Alternating-current fault control method and system for onshore converter station, and device and medium

Abstract

An alternating-current fault control method and system for an onshore converter station, and a device and a medium, which relate to the technical field of power systems. An alternating-current fault flag is determined on the basis of an alternating-current voltage at a grid side of a voltage source converter in an onshore converter station; on the basis of dq-axis outer loop control and dq-axis inner loop control of the onshore converter station, the alternating-current fault flag is introduced to identify whether the onshore converter station is in an alternating-current fault period, so that when the onshore converter station is in the alternating-current fault period, a second current reference value corresponding to a grid voltage amplitude within the alternating-current fault period is used to modulate a reactive current of the onshore converter station; and a voltage modulation wave generated by means of performing filtering and damping coefficient operations on a low-frequency component of a direct current of the onshore converter station is used for modulating the alternating-current voltage of the onshore converter station, such that the alternating-current fault ride-through of the onshore converter station of a voltage-source-converter-based high-voltage direct current grid-connected system for offshore wind power is successful, thereby reducing the risks of direct-current tripping and going off grid of a wind turbine in a wind farm, and improving the safety and reliability of the voltage-source-converter-based high-voltage direct current grid-connected system for offshore wind power.

Description

A method, system, equipment, and medium for AC fault control at land stations.

[0001] This application claims priority to Chinese Patent Application No. 202510358150.8, filed on March 25, 2025, entitled "A Method, System, Device and Medium for AC Fault Control of Land Station", the entire contents of which are incorporated herein by reference. Technical Field

[0002] This invention relates to the field of power system technology, and in particular to a method, system, device and medium for AC fault control at an onshore substation. Background Technology

[0003] In recent years, with the development of new energy power generation technology and power electronics technology, the technical solution of using flexible DC (Voltage Source Converter Based High-Voltage Direct Current System, VSC-HVDC) to transmit large-scale, long-distance offshore wind power and connect it to the onshore power grid has been widely used. At present, dozens of offshore wind power grid-connected projects via flexible DC have been put into operation worldwide.

[0004] Figure 1 shows the topology of an offshore wind power system connected to a flexible DC grid. Compared to traditional two-terminal grid-connected flexible DC systems, offshore wind power systems connected to flexible DC grids differ significantly in DC control functionality. Flexible DC systems generally require AC fault ride-through under various AC fault conditions without DC tripping. Current offshore wind power systems connected to flexible DC grids struggle to effectively control AC faults at onshore stations. This can lead to a sharp increase in DC voltage caused by surplus power delivered from the wind farm, resulting in failed AC fault ride-through at the onshore station, or even serious consequences such as DC tripping and wind turbine disconnection from the grid. These issues compromise the safety and reliability of the offshore wind power system connected to a flexible DC grid. Summary of the Invention

[0005] In view of this, the present invention provides a method, system, device and medium for AC fault control at onshore stations, which solves the technical problem that current offshore wind power connected to flexible DC grid systems cannot effectively control AC faults at onshore stations. This results in the surplus power delivered by the wind farm causing a sharp increase in DC voltage, leading to failure of AC fault ride-through at the onshore station, or even serious consequences such as DC tripping and wind turbine disconnection, affecting the safety and reliability of offshore wind power connected to flexible DC grid systems.

[0006] The first aspect of this invention provides a method for controlling AC faults at a land station, comprising:

[0007] The AC fault flag is determined based on the AC voltage on the flexible DC transformer side of the onshore station; wherein, the AC fault flag is used to indicate whether the onshore station is in the period of AC fault.

[0008] The onshore station is determined to be in an AC fault period based on the AC fault flag bit. When the onshore station is determined to be in an AC fault period, the reactive current of the onshore station is modulated based on the first current reference value output by the outer loop control of the dq axis of the onshore station and the second current reference value corresponding to the grid voltage amplitude during the AC fault period to obtain a current modulation signal.

[0009] Using the current modulation signal as the input of the dq-axis inner loop control of the land station, the AC voltage of the land station is modulated according to the first voltage modulation wave output by the dq-axis inner loop control of the land station and the second voltage modulation wave generated by filtering and damping coefficient calculation of the low-frequency component of the DC current of the land station, so as to obtain the voltage modulation wave signal.

[0010] Preferably, determining the AC fault flag bit based on the AC voltage on the onshore flexible DC transformer side includes:

[0011] The positive sequence voltage amplitude of the AC voltage on the flexible DC-DC transformer side of the onshore station is normalized to obtain the per-unit value of the positive sequence voltage amplitude of the AC voltage.

[0012] The per-unit value of the positive sequence voltage amplitude of the AC voltage is compared with a preset amplitude threshold.

[0013] When the per-unit value of the positive sequence voltage amplitude of the AC voltage is greater than the preset amplitude threshold, the AC fault flag bit is determined to be the first AC fault flag bit, which is used to indicate that the land station is not in an AC fault period.

[0014] When the per-unit value of the positive sequence voltage amplitude of the AC voltage is not greater than the preset amplitude threshold, the AC fault flag bit is determined to be the second AC fault flag bit, which is used to indicate that the land station is in the period of AC fault.

[0015] Preferably, the step of modulating the reactive current of the onshore station based on the first current reference value output by the dq-axis outer loop control of the onshore station and the second current reference value corresponding to the grid voltage amplitude during the AC fault to obtain a current modulation signal includes:

[0016] The first current reference value is output through the outer loop control of the dq axis of the land station. The first current reference value includes the d-axis current reference value and the q-axis current reference value.

[0017] The reference value of the q-axis current corresponding to the grid voltage amplitude of the onshore station during the AC fault is determined based on the preset Urms-Iq curve.

[0018] The q-axis current reference value output by the outer loop control of the dq axis and the q-axis current reference value corresponding to the grid voltage amplitude during the AC fault are used to modulate the q-axis reactive current of the onshore station to obtain the q-axis reactive current value.

