Energy-coordinated offshore wind power dc energy consumption device modulation voltage optimization method

Abstract

The present application relates to a kind of energy coordination-based offshore wind power DC energy consumption device modulation voltage optimization method, belong to power electronics field, it has solved the problem that existing modulation mode needs to increase the number of DC energy consumption device submodule for maintaining half-bridge submodule capacitor energy balance.The method comprises: when detecting that the shore receiving end ac power grid occurs ac short-circuit fault, REC reduces the external DC voltage of REC by energy decoupling control;The energy consumption bridge arm of HBSM-DCC is converted from the blocking state to the insertion state, and the controllable transfer energy in the energy consumption bridge arm of HBSM-DCC is buffered to the submodule capacitor of REC;After HBSM-DCC pre-discharge ends, energy coordination control between HBSM-DCC and REC is started, and the centralized energy consumption resistance of HBSM-DCC is driven to be periodically switched on and off;When detecting that ac power grid fault is cleared, control both to exit fault ride-through control.

Description

Technical Field

[0001] This invention relates to the field of power electronics technology, and in particular to a method for optimizing the modulation voltage of offshore wind power DC energy-consuming devices based on energy coordination. This method is applicable to the control of half-bridge modular centralized DC energy-consuming devices in high-voltage DC transmission systems based on modular multilevel converters. Background Technology

[0002] Currently, high-voltage direct current (HVDC) transmission technology based on modular multilevel converters (MMCs) has become the mainstream solution for large-scale deep-sea wind power transmission. DC choppers (DCCs) are key technological equipment for improving the fault-ride-through (FRT) capability of offshore wind power systems. Due to the difficulty and cost constraints of constructing offshore platforms, DCCs are typically installed on the DC side of the receiving-end converter (REC).

[0003] Wiring diagram of offshore wind power transmission system via two MMC-HVDC terminals is shown below. Figure 1 As shown. Based on the configuration of the centralized power dissipation resistor, DCCs can be divided into three categories: centralized, distributed, and hybrid. Among them, centralized DCCs use natural air cooling, resulting in lower heat dissipation costs than the latter two. Switching-type centralized DCCs achieve power control through direct series connection of multiple switching devices, but suffer from problems such as difficulty in dynamic voltage equalization and large DC-side impact; while modular centralized DCCs use a submodule cascade structure, which not only reduces the dv / dt effect but also has stronger scalability, making them more suitable for HVDC applications. In the comparison between half-bridge submodules (HBSM) and full-bridge submodules, centralized DCCs using HBSMs have been applied to offshore wind power projects such as Borwin3 in Germany and Rudong in Jiangsu Province due to their lower device costs.

[0004] For Half-Bridge Submodule-based DCChoppers (HBSM-DCCs), since the HBSM can only output non-negative voltages, the modulation voltage of the HBSM-DCC needs to be higher than the external DC voltage to maintain capacitor energy balance. Under existing control methods, if the HBSM-DCC uses trapezoidal wave modulation, the peak modulation voltage is 1.256 pu; if it uses sinusoidal wave modulation, the peak modulation voltage reaches as high as 1.367 pu. To modulate a bridge arm voltage higher than the rated value, existing solutions require increasing the number of submodules for the HBSM-DCC. For example, trapezoidal wave modulation and sinusoidal wave modulation require approximately 26% and 37% more submodules, respectively. This significantly increases the hardware cost and size of the DC power consumption device, contradicting the lightweight and low-cost construction requirements of offshore platforms.

[0005] Therefore, exploring an HBSM-DCC modulation voltage peak optimization scheme without increasing additional hardware costs has significant engineering and economic value. Summary of the Invention

[0006] Based on the above analysis, the present invention aims to provide a method for optimizing the modulation voltage of offshore wind power DC energy-consuming devices based on energy coordination, in order to solve the problem that existing modulation methods require increasing the number of DC energy-consuming device submodules to maintain the energy balance of the half-bridge submodule capacitors.

[0007] The objective of this invention is achieved through the following technical solution: This invention provides a method for optimizing the modulation voltage of an offshore wind power DC energy-consuming device based on energy coordination, the method comprising: When an AC short-circuit fault is detected in the onshore receiving-end AC power grid, the REC actively reduces the external DC voltage of the REC to the fault ride-through set value through energy decoupling control. The energy-consuming bridge arm of HBSM-DCC is switched from the locked state to the inserted state. Pre-discharge is performed using the discharge path formed by the voltage of the energy-consuming bridge arm being higher than the external DC voltage. The controllable transferred energy in the energy-consuming bridge arm of HBSM-DCC is buffered in the sub-module capacitor of REC. After the HBSM-DCC pre-discharge is completed, the energy coordination control between the HBSM-DCC and REC is initiated, driving the concentrated energy-consuming resistor of the HBSM-DCC to be periodically switched. When the fault in the onshore AC power grid is detected to be cleared, the HBSM-DCC and REC are controlled to exit fault ride-through control and return to normal operation. In this context, REC represents the receiving-end converter, and HBSM-DCC represents the half-bridge sub-module type centralized DC power consumption device.

[0008] Based on the above solution, the present invention also makes the following improvements: Furthermore, the energy coordination control between HBSM-DCC and REC includes an energy buffering phase and an energy dissipation phase; among which, During the energy caching phase, all sub-modules of the energy-consuming bridge arm of HBSM-DCC are in the inserted state. HBSM-DCC temporarily stores part of the energy of the bridge arm in the sub-module capacitor of REC by discharging. During the energy dissipation phase, all sub-modules of the HBSM-DCC energy-dissipating bridge arm are in a bypass state, and the energy of the REC cache is dissipated through the centralized energy-dissipating resistor.

