Method for power flow inversion of a direct current transmission system

Abstract

The application provides a power flow inversion method of a direct current power transmission system, the direct current power transmission system comprising a power sending end and a power receiving end, the power sending end comprising a first converter, the power receiving end comprising an electrically connected second converter and at least one third converter, the type of the first converter being the same as that of the second converter, the type of the second converter being different from that of the third converter, the power flow inversion method comprising: determining whether the direct current power transmission system receives a power flow inversion instruction; in the case that the direct current power transmission system receives the power flow inversion instruction, obtaining a direct current voltage instruction value, and controlling the direct current voltage of the first converter to follow the change of the direct current voltage instruction value, the direct current voltage instruction value being stepwise reduced from a first predetermined value to a second predetermined value; controlling the active power of the second converter according to the direct current voltage of the first converter; and controlling the direct current of the third converter to remain unchanged. The method can overcome the complexity and current impact problem of the power flow inversion operation.

Description

Technical Field

[0001] This application relates to the field of DC power transmission, and more specifically, to a power flow reversal method for a DC power transmission system, a power flow reversal device for a DC power transmission system, and a DC power transmission system. Background Technology

[0002] In existing DC transmission technologies, power flow reversal mechanisms face significant challenges. Traditional DC transmission systems often rely on shutdowns and mode switching to resolve power flow reversals, which is not only complex but may also introduce system instability factors.

[0003] Therefore, there is an urgent need for a power flow reversal method for DC transmission systems that can overcome the above problems. Summary of the Invention

[0004] The main objective of this application is to provide a power flow reversal method, a power flow reversal device, and a DC power transmission system, so as to at least solve the problems of complexity and current surge in power flow reversal operation in the prior art.

[0005] To achieve the above objectives, according to one aspect of this application, a power flow reversal method for a DC transmission system is provided. The DC transmission system includes a transmitting end and a receiving end. The transmitting end includes a first converter, and the receiving end includes a second converter and at least one third converter electrically connected. The type of the first converter is the same as that of the second converter, and the type of the second converter is different from that of the third converter. The power flow reversal method for the DC transmission system includes: determining whether the DC transmission system receives a power flow reversal command; if the DC transmission system receives the power flow reversal command, acquiring a DC voltage command value and controlling the DC voltage of the first converter to change in accordance with the DC voltage command value, wherein the DC voltage command value decreases stepwise from a first predetermined value to a second predetermined value, the first predetermined value is greater than 0, the second predetermined value is less than 0, and the absolute value of the difference between the DC voltage of the first converter and the DC voltage command value at each time is less than or equal to a preset threshold; controlling the active power of the second converter according to the DC voltage of the first converter; and controlling the DC current of the third converter to remain constant.

[0006] Optionally, obtaining the DC voltage command value includes: obtaining a DC voltage reference value of the first converter, a target virtual impedance value of the first converter, and a target current limiting value of the first converter, wherein the DC voltage reference value is a reference value of the DC voltage of the first converter, the target virtual impedance value is used to increase the virtual impedance of the first converter to suppress current surges in the first converter, and the target current limiting value is used to suppress the DC current of the first converter from exceeding a first current threshold; calculating the difference between the sum of the DC voltage reference value and the target virtual impedance value and the target current limiting value to obtain the DC voltage command value.

[0007] Optionally, obtaining the DC voltage reference value of the first converter includes: obtaining a predetermined slope and a predetermined duration, wherein the predetermined slope is the rate at which the first predetermined value decreases to 0, and the predetermined duration is the duration during which the first predetermined value decreases to 0; calculating the product of the predetermined slope and the predetermined duration to obtain a first product; and calculating the difference between the first predetermined value and the first product to obtain the DC voltage reference value.

[0008] Optionally, obtaining the target virtual impedance value of the first converter includes: obtaining the resistance value of the virtual resistor, the inductance value of the virtual inductor, and the output current of the first converter; calculating the product of the time derivative of the output current and the inductance value of the virtual inductor to obtain a first product; calculating the product of the resistance value of the virtual resistor and the output current to obtain a second product; and calculating the sum of the first product and the second product to obtain the target virtual impedance value. Optionally, obtaining the resistance value of the virtual resistor of the first converter includes: determining the relationship between the absolute value of the output current and a second current threshold, wherein the second current threshold is less than the first current threshold; when the absolute value of the output current is greater than the second current threshold, determining the maximum virtual resistance value corresponding to a DC voltage of 0 for the first converter as the resistance value of the virtual resistor; when the absolute value of the output current is less than or equal to the second current threshold, determining the minimum virtual resistance value corresponding to a DC voltage of 0 for the first converter as the resistance value of the virtual resistor.

[0009] Optionally, obtaining the inductance value of the virtual inductance of the first converter includes: obtaining the maximum inductance value of the first converter; calculating the ratio of the absolute value of the derivative of the output current with respect to time to a second current threshold to obtain a first ratio; and calculating the product of the maximum inductance value and the first ratio to obtain the inductance value of the virtual inductance corresponding to the DC voltage of the first converter being 0.

[0010] Optionally, obtaining the target current limiting value of the first converter includes: obtaining the output current and current limiting coefficient of the first converter; calculating the difference between the absolute value of the output current and the first current threshold to obtain a first difference; obtaining the maximum value between the first difference and 0 to obtain a third predetermined value; and calculating the product of the current limiting coefficient and the third predetermined value to obtain the target current limiting value.

[0011] Optionally, controlling the active power of the second converter based on the DC voltage of the first converter includes: obtaining the active power command value of the second converter; calculating in real time the ratio of the DC voltage command value to the first predetermined value to obtain a second ratio; calculating in real time the product of the active power command value of the second converter and the second ratio to obtain a target active power, and controlling the second converter to operate at the target active power obtained in real time.

[0012] According to another aspect of this application, a power flow reversal device for a DC transmission system is provided. The DC transmission system includes a transmitting end and a receiving end. The transmitting end includes a first converter, and the receiving end includes a second converter and at least one third converter electrically connected. The type of the first converter is the same as that of the second converter, and the type of the second converter is different from that of the third converter. The power flow reversal device for the DC transmission system includes: a determining unit, configured to determine whether the DC transmission system receives a power flow reversal command; a first control unit, configured to, when the DC transmission system receives the power flow reversal command, acquire a DC voltage command value and control the DC voltage of the first converter to follow the change of the DC voltage command value, wherein the DC voltage command value decreases stepwise from a first predetermined value to a second predetermined value, the first predetermined value is greater than 0, the second predetermined value is less than 0, and the absolute value of the difference between the DC voltage of the first converter and the DC voltage command value at each time is less than or equal to a preset threshold; a second control unit, configured to control the active power of the second converter according to the DC voltage of the first converter; and a third control unit, configured to control the DC current of the third converter to remain constant.

[0013] According to another aspect of this application, a DC transmission system is provided, including a power transmitting end, a power receiving end, and a controller. The power transmitting end includes a first converter, and the power receiving end includes a second converter and at least one third converter electrically connected. The first converter is of the same type as the second converter, and the second converter is of a different type from the third converter. The controller is communicatively connected to the first converter, the second converter, and the third converter, respectively. The controller is used to execute any of the power flow reversal methods of the DC transmission system described above.

[0014] This application provides a method for reversing power flow in a DC transmission system. The DC transmission system includes a transmitting end and a receiving end. The transmitting end includes a first converter, and the receiving end includes a second converter and at least one third converter connected electrically. The first converter is of the same type as the second converter, while the second converter is of a different type from the third converter. First, it is determined whether the DC transmission system has received a power flow reversal command. Then, if the DC transmission system has received a power flow reversal command, a DC voltage command value is acquired, and the DC voltage of the first converter is controlled to follow the change of the DC voltage command value, which is stepped down from a first predetermined value to a second predetermined value. Next, based on the DC voltage of the first converter, the active power of the second converter is controlled. Finally, the DC current of the third converter is kept constant. The power receiving end converter includes converters of the same type as the power sending end converter and converters of different types. During the power supply process, the power flow direction is smoothly switched online by changing the DC voltage of the first converter instead of changing the DC current in the existing technology. This can realize power flow reversal without stopping the system, while avoiding the transient instability caused by control mode switching and the impact of current surges and power oscillations on the equipment during power flow reversal, ensuring stable system operation and solving the problem of how to overcome the complexity of power flow reversal operation and the problem of current surges. Attached Figure Description

[0015] The accompanying drawings, which form part of this application, are used to provide a further understanding of this application. The illustrative embodiments and descriptions of this application are used to explain this application and do not constitute an undue limitation of this application. In the drawings:

[0016] Figure 1 A hardware structure block diagram of a mobile terminal for performing a power flow reversal method for a DC transmission system, according to an embodiment of this application, is shown.

