A coordinated active restoration control method for offshore wind farm grid-connected through flexible direct current

Abstract

The application discloses a kind of offshore wind farm through flexible dc grid-connected collaborative active recovery control method, belong to new energy grid-connected control technical field.The specific process of the method of the application is as follows: after power grid fault is removed, onshore converter station uses adaptive DC voltage control strategy, offshore wind turbine adopts adaptive two-stage active recovery control strategy;In the early stage of recovery, quickly promote wind power and release DC capacitor energy storage to provide inertia support to power grid, inhibit frequency drop rate;After this, wind power changes into rate-limiting recovery mode to protect unit equipment safety, while DC line voltage is inhibited by adaptive adjustment Transient overshoot;The combination of the two strategies effectively realizes the collaborative recovery of wind farm through flexible dc grid after fault.The application solves the problem that the existing method cannot improve the frequency drop level and prevent DC line voltage from exceeding the limit at the same time;It can be applied to the combined fault ride-through of new energy through flexible dc grid.

Description

Technical Field

[0001] This invention belongs to the field of new energy grid connection control technology, specifically relating to a collaborative active power recovery control method for offshore wind farms connected to the grid via flexible DC transmission. Background Technology

[0002] Flexible DC transmission based on modular multilevel converters (MMC-HVDC) has advantages in both technical and economic aspects and is widely recognized as an ideal solution for long-distance offshore wind power grid connection. During grid faults, the transmission channel for wind power is blocked, and the flexible DC converter station will face severe DC line overvoltage. In existing fault ride-through technologies, step-down control requires no additional hardware and allows for a natural response of wind power, which helps suppress DC line overvoltage. After the fault is cleared, the output power of variable-speed wind turbines exhibits a significant delayed recovery characteristic to protect equipment safety. However, the electrical decoupling characteristics between wind farms and onshore converter stations pose a challenge to DC line voltage recovery. On the other hand, the centralized grid connection of large-scale wind farms significantly reduces the system inertia level, and the delayed recovery behavior of wind power will cause continuous insufficient power generation in low-inertia regions, exacerbating frequency drops and even triggering the risk of frequency instability. Therefore, for scenarios where large-scale offshore wind power is connected to low-inertia power grids via flexible DC converters, the design of a coordinated active power recovery control method for wind farms and flexible DC converter stations is crucial for the transient response of grid frequency and DC line voltage.

[0003] Currently, there are two main approaches to the coordinated active power recovery strategy for wind farms and flexible DC transmission (DC). One approach establishes a coupling relationship between DC line voltage and wind power throughout the fault recovery process, achieving coordinated recovery of DC line voltage and wind power by controlling the active power recovery rate of the onshore DC converter station. This method effectively follows the delayed recovery behavior of wind power but does not consider the impact of the recovery strategy on grid frequency dynamics. The second approach increases the wind power injection level during the fault period, and after the fault is cleared, utilizes the active power imbalance of the DC transmission to provide inertial support to the grid. This approach follows the delayed recovery behavior of wind power while also improving the frequency sag level. However, limited by the size of the submodule capacitors, the DC converter station has a very low tolerance for unbalanced power, making this approach prone to severe DC line overvoltage and difficult to apply in practical engineering.

[0004] Therefore, existing methods cannot effectively address both the issues of coordinated active power recovery and grid-friendliness improvement in offshore wind farms connected to flexible DC grids. Summary of the Invention

[0005] To address the limitations of existing methods in effectively mitigating transient frequency drops and DC line voltage exceedances in offshore wind farms connected to flexible DC grids, this invention provides a collaborative active power recovery control method for offshore wind farms connected to flexible DC grids. This collaborative control method fully utilizes the fast recovery characteristics of the low-power segment of the wind farm and the energy storage of DC capacitors to provide more inertial power to the grid after fault clearance, thereby improving the recovery performance of DC line voltage and simultaneously improving the lowest frequency point.

[0006] This invention is achieved through the following technical solution:

[0007] A collaborative active power recovery control method for offshore wind farms connected to flexible DC grids specifically includes the following steps:

[0008] After the grid fault is cleared, the onshore converter station adopts an adaptive DC voltage control method. In the early stage of recovery, it releases the DC capacitor to store energy, provides inertial support to the grid, and suppresses the frequency drop rate. Thereafter, it suppresses the transient overshoot by adaptively adjusting the DC line voltage.

[0009] Offshore wind turbines employ an adaptive two-stage active power recovery control method. In the initial recovery phase, wind power is increased to provide inertial support to the grid and suppress frequency sag rate. Subsequently, wind power is switched to a rate-limited recovery mode to protect the safety of the turbine equipment.

[0010] By altering the coupling characteristics of DC line voltage and wind power, collaborative recovery of wind farms after faults can be achieved through flexible DC grid connection.

