Full-direct-current offshore wind power system zero-voltage ride-through method, equipment and medium

Abstract

The invention discloses a full-direct-current offshore wind power system zero-voltage ride-through method, equipment and a medium, and relates to the field of full-direct-current offshore wind power. The objective of the invention is to solve the problem that a receiving-end converter station cannot transmit power when a land AC power grid breaks down. When the high-voltage direct-current voltage of the offshore direct-current transformer exceeds the first preset threshold value, the medium-voltage direct-current voltage control of the offshore direct-current transformer is switched to the high-voltage direct-current voltage control, so that the offshore direct-current transformer absorbs power from a high-voltage direct-current link; when the medium-voltage direct-current voltage of the fan direct-current transformer exceeds a second preset threshold value, the fan direct-current transformer is switched from low-voltage direct-current voltage control to medium-voltage direct-current voltage control, so that the fan direct-current transformer can absorb power from the low-voltage direct-current voltage side through a fan grid-side direct-current transformer; and when the low-voltage direct-current voltage of the offshore direct-current transformer exceeds a third preset threshold value, the brake resistor is activated to dissipate surplus power until the low-voltage direct-current voltage is lower than a fourth preset threshold value.

Description

Zero-voltage ride-through methods, equipment, and media for all-DC offshore wind power systems Technical Field

[0001] This application pertains to the field of all-DC offshore wind power. Background Technology

[0002] Offshore wind power (OWP) has become a promising solution for achieving net-zero emissions due to its cleanliness, small footprint, and proximity to coastal load centers. Currently, high-voltage direct current (HVDC) transmission based on voltage source converters (VSCs) is the preferred technology for connecting offshore wind farms (OWFs). However, with the rapid growth of wind farm collection areas and installed capacity, relying on medium-voltage alternating current (MVAC) technology for power collection has significant drawbacks, including high power losses, limited collection area and capacity due to the large amount of reactive power consumed by AC cables, and high cable costs.

[0003] To address this issue, all-DC solutions, particularly those combining MVDC collector networks with HVDC transmission, are considered a viable solution for grid connection of large-capacity, wide-area offshore wind farms (OWFs). This configuration offers several advantages, including a more compact offshore platform, lower MVDC collector network power losses, stronger power collection capabilities, and greater overall system stability.

[0004] A major challenge facing all-DC offshore wind power (OWP) systems is zero-voltage ride-through (ZVRT) operation. When a severe symmetrical fault occurs in the onshore AC (Alternating Current) grid, the receiving-end converter station cannot supply power to the AC grid. Due to the electrical isolation between the medium-voltage direct current (MVDC) and high-voltage direct current (HVDC) links, offshore wind farms (OWFs) cannot quickly detect the fault. This leads to the accumulation of surplus power, causing a rise in DC voltage and potentially resulting in system outages and overvoltage damage.

[0005] To achieve zero-voltage ride-through, in existing offshore wind power projects using VSC-HVDC transmission systems, energy dissipation devices (EDDs) are typically installed on the DC side of onshore converter stations. Examples include the Dolvin3 project (0.9 GW, ±320 kV) in Germany, the Sofia project (1.32 GW, ±320 kV) in the UK, and the Rudong (1.1 GW, ±400 kV) and Sheyang (1 GW, ±250 kV) projects in China. Some literature also documents this centralized configuration. In contrast, research has proposed a distributed layout, integrating EDDs into each submodule (SM) of onshore modular multilevel converters (MMCs). However, both centralized and distributed EDDs increase system costs. For example, the EDDs used in the Rudong project cost over US$7 million.

[0006] To eliminate the need for additional energy dissipation devices, some literature proposes an energy decoupling method for modular multilevel converters (MMCs) that utilizes submodule capacitors to temporarily store excess power during onshore AC faults. However, the system's zero-voltage ride-through capability (ZVRT) is limited to less than 100 milliseconds. This short duration is insufficient to meet the ZVRT requirements stipulated by transmission system operators (TSOs). Other literature temporarily boosts the voltage of the SM capacitors in the MMC to 1.5 times their rated value to absorb surplus power. Nevertheless, this method is still limited by the finite unbalanced energy storage capacity of the MMC SM capacitors and subjectes the SM to excessive voltage stress during faults. Existing technology proposes a grid-forming control based on internal energy (GFM) that coordinates the SM capacitor energy with HVDC-side EDDs to achieve ZVRT. However, the use of EDDs still incurs high costs.

[0007] Some literature suggests converting excess power into kinetic energy stored in the turbine rotor and blades by reducing the electromagnetic power output of DC wind turbines. A drawback of this method is the potential risk of overspeeding, as the pitch control system's response is typically too slow for effective intervention. Onshore AC fault information can be transmitted to offshore wind farms via communication, enabling them to reduce power output and achieve zero-voltage ride-through. However, reliance on communication introduces latency and compromises system reliability due to the risk of communication failures.