[0009] Furthermore, the centralized power-dissipating resistor driving the HBSM-DCC is periodically switched on and off, performing the following: The capacitor voltages of all submodules in HBSM-DCC and REC are collected in real time to obtain the total energy of HBSM-DCC and REC. Based on the deviation between the total energy of REC and its reference value, and in combination with the power injected into the DC system and the active power transmitted to the onshore AC power grid, the total surplus factor is determined. The result of limiting the total surplus coefficient is compared with the triangular carrier wave to generate a periodic control signal; Based on the periodic control signal and the sorting result of the capacitor voltage of the HBSM-DCC submodule, the HBSM-DCC drives the centralized energy-consuming resistor of the HBSM-DCC to switch periodically.

[0010] Furthermore, the total surplus coefficient is generated using a composite method of "PI-adjusted output + feedforward estimation": Perform a PI calculation on the difference between the total energy of REC and the reference value of the total energy of REC to obtain the PI calculation result; The surplus factor is estimated for the power injected into the DC system and the active power transmitted to the onshore AC power grid, and the surplus factor estimation results are obtained. The total surplus coefficient is obtained by summing the PI calculation result and the surplus coefficient estimation result.

[0011] Furthermore, the results of the surplus coefficient estimation Represented as: (1) in, This indicates the power injected into the DC system. This represents the active power transmitted to the onshore receiving-end AC power grid. This indicates the rated power dissipation of the HBSM-DCC. This represents the anti-saturation scaling factor.

[0012] Furthermore, the HBSM-DCC drives the centralized energy-dissipating resistor of the HBSM-DCC to periodically switch on and off based on the periodic control signal and the sorting result of the capacitor voltage of the HBSM-DCC submodule, specifically including: When the result of the total surplus factor after limiting is greater than that of the triangular carrier wave, the HBSM-DCC will bypass the power dissipation bridge arm, and the centralized power dissipation resistor will be connected to the DC system to begin dissipating power, periodically controlling the signal. ; Otherwise, the HBSM-DCC will switch the power-dissipating arm to the insertion state, periodically controlling the signal. .

[0013] Furthermore, during the submodule switching process, submodules with higher capacitor voltages are bypassed first, while submodules with lower capacitor voltages are inserted first.

[0014] Furthermore, the HBSM-DCC employs trapezoidal wave modulation, and the peak value of the bridge arm modulation voltage of the HBSM-DCC does not exceed the rated value.

[0015] Furthermore, the method achieves the following control objectives during fault ride-through: The peak value of the bridge arm modulation voltage of the HBSM-DCC does not exceed 1 p.u.; The peak voltage of the submodule capacitor in REC does not exceed 1.1 pu; The peak DC voltage does not exceed 1.1 pu.

[0016] Furthermore, the maximum value of controllable transferred energy stored within the HBSM-DCC bridge arm. Represented as: (2) in, This indicates the submodule capacitor of HBSM-DCC. Indicates the number of submodules in HBSM-DCC. This indicates the rated voltage of the submodule capacitors in the HBSM-DCC. This indicates the multiple of the external DC voltage relative to its rated value.

[0017] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects: The purpose of this invention is to address the problem that existing half-bridge submodule-type centralized DC power dissipation devices require additional submodules to maintain capacitor energy balance, and to provide a method for optimizing the modulation voltage of offshore wind power DC power dissipation devices based on energy coordination. This method, through energy coordination control with the receiving-end converter, achieves the goal of ensuring that the peak value of the DCC modulation voltage does not exceed the rated value, thereby reducing hardware costs.

[0018] (1) Reduce hardware cost: By coordinating energy with REC, the peak modulation voltage of HBSM-DCC can be kept below the rated value. Therefore, only the rated number of half-bridge sub-modules need to be configured, and there is no need to add 26%~37% of the sub-modules. This can significantly reduce the hardware cost and volume of DCC, and is particularly suitable for offshore platform applications where space is limited and cost is sensitive.

[0019] (2) Make full use of existing system resources: During fault ride-through, the REC output power is limited and the voltage fluctuation range of the submodule capacitor is less than the maximum fluctuation range under rated operating conditions. By using the fluctuation margin of the original submodule capacitor voltage of the REC to assist in buffering part of the DCC energy, no additional energy storage equipment or hardware devices are required.

[0020] (3) Clear and reliable control logic: The control logic of "discharge first and then consume energy" is adopted. Through the coordinated control of the two stages of "energy buffer" and "energy dissipation", the peak value of DCC modulation voltage is effectively controlled, while ensuring the energy stability of DC system.

[0021] (4) Fast dynamic response speed of the system: REC adopts energy decoupling control, which breaks the strong coupling relationship between external DC voltage and internal energy of REC in traditional control and enhances the ability to regulate external DC voltage during fault ride-through; DCC adopts a composite control mode of "PI regulation output + feedforward estimation value", which improves the system's response speed to changes in surplus power.

[0022] (5) Multiple safety guarantees: Through the strategy proposed in this invention, three fault ride-through control objectives can be achieved simultaneously: the peak value of the DCC arm modulation voltage does not exceed 1 p.u.; the peak value of the REC submodule capacitor voltage does not exceed 1.1 pu; and the peak value of the DC voltage does not exceed 1.1 pu, ensuring the safe and stable operation of the system during fault ride-through.