[0011] Furthermore, the adaptive DC voltage control method specifically includes the following:

[0012] A. At the rated DC voltage V dcN Two sets of boundary zones are defined around it:

[0013] 1) Internal boundary zone [V] th1 V th2 ], which corresponds to the range of DC voltage during steady-state operation, V th1 and V th2 These are the upper and lower boundary values ​​of the internal boundary band, respectively;

[0014] 2) External boundary zone [V] th3 V th4 ], which corresponds to the adaptive adjustment region of DC voltage during fault ride-through, V th3 and V th4 These are the upper and lower boundary values ​​of the outer boundary band, respectively;

[0015] B. In the internal boundary band, the DC voltage V dcr and control loop bandwidth f c The relationship can be divided into two specific cases:

[0016] 1) When the DC voltage recovers from the external boundary band, the steady-state offset of the DC voltage is eliminated through adaptive adjustment until V dcr =V dcN The regulator is then reset, therefore the relationship between the loop bandwidth and the DC voltage offset is:

[0017] (3)

[0018] In the formula, k1 is the proportionality coefficient and k1 < 0, f cs For V dcN The corresponding control bandwidth;

[0019] 2) During steady-state operation, f c =f cs ;

[0020] C. In the outer boundary zone, the DC voltage V dcr and control loop bandwidth f c The relationship can be divided into three specific situations:

[0021] 1) DC voltage increases and V th2 ≤V dcr ≤V th4 At that time, the FRT control did not operate, and the DC voltage loop control bandwidth remained unchanged, i.e., f c =f cs ;

[0022] 2) DC voltage decreases and V th4 ≥V dcr ≥V th2 This corresponds to the active power recovery process after a fault. It requires adaptive adjustment of control parameters according to the DC voltage offset to release the energy stored in the DC capacitor. Therefore, the loop bandwidth is positively correlated with the DC voltage offset.

[0023] (4)

[0024] In the formula, k2 is the proportionality coefficient, and k2>0;

[0025] 3) DC voltage increases and V th3 ≤V dcr ≤V th1 This corresponds to the DC voltage recovery process. During this stage, adaptive adjustment of control parameters is also needed to suppress transient fluctuations in the DC voltage. The loop bandwidth f... c The expression is the same as that in equation (3);

[0026] Let V dcr The minimum and maximum allowed values ​​are V respectively. min and V maxThen when the DC voltage is in [V min V th3 ] and [V th4 V max When the range is defined, it can correspond to any stage after the fault, and f is given. c =f cm f cm This represents the maximum allowable control bandwidth for the DC voltage loop.

[0027] D. The adaptive parameter regulator obtains the control bandwidth f according to the DC voltage offset. c Then it is converted into the control parameter k. pdc and k idc And assign the value to the PI control loop, f c and k pdc k idc The relationship is represented as

[0028] , (5)

[0029] In the formula, C dc S is the equivalent capacitance of a DC line. B and V dcB ξ represents the rated capacity and rated DC voltage of the flexible DC system, respectively, and ξ is the damping ratio.

[0030] Furthermore, the adaptive two-stage active power recovery control method divides the wind power recovery into two stages:

[0031] (1) Active power recovery stage: DC voltage V dcr and wind farm terminal voltage V w Through droop control and the active current i of the wind turbine wd Mutual coupling, during which i wd The expression is

[0032] (7)

[0033] In the formula, V wN V wl and V wm These represent the steady-state operating value, the highest and lowest limits during the rapid recovery phase of the terminal voltage of an offshore wind farm, respectively, while i wN This represents the steady-state operating value of the active current of the wind turbine generator.

[0034] During this phase, onshore converter stations eliminate transient overshoot of DC voltage through adaptive DC voltage control, while offshore converter stations raise the voltage i through voltage droop control in conjunction with voltage-source active current control of wind turbines. wd To restore the low-power section of the wind farm;

[0035] (2) Active power recovery rate limiting stage: DC voltage V dcr And the active current i of the wind turbine wd Decoupling, i wd No longer following V w It does not change with the rise of time, but is a function of time t, and its expression is:

[0036] (8)

[0037] In the formula, i wl The first stage voltage limit V wl The corresponding active current value, k w The preset active current recovery rate, t s and t e These represent the start and end times of the recovery phase, respectively.

[0038] During this phase, offshore wind farms fully comply with the delayed recovery behavior of wind power to meet the standards for grid connection of new energy sources and protect the safe operation of the turbine equipment.

[0039] Furthermore, the active power recovery phase involves three scenarios, as follows:

[0040] 1) During steady-state operation, i wd >i wl And during the fault, i wd ≤i wl In the case of i wd wl Then the rate of ascent limit value is changed from k w It becomes ∞;

[0041] 2) During steady-state operation and fault periods, i wd >i wl Then the rate of ascent limit remains k. w The wind power remains unchanged, and at this point, wind power recovery is achieved solely through rate limiting control.

[0042] 3) During steady-state operation, i wd ≤i wl In this case, the recovery process is entirely dominated by adaptive boost control.

[0043] Compared with the prior art, the advantages of the present invention are as follows:

[0044] 1) The adaptive DC voltage control used in onshore converter stations significantly improves the transient response performance of DC line voltage and protects the safe operation of converter station equipment;

[0045] ​2) Adaptive two-stage active current recovery control for offshore direct-drive wind turbines effectively improves grid frequency offset level and enhances power system stability while following the delayed recovery behavior of wind power.