[0008] To avoid reliance on communication, some literature suggests that when the local HVDC voltage exceeds a preset threshold, the sending-end converter station can reduce the collector network voltage, thereby preventing the wind turbine from continuously injecting power into the DC side. However, voltage sags in the collector network increase mechanical stress on the wind turbine drivetrain. Furthermore, the zero-voltage ride-through capability of these methods has inherent limitations, which may prevent them from meeting increasingly stringent grid specifications. Summary of the Invention

[0009] This application addresses the issue of receiving-end converter stations being unable to transmit power to the AC grid when a severe symmetrical fault occurs. Due to the electrical isolation between the medium-voltage DC link and the high-voltage DC link, offshore wind farms cannot quickly detect the fault. This leads to the accumulation of surplus power, causing DC voltage rise and potentially resulting in system outages and overvoltage damage. The application provides a zero-voltage ride-through method, equipment, and medium for a full DC offshore wind power system.

[0010] The first aspect of this application provides a zero-voltage ride-through method for an all-DC offshore wind power system, including:

[0011] In the event of a fault in the onshore AC power grid:

[0012] When the high voltage DC voltage of the offshore DC transformer exceeds the first preset threshold, the offshore DC transformer is switched from medium voltage DC voltage control to high voltage DC voltage control, so that the offshore DC transformer absorbs power from the high voltage DC link and restores the high voltage DC voltage to the normal value.

[0013] When the medium-voltage DC voltage of the wind turbine DC transformer exceeds the second preset threshold, the wind turbine DC transformer is switched from low-voltage DC voltage control to medium-voltage DC voltage control, so that the wind turbine DC transformer can absorb power from the low-voltage DC voltage side through the wind turbine grid-side DC transformer, thereby restoring the medium-voltage DC voltage to the normal value.

[0014] When the low-voltage DC voltage of the offshore DC transformer exceeds the third preset threshold, the braking resistor is activated to dissipate the excess power until the low-voltage DC voltage is lower than the fourth preset threshold, at which point the braking resistor stops working.

[0015] In one possible design, when the offshore DC transformer switches to high-voltage DC voltage control, the d-axis current reference value of the offshore DC transformer is calculated by a proportional-integral controller based on the high-voltage DC voltage reference value and the measured value, as well as the medium-voltage DC voltage reference value and the measured value, in order to achieve power flow reversal and voltage recovery.

[0016] In one possible design, the d-axis current reference value of the offshore DC transformer is calculated according to the following formula. :

[0017] ,

[0018] in, and These are the reference value and the measured value of the high-voltage DC voltage, respectively. and These represent the reference value and the measured value of the medium-voltage DC voltage when the offshore DC transformer switches to high-voltage DC voltage control, respectively. and These are the proportional and integral parameters of the proportional-integral controller, respectively.

[0019] In one possible design, when the wind turbine DC transformer switches to medium-voltage DC voltage control, the d-axis current reference value of the wind turbine DC transformer is obtained by droop control calculation based on the medium-voltage DC voltage reference value, the measured value, and the low-voltage DC voltage reference value, so as to realize the coordinated regulation of the medium-voltage DC network voltage by multiple wind turbines.

[0020] In one possible design, the reference value of the d-axis current of the wind turbine DC transformer is calculated according to the following formula. :

[0021] ,

[0022] in, and These are the reference value and the measured value of the low-voltage DC voltage, respectively. and These represent the reference value and the measured value of the medium-voltage DC voltage when the DC transformer of the wind turbine switches to medium-voltage DC voltage control, respectively. and These are the proportional and integral parameters of the proportional-integral controller. This is a medium-voltage DC voltage control signal.

[0023] In one possible design, when the low-voltage DC voltage of the wind turbine's DC transformer exceeds a third preset threshold, the heat dissipated by the braking resistor is:

[0024] ,

[0025] in, The heat dissipated by the braking resistor This is the value of the braking resistor. and The first The start and stop times of the secondary braking resistor operation. This represents the total number of times the braking resistor operates during the fault period. This is the measured value of the low-voltage DC voltage.

[0026] In one possible design, in the event of a fault in the onshore AC power grid: the receiving-end modular multilevel converter adjusts the DC modulation index until the high-voltage DC voltage increases, enabling the offshore DC transformer to detect the high-voltage DC overvoltage. After a delay, the receiving-end modular multilevel converter adjusts the DC modulation index until the high-voltage DC voltage decreases.

[0027] After the fault in the onshore AC power grid is cleared: the DC modulation index is restored.

[0028] In one possible design, the power drop rate of the wind turbine is 5 pu / s.

[0029] In one possible design, the first preset threshold, the second preset threshold, and the third preset threshold are all 1.1 pu; the fourth preset threshold is 0.9 pu.