[0023] In this invention, the above-described technical solutions can be combined with each other to achieve more preferred combinations. Other features and advantages of this invention will be set forth in the following description, and some advantages may become apparent from the description or be learned by practicing the invention. The objects and other advantages of this invention can be realized and obtained from what is particularly pointed out in the description and drawings. Attached Figure Description

[0024] The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. Throughout the drawings, the same reference numerals denote the same parts. Figure 1 A schematic diagram of the structure of an offshore wind power transmission system via MMC-HVDC at both ends; Figure 2A flowchart illustrating the method for optimizing the modulation voltage of an offshore wind power DC energy-consuming device based on energy coordination, as provided in an embodiment of the present invention. Figure 3 Another flowchart of the modulation voltage optimization method for offshore wind power DC energy consumption device based on energy coordination provided in the embodiments of the present invention; Figure 4 This is a schematic diagram of energy flow and equivalent control of REC provided in an embodiment of the present invention; Figure 5 The REC equivalent model based on energy decoupling control provided in this embodiment of the invention; Figure 6 This is a block diagram of the energy decoupling control of REC provided in an embodiment of the present invention; Figure 7 A schematic diagram of the energy flow path provided in an embodiment of the present invention; Figure 8 This is a schematic diagram of energy PWM control for HBSM-DCC provided in an embodiment of the present invention; Figure 9 A block diagram of a closed-loop DCC energy control system provided in an embodiment of the present invention; Figure 10 This is a structural diagram of the HBSM-DCC provided in an embodiment of the present invention; Figure 11 The equivalent circuit of HBSM-DCC provided in the embodiments of the present invention; Figure 12 The waveform diagram of the trapezoidal wave modulated voltage provided in the embodiment of the present invention. Detailed Implementation

[0025] Preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings, which form part of this application and are used together with the embodiments of the present invention to illustrate the principles of the present invention, but are not intended to limit the scope of the present invention.

[0026] A specific embodiment of the present invention discloses a method for optimizing the modulation voltage of an offshore wind power DC energy-consuming device based on energy coordination. This method is applicable to offshore wind power transmission systems via two-terminal MMC-HVDC converters. A schematic diagram of such a system is shown below. Figure 1 As shown in the figure. In this embodiment, the offshore wind power DC energy dissipation device adopts a half-bridge submodule type centralized DC energy dissipation device (HBSM-DCC). The HBSM-DCC includes an energy dissipation bridge arm formed by cascading half-bridge submodules and a centralized energy dissipation resistor. The flowchart of this method is shown below. Figure 2 and Figure 3 As shown, the specific explanation is as follows.

[0027] Step S1: When an AC short-circuit fault is detected in the onshore receiving-end AC power grid, the REC actively reduces the external DC voltage of the REC to the fault ride-through set value through energy decoupling control.

[0028] During implementation, the voltage of the onshore receiving-end AC power grid is collected in real time. Once the voltage drops below 0.9 pu, it can be determined that an AC short-circuit fault has occurred in the onshore receiving-end AC power grid. At this time, the REC and HBSM-DCC are controlled to enter fault ride-through mode.

[0029] In the offshore wind power transmission system via MMC-HVDC at both ends, the REC is responsible for maintaining the stability of the external DC voltage. However, during fault ride-through, surplus power accumulates in the DC system, and the REC's ability to actively reduce the external DC voltage using traditional control methods is relatively limited. Therefore, in this embodiment, the REC employs energy decoupling control to actively lower its external DC voltage (i.e., the DC bus voltage at the REC's DC side port) to enhance its ability to regulate the external DC voltage during faults, reserving space for subsequent reduction of the HBSM-DCC modulation voltage peak.

[0030] For HBSM-DCC, the discharge of submodule capacitors within its bridge arms depends on the bridge arm modulation voltage being higher than the external DC voltage. Therefore, under the constraint that the bridge arm modulation voltage does not exceed 1 pu, reducing the external DC voltage becomes an effective way to promote capacitor discharge. REC employs energy decoupling control to enhance the ability to regulate the external DC voltage during fault ride-through. By reducing the external DC voltage during the fault ride-through phase, HBSM-DCC can release some energy before the centralized energy dissipation resistor is connected, reserving a safety margin for potential submodule charging during subsequent operations.

[0031] REC ( Take the modulation functions of the upper and lower arms of phases a, b, and c) , Decomposed into common-mode components , differential mode component They are represented as follows: (1) REC Common mode components of the upper and lower bridge arms of the phase , differential mode component It can also be further represented as the superposition of DC, fundamental frequency, and harmonic components to achieve different control objectives, as shown below: (2) in, , These represent the common-mode DC modulation and differential-mode DC modulation of REC, respectively. The common-mode DC modulation of each phase is the same, and the differential-mode DC modulation of each phase is the same. , These represent the first and second parts of REC. Common-mode fundamental frequency modulation scheme and common-mode second harmonic frequency modulation scheme, , These represent the first and second parts of REC. The differential-mode DC modulation, differential-mode fundamental frequency modulation, and differential-mode second harmonic modulation are defined. In practice, each modulation scheme can control a target variable. In this embodiment, the common-mode DC components of the three phases are set to be the same to ensure that the three phases are identical from the DC side perspective.

[0032] Figure 4 This diagram illustrates the energy flow and equivalent control of a REC (Recessed Electronic Control System), showing the composition of the modulation functions of each bridge arm and the related energy flow paths. In the control framework of a traditional REC, the differential-mode fundamental frequency modulation... Used to control external DC voltage, common-mode second harmonic modulation. Used to suppress circulating current. Common-mode DC modulation. Typically unused, this is manifested in the fact that the number of submodules that remain in the active state for each phase is always the rated value. Traditional control frameworks tightly couple external DC voltage with the internal energy of the REC, reducing the DC system's resilience to AC-side disturbances. When a fault occurs in the onshore AC grid, the REC's ability to regulate external DC voltage is limited, and it must rely on other devices in the DC system (such as DCC) to maintain the stability of the external DC voltage.

[0033] However, REC, with its modular multilevel structure and multi-degree-of-freedom control, possesses stronger fault ride-through control potential. Over a longer timescale, the internal energy of REC remains stable, and its external characteristics show that the injected DC power equals the output AC power. From a short-term perspective, the equivalent capacitance within REC (…) This can be considered as an energy storage unit. Energy transfer between the AC and DC sides needs to be achieved through the charging and discharging of this capacitor. The equivalent capacitor voltage of REC is... This reflects the energy stored inside the REC. The equivalent model of the REC based on energy decoupling control is as follows: Figure 5 As shown. The internal capacitors of the REC buffer power changes on both the AC and DC sides, making the control on both sides almost independent, thus achieving decoupling. When the internal energy of the REC is maintained within the allowable range, the top-level objective can be achieved through common-mode and differential-mode modulation, such as using a common-mode DC modulation. Control the external DC voltage and use differential mode fundamental frequency modulation. Control the total energy of the system.