[0017] Figure 2 A schematic diagram of a DC transmission system according to an embodiment of this application is shown;

[0018] Figure 3 A schematic flowchart of a power flow reversal method for a DC transmission system according to an embodiment of this application is shown.

[0019] Figure 4 A detailed flowchart of a power flow reversal method for a DC transmission system according to an embodiment of this application is shown.

[0020] Figure 5 A structural block diagram of a power flow reversal device for a DC transmission system provided according to an embodiment of this application is shown.

[0021] The above figures include the following reference numerals:

[0022] 102. Processor; 104. Memory; 106. Transmission device; 108. Input / output device; 110. Power supply end; 112. Power receiving end; 114. First converter; 116. Second converter; 118. Third converter. Detailed Implementation

[0023] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. This application will now be described in detail with reference to the accompanying drawings and embodiments.

[0024] To enable those skilled in the art to better understand the present application, the technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present application, and not all embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative effort should fall within the scope of protection of the present application.

[0025] It should be noted that the terms "first," "second," etc., in the specification, claims, and accompanying drawings of this application 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 for the embodiments of this application described herein. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover 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.

[0026] As described in the background section, power flow reversal in the prior art suffers from operational complexity and current surges. To address these issues, embodiments of this application provide a power flow reversal method for a DC transmission system, a power flow reversal device for a DC transmission system, and a DC transmission system.

[0027] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention.

[0028] The methods and embodiments provided in this application can be executed on a mobile terminal, computer terminal, or similar computing device. Taking running on a mobile terminal as an example, Figure 1 This is a hardware structure block diagram of a mobile terminal for a power flow reversal method in a DC transmission system according to an embodiment of the present invention. Figure 1 As shown, a mobile terminal may include one or more ( Figure 1 Only one is shown in the diagram. A processor 102 (which may include, but is not limited to, a microprocessor MCU or a programmable logic device FPGA, etc.) and a memory 104 for storing data are also shown. The mobile terminal may further include a transmission device 106 for communication functions and an input / output device 108. Those skilled in the art will understand that... Figure 1 The structure shown is for illustrative purposes only and does not limit the structure of the mobile terminal described above. For example, the mobile terminal may also include components that are more... Figure 1 The more or fewer components shown, or having the same Figure 1 The different configurations shown.

[0029] The memory 104 can be used to store computer programs, such as application software programs and modules, like the computer program corresponding to the power flow reversal method of the DC transmission system in this embodiment of the invention. The processor 102 executes various functional applications and data processing by running the computer program stored in the memory 104, thereby implementing the above-described method. The memory 104 may include high-speed random access memory and may also include non-volatile memory, such as one or more magnetic storage devices, flash memory, or other non-volatile solid-state memory. In some instances, the memory 104 may further include memory remotely located relative to the processor 102, and these remote memories can be connected to the mobile terminal via a network. Examples of the aforementioned networks include, but are not limited to, the Internet, corporate intranets, local area networks, mobile communication networks, and combinations thereof. The transmission device 106 is used to receive or send data via a network. Specific examples of the aforementioned networks may include wireless networks provided by the mobile terminal's communication provider. In one example, the transmission device 106 includes a network interface controller (NIC), which can be connected to other network devices via a base station to communicate with the Internet. In one example, the transmission device 106 may be a radio frequency (RF) module, which is used to communicate with the Internet wirelessly.

[0030] This embodiment provides a power flow reversal method for a DC transmission system that runs on a mobile terminal, computer terminal, or similar computing device. It should be noted that the steps shown in the flowchart in the accompanying drawings can be executed in a computer system such as a set of computer-executable instructions. Also, although a logical order is shown in the flowchart, in some cases, the steps shown or described may be executed in a different order than that shown here.

[0031] like Figure 2As shown, the DC transmission system includes a power transmitting end 110 and a power receiving end 112. The power transmitting end 110 includes a first converter 114, and the power receiving end 112 includes a second converter 116 and at least one third converter 118 that are electrically connected. The type of the first converter 114 is the same as that of the second converter 116, and the type of the second converter 116 is different from that of the third converter 118.

[0032] Specifically, the types of the first, second, and third converters mentioned above refer to voltage source converters or current source converters. For example, the first and second converters are voltage source converters, and the third converter is a current source converter. Further, the first and second converters can be modular multilevel converters (MMCs), which include both full-bridge and half-bridge submodules. The third converter can be a hybrid commutation converter (HCC). There can be one or more third converters, and the second and third converters can be connected in parallel.

[0033] Figure 3 This is a flowchart of a power flow reversal method for a DC transmission system according to an embodiment of this application. Figure 3 As shown, the method includes the following steps:

[0034] Step S201: Determine whether the DC transmission system has received a power flow reversal command;

[0035] In practical applications, a power flow reversal command is output to the DC transmission system under at least one of the following circumstances: the DC transmission system receives the aforementioned power flow reversal command (e.g., a change in transmission direction, such as temporarily switching from "West-to-East Power Transmission" to "East-to-West Power Transmission"); when the receiving-end AC system experiences a large power shortage due to large generator trips, DC unipolar faults, etc., in order to quickly provide frequency or voltage support, the original sending-end DC rectifier station can be switched to inverter operation, turning the "receiving end" into the "sending end," thus achieving emergency power reverse transmission; if the output of renewable energy at the sending end drops sharply while wind and solar power generation at the receiving end surges, resulting in a surplus of power at the "original receiving end" and a shortage of power at the "original sending end." To reduce wind and solar power curtailment and meet load balance requirements, the power flow direction can be reversed. When the original sending-end converter station, nearby AC lines, or generator units need to be shut down for maintenance, the power direction can be temporarily reversed, and power can be supplied from the opposite grid through the same DC line to avoid large-scale power outages or reduce the demand for reserve capacity. During the flood season, when hydropower is generated in large quantities, power is transmitted to other areas. During the dry season, when the near-area load is high and the hydropower output is insufficient, pumped storage or receiving-end power sources can be used to reverse the power flow, achieving "bidirectional regulation" through power flow reversal. During the black start process of the regional power grid, in order to gradually charge and load areas without power sources, the side with power sources can first supply power in reverse at the minimum power, and then restore the normal power supply direction after the units are connected to the grid.

[0036] Step S202: When the DC transmission system receives the power flow reversal command, the DC voltage command value is obtained, and the DC voltage of the first converter is controlled to follow the change of the DC voltage command value. The DC voltage command value is reduced stepwise from a first predetermined value to a second predetermined value. The first predetermined value is greater than 0, the second predetermined value is less than 0, and the absolute value of the difference between the DC voltage of the first converter and the DC voltage command value at each time is less than or equal to a preset threshold.

[0037] Specifically, the change in the aforementioned DC voltage command value includes a first stage, a second stage, and a third stage. The first stage includes a reduction from the aforementioned first predetermined value to 0; the second stage includes the aforementioned DC voltage command being at 0; and the third stage includes a reduction from 0 to the aforementioned second predetermined value. The reduction rate of the first stage is greater than the reduction rate of the third stage, for example, the reduction rate of the first stage is 600~800kV / s, and the reduction rate of the third stage is 400~500kV / s. Furthermore, the duration of the first stage and the third stage is greater than the duration of the second stage, and the ratio of the duration of the first stage, the second stage, and the third stage can be 8~12:1:8~12. For example, the duration of the first stage and the third stage can be 1~1.2s, and the duration of the second stage can be 100~120ms. The first stage is the voltage reduction stage. In the initial stage of power flow reversal, the first converter needs to slowly reduce the DC voltage from a positive value to near zero. The second stage is the buffering stage. When the DC voltage approaches zero, the system enters the sensitive region of voltage polarity switching, where the current is most prone to oscillation or overshoot. The third stage is the polarity reversal stage. The polarity reversal stage still requires the participation of virtual impedance, but its intensity is reduced, that is, the virtual impedance value is less than the voltage drop and buffering, to avoid hindering the switching speed, while the current limiter remains in operation at all times.

