A direct current collection grid-connected system and control method

Abstract

This invention provides a DC aggregation grid-connected system and control method, relating to the field of power system technology, for improving the stability margin of the DC aggregation grid-connected system during operation. The DC aggregation grid-connected system includes: at least one wind turbine generator, a DC aggregation network, a DC transformer, a high-voltage DC line, and a DC converter station; the DC aggregation network is connected to the wind turbine generator; the DC transformer includes: a low-voltage port and a high-voltage port; the low-voltage port is connected to the DC aggregation network, and the high-voltage port is connected to the high-voltage DC line; the DC converter station is connected to the high-voltage DC line and is configured to control the high-voltage DC voltage; a controller in the DC transformer is configured to control the DC voltage at the aggregation port and to enable it to improve the stability margin of the low-voltage and high-voltage ports of the DC transformer to a preset value.

Description

Technical Field

[0001] This invention relates to the field of power system technology, and in particular to a DC collection and grid connection system and control method. Background Technology

[0002] Large-scale deep-sea and desert wind power collection and transmission technology is a current research hotspot. The DC collection-DC transmission scheme solves the problem of insufficient voltage support capacity for long-distance AC transmission, while also reducing the need for large-capacity power frequency transformers, reducing the weight and volume of offshore converter platforms, and avoiding reactive power loss and overvoltage problems in long-distance AC collection, thus improving collection efficiency. Therefore, it has received widespread attention.

[0003] In a wind power DC collection and transmission system, the DC transformer (DCT) is the core power electronic device for realizing DC collection. It is used to realize the step-up and energy transmission between DC wind turbines and high-voltage flexible DC modular multilevel converters (MMCs). In order to realize high-voltage and high-capacity DC step-up and collection, DC transformers usually adopt input parallel output series topology or MMC topology, which has a complex topology and control structure.

[0004] However, in wind power DC collection and transmission systems, both ports of the DCT are at risk of oscillation. The same parameter changes have different effects on the impedance characteristics of the collection (low voltage) and transmission (high voltage) ports, and their adaptability to operating conditions is also different. Therefore, there are situations where the stability margin of the high voltage port decreases when the stability margin of the low voltage port increases, and the stability margin is high when the output power is large and low when the output power is low. These are difficult to coordinate, making it difficult to tune the oscillation suppression parameters. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention provides a DC collection grid-connected system and control method to improve the stability margin of the DC collection grid-connected system during operation.

[0006] In a first aspect, the present invention provides a DC collection and grid connection system, comprising: at least one wind turbine generator, a DC collection network, a DC transformer, a high-voltage DC line, and a DC converter station controller; the DC collection network is connected to the wind turbine generator; the DC transformer includes: a low-voltage port and a high-voltage port; the low-voltage port is connected to the DC collection network, and the high-voltage port is connected to the high-voltage DC line; the DC converter station is connected to the high-voltage DC line, and the DC converter station is configured to control the voltage of the high-voltage DC transmission system; the controller in the DC transformer is configured to control the voltage of the collection port and to enable it to increase the stability margin of the low-voltage port and the high-voltage port of the DC transformer to a preset value.

[0007] Optionally, the DC transformer includes: a first bridge arm, a second bridge arm, and a third bridge arm; the first bridge arm and the second bridge arm constitute the low-voltage port; the third bridge arm and the second bridge arm constitute the high-voltage port; the second bridge arm is a bridge arm shared by the low-voltage port and the high-voltage port.

[0008] Optionally, the first bridge arm includes at least one first bridge arm unit, the first bridge arm unit includes multiple first bridge arm sub-modules, and the multiple first bridge arm sub-modules are connected in series; the second bridge arm includes at least one second bridge arm unit, the second bridge arm unit includes multiple second bridge arm sub-modules, and the multiple second bridge arm sub-modules are connected in series; the third bridge arm includes at least one third bridge arm unit, the third bridge arm unit includes multiple third bridge arm sub-modules, and the multiple third bridge arm sub-modules are connected in series.

[0009] Optionally, the controller includes a first bridge arm controller, a second bridge arm controller, and a third bridge arm controller: the first bridge arm controller is used to control the voltage of the low-voltage port and the voltage of the first bridge arm submodule; the second bridge arm controller is used to control the voltage of the second bridge arm; the third bridge arm controller is used to control the voltage of the high-voltage port and the voltage of the third bridge arm submodule; the first bridge arm controller, the second bridge arm controller, and the third bridge arm controller each include an outer loop control layer and an inner loop control layer; the outer loop voltage control layer is configured to generate a current reference command based on a voltage reference command; the inner loop current control layer is connected to the outer loop voltage control layer and is configured to generate a modulation signal based on the current reference command, and output it to the DC transformer.

[0010] Optionally, the controller further includes: a parameter storage unit and a parameter scheduling unit; the parameter storage unit is configured to store control parameters, the control parameters including at least one adjustable parameter in the outer loop voltage control layer and / or the inner loop current control layer; the parameter scheduling unit is connected to the parameter storage unit and is configured to retrieve the corresponding control parameters from the parameter storage unit according to the real-time operating conditions and transmit them to the outer loop voltage control layer and / or the inner loop current control layer.

[0011] Optionally, the controller further includes: an oscillation monitoring unit and an oscillation suppression unit; the oscillation monitoring unit is configured to monitor harmonic components in the voltage or current signals of the electrical quantities of the low-voltage port and / or the high-voltage port in real time; the oscillation suppression unit is connected to the oscillation monitoring unit and also to the parameter scheduling unit, and is configured to adjust at least one adjustable parameter in the outer loop voltage control layer and / or the inner loop current control layer when the harmonic component exceeds a preset threshold, until the harmonic component is lower than the preset threshold; the oscillation suppression unit is further configured to determine, based on the current operating parameters, that the oscillation is mainly associated with the low-voltage port or the high-voltage port; adjust the adjustable parameters sequentially according to a preset priority order, and re-evaluate the harmonic component after each adjustment.

[0012] Optionally, the stability margin is the phase margin of the low-voltage port relative to the DC collection network and the wind turbine, and the phase margin of the high-voltage port relative to the high-voltage DC line and the DC converter station, and the preset value of the stability margin is greater than or equal to 10°.

[0013] Secondly, based on the same inventive concept, the present invention also provides a control method for a DC-DC grid-connected system, comprising: establishing a parameter library for different operating conditions, wherein each set of parameters is configured to ensure that the stability margin of the low-voltage port and the high-voltage port of the DC transformer meets a preset value under the corresponding operating condition; calling the corresponding parameters from the parameter library according to the real-time monitored operating conditions, and controlling the operation of the DC transformer based on the called parameters; and monitoring the oscillation state of the DC-DC grid-connected system in real time, and when an oscillation risk is detected, adjusting at least one parameter among the parameters until the oscillation is suppressed.

[0014] Optionally, establishing a parameter library for different operating conditions, where each set of parameters is configured to ensure that the stability margin of the low-voltage port and high-voltage port of the DC transformer meets a preset value under the corresponding operating condition, includes: constructing a two-port impedance model of the DC transformer; analyzing the oscillation risk frequency band of the low-voltage port and the high-voltage port under different operating conditions based on the model, screening control parameters whose influence on the impedance characteristics of the risk frequency band exceeds a preset threshold, and recording them as dominant control parameters; using the dominant control parameters as optimization variables, performing parameter optimization with the goal of simultaneously meeting the stability margin requirements of the low-voltage port and the high-voltage port, obtaining multiple sets of control parameters corresponding to different operating conditions, and constructing the parameter library.

[0015] Optionally, the step of calling the corresponding parameters from the parameter library according to the real-time monitored operating conditions and controlling the operation of the DC transformer based on the called parameters includes: generating a current reference command based on a voltage reference command through an outer loop voltage control layer; generating a modulation signal based on the current reference command through an inner loop current control layer, and outputting it to the DC transformer.