[0030] The second aspect of this application provides a zero-voltage ride-through device for an all-DC offshore wind power system, the all-DC offshore wind power system zero-voltage ride-through device including a processor and a memory, the memory storing at least one instruction, the at least one instruction being loaded and executed by the processor to implement the all-DC offshore wind power system zero-voltage ride-through method as described above.

[0031] A third aspect of this application provides a computer storage medium storing at least one instruction, which is loaded and executed by a processor to implement the zero-voltage ride-through method for a full DC offshore wind power system as described above.

[0032] The beneficial effects of this application are:

[0033] This application proposes a novel ZVRT strategy based on adaptive DC voltage control. In this application, when the offshore DC transformer (DCT) detects an overvoltage in the High-Voltage Direct Current (HVDC) link, it seamlessly switches to HVDC link DC voltage control mode, thereby transferring surplus power from the HVDC link to the Medium-Voltage Direct Current (MVDC) network. When the DC wind turbines (WTs) sense an overvoltage in the MVDC network, their control mode changes from internal Low-Voltage Direct Current (LVDC) bus voltage control to droop-based MVDC voltage control, allowing multiple wind turbines to collaboratively regulate the MVDC network voltage. Internal DC choppers within the wind turbines are used to dissipate unbalanced power and maintain LVDC voltage stability.

[0034] Furthermore, this application proposes a rapid fault detection scheme, enabling DC wind turbines to quickly detect onshore AC faults and reduce power generation, thereby mitigating overvoltage issues during fault periods. This application eliminates the need for onshore energy dissipation devices and achieves 350 ms zero-voltage ride-through even without communication, significantly improving system reliability and economy. Simulation results verify the effectiveness of the proposed ZVRT strategy. Attached Figure Description

[0035] Figure 1 shows the topology of a full DC offshore wind power system.

[0036] Figure 2 shows the single-phase circuit diagram of MMC;

[0037] Figure 3 shows the single-phase analytical model of the system;

[0038] Figure 4 shows a comparison of DC overvoltage between the analytical model and the time-domain simulation model;

[0039] Figure 5 shows the logic diagram of adaptive DC voltage control, (a) DC voltage curve, (b) control mode switching signal;

[0040] Figure 6 is a flowchart of the ODCT control for zero voltage ride-through;

[0041] Figure 7 is a flowchart of the DC wind turbine control for zero voltage ride-through;

[0042] Figure 8 is a control block diagram of REMMC;

[0043] Figure 9 is a flowchart of the zero-voltage ride-through strategy described in the embodiment;

[0044] Figure 10 shows the zero-voltage ride-through performance under a 350ms fault, where (a) grid-side voltage, (b) grid-side current, (c) power of OWF ODCT and REMMC, (d) DC voltage, (e) ODCT MMC1 SM capacitor voltage, and (f) REMMC SM capacitor voltage.

[0045] Figure 11 shows the zero-voltage ride-through performance under a 100ms fault, where (a) grid-side voltage, (b) grid-side current, (c) power of OWF ODCT and REMMC, (d) DC voltage, (e) voltage of ODCT MMC1 SM capacitor, and (f) voltage of REMMC SM capacitor.

[0046] Figure 12 is a schematic diagram of zero voltage ride-through performance under a 30ms fault, where (a) grid-side voltage, (b) grid-side current, (c) power of OWF ODCT and REMMC, (d) DC voltage, (e) voltage of ODCT MMC1 SM capacitor, and (f) voltage of REMMC SM capacitor.

[0047] Figure 13 is a comparison of the trigger signal times, where (a) ODCT DC voltage control switching signal SHVDC, (b) WT DC voltage control switching signal and power drop signal SMVDC, and (c) DC chopper activation signal SChopper.

[0048] Figure 14 is a schematic diagram comparing DC voltages with and without control, where (a) ODCT HVDC voltage, (b) WTMVDC voltage, and (c) WT's internal LVDC voltage.

[0049] Figure 15 is a schematic diagram comparing DC voltages under different zero-voltage ride-through strategies, where (a) is the traditional zero-voltage ride-through strategy and (b) is the zero-voltage ride-through strategy in the embodiment.

[0050] Figure 16 is a schematic diagram comparing the energy dissipation of the traditional strategy and the proposed zero-voltage ride-through strategy in the braking resistor. Detailed Implementation

[0051] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of this application, and not all of them. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application. It should be noted that, unless otherwise specified, the embodiments and features in the embodiments of this application can be combined with each other.

[0052] I. Layout of All-DC Offshore Wind Power System

[0053] The topology of the all-DC offshore wind power system is shown in Figure 1, where DC wind turbines are connected in parallel to a medium-voltage DC network. The generated wind power is stepped up to the high-voltage DC level via an offshore DC transformer (ODCT) and then transmitted to a receiving-end modular multilevel converter (REMMC). The onshore converter station maintains the HVDC voltage at a stable level. The DC / AC inverter inside the ODCT uses constant DC voltage control to regulate the voltage of the offshore MVDC network, while its AC / DC rectifier uses constant AC voltage control to maintain the AC voltage inside the ODCT.