[0019] The current modulation signal is determined based on the d-axis current reference value and the q-axis reactive current value.

[0020] Preferably, the step of controlling the output of the first current reference value through the outer loop of the dq axis of the land station includes:

[0021] The DC voltage difference is obtained by subtracting the reference value and the measured value of DC voltage at the land station.

[0022] The reactive power difference is obtained by subtracting the reference value and the measured value of reactive power at the land station.

[0023] Both the DC voltage difference and the reactive power difference are subjected to PI control to obtain the superimposed DC current difference and the superimposed reactive current difference.

[0024] Both the superimposed difference of DC current and the superimposed difference of reactive current are subjected to amplitude limiting processing to obtain the d-axis current reference value and the q-axis current reference value.

[0025] Preferably, the step of using the current modulation signal as the input to the dq-axis inner loop control of the land station, and modulating the AC voltage of the land station according to the first voltage modulation wave output by the dq-axis inner loop control of the land station, and the second voltage modulation wave generated by filtering and damping coefficient calculation of the low-frequency component of the DC current of the land station, to obtain the voltage modulation wave signal, includes:

[0026] The current modulation signal is used as the input to the dq-axis inner loop control of the land station, and a first voltage modulation wave is output through the dq-axis inner loop control of the land station; the first voltage modulation wave includes a d-axis voltage reference value and a q-axis voltage reference value;

[0027] The low-frequency component of the DC current of the land station is filtered to obtain the low-frequency filtered component.

[0028] The low-frequency filtered component is multiplied by the damping coefficient to obtain the second voltage modulation wave;

[0029] The second voltage modulation wave is superimposed with the d-axis voltage reference value to obtain the d-axis voltage superposition value;

[0030] The three-phase voltage modulation signal is obtained by performing an inverse Park operation on the superimposed d-axis voltage value and the q-axis voltage reference value.

[0031] The three-phase voltage modulation signal is used for modulation trigger control to obtain a voltage modulation wave signal.

[0032] Preferably, the current modulation signal includes a d-axis current reference value and a q-axis reactive current value;

[0033] The step of using the current modulation signal as the input to the dq-axis inner loop control of the land station, and outputting a first voltage modulation wave through the dq-axis inner loop control of the land station, includes:

[0034] The difference between the reference value of the d-axis current and the measured value of the d-axis current is obtained by subtracting the reference value of the d-axis current from the measured value of the d-axis current.

[0035] The difference between the q-axis reactive current value and the measured q-axis current value is obtained by subtracting the q-axis current difference value.

[0036] Both the d-axis current difference and the q-axis current difference are subjected to PI control to obtain the d-axis current PI output value and the q-axis current PI output value.

[0037] The d-axis current PI output value is compensated using the measured q-axis voltage value and the d-axis current compensation value to obtain the d-axis voltage reference value; wherein, the d-axis current compensation value is obtained by compensating the measured d-axis current value for angular frequency.

[0038] The q-axis current PI output value is compensated using the measured d-axis voltage value and the q-axis current compensation value to obtain the q-axis voltage reference value; wherein, the q-axis current compensation value is obtained by compensating the measured q-axis current value for angular frequency.

[0039] Preferably, the method further includes:

[0040] Determine whether the DC voltage of the land station has reached the overvoltage threshold for the DC power consumption device to be activated;

[0041] When the DC voltage of the land station reaches the overvoltage threshold for the DC energy consumption device to be activated, the DC energy consumption device is activated to consume the surplus active power of the land station.

[0042] Determine whether the DC voltage after the DC energy-consuming device is put into operation drops to a preset voltage threshold.

[0043] When it is determined that the DC voltage after the DC power consumption device is turned on drops to the preset voltage threshold, the DC power consumption device is turned off.

[0044] Secondly, the present invention also provides an AC fault control system for a land station, comprising:

[0045] The fault flag determination module is used to determine the AC fault flag bit based on the AC voltage on the flexible DC transformer side of the onshore station; wherein, the AC fault flag bit is used to indicate whether the onshore station is in the period of AC fault.

[0046] The current modulation module is used to determine whether the onshore station is in an AC fault period based on the AC fault flag bit. When it is determined that the onshore station is in an AC fault period, the reactive current of the onshore station is modulated based on the first current reference value output by the outer loop control of the dq axis of the onshore station and the second current reference value corresponding to the grid voltage amplitude during the AC fault period to obtain a current modulation signal.

[0047] The voltage modulation module is used to use the current modulation signal as the input of the dq axis inner loop control of the land station, and modulate the AC voltage of the land station according to the first voltage modulation wave output by the dq axis inner loop control of the land station and the second voltage modulation wave generated by filtering and damping coefficient calculation of the low frequency component of the DC current of the land station, so as to obtain the voltage modulation wave signal.

[0048] Thirdly, the present invention also provides an electronic device, the electronic device including a memory and a processor, the memory storing a computer program, the computer program being executed by the processor causing the processor to perform the steps of the land station AC fault control method as described in the first aspect.

[0049] Fourthly, the present invention also provides a computer-readable storage medium having a computer program stored thereon, which, when executed, implements the steps of the land station AC fault control method as described in the first aspect.