[0016] Optionally, the real-time monitoring of the oscillation state of the DC collection grid-connected system, and adjusting at least one parameter among the parameters until the oscillation is suppressed when an oscillation risk is detected, includes: real-time monitoring of harmonic components in the voltage or current signals of the low-voltage port and / or the high-voltage port; determining that there is an oscillation risk when a specific non-power frequency harmonic component exceeds a preset threshold; determining that the oscillation is mainly associated with the low-voltage port or the high-voltage port based on the current operating parameters and the established parameter library; adjusting the parameters sequentially according to a preset priority order, and re-evaluating the harmonic components after each adjustment until the harmonic components are lower than the preset threshold.

[0017] Compared with the prior art, the technical solution provided by the embodiments of the present invention has the following advantages: The present invention provides a DC collection and grid connection system. By setting up a controller connected to the wind turbine, DC collection network, DC transformer, high-voltage DC line and DC converter station, and enabling it to improve the stability margin of the low-voltage port and high-voltage port of the DC transformer to a preset value, the present invention solves the problem of difficulty in coordinating the stability margin caused by the difference in impedance characteristics of the two ports in the prior art. It realizes bidirectional stable support for the low-voltage side collection network and the high-voltage side transmission line under all operating conditions, and significantly enhances the overall operational stability and reliability of the wind power DC collection and transmission system. Attached Figure Description

[0018] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with the invention and, together with the description, serve to explain the principles of the invention.

[0019] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0020] Figure 1 A schematic diagram of a DC collection grid-connected system structure is provided in an embodiment of the present invention; Figure 2 A schematic diagram of a second bridge arm outer ring control module provided in an embodiment of the present invention; Figure 3 A schematic diagram of a first bridge arm outer ring control module provided in an embodiment of the present invention; Figure 4 A schematic diagram of a third bridge arm outer ring control module provided in an embodiment of the present invention; Figure 5 A schematic diagram of an AC current control unit provided in an embodiment of the present invention; Figure 6 A schematic diagram of a DC current control unit provided in an embodiment of the present invention; Figure 7 A flowchart of a control method based on a DC collection grid-connected system provided in an embodiment of the present invention; Figure 8 A flowchart of another control method based on a DC collection grid-connected system provided in an embodiment of the present invention; Figure 9 A flowchart illustrating another control method based on a DC collection grid-connected system provided in an embodiment of the present invention; Figure 10 A schematic diagram of a frequency sweep result provided in an embodiment of the present invention; Figure 11 A schematic diagram illustrating the influence of DC voltage control on the low-voltage port impedance, provided as an embodiment of the present invention; Figure 12 A schematic diagram illustrating a low-voltage port oscillation risk provided in an embodiment of the present invention; Figure 13 A flowchart of another control method based on a DC collection grid-connected system provided in an embodiment of the present invention; Figure 14 A schematic diagram of a low-voltage port oscillation waveform provided in an embodiment of the present invention; Figure 15 This is a schematic diagram of an oscillation waveform after suppression, provided as an embodiment of the present invention. Detailed Implementation

[0021] To better understand the above-mentioned objectives, features, and advantages of the present invention, the solutions of the present invention will be further described below. It should be noted that, unless otherwise specified, the embodiments of the present invention and the features thereof can be combined with each other.

[0022] Many specific details are set forth in the following description in order to provide a full understanding of the invention, but the invention may also be practiced in other ways different from those described herein; obviously, the embodiments in the specification are only some embodiments of the invention, and not all embodiments.

[0023] Based on this, this application provides a DC collection grid-connected system, such as... Figure 1 As shown, Figure 1 This is a schematic diagram of a DC collection and grid-connected system provided in an embodiment of the present invention. The DC collection and grid-connected system includes: at least one wind turbine, a DC collection network, a DC transformer, a high-voltage DC line, and a DC converter station.

[0024] The DC collection network is connected to the wind turbine generator; the DC transformer includes a low-voltage port and a high-voltage port; the low-voltage port is connected to the DC collection network, and the high-voltage port is connected to the high-voltage DC line; the DC converter station is connected to the high-voltage DC line and is configured to convert DC power to AC power; the controller in the DC transformer is configured to increase the stability margin of the low-voltage port and the high-voltage port to a preset value.

[0025] In some embodiments, the controller is part of the DC transformer, or the controller is independent of the DC transformer and the two are in parallel.

[0026] In some embodiments, the stability margin is the phase margin of the low-voltage port relative to the DC collection network and the wind turbine, and the phase margin of the high-voltage port relative to the high-voltage DC line and the DC converter station, and the preset value of the stability margin is greater than or equal to 10°.

[0027] Phase margin is an indicator for measuring the relative stability of a DC-DC grid-connected system.

[0028] When the impedance of the low-voltage port and the impedance of the external network it is connected to, namely the DC collection network and the wind turbine, have the same amplitude and a phase difference of 180° or close to 180° at certain frequencies, oscillation will occur. The phase margin is the distance between the actual phase difference and this 180° critical point. The calculation formula is: Phase margin = 180° - actual phase difference.

[0029] The phase margin of the low-voltage port relative to the DC collection network and the wind turbine refers to the impedance interaction between the two when looking at the external system it is connected to from the low-voltage port as the observation point; it is used to assess whether there is an impedance mismatch risk between the low-voltage port and the entire low-voltage side system (collection network + wind turbine); if the impedance of the two is close to a 180° phase difference at certain frequencies, it will cause oscillations on the low-voltage side.

[0030] The phase margin of a high-voltage port relative to a high-voltage DC line and a DC converter station refers to the impedance interaction between the high-voltage port and the external system it is connected to when viewed from the high-voltage port as the observation point. It is used to assess whether there is an impedance mismatch risk between the high-voltage port and the entire high-voltage side system (high-voltage DC line + DC converter station). If the impedances of the two are close to a 180° phase difference at certain frequencies, it will cause oscillations on the high-voltage side.

[0031] For example, taking an offshore wind farm as an example, the offshore wind farm includes 20 wind turbine units, which are connected to a DC transformer through a ±200kV DC collection network.

[0032] The low-voltage port of the DC transformer is connected to the DC collection network, and the high-voltage port is connected to the DC converter station via a ±640kV high-voltage DC line. The controller monitors the voltage of the low-voltage port, the voltage of the high-voltage port, and the current of the DC transformer in real time. As the output power of the wind farm gradually increases, the controller automatically adjusts the key parameters of the outer loop voltage controller and the inner loop current controller according to the pre-stored parameter library, such as the current loop bandwidth, voltage loop bandwidth, and feedforward coefficient, so that the phase margin of the low-voltage port relative to the DC collection network and the wind turbine is always maintained above 12°, while the phase margin of the high-voltage port relative to the high-voltage DC line and the DC converter station is always maintained above 15°.

[0033] Compared with traditional fixed parameter control, this scheme effectively avoids the problem of decreased stability margin on the low-voltage or high-voltage side due to power changes, ensuring that the system stays away from the oscillation risk area throughout the entire operating range, and significantly improves the operational reliability and stability of the wind power DC collection and transmission system.

[0034] In summary, this invention provides a DC collection and grid-connected system. By setting up a controller connected to the wind turbine, DC collection network, DC transformer, high-voltage DC line, and DC converter station, and enabling it to improve the stability margin of the low-voltage and high-voltage ports of the DC transformer to a preset value, this invention solves the problem of difficulty in coordinating the stability margin caused by the difference in impedance characteristics of the two ports in the prior art. It realizes bidirectional stable support for the low-voltage collection network and the high-voltage transmission line under all operating conditions, significantly enhancing the overall operational stability and reliability of the wind power DC collection and transmission system.

[0035] Optionally, the DC transformer includes: a first bridge arm, a second bridge arm, and a third bridge arm; the first bridge arm and the second bridge arm constitute a low-voltage port; the third bridge arm and the second bridge arm constitute a high-voltage port; the second bridge arm is a bridge arm shared by the low-voltage port and the high-voltage port.