[0038] Step S203: Control the active power of the second converter according to the DC voltage of the first converter.

[0039] Step S204: Control the DC current of the third converter to remain constant.

[0040] In other words, the first converter employs constant DC voltage control, constant reactive power control, or constant AC voltage control. The aforementioned stepped control refers to a gradual reduction of the DC voltage from a positive value to a negative value in stages. The absolute values ​​of the first and second predetermined values ​​can be the same or different. That is, the second converter employs constant active power control in its active power control stage and constant reactive power control in its reactive power control stage. The active power control stage includes constant DC voltage control and constant active power control, while the reactive power control stage includes constant AC voltage control and constant reactive power control. The third converter employs constant DC current control.

[0041] This embodiment provides a method for reversing power flow in a DC transmission system. The DC transmission system includes a transmitting end and a receiving end. The transmitting end includes a first converter, and the receiving end includes a second converter and at least one third converter electrically connected. The first converter is of the same type as the second converter, while the second converter is of a different type from the third converter. First, it is determined whether the DC transmission system has received a power flow reversal command. Then, if the DC transmission system has received a power flow reversal command, a DC voltage command value is obtained, and the DC voltage of the first converter is controlled to follow the change of the DC voltage command value, which is gradually reduced from a first predetermined value to a second predetermined value. Next, based on the DC voltage of the first converter, the active power of the second converter is controlled. Finally, the DC current of the third converter is kept constant. The power receiving end converter includes converters of the same type as the power sending end converter and converters of different types. During the power supply process, the power flow direction is smoothly switched online by changing the DC voltage of the first converter instead of changing the DC current in the existing technology. This can realize power flow reversal without stopping the system, while avoiding the transient instability caused by control mode switching and the impact of current surges and power oscillations on the equipment during power flow reversal, ensuring stable system operation and solving the problem of how to overcome the complexity of power flow reversal operation and the problem of current surges.

[0042] Step S202 of this application can be implemented through the following steps: Step S2021, obtaining the DC voltage reference value of the first converter, the target virtual impedance value of the first converter, and the target current limiting value of the first converter, wherein the DC voltage reference value is a reference value of the DC voltage of the first converter, the target virtual impedance value is used to increase the virtual impedance of the first converter to suppress the current surge of the first converter, and the target current limiting value is used to suppress the DC current of the first converter from exceeding a first current threshold; Step S2022, calculating the difference between the sum of the DC voltage reference value and the target virtual impedance value and the target current limiting value to obtain the DC voltage command value. This method combines the target virtual impedance value and the target current limiting value, that is, the virtual impedance control and the current limiting control work together. The virtual impedance can provide "soft damping", and the current limiting control acts as a "hard constraint", forming a double-layer protection of "soft damping + hard constraint", while further avoiding current overshoot and power oscillation during the power flow reversal process, so that the system power flow can be smoothly reversed.

[0043] Step S2021 can be implemented in the following ways, for example: Step S20211, obtaining a predetermined slope and a predetermined duration, wherein the predetermined slope is the rate at which the first predetermined value decreases to 0, and the predetermined duration is the duration during which the first predetermined value decreases to 0; Step S20212, calculating the product of the predetermined slope and the predetermined duration to obtain a first product; Step S20213, calculating the difference between the first predetermined value and the first product to obtain the DC voltage reference value. This method can further calculate an accurate DC voltage reference value.

[0044] Specifically, the formula for calculating the DC voltage reference value is as follows: ,in, The above-mentioned DC voltage reference value, The first predetermined value mentioned above, For the aforementioned predetermined slope, The above-mentioned scheduled duration.

[0045] Step S2021 can also be implemented in other ways, for example: Step S20214, obtaining the resistance value of the virtual resistor of the first converter, the inductance value of the virtual inductor of the first converter, and the output current of the first converter, wherein the resistance value of the virtual resistor is used to simulate the impedance characteristics of the actual resistor, and the inductance value of the virtual inductor is used to simulate the impedance characteristics of the actual inductor; Step S20215, calculating the product of the time derivative of the output current and the inductance value of the virtual inductor to obtain a first product; Step S20216, calculating the product of the resistance value of the virtual resistor and the output current to obtain a second product; Step S20217, calculating the sum of the first product and the second product to obtain the target virtual impedance value. This method can further quickly obtain the target virtual impedance value.

[0046] Specifically, the formula for calculating the target virtual impedance value is as follows: ,in, The target virtual impedance value is as described above. The resistance value of the aforementioned virtual resistor. For the above output current, The inductance value of the virtual inductor mentioned above can be different in the first stage, the second stage, and the third stage.

[0047] In practical applications, the derivative of the output current with respect to time can also be obtained in other ways, such as: Step S202151, acquiring multiple operating data of the first converter before the prediction time to obtain multiple sample data, part of which is test data and the other part is training data, wherein the operating data includes DC current, voltage, power command, ambient temperature and current change rate; Step S202152, sequentially inputting the test data into the Long Short-Term Memory Artificial Neural Network Model to obtain multiple predicted operating data of the first converter, wherein the Long Short-Term Memory Artificial Neural Network Model is trained by machine learning using multiple sets of first historical data, each set of the first historical data includes: the training data of the first time period and the training data of the second time period, the start time of the second time period is after the end time of the first time period, and the predicted operating data is the operating data after the test data; Step S202153, determining the predicted operating data as the derivative of the output current with respect to time. The above method, through a long short-term memory artificial neural network model, can further improve the accuracy of calculating the derivative of the output current with respect to time, i.e., the rate of change of the output current.

[0048] Step S20214 above can be achieved through the following steps: Step S202141, determining the relationship between the absolute value of the output current and the second current threshold, wherein the second current threshold is less than the first current threshold; Step S202142, when the absolute value of the output current is greater than the second current threshold, determining the maximum virtual resistance value corresponding to the DC voltage of the first converter being 0 as the resistance value of the virtual resistor; Step S202143, when the absolute value of the output current is less than or equal to the second current threshold, determining the minimum virtual resistance value corresponding to the DC voltage of the first converter being 0 as the resistance value of the virtual resistor. This method can further obtain an accurate resistance value of the virtual resistor.

[0049] Specifically, the formula for calculating the resistance value of the virtual resistor in the second stage mentioned above is as follows: ,in, The resistance value of the aforementioned virtual resistor. The above is the maximum virtual resistance value. The minimum virtual resistance value mentioned above. For the above output current, The second current threshold mentioned above, and All are preset values. When When the value is large, increase the virtual impedance. When the current is relatively small and gradually stabilizes, reduce the virtual impedance to avoid increasing additional losses.

[0050] Step S20214 can also be implemented in other ways, for example: Step S202144, obtain the maximum inductance value of the first converter; Step S202145, calculate the ratio of the absolute value of the derivative of the output current with respect to time to the second current threshold to obtain a first ratio; Step S202146, calculate the product of the maximum inductance value and the first ratio to obtain the inductance value of the virtual inductor corresponding to the DC voltage of the first converter being 0. This method can further calculate the accurate inductance value of the virtual inductor.

[0051] Specifically, the formula for calculating the inductance value of the aforementioned virtual inductor is as follows: ,in, The inductance value of the aforementioned virtual inductor. This is the maximum inductance value. For the above output current, This is the second current threshold mentioned above. When When the value is large, increase the virtual impedance. When the current is relatively small and gradually stabilizes, reduce the virtual impedance to avoid increasing additional losses.