[0034] The variables in the REC equivalent model also follow the power conservation relationship, as shown in equations (3) and (4): (3) (4) in, These refer to the power injected into the DC side of REC, which is the power of offshore wind power that is rectified by the sending-end converter and then transmitted to the DC side of REC through DC transmission lines; The active power transmitted from REC to the onshore receiving-end AC grid is given, while neglecting the internal losses of REC. The equivalent input current on the DC side of REC represents the equivalent value of the DC current injected into REC by offshore wind power through a DC line. The equivalent output current on the DC side of the REC represents the equivalent value of the AC current injected into the onshore receiving-end AC grid after the REC is converted. Indicating the first in the onshore receiving-end AC power grid Phase current, This represents the current flowing into the DC side of REC; Represents the total energy of REC. This represents the capacitance of each submodule within REC. This indicates the rated number of submodules in a single bridge arm within the REC.

[0035] In the energy decoupling control architecture, REC can be regarded as an energy buffer unit, which not only enhances its robustness to AC side disturbances but also improves the external DC voltage. The dynamic response speed is excellent. Especially during fault ride-through, this decoupled control enables REC to maintain regulation of the external DC voltage. This ensures the effectiveness of the DCC pre-discharge process, thus providing an effective energy buffer. The energy decoupling control block diagram of REC is shown below. Figure 6 As shown, the top-level control objective of REC includes a reference value for the total energy of REC. Reference values ​​for reactive power and reference value of external DC voltage And respectively through differential mode d-axis adjustment. Differential mode q-axis adjustment system and common-mode DC modulation The differential mode d-axis and q-axis modulation is obtained by synchronous rotation transformation of the three-phase differential mode fundamental frequency modulation. , These represent the d-axis and q-axis components of the three-phase grid voltage in the onshore receiving-end AC power grid. L It is the equivalent inductance on the AC side of REC.

[0036] The REC has two control modes: normal operation and fault ride-through. During normal operation, the three control objectives mentioned above can be adjusted independently and flexibly. During fault ride-through, the REC actively reduces its external DC voltage to the fault ride-through setpoint through the following energy decoupling control: Switch the differential d-axis and differential q-axis modulation of REC to constant current control mode (reference values ​​are respectively). and ), and switch the DC voltage reference value to the fault ride-through setting value ( The control strategies, such as circulation control, upper and lower bridge arms, and interphase energy balance, are similar to those in existing research and will not be elaborated here.

[0037] Accordingly, in step S1 of this embodiment, the energy decoupling control of REC is achieved by decomposing the modulation function of REC into common-mode components and differential-mode components: the control of the external DC voltage is completed by using the common-mode DC modulation, and the regulation of the total energy of the system is achieved by using the differential-mode fundamental frequency modulation; at the same time, a reference value switching, voltage feedforward and PI controller clearing mechanism are introduced in the DC voltage control loop to improve the control response speed of the DC voltage.

[0038] Step S2: Change the power dissipation bridge arm of HBSM-DCC from the locked state to the inserted state, and use the discharge path formed by the power dissipation bridge arm voltage being higher than the external DC voltage to perform pre-discharge, and buffer the controllable transferred energy in the power dissipation bridge arm of HBSM-DCC into the sub-module capacitor of REC.

[0039] Existing methods rely on maintaining the external DC voltage near its rated value during fault ride-through, achieving this by increasing the modulation voltage of the HBSM-DCC. In this embodiment, however, the reverse current consumption can be achieved through another approach: actively reducing the external DC voltage while ensuring the peak value of the HBSM-DCC modulation voltage does not exceed its rated value. From an energy perspective, active voltage reduction is the technical method for energy transfer from the HBSM-DCC, but energy storage still relies on other devices in the external DC system. Since the output power of the REC is limited during fault ride-through, the fluctuation range of its submodule capacitor voltage is lower than the design limit. Therefore, the fluctuation margin of the REC submodule capacitor voltage can be used to temporarily store some of the HBSM-DCC's energy.

[0040] In practice, when the HBSM-DCC enters fault ride-through mode, the bridge arm of the HBSM-DCC is switched from the locked state to the inserted state, causing energy to flow out of the internal submodule capacitors. Furthermore, when the bridge arm modulation voltage of the HBSM-DCC is higher than the external DC voltage, the energy-consuming current is forced to reverse to balance the bridge arm energy, and the half-bridge submodule capacitors release energy to the REC side through the discharge path. Therefore, the energy release step of the HBSM-DCC occurs before the dissipation of surplus power, and the energy release of the HBSM-DCC is achieved by actively reducing the DC voltage and changing the HBSM-DCC bridge arm to the inserted state during fault ride-through.

[0041] This embodiment introduces pre-discharge control to reserve energy margin for the charging process caused by subsequent switching, keeping the peak modulation voltage of the HBSM-DCC bridge arm within the rated value. Therefore, simply reducing the external DC voltage allows the HBSM-DCC to pre-discharge. The pre-discharge process continues until the HBSM-DCC submodule capacitor voltage drops to a level matching the external DC voltage due to discharge, the voltage difference disappears, the discharge current decays to zero, and the pre-discharge process naturally ends.

[0042] In this embodiment, the controllable energy transfer is determined by the time when the HBSM-DCC enters energy PWM control and the reduced DC voltage level.

[0043] Maximum controlled transfer energy stored within the HBSM-DCC bridge arm Represented as: (5) in, This indicates the submodule capacitor of HBSM-DCC. Indicates the number of submodules in HBSM-DCC. This indicates the rated voltage of the submodule capacitors in the HBSM-DCC. This indicates the multiple of the external DC voltage relative to its rated value; in this embodiment, it is taken as 0.9.