[0054] Each DC wind turbine employs a full-power converter configuration, where the grid-side DC transformer (DCT) boosts the turbine's internal low-voltage DC to MVDC levels for connection to the offshore power collection network. A DC chopper is connected in parallel to the LVDC bus of the turbine converter. Both the machine-side AC / DC converter (MSC) and the DC / AC inverter within the DCT utilize two-level voltage source converters (2L-VSCs), while the DCT's AC / DC rectifier employs an MMC structure. The MSC regulates the DC wind turbine's power output, with its power reference value determined by a Maximum Power Point Tracking (MPPT) algorithm. The DC / AC inverter within the turbine's DCT operates in constant DC voltage control mode to maintain a stable LVDC voltage, while the DCT's AC / DC rectifier uses AC voltage control.

[0055] II. System Characteristics During Shore-to-Shore Communication Failures

[0056] If the method of storing surplus energy is adopted by utilizing the voltage rise of the MMC submodule capacitor, the HVDC voltage increases to... The required time interval It is given by the following formula:

[0057] (1),

[0058] in, Refers to the HVDC voltage measured by REMMC. Indicates the system's rated power. The equivalent capacitance of the modulus MMC.

[0059] Considering the magnitude of HVDC voltage rise The capacitance requirement of the MMC at 50 Hz is 40 kJ / MVA. In the test system, the energy stored in the capacitor increased by 5.86 MJ, and its parameters are listed in Table 1. According to formula (1), when the system is at rated power... During runtime, time interval The calculated result is 35.2 ms. This duration is insufficient to ensure reliable zero-voltage ride-through in typical onshore AC fault scenarios.

[0060] To analyze system overvoltages during onshore AC faults, a single-phase equivalent circuit of the system is considered. According to Kirchhoff's Voltage Law (KVL), the voltages of the upper and lower bridge arms in Figure 2 are... and It can be represented as:

[0061] (2),

[0062] in, Indicates MMC in AC terminal voltage of phase, and These represent the currents of the upper and lower bridge arms, respectively. DC voltage For bridge arm inductance.

[0063] If circulation is ignored, then and Determined by the following formula:

[0064] (3),

[0065] in, and These are direct current and alternating current, respectively.

[0066] Subtracting the lower arm voltage from the upper arm voltage in equation (2) yields the MMC voltage at... AC terminal voltage of phase :

[0067] (4).

[0068] Based on equations (3) and (4), the desired AC output voltage of the MMC can be obtained. for:

[0069] (5).

[0070] Multiply both sides of equation (5) by the phase current. Based on the power balance relationship between the single-phase AC side and the DC side, the phase power... It can be represented as:

[0071] (6).

[0072] Therefore, direct current It can be represented as:

[0073] (7).

[0074] Based on the single-phase analysis model of the system shown in Figure 3, the current... For equivalent capacitance ( The charge is given by the following formula:

[0075] (8),

[0076] in, This refers to the capacitors of the MMC submodules. This represents the number of submodules in a single bridge arm.

[0077] From Figure 3, we can obtain the following expression:

[0078] (9),

[0079] in, This represents the desired AC output voltage on the HVDC side in the analytical model. This represents the desired AC output voltage on the MVDC side in the analytical model. Indicates the DC current on the HVDC side. Indicates the DC current on the MVDC side. express Phase power, This is the DC voltage on the HVDC side. This is the DC voltage on the MVDC side. For the bridge arm inductors of the DC / AC converter in the offshore converter station, This is the bridge arm inductor for the AC / DC converter in the offshore converter station.

[0080] Based on equation (9), the following expression can be derived:

[0081] (10)

[0082] in, and These represent the equivalent capacitance on the HVDC side of the offshore booster platform and the equivalent capacitance on the HVDC side of the onshore converter station, respectively.

[0083] therefore, The capacitor voltage at time t can be derived as follows:

[0084] (11),

[0085] in, This is the time when the fault begins.

[0086] Figure 4 shows a comparison of the DC overvoltage between the above analytical model and the system time-domain simulation model. After a land-based AC fault occurs at 1.5 s, the DC voltage in the analytical model rises from 1 pu to 1.499 pu within 100 ms. This result agrees well with the 1.442 pu overvoltage value obtained from the time-domain simulation model, with an error of 3.95%. This difference is mainly attributed to the power loss in the simulation model, thus verifying the accuracy of the derived analytical model.

[0087] In view of this, the present application provides a zero-voltage ride-through method for a full DC offshore wind power system to solve the above-mentioned problems. The embodiments of the present application will now be described in detail with reference to Figures 5 to 16.