[0046] 3) The collaborative control method of the present invention is of great significance for improving the fault recovery performance of offshore wind power directly connected to low-inertia AC power grid. Attached Figure Description

[0047] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the accompanying drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. In all the drawings, similar elements or parts are generally identified by similar reference numerals. In the drawings, the elements or parts are not necessarily drawn to scale.

[0048] Figure 1 This is a schematic diagram of a computational example system for offshore wind power connected to a low-inertia power grid via flexible DC transmission;

[0049] Among them: Z cs Z represents the equivalent impedance of the collector line in an offshore wind farm. ls For the impedance of the marine AC line, V w and P w These represent the terminal voltage and output power of an offshore wind farm, respectively, V. dch P is the DC line voltage. h and Q h These represent the active and reactive power outputs of the onshore converter station, respectively, in V. p For flexible DC connection to AC grid voltage, Z l11 Z l12 Z l3 Z l4 Z represents the line impedance of the land power grid. cl H is the equivalent impedance of the collector lines in an onshore wind farm. s0 The inertia coefficient of the synchronous generator set;

[0050] Figure 2 The simulation results show the mechanical torque of the equivalent synchronous power source during a severe power grid fault.

[0051] Wherein: T m The mechanical torque of the equivalent synchronous generator is represented by t, where t represents time.

[0052] Figure 3 These are simulation results of the onshore power grid frequency during a severe power grid failure.

[0053] Where: ω s For land-based power grid frequencies;

[0054] Figure 4The simulation results show the output power of the equivalent synchronous generator during a severe power grid fault.

[0055] Where: P g This represents the output electromagnetic power of an equivalent synchronous generator;

[0056] Figure 5 These are simulation results of the active power of the onshore power grid load during a severe power grid failure.

[0057] Among them, P l Active power representing the load of the onshore power grid;

[0058] Figure 6 These are simulation results of the active power output of flexible DC during severe power grid faults.

[0059] Among them, P h This represents the output active power of the onshore converter station;

[0060] Figure 7 These are simulation results of the active power output of onshore and offshore wind farms during severe grid outages.

[0061] Among them, P w This represents the active power output of the wind farm.

[0062] Figure 8 This invention presents the adaptive DC voltage control bandwidth adjustment curve for onshore converter stations.

[0063] Among them, V dcN V min and V max These represent the rated, minimum, and maximum DC voltage values, V. th1 and V th2 These are the upper and lower boundary values ​​of the internal boundary band, V. th3 and V th4 These are the upper and lower boundary values ​​of the outer boundary band, respectively;

[0064] Figure 9 This is a schematic diagram of the implementation scheme of the adaptive DC voltage control method (for onshore converter stations) proposed in this invention;

[0065] Figure 10 This is a schematic diagram of the adaptive two-stage active current recovery control curve proposed in this invention;

[0066] Among them, V wN V wt and V wl V wm These represent the steady-state operating value of the terminal voltage of an offshore wind farm, the load shedding threshold, and the highest and lowest limits during the rapid recovery phase, respectively. wN and i wlThese are the rated active current of the wind turbine and the two-stage switching threshold, respectively.

[0067] Figure 11 This is a schematic diagram of the implementation scheme of the adaptive two-stage active current recovery control method proposed in this invention;

[0068] Figure 12 This is a simulation comparison of the voltage at the flexible DC grid connection point using the method of this invention and existing methods when the residual voltage of the power grid fault is 0.4 pu.

[0069] Figure 13 This is a simulation comparison of the reactive power output of flexible DC using the method of this invention and existing methods when the residual voltage of the power grid fault is 0.4 pu.

[0070] Figure 14 This is a simulation comparison of the active power output of flexible DC using the method of this invention and existing methods when the residual voltage of the power grid fault is 0.4 pu.

[0071] Figure 15 This is a comparison of DC line voltage simulations using the method of this invention and existing methods when the residual voltage of a power grid fault is 0.4 pu.

[0072] Figure 16 This is a simulation comparison of the terminal voltage of an offshore wind farm using the method of this invention and existing methods when the residual voltage of the power grid fault is 0.4 pu.

[0073] Figure 17 This is a simulation comparison of the active power of an offshore wind farm using the method of this invention and existing methods when the residual voltage of the power grid fault is 0.4 pu.

[0074] Figure 18 This is a simulation comparison of the active power of an onshore wind farm using the method of this invention and existing methods when the residual voltage of the power grid fault is 0.4 pu.

[0075] Figure 19 This is a simulation comparison of the active power of a synchronous generator using the method of this invention and existing methods when the residual voltage of the power grid fault is 0.4 pu.

[0076] Figure 20 This is a comparison chart of power grid frequency simulations using the method of this invention and existing methods when the residual voltage of the power grid fault is 0.4 pu.

[0077] Figure 21 This is a simulation comparison of the frequency change rate using the method of this invention and existing methods when the residual voltage of the power grid fault is 0.4 pu.

[0078] Figure 22This is a simulation comparison of the voltage at the flexible DC grid connection point using the method of this invention and existing methods when the residual voltage of the power grid fault is 0.6 pu.