[0050] As can be seen from the above technical solutions, this invention determines the AC fault flag based on the AC voltage on the flexible DC grid side of the onshore station. By introducing the AC fault flag on the basis of the dq-axis outer loop control and dq-axis inner loop control of the onshore station, it identifies whether the onshore station is experiencing an AC fault. Therefore, when the onshore station is experiencing an AC fault, the reactive current of the onshore station is modulated using the second current reference value corresponding to the grid voltage amplitude during the AC fault period. Furthermore, the AC voltage of the onshore station is modulated by a voltage modulation wave generated by filtering and damping coefficient calculation of the low-frequency component of the DC current of the onshore station, thereby suppressing the low-frequency component on the DC side. This solves the problem of effectively controlling AC faults at the onshore station in offshore wind power connected to the flexible DC grid system, enabling successful AC fault crossing at the onshore station of offshore wind power connected to the flexible DC grid system. It can reduce the risk of DC tripping and wind turbine disconnection from the grid, and improve the safety and reliability of offshore wind power connected to the flexible DC grid system. Attached Figure Description

[0051] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0052] Figure 1 is a schematic diagram of the topology of a dual-end offshore wind power system connected to a flexible DC grid.

[0053] Figure 2 shows the application environment of an AC fault control method for land stations provided in an embodiment of the present invention;

[0054] Figure 3 is a flowchart of an AC fault control method for a land station provided in an embodiment of the present invention;

[0055] Figure 4a is a schematic diagram of the dq transformation process of the three-phase voltage on the flexible DC transformer side of the onshore station;

[0056] Figure 4b is a schematic diagram of the dq transformation process of the three-phase current on the flexible DC transformer side of the onshore station;

[0057] Figure 5 is a schematic diagram of the AC fault flag judgment logic;

[0058] Figure 6 is a schematic diagram of reactive current control logic;

[0059] Figure 7 is a schematic diagram of the Urms-Iq curve control logic;

[0060] Figure 8 is a schematic diagram of DC oscillation suppression control logic;

[0061] Figure 9 is a schematic diagram of the inner loop and modulation wave control logic of the dq axis;

[0062] Figure 10 is a schematic diagram of the DC power consumption switching control logic;

[0063] Figure 11 is a schematic diagram of the structure of an AC fault control system for a land station provided in an embodiment of the present invention;

[0064] Figure 12 is a schematic diagram of the structure of an electronic device provided in an embodiment of the present invention. Detailed Implementation

[0065] To enable those skilled in the art to better understand the present invention, the technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0066] The land station AC fault control method provided in this application embodiment can be applied to the application environment shown in Figure 2. The land station AC system communicates with server 102 via a network. The data storage system can store the data that server 102 needs to process. The data storage system can be integrated onto server 102, or it can be located in the cloud or on another network server. Server 102 determines the AC fault flag bit based on the AC voltage on the flexible DC-DC converter side of the onshore substation. The AC fault flag bit indicates whether the onshore substation is experiencing an AC fault. Based on the AC fault flag bit, if the onshore substation is determined to be experiencing an AC fault, the reactive current of the onshore substation is modulated according to the first current reference value output by the outer loop control of the dq axis of the onshore substation and the second current reference value corresponding to the grid voltage amplitude during the AC fault, resulting in a current modulation signal. This current modulation signal is then used as the input to the inner loop control of the dq axis of the onshore substation. The AC voltage of the onshore substation is modulated according to the first voltage modulation wave output by the inner loop control of the dq axis of the onshore substation and the second voltage modulation wave generated by filtering and damping coefficient calculations of the low-frequency component of the DC current of the onshore substation, resulting in a voltage modulation wave signal. Server 102 can be a standalone physical server, a server cluster or distributed system composed of multiple physical servers, or a cloud server providing cloud computing services.

[0067] As shown in Figure 3, this application provides a method for controlling AC faults at a land station. Taking the application of this method to server 102 in Figure 2 as an example, the method includes the following steps S1 to S3.

[0068] in:

[0069] Step S1: Determine the AC fault flag bit based on the AC voltage on the flexible DC transformer side of the onshore station; wherein, the AC fault flag bit is used to indicate whether the onshore station is in the period of AC fault.

[0070] This application embodiment monitors the AC voltage on the flexible DC-DC converter side of the onshore station, determines whether the onshore station is in an AC fault period based on the AC voltage on the flexible DC-DC converter side of the onshore station, and determines the AC fault flag bit.

[0071] Specifically, step S1, determining the AC fault flag based on the AC voltage on the onshore flexible DC transformer side, includes:

[0072] Step S101: Normalize the positive sequence voltage amplitude of the AC voltage on the flexible DC transformer side of the onshore station to obtain the per-unit value of the positive sequence voltage amplitude of the AC voltage.

[0073] Specifically, by collecting the three-phase AC voltage and current from the onshore flexible DC transformer, the three-phase AC voltage and current are decomposed along the d-q axis to obtain the three-phase AC d-axis voltage, AC q-axis voltage, AC d-axis current, and AC q-axis current, as shown in Figures 4a and 4b. Then, by calculating the effective values ​​of the three-phase AC d-axis voltage and AC q-axis voltage, the per-unit value of the positive-sequence voltage amplitude of the AC voltage is obtained.

[0074] Step S102: Compare the per-unit value of the positive sequence voltage amplitude of the AC voltage with the preset amplitude threshold.

[0075] Step S103: When the per-unit value of the positive sequence voltage amplitude of the AC voltage is greater than the preset amplitude threshold, the AC fault flag bit is determined to be the first AC fault flag bit. The first AC fault flag bit is used to indicate that the land station is not in an AC fault period.

[0076] Step S104: When the per-unit value of the positive sequence voltage amplitude of the AC voltage is not greater than the preset amplitude threshold, the AC fault flag bit is determined to be the second AC fault flag bit. The second AC fault flag bit is used to indicate that the land station is in the period of AC fault.

[0077] For example, as shown in Figure 5, the AC fault flag bit judgment logic sets the amplitude threshold to 0.9pu. When the per-unit value of the positive sequence voltage amplitude of the AC voltage, Udrms, is greater than 0.9pu, it is determined that the land station is not in an AC fault period, and the AC fault flag bit ACfault is set to 0. When the per-unit value of the positive sequence voltage amplitude of the AC voltage, Udrms, is not greater than 0.9pu, it is determined that the land station is in an AC fault period, and the AC fault flag bit ACfault is set to 1.

