Fault ride-through test method of offshore wind power flexible direct output system

Abstract

The invention discloses a fault ride-through test method for an offshore wind power flexible direct output system, and the method comprises the steps: building a fault equivalent circuit of the offshore wind power flexible direct output system, analyzing the operation characteristics of an AC power grid side under a low-voltage fault, and determining the incidence relation between the DC side excess energy and the power grid voltage drop; establishing a model representing each component in the fault equivalent circuit, wherein the components at least comprise a machine-side converter, a grid-side converter, a direct-current capacitor and an unloading resistor; when a low-voltage fault of an alternating-current power grid side is detected, executing the following cooperative control based on the model: synchronously starting unloading resistance control and reactive power support control, wherein an unloading resistor dissipates excess energy of a direct-current side through a direct-current chopper; the grid-side converter injects a capacitive reactive current which is deeply matched with the voltage drop of the power grid so as to improve the voltage of a grid-connected point; and if the direct-current voltage continuously exceeds the preset threshold value, starting power prompt drop control, and reducing the output power of the unit according to a gradient until the system recovers to be stable.

Description

Technical Field

[0001] This invention relates to the field of power system fault testing technology, and in particular to a fault ride-through test method for offshore wind power systems transmitted via flexible direct current. Background Technology

[0002] Offshore wind power, with its abundant resources and stable power generation efficiency, has become an important direction for global clean energy development. Its large-scale grid connection is crucial for optimizing the energy structure and promoting the achievement of "dual carbon" goals. Flexible DC transmission technology, with its characteristics of long-distance, large-capacity power transmission, flexible power regulation, and minimal disturbance to the AC grid, has become the mainstream technology for transmitting offshore wind power and is widely used in connection scenarios between large offshore wind farms and onshore power grids.

[0003] However, the safe and stable operation of offshore wind power transmission systems via flexible DC transmission faces severe challenges from grid faults. When faults such as short circuits and voltage dips occur on the AC grid side, the system's energy balance is easily disrupted: the active power input from the wind turbine to the DC link through the turbine-side converter deviates significantly from the active power output from the grid-side converter to the faulty grid. Excess energy can cause the DC capacitor voltage to rise rapidly, potentially triggering the converter's overvoltage protection. Simultaneously, a drop in grid voltage can limit the output current of the grid-side converter, further exacerbating the power imbalance. If not properly controlled, this could lead to large-scale disconnection of wind turbines from the grid, causing a chain reaction on the stability of the power system.

[0004] Low voltage ride-through (LVRT) capability is a core indicator for measuring the ability of wind power systems to cope with grid faults and is also a basic requirement of industry standards for wind turbines and transmission systems. Currently, the industry generally requires wind turbines to maintain grid-connected operation for at least several hundred milliseconds when the voltage drop depth is certain (such as the terminal voltage dropping to 20% or below the rated value) and to provide necessary reactive power support to help the grid voltage recover during the fault. However, for offshore wind power systems transmitted via flexible DC transmission, their topology is more complex (including wind turbines, turbine-side converters, DC links, grid-side converters, and other components), and the dynamic responses of each component are coupled. Traditional LVRT control strategies for single wind turbines (such as relying solely on unloading resistors or reactive power compensation) are difficult to adapt: ​​relying solely on unloading resistors may not be able to quickly dissipate a large amount of excess energy, and relying solely on reactive power support is insufficient to solve the problem of DC-side voltage runaway, resulting in the system still facing the risk of grid disconnection during deep voltage drops.

[0005] Furthermore, the dynamic characteristics of the DC link in a flexible DC system place extremely high demands on fault response speed. The energy storage characteristics of DC capacitors enable voltage fluctuations to propagate rapidly. If the timing of control strategies is not properly coordinated after a fault occurs, secondary problems such as converter lockout and wind turbine speed instability may occur. Therefore, considering the characteristics of offshore wind power transmission systems via flexible DC, researching fault ride-through technologies suitable for its topology and dynamic characteristics has become a key issue in ensuring the safe grid connection of large-scale offshore wind power. Summary of the Invention

[0006] To address the shortcomings and deficiencies of existing technologies, this invention provides a fault ride-through test method for offshore wind power transmission systems via flexible direct transmission, aiming to improve the stability and safety of offshore wind power transmission systems via flexible direct transmission when low-voltage faults occur on the AC grid side.

[0007] This method establishes a fault equivalent circuit for offshore wind power transmission via flexible direct current, analyzes the system operation characteristics under low voltage faults, and clarifies the correlation between excess energy on the DC side and grid voltage drop. At the same time, it constructs models representing each core component in the fault equivalent circuit (including turbine-side converters, grid-side converters, DC capacitors, and unloading resistors, etc.), providing a theoretical basis for fault ride-through testing.

[0008] During the test, a coordinated control strategy combining unloading resistors, reactive power support, and power descent was adopted: when a low voltage fault was detected on the AC grid side, unloading resistor control and reactive power support control were activated simultaneously. The unloading resistor was quickly connected through a DC chopper to dissipate excess energy on the DC side, while the grid-side converter injected capacitive reactive current matching the voltage drop depth of the grid to help raise the voltage at the grid connection point. If the DC voltage continued to exceed the preset threshold, power descent control was activated to reduce the unit's output power at a certain gradient until the system returned to stability.