[0036] In other words, a DC transformer couples two DC systems of different voltage levels together through a shared second bridge arm; the first bridge arm works with the second bridge arm to form a low-voltage port that bears the low-voltage side voltage; the third bridge arm works with the second bridge arm to form a high-voltage port that bears the high-voltage side voltage; after energy flows in from the low-voltage port, it is transferred to the high-voltage port through the three bridge arms, with the second bridge arm acting as an intermediate bridge, thus realizing the DC power conversion and transmission from low voltage to high voltage.

[0037] This shared bridge arm structure eliminates the need for a separate bridge arm. Since the second bridge arm is reused by two ports, the total number of submodules required is significantly reduced. In high-voltage, high-capacity applications, the cost of submodules accounts for a large proportion of the cost, so this structure can effectively reduce the hardware cost and footprint of DC transformers.

[0038] For example, taking an offshore wind power DC collection system as an example, its core equipment, the DC transformer, adopts a T-type modular multilevel DC converter station (MMC) topology. The DC transformer specifically includes three arms: the first arm (L arm) is composed of multiple sub-modules connected in series, the second arm (W arm) is composed of multiple sub-modules connected in series, and the third arm (H arm) is composed of multiple sub-modules connected in series.

[0039] The first bridge arm is connected at one end to the positive terminal of the ±200kV DC collection network and at the other end to the midpoint of the second bridge arm; the third bridge arm is connected at one end to the positive terminal of the ±640kV high-voltage DC line and at the other end to the midpoint of the second bridge arm; the other end of the second bridge arm is connected to both the negative terminal of the DC collection network and the negative terminal of the high-voltage DC line.

[0040] The first and second bridge arms together form the low-voltage port connecting the low-voltage collection network, while the third and second bridge arms together form the high-voltage port connecting the high-voltage DC line. The second bridge arm becomes a shared multiplexed bridge arm for both the low-voltage and high-voltage ports. The controller coordinates the switching of the three bridge arm sub-modules, utilizing the multiplexing characteristic of the second bridge arm to achieve energy transfer from the low-voltage side to the high-voltage side. Simultaneously, it optimizes the impedance characteristics of both the low-voltage and high-voltage ports, ensuring that their phase margins relative to their respective connected systems are consistently above 12°. Compared to solutions using independent transformers, this shared bridge arm structure not only reduces the number of sub-modules by approximately one-third, lowering hardware costs, but also achieves simultaneous improvement in the stability margins of both ports through controller-based collaborative optimization. This avoids the problem of sacrificing one port's stability for another, a common issue in traditional solutions, significantly enhancing the system's operational reliability under all operating conditions.

[0041] Optionally, the first bridge arm includes at least one first bridge arm unit, and the first bridge arm unit includes multiple first bridge arm sub-modules connected in series.

[0042] The second bridge arm includes at least one second bridge arm unit, and the second bridge arm unit includes multiple second bridge arm sub-modules connected in series.

[0043] The third bridge arm includes at least one third bridge arm unit, and the third bridge arm unit includes multiple third bridge arm sub-modules connected in series.

[0044] First, the voltage withstand capability of a single power electronic switching device (such as an IGBT) is limited and cannot directly withstand the voltage of a high-voltage DC line (such as ±200kV or even higher). By connecting multiple sub-modules in series, each sub-module shares a portion of the voltage, making the total voltage that the entire bridge arm can withstand equal to the sum of the voltages of all sub-modules. The more sub-modules there are, the higher the voltage level that the bridge arm can withstand, thus achieving a step-up conversion from low voltage to high voltage.

[0045] Meanwhile, in traditional two-level or three-level DC converter stations, the failure of any switching device can cause the entire system to shut down; however, in the sub-module series structure of this application, each bridge arm sub-module is equipped with a bypass switch; when a bridge arm sub-module fails, the controller can bypass it, and the remaining bridge arm sub-modules continue to operate; this design enables the system to have fault tolerance, and even if individual bridge arm sub-modules fail, the system can still operate at reduced capacity or maintain normal operation.

[0046] In some embodiments, taking an offshore wind power DC collection system as an example, its DC transformer adopts a T-type MMC topology, and the specific bridge arm structure is as follows: The first bridge arm (L bridge arm) includes a first bridge arm unit, which is composed of 24 first bridge arm sub-modules connected in series. The rated voltage of each sub-module is 16kV, which together support the voltage requirements of the ±200kV low-voltage port; The second bridge arm (W bridge arm) includes two second bridge arm units, each of which is composed of 24 second bridge arm sub-modules connected in series. The two units are then connected in series to form a multiplexed bridge arm with a total of 48 sub-modules to meet the requirement of simultaneously bearing the voltage stress of the low-voltage side and the high-voltage side; The third bridge arm (H bridge arm) includes a third bridge arm unit, which is composed of 24 third bridge arm sub-modules connected in series, which together support the voltage output of the ±640kV high-voltage port.

[0047] All bridge arm submodules adopt the same structural parameters, facilitating mass production and maintenance. This modular series structure allows the system to flexibly configure the number of submodules according to voltage level requirements. When a single bridge arm submodule fails, it can be disconnected via a bypass switch, while the remaining submodules can maintain system operation, significantly improving the system's fault tolerance and availability. Simultaneously, the series connection of multiple submodules uniformly reduces the voltage stress on each switching device, facilitating the selection of mature devices with lower voltage levels, reducing equipment costs and design complexity, and providing a reliable hardware foundation for DC transformers to achieve high-voltage, high-capacity power transmission.

[0048] Reference Figure 2 and Figure 3 In some embodiments, the controller includes an outer loop voltage control layer and an inner loop current control layer.

[0049] The outer loop voltage control layer is configured to generate a current reference command based on a voltage reference command; the inner loop current control layer is connected to the outer loop voltage control layer and is configured to generate a modulation signal based on a current reference command, which is then output to the DC transformer.

[0050] in, Figures 2-4 Corresponding to the outer loop voltage control layer, Figure 5 and Figure 6 Corresponding to the inner loop current control layer.

[0051] first, Figures 2-4 It includes three parallel control modules, corresponding to the W bridge arm (second bridge arm), L bridge arm (first bridge arm) and H bridge arm (third bridge arm) respectively.

[0052] Reference Figure 2 W-arm: The input signal has reference values ​​and feedback values; Reference values: d-axis AC voltage reference value vdsWref and q-axis AC voltage reference value vqsWref; Feedback values: d-axis actual voltage vdsW and q-axis actual voltage vqsW.

[0053] Among them, the actual voltage vdsW on the d-axis and the actual voltage vqsW on the q-axis are obtained by converting vxsW and θs.

[0054] Then, the voltage error is calculated: Δvd = vdsWref – vdsW; Δvq = vqsWref – vqsW. The error signals are fed into the AC voltage regulator Hvw(s) for proportional-integral calculation; finally, the AC voltage regulator outputs the d-axis reference current idWref and the q-axis reference current iqWref. The d-axis reference current idWref and the q-axis reference current iqWref will be used as… Figure 5 Input to the AC current control unit.

[0055] Reference Figure 3 L-arm: global voltage control and low-voltage port DC voltage control.

[0056] Global voltage control: The input signal has a reference value and a feedback value; the reference value is the target value Vclref of the submodule capacitor voltage; the feedback value is the actual acquired submodule capacitor voltage Vcl.

[0057] Then calculate the capacitor voltage error: ΔVc=Vclref-Vcl; send the error signal to the global voltage regulator Hvcl(s); the global voltage regulator outputs the d-axis reference current idlref and the q-axis reference current iqlref; and the q-axis reference current iqlref=0, indicating that no reactive component is introduced to interfere with the capacitor voltage control.

