Offshore wind power flexible direct current converter valve fault ride-through control method for reducing transient current overshoot

Abstract

The invention discloses an offshore wind power flexible direct current converter valve fault ride-through control method for reducing transient current overshoot, relates to the technical field of new energy grid connection, and solves the problem of transient current overshoot of an offshore wind power flexible direct current converter valve (MMC) during a fault ride-through period. A mathematical model of the offshore wind power flexible direct current converter valve is established; separating positive and negative sequence components by adopting a bisynchronous rotation dq coordinate system transformation mode, and designing a corresponding positive sequence inner ring current controller and a corresponding negative sequence inner ring current controller; detecting the power grid voltage drop condition through an effective value algorithm, adjusting active and reactive power reference values according to the detected power grid voltage value, and controlling the voltage to be stable; the method comprises the following steps: estimating the impedance of a power grid in real time by using a Kalman filter, introducing adaptive virtual impedance, dynamically adjusting a virtual impedance value through model predictive control (MPC), and adaptively inhibiting transient current; according to the method, it can be ensured that transient current overshoot is effectively reduced during the fault ride-through period of the offshore wind power flexible direct current converter valve, the stability and reliability of the system are improved, and a safer and more efficient fault ride-through control strategy is provided for offshore wind power grid connection.

Description

Technical Field

[0001] The present invention belongs to the technical field of new energy grid connection, and relates to a fault ride-through control method for a flexible DC converter valve of an offshore wind farm to reduce transient current overshoot. Background Art

[0002] With the rapid development of offshore wind power, High-Voltage Direct Current (HVDC) technology has been widely applied in the offshore wind power field due to its significant advantages in long-distance power transmission and grid connection. However, in actual operation, the offshore wind power flexible DC system inevitably faces various fault challenges, especially the transient current impact problem caused by AC side short-circuit faults. If this problem cannot be effectively controlled, it may lead to overcurrent damage to the internal devices of the offshore wind power flexible DC converter valve (Modular Multilevel Converter, MMC), misoperation of system protection, and even large-scale chain reactions in the power system, seriously affecting the stability of the power grid. The traditional Virtual Synchronous Generator (VSG) control method maintains the power setting before the fault during low voltage ride-through, which may result in a large impact current and cannot effectively suppress the transient current overshoot. Although measures such as fault isolation and overcurrent protection have been taken in the offshore wind power flexible DC system, these measures are still not effective enough in suppressing the transient current overshoot. The converter valve suffers severe impacts during fault ride-through, affecting the system stability and may cause equipment damage.

[0003] Although the existing fault ride-through technologies can support the grid connection operation of the offshore wind power flexible DC converter valve, it is still difficult to ensure the stability of the system output power while effectively suppressing the transient current overshoot. In addition, there are significant differences in the current suppression effects under different fault types, and there is a lack of flexible adjustment ability in the face of system disturbances. Therefore, there is an urgent need to develop a new method that can effectively reduce the transient current overshoot and improve the reliability and stability of the system during fault occurrence.

[0004] For this reason, the present invention proposes a fault ride-through control method for a flexible HVDC converter valve of an offshore wind farm to reduce transient current overshoot. This method mathematically models the flexible HVDC converter valve of the offshore wind farm, generates active and reactive power in combination with the grid voltage amplitude as the power reference value during low voltage ride-through. Further, considering the influence of line impedance, a Kalman filter is used to estimate the grid impedance in real time and dynamically update the control parameters. The model predictive control (MPC) method is introduced, and in combination with the adaptive virtual impedance, the virtual impedance value is dynamically adjusted based on the prediction of the future grid state, effectively suppressing the transient current overshoot, significantly improving the fault ride-through ability of the flexible HVDC system of the offshore wind farm under grid faults, ensuring equipment safety, enhancing system stability, and having important application prospects and practical significance. Summary of the Invention

[0005] The object of the present invention is to provide a fault ride-through control method for a flexible HVDC converter valve of an offshore wind farm to reduce transient current overshoot, which solves the problem of transient current impact of the converter valve during fault ride-through, aims to improve the fault ride-through ability of the offshore wind power system during converter valve faults, reduce transient current overshoot, protect equipment from overcurrent, and thus ensure the stable operation of the system.