[0009] This method effectively addresses energy imbalance issues under low-voltage faults through multi-level control coordination, providing a reliable test basis for the safe operation of offshore wind power transmission systems via flexible direct current transmission and the optimized design of related equipment.

[0010] The present invention specifically adopts the following technical solution:

[0011] A fault ride-through test method for offshore wind power transmitted via a flexible direct current system includes:

[0012] Establish a fault equivalent circuit for an offshore wind power transmission system via flexible direct current, analyze the operating characteristics under low voltage faults on the AC grid side, and determine the correlation between excess energy on the DC side and grid voltage drop.

[0013] Establish a model to characterize each component in the fault equivalent circuit, wherein the components include at least a machine-side converter, a grid-side converter, a DC capacitor, and a load-relief resistor;

[0014] Upon detecting a low-voltage fault on the AC grid side, the following coordinated control is executed based on the model:

[0015] The unloading resistor control and reactive power support control are started simultaneously. The unloading resistor dissipates excess energy on the DC side through a DC chopper, and the grid-side converter injects capacitive reactive current that matches the voltage drop depth of the grid to improve the voltage at the grid connection point.

[0016] If the DC voltage continues to exceed the preset threshold, power throttling control is activated to gradually reduce the unit's output power until the system returns to stability.

[0017] Furthermore, the fault equivalent circuit includes a gearbox, a generator, a generator-side converter, a DC capacitor, a DC chopper connected in parallel with the unloading resistor, and a grid-side converter connected in sequence. The DC chopper is connected in parallel with the unloading resistor and then connected to the DC capacitor, and finally connected to the power grid.

[0018] Furthermore, the models representing each component include:

[0019] A power outer loop + current inner loop control model for the generator-side converter is used to describe the dynamic adjustment relationship between active power and DC voltage.

[0020] A voltage outer loop + reactive power inner loop control model for grid-side converters is used to describe the relationship between reactive current injection and grid voltage.

[0021] A DC capacitor energy storage model is used to characterize the dynamic response characteristics of DC voltage and excess energy.

[0022] Furthermore, the triggering condition for the synchronous start-up unloading resistor control and reactive power support control is: when the AC grid voltage drops to 0.2 times or less of the rated voltage, a control command is synchronously issued through the fault detection module.

[0023] Furthermore, in the unloading resistor control, the unloading resistor is activated within 5 milliseconds after fault detection, with a resistance value of 0.5 to 2 ohms. Energy dissipation is achieved by turning on the IGBT switch of the DC chopper, and the proportion of excess energy dissipated is 20%-30%.

[0024] Furthermore, the reference value of the capacitive reactive current injected by the grid-side converter is not less than the product of the dynamic reactive current proportional coefficient and 0.9 minus the per-unit value of the grid voltage during the fault period, and then the product of the rated current of the grid-side converter.

[0025] Furthermore, the preset threshold is 1.1 times the rated value of the DC voltage, and the gradient of the power descent control is to reduce the rated power of the unit by 10% every 10 milliseconds until the output power drops to 50% or below the rated value.

[0026] Furthermore, the machine-side converter includes a power sag module, a power outer loop, a current inner loop, and a dq conversion module. The power sag module generates active power regulation based on the DC voltage deviation and achieves power gradient reduction through the coordinated control of the power outer loop and the current inner loop.

[0027] The grid-side converter includes a reactive current compensation module, a DC voltage outer loop, and a current inner loop. The reactive current compensation module generates a q-axis current reference value based on the grid voltage drop depth and achieves dynamic injection of capacitive reactive current through the coordinated control of the DC voltage outer loop and the current inner loop.

[0028] Furthermore, the analysis of the operating characteristics under low-voltage faults on the AC grid side includes: based on the principle of energy conservation, quantifying the correlation between the grid voltage drop depth, the overcurrent capacity of the grid-side converter, and DC voltage fluctuations, and describing the DC-side energy change through a power balance equation; the power balance equation is that the product of the DC-side capacitance and the DC voltage change rate is equal to the difference between the machine-side input power and the grid-side output power.

[0029] And, a fault ride-through control device for an offshore wind power transmission system via flexible direct current transmission, comprising:

[0030] Fault modeling unit: used to establish fault equivalent circuits, analyze the operating characteristics under low voltage faults on the AC grid side, and determine the correlation between excess energy on the DC side and grid voltage drop;

[0031] Component modeling unit: used to establish models representing each component in the fault equivalent circuit, wherein the components include at least a machine-side converter, a grid-side converter, a DC capacitor, and a load-relief resistor;

[0032] Cooperative control unit: used to perform coordinated operations of unloading resistor control, reactive power support control, and power rate descent control based on the model when a low-voltage fault is detected on the AC grid side, wherein:

[0033] Synchronous start-up of unloading resistor control and reactive power support control, dissipating excess energy through DC chopper and injecting capacitive reactive current through grid-side converter;

[0034] If the DC voltage continues to exceed the preset threshold, power descent control will be activated to gradually reduce the unit's output power.

[0035] And an electronic device, including a memory, a processor, and a computer program stored in the memory and executable on the processor, characterized in that the processor executes the program to implement the steps of the method described above.