[0052] Step S20214 above can also be implemented in other ways, for example: step S202147, obtaining the output current and current limiting coefficient of the first converter; step S202148, calculating the difference between the absolute value of the output current and the first current threshold to obtain a first difference; step S202149, obtaining the maximum value between the first difference and 0 to obtain a third predetermined value; step S2021410, calculating the product of the current limiting coefficient and the third predetermined value to obtain the target current limiting value. This method can further calculate a precise target current limiting value.

[0053] Specifically, the formula for calculating the target current limit value is as follows: ,in, The target current limit is set above. The above-mentioned current limiting coefficient, For the above output current, The first current threshold is as described above.

[0054] Step S203 above can be implemented through the following steps: Step S2031, obtain the active power command value of the second converter; Step S2042, calculate the ratio of the DC voltage command value to the first predetermined value in real time to obtain a second ratio; Step S2043, calculate the product of the active power command value of the second converter and the second ratio in real time to obtain the target active power, and control the second converter to operate at the target active power obtained in real time. This method can further achieve precise control of the second converter.

[0055] Specifically, the formula for calculating the active power command value of the second converter mentioned above is as follows: ,in, DC voltage is The active power command value at that time, This is the DC voltage command value. The first predetermined value is as described above.

[0056] In some embodiments, step S2031 can also be implemented through the following steps: the first converter is connected in parallel with the second and third converters respectively; in step S20311, the target DC voltage value of the second converter is obtained to obtain the DC voltage command value. This method, by obtaining only the DC voltage value of the receiving end of the second converter, can simultaneously obtain the two DC voltage command values ​​of the first and second converters, eliminating the need to obtain the DC command values ​​of the first and second converters separately. Therefore, it can further simplify the DC voltage command value acquisition process.

[0057] The first converter is connected in parallel with the second and third converters. Since the voltages are equal in parallel, when the DC voltage of the first converter changes, the voltage of the second converter will change accordingly. Ignoring the line voltage drop between the second and third converters, the voltage of the first converter is Udc = Udc1, and the voltage of the second converter is Udc = Udc(n-1) + ΔU. The DC circuit voltage drop ΔU is generally very small; therefore, when the line voltage drop is ignored, Udc = Udc(n-1), meaning the DC voltages of the first and second converters are equal.

[0058] To enable those skilled in the art to better understand the technical solution of this application, the implementation process of the power flow reversal method for DC transmission systems of this application will be described in detail below with reference to specific embodiments.

[0059] This embodiment relates to a specific method for reversing power flow in a DC transmission system, such as... Figure 4 As shown, it includes the following steps:

[0060] Step S1: Determine whether a power flow reversal command has been received. If yes, proceed to step S2; otherwise, no reversal is required.

[0061] Step S2: The sending-end MMC sets the DC voltage and AC voltage for control, and the receiving-end MMC synchronizes the power command.

[0062] Step S3: The MMC at the sending end is stepped down, and virtual impedance and current limiting control are activated;

[0063] Step S4: When the MMC voltage at the sending end is near 0, adaptive virtual impedance control is implemented;

[0064] Step S5: The polarity of the MMC voltage at the sending end reverses and rises to the negative rated value, with weak virtual impedance control;

[0065] Step S6: Polarity reversal complete, virtual impedance and current limiting control exit;

[0066] Step S7: Adjust the MMC reactive power control as needed;

[0067] Step S8: Reversal complete.

[0068] This application also provides a power flow reversal device for a DC transmission system. It should be noted that the power flow reversal device for a DC transmission system in this application can be used to execute the power flow reversal method for a DC transmission system provided in this application. This device is used to implement the above embodiments and preferred embodiments; details already described will not be repeated. As used below, the term "module" can refer to a combination of software and / or hardware that performs a predetermined function. Although the device described in the following embodiments is preferably implemented in software, hardware implementation, or a combination of software and hardware, is also possible and contemplated.

[0069] The following describes the power flow reversal device for a DC transmission system provided in the embodiments of this application.

[0070] Figure 5 This is a schematic diagram of a power flow reversal device for a DC transmission system according to an embodiment of this application. Figure 5 As shown, the device includes:

[0071] Determining unit 10 is used to determine whether the above-mentioned DC transmission system has received a power flow reversal command;

[0072] In practical applications, a power flow reversal command is output to the DC transmission system under at least one of the following circumstances: the DC transmission system receives the aforementioned power flow reversal command (e.g., a change in transmission direction, such as temporarily switching from "West-to-East Power Transmission" to "East-to-West Power Transmission"); when the receiving-end AC system experiences a large power shortage due to large generator trips, DC unipolar faults, etc., in order to quickly provide frequency or voltage support, the original sending-end DC rectifier station can be switched to inverter operation, turning the "receiving end" into the "sending end," thus achieving emergency power reverse transmission; if the output of renewable energy at the sending end drops sharply while wind and solar power generation at the receiving end surges, resulting in a surplus of power at the "original receiving end" and a shortage of power at the "original sending end." To reduce wind and solar power curtailment and meet load balance requirements, the power flow direction can be reversed. When the original sending-end converter station, nearby AC lines, or generator units need to be shut down for maintenance, the power direction can be temporarily reversed, and power can be supplied from the opposite grid through the same DC line to avoid large-scale power outages or reduce the demand for reserve capacity. During the flood season, when hydropower is generated in large quantities, power is transmitted to other areas. During the dry season, when the near-area load is high and the hydropower output is insufficient, pumped storage or receiving-end power sources can be used to reverse the power flow, achieving "bidirectional regulation" through power flow reversal. During the black start process of the regional power grid, in order to gradually charge and load areas without power sources, the side with power sources can first supply power in reverse at the minimum power, and then restore the normal power supply direction after the units are connected to the grid.

[0073] The first control unit 20 is configured to, when the DC power transmission system receives the power flow reversal command, acquire a DC voltage command value and control the DC voltage of the first converter to follow the change of the DC voltage command value, wherein the DC voltage command value is reduced stepwise from a first predetermined value to a second predetermined value, the first predetermined value is greater than 0, the second predetermined value is less than 0, and the absolute value of the difference between the DC voltage of the first converter and the DC voltage command value at each time is less than or equal to a preset threshold.

[0074] Specifically, the change in DC voltage of the first converter includes a first stage, a second stage, and a third stage. The first stage includes a reduction from the first predetermined value to 0; the second stage includes the DC voltage command being 0; and the third stage includes a reduction from 0 to the second predetermined value. The reduction rate of the first stage is greater than that of the third stage, for example, the reduction rate of the first stage is 600~800kV / s, and the reduction rate of the third stage is 400~500kV / s. Furthermore, the duration of the first and third stages is greater than the duration of the second stage, and the ratio of the duration of the first, second, and third stages can be 8~12:1:8~12. For example, the duration of the first and third stages can be 1~1.2s, and the duration of the second stage can be 100~120ms. The first stage is the voltage reduction stage; at the initial stage of power flow reversal, the first converter needs to slowly reduce the DC voltage from a positive value to near zero. The second stage is the buffering stage. When the DC voltage approaches zero, the system enters the sensitive region of voltage polarity switching, where the current is most prone to oscillation or overshoot. The third stage is the polarity reversal stage. The polarity reversal stage still requires the participation of virtual impedance, but its intensity is reduced, that is, the virtual impedance value is less than the voltage drop and buffering, to avoid hindering the switching speed, while the current limiter remains in operation at all times.

[0075] Specifically, the change in the aforementioned DC voltage command value includes a first stage, a second stage, and a third stage. The first stage includes a reduction from the aforementioned first predetermined value to 0; the second stage includes the aforementioned DC voltage command being at 0; and the third stage includes a reduction from 0 to the aforementioned second predetermined value. The first stage is the voltage reduction stage, where, at the initial stage of power flow reversal, the first converter needs to slowly reduce the DC voltage from a positive value to near zero. The second stage is the buffering stage, where, when the DC voltage approaches zero, the system enters the sensitive region of voltage polarity switching, and the current is most prone to oscillation or overshoot. The third stage is the polarity reversal stage, where virtual impedance still needs to participate, but its intensity is reduced, i.e., the virtual impedance value is less than that of the voltage reduction and buffering, to avoid hindering the switching speed, while the current limiter remains in operation at all times.