[0006] The present invention solves the above technical problems through the following technical solutions:

[0007] A fault ride-through control method for a flexible HVDC converter valve of an offshore wind farm to reduce transient current overshoot, comprising the following steps:

[0008] S1. Establish a corresponding mathematical model according to the topology of the flexible HVDC converter valve of the offshore wind farm;

[0009] S2. Based on the mathematical model in step S1, when a grid fault occurs, use a double synchronous rotating dq coordinate system transformation to separate the positive and negative sequence components of voltage and current, and design a positive sequence inner loop current controller and a negative sequence inner loop current controller respectively to achieve independent regulation of positive and negative sequence currents;

[0010] S3. Calculate the effective value U of the grid voltage in real time through the effective value algorithm rms , calculate the voltage dip depth, and generate the active power reference value and reactive power reference value during low voltage ride-through based on the voltage dip depth;

[0011] S4. Considering the influence of line impedance, use a Kalman filter to identify the grid impedance in real time, combine with the model predictive control algorithm, roll-optimize the virtual impedance value in multiple future control cycles, generate an optimal control sequence, and achieve active suppression of transient current;

[0012] S5. Verify and analyze the effectiveness of the fault ride-through control method for the HVDC converter valve of offshore wind power in reducing the transient current overshoot under two fault conditions.

[0013] Furthermore, the method for establishing the mathematical model of the HVDC converter valve of offshore wind power is as follows:

[0014] According to the topological structure of the HVDC converter valve of offshore wind power, the calculation formulas for the voltage and current relationships of the upper and lower bridge arms are as follows:

[0015]

[0016] The calculation formulas for the differential-mode voltage and common-mode voltage of the upper and lower bridge arms are as follows:

[0017]

[0018] Rewrite the above formulas into three-phase form, and the calculation formula for the HVDC converter valve of offshore wind power is as follows:

[0019]

[0020] In the formula, L is the equivalent inductance after simplification, and R is the equivalent resistance after simplification.

[0021] Furthermore, when a fault occurs in the power grid, the method for separating the positive and negative sequence components of voltage and current by using the double synchronous rotating dq coordinate system transformation and respectively designing the positive sequence inner loop current controller and negative sequence inner loop current controller to realize the independent regulation of positive and negative sequence currents is as follows:

[0022] The calculation formula of the mathematical model of the HVDC converter valve of offshore wind power can be expressed in the form of the superposition of positive and negative sequence current and voltage components. To realize the separation of positive and negative sequence components, the double synchronous rotating dq coordinate system transformation method is adopted. The positive sequence components are mapped to the dq coordinate system through the forward rotating coordinate transformation, and the negative sequence components are mapped to the dq coordinate system through the reverse rotating coordinate transformation. Then the specific formulas after transformation are as follows:

[0023]

[0024] Perform Laplace transform, and the specific formula of the control equation is as follows:

[0025]

[0026] In the formula: are respectively the reference values of the positive sequence d and q components of the differential-mode voltage of the bridge arm, are respectively the reference values of the negative sequence d and q components of the differential-mode voltage of the bridge arm, are respectively the reference values of the positive sequence d and q components of the MMC output current, They are the reference values of the negative-sequence d and q components of the MMC output current respectively. The positive-sequence and negative-sequence inner-loop current controllers are obtained according to the control equations.

[0027] Further, the effective value U of the grid voltage is calculated in real time by the effective value algorithm rms , and the specific method for calculating the voltage dip depth and generating the reference values of the active power and reactive power during the low-voltage ride-through period based on the voltage dip depth is as follows:

[0028] Since the VSC-HVDC converter valve of the offshore wind power adopts the VSG control technology, when the grid operates normally, the VSG is also in the normal working state. In this case, the fault detection software switch S1 is 0, and the set active power is P n , and the set reactive power reference value is Q n . Once it is detected at the grid connection point that the grid voltage drops below 0.9 p.u. due to a fault, the fault detection software switch S1 turns to 1. In this case, the set active power reference value switches from P n to P ref , and the set reactive power reference value switches from Q n to Q ref . At the same time, the reactive power droop control link is blocked, avoiding the deviation of the reactive power output. When the grid voltage returns to normal, the system will automatically return to the initial control state.