[0044] Formula (5) quantitatively describes the magnitude of the REC active voltage reduction in step S1. The mathematical relationship between the transferable energy of the HBSM-DCC pre-discharge in step S2 and the energy of the pre-discharge: When The smaller the value (i.e., the greater the drop in REC pressure). The larger the value, the more energy the HBSM-DCC arm releases, and the greater the energy margin reserved for subsequent energy consumption phases; when When =1 (i.e., REC does not reduce voltage), =0, HBSM-DCC cannot release energy through pre-discharge, which is the operating condition of the existing technical solution—it is necessary to increase the modulation voltage by adding sub-modules to maintain the energy balance of the bridge arms; when When the voltage is 0.9 (in this embodiment), the HBSM-DCC can release 19% of the total energy. Combined with the "discharge first, then consume energy" control logic, this is sufficient to ensure that the HBSM-DCC achieves bridge arm energy balance under the condition that the modulation voltage does not exceed 1 p.u. This quantitative relationship provides mathematical support and theoretical basis for the present invention's "fault ride-through without adding sub-modules".

[0045] Step S3: After the HBSM-DCC pre-discharge is completed, the energy coordination control between HBSM-DCC and REC is started, driving the concentrated energy-consuming resistor of HBSM-DCC to be periodically switched.

[0046] In this embodiment, a "discharge first, then dissipate energy" control logic is adopted. Through coordinated control of the "energy buffering" and "energy dissipation" stages, DC system energy stability is achieved. Specifically, the energy coordination control between HBSM-DCC and REC consists of two parts: HBSM-DCC energy control and REC coordination control. REC coordination control assists HBSM-DCC in completing pre-discharge before initiating energy dissipation. Specifically, the energy coordination control between HBSM-DCC and REC executes as follows: HBSM-DCC uses the total energy of REC as the top-level direct control target and the bridge arm energy of HBSM-DCC as the indirect control target. It generates periodic control signals through a pulse width modulation (PWM) strategy with adjustable duty cycle, driving the centralized energy dissipation resistor of HBSM-DCC to periodically switch on and off to dissipate the surplus power of the DC system.

[0047] In the energy coordination control of DCC, the primary target of DCC control is not its own energy, but rather the energy of REC. The energy of REC is the direct control target, while the energy of DCC itself is used as an indirect control target (in fact, the energy control of DCC is automatically achieved by REC reducing the DC voltage). A schematic diagram of the energy flow path is shown below. Figure 7 As shown. Energy coordination control mainly includes two energy flow processes: energy buffering and energy dissipation; where: 1) Before each power-consuming resistor is engaged, the energy buffering phase is entered. The conditions required to enter this phase are: Gctrl=0, all sub-modules of the DCC bridge arm are in the inserted state, and the DC voltage has been reduced.

[0048] During the energy caching phase, DCC does not start energy-consuming operations, but instead temporarily stores part of the bridge arm energy in the submodule capacitor of REC by discharging.

[0049] 2) When Gcrtl=1, the energy dissipation stage begins. The controller uses a PI controller and a triangular carrier wave to generate periodic control signals. Specifically, the condition for entering the energy dissipation stage is: Gcrtl=1, and all DCC submodules are in bypass mode.

[0050] During the energy dissipation phase, DCC initiates energy-consuming operations, while the energy previously buffered by REC is dissipated through centralized energy-dissipating resistors. In both of these processes, U dc All remained at levels slightly below the rated values.

[0051] During the energy buffering phase, all DCC bridge arm submodules are in the inserted state; while during the energy dissipation phase, all DCC bridge arm submodules are in the bypass state. Therefore, both the stable energy buffering phase and the stable energy dissipation phase are steady-state processes. In addition to these two steady-state processes, the DCC also undergoes two transient processes with different state transitions: energy consumption input and energy consumption output. The DCC bridge arm energy gradually increases during the transient period and gradually recovers after energy consumption output by discharging into the DC system. The transient process is a switching process between different steady-state phases: the energy consumption output process is when the DCC bridge arm submodules gradually switch from all bypassed states to all inserted states. The energy consumption input process is when the DCC bridge arm submodules gradually switch from all inserted states to all bypassed states.

[0052] Step S31: Real-time acquisition of capacitor voltages of all sub-modules in HBSM-DCC and REC to obtain the total energy of HBSM-DCC and REC, and to determine the power injected into the DC system and the active power transmitted to the onshore AC power grid.

[0053] Total energy of HBSM-DCC Represented as: (6) in, Indicates the first in HBSM-DCC The capacitor voltage of each submodule.

[0054] Total energy of REC Represented as: (7) in, This indicates the capacitor of the REC submodule. Indicates the rated number of submodules in a single bridge arm of the REC; , They are REC The middle section of the upper and lower bridge arms i The capacitor voltage of each submodule.

[0055] In addition, the power injected into the DC system can be calculated based on the current injected into the DC line (the DC current at the DC side outlet of the sending-end converter) and the DC line voltage (the DC voltage at the DC side outlet of the sending-end converter).

[0056] Power injected into the DC system Represented as: (8) in, Indicates the DC line voltage. This indicates the current injected into the DC line.

[0057] Calculate the active power transmitted to the onshore AC power grid based on the three-phase voltage and three-phase current of the onshore receiving-end AC grid.

[0058] It should be noted that the DCC control structure is mainly divided into two layers. The lowest layer is the power switching control of semiconductor devices and the voltage balancing control between sub-modules, while the top layer is the energy closed-loop control, which is responsible for realizing energy coordination between DCC and REC. For the specific implementation process, please refer to steps S32 and S33.

[0059] Step S32: Based on the deviation between the total energy of REC and its reference value, and combined with the power injected into the DC system and the active power transmitted to the onshore AC power grid, determine the total surplus coefficient; compare the result of the total surplus coefficient after limiting with the triangular carrier wave to generate a periodic control signal.