[0078] Step S2: Determine whether the onshore station is in an AC fault period based on the AC fault flag bit. When it is determined that the onshore station is in an AC fault period, modulate the reactive current of the onshore station according to the first current reference value output by the outer loop control of the dq axis of the onshore station and the second current reference value corresponding to the grid voltage amplitude during the AC fault period to obtain a current modulation signal.

[0079] It is understood that, based on the outer loop control of the dq axis of the land station, the embodiments of this application introduce reactive current control of AC fault mode with AC fault flag bit as criterion, and the specific control logic is shown in Figure 6.

[0080] When it is determined that the onshore station is not in an AC fault period, the reactive current of the onshore station can be modulated according to the first current reference value of the outer loop control output of the dq axis of the onshore station, without the need to adjust the reactive current according to the degree of AC voltage drop.

[0081] When it is determined that the onshore station is in the period of AC fault, the reactive current of the onshore station is modulated according to the first current reference value of the outer loop control output of the dq axis of the onshore station and the second current reference value corresponding to the grid voltage amplitude during the AC fault, so as to provide reactive power support for the grid voltage recovery.

[0082] Specifically, in step S2, the reactive current of the onshore station is modulated based on the first current reference value output by the outer loop control of the dq axis of the onshore station and the second current reference value corresponding to the grid voltage amplitude during the AC fault, to obtain the current modulation signal, which includes:

[0083] Step S201: Output the first current reference value through the outer loop control of the dq axis of the land station. The first current reference value includes the d-axis current reference value and the q-axis current reference value.

[0084] As shown in Figure 6, the first current reference value is output through the outer loop control of the dq axis of the land station, including:

[0085] Step S2011: Subtract the DC voltage reference value Udcref and the measured DC voltage value Udc of the land station to obtain the DC voltage difference value.

[0086] Step S2012: Subtract the reactive power reference value and the measured reactive power value of the land station to obtain the reactive power difference value.

[0087] Step S2013: Perform PI control on both the DC voltage difference and the reactive power difference to obtain the superimposed DC current difference and the superimposed reactive current difference.

[0088] PI control includes a proportional element and an integral element. The DC voltage difference and the reactive power difference are both processed by the proportional element and the integral element to obtain the superimposed difference of DC current and the superimposed difference of reactive current.

[0089] Step S2014: Both the superimposed difference of DC current and the superimposed difference of reactive current are subjected to amplitude limiting processing to obtain the d-axis current reference value Idref and the q-axis current reference value Iqref0.

[0090] Step S202: Determine the reference value of the q-axis current corresponding to the grid voltage amplitude during an AC fault at the onshore station based on the preset Urms-Iq curve.

[0091] As shown in Figure 7, the reactive current will be controlled according to the AC voltage drop and the Urms-Iq curve designed according to the grid demand, so as to obtain the q-axis current reference value Iqref1 corresponding to the grid voltage amplitude during the AC fault of the onshore station.

[0092] The Urms-Iq curve is provided by different engineering designs. Specifically, it is a curve formed by the reactive current provided under different voltage amplitudes, as shown in Figure 7. As the voltage amplitude Urms decreases, the required reactive current increases, and there is a maximum limit value.

[0093] Input a voltage amplitude using the Urms-Iq curve, find the corresponding output current Iq value from the curve, and use it as the q-axis current reference value Iqref1.

[0094] Step S203: Modulate the q-axis reactive current of the onshore station with the q-axis current reference value Iqref0 output by the outer loop control of the dq axis and the q-axis current reference value Iqref1 corresponding to the grid voltage amplitude during the AC fault, so as to obtain the q-axis reactive current value Iqref.

[0095] Step S204: Determine the current modulation signal based on the d-axis current reference value Idref and the q-axis reactive current value Iqref.

[0096] Among them, the d-axis current reference value Idref and the q-axis reactive current value Iqref are used as current modulation signals input to the dq-axis inner loop control of the land station.

[0097] Step S3: Using the current modulation signal as the input of the dq axis inner loop control of the land station, the AC voltage of the land station is modulated according to the first voltage modulation wave output by the dq axis inner loop control of the land station and the second voltage modulation wave generated by filtering and damping coefficient calculation of the low frequency component of the DC current of the land station, so as to obtain the voltage modulation wave signal.

[0098] When it is determined that the land station is not in an AC fault period, the current modulation signal is used as the input of the dq axis inner loop control of the land station. The AC voltage of the land station can be modulated according to the first voltage modulation wave output by the dq axis inner loop control of the land station, without the need to suppress the low frequency component of the DC side.

[0099] As shown in Figure 8, when the onshore station is determined to be in an AC fault period, the configured DC oscillation suppression control function extracts the low-frequency component of the DC current, multiplies it by the damping coefficient, and generates Udref1 which is superimposed on the first voltage modulation wave of the inner loop control output of the dq axis, thereby achieving the purpose of suppressing the low-frequency component of the DC side. The voltage modulation wave signal is then transmitted to the converter of the onshore station for pulse modulation.

[0100] During steady-state operation, DC oscillation suppression control effectively controls low-frequency oscillations in DC side current and voltage. However, during AC faults at the onshore station, if the DC oscillation suppression function is not deactivated during repeated switching of DC power consumption, it will work together with the switching of DC power consumption to cause fluctuations in DC voltage, leading to instability in the DC system grid. Therefore, it is necessary to deactivate the DC oscillation suppression function when the AC fault flag bit of the onshore station is active.

[0101] The damping coefficient is set according to the DC engineering requirements and can be between 0.1 and 0.5.

