Power flow reversal method of direct-current power transmission system

Abstract

The invention provides a power flow reversal method of a direct-current power transmission system, the direct-current power transmission system comprises a power transmitting end and a power receiving end, the power transmitting end comprises a first converter, the power receiving end comprises a second converter and at least one third converter which are electrically connected, and the type of the first converter is the same as that of the second converter. The type of the second current converter is different from that of the third current converter. The power flow reversal method of the direct-current power transmission system comprises the steps of determining whether the direct-current power transmission system receives a power flow reversal instruction or not; under the condition that the direct-current power transmission system receives a power flow reversal instruction, a direct-current voltage instruction value is obtained, the direct-current voltage of the first converter is controlled to change along with the direct-current voltage instruction value, and the direct-current voltage instruction value is reduced to a second preset value from a first preset value in a stepped mode; controlling the active power of the second current converter according to the DC voltage of the first current converter; and controlling the direct current of the third current converter to be unchanged. The method can overcome the problems of complexity and current impact of power flow reversal operation.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of direct current transmission, in particular to a power flow reversal method of a direct current transmission system, a power flow reversal device of a direct current transmission system and a direct current transmission system. BACKGROUND

[0002] In the existing direct current transmission technology, the power flow reversal mechanism faces significant challenges. The traditional direct current transmission system often relies on shutdown and mode switching during power flow reversal, which is not only complex but also may introduce system instability factors.

[0003] Therefore, there is an urgent need for a power flow reversal method of a direct current transmission system to overcome the above problems. SUMMARY

[0004] The main purpose of the present application is to provide a power flow reversal method of a direct current transmission system, a power flow reversal device of a direct current transmission system and a direct current transmission system to at least solve the problems of complex power flow reversal operation and current impact in the prior art.

[0005] In order to achieve the above-mentioned purpose, according to one aspect of the present application, a power flow reversal method of a direct current transmission system is provided, the direct current 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 a second converter and at least one third converter connected in series, 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 reversal method of the direct current transmission system comprising: determining whether the direct current transmission system receives a power flow reversal instruction; in the case that the direct current transmission system receives the power flow reversal instruction, obtaining a direct current voltage instruction value, and controlling the direct current voltage of the first converter to change following the direct current voltage instruction value, wherein the direct current voltage instruction value is lowered from a first predetermined value to a second predetermined value in steps, the first predetermined value being greater than 0, the second predetermined value being less than 0, the absolute value of the difference between the direct current voltage of the first converter at each moment and the direct current voltage instruction value being less than or equal to a preset threshold; controlling the active power of the second converter according to the direct current voltage of the first converter; controlling the direct current of the third converter to remain unchanged.

[0006] Optionally, the DC voltage instruction value is obtained by: 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 a DC voltage of the first converter, the target virtual impedance value is used to increase a virtual impedance of the first converter to suppress a current mutation of the first converter, and the target current limiting value is used to suppress the DC current of the first converter being greater than a first current threshold; and calculating a difference between a sum of the DC voltage reference value and the target virtual impedance value and the target current limiting value to obtain the DC voltage instruction value.

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

[0008] Optionally, the target virtual impedance value of the first converter is obtained by: obtaining a resistance value of a virtual resistor of the first converter, an inductance value of a virtual inductor of the first converter, and an output current of the first converter; calculating a product of a derivative of the output current with respect to time and the inductance value of the virtual inductor to obtain a first product; calculating a product of the resistance value of the virtual resistor and the output current to obtain a second product; and calculating a sum of the first product and the second product to obtain the target virtual impedance value. Optionally, the resistance value of the virtual resistor of the first converter is obtained by: determining a size relationship between an absolute value of the output current and a second current threshold, wherein the second current threshold is less than the first current threshold; in a case where the absolute value of the output current is greater than the second current threshold, determining that a maximum virtual resistor value corresponding to a case where the DC voltage of the first converter is 0 is the resistance value of the virtual resistor; and in a case where the absolute value of the output current is less than or equal to the second current threshold, determining that a minimum virtual resistor value corresponding to a case where the DC voltage of the first converter is 0 is the resistance value of the virtual resistor.

