A method and system for suppressing switching overvoltage of a flexible low-frequency power transmission system

Abstract

This invention discloses a method and system for suppressing closing overvoltage in a flexible low-frequency transmission system. The method includes: real-time acquisition of line circuit breaker status signals and line voltage to obtain the line circuit breaker status and line voltage values; determining whether closing overvoltage has occurred based on the line circuit breaker status and line voltage values; if no closing overvoltage has occurred, controlling a virtual impedance to be connected to the transmission circuit; determining whether the closing overvoltage has disappeared based on the acquired line voltage values; if it has disappeared, controlling the virtual impedance to be removed from the transmission circuit. By modifying the control logic of the M3C converter, a virtual impedance is connected in series on the transmission circuit, that is, by controlling power electronic devices to achieve the effect of equivalently connecting a real impedance in series on the circuit, using the virtual impedance to suppress closing overvoltage, effectively suppressing line closing overvoltage. The switching of the virtual impedance is controllable and does not affect the normal operation of the low-frequency transmission system.

Description

Technical Field

[0001] This invention relates to the field of low-frequency power transmission technology, specifically to a method and system for suppressing overvoltage during closing in a flexible low-frequency power transmission system. Background Technology

[0002] In recent years, with the increasingly urgent need for large-scale development and grid connection of offshore wind power, flexible low-frequency power transmission technology has received widespread attention as an innovative solution. The core of this technology lies in using power electronic frequency converters to convert the low-frequency electricity (e.g., 20Hz) transmitted from offshore wind farms to land into 50Hz industrial frequency electricity. Its development is primarily driven by the need to solve the economic and technical challenges of offshore wind power transmission: compared to industrial frequency AC transmission, low-frequency transmission can significantly reduce the reactive power charging power of submarine cables, greatly increase transmission distance and capacity, and effectively overcome the bottleneck of medium- and long-distance submarine cable power transmission; and compared to high-voltage direct current transmission, it has advantages such as lower cost, easier grid connection, and flexible control of system power flow.

[0003] Currently, this technology primarily involves transmitting low-frequency electrical energy generated by offshore booster stations to onshore converter stations via submarine cables. The converter stations then convert the low-frequency electricity into 50Hz power frequency electricity, which is subsequently transmitted to the main power grid. The converter stations utilize M3C converters for frequency conversion, and these converters are connected to the submarine cable lines and overhead lines via low-frequency and power frequency transformers, respectively. The submarine cable and overhead lines serve as the low-frequency and power frequency transmission channels for the flexible low-frequency power transmission system. During system operation, if a fault occurs and triggers the automatic reclosing of the circuit breaker, it can cause significant closing overvoltages. These overvoltages propagate to both sides of the circuit breaker, easily forming localized voltage spikes at critical equipment within the onshore converter station. Furthermore, because it is difficult to find limiting and protective measures for this type of overvoltage in high-voltage or ultra-high-voltage systems, it becomes a dominant factor affecting the insulation level of high-voltage or ultra-high-voltage systems.

[0004] Existing methods for suppressing closing overvoltage mainly involve installing surge arresters and circuit breaker closing resistors, which limit closing overvoltage at the physical structure level. Some studies have also focused on overvoltage caused by asymmetrical short-circuit faults, making improvements at the control level to suppress asymmetrical short-circuit fault overvoltages. However, these methods are less effective at suppressing line closing overvoltages and can easily affect the normal operation of low-frequency transmission systems. Summary of the Invention

[0005] This invention proposes a method and system for suppressing overvoltage during closing in a flexible low-frequency power transmission system, in order to solve the technical problems mentioned in the background.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: A method for suppressing closing overvoltage in a flexible low-frequency power transmission system according to the present invention includes the following steps: S1. Real-time acquisition of circuit breaker status signals and line voltage to obtain circuit breaker status and line voltage values. S2. Determine whether there is a closing overvoltage on the line based on the status of the line circuit breaker and the line voltage value. If no closing overvoltage is found, return to S1; otherwise, proceed to S3. S3. Control the virtual impedance connection to the power transmission circuit; S4. Determine whether the closing overvoltage has disappeared based on the collected line voltage value. If it has not disappeared, return to S4; otherwise, proceed to S5. S5. Control the virtual impedance to exit the transmission circuit.