[0102] Specifically, as shown in Figure 9, in step S3, the current modulation signal is used as the input to the dq-axis inner loop control of the land station. Based on the first voltage modulation wave output from the dq-axis inner loop control of the land station, and the second voltage modulation wave generated by filtering and damping coefficient calculation of the low-frequency component of the DC current of the land station, the AC voltage of the land station is modulated to obtain the voltage modulation wave signal, including:

[0103] Step S301: Using the current modulation signals Idref and Iqref as inputs to the dq-axis inner loop control of the land station, the first voltage modulation wave is output through the dq-axis inner loop control of the land station; the first voltage modulation wave includes the d-axis voltage reference value Udref and the q-axis voltage reference value Uqref.

[0104] Step S302: Filter the low-frequency component of the DC current of the land station to obtain the low-frequency filtered component;

[0105] Step S303: Multiply the low-frequency filter component with the damping coefficient to obtain the second voltage modulation wave Udref1;

[0106] Step S304: Superimpose the second voltage modulation wave Udref1 with the d-axis voltage reference value Udref to obtain the superimposed d-axis voltage value;

[0107] Step S305: Perform inverse Parking operation on the superimposed d-axis voltage value and the q-axis voltage reference value Uqref to obtain the three-phase voltage modulation signals UAref, UBref, and UCref;

[0108] Step S306: Use the three-phase voltage modulation signals UAref, UBref, and UCref for modulation trigger control to obtain the voltage modulation wave signal.

[0109] In some embodiments, the current modulation signal includes a d-axis current reference value and a q-axis reactive current value;

[0110] Step S301, which uses the current modulation signal as the input to the dq-axis inner loop control of the land station and outputs the first voltage modulation wave through the dq-axis inner loop control of the land station, includes:

[0111] Step S3011: Subtract the d-axis current reference value Idref from the measured d-axis current value Id to obtain the d-axis current difference value;

[0112] Step S3012: Subtract the q-axis reactive current value Iqref from the measured q-axis current value Iq to obtain the q-axis current difference value;

[0113] Step S3013: Perform PI control on both the d-axis current difference and the q-axis current difference to obtain the d-axis current PI output value and the q-axis current PI output value;

[0114] The PI control includes a proportional element and an integral element. The d-axis current difference and the q-axis current difference are both processed by the proportional element and the integral element to obtain the d-axis current PI output value and the q-axis current PI output value.

[0115] Step S3014: Compensate the d-axis current PI output value using the measured q-axis voltage value Uq and the d-axis current compensation value to obtain the d-axis voltage reference value Udref; wherein, the d-axis current compensation value is obtained by compensating the measured d-axis current value Id with angular frequency wl.

[0116] Step S3015: Use the measured d-axis voltage value Ud and the q-axis current compensation value to compensate the q-axis current PI output value to obtain the q-axis voltage reference value Uqref; wherein, the q-axis current compensation value is obtained by compensating the measured q-axis current value with angular frequency wl.

[0117] It should be noted that the embodiments of this application determine the AC fault flag bit based on the AC voltage on the flexible DC grid side of the onshore station. By introducing the AC fault flag bit on the basis of the dq axis outer loop control and dq axis inner loop control of the onshore station, it is possible to identify whether the onshore station is in an AC fault period. Thus, when the onshore station is in an AC fault period, the reactive current of the onshore station is modulated using the second current reference value corresponding to the grid voltage amplitude during the AC fault period. In addition, the AC voltage of the onshore station is modulated by the voltage modulation wave generated by filtering and damping coefficient calculation of the low frequency component of the DC current of the onshore station, thereby achieving the purpose of suppressing the low frequency component of the DC side. This solves the problem that it is difficult to effectively control the AC fault of the onshore station in offshore wind power through the flexible DC grid connection system. It enables the successful passage of AC faults of the onshore station of offshore wind power through the flexible DC grid connection system, which can reduce the risk of DC tripping and wind turbine disconnection from the grid, and improve the safety and reliability of offshore wind power through the flexible DC grid connection system.

[0118] In some embodiments, under high-power and severe AC faults at onshore stations, the DC voltage will rise sharply because the power of offshore stations cannot be sent to the grid in time. It is necessary to configure DC energy-consuming equipment on the DC side to absorb the surplus active power, effectively reduce the DC voltage, and avoid equipment overvoltage and DC tripping.

[0119] In some embodiments, the present application embodiments configure DC power consumption switching so that when the DC voltage of the land station reaches the overvoltage setpoint for DC power consumption, DC power consumption is activated; when the DC voltage drops to the DC power consumption cutoff setpoint, DC power consumption is cut off. If the AC fault is still not cleared, the DC voltage will continue to rise after the DC power consumption is cut off, and DC power consumption may still be activated and cut off. During the AC fault, the DC voltage will show repeated increases and decreases with the switching of DC power consumption.

[0120] Specifically, the embodiments of this application further include the following through the DC power consumption switching process:

[0121] Step S401: Determine whether the DC voltage of the land station has reached the overvoltage threshold for the DC energy consumption device to be put into operation.

[0122] Step S402: When the DC voltage of the land station reaches the overvoltage threshold for the DC energy consumption device to be activated, the DC energy consumption device is activated to consume the surplus active power of the land station.

[0123] Step S403: Determine whether the DC voltage after the DC energy consumption device is put into operation has dropped to the preset voltage threshold.

[0124] Step S404: When it is determined that the DC voltage after the DC energy consumption device is put into operation drops to the preset voltage threshold, the DC energy consumption device is disconnected.

[0125] For example, as shown in Figure 10, when the DC voltage Udc of the land station reaches the overvoltage threshold UdcH for DC power consumption activation, DC power consumption is activated; when the DC voltage drops to the return threshold UdcL, DC power consumption is then deactivated. If the AC fault is not cleared, the DC voltage will continue to rise after DC power consumption is deactivated, and DC power consumption may still be activated and deactivated. During the AC fault, the DC voltage will repeatedly rise and fall with the switching of DC power consumption.