[0036] A non-transitory computer-readable storage medium having a computer program stored thereon, characterized in that the computer program, when executed by a processor, implements the steps of the method described above.

[0037] Compared with existing technologies, this invention and its preferred solution form a progressive fault response mechanism through a coordinated control strategy of unloading resistor, reactive power support, and power velocity reduction. This mechanism can effectively balance the energy imbalance of the system when a low-voltage fault occurs on the AC grid side. Specifically, the unloading resistor quickly dissipates excess energy to suppress DC voltage fluctuations, the reactive power support helps the grid voltage recover to maintain grid connection stability, and the power velocity reduction acts as a backup regulation to further ensure system safety. The synergy of these three factors significantly improves the fault ride-through capability of the offshore wind power transmission system via flexible DC transmission.

[0038] Meanwhile, by establishing fault equivalent circuits and component models, precise theoretical support is provided for fault characteristic analysis and control strategy implementation, ensuring the pertinence and effectiveness of coordinated control. The dual closed-loop control design of the generator-side converter and grid-side converter further optimizes the dynamic response of power regulation and reactive power injection, making the operating characteristics of the entire system more controllable during faults. This provides a reliable technical basis for the safe operation of offshore wind power transmission systems via flexible direct current transmission, equipment optimization design, and fault response strategy formulation. Attached Figure Description

[0039] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments:

[0040] Figure 1 This is a schematic diagram of the fault equivalent circuit structure according to an embodiment of the present invention;

[0041] Figure 2 This is a schematic diagram of the machine-side converter topology in an embodiment of the present invention;

[0042] Figure 3 This is a schematic diagram of the machine-side converter control system according to an embodiment of the present invention;

[0043] Figure 4 This is a schematic diagram of the grid-side converter topology according to an embodiment of the present invention;

[0044] Figure 5 This is a schematic diagram of the grid-side converter control system according to an embodiment of the present invention;

[0045] Figure 6 This is a schematic diagram illustrating the principle of unloading resistor activation in an embodiment of the present invention;

[0046] Figure 7 This is a schematic diagram of the process of the present invention using a combination of unloading resistor, reactive power support, and power rapid reduction.

[0047] Figure 8This is a schematic diagram of the electrical quantity changes during fault ride-through using a combination of unloading resistor, reactive power support, and power rapid reduction in an embodiment of the present invention. Figure (a) shows the active power (MVA) on the generator side, Figure (b) shows the active power (MVA) on the grid side, Figure (c) shows the reactive power (MVA) on the generator side, Figure (d) shows the reactive power (MVA) on the grid side, Figure (e) shows the generator frequency (pu), Figure (f) shows the DC voltage (kV), Figure (g) shows the grid voltage (pu), and Figure (h) shows the generator terminal voltage (kV). Detailed Implementation

[0048] In the following, specific embodiments of this application will be described in detail with reference to the accompanying drawings. Based on these detailed descriptions, those skilled in the art will be able to clearly understand and implement this application. Without departing from the principles of this application, features from various embodiments can be combined to obtain new implementations, or certain features from some embodiments can be substituted to obtain other preferred implementations.

[0049] To make the features and advantages of the present invention more apparent and understandable, specific embodiments are described below in conjunction with the accompanying drawings:

[0050] This invention provides a fault ride-through testing method for offshore wind power systems transmitted via flexible DC transmission. By establishing a fault equivalent circuit for the offshore wind power system via flexible DC transmission, the operating characteristics under different types of grid faults are analyzed. Mathematical models of each component in the fault equivalent circuit are established. Based on these models, when a fault occurs on the AC grid side of the offshore wind power system via flexible DC transmission, a combination of unloading resistors, reactive power support, and power velocity reduction is used to dynamically adjust the voltage to cope with low-voltage faults. This invention, through in-depth analysis of the unit's operating characteristics under fault conditions, utilizes the combination of unloading resistors, reactive power support, and power velocity reduction to improve the stability and safety of the offshore wind power system via flexible DC transmission under grid fault conditions. It provides technical support for the safe operation and grid regulation of future offshore wind power systems via flexible DC transmission during grid faults, and provides theoretical and practical basis for the optimized design of related equipment and the formulation of fault response strategies.

[0051] Its specific implementation mechanism includes: establishing a fault equivalent circuit for the offshore wind power transmission system via flexible direct transmission, analyzing the operating characteristics under different types of grid faults, and studying the impact of different types of grid faults on the operating rules, control performance, and grid support capabilities of the offshore wind power transmission system via flexible direct transmission.

[0052] Mathematical models of each component in the fault equivalent circuit are established. The offshore wind power transmission system via flexible DC involves numerous components and control methods, including permanent magnet synchronous generators, converters, flexible DC equipment, and AC power grids. To achieve rapid fault estimation, the entire system must be simplified and equivalently represented to analyze the effectiveness of the methods used in the event of a severe fault.

[0053] First, a fault is simulated on the AC grid side in the equivalent circuit. The effectiveness of the method is determined by observing changes in a series of electrical quantities such as DC voltage, active and reactive power on the turbine side and grid side. Then, the simulation waveform is observed to see if it fluctuates violently and causes a breakdown, thus proving whether the offshore wind power transmission system via flexible DC transmission has achieved fault ride-through after a fault occurs on the AC grid side.