[0009] Optionally, the inductance value of the virtual inductor of the first converter is obtained by: obtaining a maximum inductance value of the first converter; calculating a ratio of an 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 a product of the maximum inductance value and the first ratio to obtain the inductance value of the virtual inductor corresponding to a case where the DC voltage of the first converter is 0.

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

[0011] Optionally, the active power of the second converter is controlled according to the DC voltage of the first converter by: obtaining an active power instruction value of the second converter; calculating a ratio of the DC voltage instruction value and the first predetermined value in real time to obtain a second ratio value; calculating a product of the active power instruction value of the second converter and the second ratio value in real time 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 the present application, there is provided a power flow inversion device for a DC power transmission system, the DC 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 a second converter and at least one third converter electrically connected, the first converter being of the same type as the second converter, the second converter being of a different type from the third converter, the power flow inversion device comprising: a determination unit configured to determine whether the DC power transmission system receives a power flow inversion instruction; a first control unit configured to, in the case that the DC power transmission system receives the power flow inversion instruction, obtain a DC voltage instruction value and control a DC voltage of the first converter to change following the DC voltage instruction value, wherein the DC voltage instruction value is stepwise reduced from a first predetermined value to a second predetermined value, the first predetermined value being greater than 0 and the second predetermined value being less than 0, and an absolute value of a difference between the DC voltage of the first converter at each moment and the DC voltage instruction value being less than or equal to a preset threshold value; a second control unit configured to control an active power of the second converter according to the DC voltage of the first converter; and a third control unit configured to control a DC current of the third converter to remain unchanged.

[0013] According to still another aspect of the present application, there is provided a DC power transmission system comprising a power sending end, a power receiving end and a controller, the power sending end comprising a first converter, the power receiving end comprising a second converter and at least one third converter electrically connected, the first converter being of the same type as the second converter, the second converter being of a different type from the third converter, the controller being communicatively connected to the first converter, the second converter and the third converter respectively, and the controller being configured to perform any of the power flow inversion methods for the DC power transmission system.

[0014] The technical scheme of the application provides a power flow inversion method of a direct current transmission system, the direct current 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 a second converter and at least one third converter connected in series, the type of the first converter being the same as that of the second converter, and the type of the second converter being different from that of the third converter. First, it is determined whether the direct current transmission system receives a power flow inversion instruction. Then, in the case that the direct current transmission system receives the power flow inversion instruction, a direct current voltage instruction value is obtained, and the direct current voltage of the first converter is controlled to change with the direct current voltage instruction value, the direct current voltage instruction value being lowered from a first predetermined value to a second predetermined value in steps. Then, the active power of the second converter is controlled according to the direct current voltage of the first converter. Finally, the direct current of the third converter is controlled to remain unchanged. The converters of the power receiving end comprise converters of the same type as the converters of the power sending end and converters of different types. In the process of power supply, the direct current voltage of the first converter is changed to replace the mode of changing the direct current in the prior art, so as to realize the online smooth conversion of the power flow direction. The power flow inversion can be realized without stopping the system, the transient instability caused by the switching of the control mode is avoided, the influence of the current impact and power oscillation in the process of power flow inversion on the equipment is avoided, the stable operation of the system is ensured, and the complexity of the power flow inversion operation and the current impact problem are solved. BRIEF DESCRIPTION OF DRAWINGS

[0015] The accompanying drawings, which form a part of the specification, are included to provide a further understanding of the application and are incorporated herein in conjunction with the description of the application. The drawings illustrate an exemplary embodiment of the present application and, together with the description, serve to explain the application. In the drawings:

[0016] Figure 1 A hardware structure block diagram of a mobile terminal for executing a power flow inversion method of a direct current transmission system is shown according to an embodiment of the application;

[0017] Figure 2 A structure schematic diagram of a direct current transmission system is shown according to an embodiment of the application;

[0018] Figure 3 A flowchart of a power flow inversion method of a direct current transmission system is shown according to an embodiment of the application;

[0019] Figure 4 A specific flowchart of a power flow inversion method of a direct current transmission system is shown according to an embodiment of the application;

[0020] Figure 5 A structure block diagram of a power flow inversion device of a direct current transmission system is shown according to an embodiment of the application.

[0021] In the above drawings, the following reference signs are used:

[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 DESCRIPTION

[0023] It should be noted that the embodiments in the present application and the features in the embodiments can be combined with each other without conflict. The technical solutions in the embodiments of the present application will be described in detail below with reference to the drawings and in combination with the embodiments.