[0088] Specific Implementation Method 1: The zero-voltage ride-through method for a full DC offshore wind power system described in this implementation method includes:

[0089] I. Adaptive DC Voltage Control During Zero Voltage Ride

[0090] Adaptive DC voltage control enables the all-DC offshore wind power system to overcome AC grid faults onshore. Assuming the fault initiation time... A three-phase ground fault occurred in the onshore AC power grid:

[0091] 1. Control method for offshore DC transformers used for zero voltage ride-through

[0092] When ODCT is Local HVDC voltage is detected at all times. When the value exceeds its preset threshold (1.1 pu in this embodiment), the DC voltage control switching signal is activated. The value is set to 1, as shown in Figure 5. Therefore, the ODCT switches from MVDC voltage control to HVDC voltage control, as shown in Figure 6. This causes the power flow to reverse; the ODCT changes from injecting power into the HVDC link to absorbing power from it to restore the HVDC voltage. According to the control scheme shown in Figure 6, the d-axis current reference value is... This can be expressed as:

[0093] (12),

[0094] in, and These represent the voltage reference values ​​for MVDC and HVDC, respectively. The voltage measurement value for MVDC. and These are the proportional and integral parameters of a proportional-integral (PI) controller.

[0095] ODCT's MMC1 and MMC2 share a common phase angle. Used for coordinate transformation, this phase angle It is generated by integrating a preset 250 Hz frequency, thus avoiding the use of a phase-locked loop (PLL).

[0096] 2. Control method for DC wind turbines used in zero-voltage ride-through operation

[0097] As the ODCT actively absorbs surplus power from the HVDC link and transmits it to the MVDC side, while the wind turbine WT continuously injects power into the MVDC collector network, the MVDC voltage rises accordingly. At that moment, the wind turbine's DC transformer detected the local MVDC voltage. If the preset threshold is exceeded (1.1 pu in this embodiment), then as shown in Figure 5, the control signal will be... Set to 1. Therefore, the wind turbine DCT switches from LVDC voltage control to MVDC voltage control with droop characteristics, enabling a large number of wind turbines to simultaneously regulate the MVDC voltage, as shown in Figure 7. The power flow direction is reversed accordingly, allowing surplus power to be absorbed from the MVDC network to the LVDC side through the wind turbine DCT. Therefore, the MVDC voltage... Returned to normal value. Shaft current reference value This can be expressed as:

[0098] (13)

[0099] in, This indicates the LVDC voltage inside the fan. and These are the voltage reference values ​​for LVDC and MVDC, respectively.

[0100] exist Upon detection of a fault, the output power of the turbine-side converter (MSC) simultaneously begins to decrease. An excessively rapid power decrease rate can impose severe mechanical stress on the turbine and accelerate turbine speed, while an excessively slow rate will cause more surplus power to accumulate on the LVDC side, exacerbating thermal stress on the braking resistor. Therefore, the advantages and disadvantages must be carefully weighed; this embodiment limits the turbine's power decrease rate to 5 pu / s.

[0101] from From that moment on, due to power absorption from the MVDC side and continuous power generation from the wind turbine, the LVDC voltage... It begins to rise, as shown in Figure 5. The fan detects the local LVDC voltage. exist When the time exceeds the upper limit threshold (1.1 pu in this implementation), the internal braking resistor... Activated (power-consuming resistor input cut-off signal) This dissipates surplus power, leading to a decrease. Once When the voltage drops below the lower threshold of 0.9 pu, the braking resistor stops working. If the wind power has not yet dropped to 0 or the ODCT is still absorbing power from the HVDC link, It may rise again to 1.1 pu, thereby reactivating the braking resistor. During this process, the braking resistor... Dissipated heat energy It can be represented as:

[0102] (14)

[0103] in, and They represent the first The start and stop times of the secondary braking resistor operation. This represents the total number of times the braking resistor operates during the fault. Throughout the fault process, the system's surplus power is dissipated through the braking resistor of the DC fan, thus eliminating the need to install an energy dissipation device on the HVDC side and improving the system's economy.

[0104] 3. Rapid Onshore Fault Detection Method for Offshore Wind Farms

[0105] To alleviate the surplus power problem during onshore AC faults, this embodiment also proposes an onshore fault detection scheme, enabling offshore wind farms to quickly identify such faults, thereby reducing the risk of system overvoltage.

[0106] Number of sub-modules deployed in the upper and lower arms of the MMC and It can be represented as:

[0107] (15)

[0108] in, Rated DC voltage This is the rated voltage of the submodule capacitor. Indicates the DC modulation index. and These represent the modulation indices generated by AC current control and circulating current suppression control (CCSC), respectively.