[0007] Preferably, the logic for the virtual impedance switching control module to determine whether a closing overvoltage has occurred on the line based on the status of the line circuit breaker and the line voltage value is as follows: When the virtual impedance switching control module receives a change in the circuit breaker status from open to closed, and the line voltage is greater than 1.15 times the line rated voltage, it determines that the system has experienced a closing overvoltage.

[0008] Preferably, the method by which the virtual impedance switching control module controls the connection of the virtual impedance to the transmission circuit is as follows: The virtual impedance switching control module controls the switching of virtual impedance. The virtual impedance suppresses overvoltage by modifying the control logic on both sides of the M3C converter. The modification of the control logic on both sides of the M3C converter includes modification operations on the low-frequency side and modification operations on the power frequency side.

[0009] Preferably, the modification operation on the low-frequency side includes the following steps: Let the input virtual impedance be inductive impedance. The connection point is the valve side of the low-frequency transformer in the converter station. Incorporating this virtual impedance into the low-frequency side control loop of the M3C causes the voltage reference value on the low-frequency side of the M3C converter to change as follows:

[0010] in, The voltage reference value on the low-frequency side of the M3C was modified before the change. This is the modified voltage reference value for the low-frequency side of the M3C. This refers to the angular frequency on the low-frequency side. This refers to the valve-side current of the low-frequency transformer in the converter station. This is the low-frequency side-switching control coefficient. This refers to the resistance parameter in the virtual impedance. The inductance parameter in the virtual impedance. The imaginary unit; The relationship between the value and the closing overvoltage determination result is as follows: If the virtual impedance switching control module determines that a closing overvoltage has occurred on the low-frequency side of the system, it will output... Invest the virtual impedance on the low-frequency side; If the virtual impedance switching control module determines that the low-frequency side closing overvoltage of the system has disappeared, then it outputs... Cut off the virtual impedance on the low-frequency side; Voltage reference value for the low-frequency side of M3C Perform the park transformation to obtain d-axis and q-axis components:

[0011] in, The d-axis component of the low-frequency side voltage reference value of M3C; The q-axis component of the low-frequency side voltage reference value of M3C; The low-frequency grid voltage phase is calculated using a phase-locked loop. , as well as These are the voltage reference values ​​for the u, v, and w phase voltages on the low-frequency side, respectively. The result , As the reference values ​​for the d-axis and q-axis voltages in the outer loop control of the voltage-current dual closed-loop control, the d-axis and q-axis components of the low-frequency side voltage are controlled by a PI controller. , Follow voltage reference value , The d-axis and q-axis current reference values ​​of the low-frequency side current are obtained. , ; The d-axis and q-axis components of the low-frequency side current are controlled by a PI controller. , Follow current reference value , Furthermore, voltage feedforward control is incorporated to obtain the d-axis and q-axis components of the low-frequency side modulation voltage. , The expression is as follows:

[0012]

[0013] in, This is the equivalent inductance of the low-frequency transformer, virtual impedance, and bridge arm reactor. This is the transfer function of the PI controller. This is the proportionality coefficient. The integral coefficient is... For the integral operator in the Laplace transform; Will , The modulation voltages of each phase on the low-frequency side are obtained by inverse Park transform. .

[0014] Preferably, the modification operation on the power frequency side includes the following steps: Let the input virtual impedance be inductive impedance. The connection point is the valve side of the power frequency transformer in the converter station. This virtual impedance is incorporated into the power frequency side control loop of the M3C, causing the voltage reference value on the power frequency side of the M3C converter to change as follows:

[0015] in, The voltage reference value on the M3C power frequency side before modification; This is the modified voltage reference value for the M3C's power frequency side; This refers to the angular frequency on the power frequency side. This refers to the valve-side current of the power frequency transformer in the converter station. This refers to the switching control coefficient on the power frequency side. The relationship between the value and the closing overvoltage determination result is as follows: If the virtual impedance switching control module determines that a closing overvoltage has occurred on the power frequency side of the system, it will output... Invest the virtual impedance on the power frequency side; If the virtual impedance switching control module determines that the overvoltage on the power frequency side of the system has disappeared, then it outputs... Cut off the virtual impedance on the power frequency side; Voltage reference value on the M3C power frequency side Perform the park transformation to obtain d-axis and q-axis components:

[0016] in, The d-axis component of the M3C power frequency side voltage reference value; The q-axis component of the M3C power frequency side voltage reference value; The phase of the power grid voltage calculated by the phase-locked loop. , as well as These are the voltage reference values ​​for phases a, b, and c on the power frequency side, respectively. Will Reference value of capacitor voltage of sub-module As the reference value for the outer loop control voltage in the voltage and current dual closed-loop control, the average value of the capacitor voltage of the control submodule is used by the PI controller. q-axis component of power frequency side voltage Follow voltage reference value , The d-axis and q-axis current reference values ​​of the power frequency side current are obtained. , ; The d-axis and q-axis components of the power frequency side current are controlled by a PI controller. , Follow current reference value , And by incorporating voltage feedforward control, the d-axis and q-axis components of the modulated voltage on the power frequency side are obtained. , The expression is as follows:

[0017]

[0018] in, This is the equivalent inductance of the power frequency transformer, virtual impedance, and bridge arm reactor; This is the transfer function of the PI controller. This is the proportionality coefficient. The integral coefficient; Will , The modulation voltages of each phase on the power frequency side are obtained by inverse Park transform. .

[0019] Preferably, the logic for the virtual impedance switching control module to determine whether the closing overvoltage has disappeared based on the collected line voltage is as follows: When the virtual impedance switching control module receives a line voltage less than 1.1 times the line rated voltage, it determines that the system closing overvoltage has disappeared.

[0020] A closing overvoltage suppression system for a flexible low-frequency power transmission system, the system comprising: The status acquisition module is used to acquire the status signals of the line circuit breaker and the line voltage in real time, and send the acquired signals to the virtual impedance switching control module. The virtual impedance switching control module is used to determine whether there is a closing overvoltage on the line based on the status of the line circuit breaker and the line voltage value. If no closing overvoltage is detected, it returns to S1; otherwise, it enters S3. Then, it controls the virtual impedance to connect to the transmission circuit and determines whether the closing overvoltage has disappeared based on the collected line voltage value. If it has not disappeared, it returns to S4; otherwise, it enters S5. Finally, it controls the virtual impedance to exit the transmission circuit.

[0021] As can be seen from the above technical solution, the present invention provides a method and system for suppressing closing overvoltage in a flexible low-frequency transmission system. Compared with the prior art, the present invention has the following advantages: by modifying the control logic of the M3C converter, a virtual impedance is connected in series in the transmission circuit, that is, by controlling the power electronic devices to achieve the effect of connecting a real impedance in series in the equivalent circuit, the closing overvoltage is suppressed by the virtual impedance, effectively suppressing the line closing overvoltage. The switching of the virtual impedance is controllable and does not affect the normal operation of the low-frequency transmission system. Attached Figure Description

[0022] Figure 1 This is a flowchart of a method for suppressing overvoltage during closing in a flexible low-frequency power transmission system according to the present invention.

[0023] Figure 2 A topology diagram illustrating a flexible low-frequency power transmission system for offshore wind power incorporating the method of this invention.

[0024] Figure 3 The control block diagram for the low-frequency side control of voltage and current dual closed-loop control with virtual impedance introduced in this invention is shown.

[0025] Figure 4 The control block diagram for the voltage and current dual closed-loop power frequency side control with virtual impedance introduced in this invention is shown.

[0026] Figure 5 This is a comparison diagram of the simulation waveforms of the low-frequency side closing overvoltage of the present invention.

[0027] Figure 6 This is a comparison diagram of the simulated overvoltage waveforms on the power frequency side of the present invention. Detailed Implementation

[0028] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are some embodiments of the present invention, but not all embodiments.