[0058] The function of this circuit is to maintain the internal energy balance of the DC transformer; when the capacitor voltage in the bridge arm submodule is too low, more energy needs to be absorbed from the system to charge the capacitor; when the capacitor voltage is too high, excess energy needs to be released; the d-axis reference current idlref is the current command used to regulate this energy exchange.

[0059] Low-voltage port DC voltage control: The input signal has a reference value and a feedback value; Reference value: low-voltage port DC voltage reference value vdcLref; Feedback value: low-voltage port actual DC voltage vdcL.

[0060] Then, the voltage error is calculated: ΔvdcL = vdcLref - vdcL; the error signal is sent to the DC voltage regulator HvdCL(s) for proportional-integral calculation; finally, the DC voltage regulator outputs the L-arm reference DC current ioLref; the L-arm reference DC current will be used as... Figure 6 Input to the DC current control unit.

[0061] The function of this loop is to maintain the voltage stability of the DC collector network. When the output power of the wind turbine changes, causing the low-voltage side voltage to fluctuate, ioLref will adjust accordingly, telling the inner loop how much current needs to be drawn in or released from the low-voltage side to maintain voltage stability.

[0062] Reference Figure 4 H-arm: global voltage control and high-voltage port voltage control.

[0063] Global voltage control: The input signal has a reference value and a feedback value; the reference value is the target value Vchref of the capacitor voltage of the H-bridge arm submodule; the feedback value is the actual acquired capacitor voltage Vch of the H-bridge arm submodule.

[0064] Then calculate the capacitor voltage error: ΔVch=Vchref–Vch; send the error signal to the global voltage regulator Hvch(s) for proportional-integral operation; the global voltage regulator Hvch(s) outputs the d-axis reference current idhref and the q-axis reference current iqhref; and the q-axis reference current iqhref=0, indicating that no reactive component is introduced to interfere with the capacitor voltage control.

[0065] When the capacitor voltage of an H-arm submodule is too low, energy needs to be drawn from the system to charge the capacitor, increasing idhref; when the capacitor voltage is too high, energy needs to be released, decreasing idhref. This loop ensures that the voltage of the submodules inside the H-arm is always maintained near the rated value, avoiding device overvoltage or system oscillation caused by capacitor voltage imbalance.

[0066] High-voltage port voltage control: The input signal has a reference value; the reference value is the DC voltage reference value VHref at the high-voltage port; the output is mdcH through a fixed proportional coefficient 3 / VCHREF.

[0067] The relationship between the modulation signal mdcH and the output voltage is: Vout = mdcH × N × Vc; Vout refers to the DC voltage output by the bridge arm; N is the number of bridge arm sub-modules; Vc is the capacitor voltage of the bridge arm sub-module.

[0068] To enable the H-bridge arm to output the voltage VHref, the required modulation ratio is: mdcH = VHref / (N × Vc).

[0069] Reference Figures 2-4In some embodiments, the outer loop voltage control layer includes: a first outer loop control module, a second outer loop control module, and a third outer loop control module.

[0070] The first outer loop control module, connected to the second bridge arm, is configured to generate a current reference command for the second bridge arm based on the difference between the AC voltage reference command and the AC voltage feedback value.

[0071] The second outer loop control module, connected to the first bridge arm, is configured to generate a DC current reference command for the first bridge arm based on the difference between the DC voltage reference command and the DC voltage feedback value.

[0072] The third outer loop control module, connected to the third bridge arm, is configured to generate the modulation signal for the third bridge arm based on the bridge arm DC voltage reference command and the proportional coefficient.

[0073] The first outer loop control module corresponds to Figure 2 The AC voltage reference commands are the d-axis AC voltage reference value vdsWref and the q-axis AC voltage reference value vqsWref; the AC voltage feedback values ​​are the d-axis actual voltage vdsW and the q-axis actual voltage vqsW; the voltage error is calculated as follows: Δvd = vdsWref – vdsW; Δvq = vqsWref – vqsW; the error signal is sent to the AC voltage regulator Hvw(s) for proportional-integral calculation; finally, the AC voltage regulator outputs the d-axis reference current idWref and the q-axis reference current iqWref.

[0074] The first outer loop control module uses AC voltage control to precisely adjust the internal circulating current, ensuring smooth energy transfer between high and low voltage ports.

[0075] The second outer loop control module corresponds to Figure 3 The DC voltage reference command is the low-voltage port DC voltage reference value vdcLref; the DC voltage feedback value is the actual low-voltage port DC voltage vdcL; then the voltage error is calculated: ΔvdcL = vdcLref - vdcL; the error signal is sent to the DC voltage regulator HvdCL(s) for proportional-integral operation; finally, the DC voltage regulator outputs the L-bridge arm reference DC current ioLref.

[0076] The second outer loop control module uses DC voltage control to directly stabilize the low-voltage collection network voltage, ensuring reliable collection of wind turbine power.

[0077] The third outer loop control module corresponds to Figure 4 The DC voltage reference command for the bridge arm is the DC voltage reference value VHref at the high-voltage port; the proportional coefficient is 3 / VCHREF; and the modulation signal for the third bridge arm is mdcH.

[0078] The third outer loop control module adopts a composite control (capacitor voltage balancing + port voltage control), which takes into account both the safety of internal components and the external output requirements.

[0079] Reference Figure 5 and Figure 6 In some embodiments, the inner loop current control layer includes an AC current control unit and a DC current control unit.

[0080] The AC current control unit, connected to the first outer loop control module, is configured to generate an AC modulation signal and output it to the second and / or third bridge arm based on the current reference command of the second bridge arm and the actual current of the second bridge arm collected.

[0081] The DC current control unit, connected to the second outer loop control module, is configured to generate a DC modulation signal and output it to the first bridge arm based on the DC current reference command of the first bridge arm and the collected DC current of the first bridge arm.

[0082] Among them, the AC current control unit corresponds to Figure 5 The current reference commands based on the second bridge arm are the d-axis reference current idWref and the q-axis reference current iqWref; the actual current of the second bridge arm is the collected three-phase AC current ixz of the second bridge arm; finally, the modulation signal mxz is generated; the modulation signal mxz is output to the second bridge arm to control the AC current of the second bridge arm, and the modulation signal mxz is output to the third bridge arm to control the AC component of the high-voltage side bridge arm.

[0083] DC current control unit Figure 6 The DC current reference command is the L-arm reference DC current ioLref; the first arm DC current is ioL; a DC modulation signal mdcL is generated and output to the first arm to control the DC current of the low-voltage side arm, thereby affecting the low-voltage port voltage.

[0084] This invention achieves differentiated control of different current characteristics in a T-type MMC DC transformer by dividing the inner loop current control layer into an AC current control unit and a DC current control unit. The AC current control unit adopts a decoupled control strategy based on a synchronous rotating coordinate system, converting the three-phase AC quantities into DC quantities through coordinate transformation for zero steady-state error regulation. It also introduces voltage feedforward and current decoupling compensation, which can effectively suppress circulating current components and high-frequency harmonics between bridge arms, ensuring that the AC current of the W and H bridge arms quickly and accurately tracks the outer loop command. The DC current control unit uses a simplified DC PI regulator with voltage feedforward compensation to directly control the DC current of the L bridge arm, providing rapid current response support for the stability of the low-voltage port voltage. Ultimately, the controller can achieve a comprehensive improvement in the stability margin of both ports through parameter scheduling across the entire operating range, and quickly suppress risks when oscillations occur by precisely adjusting the sensitive parameters of the corresponding units. Finally, it achieves comprehensive optimization of system stability, dynamic response capability, and control simplicity.

[0085] like Figure 5 As shown, in some embodiments, the AC current control unit includes: a first coordinate transformation module, a first current regulation module, a first compensation synthesis module, and a first inverse transformation module.

[0086] The first coordinate transformation module is configured to transform the collected three-phase AC current of the second bridge arm to the synchronous rotating coordinate system based on the synchronous rotating reference angle, so as to obtain the direct-axis current feedback value and the quadrature-axis current feedback value.