[0126] Understandably, by coordinating DC power consumption switching with onshore converter control, offshore wind power can successfully overcome AC faults at the onshore station of the flexible DC grid-connected system. This provides a reference and guidance for the design of the control system for offshore wind power flexible DC grid-connected projects, and has the advantages of strong operability and simple and convenient implementation.

[0127] Based on the same inventive concept, this application also provides a land station AC fault control system for implementing the land station AC fault control method described above.

[0128] The solution provided by this system is similar to the solution described in the above method. Therefore, the specific limitations of one or more embodiments of the land station AC fault control system provided below can be found in the limitations of the land station AC fault control method described above, and will not be repeated here.

[0129] As shown in Figure 11, this application embodiment also provides an AC fault control system for a land station, including:

[0130] The fault flag determination module 100 is used to determine the AC fault flag bit based on the AC voltage on the flexible DC transformer side of the onshore station; wherein, the AC fault flag bit is used to indicate whether the onshore station is in the period of AC fault.

[0131] The current modulation module 200 is used to determine whether the onshore station is in an AC fault period based on the AC fault flag bit. When it is determined that the onshore station is in an AC fault period, the reactive current of the onshore station is modulated according to the first current reference value output by the outer loop control of the dq axis of the onshore station and the second current reference value corresponding to the grid voltage amplitude during the AC fault period to obtain a current modulation signal.

[0132] The voltage modulation module 300 is used to use the current modulation signal as the input of the dq axis inner loop control of the land station. Based on the first voltage modulation wave output by the dq axis inner loop control of the land station and the second voltage modulation wave generated by filtering and damping coefficient calculation of the low frequency component of the DC current of the land station, the AC voltage of the land station is modulated to obtain the voltage modulation wave signal.

[0133] In some embodiments, the fault flag determination module 100 is used to normalize the positive sequence voltage amplitude of the AC voltage on the flexible DC transformer side of the onshore station to obtain the positive sequence voltage amplitude per unit value of the AC voltage.

[0134] The per-unit value of the positive sequence voltage amplitude of the AC voltage is compared with the preset amplitude threshold.

[0135] When the per-unit value of the positive sequence voltage amplitude of the AC voltage is greater than the preset amplitude threshold, the AC fault flag bit is determined to be the first AC fault flag bit. The first AC fault flag bit is used to indicate that the land station is not in the period of AC fault.

[0136] When the per-unit value of the positive sequence voltage amplitude of the AC voltage is not greater than the preset amplitude threshold, the AC fault flag bit is determined to be the second AC fault flag bit. The second AC fault flag bit is used to indicate that the land station is in the period of AC fault.

[0137] In some embodiments, the current modulation module 200 is used to output a first current reference value through the dq-axis outer loop control of the land station. The first current reference value includes a d-axis current reference value and a q-axis current reference value.

[0138] The reference value of the q-axis current corresponding to the grid voltage amplitude during AC faults at the onshore station is determined based on the preset Urms-Iq curve.

[0139] The q-axis current reference value output by the outer loop control of the dq axis and the q-axis current reference value corresponding to the grid voltage amplitude during AC faults are used to modulate the q-axis reactive current of the onshore station to obtain the q-axis reactive current value.

[0140] The current modulation signal is determined based on the d-axis current reference value and the q-axis reactive current value.

[0141] In some embodiments, the first current reference value is output via the outer loop control of the dq axis of the land station, including:

[0142] The DC voltage difference is obtained by subtracting the reference value and the measured value of DC voltage at the land station.

[0143] The reactive power difference is obtained by subtracting the reference value and the measured value of reactive power at the land station.

[0144] Both the DC voltage difference and the reactive power difference are controlled by PI to obtain the superimposed difference of DC current and the superimposed difference of reactive current.

[0145] Both the superimposed difference of DC current and the superimposed difference of reactive current are subjected to amplitude limiting processing to obtain the d-axis current reference value and the q-axis current reference value.

[0146] In some embodiments, the voltage modulation module 300 is used to use the current modulation signal as the input of the dq-axis inner loop control of the land station, and outputs a first voltage modulation wave through the dq-axis inner loop control of the land station; the first voltage modulation wave includes a d-axis voltage reference value and a q-axis voltage reference value.

[0147] The low-frequency component of the DC current at the land station is filtered to obtain the low-frequency filtered component.

[0148] The low-frequency filter component is multiplied by the damping coefficient to obtain the second voltage modulation wave;

[0149] The second voltage modulation wave is superimposed with the d-axis voltage reference value to obtain the superimposed d-axis voltage value;

[0150] The three-phase voltage modulation signal is obtained by performing an inverse Park operation on the superimposed d-axis voltage value and the q-axis voltage reference value.

[0151] A voltage modulation wave signal is obtained by using a three-phase voltage modulation signal for modulation trigger control.

[0152] In some embodiments, the current modulation signal includes a d-axis current reference value and a q-axis reactive current value;

[0153] Using a current-modulated signal as the input to the dq-axis inner loop control of the land station, the first voltage-modulated wave is output through the dq-axis inner loop control of the land station, including:

[0154] The difference between the reference value and the measured value of the d-axis current is obtained by subtracting the reference value and the measured value of the d-axis current.

[0155] The difference between the q-axis reactive current value and the measured q-axis current value is obtained by subtracting the q-axis current difference value.

[0156] Both the d-axis current difference and the q-axis current difference are controlled by PI to obtain the d-axis current PI output value and the q-axis current PI output value.

[0157] The d-axis current PI output value is compensated using the measured q-axis voltage value and the d-axis current compensation value to obtain the d-axis voltage reference value; where the d-axis current compensation value is obtained by compensating the measured d-axis current value for angular frequency.