[0054] Based on the mathematical models of each component, when a fault occurs on the AC grid side of the flexible DC transmission system for offshore wind power, a combination of unloading resistor, reactive power support, and power rate reduction is used to dynamically adjust the voltage to cope with the low-voltage fault.

[0055] In some embodiments, the fault equivalent circuit includes a gearbox, a generator, a generator-side converter, a DC capacitor, a DC chopper, a grid-side converter, and a power grid connected in sequence.

[0056] The generator-side converter includes a power sag module for generating the final active power reference value, a power outer loop for generating the current reference value through a PI control loop, a current inner loop for generating the voltage reference value after cooperating with the decoupling module, and a dq conversion module. The dq conversion module generates the corrected three-phase voltage from the d-axis and q-axis voltage correction values ​​and then returns them to the system for adjustment until the reference value is the same as the actual value.

[0057] The power outer loop control system used is:

[0058]

[0059]

[0060] The current inner loop control system is as follows:

[0061]

[0062] In the formula, This is the reference value for the d-axis voltage generated by the inner current loop of the machine-side converter. This is the reference value for the q-axis voltage generated by the inner current loop of the machine-side converter. This refers to the stator winding resistance. This represents the actual value of the d-axis current of the machine-side converter; This represents the actual value of the q-axis current of the machine-side converter; The d-axis inductance component on the stator side; This refers to the q-axis inductance component on the stator side. The rotor's dynamic electrical angular velocity; This represents the actual value of the d-axis voltage of the machine-side converter; This is the reference value for the current generated in the outer power loop of the machine-side converter. This is the proportionality coefficient; The integral coefficient; For complex variables in the Laplace transform; This is the final reference value generated after adjustment based on DC voltage control. This represents the actual value of the active power output of the machine-side converter; To generate the reference quantity that needs to be changed according to the change of DC voltage; This is the proportionality coefficient; This is a reference value for DC voltage. This is the actual value of the DC voltage.

[0063] The grid-side converter used includes a reactive current compensation module for generating q-axis current reference values, a DC voltage outer loop for generating q-axis current reference values, a current inner loop for generating corresponding voltage reference values ​​through a PI control loop and in cooperation with a decoupling module, and a dq conversion module. The dq conversion module generates corrected three-phase voltages from the d-axis and q-axis voltage correction values ​​and then returns them to the system for adjustment until the reference values ​​are the same as the actual values.

[0064] The DC voltage outer loop control system used is as follows:

[0065]

[0066] The current inner loop control is as follows:

[0067]

[0068] In the formula, The d-axis voltage reference value generated by the grid-side converter; The q-axis voltage reference value generated by the grid-side converter; , This refers to the current control coefficient; For complex variables in the Laplace transform; The d-axis current reference value generated for the grid-side converter; The reference value for the q-axis current generated by the grid-side converter; This represents the actual d-axis current generated by the grid-side converter. This represents the actual value of the q-axis current generated by the grid-side converter. The power grid frequency angular velocity; This refers to the value of the filter inductance on the grid side. The d-axis component of the grid-coupled electromotive force; The q-axis component of the grid-coupled electromotive force; This is the reference value for the d-axis current. This is the reference value for the q-axis current generated by the outer loop of the DC voltage. , This refers to the DC voltage control coefficient. , This is the reactive power control coefficient; This is a reference value for DC voltage. This is the actual value of the DC voltage; This is a reference value for reactive power. This is the measured value of reactive power.

[0069] During low-voltage fault ride-through control, the power of the generator-side converter is rapidly reduced by unloading resistors, and the grid voltage is increased by utilizing the reactive power support of the grid-side converter. If the DC voltage still exceeds the threshold, the power descent module is activated to reduce the unit's output power to a safe level.

[0070] This invention, through in-depth analysis of the unit's operating characteristics under fault conditions, utilizes a combination of unloading resistor, reactive power support, and power rate reduction to improve the stability and safety of offshore wind power transmission systems under grid fault conditions, providing technical support for the safe operation of offshore wind power transmission systems and grid regulation during grid faults.

[0071] The implementation process of the present invention will be demonstrated and described below through a more specific embodiment:

[0072] Please see Figures 1-8 A fault ride-through test method for offshore wind power transmitted via a flexible direct transmission system, the implementation process of which includes the following steps:

[0073] S1. Establish the fault equivalent circuit of the offshore wind power transmission system via flexible direct transmission, and analyze the operating characteristics under different types of grid faults.

[0074] like Figure 1 As shown, the fault equivalent circuit of the offshore wind power transmission system includes a gearbox, generator, generator-side converter, DC capacitor, DC chopper, grid-side converter and grid connected in sequence.

[0075] Under steady-state operation of the offshore wind power transmission system via flexible direct current, neglecting losses in power switching devices, the active power injected by the generator into the turbine-side converter is: Power transferred from the machine-side converter to the DC link (including DC capacitors and DC choppers) Power transmitted from the DC link to the grid-side converter and the power fed into the grid by the grid-side converter. All satisfy the energy conservation principle. At this time, the energy of each port achieves dynamic balance, and the DC link voltage... When the offshore wind power output via the flexible direct current transmission system is stable within the set value range, it operates in a stable state. Its power balance equation can be expressed as:

[0076]

[0077] In the formula, For the DC side capacitor, This represents a positive power deviation.