[0079] Figure 23 This is a simulation comparison of the reactive power output of flexible DC using the method of this invention and existing methods when the residual voltage of the power grid fault is 0.6 pu.

[0080] Figure 24 This is a simulation comparison of the active power output of flexible DC using the method of this invention and existing methods when the residual voltage of the power grid fault is 0.6 pu.

[0081] Figure 25 This is a comparison of DC line voltage simulations using the method of this invention and existing methods when the residual voltage of a power grid fault is 0.6 pu.

[0082] Figure 26 This is a simulation comparison of the terminal voltage of an offshore wind farm using the method of this invention and existing methods when the residual voltage of a grid fault is 0.6 pu.

[0083] Figure 27 This is a simulation comparison of the active power of an offshore wind farm using the method of this invention and existing methods when the residual voltage of the power grid fault is 0.6 pu.

[0084] Figure 28 This is a simulation comparison of the active power of an onshore wind farm using the method of this invention and existing methods when the residual voltage of the power grid fault is 0.6 pu.

[0085] Figure 29 This is a simulation comparison of the active power of a synchronous generator using the method of this invention and existing methods when the residual voltage of the power grid fault is 0.6 pu.

[0086] Figure 30 This is a comparison chart of power grid frequency simulations using the method of this invention and existing methods when the residual voltage of the power grid fault is 0.6 pu. Detailed Implementation

[0087] To clearly and completely describe the technical solution and its specific working process of the present invention, the specific embodiments of the present invention are as follows, in conjunction with the accompanying drawings:

[0088] Example 1

[0089] Figure 1 The structure of offshore wind farms connected to a low-inertia system via flexible DC transmission is as follows: the onshore power grid is represented by an equivalent steam turbine generator, the onshore wind farm is composed of a 1:1 mixture of direct-drive and doubly-fed wind turbine units, while the offshore wind farm consists of direct-drive wind turbines. The maximum proportion of onshore and offshore wind power in the total power generation is 57.1%.

[0090] Low inertia systems are mainly caused by two factors: first, the centralized access of a high proportion of new energy sources to the grid will replace a portion of the traditional power supply capacity, resulting in a reduction in the rotational kinetic energy of the system; second, high-voltage direct current transmission will separate the interconnected grid into multiple asynchronous sub-networks, leading to a decrease in system inertia.

[0091] Studies have shown that the penetration rate of new energy sources η is related to the inertia coefficient H of the power system. s Approximate inverse proportional relationship, that is

[0092] (1)

[0093] In the formula, H s0 This represents the inertia coefficient when only synchronous power sources are present. Generally, when H... s When the time is greater than 4 seconds, the system inertia is considered sufficient, while H s A value less than 2 seconds indicates a severe deficiency in system inertia. Therefore, when H... s When =4 s and η>50%, Figure 1 The example system of offshore wind power directly connected to the grid via flexible transmission will simultaneously meet the above conditions, forming a typical low inertia operation scenario. Through calculation, its inertia coefficient can be obtained as 1.714 s.

[0094] Figure 1 In the system shown, the dynamic processes of active power and frequency in the power grid can be described by the rotor motion equations of the synchronous generator.

[0095] (2)

[0096] In the formula, f s P represents the system frequency. m and P e P represents the mechanical and electromagnetic power of the equivalent generator, respectively. w and P h P represents onshore wind power and flexible DC transmission power, respectively. l This represents the load power.

[0097] During grid outages, due to the limited short-circuit capacity of wind turbine converters and flexible DC converter stations, P w and P h All follow P l Significantly reduced, while synchronous generators have a very high short-circuit capacity, P e There will be no drastic changes, so the power loss in the system will be minimal; after the fault is cleared, to prevent DC line voltage from exceeding limits and to protect the unit equipment, P w and P h Both exhibit significant delayed recovery behavior; meanwhile, P l As the load-side voltage recovers rapidly, then P e It will rise sharply to quickly balance P.l And P m Since the speed regulation delay cannot be changed immediately, f in equation (1) will drop. From equations (1) and (2), it can be seen that in a low-inertia power system with high-penetration wind power integration, H... s The value of f is very small, so the drop in f is more severe after the fault is cleared.

[0098] Figures 2-7 To utilize Figure 1 The system shown is simulated on the PSCAD / EMTDC platform, demonstrating the operation of the above process. Considering the delayed recovery behavior of wind power, the frequency shift process caused by grid faults can be divided into three stages:

[0099] 1) Frequency increase phase: corresponding to Figure 3 The time from the occurrence of a fault to its clearance. Since onshore converter stations lack the capability for disturbance power distribution, the active power injected into the grid by the flexible DC transmission system during a fault depends solely on the voltage drop level, such as... Figure 6 As shown. To prevent DC overvoltage at the converter station, offshore converter stations employ step-down control to reduce the active power of the wind farm, such as... Figure 7 As shown. During this stage, the mechanical torque of the equivalent steam turbine generator remains constant, as... Figure 1 As shown; electromagnetic torque (power) decreases with load power, such as Figure 4 and Figure 5 As shown, the imbalance of electromechanical power causes the system frequency to rise.