[0029] like Figure 1 As shown in this embodiment, a method for suppressing closing overvoltage in a flexible low-frequency transmission system includes the following steps: S1. Real-time acquisition of circuit breaker status signals and line voltage to obtain circuit breaker status and line voltage values. S2. Determine whether there is a closing overvoltage on the line based on the status of the line circuit breaker and the line voltage value. If no closing overvoltage is found, return to S1; otherwise, proceed to S3. S3. Control the virtual impedance connection to the power transmission circuit; S4. Determine whether the closing overvoltage has disappeared based on the collected line voltage value. If it has not disappeared, return to S4; otherwise, proceed to S5. S5. Control the virtual impedance to exit the transmission circuit.

[0030] Furthermore, the virtual impedance switching control module determines whether a closing overvoltage has occurred on the line based on the status of the line circuit breaker and the line voltage value, according to the following logic: When the virtual impedance switching control module receives a change in the circuit breaker status from open to closed, and the line voltage is greater than 1.15 times the line rated voltage, it determines that the system has experienced a closing overvoltage.

[0031] Figure 2 This is a topology diagram illustrating the flexible low-frequency power transmission system for offshore wind power, which incorporates the present invention. It includes a status acquisition module 1, a virtual impedance switching control module 2, a low-frequency side virtual impedance 3, a power frequency side virtual impedance 4, a 330kV submarine cable line 6, submarine cable circuit breakers 5 and 7, a 330 / 66kV energy-consuming transformer 8, a 330 / 160kV low-frequency transformer 9, a bridge arm reactor 10, a full-bridge submodule 11, a 160 / 500kV power frequency transformer 12, a 500kV overhead line 14, and overhead line circuit breakers 13 and 15.

[0032] The status acquisition module 1 is used to acquire the status signals of circuit breakers 5, 7, 13 and 15 and the voltage of submarine cable line 6 and overhead line 14 in real time, and send the acquired signals to the virtual impedance switching control module 2.

[0033] The virtual impedance switching control module 2 is used to receive and process the status signal of the line circuit breaker and the line voltage sent by the status acquisition module 1, and to control the switching of the low-frequency side virtual impedance 3 and the power frequency side virtual impedance 4 according to the status signal of the line circuit breaker and the line voltage.

[0034] Furthermore, the method by which the virtual impedance switching control module controls the connection of virtual impedance to the transmission circuit is as follows: The virtual impedance switching control module controls the switching of virtual impedance. The virtual impedance suppresses overvoltage by modifying the control logic on both sides of the M3C converter. The modification of the control logic on both sides of the M3C converter includes modification operations on the low-frequency side and modification operations on the power frequency side.

[0035] Further modifications to the low-frequency side include the following steps: like Figure 3 As shown, in this embodiment, the input virtual impedance is assumed to be inductive impedance. The connection point is the valve side of the low-frequency transformer in the converter station. Incorporating this virtual impedance into the low-frequency side control loop of the M3C causes the voltage reference value on the low-frequency side of the M3C converter to change as follows:

[0036] in, The voltage reference value on the low-frequency side of the M3C was modified before the change. This is the modified voltage reference value for the low-frequency side of the M3C. This refers to the angular frequency on the low-frequency side. This refers to the valve-side current of the low-frequency transformer in the converter station. This is the low-frequency side-switching control coefficient. This refers to the resistance parameter in the virtual impedance. The inductance parameter in the virtual impedance. The imaginary unit; The relationship between the value and the closing overvoltage determination result is as follows: If the virtual impedance switching control module determines that a closing overvoltage has occurred on the low-frequency side of the system, it will output... Invest the virtual impedance on the low-frequency side; If the virtual impedance switching control module determines that the low-frequency side closing overvoltage of the system has disappeared, then it outputs... Cut off the virtual impedance on the low-frequency side; Voltage reference value for the low-frequency side of M3C Perform the park transformation to obtain d-axis and q-axis components:

[0037] in, The d-axis component of the low-frequency side voltage reference value of M3C; The q-axis component of the low-frequency side voltage reference value of M3C; The low-frequency grid voltage phase is calculated using a phase-locked loop. , as well as These are the voltage reference values ​​for the u, v, and w phase voltages on the low-frequency side, respectively. The result , As the reference values ​​for the d-axis and q-axis voltages in the outer loop control of the voltage-current dual closed-loop control, the d-axis and q-axis components of the low-frequency side voltage are controlled by a PI controller. , Follow voltage reference value , The d-axis and q-axis current reference values ​​of the low-frequency side current are obtained. , ; The d-axis and q-axis components of the low-frequency side current are controlled by a PI controller. , Follow current reference value , Furthermore, voltage feedforward control is incorporated to obtain the d-axis and q-axis components of the low-frequency side modulation voltage. , The expression is as follows:

[0038]

[0039] in, This is the equivalent inductance of the low-frequency transformer, virtual impedance, and bridge arm reactor. This is the transfer function of the PI controller. This is the proportionality coefficient. The integral coefficient is... For the integral operator in the Laplace transform; Will , The modulation voltages of each phase on the low-frequency side are obtained by inverse Park transform. .

[0040] Figure 5 The following is a comparison of the simulation waveforms of the closing overvoltage when the method of this invention is used on the low-frequency side; Low-frequency side reclosing process: A U-phase short-circuit ground fault occurred on the low-frequency submarine cable line 6, causing circuit breakers 5 and 7 to trip successively, and then circuit breaker 7 reclosed.

[0041] At 0.081s, the submarine cable line voltage was detected to exceed 1.15 times the rated voltage. The virtual impedance switching control module determined that a closing overvoltage had occurred on the low-frequency side of the system and output... , Input the virtual impedance on the low-frequency side 3.

[0042] At 0.095s, the line voltage is less than 1.1 times the rated line voltage. The virtual impedance switching control module determines that the low-frequency side closing overvoltage has disappeared and outputs... , cut off the virtual impedance on the low-frequency side 3.

[0043] Figure 5 The intermediate voltage is the phase W voltage on the valve side of the low-frequency transformer; without limiting measures, the peak value of phase W voltage is 265kV; after adopting the suppression strategy proposed in this invention, the peak value of phase W voltage is 172kV. Further analysis shows that after adopting the suppression strategy of this invention, the overvoltage multiple of automatic reclosing can be limited from 1.76pu to 1.14pu.

[0044] Furthermore, the modification operations on the power frequency side include the following steps: like Figure 4 As shown, in this embodiment, the input virtual impedance is assumed to be inductive impedance. The connection point is the valve side of the power frequency transformer in the converter station. This virtual impedance is incorporated into the power frequency side control loop of the M3C, causing the voltage reference value on the power frequency side of the M3C converter to change as follows:

[0045] in, The voltage reference value on the M3C power frequency side before modification; This is the modified voltage reference value for the M3C's power frequency side; This refers to the angular frequency on the power frequency side. This refers to the valve-side current of the power frequency transformer in the converter station. This refers to the switching control coefficient on the power frequency side. The relationship between the value and the closing overvoltage determination result is as follows: If the virtual impedance switching control module determines that a closing overvoltage has occurred on the power frequency side of the system, it will output... Invest the virtual impedance on the power frequency side; If the virtual impedance switching control module determines that the overvoltage on the power frequency side of the system has disappeared, then it outputs... Cut off the virtual impedance on the power frequency side; Voltage reference value on the M3C power frequency side Perform the park transformation to obtain d-axis and q-axis components:

[0046] in, The d-axis component of the M3C power frequency side voltage reference value; The q-axis component of the M3C power frequency side voltage reference value; The phase of the power grid voltage calculated by the phase-locked loop. , as well as These are the voltage reference values ​​for phases a, b, and c on the power frequency side, respectively. Will Reference value of capacitor voltage of sub-module As the reference value for the outer loop control voltage in the voltage and current dual closed-loop control, the average value of the capacitor voltage of the control submodule is used by the PI controller. q-axis component of power frequency side voltage Follow voltage reference value , The d-axis and q-axis current reference values ​​of the power frequency side current are obtained. , ; The d-axis and q-axis components of the power frequency side current are controlled by a PI controller. , Follow current reference value , And by incorporating voltage feedforward control, the d-axis and q-axis components of the modulated voltage on the power frequency side are obtained. , The expression is as follows:

[0047]

[0048] in, This is the equivalent inductance of the power frequency transformer, virtual impedance, and bridge arm reactor; This is the transfer function of the PI controller. This is the proportionality coefficient. The integral coefficient; Will , The modulation voltages of each phase on the power frequency side are obtained by inverse Park transform. .

[0049] like Figure 6 As shown, the reclosing process is as follows: A phase a short-circuit ground fault occurred on the power frequency overhead line 14, causing circuit breakers 13 and 15 to trip successively, and then circuit breaker 13 reclosed.

[0050] At 0.051s, the submarine cable line voltage exceeded 1.15 times the rated voltage. The virtual impedance switching control module determined that a closing overvoltage had occurred on the power frequency side of the system and output... , Input the virtual impedance on the power frequency side 4.

[0051] At 0.057s, the line voltage is less than 1.1 times the rated line voltage. The virtual impedance switching control module determines that the overvoltage on the power frequency side of the system has disappeared and outputs... , cut off the virtual impedance on the power frequency side 4.

[0052] Figure 6 The intermediate voltage is the phase b voltage on the valve side of the power frequency transformer; without limiting measures, the peak value of the phase b voltage is 220kV; after adopting the suppression strategy proposed in this invention, the peak value of the phase b voltage is 193kV. Further analysis shows that after adopting the suppression strategy of this invention, the overvoltage multiple of automatic reclosing can be limited from 1.46pu to 1.28pu.

[0053] The following table shows the overvoltage characteristics before and after suppression on the power frequency side and low frequency side:

[0054] The table above shows the overvoltage characteristics before and after suppression on the power frequency side and low frequency side. Furthermore, the virtual impedance switching control module determines whether the closing overvoltage has disappeared based on the collected line voltage as follows: When the virtual impedance switching control module receives a line voltage less than 1.1 times the line rated voltage, it determines that the system closing overvoltage has disappeared.

[0055] A flexible low-frequency power transmission system closing overvoltage suppression system, the system comprising: The status acquisition module is used to acquire the status signals of the line circuit breaker and the line voltage in real time, and send the acquired signals to the virtual impedance switching control module. The virtual impedance switching control module is used to determine whether there is a closing overvoltage on the line based on the status of the line circuit breaker and the line voltage value. If no closing overvoltage is detected, it returns to S1; otherwise, it enters S3. Then, it controls the virtual impedance to connect to the transmission circuit and determines whether the closing overvoltage has disappeared based on the collected line voltage value. If it has not disappeared, it returns to S4; otherwise, it enters S5. Finally, it controls the virtual impedance to exit the transmission circuit.

[0056] In summary, this invention modifies the control logic of the M3C converter to achieve the effect of connecting a virtual impedance in series on the transmission circuit. That is, by controlling the power electronic devices to achieve the effect of connecting a real impedance in series on the equivalent circuit, the virtual impedance suppresses the closing overvoltage, effectively suppressing the line closing overvoltage. The switching of the virtual impedance is controllable and does not affect the normal operation of the low-frequency transmission system.

[0057] In the above embodiments, implementation can be achieved, in whole or in part, through software, hardware, firmware, or any combination thereof. When implemented in software, it can be implemented, in whole or in part, as a computer program product. A computer program product includes one or more computer instructions. When the computer program instructions are loaded and executed on a computer, all or part of the flow or function according to the embodiments of this application is generated. The computer can be a general-purpose computer, a special-purpose computer, a computer network, or other programmable device. The computer instructions can be stored in a computer-readable storage medium or transmitted from one computer-readable storage medium to another. For example, computer instructions can be transmitted from one website, computer, server, or data center to another website, computer, server, or data center via wired (e.g., coaxial cable, fiber optic, digital subscriber line (DSL)) or wireless (e.g., infrared, wireless, microwave, etc.) means. The computer-readable storage medium can be any available medium that a computer can access or a data storage device such as a server or data center that integrates one or more available media. The available medium can be a magnetic medium (e.g., floppy disk, hard disk, magnetic tape), an optical medium (e.g., DVD), or a semiconductor medium (e.g., solid-state disk), etc.