[0024] In order for those skilled in the art to better understand the present application, the technical solutions in the embodiments of the present application will be described clearly and completely below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, not all. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor should be within the scope of protection of the present application.

[0025] It should be noted that the terms "first", "second" and the like in the specification and claims of the present application and the above-mentioned drawings are used to distinguish similar objects, and do not necessarily indicate a specific order or sequence. It should be understood that the data thus used can be interchanged under appropriate circumstances, so that the embodiments of the present application described herein can be implemented. In addition, the terms "include" and "have" and any variations thereof are intended to cover non-exclusive inclusion, for example, a process, method, system, product or device including a series of steps or units does not have to be limited to those steps or units clearly listed, but can include other steps or units not clearly listed or inherent to these processes, methods, products or devices.

[0026] As introduced in the background, the existing technology has the problems of complex operation and current impact in power flow reversal. To solve the above problems, the embodiments of the present application provide a power flow reversal method of a DC power transmission system, a power flow reversal device of a DC power transmission system and a DC power transmission system.

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

[0028] The method embodiments provided in the embodiments of the present application can be executed in a mobile terminal, a computer terminal or a similar computing device. Taking the case of running on a mobile terminal, Figure 1 is a hardware structure block diagram of a mobile terminal of a power flow reversal method of a DC power transmission system according to an embodiment of the present application. As Figure 1 shown, the mobile terminal can include one or moreFigure 1 The mobile terminal can further include a transmission device 106 for communication function and an input / output device 108. Those skilled in the art can understand that, Figure 1 The structure shown is only schematic and does not limit the structure of the mobile terminal. For example, the mobile terminal can include more or less components than those shown, or have a different configuration or arrangement of the components. Figure 1 The structure shown is only schematic and does not limit the structure of the mobile terminal. For example, the mobile terminal can include more or less components than those shown, or have a different configuration or arrangement of the components. Figure 1 The structure shown is only schematic and does not limit the structure of the mobile terminal. For example, the mobile terminal can include more or less components than those shown, or have a different configuration or arrangement of the components.

[0029] The memory 104 can be used to store computer programs, such as software programs of application software and modules, for example, the computer program corresponding to the power flow inversion method of the DC power transmission system in the embodiments of the present application. The processor 102 executes various functional applications and data processing by running the computer programs stored in the memory 104, that is, implements the method described above. The memory 104 can include a high-speed random access memory, and can further include a non-volatile memory, such as one or more magnetic storage devices, flash memories, or other non-volatile solid-state memories. In some examples, the memory 104 can further include a memory remotely arranged with respect to the processor 102, and these remote memories can be connected to the mobile terminal through a network. Examples of the network include, but are not limited to, the Internet, an intranet, a local area network, a mobile communication network, and a combination thereof. The transmission device 106 is used to receive or send data via a network. The specific examples of the network can include a wireless network provided by a communication provider of the mobile terminal. In one example, the transmission device 106 includes a network adapter (NIC), which can be connected to other network devices through a base station so as to communicate with the Internet. In one example, the transmission device 106 can be a radio frequency (RF) module, which is used to communicate with the Internet in a wireless manner.

[0030] In the embodiments, a power flow inversion method of a DC power transmission system running on a mobile terminal, a computer terminal or a similar computing device is provided. It should be noted that the steps shown in the flowchart of the accompanying drawings can be executed in a computer system such as a set of computer executable instructions, and although a logical order is shown in the flowchart, in some cases, the steps shown or described can be executed in an order different from that shown here.

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

[0032] Specifically, the type of the first converter, the type of the second converter and the type of the third converter refer to voltage source type converters or current source type converters, for example, the first converter and the second converter are voltage source type converters, and the third converter is a current source type converter. Further, the first converter and the second converter can be modular multilevel converters (MMC), wherein the MMC includes both full-bridge sub-modules and half-bridge sub-modules. The third converter can be a hybrid commutation converter (HCC). The third converter can be one or multiple, and the second converter and the third converter can be in parallel.