[0060] In this embodiment, when the total energy of REC Increase to the set value or the total energy of DCC If the value drops to the set value, the HBSM-DCC energy control will be activated.

[0061] It's important to note that the energy control here uses a two-condition OR logic (REC total energy exceeds the upper limit or DCC energy exceeds the lower limit). Before entering DCC energy control, DCC is in an energy buffering phase. After entering energy control, there will be periodic energy buffering and energy dissipation switching. Furthermore, the parameter sets the upper limit requirement for REC energy, such as setting the REC total energy limit to 1.05 pu. The total DCC energy is related to the discharge level, and can be set to 0.9 pu.

[0062] That is, HBSM-DCC startup is based on The energy PWM control mode. Specifically, the energy control of DCC mainly uses the total energy of REC. As the controlled object, the energy PWM control schematic diagram of HBSM-DCC is as follows: Figure 8 As shown, the closed-loop system block diagram of HBSM-DCC energy control is as follows: Figure 9 As shown. In this embodiment, to improve the dynamic response speed, the total surplus coefficient adopts a composite generation method of "PI control output + feedforward estimation value". The PI controller is used to accurately track the reference value of the total REC energy, and the feedforward estimation value is used to provide the static operating point for the system, improving the response speed of the closed-loop system to the control target. The specific energy PWM control scheme is as follows.

[0063] Total energy of REC Reference value of total energy of REC The difference is used to perform a PI operation to obtain the PI result.

[0064] Power injected into the DC system and active power transmitted to the onshore receiving-end AC power grid The earnings ratio is estimated, and the earnings ratio estimation results are obtained. .

[0065] Surplus coefficient estimation results Represented as: (9) in, Indicates surplus power. This indicates the rated power dissipation of the HBSM-DCC. This represents the anti-saturation scaling factor.

[0066] Results of PI calculation and surplus factor estimation Summing them up yields the total surplus coefficient. .

[0067] Total surplus factor (or the duty cycle of centralized energy-consuming resistors) After the limiting stage, the result is compared with the triangular carrier wave. When the total surplus coefficient after limiting is greater than that of the triangular carrier wave, the HBSM-DCC bypasses the power dissipation bridge arm. At this point, the centralized power dissipation resistor is directly connected to the DC system to dissipate power, i.e., the periodic control signal. Conversely, HBSM-DCC switches the power-consuming bridge arm to the insertion state, periodically controlling the signal. .

[0068] As can be seen from the above, in this embodiment, the controller will... Compared with the triangular carrier wave, a periodic control signal is generated. This drives the switching of the HBSM-DCC. The control objective of the HBSM-DCC is to maximize the total REC energy. Stabilized at the reference value Nearby, while ensuring that the arm energy of the HBSM-DCC does not exceed the rated value.

[0069] Step S33: The HBSM-DCC drives the centralized energy-consuming resistor of the HBSM-DCC to switch periodically according to the periodic control signal and the sorting result of the capacitor voltage of the HBSM-DCC submodule.

[0070] In specific implementation, HBSM-DCC employs trapezoidal wave modulation to complete the switching operations of all submodules based on the periodic control signal and the HBSM-DCC submodule capacitor voltage sorting results. The trapezoidal wave modulation of HBSM-DCC satisfies the following: all submodule voltages of HBSM-DCC are sorted; when a bridge arm needs to be bypassed, the submodule with the higher capacitor voltage is bypassed first; when a bridge arm needs to be inserted, the submodule with the lower capacitor voltage is inserted first. That is, the submodule switching operation in step S3 follows these principles: during stepped power consumption, the submodule with the higher capacitor voltage is bypassed first; during stable power consumption, all submodules are in a bypass state; during stepped power withdrawal, the submodule with the lower capacitor voltage is inserted first into the DC system.

[0071] When all submodules of the HBSM-DCC are connected to a DC system, the per-unit value of the bridge arm energy and the modulation voltage is the same. Under trapezoidal wave modulation, both the peak value of the bridge arm energy and the peak value of the modulation voltage occur when the energy dissipation current decays to 0 after the centralized energy dissipation resistor is removed.

[0072] The structural diagram of HBSM-DCC is as follows: Figure 10 As shown, it mainly includes: A control bridge arm composed of HBSMs connected in series and a centralized power dissipation resistor The equivalent circuit of HBSM-DCC is as follows: Figure 11 As shown, it changes the modulation voltage of the control bridge arm. Realize centralized energy dissipation resistor Investment and exit: When Equal to external DC voltage When the power dissipation is zero; when When the value is 0, the power dissipation reaches its maximum.

[0073] HBSM-DCC power consumption current Represented as: (10) Due to the structural characteristics of HBSMs, HBSM-DCC requires an external DC system to establish a discharge path. Because there is a time difference between the switching of each HBSM in the control arm of HBSM-DCC, the capacitors in the HBSMs will charge during the switching on and off of HBSM-DCC.

[0074] This embodiment takes the offshore wind power transmission system via two MMC-HVDC terminals as the application background and HBSM-DCC as the core research object. It analyzes the energy balance constraints of the DCC arms under trapezoidal wave modulation, as shown in equations (11) and (12). To ensure the safe and stable operation of the HBSM-DCC, the modulation voltage of the arms is controlled. Must meet unit period Internal control arm energy balance, i.e., the average power of the control arm within a unit cycle. A value of 0 indicates: (11) Since the HBSM cannot output a negative voltage, the DCC bridge arm can only modulate a non-negative voltage. Therefore, the energy balance of the DCC bridge arm needs to be achieved through the energy-consuming current. The reverse implementation, i.e., the modulation voltage is higher than the external DC voltage.

[0075] To ensure energy stability in the DC system, the modulation voltage is... The power dissipation of the DCC per unit period should also be satisfied. It equals the system surplus power, which means it must satisfy constraint (12).

[0076] (12) In the formula, , These represent the power injected into the DC system by the wind power and the active power sent from the REC to the onshore receiving-end AC grid, respectively, while neglecting the internal losses of the REC.