[0109] Based on the decoupling mechanism of DC and AC components in MMC as shown in Equation (15), the proposed fast fault detection scheme actively adjusts the DC modulation index. This causes a slight change in DC voltage, enabling offshore wind farms to quickly detect onshore AC faults. Specifically, this includes:

[0110] Under normal operating conditions, land-based fault signals With delay signal All remain at 0, and the DC modulation index Set to 1, as shown in Figure 8. Once a land-based fault is detected, the land-based fault signal... Set to 1, REMMC will... Set to 1.05, temporarily boosting the HVDC voltage to 1.05 pu, enabling the ODCT to quickly detect HVDC overvoltage. After a 15 ms delay, the delayed signal... Set to 1, REMMC will... The value is set to 0.95 to actively reduce the HVDC voltage. This not only alleviates HVDC overvoltage but also establishes a voltage difference between the HVDC terminals of the ODCT and REMMC, thereby storing some of the surplus power in the submodule capacitors of the onshore REMMC.

[0111] After the land-based AC fault is cleared, the fault signal Reset to 0, It recovered from 0.95 to the nominal value of 1. (Figure 8) The expression for is given by the following formula:

[0112] (16).

[0113] When ODCT detects local HVDC voltage When the threshold of 1.1 pu is exceeded, it switches to HVDC voltage control mode (i.e., The ODCT actively boosts the MVDC voltage to 1.05 pu using the decoupling control of the DC and AC components of the MMC, as shown in Figure 6. After a 15 ms delay, the ODCT actively reduces the DC bias of the MMC arm voltage from 1.05 pu to 0.95 pu. (DC modulation index) It is given by the following formula:

[0114] (17).

[0115] The zero-voltage ride-through process described above is shown in Figure 9. During a fault, this strategy enables offshore wind turbines to quickly detect onshore faults by fine-tuning the HVDC and MVDC voltages. As a result, the wind farm's output power is effectively reduced, and the braking resistors inside the turbine are rapidly activated to dissipate excess power. This alleviates overvoltage issues on the HVDC link, MVDC collector network, and the LVDC bus inside the turbine. Therefore, this strategy ensures reliable zero-voltage ride-through while providing the possibility of eliminating the need for energy dissipation devices on the HVDC side of the onshore MMC.

[0116] To verify the effectiveness of the proposed zero-voltage ride-through strategy, a full DC offshore wind power system simulation model, as shown in Figure 1, was built in the PSCAD / EMTDC simulation platform. Detailed system parameters are shown in Table 1.

[0117] Table 1. Main parameters of the all-DC offshore wind power system

[0118]

[0119] To improve computational efficiency, an average value model was used for MMC. The offshore wind farm is represented by three convergent wind turbines, each with a rated capacity of 667 MW.

[0120] 1. Zero voltage ride-through capability

[0121] In the simulation scenario, A three-phase short-circuit fault is applied to the onshore AC power grid. Figures 10, 11, and 12 show the zero-voltage ride-through of the proposed control scheme under different fault durations.

[0122] As shown in Figures 10(a) and (b), in Following the fault, the onshore power grid voltage collapsed, and the REMMC limited the AC current to 1.2 pu. Simultaneously, the REMMC actively boosted its HVDC terminal voltage to 1.05 pu. At this time, the ODCT detects that the local HVDC voltage exceeds the threshold of 1.1 pu and switches to HVDC voltage control mode. This causes the ODCT power (PODCT) direction to reverse to restore the HVDC voltage, as shown in Figures 10(c) and (d). This action causes the MVDC voltage to rise. At the same time, the ODCT slightly increases the MVDC voltage to quickly transmit the onshore fault signal to the DC wind turbine.

[0123] DC fans in When the MVDC terminal voltage is detected to exceed the 1.1 pu threshold, the system switches to MVDC voltage droop control mode. As shown in Figures 10(c) and (d), this causes a reversal in the power output power (POWF) of the offshore wind farm, which aims to restore the MVDC voltage and induce an increase in the LVDC voltage. Simultaneously, the turbine-side converter begins to reduce the turbine's output power. When the internal LVDC voltage UL of the wind turbine exceeds the threshold of 1.1 pu, the braking resistor is activated, causing... Decline. When exist When the pressure drops to 0.9 pu, the braking resistor is disconnected. After the braking resistor is finally removed, as the output power PWT of the wind turbine decays to zero, the LVDC voltage tends to stabilize, as shown in Figures 10(c) and (d).

[0124] Furthermore, as shown in Figures 10(e) and (f), during the fault, the voltage of the submodule capacitors of ODCT MMC1 and REMMC rises to temporarily store some of the surplus energy, reaching a peak of 1.25 pu.

[0125] Figure 11 illustrates the zero-voltage ride-through performance during an onshore AC fault lasting 100 ms. The system behavior is largely consistent with the aforementioned scenarios. A significant difference is that, due to the shorter fault duration, the braking resistor on the wind turbine LVDC bus is activated only once, as shown in Figure 11(d).

[0126] When the fault duration is further reduced to 30 ms, zero-voltage ride-through can be achieved solely through the coordinated action of REMMC and ODCT. As shown in Figures 12(c) and (d), the MVDC voltage at the DC wind turbine remains below the activation threshold of 1.1 pu, thus ensuring uninterrupted turbine operation. The proposed control strategy demonstrates independence from fault duration, ensuring reliable ZVRT operation even under prolonged onshore AC faults.