[0158] The q-axis current PI output value is compensated using the measured d-axis voltage value and the q-axis current compensation value to obtain the q-axis voltage reference value; wherein, the q-axis current compensation value is obtained by compensating the measured q-axis current value for angular frequency.

[0159] In some embodiments, the system further includes: a DC power consumption switching module, used to determine whether the DC voltage of the onshore station reaches the overvoltage threshold for the DC power consumption device to be switched on; when the DC voltage of the onshore station reaches the overvoltage threshold for the DC power consumption device to be switched on, the DC power consumption device is switched on to consume the surplus active power of the onshore station; it is determined whether the DC voltage after the DC power consumption device is switched on drops to a preset voltage threshold; when it is determined that the DC voltage after the DC power consumption device is switched on drops to the preset voltage threshold, the switching on of the DC power consumption device is switched off.

[0160] As shown in Figure 12, this application embodiment also provides an electronic device. The electronic device 10 includes a memory 20 and a processor 30. The memory 20 stores a computer program. When the computer program is executed by the processor 30, the processor 30 performs the steps of the land station AC fault control method as described in any of the above embodiments.

[0161] This application also provides a computer-readable storage medium storing a computer program thereon, which, when executed, implements the steps of the land station AC fault control method as described in any of the above embodiments.

[0162] Those skilled in the art will clearly understand that, for the sake of convenience and brevity, the specific working processes of the systems, electronic devices, and computer storage media described above can be referred to the corresponding processes in the foregoing method embodiments, and will not be repeated here.

[0163] It should be noted that the terms "first," "second," etc., in the specification, claims, and accompanying drawings of this invention are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such data can be interchanged where appropriate so that the embodiments of the invention described herein can be implemented in orders other than those illustrated or described herein. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover a non-exclusive inclusion; for example, a process, method, system, product, or apparatus that comprises a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to such processes, methods, products, or apparatus.

[0164] In the several embodiments provided by this invention, it will be understood that each block in the flowchart or block diagram may represent a module, segment, or portion of code, which contains one or more executable instructions for implementing a specified logical function. It should also be noted that in some alternative implementations, the functions marked in the blocks may occur in a different order than those shown in the figures. For example, two consecutive blocks may actually be executed substantially in parallel, and they may sometimes be executed in reverse order, depending on the functions involved.

[0165] In the embodiments provided by this invention, it should be understood that the disclosed systems, electronic devices, computer storage media, and methods can be implemented in other ways. For example, the device embodiments described above are merely illustrative; for instance, the division of units is only a logical functional division, and in actual implementation, there may be other division methods. For example, multiple units or components may be combined or integrated into another system, or some features may be ignored or not executed. Furthermore, the coupling or direct coupling or communication connection shown or discussed may be through some interfaces, or indirect coupling or communication connection between devices or units, and may be electrical, mechanical, or other forms.

[0166] The units described as separate components may or may not be physically separate. The components shown as units may or may not be physical units; that is, they may be located in one place or distributed across multiple network units. Some or all of the units can be selected to achieve the purpose of this embodiment according to actual needs.

[0167] Furthermore, the functional units in the various embodiments of the present invention can be integrated into one processing unit, or each unit can exist physically separately, or two or more units can be integrated into one unit. The integrated unit can be implemented in hardware or as a software functional unit.

[0168] If the integrated unit is implemented as a software functional unit and sold or used as an independent product, it can be stored in a computer-readable storage medium. Based on this understanding, the technical solution of the present invention, in essence, or the part that contributes to the prior art, or all or part of the technical solution, can be embodied in the form of a software product. This computer software product is stored in a storage medium and includes several instructions for executing all or part of the steps of the methods described in the various embodiments of the present invention through a computer device (which may be a personal computer, server, or network device, etc.). The aforementioned storage medium includes: USB flash drives, portable hard drives, read-only memory (ROM), random access memory (RAM), magnetic disks, optical disks, and other media capable of storing program code.

[0169] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for controlling AC faults at a land-based station, characterized in that, include: The AC fault flag is determined based on the AC voltage on the flexible DC transformer side of the onshore station; wherein, the AC fault flag is used to indicate whether the onshore station is in the period of AC fault. The onshore station is determined to be in an AC fault period based on the AC fault flag bit. When the onshore station is determined to be in an AC fault period, the reactive current of the onshore station is modulated based on the first current reference value output by the outer loop control of the dq axis of the onshore station and the second current reference value corresponding to the grid voltage amplitude during the AC fault period to obtain a current modulation signal. Using the current modulation signal as the input of the dq-axis inner loop control of the land station, the AC voltage of the land station is modulated according to the first voltage modulation wave output by the dq-axis inner loop control of the land station and the second voltage modulation wave generated by filtering and damping coefficient calculation of the low-frequency component of the DC current of the land station, so as to obtain the voltage modulation wave signal.

2. The method for controlling AC faults at land stations according to claim 1, characterized in that, The method of determining the AC fault flag bit based on the AC voltage on the onshore flexible DC transformer side includes: The positive sequence voltage amplitude of the AC voltage on the flexible DC-DC transformer side of the onshore station is normalized to obtain the per-unit value of the positive sequence voltage amplitude of the AC voltage. The per-unit value of the positive sequence voltage amplitude of the AC voltage is compared with a preset amplitude threshold. When the per-unit value of the positive sequence voltage amplitude of the AC voltage is greater than the preset amplitude threshold, the AC fault flag bit is determined to be the first AC fault flag bit, which is used to indicate that the land station is not in an AC fault period. When the per-unit value of the positive sequence voltage amplitude of the AC voltage is not greater than the preset amplitude threshold, the AC fault flag bit is determined to be the second AC fault flag bit, which is used to indicate that the land station is in the period of AC fault.