[0100] 2) Frequency drop phase: corresponding to Figure 3 The time from the clearing of the fault to the lowest frequency point. Frequency drops are mainly caused by the delayed recovery behavior of wind power after a voltage drop, such as... Figure 7 As shown. Based on the timing of the prime mover-governor operation, this stage is further subdivided into the inertia-dominant stage and the speed-governing stage, with the inertia-dominant stage corresponding to the frequency response approximately 1 second after the fault. After the fault is cleared, as... Figure 5 As shown, the load power rises rapidly with the voltage, and the network imbalance is mainly compensated by the generator releasing rotor kinetic energy, thus causing the frequency to drop rapidly. In the subsequent speed regulation stage, the system inertia support decreases, and the synchronous generator's speed regulation system suppresses the frequency drop rate by significantly increasing the input power of the prime mover. When the electromechanical power of the synchronous generator is equal, the frequency drops to its lowest point.

[0101] 3) Frequency recovery stage: In this stage, the inertia support disappears, and the frequency recovery is mainly dominated by the speed control system, which eventually adjusts the frequency to the allowable range.

[0102] In the above stages, the method of this embodiment focuses on the inertial response process after a grid fault. The coordinated active power recovery control after faults in offshore wind power and flexible DC transmission needs to consider three factors:

[0103] 1) The transient frequency drop rate and the lowest frequency point of the system;

[0104] 2) The speed and safety constraints of DC voltage dynamic response;

[0105] 3) Compliance of offshore wind power and flexible DC transmission with active power delay recovery behavior;

[0106] Based on the above considerations, the design concept of this control method is as follows: In the early stage of fault recovery, the coupling characteristics of DC voltage and wind power are fully utilized to provide inertial support to the grid by rapidly increasing wind power and releasing DC capacitor energy storage, thereby suppressing the frequency drop rate and reducing the oscillation of active current in the onshore converter station; thereafter, wind power and DC voltage are decoupled: the recovery of wind power is transformed into a rate limiting mode to protect equipment safety, while the dynamic response of DC voltage is improved through adaptive adjustment to suppress transient overshoot of DC voltage.

[0107] Example 2

[0108] After the grid fault is cleared, the onshore converter station adopts an adaptive DC voltage control method. In the initial stage of recovery, it releases the DC capacitor energy storage to provide inertial support to the grid and suppress the frequency drop rate. Thereafter, it suppresses the transient overshoot of the DC line voltage through adaptive adjustment. Specifically, by adjusting the loop bandwidth of proportional-integral (PI) control in real time, it can quickly release the DC capacitor energy storage to the grid during the fault recovery period, minimize the transient process of DC voltage, and ensure the safe operation of the flexible DC converter station.

[0109] like Figure 8 As shown, the adaptive DC voltage control method specifically includes the following:

[0110] A. At the rated DC voltage V dcN Two sets of boundary zones are defined around it:

[0111] 1) Internal boundary zone [V] th1 V th2 ], which corresponds to the range of DC voltage during steady-state operation, V th1 and V th2 These are the upper and lower boundary values ​​of the internal boundary band, respectively;

[0112] 2) External boundary zone [V] th3 V th4 ], which corresponds to the adaptive adjustment region of DC voltage during fault ride-through, V th3 and V th4These are the upper and lower boundary values ​​of the outer boundary band, respectively;

[0113] To simplify the design, Figure 8 Chinese f c With V dcr The relationship is linear, but it can also be implemented as a power series or logarithmic function. Switching between boundary bands is achieved through hysteresis control.

[0114] B. In the internal boundary band, the DC voltage V dcr and control loop bandwidth f c The relationship can be divided into two specific cases:

[0115] 1) When the DC voltage recovers from the external boundary band, it is necessary to quickly eliminate the steady-state offset of the DC voltage through adaptive adjustment, until V dcr =V dcN The regulator is then reset, therefore the relationship between the loop bandwidth and the DC voltage offset is:

[0116] (3)

[0117] In the formula, k1 is the proportionality coefficient and k1 < 0, f cs For V dcN The corresponding control bandwidth;

[0118] 2) During steady-state operation, f c =f cs ;

[0119] C. In the outer boundary zone, the DC voltage V dcr and control loop bandwidth f c The relationship can be divided into three specific situations:

[0120] 1) DC voltage increases and V th2 ≤V dcr ≤V th4 At that time, the FRT control did not operate, and the DC voltage loop control bandwidth remained unchanged, i.e., f c =f cs ;

[0121] 2) DC voltage decreases and V th4 ≥V dcr ≥V th2 This corresponds to the active power recovery process after a fault. It requires adaptive adjustment of control parameters according to the DC voltage offset to achieve rapid release of energy stored in the DC capacitor. Therefore, the loop bandwidth is positively correlated with the DC voltage offset.