[0087] The first current regulation module, connected to the first coordinate transformation module and the first outer loop control module, is configured to generate a direct-axis preliminary voltage command based on the difference between the direct-axis current reference command and the direct-axis current feedback value of the second bridge arm, and to generate a quadrature-axis preliminary voltage command based on the difference between the quadrature-axis current reference command and the quadrature-axis current feedback value of the second bridge arm.

[0088] The first compensation synthesis module, connected to the first current regulation module, is configured to introduce a voltage feedforward compensation term based on the common connection point voltage and the first feedforward coefficient, and a current decoupling compensation term based on the direct-axis and quadrature-axis current feedback values ​​and decoupling coefficients, and synthesize them with the initial direct-axis and quadrature-axis voltage commands to generate direct-axis and quadrature-axis modulated signals.

[0089] The first inverse transformation module, connected to the first compensation synthesis module, is configured to inverse transform the direct-axis and quadrature-axis modulation signals to a three-phase stationary coordinate system to generate an AC modulation signal.

[0090] The first coordinate transformation module will convert the three-phase time-varying AC current ixz (a, b, c) of the second bridge arm (W bridge arm) into two DC quantities i_dZ and i_qZ in the synchronous rotating coordinate system; the input signal also includes the synchronous rotating reference angle θs.

[0091] Three-phase AC quantities are sinusoidal waves that vary with time in a stationary coordinate system, making it impossible to achieve zero steady-state error tracking with direct PI control. After being converted to DC quantities through coordinate transformation, zero steady-state error regulation can be achieved using a PI controller, greatly simplifying the controller design.

[0092] The input signals of the first current regulation module include: the direct-axis current reference command idWref (d-axis reference current idWref) from the first outer loop control module, the direct-axis current feedback value idZ from the first coordinate transformation module, the quadrature-axis current reference command iqWref (q-axis reference current iqWref) from the first outer loop control module, and the quadrature-axis current feedback value iqZ from the first coordinate transformation module.

[0093] Calculate the direct-axis current error: Δid = idWref - idZ; calculate the quadrature-axis current error: Δiq = iqWref -iqZ; the two error signals are respectively sent to the PI controller (Hiz(s)) for proportional-integral operation; output the initial direct-axis voltage command vdsW and the initial quadrature-axis voltage command vqsW.

[0094] The first compensation synthesis module corresponds to the additive synthesis point of feedforward compensation (Kfz multiplication) and decoupling compensation (KdZ related terms); the input signals include: the direct-axis preliminary voltage command vdsW from the first current regulation module, the quadrature-axis preliminary voltage command vqsW from the first current regulation module, the first feedforward coefficient Kfz, the direct-axis current feedback value idZ, the quadrature-axis current feedback value iqZ, and the decoupling coefficient KdZ. The common coupling point voltage is also the direct-axis preliminary voltage command vdsW and the quadrature-axis preliminary voltage command vqsW.

[0095] First, the common point voltage is multiplied by the first feedforward coefficient Kfz to generate the feedforward compensation term; feedforward d = Kfz × vdsW; feedforward q = Kfz × vqsW.

[0096] Then, based on the current feedback value and system parameters, decoupling compensation terms are generated; decoupling d = -ωL × iqZ, decoupling q = +ωL × idZ, where ωL is equivalently characterized by the decoupling coefficient KdZ.

[0097] Finally, the direct-axis modulated signal mdZ = vdsW + feedforward d + decoupling d is synthesized; the quadrature-axis modulated signal mqZ = vqsW + feedforward q + decoupling q is synthesized.

[0098] The parameters Kfz and KdZ of the compensation term are key parameters that can be optimized. By adjusting them, the damping characteristics and disturbance rejection capability of the system can be changed.

[0099] The input signals of the first inverse transformation module include: direct-axis modulation signal mdZ, quadrature-axis modulation signal mqZ, and synchronous rotation reference angle θs (consistent with the angle used during coordinate transformation); the final output signal is a three-phase AC modulation signal mxZ (x = a, b, c).

[0100] Among them, mxZ is sent to the PWM modulator of the second bridge arm (W bridge arm) and / or the third bridge arm (H bridge arm) to generate specific switching pulses.

[0101] This application constructs a complete and efficient AC current closed-loop control link by finely dividing the AC current control unit into a first coordinate transformation module, a first current regulation module, a first compensation synthesis module, and a first inverse transformation module. The first coordinate transformation module converts the time-varying three-phase AC quantity into an easily controllable DC quantity, laying the foundation for zero steady-state error control. The first current regulation module ensures that the current tracks the outer loop command quickly and accurately through a PI controller. The first compensation synthesis module introduces voltage feedforward and current decoupling compensation, significantly enhancing the system's ability to suppress grid voltage fluctuations and internal coupling disturbances. The first inverse transformation module restores the control quantity in the dq domain to the actual three-phase modulation signal, completing the final step from algorithm to physical execution. Ultimately, this enables the controller to accurately adjust these parameters according to different operating conditions, thereby achieving fast, accurate, and stable control of the AC current across the entire operating range. This provides a solid inner-loop execution guarantee for the overall improvement of the stability margin of the DC transformer's two ports. Furthermore, when system oscillation occurs, the oscillation suppression unit can accurately locate the sensitive parameters within that unit for fine-tuning, achieving rapid and effective suppression of oscillations.

[0102] like Figure 6 As shown, in some embodiments, the DC current control unit includes: a second current regulation module and a second compensation synthesis module.

[0103] The second current regulation module, connected to the second outer loop control module, is configured to generate a preliminary DC voltage command based on the difference between the DC current reference command of the first bridge arm and the collected DC current feedback value of the first bridge arm.

[0104] The second compensation synthesis module, connected to the second current regulation module, is configured to generate a DC modulation signal based on the difference between the low-voltage side voltage reference value and the low-voltage side voltage feedback value, the difference, the voltage feedforward compensation term of the second feedforward coefficient, and the preliminary DC voltage command.

[0105] First, the second current regulation module receives the first bridge arm DC current reference command iolref from the second outer loop control module and compares it with the collected first bridge arm actual DC current feedback value iol. Based on the difference between the two, a preliminary DC voltage command is generated through proportional-integral regulation. This step ensures that the DC current can quickly and without steady-state error track the command value given by the outer loop. Then, the second compensation synthesis module receives the initial DC voltage command and introduces a voltage feedforward compensation term generated by multiplying the difference between the low-voltage side voltage reference value VOL and the low-voltage side voltage feedback value VdcL by the second feedforward coefficient KfL. The two are then synthesized to generate the final DC modulation signal mdcL, which is output to the PWM modulator of the first bridge arm, thereby completing the complete conversion process from DC current command to actual drive pulse.

[0106] By dividing the DC current control unit into a second current regulation module and a second compensation synthesis module, the second current regulation module employs proportional-integral control to ensure that the DC current of the first bridge arm can accurately track the outer loop command, providing rapid current response support for the stability of the low-voltage port voltage. The second compensation synthesis module introduces feedforward compensation based on the low-voltage side voltage deviation. When the low-voltage bus voltage is disturbed by wind power fluctuations or load changes, this feedforward term can correct the modulation signal in advance, enabling the current loop to respond before the voltage disturbance affects the current, significantly enhancing the system's ability to suppress voltage fluctuations. By optimizing the bandwidth parameters of the second current regulation module, the speed and damping characteristics of the low-voltage port current response can be adjusted; by optimizing the second feedforward coefficient, the system's ability to suppress low-voltage side voltage disturbances can be enhanced, thereby indirectly improving the phase margin of the low-voltage port relative to the collection network.

[0107] In some embodiments, the controller further includes a parameter storage unit and a parameter scheduling unit.

[0108] The parameter storage unit is configured to store control parameters, including at least one adjustable parameter in the outer loop voltage control layer and / or the inner loop current control layer.