3. The method for controlling AC faults at land stations according to claim 1, characterized in that, The process of modulating the reactive current of the onshore station based on the first current reference value output by the dq-axis outer loop control of the onshore station and the second current reference value corresponding to the grid voltage amplitude during the AC fault to obtain a current modulation signal includes: The first current reference value is output through the outer loop control of the dq axis of the land station. The first current reference value includes the d-axis current reference value and the q-axis current reference value. The reference value of the q-axis current corresponding to the grid voltage amplitude of the onshore station during the AC fault is determined based on the preset Urms-Iq curve. The q-axis current reference value output by the outer loop control of the dq axis and the q-axis current reference value corresponding to the grid voltage amplitude during the AC fault are used to modulate the q-axis reactive current of the onshore station to obtain the q-axis reactive current value. The current modulation signal is determined based on the d-axis current reference value and the q-axis reactive current value.

4. The method for controlling AC faults at land stations according to claim 3, characterized in that, The first current reference value is output through the outer loop control of the dq axis of the land station, including: The DC voltage difference is obtained by subtracting the reference value and the measured value of DC voltage at the land station. The reactive power difference is obtained by subtracting the reference value and the measured value of reactive power at the land station. Both the DC voltage difference and the reactive power difference are subjected to PI control to obtain the superimposed DC current difference and the superimposed reactive current difference. Both the superimposed difference of DC current and the superimposed difference of reactive current are subjected to amplitude limiting processing to obtain the d-axis current reference value and the q-axis current reference value.

5. The method for controlling AC faults at land stations according to claim 1, characterized in that, The method involves using the current modulation signal as the input to the dq-axis inner loop control of the land station, and modulating the AC voltage of the land station based on the first voltage modulation wave output from the dq-axis inner loop control of the land station, and the second voltage modulation wave generated by filtering and damping coefficient calculation of the low-frequency component of the DC current of the land station, to obtain a voltage modulation wave signal, including: The current modulation signal is used as the input to the dq-axis inner loop control of the land station, and a first voltage modulation wave is output through the dq-axis inner loop control of the land station; the first voltage modulation wave includes a d-axis voltage reference value and a q-axis voltage reference value; The low-frequency component of the DC current of the land station is filtered to obtain the low-frequency filtered component. The low-frequency filtered component is multiplied by the damping coefficient to obtain the second voltage modulation wave; The second voltage modulation wave is superimposed with the d-axis voltage reference value to obtain the d-axis voltage superposition value; The three-phase voltage modulation signal is obtained by performing an inverse Park operation on the superimposed d-axis voltage value and the q-axis voltage reference value. The three-phase voltage modulation signal is used for modulation trigger control to obtain a voltage modulation wave signal.

6. The method for controlling AC faults at land stations according to claim 5, characterized in that, The current modulation signal includes a d-axis current reference value and a q-axis reactive current value. The step of using the current modulation signal as the input to the dq-axis inner loop control of the land station, and outputting a first voltage modulation wave through the dq-axis inner loop control of the land station, includes: The difference between the reference value of the d-axis current and the measured value of the d-axis current is obtained by subtracting the reference value of the d-axis current from the measured value of the d-axis current. The difference between the q-axis reactive current value and the measured q-axis current value is obtained by subtracting the q-axis current difference value. Both the d-axis current difference and the q-axis current difference are subjected to PI control to obtain the d-axis current PI output value and the q-axis current PI output value. The d-axis current PI output value is compensated using the measured q-axis voltage value and the d-axis current compensation value to obtain the d-axis voltage reference value; wherein, the d-axis current compensation value is obtained by compensating the measured d-axis current value for angular frequency. The q-axis current PI output value is compensated using the measured d-axis voltage value and the q-axis current compensation value to obtain the q-axis voltage reference value; wherein, the q-axis current compensation value is obtained by compensating the measured q-axis current value for angular frequency.

7. The method for controlling AC faults at land stations according to claim 1, characterized in that, Also includes: Determine whether the DC voltage of the land station has reached the overvoltage threshold for the DC power consumption device to be activated; When the DC voltage of the land station reaches the overvoltage threshold for the DC energy consumption device to be activated, the DC energy consumption device is activated to consume the surplus active power of the land station. Determine whether the DC voltage after the DC energy-consuming device is put into operation drops to a preset voltage threshold. When it is determined that the DC voltage after the DC energy-consuming device is put into operation drops to the preset voltage threshold, the DC energy-consuming device is disconnected.

8. A land-based station AC fault control system, characterized in that, include: The fault flag determination module is used to determine the AC fault flag bit based on the AC voltage on the flexible DC transformer side of the onshore station; wherein, the AC fault flag bit is used to indicate whether the onshore station is in the period of AC fault. The current modulation module is used to determine whether the onshore station is in an AC fault period based on the AC fault flag bit. When it is determined that the onshore station is in an AC fault period, the reactive current of the onshore station is modulated based on the first current reference value output by the outer loop control of the dq axis of the onshore station and the second current reference value corresponding to the grid voltage amplitude during the AC fault period to obtain a current modulation signal. The voltage modulation module is used to use the current modulation signal as the input of the dq axis inner loop control of the land station, and modulate the AC voltage of the land station according to the first voltage modulation wave output by the dq axis inner loop control of the land station and the second voltage modulation wave generated by filtering and damping coefficient calculation of the low frequency component of the DC current of the land station, so as to obtain the voltage modulation wave signal.

9. An electronic device, characterized in that, The electronic device includes a memory and a processor. The memory stores a computer program, which, when executed by the processor, causes the processor to perform the steps of the land station AC fault control method as described in any one of claims 1-7.

10. A computer-readable storage medium having a computer program stored thereon, characterized in that, When the computer program is executed, it implements the steps of the land station AC fault control method as described in any one of claims 1-7.

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