[0078] During the steady-state operation of the system, the active power injected by the generator into the generator-side converter. Power fed into the grid by the grid-side converter The two power sources maintain a dynamic energy balance, with a positive power deviation. DC voltage approaches zero Maintaining stability. When a voltage dip occurs in the grid, the output current of the grid-side converter cannot change abruptly due to the current constraint threshold. At this time, the active power transfer of the grid-side converter... It will decrease as the voltage amplitude decreases. This is due to the active power component on the generator side. To maintain constant operation, the system generates a positive power deviation. ( This energy difference is absorbed by the DC capacitor, causing the DC link voltage to show an upward trend.

[0079] The operating characteristics of the power grid under different types of faults are analyzed as follows:

[0080] Let the reference operating voltage of the power grid be The corresponding grid-side converter current is When the grid voltage drops to And the current reaches When the inverter's current limiting protection function is ignored and a zero reactive power control strategy is adopted, the power transmitted by the grid-side inverter in a fault state can be derived based on the principle of energy conservation. The mathematical expression is:

[0081]

[0082] Considering the minimum voltage during the fault period is The grid-side converter has a maximum overcurrent capacity of [number] times the rated voltage. After the current limiting control of the converter is activated, the power transmitted by the grid-side converter is reduced to twice the rated current. for:

[0083]

[0084] Assuming that the DC voltage does not change abruptly at the moment of the fault, the above equation becomes:

[0085]

[0086] DC voltage is:

[0087]

[0088] In the formula, The time before the failure occurred; This refers to the time after the fault ends. This is the initial value of the DC voltage before the fault.

[0089] It can be seen that the greater the voltage drop in the grid, the smaller the overcurrent capacity of the grid-side converter and the higher the DC overvoltage.

[0090] During a fault, if the grid-side voltage drops to Considering the maximum overcurrent capacity k of the grid-side converter i =1.2pu, then the capacitor voltage is:

[0091]

[0092] In summary, the voltage across the DC capacitor will increase significantly, seriously threatening the safety of the converter.

[0093] S2. Establish mathematical models of each component in the fault equivalent circuit.

[0094] like Figure 2 As shown, the machine-side converter uses six IGBT switches as a three-phase fully controlled bridge circuit to form the rectifier. In the converter topology, the rotor-side flux-induced electromotive force is generated by... , , Parameter characterization, stator-side equivalent impedance parameters include winding resistance With inductive component The system operating parameters include the three-phase stator current. , , and stator terminal voltage , , In the DC bus configuration, C represents the energy storage capacitor element. Define the operating DC voltage of the bus. The control system block diagram of the generator-side converter is as follows: Figure 3 As shown, the d-axis grid voltage orientation is adopted, the q-axis current is set to 0, the active power deviation value determines the d-axis current, and a PI controller is used to achieve rapid tracking of current and power.

[0095] When a fault occurs, once the DC voltage rises to a certain level, the machine-side converter will initiate a power throttling process, reducing the DC voltage. After rising, according to the reference value After correction, the active power reference value that needs to be corrected is generated through the PI circuit. Compared with the active power reference value before the fault occurred After combining these values, a final active power reference value is generated, which is then adjusted in the power outer loop. The expression for the d-axis power outer loop is:

[0096]

[0097]

[0098] In the formula, This is the reference value for the d-axis voltage generated by the inner current loop of the machine-side converter. This is the reference value for the q-axis voltage generated by the inner current loop of the machine-side converter. This refers to the stator winding resistance. This represents the actual value of the d-axis current of the machine-side converter; This represents the actual value of the q-axis current of the machine-side converter; The d-axis inductance component on the stator side; This refers to the q-axis inductance component on the stator side. The rotor's dynamic electrical angular velocity; This represents the actual value of the d-axis voltage of the machine-side converter; This is the reference value for the current generated in the outer power loop of the machine-side converter. This is the proportionality coefficient; The integral coefficient; For complex variables in the Laplace transform; This is the final reference value generated after adjustment based on DC voltage control. This represents the actual value of the active power output of the machine-side converter; To generate the reference quantity that needs to be changed according to the change of DC voltage; This is the proportionality coefficient; This is a reference value for DC voltage. This is the actual value of the DC voltage.

[0099] The power outer loop generates the final reference value. Compared with actual active power After correction, the principle of the d-axis power outer loop is that the final reference value of the d-axis active power is obtained through the power outer loop. By comparing with the actual value of active power After correction, a current reference value is generated through a PI control circuit. Meanwhile, the reactive power on the q-axis... Reactive power reference value After correction, the principle of the q-axis power outer loop, after passing through the power outer loop, is that the q-axis reactive power... Reactive power reference value The corrected value is used to generate a corresponding current reference value through a PI control circuit. The expression for the q-axis power outer loop is:

[0100]

[0101] In the formula, This is the reference value for the current generated through the q-axis power outer loop. This is the reactive power reference value, which is 0. This represents the actual value of reactive power.