[0109] The parameter scheduling unit, connected to the parameter storage unit, is configured to retrieve the corresponding control parameters from the parameter storage unit according to the real-time operating conditions and transmit them to the outer loop voltage control layer and / or the inner loop current control layer.

[0110] In other words, the parameter storage unit in the controller pre-stores multiple sets of control parameters. These parameters cover all adjustable key parameters in the outer loop voltage control layer (such as the proportional gain and integral time of the first voltage regulator, the second voltage regulator, and the third outer loop control module) and the inner loop current control layer (such as the proportional gain and integral time of the first and second current regulation modules, the first feedforward coefficient, the second feedforward coefficient, the decoupling coefficient, etc.). The parameter scheduling unit is connected to the parameter storage unit and monitors the current operating conditions (such as output power level, number of converged branches, voltage level, etc.) in real time during system operation. Based on the preset correspondence, it retrieves the set of control parameters that best matches the current operating conditions from the parameter storage unit and accurately transmits them to the corresponding regulation modules in the outer loop voltage control layer and / or the inner loop current control layer, thereby realizing the dynamic updating of the controller parameters.

[0111] In this way, the parameter storage unit in this application will solidify and store multiple sets of optimized control parameters obtained through dual-port impedance modeling and multi-objective optimization in the offline stage, forming an optimal parameter library covering all operating conditions; the parameter scheduling unit will dynamically call the corresponding parameter combination according to the real-time operating conditions, so that the controller can always operate in the parameter state optimized for the operating condition under different power levels, different numbers of converged branches, etc., thereby fundamentally solving the problem that traditional fixed parameter control cannot adapt to changes in operating conditions and is difficult to take into account dual-port stability margin.

[0112] In some embodiments, the controller further includes an oscillation monitoring unit and an oscillation suppression unit.

[0113] The oscillation monitoring unit is configured to monitor harmonic components in the voltage or current signals of electrical quantities at the low-voltage and / or high-voltage ports in real time.

[0114] The oscillation suppression unit, connected to the oscillation monitoring unit and the parameter scheduling unit, is configured to adjust at least one adjustable parameter in the outer loop voltage control layer and / or the inner loop current control layer when the harmonic component exceeds a preset threshold, until the harmonic component is lower than the preset threshold.

[0115] The oscillation suppression unit is also configured to determine whether the oscillation is mainly associated with the low-voltage port or the high-voltage port based on the current operating parameters; adjust the adjustable parameters in sequence according to the preset priority order, and re-evaluate the harmonic components after each adjustment.

[0116] In other words, in the controller, the oscillation monitoring unit is configured to monitor the voltage or current signals of the low-voltage port and / or the high-voltage port in real time, extract the harmonic components through spectrum analysis, and continuously evaluate the oscillation state of the system. When a specific non-power frequency harmonic component is detected to exceed a preset safety threshold, it is determined that the system has an oscillation risk, and then the oscillation suppression unit is triggered. The oscillation suppression unit first intelligently determines whether the oscillation is mainly related to the low-voltage port or the high-voltage port based on the current operating parameters (such as the currently called control parameters, the amplitude and phase of the port voltage and current, etc.) and the parameter characteristics stored in the parameter library. Then, it adjusts the adjustable parameters in the outer loop voltage control layer and / or the inner loop current control layer in a preset priority order. After each adjustment, it re-evaluates whether the harmonic component has dropped below the threshold. This process is iterated until the oscillation is completely suppressed and the system returns to stable operation.

[0117] In this way, by setting up an oscillation monitoring unit and an oscillation suppression unit, the present invention continuously performs harmonic analysis on the port voltage and current, accurately identifying risks in the early stages of oscillation and avoiding the serious consequences of system shutdown caused by oscillation deterioration. The oscillation suppression unit makes full use of the parameter characteristic information stored in the offline optimization parameter library, accurately locating the oscillation source (low-voltage port or high-voltage port) based on the current operating status, and scientifically and orderly adjusting the parameters according to the priority order of their impact on stability margin, rather than blindly trying different approaches. This achieves effective oscillation suppression in the shortest time and with the smallest parameter variation. This mechanism fundamentally solves the problems of difficult parameter tuning, slow response speed, and easy omission in traditional oscillation suppression methods: it achieves rapid, accurate, and adaptive suppression of system oscillation, significantly improving the operational reliability and stability of DC transformers in the face of various disturbances and changes in operating conditions throughout their entire life cycle, and providing a solid guarantee for the safe and stable operation of wind power DC collection systems.

[0118] Reference Figure 7 Secondly, based on the same inventive concept, the present invention also provides a control method based on a DC collection grid-connected system, comprising: S1. Establish a parameter library for different operating conditions. Each set of parameters is configured to ensure that the stability margin of the low-voltage port and high-voltage port of the DC transformer meets the preset value under the corresponding operating condition.

[0119] Step S1 first establishes a parameter library associated with different operating conditions through offline analysis. Each set of parameters is optimized and configured to ensure that the stability margin of the low-voltage port and high-voltage port of the DC transformer meets the preset requirements (such as a phase margin greater than or equal to 10°) under the corresponding operating conditions. This step completes the complex stability analysis and parameter tuning work in advance, laying a scientific data foundation for subsequent online control.

[0120] S2. Based on the real-time monitored operating conditions, the corresponding parameters are retrieved from the parameter library, and the operation of the DC transformer is controlled based on the retrieved parameters.

[0121] In step S2, when the system is running online, the corresponding optimized parameters are accurately called from the parameter library based on the real-time monitored operating conditions (such as the current output power level, the number of converged branches, etc.). Based on these parameters, the outer loop voltage control layer and the inner loop current control layer of the DC transformer are controlled to operate in coordination to generate modulation signals to drive each bridge arm, thereby realizing the adaptive optimized operation of the system under all operating conditions.

[0122] S3. Monitor the oscillation status of the DC collection grid-connected system in real time. When an oscillation risk is detected, adjust at least one parameter until the oscillation is suppressed.

[0123] Step S3 continuously monitors the oscillation status of the system in real time. By performing harmonic analysis on the voltage or current signals of the low-voltage port and / or high-voltage port, once a specific non-power frequency harmonic component is detected to exceed a preset threshold, it is determined that there is an oscillation risk. Then, an adaptive adjustment mechanism is activated. Based on the current operating parameters and the parameter characteristics stored in the parameter library, the port mainly associated with the oscillation is determined, and the key parameters in the outer loop voltage control layer and / or inner loop current control layer are adjusted in sequence according to the preset priority order. After each adjustment, the harmonic components are re-evaluated until the oscillation is completely suppressed and the system returns to stable operation.

[0124] In summary, this application pre-completes the complex dual-port impedance modeling, oscillation risk analysis, and multi-objective parameter optimization work through offline library construction in step S1, fundamentally solving the drawbacks of traditional methods, such as difficult parameter tuning and reliance on manual experience. Step S2, online invocation, enables the controller to intelligently switch to the optimal parameters based on real-time operating conditions, ensuring sufficient stability margins for both low-voltage and high-voltage ports under various operating scenarios, including light load, half load, and full load. Step S3 achieves early identification of oscillation risks through real-time harmonic monitoring and performs precise and orderly parameter fine-tuning based on priority information in the parameter library, quelling oscillations in the shortest time with minimal control costs. This not only completely solves the core problem of balancing dual-port stability margins in wind power DC collection systems but also significantly improves the system's robustness to complex operating conditions and sudden disturbances, providing a systematic solution with high stability, strong adaptability, and engineering practicality for the reliable transmission of large-scale deep-sea and desert wind power.

[0125] like Figure 8 As shown, in some embodiments, S1, establishing a parameter library for different operating conditions, where each set of parameters is configured to ensure that the stability margin of the low-voltage port and high-voltage port of the DC transformer meets a preset value under the corresponding operating condition includes: S11. Construct a two-port impedance model for a DC transformer.