[0122] (4)

[0123] In the formula, k2 is the proportionality coefficient, and k2>0;

[0124] 3) DC voltage increases and V th3 ≤V dcr ≤V th1 This corresponds to the DC voltage recovery process. During this stage, adaptive adjustment of control parameters is also needed to suppress transient fluctuations in the DC voltage. The loop bandwidth f... c The expression is the same as that in equation (3);

[0125] Let V dcr The minimum and maximum allowed values ​​are V respectively. min and V max Then when the DC voltage is in [V min V th3 ] and [V th4 V max When the range is defined, it can correspond to any stage after the fault, and f is given. c =f cm f cm This represents the maximum allowable control bandwidth for the DC voltage loop, with the aim of quickly eliminating excessive DC voltage offset. It should be noted that f... cs and f cm The selection of the converter should take into account factors such as the AC current harmonic level, the DC capacitor size, and the converter station switching frequency.

[0126] Figure 9 This is a comprehensive adaptive DC voltage control implementation scheme based on the above analysis. The adaptive parameter regulator obtains the control bandwidth f according to the DC voltage offset. c Then it is converted into the control parameter k. pdc and k idc (Control parameter conversion), assigned to the PI control loop, f c and k pdc k idc The relationship is represented as

[0127] (5)

[0128] In the formula, C dc S is the equivalent capacitance of a DC line. B and V dcB ξ represents the rated capacity and rated DC voltage of the flexible DC system, respectively, and ξ is the damping ratio.

[0129] It should be noted that after the fault occurs, the onshore converter station switches to reactive current support mode. Therefore, the reference value of the active current generated by DC voltage control is limited.

[0130] (6)

[0131] Due to the effect of the limiting element, the PI control element is effectively disabled, although k pdc and k idc The parameters have changed, but this will not affect the dynamic performance of the wind power-flexible DC system during the fault. After the fault is cleared, the limiting circuit fails, and the adaptive DC voltage control quickly adjusts the active current I... hd Improve V dcr The transient performance is improved, and voltage overshoot is minimized.

[0132] Example 3

[0133] Offshore wind turbines employ an adaptive two-stage active power recovery control method. In the initial recovery phase, wind power is increased to provide inertial support to the grid and suppress frequency sag rate. Subsequently, wind power is switched to a rate-limited recovery mode to protect the safety of the turbine equipment.

[0134] The two-stage active current recovery control curve of offshore wind turbine is as follows: Figure 10 As shown, during a grid fault, the offshore converter station uses voltage droop control to reduce the wind farm terminal voltage V. w At the same time, the wind turbine switches to voltage-type active current control mode, according to V w Automatic adjustment of active current i wd After a brief delay following a grid fault, the two-stage active current recovery control is activated. Once the fault is cleared, the offshore wind turbines employ an adaptive two-stage active current recovery control method, including a rapid active current recovery stage and an active current recovery rate limiting stage, as detailed below:

[0135] 1) Active power rapid recovery phase: DC voltage V dcr and wind farm terminal voltage V w Through droop control and the active current i of the wind turbine wd Mutual coupling, during which the wind turbine still maintains the voltage-type active current control mode during the fault period, i wd The expression is

[0136] (7)

[0137] In the formula, V wN V wl and V wm These represent the steady-state operating value, the highest and lowest limits during the rapid recovery phase of the terminal voltage of an offshore wind farm, respectively, while i wN This represents the steady-state operating value of the active current of the wind turbine generator.

[0138] During this stage, DC voltage and wind power are coupled. Onshore converter stations quickly eliminate transient overshoot of DC voltage through adaptive DC voltage control, while offshore converter stations rapidly raise the voltage droop control in conjunction with the voltage-type active current control of the wind turbine. wd This enables rapid recovery of wind farms during low-power periods by providing inertial power to the grid via flexible DC transmission. The low-power threshold P in wind farm turbines... wl For the corresponding voltage threshold V wl and corresponding current threshold i wl The product of these terms generally falls within the range of 0.2 ≤ P. wl ≤0.3 pu.

[0139] 2) Active power recovery rate limiting stage: DC voltage V dcr and the active current i of each unit wd Decoupling, i wd No longer following V w It does not change with the rise of time, but is a function of time t, and its expression is:

[0140] (8)

[0141] In the formula, i wl The first stage voltage limit V wl The corresponding active current value, k w The preset active current recovery rate, t c and t r These represent the start and end times of the recovery phase, respectively.

[0142] During this phase, offshore wind farms can fully adhere to the power delay recovery behavior to protect the safety of the turbine equipment.

[0143] Figure 11 This is a structural diagram of a two-stage active current recovery control system, implemented by embedding a variable rate limiter in the voltage-type active current control loop. According to... Figure 11 The control method shown may result in three scenarios for wind power recovery:

[0144] 1) During steady-state operation, i wd >i wl And i occurred during the fault wd ≤i wl In the case of i wd wl Then the rate of ascent limit value is changed from k w It becomes ∞.

[0145] 2) i exists during both steady-state operation and fault periods. wd >i wl Then the rate of ascent limit remains k.​w The value remains unchanged; at this point, wind power recovery is achieved solely through rate limiting control. This is because i wd >i wl This indicates that the severity of the power grid fault is not high, and the system does not need to utilize the fast recovery characteristics of the low-power section of the wind farm to alleviate the frequency offset of the onshore power grid.