[0029] Further, the specific method for actively suppressing the transient current by using the Kalman filter to identify the grid impedance in real time, combining with the model predictive control (MPC) algorithm, and rolling to optimize the virtual impedance value in multiple future control cycles to generate the optimal control sequence is as follows:

[0030] The Kalman filter is used for real-time impedance estimation, and the specific formulas of its state equation and observation equation are as follows:

[0031] x(k + 1) = Ax(k) + Bu(k) + w(k)

[0032] y(k) = Cx(k) + v(k)

[0033] where x(k) is the state vector, u(k) is the input vector, y(k) is the observation vector, and w(k) and v(k) are the process noise and observation noise respectively;

[0034] The estimated value of the grid impedance is updated in real time through the Kalman filter, providing accurate impedance information for the MPC algorithm. The goal of MPC is to minimize the current overshoot and ensure stable power output. By optimizing the adjustment of the virtual impedance, the objective function of MPC can be expressed as:

[0035]

[0036] Among them, λ is the weight coefficient used to balance transient current suppression and impedance matching, and i t is the transient current.

[0037] During the optimization process, the following constraints need to be considered:

[0038] ΔZ min ≤ΔZ v ≤ΔZ max

[0039] ‖i t (k)‖≤i max

[0040] Z grid,min ≤Z grid (k)≤Z grid,max

[0041] The virtual impedance value optimized by MPC is fed back to the VSG control loop to dynamically adjust the virtual impedance value and suppress the transient current overshoot. The voltage components generated by the virtual resistor in the dq coordinate system are fed back to the double closed-loop control circuit of the VSG. The specific formula for the feedback quantity generated by the virtual resistor is as follows:

[0042]

[0043] When the grid fault amplitude drops, the Kalman filter is used to identify the grid impedance in real time, so as to dynamically adjust the virtual impedance value and adaptively suppress the transient current.

[0044] The advantages of the present invention are as follows: The technical solution of the present invention considers the problem of transient current overshoot of the HVDC converter valve for offshore wind power during fault ride-through, uses the Kalman filter for impedance grid identification, introduces the model predictive control method, combines the adaptive virtual impedance, and dynamically adjusts the virtual impedance value based on the prediction of the future grid state, effectively suppressing the transient current overshoot and ensuring the stable operation of the system; compared with the traditional VSG control method, the present invention can effectively reduce the transient current overshoot, significantly improve the fault ride-through ability of the HVDC system for offshore wind power under grid faults, ensure the safety of equipment, improve the system stability, and has important application prospects and practical significance. Brief Description of the Drawings

[0045] Figure 1 is the flowchart of the fault ride-through control method for the HVDC converter valve of offshore wind power to reduce transient current overshoot according to the embodiment of the present invention;

[0046] Figure 2 is the topological structure diagram of the HVDC converter valve of offshore wind power according to the embodiment of the present invention;

[0047] Figure 3 is the block diagram of the positive sequence inner loop current controller according to the embodiment of the present invention;

[0048] Figure 4 Block diagram of the negative-sequence inner-loop current controller for the embodiment of the present invention;

[0049] Figure 5 Block diagram of the fault ride-through control structure based on VSG control for the embodiment of the present invention;

[0050] Figure 6 Block diagram of the control with an adaptive virtual impedance under the introduction of the MPC algorithm for the embodiment of the present invention;

[0051] Figure 7 Output voltage waveform under the fault condition of the conventional HVDC converter valve for offshore wind power in the embodiment of the present invention;

[0052] Figure 8 Output current waveform under the fault condition of the conventional HVDC converter valve for offshore wind power in the embodiment of the present invention;

[0053] Figure 9 Output voltage waveform under Fault Condition 1 using the control method of the present invention in the embodiment of the present invention;

[0054] Figure 10 Output current waveform under Fault Condition 1 using the control method of the present invention in the embodiment of the present invention;

[0055] Figure 11 Analysis diagram of the output voltage THD under Fault Condition 1 using the control method of the present invention in the embodiment of the present invention;

[0056] Figure 12 Analysis diagram of the output current THD under Fault Condition 1 using the control method of the present invention in the embodiment of the present invention;

[0057] Figure 13 Output power waveform under Fault Condition 1 using the control method of the present invention in the embodiment of the present invention;

[0058] Figure 14 Output voltage waveform under Fault Condition 2 using the control method of the present invention in the embodiment of the present invention;

[0059] Figure 15 Output current waveform under Fault Condition 2 using the control method of the present invention in the embodiment of the present invention;