[0126] Step S11 first constructs a two-port impedance model of the DC transformer, using accurate modeling methods such as harmonic linearization to ensure that the model can accurately reflect the impedance frequency characteristics from the low-voltage port to the collection network and wind turbine, and from the high-voltage port to the DC line and DC converter station, providing a reliable mathematical basis for subsequent analysis.

[0127] The DCT parameters in this embodiment are shown in Table 1: Table 1 DCT parameters

[0128] Using the harmonic linearized impedance modeling method, an analytical model of the impedance at the high and low voltage ports of the DCT can be obtained, and an electromagnetic transient simulation model of the offshore wind power DC collection and transmission system can be built. Based on the simulation model, a perturbation sinusoidal voltage of 0.03 pu is injected into the high and low voltage DC ports of the DCT, with a frequency range of 1Hz-1000Hz, and the frequency sweep results are obtained as follows. Figure 10 As shown, the frequency sweep results verified the correctness of the analytical model, and the correctness of the analytical model is the basis for accurate selection of the parameter library.

[0129] S12. Based on model analysis of the oscillation risk frequency bands of low-voltage and high-voltage ports under different operating conditions, select control parameters whose impact on impedance characteristics of risk frequency bands exceeds the preset threshold and record them as dominant control parameters.

[0130] Step S12, based on the model, performs impedance characteristic analysis on the low-voltage port and high-voltage port under different operating conditions, identifies the risk frequency bands that may oscillate, and further filters out the control parameters that affect the impedance characteristics of the risk frequency bands beyond the preset threshold (such as amplitude change exceeding 10dB or phase change exceeding 10°), and records these parameters as the dominant control parameters.

[0131] According to this embodiment, Figure 11 This is a schematic diagram illustrating the influence of DC voltage control on the low-voltage port impedance, provided by an embodiment of the present invention. Figure 11 The controller bandwidth is represented by fvdcL. As shown in the figure, the low-voltage port is affected by the DC voltage control parameters across the entire frequency band. Other controllers are similar and will not be elaborated further.

[0132] Based on impedance characteristic analysis, it can be seen that the impedance characteristics of the low-voltage port are greatly affected by the DC voltage, DC current, W-arm circulating current, L-arm circulating current, H-arm global voltage, and L-arm global voltage control parameters; the impedance characteristics of the output port are greatly affected by the W-arm circulating current, L-arm circulating current, H-arm circulating current, L-arm global voltage, H-arm global voltage, and DC current control parameters.

[0133] Figure 12The analysis of low-voltage port oscillation risk shows that there is an oscillation risk in the 100-300Hz frequency band. Therefore, the control parameters that affect the impedance characteristics of the relevant frequency band are the dominant control parameters, as shown in Table 2.

[0134] Table 2. Influence of various parameters on the two-port terminal

[0135] S13. Using the dominant control parameters as optimization variables, and aiming to simultaneously meet the stability margin requirements of both the low-pressure port and the high-pressure port, perform parameter optimization to obtain multiple sets of control parameters corresponding to different operating conditions, and construct a parameter library.

[0136] Step S13 uses the selected dominant control parameters as optimization variables, with the goal of simultaneously meeting the stability margin requirements of both the low-pressure port and the high-pressure port (such as a phase margin greater than or equal to 10°). A multi-objective optimization algorithm is used to optimize the parameters, obtaining multiple sets of optimal control parameter combinations for different operating conditions, and finally constructing an optimization parameter library covering all operating conditions.

[0137] Step S11, with its two-port impedance modeling, provides a precise quantitative tool for stability analysis, avoiding the errors inherent in traditional methods that rely on approximate models. Step S12, with its risk frequency band identification and dominant parameter screening, accurately identifies the key links and core parameters affecting system stability, avoiding the ineffective effort of blindly optimizing all parameters. Step S13, with its multi-objective parameter optimization, uses the simultaneous guarantee of two-port stability margin as a constraint, completely solving the problem in traditional methods where improving the stability of one port comes at the expense of the other.

[0138] like Figure 9 As shown, in some embodiments, S2, calling the corresponding parameters from the parameter library according to the real-time monitored operating conditions, and controlling the operation of the DC transformer based on the called parameters includes: S21. Generate current reference command based on voltage reference command through outer loop voltage control layer.

[0139] Step S21 is first executed by the outer loop voltage control layer. This layer receives system-level voltage reference commands (such as AC voltage reference commands, low-voltage port DC voltage reference commands, and bridge arm DC voltage reference commands) and compares them with the corresponding voltage feedback values. Based on the difference between the two, current reference commands (such as direct-axis / quadrature-axis current reference commands for the second bridge arm and DC current reference commands for the first bridge arm) are generated through voltage regulators (such as the first voltage regulator and the second voltage regulator). This step converts the voltage stability target into an inner loop trackable current target.

[0140] S22. Through the inner loop current control layer, a modulation signal is generated based on the current reference command and output to the DC transformer.

[0141] Step S22 is then executed by the inner loop current control layer, which receives the current reference command generated by the outer loop and compares it with the actual current feedback value collected. Based on the difference between the two, the final modulation signal is generated by the current regulator (such as the first current regulation module and the second current regulation module) and the compensation synthesis module (introducing voltage feedforward compensation and current decoupling compensation), and output to the PWM modulator of each bridge arm of the DC transformer to drive the sub-module to switch, thereby accurately controlling the bridge arm current and port voltage.

[0142] In this way, the outer-loop voltage control layer focuses on steady-state accuracy, generating current commands based on the system voltage target to ensure that the voltage stability margins of the low-voltage and high-voltage ports meet the preset requirements; the inner-loop current control layer focuses on dynamic response, quickly and accurately tracking the current commands given by the outer loop, and significantly enhancing the anti-disturbance capability by introducing feedforward compensation and decoupling compensation; this not only ensures the coordinated improvement of the stability margins of both ports under all operating conditions, but also significantly simplifies the control logic, providing a solid execution guarantee for the reliable operation of the system.

[0143] like Figure 13 As shown, in some embodiments, S3, real-time monitoring of the oscillation state of the DC collection grid-connected system, and adjusting at least one parameter among the parameters until the oscillation is suppressed when an oscillation risk is detected, includes: S31. Monitor the harmonic components in the voltage or current signals of the low-voltage port and / or high-voltage port in real time.

[0144] Step S31 First, the oscillation monitoring unit continuously acquires and performs spectrum analysis on the voltage or current signals of the low-voltage port and / or the high-voltage port in real time, and extracts the harmonic components as a quantitative indicator for evaluating the oscillation state of the system.

[0145] S32. When a specific non-power frequency harmonic component exceeds a preset threshold, it is determined that there is a risk of oscillation.

[0146] When the amplitude of a specific non-power frequency harmonic component (such as a certain characteristic harmonic) exceeds the preset safety threshold (e.g., 2% of the fundamental frequency amplitude) is detected in step S32, the system determines that there is an oscillation risk and immediately triggers the oscillation suppression logic.

[0147] S33. Based on the current operating parameters and the established parameter library, determine whether the oscillation is mainly associated with the low-voltage port or the high-voltage port.

[0148] In step S33, the oscillation suppression unit intelligently determines whether the oscillation is mainly associated with the low-voltage port or the high-voltage port based on the current operating parameters (such as the currently called control parameters, port voltage, current amplitude, phase, etc.) and the parameter characteristic information stored in the optimized parameter library established in step S1, thereby accurately locating the oscillation source.

[0149] S34. Adjust the key parameters in sequence according to the preset priority order, and re-evaluate the harmonic components after each adjustment until the harmonic components are lower than the preset threshold.

[0150] Step S34: Based on the location results of the oscillation source, adjust the key parameters in the outer loop voltage control layer and / or inner loop current control layer in a preset priority order. After each adjustment, immediately re-evaluate whether the harmonic components have dropped below the threshold. Iterate in this way until the oscillation is completely suppressed and the system returns to stable operation.