[0146] 3) i during steady-state operation wd ≤i wl In this case, the recovery process is entirely dominated by adaptive boost control. This is because the wind turbines in the wind farm are operating at low power at this time, and there is no need for power delay recovery protection equipment.

[0147] Therefore, the two-stage active current recovery control scheme is adaptive to the severity of grid faults and is easy to implement without requiring complex control logic switching.

[0148] To verify the effectiveness of the cooperative active power recovery control method of the present invention, a system was built on the PSCAD / EMTDC platform. Figure 1 The simulation model shown illustrates an offshore wind farm connected to a low-inertia power grid via flexible DC transmission. The simulation considers both minor and severe fault operating conditions. Both onshore and offshore wind farms operate at maximum power in steady state. A three-phase short-circuit fault occurs on the AC line connecting the flexible DC transmission to the grid, lasting 0.2 seconds. The simulation assumes a DC voltage fluctuation rate ε = 10% for the MMC (Medium-to-Cycle Controller), and a corresponding equal-capacity discharge time constant H. dc =45ms. The typical method of reference [1] is selected for comparison with the method of this invention. It should be noted that the existing technology (Advanced LVRT Control Scheme for Offshore Wind Power Plant) provides an existing active power recovery control method that considers the transient frequency offset of the power grid. Its basic idea is: during the fault, the active power imbalance of the flexible DC body is used to improve the wind power injection level, that is, by reducing the load reduction efficiency of the wind farm, the wind power injection power is artificially formed to be higher than the transmission power of the onshore converter station. The magnitude of the unbalanced power depends on the size and withstand voltage level of the DC equivalent capacitor. After the fault is cleared, the active power recovery of the onshore converter station is realized by using the rapid rise characteristics of AC voltage. The DC capacitor energy is released through HVDC to provide inertial power to the power grid. After the offshore wind turbine detects the rise in terminal voltage, its active power rises to the steady-state operating value at a preset rate. In this embodiment, the control method of this invention and the existing method are compared in the same model system. The simulation results are as follows. Figures 12-21 As shown.

[0149] Figures 12-21The transient responses of the wind power-flexible DC system and the onshore power grid are shown when the residual fault voltage is 0.4 pu and the active power recovery rate is 0.5 pu / s, respectively. According to grid connection standards, after a fault occurs, the onshore converter station injects as much reactive power as possible into the grid; at this point, the active power imbalance of the flexible DC system is at its maximum. Figure 13 and Figure 14 As shown. Subject to the size (H) of the MMC-HVDC DC capacitor. dc Limited by the ≤45ms threshold and maximum withstand voltage level (typically 1.1 pu), flexible DC transmission lines have very low tolerance to active power imbalance. Therefore, existing methods lead to more severe DC line overvoltages, such as... Figure 15 As shown. The method of this invention significantly improves the overvoltage level of DC lines during faults and reduces the electrical stress of the flexible DC converter station by rapidly balancing the input and output power of the flexible DC converter, such as... Figure 14 , Figure 15 and Figure 17 As shown.

[0150] After the fault was cleared, both methods provided short-term frequency support to the grid, following the delayed recovery characteristics of wind power, such as... Figure 14 and 17 As shown. Existing methods only provide inertial power to the grid by releasing the energy stored in DC capacitors, while the method of this invention utilizes both DC energy storage and the rapid rise characteristics of wind farms in the low-power phase, improving V through adaptive DC voltage control. dch It improves transient response performance and enhances the active power recovery efficiency of offshore wind power and flexible DC transmission, while reducing power impact on the power grid. Figure 20 This indicates that the method of the present invention has a better effect on improving transient frequency shift than the methods in the literature.

[0151] Figures 22-30 The transient response of the system is demonstrated when the fault residual voltage is 0.6 pu and the active power recovery rate is 0.5 pu / s. When the voltage drop is not severe, due to the limitations of DC capacitor size and maximum withstand voltage level, existing methods have limited effectiveness in improving wind power output during faults, such as... Figure 27 As shown. Therefore, Figure 25 The maximum voltage of the medium-voltage DC line is only slightly higher than that of the method of this invention. After the fault is cleared, existing methods completely follow the delayed recovery behavior of wind power, while the method of this invention improves the recovery efficiency of wind power by utilizing adaptive two-stage active current recovery control, effectively improving the minimum frequency drop point, such as... Figure 30 As shown.

[0152] The above examples of the present invention are merely illustrative of the computational model and process of the present invention, and are not intended to limit the implementation of the present invention. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is impossible to exhaustively list all possible implementations here. Any obvious variations or modifications derived from the technical solutions of the present invention are still within the scope of protection of the present invention.