[0076] The second control unit 30 is used to control the active power of the second converter according to the DC voltage of the first converter.

[0077] The third control unit 40 is used to keep the DC current of the third converter constant.

[0078] In other words, the first converter employs constant DC voltage control, constant reactive power control, or constant AC voltage control. The aforementioned stepped control refers to a gradual reduction of the DC voltage from a positive value to a negative value in stages. The absolute values ​​of the first and second predetermined values ​​can be the same or different. That is, the second converter employs constant active power control in its active power control stage and constant reactive power control in its reactive power control stage. The active power control stage includes constant DC voltage control and constant active power control, while the reactive power control stage includes constant AC voltage control and constant reactive power control. The third converter employs constant DC current control.

[0079] This embodiment provides a power flow reversal device for a DC transmission system. The DC transmission system includes a transmitting end and a receiving end. The transmitting end includes a first converter, and the receiving end includes a second converter and at least one third converter electrically connected. The first converter is of the same type as the second converter, while the second converter is of a different type from the third converter. A determining unit determines whether the DC transmission system has received a power flow reversal command. When the DC transmission system receives a power flow reversal command, a first control unit acquires a DC voltage command value and controls the DC voltage of the first converter to follow the change of the DC voltage command value, with the DC voltage command value decreasing in a stepwise manner from a first predetermined value to a second predetermined value. The second control unit controls the active power of the second converter based on the DC voltage of the first converter. The third control unit controls the DC current of the third converter to remain constant. The power receiving end converter includes converters of the same type as the power sending end converter and converters of different types. During the power supply process, the power flow direction is smoothly switched online by changing the DC voltage of the first converter instead of changing the DC current in the existing technology. This can realize power flow reversal without stopping the system, while avoiding the transient instability caused by control mode switching and the impact of current surges and power oscillations on the equipment during power flow reversal, ensuring stable system operation and solving the problem of how to overcome the complexity of power flow reversal operation and the problem of current surges.

[0080] The first control unit of this application includes a first acquisition module and a first calculation module. The first acquisition module acquires a DC voltage reference value, a target virtual impedance value, and a target current limit value for the first converter. The DC voltage reference value is a reference value for the DC voltage of the first converter. The target virtual impedance value is used to increase the virtual impedance of the first converter to suppress current surges. The target current limit value is used to suppress the DC current of the first converter from exceeding a first current threshold. The first calculation module calculates the difference between the sum of the DC voltage reference value and the target virtual impedance value and the target current limit value to obtain the DC voltage command value. This method combines the target virtual impedance value and the target current limit value, i.e., virtual impedance control and current limit control work synergistically. The virtual impedance provides "soft damping," while the current limit control acts as a "hard constraint," forming a dual-layer protection of "soft damping + hard constraint." This further avoids current overshoot and power oscillation during power flow reversal, allowing for smooth power flow reversal.

[0081] The aforementioned first acquisition module includes a first acquisition submodule, a first calculation submodule, and a second calculation submodule. The first acquisition submodule acquires a predetermined slope and a predetermined duration, where the predetermined slope is the rate at which the first predetermined value decreases to 0, and the predetermined duration is the duration during which the first predetermined value decreases to 0. The first calculation submodule calculates the product of the predetermined slope and the predetermined duration to obtain a first product. The second calculation submodule calculates the difference between the first predetermined value and the first product to obtain the DC voltage reference value. This device can further calculate an accurate DC voltage reference value.

[0082] Specifically, the formula for calculating the DC voltage reference value is as follows: ,in, The above-mentioned DC voltage reference value, The first predetermined value mentioned above, For the aforementioned predetermined slope, The above-mentioned scheduled duration.

[0083] The aforementioned first acquisition module further includes a second acquisition submodule, a third calculation submodule, a fourth calculation submodule, and a fifth calculation submodule. The second acquisition submodule acquires the resistance value of the virtual resistor, the inductance value of the virtual inductor, and the output current of the first converter. The resistance value of the virtual resistor simulates the impedance characteristics of an actual resistor, and the inductance value of the virtual inductor simulates the impedance characteristics of an actual inductor. The third calculation submodule calculates the product of the time derivative of the output current and the inductance value of the virtual inductor to obtain a first product. The fourth calculation submodule calculates the product of the resistance value of the virtual resistor and the output current to obtain a second product. The fifth calculation submodule calculates the sum of the first and second products to obtain the target virtual impedance value. This device can further rapidly acquire the target virtual impedance value.

[0084] Specifically, the formula for calculating the target virtual impedance value is as follows: ,in, The target virtual impedance value is as described above. The resistance value of the aforementioned virtual resistor. For the above output current, The inductance value of the virtual inductor mentioned above can be different in the first stage, the second stage, and the third stage.

[0085] The second acquisition submodule includes a first determination submodule, a second determination submodule, and a third determination submodule. The first determination submodule is used to determine the relationship between the absolute value of the output current and a second current threshold, wherein the second current threshold is less than the first current threshold. The second determination submodule is used to determine, when the absolute value of the output current is greater than the second current threshold, the maximum virtual resistance value corresponding to the DC voltage of the first converter being 0, as the resistance value of the virtual resistor. The third determination submodule is used to determine, when the absolute value of the output current is less than or equal to the second current threshold, the minimum virtual resistance value corresponding to the DC voltage of the first converter being 0, as the resistance value of the virtual resistor. This device can further obtain accurate virtual resistance values.

[0086] Specifically, the formula for calculating the resistance value of the virtual resistor in the second stage mentioned above is as follows: ,in, The resistance value of the aforementioned virtual resistor. The above is the maximum virtual resistance value. The minimum virtual resistance value mentioned above. For the above output current, The second current threshold mentioned above, and All are preset values. When When the value is large, increase the virtual impedance. When the current is relatively small and gradually stabilizes, reduce the virtual impedance to avoid increasing additional losses.

[0087] The second acquisition submodule further includes a third acquisition submodule, a sixth calculation submodule, and a seventh calculation submodule. The third acquisition submodule acquires the maximum inductance value of the first converter. The sixth calculation submodule calculates the ratio of the absolute value of the derivative of the output current with respect to time to a second current threshold to obtain a first ratio. The seventh calculation submodule calculates the product of the maximum inductance value and the first ratio to obtain the inductance value of the virtual inductor when the DC voltage of the first converter is 0. This device can further calculate the accurate inductance value of the virtual inductor.

[0088] Specifically, the formula for calculating the inductance value of the aforementioned virtual inductor is as follows: ,in, The inductance value of the aforementioned virtual inductor. This is the maximum inductance value. For the above output current, This is the second current threshold mentioned above. When When the value is large, increase the virtual impedance. When the current is relatively small and gradually stabilizes, reduce the virtual impedance to avoid increasing additional losses.

[0089] The aforementioned second acquisition submodule further includes a fourth acquisition submodule, an eighth calculation submodule, a fifth acquisition submodule, and a ninth calculation submodule. The fourth acquisition submodule acquires the output current and current limiting coefficient of the first converter. The eighth calculation submodule calculates the difference between the absolute value of the output current and the first current threshold to obtain a first difference value. The fifth acquisition submodule acquires the maximum value between the first difference and 0 to obtain a third predetermined value. The ninth calculation submodule calculates the product of the current limiting coefficient and the third predetermined value to obtain the target current limiting value. This device can further calculate a precise target current limiting value.

[0090] Specifically, the formula for calculating the target current limit value is as follows: ,in, The target current limit is set above. The above-mentioned current limiting coefficient, For the above output current, The first current threshold is as described above.

[0091] The aforementioned second control unit includes a second acquisition module, a second calculation module, and a third calculation module. The second acquisition module acquires the active power command value of the second converter. The second calculation module calculates in real-time the ratio of the DC voltage command value to the first predetermined value to obtain a second ratio. The third calculation module calculates in real-time the product of the active power command value of the second converter and the second ratio to obtain a target active power, and controls the second converter to operate at the obtained target active power. This device can further achieve precise control of the second converter.