[0077] In the specific implementation process, different modulation methods correspond to different modulation voltages, but all must simultaneously satisfy the constraints of equations (11) and (12), otherwise DCC and DC system will face the risk of overvoltage.

[0078] The waveform of the trapezoidal wave modulated voltage is as follows Figure 12 As shown, Figure 12 middle This refers to the ramp time of the trapezoidal wave. DCC can be used to change the peak time of the trapezoidal wave. It enables adaptive following of dissipation for different surplus power.

[0079] Rated number of submodules Defined as when the rated DC voltage is The number of submodules connected to the HVDC system via the DCC bridge arm is expressed as follows: (13) At the rated submodule capacitor voltage Below, the peak value of the trapezoidal wave modulated voltage actually depends on the number of sub-modules configured in the DCC. , is represented as: (14) The standard start criterion for DCC is Exceeding the set upper limit. To maintain the energy balance of the bridge arms in a half-bridge centralized DCC system, existing solutions... All must be higher than 1 pu, which means the required number of submodules in the DCC configuration. Greater than For example, for trapezoidal wave modulation and sinusoidal wave modulation under existing control strategies, the number of sub-modules configured in DCC is 1.256 respectively. and 1.367 If the peak modulation voltage can be reduced to within the rated value, then the DCC only needs to be configured with the rated number of sub-modules to meet the energy balance constraint.

[0080] The energy balance relationship of the DC system is shown in formula (15), where the surplus factor is related to the duty cycle of the energy-consuming resistor. The relationship between them is shown in equation (16).

[0081] (15) (16) in, , These represent the switching cycle of the energy-consuming resistor and the time invested within a unit cycle, respectively. This represents the energy increment of REC.

[0082] In this embodiment, the HBSM-DCC employs a trapezoidal wave modulation strategy with adjustable duty cycle. Its duty cycle is adaptively matched to different surplus power levels via a PI controller. When the ramp time of the trapezoidal wave is much shorter than the control period, the average value of the modulation voltage of the HBSM-DCC arm within one unit period can be approximately expressed as shown in equation (17). When the HBSM-DCC uses an energy-consuming input duty cycle of... After the modulation voltage is activated, the average power dissipation of the HBSM-DCC is... Average charging power of REC and REC total energy The parsing expressions are respectively expressed as: (17) (18) in, The initial value of the total REC energy before control.

[0083] Equation (18) represents the physical model of the relationship between the power dissipation of HBSM-DCC and the total energy of REC.

[0084] The duty cycle of the HBSM-DCC controller can be calculated from this physical model. The calculation method is shown in equations (19) and (20).

[0085] (19) (20) According to equation (20), the duty cycle The relationship between the total energy change rate of REC and the total energy of REC contains a nonlinear term. To control the total energy of REC using a linear controller (such as a PI controller), the relationship in equation (20) needs to be linearized. Assume the static operating point of the duty cycle is... Then the duty cycle Expandable to and disturbance quantity d The superposition form is shown in equation (21). Substituting equation (21) into equation (20), we obtain relation (22). Ignoring second-order nonlinear terms ( d 2 (Item), sorted out d and The approximate relationship between them is (23).

[0086] (twenty one) (twenty two) (twenty three) In equation (23), d and , , The variables are correlated. Under trapezoidal wave modulation, the peak value of the modulation voltage... This occurs after the HBSM-DCC power dissipation resistor is removed. It happens when all submodules of the HBSM-DCC bridge arm are connected to the DC system. Ultimately, it will be clamped to... At the same level. At this point, equation (23) can be further simplified to the form of equation (24). In equation (24), d and Since it exhibits a negative correlation, a PI controller as shown in equation (25) can be designed to track the reference value of the total REC energy. .in, , These are the proportional and integral coefficients of the PI controller, respectively.

[0087] (twenty four) (25) For the static operating point of the duty cycle One can choose a value near the surplus coefficient, such as by calculating according to formula (26), where the coefficient is... The value range is 0.9 to 1.

[0088] (26) In this embodiment, the HBSM-DCC employs trapezoidal wave modulation with adjustable duty cycle to match different surplus power requirements, thereby achieving the explicit control objective of REC total energy tracking the reference value. From a system perspective, the energy coordination between HBSM-DCC and REC not only achieves DC voltage control during fault ride-through but also controls the peak value of the HBSM-DCC modulation voltage and the total energy of REC.

[0089] Step S4: After the fault in the onshore AC power grid is detected to be cleared, control HBSM-DCC and REC to exit fault ride-through control and return to normal operation.

[0090] Through the strategy proposed in this embodiment, the coordinated control of HBSM-DCC and REC achieves three fault ride-through control objectives: the peak value of the DCC bridge arm modulation voltage does not exceed 1 pu; the peak value of the REC submodule capacitor voltage does not exceed 1.1 pu; and the peak value of the DC voltage does not exceed 1.1 pu.

[0091] In summary, the energy coordination-based modulation voltage optimization method for offshore wind power DC energy dissipation devices provided in this embodiment addresses the issue of DCC entering fault ride-through control mode when a fault occurs in the onshore receiving-end AC grid and the total energy of the receiving-end converter (REC) exceeds a set value. This mode employs PWM control based on the total energy of the receiving-end converter, periodically switching the centralized energy dissipation resistor to maintain stable total energy of the REC. The proposed DCC modulation voltage peak optimization strategy, based on energy coordination control with the REC, utilizes a "discharge first, dissipate later" control logic for the DCC, achieving DCC arm energy balance at a lower cost than existing solutions. During AC faults, instead of directly switching the centralized energy dissipation resistor, the DC voltage is proactively reduced to pre-release some arm energy from the DCC; even if subsequent switching still leads to arm charging, the modulation voltage peak remains within the rated range.