[0102] Current reference value generated through the d-axis power outer loop The current reference value generated through the q-axis power outer loop Through the inner current loop. The principle of the inner current loop is that the d-axis current reference value... Compared with the actual current value The corrected value is passed through a PI control circuit and compared with the q-axis current. Decoupling is performed by multiplying by wL to finally generate the d-axis voltage reference value. q-axis current reference value Compared with the actual current value The corrected value is passed through a PI control circuit and compared with the d-axis current. Decoupling is performed by multiplying by wL to finally generate the q-axis voltage reference value. In a dual-loop control system, the expression for the inner current loop is:

[0103]

[0104] Generate d-axis voltage reference value q-axis voltage reference value Relative to the actual voltage of the d-axis q-axis actual voltage value After correction, the voltage enters the dq converter. The working principle of the dq converter is to generate corrected three-phase voltages from the generated d-axis and q-axis voltage correction values. Then return to the system for adjustment until the reference value matches the actual value.

[0105] The d-axis of the machine-side converter adopts a constant DC voltage control method, which can make timely adjustments to changes in the DC side voltage and, in conjunction with the output of active power, achieve the purpose of regulating the DC voltage.

[0106] As the interface device between the DC link and the power grid, the core function of the grid-side converter is to efficiently transfer the active power generated by the generator-side converter to the power grid. The structure of the grid-side converter is as follows: Figure 4As shown, the grid-side converter is also a three-phase fully controlled bridge circuit composed of six IGBTs. It functions as an inverter during power generation, and its operation must strictly adhere to grid synchronization constraints to ensure that the amplitude, phase, and frequency of the output electrical parameters meet grid connection specifications. Power coupling characteristics lead to interactive effects between active and reactive components, creating multi-dimensional control challenges.

[0107] The grid-side converter control system needs to achieve two objectives: maintaining DC link voltage stability to ensure continuous energy transmission; and precisely adjusting grid-side reactive power to optimize power quality. This is achieved by implementing a grid voltage vector-oriented control strategy to align the grid voltage vector with the d-axis reference, thereby obtaining… , The coordinate conditions, where , To describe the orthogonal components of the grid-coupled electromotive force, since the reactive power parameter is set to 0, the grid-coupled electromotive force... With d-axis components They are equal. In this coordinate system, the power equation can be effectively decoupled, and its active and reactive component expressions will degenerate into a simplified form:

[0108]

[0109] In the formula, This refers to the active power between the grid-side converter and the power grid. This indicates the reactive power between the grid-side converter and the power grid.

[0110] In the event of a transient voltage drop in the power grid, generator sets must inject specific reactive current according to technical specifications. Based on existing standards, when the system voltage drops to the range of 0.2 to 0.9 per-unit value, the standard specifies the following requirements for the reactive current output characteristics of the generator sets:

[0111]

[0112]

[0113] In the formula, This is a reference value for the reactive current on the grid side during a fault. This refers to the per-unit value of the grid voltage during the fault. This is the rated current of the GSC; This is the proportional coefficient for dynamic reactive current. This represents the maximum operating current on the grid side. When a fault occurs on the AC grid side and the grid voltage drops, a corresponding grid-side reactive current reference value is generated. .

[0114] like Figure 4As shown, a reactive current compensation module has been added to the grid-side converter control architecture. The DC voltage outer loop and the reactive power control loop form a dual closed-loop control. When a grid fault occurs, reactive current compensation is activated to generate a corresponding reactive power reference value. The reactive current compensation works similarly to the power outer loop of the machine-side converter, essentially generating a q-axis current reference value in the reactive power outer loop, while the d-axis DC voltage... Once it rises to a certain level, the d-axis power outer loop will be activated, and the DC voltage will be... After rising, the DC voltage reference value After correction, the d-axis current reference value is generated through a PI circuit. The outer loop of DC voltage can be represented as:

[0115]

[0116] The grid-side converter system operates under standard conditions in unity power factor control mode. During normal operation, the reactive power reference value... Set to zero.

[0117] The d-axis is also adjusted based on the DC voltage outer loop (similar to the generator-side converter, but the limiting may not reach the ideal value). When reactive power compensation is started, a d-axis current reference value is generated. Based on reactive current reference value The size of the generated data is the primary factor, with the current reference value along the d-axis. It will be related to the actual value of the d-axis current. After correction, the current enters the d-axis current inner loop, and the current reference value for the q-axis is... It will be related to the actual value of the q-axis current. After correction, the current also enters the q-axis current inner loop. The current inner loop is similar to that of the machine-side converter. The correction value of the d-axis is controlled by a PI controller and compared with the actual current value of the q-axis. Decoupling is performed by multiplying by wL to finally generate the d-axis voltage reference value. The q-axis correction value is controlled by a PI controller and compared with the actual current value on the d-axis. Decoupling is performed by multiplying by wL to finally generate the q-axis voltage reference value. In grid-side converter control, the DC voltage deviation determines the d-axis current, and the reactive power deviation determines the q-axis current; both are controlled using a PI controller. The inner-loop current control system can be represented as:

[0118]

[0119] Generated d-axis voltage reference value q-axis voltage reference value Relative to the actual voltage of the d-axis q-axis actual voltage After correction, it enters the dq transformation. After the dq transformation, the d-axis voltage reference value is... q-axis voltage reference value The pulse signal is input into the SVPWM system to obtain the IGBT pulse signal. The grid frequency and angle information required for this process are obtained by a phase-locked loop.