[0092] Specifically, the formula for calculating the active power command value of the second converter mentioned above is as follows: ,in, DC voltage is The active power command value at that time. This is the DC voltage command value. The first predetermined value is as described above.

[0093] In other embodiments, the first converter is connected in parallel with the second and third converters, respectively. The second acquisition module further includes a sixth acquisition submodule for acquiring the target DC voltage value of the second converter to obtain the DC voltage command value. By acquiring only the DC voltage value of the receiving end of the second converter, both DC voltage command values ​​of the first and second converters can be obtained simultaneously, eliminating the need to acquire the DC command values ​​of the first and second converters separately. Therefore, the DC voltage command value acquisition process can be further simplified.

[0094] The first converter is connected in parallel with the second and third converters. Since the voltages are equal in parallel, when the DC voltage of the first converter changes, the voltage of the second converter will change accordingly. Ignoring the line voltage drop between the second and third converters, the voltage of the first converter is Udc = Udc1, and the voltage of the second converter is Udc = Udc(n-1) + ΔU. The DC circuit voltage drop ΔU is generally very small; therefore, when the line voltage drop is ignored, Udc = Udc(n-1), meaning the DC voltages of the first and second converters are equal.

[0095] The power flow reversal device of the aforementioned DC transmission system includes a processor and a memory. The aforementioned determining unit, first control unit, second control unit, and third control unit are all stored as program units in the memory, and the processor executes the aforementioned program units stored in the memory to achieve the corresponding functions. All of the above modules are located in the same processor; or, the above modules are located in different processors in any combination.

[0096] The processor contains a core, which retrieves the corresponding program unit from memory. One or more cores can be configured, and power flow reversal in the DC transmission system can be achieved by adjusting the core parameters.

[0097] The memory may include non-permanent memory in computer-readable media, such as random access memory (RAM) and / or non-volatile memory, such as read-only memory (ROM) or flash RAM, and the memory includes at least one memory chip.

[0098] This invention provides a computer-readable storage medium including a stored program, wherein, when the program is executed, it controls the device containing the computer-readable storage medium to perform the power flow reversal method of the DC transmission system.

[0099] Specifically, the power flow reversal methods for DC transmission systems include:

[0100] Step S201: Determine whether the DC transmission system has received a power flow reversal command;

[0101] Step S202: When the DC transmission system receives the power flow reversal command, the DC voltage command value is obtained, and the DC voltage of the first converter is controlled to follow the change of the DC voltage command value. The DC voltage command value is reduced stepwise from a first predetermined value to a second predetermined value. The first predetermined value is greater than 0, the second predetermined value is less than 0, and the absolute value of the difference between the DC voltage of the first converter and the DC voltage command value at each time is less than or equal to a threshold.

[0102] Step S203: Control the active power of the second converter according to the DC voltage of the first converter.

[0103] Step S204: Control the DC current of the third converter to remain constant.

[0104] Optionally, controlling the DC voltage of the first converter to decrease from a first predetermined value to a second predetermined value in a stepwise manner includes: acquiring a DC voltage command value, wherein the DC voltage command value is a target DC voltage value of the first converter, and the change of the DC voltage command value includes a first stage, a second stage, and a third stage, wherein the first stage includes decreasing from the first predetermined value to 0, the second stage includes the absolute value of the difference between the DC voltage command value and 0 being less than a first absolute value threshold, and the third stage includes decreasing from 0 to the second predetermined value; and controlling the absolute value of the difference between the DC voltage of the first converter and the DC voltage command value to be less than the second absolute value threshold.

[0105] Optionally, obtaining the DC voltage command value includes: obtaining a DC voltage reference value of the first converter, a target virtual impedance value of the first converter, and a target current limit value of the first converter, wherein the DC voltage reference value is a reference value of the DC voltage of the first converter, the target virtual impedance value is used to increase the virtual impedance of the first converter to suppress current surges in the first converter, and the target current limit value is used to suppress the DC current of the first converter from exceeding a first current threshold; calculating the difference between the sum of the DC voltage reference value and the target virtual impedance value and the target current limit value to obtain the DC voltage command value.

[0106] Optionally, obtaining the DC voltage reference value of the first converter includes: obtaining a predetermined slope and a predetermined duration, wherein the predetermined slope is the rate at which the first predetermined value decreases to 0, and the predetermined duration is the duration during which the first predetermined value decreases to 0; calculating the product of the predetermined slope and the predetermined duration to obtain a first product; and calculating the difference between the first predetermined value and the first product to obtain the DC voltage reference value.

[0107] Optionally, obtaining the target virtual impedance value of the first converter includes: obtaining the resistance value of the virtual resistor, the inductance value of the virtual inductor, and the output current of the first converter, wherein the resistance value of the virtual resistor is used to simulate the impedance characteristics of the actual resistor, and the inductance value of the virtual inductor is used to simulate the impedance characteristics of the actual inductor; calculating the product of the time derivative of the output current and the inductance value of the virtual inductor to obtain a first product; calculating the product of the resistance value of the virtual resistor and the output current to obtain a second product; and calculating the sum of the first product and the second product to obtain the target virtual impedance value.

[0108] Optionally, obtaining the resistance value of the virtual resistor of the first converter includes: determining the relationship between the absolute value of the output current and a second current threshold, wherein the second current threshold is less than the first current threshold; when the absolute value of the output current is greater than the second current threshold, determining the maximum virtual resistance value corresponding to the DC voltage of the first converter being 0 as the resistance value of the virtual resistor; when the absolute value of the output current is less than or equal to the second current threshold, determining the minimum virtual resistance value corresponding to the DC voltage of the first converter being 0 as the resistance value of the virtual resistor.

[0109] Optionally, obtaining the inductance value of the virtual inductance of the first converter includes: obtaining the maximum inductance value of the first converter; calculating the ratio of the absolute value of the derivative of the output current with respect to time to a second current threshold to obtain a first ratio; and calculating the product of the maximum inductance value and the first ratio to obtain the inductance value of the virtual inductance corresponding to the DC voltage of the first converter being 0.

[0110] Optionally, obtaining the target current limiting value of the first converter includes: obtaining the output current and current limiting coefficient of the first converter; calculating the difference between the absolute value of the output current and the first current threshold to obtain a first difference; obtaining the maximum value between the first difference and 0 to obtain a third predetermined value; and calculating the product of the current limiting coefficient and the third predetermined value to obtain the target current limiting value.

[0111] Optionally, controlling the active power of the second converter based on the DC voltage of the first converter includes: obtaining the active power command value of the second converter; calculating in real time the ratio of the DC voltage command value to the first predetermined value to obtain a second ratio; calculating in real time the product of the active power command value of the second converter and the second ratio to obtain a target active power, and controlling the second converter to operate at the real-time obtained target active power.

[0112] This invention provides a processor for running a program, wherein the program executes the power flow reversal method of the DC transmission system.

[0113] Specifically, the power flow reversal methods for DC transmission systems include:

[0114] Step S201: Determine whether the DC transmission system has received a power flow reversal command;

[0115] Step S202: When the DC transmission system receives the power flow reversal command, the DC voltage command value is obtained, and the DC voltage of the first converter is controlled to follow the change of the DC voltage command value. The DC voltage command value is reduced stepwise from a first predetermined value to a second predetermined value. The first predetermined value is greater than 0, the second predetermined value is less than 0, and the absolute value of the difference between the DC voltage of the first converter and the DC voltage command value at each time is less than or equal to a preset threshold.

[0116] Step S203: Control the active power of the second converter according to the DC voltage of the first converter.

[0117] Step S204: Control the DC current of the third converter to remain constant.

[0118] Optionally, obtaining the DC voltage command value includes: obtaining a DC voltage reference value of the first converter, a target virtual impedance value of the first converter, and a target current limit value of the first converter, wherein the DC voltage reference value is a reference value of the DC voltage of the first converter, the target virtual impedance value is used to increase the virtual impedance of the first converter to suppress current surges in the first converter, and the target current limit value is used to suppress the DC current of the first converter from exceeding a first current threshold; calculating the difference between the sum of the DC voltage reference value and the target virtual impedance value and the target current limit value to obtain the DC voltage command value.