[0058] It should be noted that, in this document, relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes the element.

[0059] The various embodiments in this specification are described in a related manner. Similar or identical parts between embodiments can be referred to mutually. Each embodiment focuses on describing the differences from other embodiments. In particular, the system embodiments are basically similar to the method embodiments, so the description is relatively simple; relevant parts can be referred to the descriptions of the method embodiments.

[0060] 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 suppressing closing overvoltage in a flexible low-frequency power transmission system, characterized in that, Includes the following steps: S1. Real-time acquisition of circuit breaker status signals and line voltage to obtain circuit breaker status and line voltage values. S2. Determine whether there is a closing overvoltage on the line based on the status of the line circuit breaker and the line voltage value. If it is determined that there is no closing overvoltage, return to S1; otherwise, proceed to S3. S3. Control the virtual impedance to be connected to the transmission circuit. The virtual impedance is used to suppress overvoltage by modifying the control logic on both sides of the M3C converter. S4. Determine whether the overvoltage after closing has disappeared based on the collected line voltage value. If it has not disappeared, return to S4; otherwise, proceed to S5. S5. Control the virtual impedance to exit the transmission circuit.

2. The method for suppressing closing overvoltage in a flexible low-frequency power transmission system according to claim 1, characterized in that: The logic for determining whether a closing overvoltage has occurred on the line based on the status of the line circuit breaker and the line voltage value is as follows: When the virtual impedance switching control module receives a change in the circuit breaker status from open to closed, and the line voltage is greater than 1.15 times the line rated voltage, it determines that the system has experienced a closing overvoltage.

3. The method for suppressing closing overvoltage in a flexible low-frequency transmission system according to claim 1, characterized in that: The modification of the control logic on both sides of the M3C converter includes modification operations on the low-frequency side and modification operations on the power frequency side.

4. The method for suppressing closing overvoltage in a flexible low-frequency transmission system according to claim 3, characterized in that: The modification operation on the low-frequency side Includes the following steps: Let the input virtual impedance be inductive impedance. The connection point is the valve side of the low-frequency transformer in the converter station. Incorporating this virtual impedance into the low-frequency side control loop of the M3C causes the voltage reference value on the low-frequency side of the M3C converter to change as follows: in, To modify the voltage reference value on the low-frequency side of the M3C before modification; This is the modified voltage reference value for the low-frequency side of the M3C. This refers to the angular frequency on the low-frequency side. This refers to the valve-side current of the low-frequency transformer in the converter station. This is the low-frequency side-switching control coefficient. This refers to the resistance parameter in the virtual impedance. The inductance parameter in the virtual impedance. The imaginary unit; The relationship between the value and the closing overvoltage determination result is as follows: If the virtual impedance switching control module determines that a closing overvoltage has occurred on the low-frequency side of the system, it will output... Invest the virtual impedance on the low-frequency side; If the virtual impedance switching control module determines that the low-frequency side closing overvoltage of the system has disappeared, then it outputs... Cut off the virtual impedance on the low-frequency side; Voltage reference value for the low-frequency side of M3C Performing the Park transformation yields d-axis and q-axis components: in, The d-axis component of the low-frequency side voltage reference value of M3C; The q-axis component of the low-frequency side voltage reference value of M3C; The low-frequency grid voltage phase is calculated using a phase-locked loop. , as well as These are the voltage reference values ​​for the u, v, and w phase voltages on the low-frequency side, respectively. The result , As reference values ​​for the d-axis and q-axis voltages in the outer voltage loop of the voltage-current dual closed-loop control, the d-axis and q-axis components of the low-frequency side voltage are controlled by a PI controller. , Follow voltage reference value , The d-axis and q-axis current reference values ​​of the low-frequency side current are obtained. , ; The d-axis and q-axis components of the low-frequency side current are controlled by a PI controller. , Follow current reference value , Furthermore, voltage feedforward control is incorporated to obtain the d-axis and q-axis components of the low-frequency side modulation voltage. , The expression is as follows: in, This is the equivalent inductance of the low-frequency transformer, virtual impedance, and bridge arm reactor. This is the transfer function of the PI controller. This is the proportionality coefficient. The integral coefficient is... For the integral operator in the Laplace transform; Will , The modulation voltages of each phase on the low-frequency side are obtained by inverse Park transform. .