[0033] Figure 3 is a flow chart of a power flow reversal method of a DC power transmission system according to an embodiment of the present application. As shown in the figure, the method includes the following steps: Figure 3

[0034] Step S201, determining whether the DC power transmission system receives a power flow reversal instruction;

[0035] ​In practical applications, the power flow reversal instruction is output to the DC power transmission system when at least one of the following occurs, i.e., the DC power transmission system receives the power flow reversal instruction, for example: the power transmission direction changes, such as temporarily changing from "power transmission from west to east" to "power transmission from east to west"; when a large power shortage occurs in the receiving end AC system due to large unit tripping, DC single-pole fault, etc., in order to quickly provide frequency or voltage support, the original sending end DC rectifier station can be converted to inverter operation, changing the "receiving end" to the "sending end", and realizing emergency power reverse sending; if the sending end new energy output suddenly drops and the receiving end wind power and photovoltaic power are large, the situation of power surplus in the original receiving end and power deficiency in the original sending end occurs, in order to reduce wind and light curtailment and meet load balance, the power flow direction can be reversed; when the original sending end converter station, the nearby AC line or generator unit needs to be shut down for maintenance, the power direction can be temporarily reversed, and the opposite power grid can send power through the same DC line to avoid large-scale power outage or reduce standby capacity demand; when there is a large amount of power generation in the flood season, and the load is high in the dry season and the water power output is insufficient, the power can be sent back through the power flow reversal to realize "bidirectional regulation"; in the process of black start of the partition power grid, in order to gradually charge the area without power and carry the load, the power can be supplied in reverse by the power side with power at the minimum power, and then the normal direction power transmission can be restored after the unit is connected to the grid.

[0036] In step S202, in the case where the DC power transmission system receives the power flow reversal instruction, a DC voltage instruction value is obtained, and the DC voltage of the first converter is controlled to follow the change of the DC voltage instruction value, wherein the DC voltage instruction value is stepped down from a first predetermined value to a second predetermined value, the first predetermined value is greater than 0, and the second predetermined value is less than 0, and the absolute value of the difference between the DC voltage of the first converter at each moment and the DC voltage instruction value is less than or equal to a preset threshold value.

[0037] Specifically, the change of the direct current voltage command value includes a first stage, a second stage and a third stage, the first stage includes a decrease from the first predetermined value to 0, the second stage includes that the direct current voltage command is at 0, and the third stage includes a decrease from 0 to the second predetermined value. The decrease rate of the first stage is greater than the decrease rate of the third stage, for example, the decrease rate of the first stage is 600-800 kV / s, and the decrease rate of the third stage is 400-500 kV / s. Moreover, 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 duration of the second stage and the duration of the third stage can be 8-12:1:8-12. For example, the duration of the first stage and the third stage can be 1-1.2 s, and the duration of the second stage can be 100-120 ms. The first stage is a voltage decrease stage, and in the initial stage of power flow reversal, the first converter needs to slowly decrease the direct current voltage from a positive value to close to zero. The second stage is a buffer stage, and when the direct current voltage is close to zero, the system enters a sensitive area of voltage polarity switching, and the current is most likely to oscillate or overshoot. The third stage is a polarity reversal stage, and the virtual impedance still needs to participate in the polarity reversal stage, but the decrease strength is smaller than that in the voltage decrease stage and the buffer stage, that is, the value of the virtual impedance is smaller than that in the voltage decrease stage and the buffer stage, so as to avoid hindering the reversal speed, and at the same time, the current limiter is kept full-time investment.

[0038] In step S203, the active power of the second converter is controlled according to the direct current voltage of the first converter.

[0039] In step S204, the direct current of the third converter is controlled to remain unchanged.

[0040] That is, the first converter adopts fixed direct current voltage control, fixed reactive power control or fixed alternating current voltage control. The step-by-step refers to gradually decreasing the direct current voltage from a positive value to a negative value in stages, and the absolute values of the first predetermined value and the second predetermined value can be the same or different. That is, the active control link of the second converter adopts fixed active power control, and the reactive control link adopts fixed reactive power control, wherein the active control link includes fixed direct current voltage control and fixed active power control, and the reactive control link includes fixed alternating current voltage control and fixed reactive power control. The third converter adopts fixed direct current control.

