintain a set point grid frequency (ω1) of the subgrid; Sense the grid frequency (ω) in the subgrid g ); determining a reference quantity based on a difference between the sensed grid frequency and the set point grid frequency; as well as The energy conversion device is controlled according to the determined reference quantity.

15. A computer program comprising instructions for causing a controller (41, 42) according to claim 14 to perform a method (200) according to any one of claims 1 to 13.

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