5. The method for suppressing closing overvoltage in a flexible low-frequency transmission system according to claim 3, characterized in that: The modification operation on the power frequency side includes the following steps: Let the input virtual impedance be inductive impedance. The connection point is the valve side of the power frequency transformer in the converter station. This virtual impedance is incorporated into the power frequency side control loop of the M3C, causing the voltage reference value on the power frequency side of the M3C converter to change as follows: in, The voltage reference value on the M3C power frequency side before modification; This is the modified voltage reference value for the M3C's power frequency side; This refers to the angular frequency on the power frequency side; This refers to the valve-side current of the power frequency transformer in the converter station. This refers to the switching control coefficient on the power frequency side. The relationship between the value and the closing overvoltage determination result is as follows: If the virtual impedance switching control module determines that a closing overvoltage has occurred on the power frequency side of the system, it will output... , input the virtual impedance on the power frequency side; If the virtual impedance switching control module determines that the overvoltage on the power frequency side of the system has disappeared, then it outputs... Cut off the virtual impedance on the power frequency side; Voltage reference value for the M3C power frequency side Performing the Park transformation yields d-axis and q-axis components: in, The d-axis component of the M3C power frequency side voltage reference value; The q-axis component of the M3C power frequency side voltage reference value; The phase of the power grid voltage calculated by the phase-locked loop. , as well as These are the voltage reference values ​​for phases a, b, and c on the power frequency side, respectively. Will Reference value of capacitor voltage of sub-module As the reference value for the outer loop control voltage in the voltage and current dual closed-loop control, the average value of the capacitor voltage of the control submodule is used by the PI controller. q-axis component of power frequency side voltage Follow voltage reference value , The d-axis and q-axis current reference values ​​of the power frequency side current are obtained. , ; The d-axis and q-axis components of the power frequency side current are controlled by a PI controller. , Follow current reference value , And by incorporating voltage feedforward control, the d-axis and q-axis components of the modulated voltage on the power frequency side are obtained. , The expression is as follows: in, This is the equivalent inductance of the power frequency transformer, virtual impedance, and bridge arm reactor; This is the transfer function of the PI controller. This is the proportionality coefficient. The integral coefficient; Will , The modulation voltages of each phase on the power frequency side are obtained by inverse Park transform. .

6. The method for suppressing closing overvoltage in a flexible low-frequency transmission system according to claim 1, characterized in that: The logic for determining whether the closing overvoltage has disappeared based on the collected line voltage is as follows: When the virtual impedance switching control module receives a line voltage less than 1.1 times the line rated voltage, it determines that the system closing overvoltage has disappeared.

7. A closing overvoltage suppression system for a flexible low-frequency transmission system, employing a closing overvoltage suppression method for a flexible low-frequency transmission system as described in claims 1-6, characterized in that: The system includes: The status acquisition module is used to acquire the status signals of the line circuit breaker and the line voltage in real time, and send the acquired signals to the virtual impedance switching control module. The virtual impedance switching control module is used to determine whether there is a closing overvoltage on the line based on the status of the line circuit breaker and the line voltage value. If it is determined that there is no closing overvoltage, it returns to S1; otherwise, it enters S3. Then, control the virtual impedance to connect to the transmission circuit, and determine whether the closing overvoltage has disappeared based on the collected line voltage value. If it has not disappeared, return to S4; otherwise, enter S5, and finally control the virtual impedance to exit the transmission circuit.

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