[0041] The embodiment provides a power flow inversion method of a direct current transmission system, the direct current 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 a second converter and at least one third converter connected in an electric manner, 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 direct current transmission system receives a power flow inversion instruction. Then, in the case that the direct current transmission system receives the power flow inversion instruction, a direct current voltage instruction value is obtained, and a direct current voltage of the first converter is controlled to change along with the direct current voltage instruction value, the direct current voltage instruction value is lowered from a first predetermined value to a second predetermined value in a stepwise manner. Then, the active power of the second converter is controlled according to the direct current voltage of the first converter. Finally, the direct current of the third converter is controlled to remain unchanged. The converters of the power receiving end comprise converters of the same type as the converters of the power sending end and converters of different types. In the process of power supply, the direct current voltage of the first converter is changed to replace the mode of changing the direct current in the prior art, so as to realize the online smooth conversion of the power flow direction. The power flow inversion can be realized without stopping the system, the transient instability caused by the switching of the control mode and the influence of the current impact and power oscillation in the process of the power flow inversion on the equipment are avoided, the stable operation of the system is ensured, and the complexity of the power flow inversion operation and the current impact problem are solved.

[0042] The step S202 of the application can be realized by the following steps: in step S2021, a direct current 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 are obtained, wherein the direct current voltage reference value is a reference value of the direct current 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 mutation of the first converter, and the target current limiting value is used to suppress the direct current of the first converter greater than a first current threshold; in step S2022, a difference between the sum of the direct current voltage reference value and the target virtual impedance value and the target current limiting value is calculated to obtain the direct current voltage instruction value. The method combines the target virtual impedance value and the target current limiting value, that is, the virtual impedance control and the current limiting control are cooperated, the virtual impedance can provide "soft damping", the current limiting control serves as "hard constraint", the double-layer protection of "soft damping + hard constraint" is formed, the current overshoot and power oscillation in the process of the power flow inversion are further avoided, and the system power flow is smoothly inverted.

[0043] The step S2021 can be implemented by the following steps, for example: a step S20211 of obtaining a predetermined slope and a predetermined time length, the predetermined slope being a rate at which the first predetermined value decreases to 0, and the predetermined time length being a time length during which the first predetermined value decreases to 0; a step S20212 of calculating a product of the predetermined slope and the predetermined time length to obtain a first product; and a step S20213 of calculating a difference between the first predetermined value and the first product to obtain the direct-current voltage reference value. This method can further calculate an accurate direct-current voltage reference value.

[0044] Specifically, the calculation formula of the direct-current voltage reference value is as follows: wherein, is the direct-current voltage reference value, is the first predetermined value, is the predetermined slope, is the predetermined time length.

[0045] The step S2021 can also be implemented by other methods, for example: a step S20214 of obtaining a resistance value of a virtual resistor of the first converter, an inductance value of a virtual inductor of the first converter, and an output current of the first converter, the resistance value of the virtual resistor being used to simulate the impedance characteristics of an actual resistor, and the inductance value of the virtual inductor being used to simulate the impedance characteristics of an actual inductor; a step S20215 of calculating a product of a derivative of the output current with respect to time and the inductance value of the virtual inductor to obtain a first product; a step S20216 of calculating a product of the resistance value of the virtual resistor and the output current to obtain a second product; and a step S20217 of calculating a 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 calculation formula of the target virtual impedance value is as follows: wherein, is the target virtual impedance value, is the resistance value of the virtual resistor, is the output current, is the inductance value of the virtual inductor, and the resistance value of the virtual resistor and the inductance value of the virtual inductor 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 achieved in other ways, for example: in step S202151, a plurality of operating data of the first converter at a predicted time are obtained to obtain a plurality of sample data, part of the plurality of sample data is test data, and the other part is training data, wherein the operating data includes DC current, voltage, power instruction, environmental temperature, and current change rate; in step S202152, the test data are sequentially input into a long short-term memory artificial neural network model to obtain a plurality of predicted operating data of the first converter, wherein the long short-term memory artificial neural network model is trained by machine learning using a plurality of sets of first historical data, each set of data in the plurality of sets of first historical data includes: the training data in a first time period and the training data in a 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 operating data after the test data; in step S202153, it is determined that the predicted operating data is the derivative of the output current with respect to time. The long short-term memory artificial neural network model can further improve the accuracy of the calculation of the derivative of the output current with respect to time, i.e., the output current change rate.