[0119] Optionally, obtaining the DC voltage reference value of the first converter includes: obtaining a predetermined slope and a predetermined duration, wherein the predetermined slope is the rate at which the first predetermined value decreases to 0, and the predetermined duration is the duration during which the first predetermined value decreases to 0; calculating the product of the predetermined slope and the predetermined duration to obtain a first product; and calculating the difference between the first predetermined value and the first product to obtain the DC voltage reference value.

[0120] Optionally, obtaining the target virtual impedance value of the first converter includes: obtaining the resistance value of the virtual resistor, the inductance value of the virtual inductor, and the output current of the first converter, wherein the resistance value of the virtual resistor is used to simulate the impedance characteristics of the actual resistor, and the inductance value of the virtual inductor is used to simulate the impedance characteristics of the actual inductor; calculating the product of the time derivative of the output current and the inductance value of the virtual inductor to obtain a first product; calculating the product of the resistance value of the virtual resistor and the output current to obtain a second product; and calculating the sum of the first product and the second product to obtain the target virtual impedance value.

[0121] Optionally, obtaining the resistance value of the virtual resistor of the first converter includes: determining the relationship between the absolute value of the output current and a second current threshold, wherein the second current threshold is less than the first current threshold; when the absolute value of the output current is greater than the second current threshold, determining the maximum virtual resistance value corresponding to the DC voltage of the first converter being 0 as the resistance value of the virtual resistor; when the absolute value of the output current is less than or equal to the second current threshold, determining the minimum virtual resistance value corresponding to the DC voltage of the first converter being 0 as the resistance value of the virtual resistor.

[0122] Optionally, obtaining the inductance value of the virtual inductance of the first converter includes: obtaining the maximum inductance value of the first converter; calculating the ratio of the absolute value of the derivative of the output current with respect to time to a second current threshold to obtain a first ratio; and calculating the product of the maximum inductance value and the first ratio to obtain the inductance value of the virtual inductance corresponding to the DC voltage of the first converter being 0.

[0123] Optionally, obtaining the target current limiting value of the first converter includes: obtaining the output current and current limiting coefficient of the first converter; calculating the difference between the absolute value of the output current and the first current threshold to obtain a first difference; obtaining the maximum value between the first difference and 0 to obtain a third predetermined value; and calculating the product of the current limiting coefficient and the third predetermined value to obtain the target current limiting value.

[0124] Optionally, controlling the active power of the second converter based on the DC voltage of the first converter includes: obtaining the active power command value of the second converter; calculating in real time the ratio of the DC voltage command value to the first predetermined value to obtain a second ratio; calculating in real time the product of the active power command value of the second converter and the second ratio to obtain a target active power, and controlling the second converter to operate at the real-time obtained target active power.

[0125] This invention provides a device including a processor, a memory, and a program stored in the memory and executable on the processor. When the processor executes the program, it performs at least the following steps:

[0126] Step S201: Determine whether the DC transmission system has received a power flow reversal command;

[0127] Step S202: When the DC transmission system receives the power flow reversal command, the DC voltage command value is obtained, and the DC voltage of the first converter is controlled to follow the change of the DC voltage command value. The DC voltage command value is reduced stepwise from a first predetermined value to a second predetermined value. The first predetermined value is greater than 0, the second predetermined value is less than 0, and the absolute value of the difference between the DC voltage of the first converter and the DC voltage command value at each time is less than or equal to a preset threshold.

[0128] Step S203: Control the active power of the second converter according to the DC voltage of the first converter.

[0129] Step S204: Control the DC current of the third converter to remain constant.

[0130] The devices mentioned in this article can be servers, PCs, tablets, mobile phones, etc.

[0131] This application also provides a computer program product, which, when executed on a data processing device, is suitable for executing an initialization program having at least the following method steps:

[0132] Step S201: Determine whether the DC transmission system has received a power flow reversal command;

[0133] Step S202: When the DC transmission system receives the power flow reversal command, the DC voltage command value is obtained, and the DC voltage of the first converter is controlled to follow the change of the DC voltage command value. The DC voltage command value is reduced stepwise from a first predetermined value to a second predetermined value. The first predetermined value is greater than 0, the second predetermined value is less than 0, and the absolute value of the difference between the DC voltage of the first converter and the DC voltage command value at each time is less than or equal to a preset threshold.

[0134] Step S203: Control the active power of the second converter according to the DC voltage of the first converter.

[0135] Step S204: Control the DC current of the third converter to remain constant.

[0136] It is obvious to those skilled in the art that the modules or steps of the present invention described above can be implemented using general-purpose computing devices. They can be centralized on a single computing device or distributed across a network of multiple computing devices. They can be implemented using computer-executable program code, and thus can be stored in a storage device for execution by a computing device. In some cases, the steps shown or described can be performed in a different order than those described herein, or they can be fabricated as separate integrated circuit modules, or multiple modules or steps can be fabricated as a single integrated circuit module. Thus, the present invention is not limited to any particular combination of hardware and software.

[0137] Those skilled in the art will understand that embodiments of this application can be provided as methods, systems, or computer program products. Therefore, this application can take the form of a completely hardware embodiment, a completely software embodiment, or an embodiment combining software and hardware aspects. Furthermore, this application can take the form of a computer program product embodied on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code.

[0138] This application is described with reference to flowchart illustrations and / or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of this application. It will be understood that each block of the flowchart illustrations and / or block diagrams, and combinations of blocks in the flowchart illustrations and / or block diagrams, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, special-purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, generate instructions for implementing the flowchart... Figure 1 One or more processes and / or boxes Figure 1 A device that provides the functions specified in one or more boxes.

[0139] These computer program instructions may also be stored in a computer-readable storage medium that can direct a computer or other programmable data processing device to function in a particular manner, such that the instructions stored in the computer-readable storage medium produce an article of manufacture including instruction means, which are implemented in a process Figure 1 One or more processes and / or boxes Figure 1 The function specified in one or more boxes.

[0140] These computer program instructions may also be loaded onto a computer or other programmable data processing equipment to cause a series of operational steps to be performed on the computer or other programmable equipment to produce a computer-implemented process, thereby providing instructions that execute on the computer or other programmable equipment for implementing the process. Figure 1 One or more processes and / or boxes Figure 1 The steps of the function specified in one or more boxes.

[0141] In a typical configuration, a computing device includes one or more processors (CPU), input / output interfaces, network interfaces, and memory.

[0142] Memory may include non-persistent memory in computer-readable media, such as random access memory (RAM) and / or non-volatile memory, like read-only memory (ROM) or flash RAM. Memory is an example of computer-readable media.

[0143] Computer-readable media includes both permanent and non-permanent, removable and non-removable media that can store information using any method or technology. Information can be computer-readable instructions, data structures, modules of programs, or other data. Examples of computer storage media include, but are not limited to, phase-change memory (PRAM), static random access memory (SRAM), dynamic random access memory (DRAM), other types of random access memory (RAM), read-only memory (ROM), electrically erasable programmable read-only memory (EEPROM), flash memory or other memory technologies, CD-ROM, digital versatile optical disc (DVD) or other optical storage, magnetic tape, disk storage or other magnetic storage devices, or any other non-transferable medium that can be used to store information accessible by a computing device. As defined herein, computer-readable media does not include transient computer-readable media, such as modulated data signals and carrier waves.

[0144] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0145] It should also be noted that 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 process, method, article, or apparatus. Unless otherwise specified, 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 that element.