[0060] Figure 16 Analysis diagram of the output voltage THD under Fault Condition 2 using the control method of the present invention in the embodiment of the present invention;

[0061] Figure 17 Analysis diagram of the output current THD under Fault Condition 2 using the control method of the present invention in the embodiment of the present invention;

[0062] Figure 18 Output power waveform under fault condition 2 with the control method of the present invention in the embodiment of the present invention Detailed implementation manners

[0063] To make the objectives, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0064] The technical solutions of the present invention will be further described below in conjunction with the accompanying drawings of the specification and specific embodiments:

[0065] As Figure 1 shown, a fault ride-through control method for a flexible HVDC converter valve of an offshore wind farm to reduce transient current overshoot provided in this embodiment includes the following steps:

[0066] Step 1: Establish a corresponding mathematical model according to the topology of the flexible HVDC converter valve of the offshore wind farm. The specific process is as follows:

[0067] As Figure 2 shown is the topology of the flexible HVDC converter valve of the offshore wind farm. According to its topology, the calculation formulas for the voltage and current relationships of the upper and lower bridge arms are as follows:

[0068]

[0069] The calculation formulas for the differential-mode voltage and common-mode voltage of the upper and lower bridge arms are as follows:

[0070]

[0071] Rewrite the above formulas into three-phase form to obtain the calculation formulas for the flexible HVDC converter valve of the offshore wind farm as follows:

[0072]

[0073] In the formula, L is the equivalent inductance after simplification, and R is the equivalent resistance after simplification.

[0074] It can be seen from the above formula that the output current of the flexible HVDC converter valve of the offshore wind farm is only related to the grid voltage and the differential-mode voltage of the bridge arm. Therefore, controlling the output current is ultimately achieved by controlling the differential-mode voltage of the bridge arm.

[0075] Step 2: Based on the mathematical model in Step 1, when a fault occurs in the power grid, taking the suppression of the negative-sequence current on the valve side of the HVDC converter valve for offshore wind power as the control objective, the positive- and negative-sequence components are separated by means of double synchronous rotating dq coordinate system transformation, and the corresponding positive-sequence inner-loop current controller and negative-sequence inner-loop current controller are designed. The specific process is as follows:

[0076] When an asymmetric fault occurs in the AC power grid, the bus voltage of the HVDC converter valve for offshore wind power is unbalanced, and negative-sequence and zero-sequence components will appear. Compared with the normal operating state, since the transformer on the valve side of the HVDC converter valve for offshore wind power adopts Yd11 wiring, there is no zero-sequence current path on the valve side, and a large negative-sequence current will be generated. After superimposing the positive-sequence component and the DC current, it may exceed the current capacity of the power electronic devices, threatening the safe and stable operation of itself. Therefore, for the HVDC converter valve for offshore wind power after an asymmetric fault, the commonly adopted control objective is to suppress the negative-sequence current on the valve side to avoid overcurrent of the power components.

[0077] The mathematical model calculation formula of the HVDC converter valve for offshore wind power can be expressed in the form of the superposition of positive- and negative-sequence current and voltage components. To separate the positive- and negative-sequence components, the double synchronous rotating dq coordinate system transformation method is used. The positive-sequence component is mapped to the dq coordinate system through the forward-rotating coordinate transformation, and the negative-sequence component is mapped to the dq coordinate system through the reverse-rotating coordinate transformation. The specific formulas after transformation are as follows:

[0078]

[0079] Performing Laplace transform, the specific formula of the control equation is as follows:

[0080]

[0081] In the formula: are the reference values of the positive-sequence d and q components of the bridge arm differential-mode voltage respectively, are the reference values of the negative-sequence d and q components of the bridge arm differential-mode voltage respectively, are the reference values of the positive-sequence d and q components of the MMC output current respectively, are the reference values of the negative-sequence d and q components of the MMC output current respectively. According to the control equation, the positive-sequence and negative-sequence inner-loop current controllers are obtained. The block diagram of the positive-sequence inner-loop current controller is as shown in Figure 3 shown, and the block diagram of the negative-sequence inner-loop current controller is as shown in Figure 4 shown.