Claims

1. A collaborative active power recovery control method for offshore wind farms connected to flexible DC grids, characterized in that, Specifically, the steps include the following: After the grid fault is cleared, the onshore converter station adopts an adaptive DC voltage control method. In the early stage of recovery, it releases the DC capacitor to store energy, provides inertial support to the grid, and suppresses the frequency drop rate. Thereafter, it suppresses the transient overshoot by adaptively adjusting the DC line voltage. Offshore wind turbines employ an adaptive two-stage active power recovery control method. In the initial recovery phase, wind power is increased to provide inertial support to the grid and suppress frequency sag rate. Subsequently, wind power is switched to a rate-limited recovery mode to protect the safety of the turbine equipment. The coordinated recovery of wind farms after faults via flexible DC grid connection is achieved by changing the coupling characteristics of DC line voltage and wind power. The adaptive DC voltage control method specifically includes the following: A. At the rated DC voltage V dcN Two sets of boundary zones are defined around it: 1) Internal boundary zone [V] th1 V th2 ], which corresponds to the range of DC voltage during steady-state operation, V th1 and V th2 These are the upper and lower boundary values ​​of the internal boundary band, respectively; 2) External boundary zone [V] th3 V th4 ], which corresponds to the adaptive adjustment region of DC voltage during fault ride-through, V th3 and V th4 These are the upper and lower boundary values ​​of the outer boundary band, respectively; B. In the internal boundary band, the DC voltage V dcr and control loop bandwidth f c The relationship can be divided into two specific cases: 1) When the DC voltage recovers from the external boundary band, the steady-state offset of the DC voltage is eliminated through adaptive adjustment until V dcr =V dcN The regulator is then reset, therefore the relationship between the loop bandwidth and the DC voltage offset is: (3) In the formula, k1 is the proportionality coefficient and k1 < 0, f cs For V dcN The corresponding control bandwidth; 2) During steady-state operation, f c =f cs ; C. In the outer boundary zone, the DC voltage V dcr and control loop bandwidth f c The relationship can be divided into three specific situations: 1) DC voltage increases and V th2 ≤V dcr ≤V th4 At that time, the FRT control did not operate, and the DC voltage loop control bandwidth remained unchanged, i.e., f c =f cs ; 2) DC voltage decreases and V th4 ≥V dcr ≥V th2 This corresponds to the active power recovery process after a fault. It requires adaptive adjustment of control parameters according to the DC voltage offset to release the energy stored in the DC capacitor. Therefore, the loop bandwidth is positively correlated with the DC voltage offset. (4) In the formula, k2 is the proportionality coefficient, and k2>0; 3) DC voltage increases and V th3 ≤V dcr ≤V th1 This corresponds to the DC voltage recovery process. During this stage, adaptive adjustment of control parameters is also needed to suppress transient fluctuations in the DC voltage. The loop bandwidth f... c The expression is the same as that in equation (3); Let V dcr The minimum and maximum allowed values ​​are V respectively. min and V max Then when the DC voltage is in [V min V th3 ] and [V th4 V max When the range is defined, it can correspond to any stage after the fault, and f is given. c =f cm f cm This represents the maximum allowable control bandwidth for the DC voltage loop. D. The adaptive parameter regulator obtains the control bandwidth f according to the DC voltage offset. c Then it is converted into the control parameter k. pdc and k idc And assign the value to the PI control loop, f c and k pdc k idc The relationship is represented as 、 (5) In the formula, C dc S is the equivalent capacitance of a DC line. B and V dcB ξ represents the rated capacity and rated DC voltage of the flexible DC system, respectively, and ξ is the damping ratio.

2. The method for coordinated active power recovery control of offshore wind farms connected to flexible DC grids as described in claim 1, characterized in that, The adaptive two-stage active power recovery control method divides the wind power recovery into two stages: (1) Active power recovery stage: DC voltage V dcr and wind farm terminal voltage V w Through droop control and the active current i of the wind turbine wd Mutual coupling, during which i wd The expression is (7) In the formula, V wN V wl and V wm These represent the steady-state operating value, the highest and lowest limits during the rapid recovery phase of the terminal voltage of an offshore wind farm, respectively, while i wN This represents the steady-state operating value of the active current of the wind turbine generator. During this phase, onshore converter stations eliminate transient overshoot of DC voltage through adaptive DC voltage control, while offshore converter stations raise the voltage i through voltage droop control in conjunction with voltage-source active current control of wind turbines. wd To restore the low-power section of the wind farm; (2) Active power recovery rate limiting stage: DC voltage V dcr And the active current i of the wind turbine wd Decoupling, i wd No longer following V w It does not change with the rise of time, but is a function of time t, and its expression is: (8) In the formula, i wl The first stage voltage limit V wl The corresponding active current value, k w For the preset active current recovery rate, t s and t e These represent the start and end times of the recovery phase, respectively. During this phase, offshore wind farms fully comply with the delayed recovery behavior of wind power to meet the standards for grid connection of new energy sources and protect the safe operation of the turbine equipment.

3. The method for coordinated active power recovery control of offshore wind farms connected to flexible DC grids as described in claim 1, characterized in that, There are three scenarios in the active power recovery phase, as follows: 1) During steady-state operation, i wd >i wl And during the fault, i wd ≤i wl In the case of i wd wl Then the rate of ascent limit value is changed from k w It becomes ∞;​ 2) During steady-state operation and fault periods, i wd >i wl Then the rate of ascent limit remains k. w The wind power remains unchanged, and at this point, wind power recovery is achieved solely through rate limiting control. 3) During steady-state operation, i wd ≤i wl In this case, the recovery process is entirely dominated by adaptive boost control.

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