[0146] As can be seen from the above description, the embodiments of this application achieve the following technical effects:

[0147] 1) The power flow reversal method for a DC transmission system of this application includes a power transmitting end and a power receiving end. The power transmitting end includes a first converter, and the power receiving end includes a second converter and at least one third converter connected electrically. The type of the first converter is the same as that of the second converter, and the type of the second converter is different from that of the third converter. First, it is determined whether the DC transmission system has received a power flow reversal command. Then, if the DC transmission system has received a power flow reversal command, a DC voltage command value is obtained, and the DC voltage of the first converter is controlled to follow the change of the DC voltage command value. The DC voltage command value is reduced stepwise from a first predetermined value to a second predetermined value. Then, based on the DC voltage of the first converter, the active power of the second converter is controlled. Finally, the DC current of the third converter is kept constant. The power receiving end converter includes converters of the same type as the power sending end converter and converters of different types. During the power supply process, the power flow direction is smoothly switched online by changing the DC voltage of the first converter instead of changing the DC current in the existing technology. This can realize power flow reversal without stopping the system, while avoiding the transient instability caused by control mode switching and the impact of current surges and power oscillations on the equipment during power flow reversal, ensuring stable system operation and solving the problem of how to overcome the complexity of power flow reversal operation and the problem of current surges.

[0148] 2) The power flow reversal device for a DC transmission system of this application includes a power transmitting end and a power receiving end. The power transmitting end includes a first converter, and the power receiving end includes a second converter and at least one third converter connected electrically. The type of the first converter is the same as that of the second converter, and the type of the second converter is different from that of the third converter. A determining unit determines whether the DC transmission system receives a power flow reversal command. When the DC transmission system receives a power flow reversal command, the first control unit acquires a DC voltage command value and controls the DC voltage of the first converter to follow the change of the DC voltage command value. The DC voltage command value is reduced stepwise from a first predetermined value to a second predetermined value. The second control unit controls the active power of the second converter according to the DC voltage of the first converter. The third control unit controls the DC current of the third converter to remain constant. The power receiving end converter includes converters of the same type as the power sending end converter and converters of different types. During the power supply process, the power flow direction is smoothly switched online by changing the DC voltage of the first converter instead of changing the DC current in the existing technology. This can realize power flow reversal without stopping the system, while avoiding the transient instability caused by control mode switching and the impact of current surges and power oscillations on the equipment during power flow reversal, ensuring stable system operation and solving the problem of how to overcome the complexity of power flow reversal operation and the problem of current surges.

[0149] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A method for reversing power flow in a DC transmission system, characterized in that, The DC transmission system includes a transmitting end and a receiving end. The transmitting end includes a first converter, and the receiving end includes a second converter and at least one third converter electrically connected. The first converter is of the same type as the second converter, and the second converter is of a different type from the third converter. The power flow reversal method of the DC transmission system includes: Determine whether the DC transmission system has received a power flow reversal command; When the DC transmission system receives the power flow reversal command, it acquires the DC voltage reference value of the first converter, the resistance value of the virtual resistor of the first converter, the inductance value of the virtual inductor of the first converter, the output current of the first converter, the output current of the first converter, and the current limiting coefficient, wherein the DC voltage reference value is the reference value of the DC voltage of the first converter. Calculate the product of the time derivative of the output current and the inductance value of the virtual inductor to obtain the first product; calculate the product of the resistance value of the virtual resistor and the output current to obtain the second product; calculate the sum of the first product and the second product to obtain the target virtual impedance value. Calculate the difference between the absolute value of the output current and the first current threshold to obtain the first difference; obtain the maximum value between the first difference and 0 to obtain the third predetermined value; calculate the product of the current limiting coefficient and the third predetermined value to obtain the target current limiting value; The difference between the sum of the DC voltage reference value and the voltage corresponding to the target virtual impedance value and the voltage corresponding to the target current limit value is calculated to obtain the DC voltage command value; and the DC voltage of the first converter is controlled to follow the change of the DC voltage command value, wherein the DC voltage command value is reduced stepwise from a first predetermined value to a second predetermined value, the first predetermined value is greater than 0, the second predetermined value is less than 0, and the absolute value of the difference between the DC voltage of the first converter and the DC voltage command value at each time is less than or equal to a preset threshold. Based on the DC voltage of the first converter, the active power of the second converter is controlled; the DC current of the third converter is kept constant.

2. The method according to claim 1, characterized in that, Obtaining the DC voltage reference value of the first converter includes: Obtain a predetermined slope and a predetermined duration, wherein the predetermined slope is the rate at which the first predetermined value decreases to 0, and the predetermined duration is the duration during which the first predetermined value decreases to 0; Calculate the product of the predetermined slope and the predetermined duration to obtain the first product; The difference between the first predetermined value and the first product is calculated to obtain the DC voltage reference value.

3. The method according to claim 1, characterized in that, Obtaining the resistance value of the virtual resistance of the first converter includes: Determine the relationship between the absolute value of the output current and a second current threshold, wherein the second current threshold is less than the first current threshold; If the absolute value of the output current is greater than the second current threshold, the maximum virtual resistance value corresponding to the DC voltage of the first converter being 0 is determined to be the resistance value of the virtual resistance. If the absolute value of the output current is less than or equal to the second current threshold, the minimum virtual resistance value corresponding to the DC voltage of the first converter being 0 is determined to be the resistance value of the virtual resistor.

4. The method according to claim 1, characterized in that, Obtaining the inductance value of the virtual inductance of the first converter includes: Obtain the maximum inductance value of the first converter; The first ratio is obtained by calculating the ratio of the absolute value of the derivative of the output current with respect to time to the second current threshold. Calculate the product of the maximum inductance value and the first ratio to obtain the inductance value of the virtual inductor when the DC voltage of the first converter is 0.

5. The method according to claim 1, characterized in that, Controlling the active power of the second converter based on the DC voltage of the first converter includes: Obtain the active power command value of the second converter; The ratio of the DC voltage command value to the first predetermined value is calculated in real time to obtain a second ratio; The active power command value of the second converter is calculated in real time and the product of the second ratio to obtain the target active power, and the second converter is controlled to operate at the target active power obtained in real time.

6. A power flow reversal device for a DC transmission system, characterized in that, The DC transmission system includes a power transmitting end and a power receiving end. The power transmitting end includes a first converter, and the power receiving end includes a second converter and at least one third converter electrically connected. The first converter is of the same type as the second converter, and the second converter is of a different type from the third converter. The power flow reversal device of the DC transmission system includes: The determining unit is used to determine whether the DC transmission system has received a power flow reversal command; A first control unit is configured to, upon receiving the power flow reversal command from the DC transmission system, acquire the following: a reference value for the DC voltage of the first converter; the resistance value of the virtual resistor of the first converter; the inductance value of the virtual inductor of the first converter; the output current of the first converter; and a current limiting coefficient, wherein the reference value for the DC voltage is a reference value for the DC voltage of the first converter; calculate the product of the time derivative of the output current and the inductance value of the virtual inductor to obtain a first product; calculate the product of the resistance value of the virtual resistor and the output current to obtain a second product; calculate the sum of the first product and the second product to obtain a target virtual impedance value; and calculate the absolute value of the output current. The difference between the first current threshold and the first current threshold is used to obtain the first difference; the maximum value between the first difference and 0 is obtained to obtain the third predetermined value; the product of the current limiting coefficient and the third predetermined value is calculated to obtain the target current limiting value; the difference between the sum of the voltages corresponding to the DC voltage reference value and the voltage corresponding to the target virtual impedance value and the voltage corresponding to the target current limiting value is calculated to obtain the DC voltage command value, and the DC voltage of the first converter is controlled to follow the change of the DC voltage command value, wherein the DC voltage command value is reduced stepwise from the first predetermined value to the second predetermined value, the first predetermined value is greater than 0, the second predetermined value is less than 0, and the absolute value of the difference between the DC voltage of the first converter and the DC voltage command value at each time is less than or equal to a preset threshold. The second control unit is used to control the active power of the second converter according to the DC voltage of the first converter; The third control unit is used to keep the DC current of the third converter constant.

7. A DC transmission system, characterized in that, The system includes a power transmitting end, a power receiving end, and a controller. The power transmitting end includes a first converter, and the power receiving end includes a second converter and at least one third converter that are electrically connected. The first converter is of the same type as the second converter, and the second converter is of a different type from the third converter. The controller is communicatively connected to the first converter, the second converter, and the third converter, respectively. The controller is used to execute the power flow reversal method of the DC transmission system according to any one of claims 1 to 5.

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