[0082] Step 3: Detect the power grid voltage sag situation through the effective value algorithm, generate the reference values of the active power and reactive power during the low voltage ride-through period based on the voltage sag depth, and control the voltage stability. The specific process is as follows:

[0083] By using the effective value algorithm, the average voltage of the grid connection point within half a cycle is detected, and the grid voltage sag condition is detected accordingly. The specific formula is as follows:

[0084]

[0085] Since the VSC-HVDC converter valve for offshore wind power adopts the VSG control technology, when the power grid operates normally, the VSG is also in a normal working state. In this case, the fault detection software switch S1 is 0, and the set active power is P n , and the set reactive power reference value is Q n . Once it is detected at the grid connection point that the grid voltage drops below 0.9 p.u. due to a fault, the fault detection software switch S1 turns to 1. In this case, the set active power reference value changes from P n to P ref , and the set reactive power reference value changes from Q n to Q ref . At the same time, the reactive power droop control link is blocked, avoiding the deviation of reactive power output. When the grid voltage returns to normal, the system will automatically return to the initial control state. The fault ride-through control block diagram based on VSG control is as Figure 5 shown.

[0086] Step 4: Considering the influence of line impedance, use the Kalman filter to identify the grid impedance in real time, and combine with the model predictive control algorithm to roll-optimize the virtual impedance value in multiple future control cycles to generate the optimal control sequence and achieve the active suppression of transient current; the specific process is as follows:

[0087] The Kalman filter is used for real-time impedance estimation, and the specific formulas of its state equation and observation equation are as follows:

[0088] x(k + 1) = Ax(k) + Bu(k) + w(k)

[0089] y(k) = Cx(k) + v(k)

[0090] where x(k) is the state vector, u(k) is the input vector, y(k) is the observation vector, and w(k) and v(k) are the process noise and observation noise respectively;

[0091] The estimated value of the grid impedance is updated in real time through the Kalman filter to provide accurate impedance information for the MPC algorithm. The goal of MPC is to minimize the current overshoot and ensure stable power output. By optimizing the adjustment of the virtual impedance, the objective function of MPC can be expressed as:

[0092]

[0093] where λ is the weight coefficient used to balance the transient current suppression and impedance matching, it is the transient current.

[0094] During the optimization process, the following constraint conditions need to be considered:

[0095] ΔZ min ≤ΔZ v ≤ΔZ max

[0096] ‖i t (k)‖≤i max

[0097] Z grid,min ≤Z grid (k)≤Z grid,max

[0098] The voltage components generated by the virtual resistor in the dq coordinate system are fed back to the double closed-loop control circuit of the VSG. The specific formula for the feedback quantity generated by the virtual resistor is as follows:

[0099]

[0100] When the grid fault amplitude drops, the Kalman filter is used to identify the grid impedance in real time, so as to dynamically adjust the virtual impedance value and adaptively suppress the transient current overshoot. The control block diagram of the adaptive virtual impedance is as Figure 6 shown.

[0101] Simulation analysis

[0102] To verify the effectiveness of the proposed control method, a simulation model based on MATLAB / SIMULINK was built to simulate and analyze the fault ride-through control method of the flexible DC converter valve of the offshore wind power under VSG control. The simulation parameters are shown in Table 1.

[0103] Table 1 Simulation system parameters

[0104]

[0105] First, a simulation analysis was carried out on the case where the three-phase voltage of the conventional offshore wind power flexible DC converter valve symmetrically dropped to 50% of the rated voltage. The output voltage waveform is as Figure 7 shown, and the output current waveform is as Figure 8 shown.

[0106] Figure 7 and Figure 8 show the voltage and current waveforms when the three-phase voltage symmetrically drops to 50% of the rated voltage. The voltage drops at 0.2 s and returns to normal at 0.5 s. During the process of grid voltage drop and recovery, overcurrent phenomena occur, and the maximum impact current is about 7.5 times the rated value, which will cause irreversible damage to the hardware circuit.