[0120] like Figure 5 As shown, when the unloading resistor is not engaged ( (This can be considered as 0). When the system detects a fault on the AC grid side, the connection of the unloading resistor can divert some of the current flowing through the DC capacitor. This allows the power passing through the DC capacitor to... This reduces the power consumption on the generator side. The function of conveying is shown in the following formula:

[0121]

[0122] In the formula, This is the current flowing through the unloading resistor when it is switched on; For grid-side power transmission; This represents the active power required to activate the unloading resistor in the event of a fault.

[0123] S3. Based on the mathematical models of each device, when a fault occurs on the AC grid side of the flexible DC transmission system for offshore wind power, the voltage is dynamically adjusted to cope with the low voltage fault by using a combination of unloading resistor, reactive power support, and power rate reduction.

[0124] During low-voltage control, the generator-side power is rapidly reduced using unloading resistors, and the grid voltage is boosted by reactive power support from the grid-side converter. Furthermore, if the DC overvoltage remains excessive, a power descent strategy will be activated to further stabilize the system.

[0125] The power reduction strategy is as follows: the DC voltage deviation passes through the dead zone and limiting stage, and after being adjusted and limited by the PI controller, the required power adjustment amount is output. This is then superimposed with the power reference command on the machine side to form a new power adjustment command.

[0126] After implementing the above combination of measures, the system voltage depth dropped by 0.15 pu. After a fault, the generator-side power decreased rapidly, and the grid-side converter output reactive power to boost the voltage. Overall, the unit remained stable, and the DC voltage remained within a safe range. However, due to the introduction of the power descent strategy, the unit frequency fluctuated. In contrast, without any measures, the unit's stability was compromised. The combined strategy demonstrated good low-voltage control performance.

[0127] like Figure 7As shown: The combination of unloading resistor + reactive power support + power descent achieves graded energy dissipation through a multi-level control architecture: When a voltage drop depth <0.2pu is detected, the unloading resistor module (resistance R=0.5-2Ω) controlled by the IGBT is first triggered, and it is put into operation within 5ms to dissipate 20%-30% of the excess energy; at the same time, the grid-side converter switches to voltage support mode, according to the formula Injecting capacitive reactive current ( The system improves the voltage recovery rate at the grid connection point. If the DC voltage still exceeds the 1.1 pu threshold, the power descent module is activated to reduce the unit's output power to a safe level in increments of 10% / 10ms. Experimental data shows that this combined strategy can suppress the DC overvoltage peak to within 1.08 pu when the voltage drop depth is 0.15 pu, a 35% reduction compared to the uncontrolled condition, with unit frequency fluctuations ≤ ±0.2Hz. The reactive power dynamic support control response time is no less than 200ms.

[0128] like Figure 8 As shown (when the grid voltage drops to a depth of 0.15 pu), the unloading resistor is first activated, and the grid-side converter reactive power support control is activated at the same time. If the DC overvoltage is still higher than the tolerable value after both the unloading resistor and the grid-side converter reactive power support control are activated, the unit power rapid reduction strategy is activated to achieve the combination of unloading resistor + reactive power support + power rapid reduction.

[0129] Based on the same inventive concept, this invention also provides a computer device, comprising: one or more processors, and a memory for storing one or more computer programs; the programs include program instructions, and the processor executes the program instructions stored in the memory. The processor may be a Central Processing Unit (CPU), or other general-purpose processors, digital signal processors (DSPs), application-specific integrated circuits (ASICs), field-programmable gate arrays (FPGAs), or other programmable logic devices, discrete gate or transistor logic devices, discrete hardware components, etc. It is the computing and control core of the terminal, used to implement one or more instructions, specifically for loading and executing one or more instructions stored in a computer storage medium to implement the above-described method.

[0130] It should be further explained that, based on the same inventive concept, the present invention also provides a computer storage medium storing a computer program, which, when executed by a processor, performs the above-described method. This storage medium can be any combination of one or more computer-readable media. A computer-readable medium can be a computer-readable signal medium or a computer-readable storage medium. A computer-readable storage medium can be, for example, but not limited to, an electrical, magnetic, optical, infrared, or semiconductor system, apparatus, or device, or any combination thereof. More specific examples of computer-readable storage media (a non-exhaustive list) include: an electrical connection having one or more wires, a portable computer disk, a hard disk, random access memory (RAM), read-only memory (ROM), erasable programmable read-only memory (EPROM or flash memory), optical fiber, portable compact disk read-only memory (CD-ROM), optical storage device, magnetic storage device, or any suitable combination thereof. In the present invention, a computer-readable storage medium can be any tangible medium containing or storing a program that can be used by or in conjunction with an instruction execution system, apparatus, or device.

[0131] In the description of this specification, references to terms such as "an embodiment," "example," "specific example," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of this disclosure. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0132] The foregoing has shown and described the basic principles, main features, and advantages of this disclosure. Those skilled in the art should understand that this disclosure is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of this disclosure. Various changes and modifications can be made to this disclosure without departing from its spirit and scope, and all such changes and modifications fall within the scope of this disclosure as claimed.

[0133] This invention is not limited to the preferred embodiment described above. Anyone inspired by this invention can derive various other forms of fault ride-through testing methods for offshore wind power systems via flexible direct transmission. All equivalent variations and modifications made within the scope of the claims of this invention shall fall within the scope of this invention.