[0127] 2. Verification of rapid onshore fault detection for offshore wind farms

[0128] To verify the feasibility of the proposed fast onshore AC fault detection scheme, the zero-voltage ride-through performance of the system with and without the proposed method was compared under the condition that the AC fault duration was 350 ms.

[0129] As shown in Figure 13, the proposed scheme significantly accelerates the system response. After applying the scheme, the ODCT detection time of the fault is advanced by 10 ms, as shown in Figure 13(a). The moment when the DC fan begins to reduce its output power is advanced from 1.551 s to 1.539 s, a reduction of 12 ms, as shown in Figure 13(b). Similarly, after detecting LVDC overvoltage, the response time of activating the fan brake resistor is shortened from 1.562 s to 1.55 s, also an improvement of 12 ms, as shown in Figure 13(c). This accelerated response effectively mitigates the overvoltage risk of the system.

[0130] The overvoltage suppression capability of this embodiment is further demonstrated in Figure 14. As shown in Figure 14(a), the HVDC overvoltage measured at the ODCT decreased from 1.11 pu to 1.103 pu (a reduction of 7 kV). The overvoltage measured at the wind turbine DCT decreased from 1.16 pu to 1.1 pu (a reduction of 6 kV), while the LVDC voltage inside the wind turbine decreased by 40 V, as shown in Figures 14(b) and (c). These results confirm that the proposed scheme enables offshore wind farms to quickly detect onshore AC faults and rapidly reduce wind turbine output power, thereby effectively suppressing overvoltages in the HVDC link, MVDC collector network, and LVDC bus inside the wind turbine.

[0131] 3. Performance comparison with traditional zero-voltage ride-through methods

[0132] To further demonstrate the superiority of this embodiment, its performance is compared with that of the conventional zero-voltage ride-through method, which involves installing an energy dissipation device at the HVDC terminal of the onshore converter station. The EDD has a resistance of 500 Ω to dissipate the rated power of the system. In this simulation, the onshore AC fault occurs at 1.5 s and is cleared at 1.85 s. The energy dissipated by the braking resistor during the zero-voltage ride-through is calculated using equation (14).

[0133] In conventional methods, once the onshore MMC station detects an HVDC overvoltage following a fault, it activates the energy dissipation device to dissipate excess power. As shown in Figure 15(a), the HVDC voltage subsequently decreases and eventually stabilizes at a level slightly below 1 pu. As shown in Figure 16, the EDD's braking resistor remains engaged throughout the fault duration, resulting in a total heat generation of 662 MJ. In contrast, the proposed strategy utilizes the braking resistor within the wind turbine, with a total activation time of only 120.8 ms (activated in three stages), as shown in Figures 15(b) and 16. This results in a total heat dissipation of 242 MJ, a 63.5% reduction compared to the conventional method. Compared to the conventional method, this embodiment significantly reduces the excess energy dissipated by the braking resistor. This reduction allows for a substantial decrease in the required braking resistor capacity, thereby improving the overall economic feasibility of the system.

[0134] In summary, this embodiment proposes a zero-voltage ride-through (ZVRT) method based on adaptive DC voltage control for a full DC offshore wind power system. This method enables the offshore DC transformer and DC wind turbine to autonomously switch operating modes, thereby transferring excess power to the low-voltage DC bus of the wind turbine. Subsequently, the excess power is safely dissipated by the DC chopper inside the wind turbine, stabilizing the LVDC voltage and eliminating the need for onshore energy dissipation devices. Furthermore, the proposed rapid fault detection scheme allows the wind turbine to quickly respond to faults and reduce power generation, effectively mitigating overvoltage stress. Simulation results verify the effectiveness of the proposed strategy, successfully achieving ZVRT operation within 350 ms. This implementation relies solely on local measurements, requiring no communication or expensive onshore hardware, thus significantly improving system reliability and economy.

[0135] Specific Implementation Method Two: The zero-voltage ride-through device for the all-DC offshore wind power system described in this implementation method includes a processor and a memory. The memory stores at least one instruction, which is loaded and executed by the processor to implement the zero-voltage ride-through method for the all-DC offshore wind power system as described in Specific Implementation Method One.

[0136] Specific Implementation Method 3: A computer storage medium as described in this embodiment stores at least one instruction, which is loaded and executed by a processor to implement the zero-voltage ride-through method for a full DC offshore wind power system as described in Specific Implementation Method 1.

[0137] While specific embodiments of this application have been described herein with reference to them, it should be understood that these embodiments are merely examples of the principles and applications of this application. Therefore, it should be understood that many modifications can be made to the exemplary embodiments, and other arrangements can be designed without departing from the spirit and scope of this application as defined by the appended claims. It should be understood that different dependent claims and features described herein can be combined in ways different from those described in the original claims. It is also understood that features described in conjunction with individual embodiments can be used in other described embodiments.