[0107] The fault ride-through control method of the HVDC converter valve for offshore wind power proposed by the present invention is used for simulation analysis, and two simulation conditions are preset as shown in Table 2:

[0108] Table 2 Fault condition settings

[0109]

[0110] In the case of fault condition 1, it is set that the three-phase voltage symmetrically drops at 0.2 s of the grid voltage, and the voltage returns to normal at 0.5 s. During the fault condition, the grid voltage drops to 50%. Figure 9 and Figure 10 shows the voltage and current waveforms of the HVDC converter valve for offshore wind power when the voltage at the grid connection point drops to 50% of the rated voltage and lasts for 0.3 s. The voltage returns to normal at 0.5 s. During the process of the grid voltage drop and recovery, the instantaneous current amplitude of the HVDC converter valve for offshore wind power is limited to the set value, and no overcurrent phenomenon occurs. Figure 11 and Figure 12 shows the THD analysis of its output voltage and current. During the low voltage ride-through period, the harmonic distortion of the current is within the normal range of 5% allowed by the country, and the current waveform is stable. Figure 13 shows the power waveform under the fault condition. It can be seen from the figure that when the grid voltage fails, the active power will decrease accordingly. After adopting the fault ride-through control method for reducing the transient current overshoot proposed by the present invention, the reactive power will be significantly improved, thereby providing necessary reactive power support for the system until the grid voltage returns to the normal level. After the grid voltage is restored, the active power of the HVDC converter valve for offshore wind power returns to the rated value of 100 MW, and the reactive power returns to 0 var.

[0111] In the case of fault condition 2, it is set that the three-phase voltage symmetrically drops at 0.2 s of the grid voltage, and the voltage returns to normal at 0.5 s. During the fault condition, the grid voltage drops to 20%. Figure 14 shows the output voltage waveform of the HVDC converter valve for offshore wind power during the three-phase voltage symmetric drop, Figure 15 shows its output current waveform. When the grid voltage drops to 20% of the rated voltage, this control method still shows good performance. During the process of the grid voltage drop and recovery, the instantaneous current amplitude of the HVDC converter valve for offshore wind power is limited to the set value, effectively limiting the transient current overshoot current phenomenon. Figure 16 and Figure 17 shows the THD analysis of the output voltage and current during the fault ride-through. It can be seen from the figure that during the fault ride-through, the harmonic distortion of the current is within the normal range of 5% allowed by the standard, providing a good sinusoidal current waveform diagram for the grid.Figure 18 It shows the output power waveform during fault ride-through. During the fault occurrence, the active power significantly decreases while the reactive power increases substantially, which provides necessary reactive power support for the power grid and meets the requirements of fault ride-through. After the grid voltage recovers, the active power of the MMC of the offshore wind power returns to the rated value of 100 MW, and the reactive power returns to 0 var.

[0112] By adopting the fault ride-through control method for the MMC of the offshore wind power proposed by the present invention to reduce the transient current overshoot, during the fault ride-through, the instantaneous current amplitude of the MMC of the offshore wind power is effectively controlled within the preset limit value, avoiding the occurrence of inrush current, and the output power and current waveforms are stable. After the grid voltage recovers to the normal level, its active power quickly returns to the rated level, and the reactive power also returns to 0 var. Therefore, the effectiveness of the control method proposed by the present invention is verified.

Claims

1. A fault ride-through control method for offshore wind power flexible direct current converter valves to reduce transient current overshoot, characterized in that: The following steps are involved: S1. Establish a corresponding mathematical model based on the topological structure of the offshore wind power flexible converter valve (MMC); S2. Based on the mathematical model of step S1, when a fault occurs in the power grid, a dual synchronous rotating dq coordinate system transformation is used to separate the positive and negative sequence components of the voltage and current, and a positive sequence inner loop current controller and a negative sequence inner loop current controller are designed respectively to realize independent regulation of the positive and negative sequence currents; S3, calculate the effective value U of the grid voltage in real time through the effective value algorithm rms , calculate the voltage sag depth, and generate the active power reference value and reactive power reference value during the low voltage ride-through period based on the voltage sag depth; S4. Considering the influence of line impedance, the Kalman filter is used to identify the grid impedance in real time. Combined with the model predictive control (MPC) algorithm, the virtual impedance value in multiple future control cycles is optimized in a rolling manner to generate the optimal control sequence and realize the active suppression of transient current. S5. Verify and analyze the effectiveness of the fault ride-through control method of the offshore wind power flexible DC converter valve in reducing transient current overshoot under two fault conditions.

2. The offshore wind power flexible direct current converter valve fault ride-through control method for reducing transient current overshoot according to claim 1 is characterized in that: The method for establishing a mathematical model of an offshore wind power flexible direct current converter valve is specifically as follows: According to the topological structure of the offshore wind power flexible direct current converter valve, the voltage and current relationship calculation formula of the upper and lower bridge arms is as follows: The calculation formulas for the differential mode voltage and common mode voltage of the upper and lower bridge arms are as follows: Rewriting the above formula into a three-phase form, the calculation formula for the offshore wind power flexible direct current converter valve is as follows: Where L is the simplified equivalent inductance, and R is the simplified equivalent resistance.