Claims

1. A fault ride-through test method for an offshore wind power transmission system via a flexible direct power transmission system, characterized in that, include: Establish a fault equivalent circuit for an offshore wind power transmission system via flexible direct current, analyze the operating characteristics under low voltage faults on the AC grid side, and determine the correlation between excess energy on the DC side and grid voltage drop. Establish a model to characterize each component in the fault equivalent circuit, wherein the components include at least a machine-side converter, a grid-side converter, a DC capacitor, and a load-relief resistor; Upon detecting a low-voltage fault on the AC grid side, the following coordinated control is executed based on the model: The unloading resistor control and reactive power support control are started simultaneously. The unloading resistor dissipates excess energy on the DC side through a DC chopper, and the grid-side converter injects capacitive reactive current that matches the voltage drop depth of the grid to improve the voltage at the grid connection point. If the DC voltage continues to exceed the preset threshold, power throttling control is activated to gradually reduce the unit's output power until the system returns to stability.

2. The fault ride-through test method for an offshore wind power transmission system via flexible direct transmission according to claim 1, characterized in that: The fault equivalent circuit includes a gearbox, a generator, a generator-side converter, a DC capacitor, a DC chopper connected in parallel with an unloading resistor, and a grid-side converter connected in sequence. The DC chopper is connected in parallel with the unloading resistor and then connected to the DC capacitor, and finally connected to the power grid.

3. The fault ride-through test method for an offshore wind power transmission system via flexible direct transmission according to claim 1, characterized in that: The models representing each component include: A power outer loop + current inner loop control model for the generator-side converter is used to describe the dynamic adjustment relationship between active power and DC voltage. A voltage outer loop + reactive power inner loop control model for grid-side converters is used to describe the relationship between reactive current injection and grid voltage. A DC capacitor energy storage model is used to characterize the dynamic response characteristics of DC voltage and excess energy.

4. The fault ride-through test method for an offshore wind power transmission system via flexible direct transmission according to claim 1, characterized in that: The triggering condition for the synchronous start-up unloading resistor control and reactive power support control is: when the AC grid voltage drops to 0.2 times or less of the rated voltage, a control command is synchronously issued through the fault detection module.

5. The fault ride-through test method for an offshore wind power transmission system via flexible direct transmission according to claim 1, characterized in that: In the unloading resistor control, the unloading resistor is activated within 5 milliseconds after fault detection, with a resistance of 0.5 to 2 ohms. Energy is dissipated by turning on the IGBT switch of the DC chopper, and the proportion of excess energy dissipated is 20%-30%.

6. The fault ride-through test method for an offshore wind power transmission system via flexible direct transmission according to claim 1, characterized in that: The reference value of the capacitive reactive current injected by the grid-side converter shall not be less than the product of the dynamic reactive current proportional coefficient, 0.9 minus the per-unit value of the grid voltage during the fault, and the product of the rated current of the grid-side converter.

7. The fault ride-through test method for an offshore wind power transmission system via flexible direct transmission according to claim 1, characterized in that: The preset threshold is 1.1 times the rated value of DC voltage, and the power descent control gradient is to reduce the rated power of the unit by 10% every 10 milliseconds until the output power drops to 50% or below the rated value.

8. The fault ride-through test method for an offshore wind power transmission system via flexible direct transmission according to claim 1, characterized in that: The machine-side converter includes a power rate reduction module, a power outer loop, a current inner loop, and a dq conversion module. The power rate reduction module generates active power regulation based on the DC voltage deviation and achieves power gradient reduction through the coordinated control of the power outer loop and the current inner loop. The grid-side converter includes a reactive current compensation module, a DC voltage outer loop, and a current inner loop. The reactive current compensation module generates a q-axis current reference value based on the grid voltage drop depth and achieves dynamic injection of capacitive reactive current through the coordinated control of the DC voltage outer loop and the current inner loop.

9. The fault ride-through test method for an offshore wind power transmission system via flexible direct transmission according to claim 1, characterized in that: The analysis of the operating characteristics under low voltage faults on the AC grid side includes: based on the principle of energy conservation, quantifying the correlation between the grid voltage drop depth, the overcurrent capacity of the grid-side converter, and DC voltage fluctuations, and describing the DC side energy change through a power balance equation; the power balance equation is that the product of the DC side capacitance and the DC voltage change rate is equal to the difference between the machine-side input power and the grid-side output power.

10. A fault ride-through control device for an offshore wind power transmission system via flexible direct transmission, characterized in that, include: Fault modeling unit: used to establish fault equivalent circuits, analyze the operating characteristics under low voltage faults on the AC grid side, and determine the correlation between excess energy on the DC side and grid voltage drop; Component modeling unit: used to establish models representing each component in the fault equivalent circuit, wherein the components include at least a machine-side converter, a grid-side converter, a DC capacitor, and a load-relief resistor; Cooperative control unit: used to perform coordinated operations of unloading resistor control, reactive power support control, and power rate descent control based on the model when a low-voltage fault is detected on the AC grid side, wherein: Synchronous start-up of unloading resistor control and reactive power support control, dissipating excess energy through DC chopper and injecting capacitive reactive current through grid-side converter; If the DC voltage continues to exceed the preset threshold, power descent control will be activated to gradually reduce the unit's output power.

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