Claims

1. A zero-voltage ride-through method for an all-DC offshore wind power system, characterized in that, include: In the event of a fault in the onshore AC power grid: when the high-voltage DC voltage of the offshore DC transformer exceeds the first preset threshold, the offshore DC transformer is switched from medium-voltage DC voltage control to high-voltage DC voltage control, allowing the offshore DC transformer to absorb power from the high-voltage DC link and thus restore the high-voltage DC voltage to its normal value; when the medium-voltage DC voltage of the wind turbine DC transformer exceeds the second preset threshold, the wind turbine DC transformer is switched from low-voltage DC voltage control to medium-voltage DC voltage control, allowing the wind turbine DC transformer to absorb power from the low-voltage DC voltage side through the wind turbine grid-side DC transformer and thus restore the medium-voltage DC voltage to its normal value; when the low-voltage DC voltage of the wind turbine DC transformer exceeds the third preset threshold, the braking resistor is activated to dissipate excess power until the low-voltage DC voltage falls below the fourth preset threshold, at which point the braking resistor stops working.

2. The zero-voltage ride-through method for an all-DC offshore wind power system according to claim 1, characterized in that, When the offshore DC transformer switches to high voltage DC voltage control, the d-axis current reference value of the offshore DC transformer is calculated by a proportional-integral controller based on the high voltage DC voltage reference value and the measured value, as well as the medium voltage DC voltage reference value and the measured value, so as to realize the power flow reversal and voltage recovery.

3. The zero-voltage ride-through method for an all-DC offshore wind power system according to claim 2, characterized in that, The reference value of the d-axis current of the offshore DC transformer is calculated according to the following formula. ： ,in, and These are the reference value and the measured value of the high-voltage DC voltage, respectively. and These represent the reference value and the measured value of the medium-voltage DC voltage when the offshore DC transformer switches to high-voltage DC voltage control, respectively. and These are the proportional and integral parameters of the proportional-integral controller, respectively.

4. The zero-voltage ride-through method for an all-DC offshore wind power system according to claim 1, characterized in that, When the wind turbine DC transformer switches to medium-voltage DC voltage control, the d-axis current reference value of the offshore DC transformer is obtained by droop control calculation based on the medium-voltage DC voltage reference value, the measured value, and the low-voltage DC voltage reference value, so as to realize the coordinated adjustment of the medium-voltage DC network voltage by multiple wind turbines.

5. The zero-voltage ride-through method for a full DC offshore wind power system according to claim 4, characterized in that, The reference value of the d-axis current of the DC transformer of the wind turbine is calculated according to the following formula. ： ,in, and These are the reference value and the measured value of the low-voltage DC voltage, respectively. and These represent the reference value and the measured value of the medium-voltage DC voltage when the DC transformer of the wind turbine switches to medium-voltage DC voltage control, respectively. and These are the proportional and integral parameters of the proportional-integral controller. This is a medium-voltage DC voltage control signal.

6. The zero-voltage ride-through method for an all-DC offshore wind power system according to claim 1, characterized in that, When the low-voltage DC voltage of the wind turbine's DC transformer exceeds a third preset threshold, the heat dissipated by the braking resistor is: ,in, The heat dissipated by the braking resistor This is the value of the braking resistor. and The first The start and stop times of the secondary braking resistor operation. This represents the total number of times the braking resistor operates during the fault period. This is the measured value of the low-voltage DC voltage.

7. The zero-voltage ride-through method for an all-DC offshore wind power system according to claim 1, characterized in that, Also includes: In the event of a fault in the onshore AC power grid: the receiving-end modular multilevel converter adjusts the DC modulation index until the high-voltage DC voltage increases, enabling the offshore DC transformer to detect the high-voltage DC overvoltage. After a delay, the receiving-end modular multilevel converter adjusts the DC modulation index until the high-voltage DC voltage decreases. After the fault in the onshore AC power grid is cleared, the DC modulation index is restored.

8. The zero-voltage ride-through method for an all-DC offshore wind power system according to claim 1, characterized in that, The power reduction rate of the wind turbine is 5 pu / s; the first preset threshold, the second preset threshold, and the third preset threshold are all 1.1 pu; the fourth preset threshold is 0.9 pu.

9. A zero-voltage ride-through device for a full DC offshore wind power system, characterized in that, The all-DC offshore wind power system zero-voltage ride-through device includes a processor and a memory, wherein the memory stores at least one instruction, which is loaded and executed by the processor to implement the all-DC offshore wind power system zero-voltage ride-through method as described in any one of claims 1 to 8.

10. A computer storage medium, characterized in that, The computer storage medium stores at least one instruction, which is loaded and executed by a processor to implement the zero-voltage ride-through method for an all-DC offshore wind power system as described in any one of claims 1 to 8.

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