[0092] Those skilled in the art will understand that all or part of the processes of the methods described in the above embodiments can be implemented by a computer program instructing related hardware, and the program can be stored in a computer-readable storage medium. The computer-readable storage medium may be a disk, optical disk, read-only memory, or random access memory, etc.

[0093] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for optimizing the modulation voltage of offshore wind power DC energy-consuming devices based on energy coordination, characterized in that, The method includes: When an AC short-circuit fault is detected in the onshore receiving-end AC power grid, the REC actively reduces the external DC voltage of the REC to the fault ride-through set value through energy decoupling control. The energy-consuming bridge arm of HBSM-DCC is switched from the locked state to the inserted state. Pre-discharge is performed using the discharge path formed by the voltage of the energy-consuming bridge arm being higher than the external DC voltage. The controllable transferred energy in the energy-consuming bridge arm of HBSM-DCC is buffered in the sub-module capacitor of REC. After the HBSM-DCC pre-discharge is completed, the energy coordination control between the HBSM-DCC and REC is initiated, driving the concentrated energy-consuming resistor of the HBSM-DCC to be periodically switched. When the fault in the onshore AC power grid is detected to be cleared, the HBSM-DCC and REC are controlled to exit fault ride-through control and return to normal operation. In this context, REC represents the receiving-end converter, and HBSM-DCC represents the half-bridge sub-module type centralized DC power dissipation device.

2. The method for optimizing the modulation voltage of offshore wind power DC energy-consuming devices based on energy coordination according to claim 1, characterized in that, Energy coordination control between HBSM-DCC and REC includes an energy buffering phase and an energy dissipation phase; among which, During the energy caching phase, all sub-modules of the energy-consuming bridge arm of HBSM-DCC are in the inserted state. HBSM-DCC temporarily stores part of the energy of the bridge arm in the sub-module capacitor of REC by discharging. During the energy dissipation phase, all sub-modules of the HBSM-DCC energy-dissipating bridge arm are in a bypass state, and the energy of the REC cache is dissipated through the centralized energy-dissipating resistor.

3. The method for optimizing the modulation voltage of offshore wind power DC energy-consuming devices based on energy coordination according to claim 2, characterized in that, The concentrated energy-dissipating resistor driving the HBSM-DCC is periodically switched on and off, performing the following: The capacitor voltages of all submodules in HBSM-DCC and REC are collected in real time to obtain the total energy of HBSM-DCC and REC. Based on the deviation between the total energy of REC and its reference value, and in combination with the power injected into the DC system and the active power transmitted to the onshore AC power grid, the total surplus factor is determined. The result of limiting the total surplus coefficient is compared with the triangular carrier wave to generate a periodic control signal; Based on the periodic control signal and the sorting result of the capacitor voltage of the HBSM-DCC submodule, the HBSM-DCC drives the centralized energy-consuming resistor of the HBSM-DCC to switch periodically.

4. The method for optimizing the modulation voltage of offshore wind power DC energy-consuming devices based on energy coordination according to claim 3, characterized in that, The total surplus coefficient is generated using a composite method of "PI-adjusted output + feedforward estimation". Perform a PI calculation on the difference between the total energy of REC and the reference value of the total energy of REC to obtain the PI calculation result; The surplus factor is estimated for the power injected into the DC system and the active power transmitted to the onshore AC power grid, and the surplus factor estimation results are obtained. The total surplus coefficient is obtained by summing the PI calculation result and the surplus coefficient estimation result.

5. The method for optimizing the modulation voltage of offshore wind power DC energy-consuming devices based on energy coordination according to claim 4, characterized in that, Surplus coefficient estimation results Represented as: (1) in, This indicates the power injected into the DC system. This represents the active power transmitted to the onshore receiving-end AC power grid. This indicates the rated power dissipation of the HBSM-DCC. This represents the anti-saturation scaling factor.

6. The method for optimizing the modulation voltage of offshore wind power DC energy-consuming devices based on energy coordination according to claim 5, characterized in that, The HBSM-DCC drives the centralized energy-dissipating resistor of the HBSM-DCC to periodically switch on and off based on the periodic control signal and the sorting result of the capacitor voltage of the HBSM-DCC submodule, specifically including: When the result of the total surplus factor after limiting is greater than that of the triangular carrier wave, the HBSM-DCC will bypass the power dissipation bridge arm, and the centralized power dissipation resistor will be connected to the DC system to begin dissipating power, periodically controlling the signal. ; Otherwise, the HBSM-DCC will switch the power-dissipating arm to the insertion state, periodically controlling the signal. .

7. The method for optimizing the modulation voltage of offshore wind power DC energy-consuming devices based on energy coordination according to claim 6, characterized in that, During the submodule switching process, submodules with higher capacitor voltages are bypassed first, while submodules with lower capacitor voltages are inserted first.

8. The method for optimizing the modulation voltage of offshore wind power DC energy-consuming devices based on energy coordination according to claim 7, characterized in that, The HBSM-DCC uses trapezoidal wave modulation, and the peak value of the bridge arm modulation voltage of the HBSM-DCC does not exceed the rated value.

9. The method for optimizing the modulation voltage of offshore wind power DC energy-consuming devices based on energy coordination according to any one of claims 1-8, characterized in that, The method achieves the following control objectives during fault ride-through: The peak value of the bridge arm modulation voltage of the HBSM-DCC does not exceed 1 p.u.; The peak voltage of the submodule capacitor in REC does not exceed 1.1 pu; The peak DC voltage does not exceed 1.1 pu.

10. The method for optimizing the modulation voltage of offshore wind power DC energy-consuming devices based on energy coordination according to claim 9, characterized in that, Maximum controlled transfer energy stored within the HBSM-DCC bridge arm Represented as: (2) in, This indicates the submodule capacitor of HBSM-DCC. Indicates the number of submodules in HBSM-DCC. This indicates the rated voltage of the submodule capacitors in the HBSM-DCC. This indicates the multiple of the external DC voltage relative to its rated value.

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