[0151] In summary, the real-time harmonic monitoring in step S31 enables the system to accurately identify risks in the early stages of oscillation (when the harmonic component just exceeds the 2% threshold), avoiding the serious consequence of system shutdown due to oscillation deterioration; the threshold determination in step S32 provides a clear trigger condition for oscillation suppression, ensuring that the suppression action is neither too sensitive (avoiding false triggers) nor too sluggish; the oscillation source localization in step S33 fully utilizes the parameter characteristic information stored in the offline optimization parameter library, enabling it to quickly and accurately determine whether the oscillation is mainly related to the low-voltage port or the high-voltage port based on the current operating status, thus pointing the way for subsequent precise adjustments; step S34 achieves effective oscillation suppression with minimal parameter variation and the fastest convergence speed, and the re-evaluation after each adjustment forms a closed-loop verification, ensuring the controllability and effectiveness of the suppression process.

[0152] To demonstrate the oscillation suppression effect, Figure 14 The low-voltage port oscillation waveform is given. As can be seen from the figure, under this operating condition and control parameters, there is a 201Hz DC oscillation with a 5% harmonic component at the low-voltage port.

[0153] According to the method described in this article, parameters can be modified online. Figure 14 The oscillations are suppressed, such as Figure 15 As shown.

[0154] The above description is merely a specific embodiment of the present invention, enabling those skilled in the art to understand or implement the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the present invention is not to be limited to the embodiments described herein, but is to be accorded the widest scope consistent with the principles and novel features of the invention herein.

Claims

1. A DC collection grid-connected system, characterized in that, include: At least one wind turbine generator, DC collection network, DC transformer, high-voltage DC line and DC converter station; The DC collection network is connected to the wind turbine generator; The DC transformer includes a low-voltage port and a high-voltage port; the low-voltage port is connected to the DC collection network, and the high-voltage port is connected to the high-voltage DC line. The DC converter station is connected to the high-voltage DC line, and the DC converter station is configured to control the high-voltage DC voltage; The controller in the DC transformer is configured to increase the DC voltage at the control collection port and to increase the stability margin of the low-voltage and high-voltage ports of the DC transformer to a preset value.

2. The DC collection grid-connected system according to claim 1, characterized in that, The DC transformer includes: a first bridge arm, a second bridge arm, and a third bridge arm; The first bridge arm and the second bridge arm constitute the low-voltage port; the third bridge arm and the second bridge arm constitute the high-voltage port; the second bridge arm is the bridge arm shared by the low-voltage port and the high-voltage port.

3. The DC collection and grid-connected system according to claim 2, characterized in that, The first bridge arm includes at least one first bridge arm unit, and the first bridge arm unit includes multiple first bridge arm sub-modules, which are connected in series. The second bridge arm includes at least one second bridge arm unit, and the second bridge arm unit includes multiple second bridge arm sub-modules, which are connected in series. The third bridge arm includes at least one third bridge arm unit, and the third bridge arm unit includes multiple third bridge arm sub-modules, which are connected in series.

4. The DC collection and grid-connected system according to claim 3, characterized in that, The controller includes a first arm controller, a second arm controller, and a third arm controller: The first bridge arm controller is used to control the voltage of the low-voltage port and the voltage of the first bridge arm submodule; The second bridge arm controller is used to control the voltage of the second bridge arm; The third bridge arm controller is used to control the voltage of the high-voltage port and the third bridge arm submodule; The first arm controller, the second arm controller, and the third arm controller all include an outer loop control layer and an inner loop control layer; The outer loop voltage control layer is configured to generate a current reference command based on a voltage reference command. The inner loop current control layer is connected to the outer loop voltage control layer and is configured to generate a modulation signal based on the current reference command and output it to the DC transformer.

5. The DC collection and grid-connected system according to claim 4, characterized in that, The controller includes: A parameter storage unit is configured to store control parameters, the control parameters including at least one adjustable parameter in the outer loop voltage control layer and / or the inner loop current control layer; The parameter scheduling unit, connected to the parameter storage unit, is configured to retrieve the corresponding control parameters from the parameter storage unit according to the real-time operating conditions and transmit them to the outer loop voltage control layer and / or the inner loop current control layer.

6. The DC collection and grid-connected system according to claim 5, characterized in that, The controller also includes: The oscillation monitoring unit is configured to monitor in real time the harmonic components in the voltage or current signals of the electrical quantities at the low-voltage port and / or the high-voltage port. An oscillation suppression unit, connected to the oscillation monitoring unit and the parameter scheduling unit, is configured to adjust at least one adjustable parameter in the outer loop voltage control layer and / or the inner loop current control layer when the harmonic component exceeds a preset threshold, until the harmonic component is lower than the preset threshold. The oscillation suppression unit is also configured to determine whether the oscillation is mainly associated with the low-voltage port or the high-voltage port based on the current operating parameters; adjust the adjustable parameters in sequence according to a preset priority order, and re-evaluate the harmonic components after each adjustment.

7. The DC collection and grid-connected system according to claim 1, characterized in that, The stability margin is the phase margin of the low-voltage port relative to the DC collection network and the wind turbine, and the phase margin of the high-voltage port relative to the high-voltage DC line and the DC converter station. The preset value of the stability margin is greater than or equal to 10°.

8. A control method based on a DC collection grid-connected system, characterized in that, The control method, applied to the DC collection grid-connected system as described in any one of claims 1 to 7, comprises: A parameter library for different operating conditions is established, and each set of parameters is configured to ensure that the stability margin of the low-voltage port and high-voltage port of the DC transformer meets the preset value under the corresponding operating condition. The corresponding parameters are retrieved from the parameter library based on the real-time monitored operating conditions, and the operation of the DC transformer is controlled based on the retrieved parameters. The oscillation status of the DC collection grid-connected system is monitored in real time. When an oscillation risk is detected, at least one of the parameters is adjusted until the oscillation is suppressed.

9. The control method according to claim 8, characterized in that, The establishment of a parameter library for different operating conditions, wherein each set of parameters is configured to ensure that the stability margin of the low-voltage port and high-voltage port of the DC transformer meets a preset value under the corresponding operating condition, includes: Construct the two-port impedance model of the DC transformer; Based on the model analysis, the oscillation risk frequency bands of the low-voltage port and the high-voltage port under different operating conditions are analyzed, and control parameters that have an impact on the impedance characteristics of the risk frequency band exceeding a preset threshold are selected and recorded as the dominant control parameters. Using the dominant control parameters as optimization variables, and with the goal of simultaneously satisfying the stability margin requirements of the low-pressure port and the high-pressure port, parameter optimization is performed to obtain multiple sets of control parameters corresponding to different operating conditions, and the parameter library is constructed.

10. The control method according to claim 8, characterized in that, The step of retrieving corresponding parameters from the parameter library based on real-time monitored operating conditions and controlling the operation of the DC transformer based on the retrieved parameters includes: The current reference command is generated based on the voltage reference command through the outer loop voltage control layer; The modulation signal is generated based on the current reference command through the inner loop current control layer and output to the DC transformer.

11. The control method according to claim 8, characterized in that, The real-time monitoring of the oscillation status of the DC collection grid-connected system, and the adjustment of at least one of the parameters until the oscillation is suppressed when an oscillation risk is detected, includes: Real-time monitoring of harmonic components in the voltage or current signals of the low-voltage port and / or the high-voltage port; When the non-power frequency harmonic components exceed the preset threshold, it is determined that there is a risk of oscillation. Based on the current operating parameters and the established parameter library, it is determined that the oscillation is mainly associated with the low-voltage port or the high-voltage port; The parameters are adjusted sequentially according to a preset priority order, and the harmonic components are re-evaluated after each adjustment until the harmonic components are lower than the preset threshold.

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