3. The offshore wind power flexible direct current converter valve fault ride-through control method for reducing transient current overshoot according to claim 1 is characterized in that: When the power grid fails, the dual synchronous rotating dq coordinate system is used to separate the positive and negative sequence components of the voltage and current, and the positive sequence inner loop current controller and the negative sequence inner loop current controller are designed respectively to realize the method of independent regulation of the positive and negative sequence currents as follows: The mathematical model calculation formula of the offshore wind power flexible direct current converter valve can be expressed as the superposition of positive and negative sequence current and voltage components. In order to separate the positive and negative sequence components, the dual synchronous rotating dq coordinate system transformation method is adopted to map the positive sequence component to the dq coordinate system through the forward rotating coordinate transformation, and map the negative sequence component to the dq coordinate system through the reverse rotating coordinate system transformation. The specific formula after the transformation is as follows: Perform Laplace transform and obtain the specific formula of the control equation as follows: Where: are the reference values ​​of the positive sequence d and q components of the bridge arm differential mode voltage, respectively. are the reference values ​​of the negative sequence d and q components of the bridge arm differential mode voltage, respectively. They are the reference values ​​of the positive sequence d and q components of the MMC output current, are the reference values ​​of the negative-sequence d and q components of the MMC output current respectively. The positive-sequence and negative-sequence inner-loop current controllers are obtained according to the control equation.

4. The offshore wind power flexible direct current converter valve fault ride-through control method for reducing transient current overshoot according to claim 1 is characterized in that: The specific method for generating active power reference value and reactive power reference value during low voltage ride-through based on voltage sag depth is as follows: Since the offshore wind power flexible direct current converter valve adopts VSG control technology, when the power grid is operating normally, VSG is also in normal working state. In this case, the fault detection software switch S1 is 0, and the set active power is P n , the reactive power reference value is set to Q n Once the grid voltage drops below 0.9pu due to a fault at the grid connection point, the fault detection software switch S1 turns to 1. In this case, the set active power reference value is P n Switch to P ref , the reactive power reference value is set by Q n Switch to Q ref At the same time, the reactive power droop control link is shielded to avoid the deviation of reactive power output. When the grid voltage returns to normal, the system will automatically return to the initial control state.

5. The offshore wind power flexible direct current converter valve fault ride-through control method for reducing transient current overshoot according to claim 1 is characterized in that: The above-mentioned method of considering the influence of line impedance, using Kalman filter to identify the grid impedance in real time, combining with model predictive control (MPC) algorithm, rolling optimization of virtual impedance values ​​in multiple future control cycles, generating optimal control sequence, and realizing active suppression of transient current is as follows: The Kalman filter is used for real-time impedance estimation, and the specific formulas of its state equation and observation equation are as follows: x(k+1)=Ax(k)+Bu(k)+w(k) y(k)=Cx(k)+v(k) Among them, x(k) is the state vector, u(k) is the input vector, y(k) is the observation vector, w(k) and v(k) are process noise and observation noise respectively; The estimated value of the grid impedance is updated in real time through the Kalman filter to provide accurate impedance information for the MPC algorithm. The goal of MPC is to minimize current overshoot and ensure stable power output. By optimizing the adjustment of virtual impedance, the objective function of MPC can be expressed as: Where λ is the weight coefficient, which is used to balance transient current suppression and impedance matching, i t is the transient current, During the optimization process, the following constraints need to be considered: ΔZ min ≤ΔZ v ≤ΔZ max ‖i t (k)‖≤i max WITH grid,min ≤Z grid (k)≤Z grid,max The voltage component generated by the virtual impedance in the dq coordinate system is fed back to the dual closed-loop control circuit of the VSG. The specific formula for the feedback amount generated by the virtual resistance is as follows: When the fault amplitude drops in the power grid, the Kalman filter is used to identify the grid impedance in real time. Through MPC control, the virtual impedance value optimized by MPC is fed back to the VSG control loop to dynamically adjust the virtual impedance value and adaptively suppress transient current.

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