[0048] The step S20214 can be achieved by the following steps: in step S202141, the size relationship between the absolute value of the output current and a second current threshold is determined, wherein the second current threshold is smaller than the first current threshold; in step S202142, in the case that the absolute value of the output current is greater than the second current threshold, it is determined that the resistance value of the virtual resistance is the maximum virtual resistance value corresponding to the DC voltage of 0 of the first converter; in step S202143, in the case that the absolute value of the output current is less than or equal to the second current threshold, it is determined that the resistance value of the virtual resistance is the minimum virtual resistance value corresponding to the DC voltage of 0 of the first converter. This method can further obtain an accurate resistance value of the virtual resistance.

[0049] Specifically, the calculation formula of the resistance value of the virtual resistance in the second stage is as follows: wherein, is the resistance value of the virtual resistance, is the maximum virtual resistance value, is the minimum virtual resistance value, is the output current, is the second current threshold, and are predetermined values. When is large, the virtual impedance is increased, and when is small, i.e., when the current gradually stabilizes, the virtual impedance is reduced to avoid increasing additional loss.

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

[0051] Specifically, the calculation formula of the inductance value of the virtual inductance is as follows: , wherein, is the inductance value of the virtual inductance, is the maximum inductance value, is the output current, is the second current threshold value. When is large, the virtual impedance is increased, and when is small, that is, the current gradually stabilizes, the virtual impedance is reduced to avoid increasing additional loss.

[0052] Step S20214 can also be implemented in other ways, for example: step S202147, obtaining the output current of the first converter and the current limiting coefficient; step S202148, calculating the difference between the absolute value of the output current and the first current threshold value to obtain a first difference; step S202149, obtaining the maximum of 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 the accurate target current limiting value.

[0053] Specifically, the calculation formula of the target current limiting value is as follows: , wherein, is the target current limiting value, is the current limiting coefficient, is the output current, is the first current threshold value.

[0054] Step S203 can be implemented by the following steps: step S2031, obtaining the active power instruction value of the second converter; step S2042, calculating the ratio of the DC voltage instruction value to the first predetermined value in real time to obtain a second ratio; step S2043, calculating the product of the active power instruction value of the second converter and the second ratio in real time to obtain a target active power, and controlling the second converter to operate at the target active power obtained in real time. This method can further achieve accurate 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 sending end MMC voltage polarity is reversed to rise to a negative rated value, and weak virtual impedance control is performed;

[0065] Step S6: the polarity reversal is completed, and the virtual impedance and current limiting control are exited;

[0066] Step S7: the MMC reactive power control is adjusted as required;

[0067] Step S8: the reversal is completed.

[0068] The embodiment of the present application further provides a power flow reversal device of a DC power transmission system. It should be noted that the power flow reversal device of the DC power transmission system of the embodiment of the present application can be used to execute the power flow reversal method for the DC power transmission system provided by the embodiment of the present application. The device is used to realize the above-mentioned embodiment and preferred embodiment, and details are not described herein. As used below, the term "module" can be a combination of software and / or hardware that realizes a predetermined function. Although the device described in the following embodiment is preferably realized in software, realization in hardware, or a combination of software and hardware is also possible and is contemplated.

[0069] The power flow reversal device of the DC power transmission system provided by the embodiment of the present application is described below.

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

[0071] A determination unit 10 is configured to determine whether the DC power transmission system receives a power flow reversal instruction.

[0072] In practical applications, the power flow reversal instruction is output to the DC power transmission system when at least one of the following occurs: the DC power transmission system receives the power flow reversal instruction, for example, the power transmission direction changes, such as temporarily changing from "power transmission from west to east" to "power transmission from east to west"; when the receiving end AC system has a large power shortage due to large unit tripping, DC single-pole fault, etc., in order to quickly provide frequency or voltage support, the original sending end DC rectifier station can be converted to inverter operation, changing the "receiving end" to the "sending end", and realizing emergency power reverse sending; if the sending end new energy output suddenly drops and the receiving end wind power and photovoltaic power are large, the situation of power surplus in the original receiving end and power deficiency in the original sending end occurs, in order to reduce wind and light curtailment and meet load balance, the power flow direction can be reversed; when the original sending end converter station, the nearby AC line or generator unit needs to be shut down for maintenance, the power direction can be temporarily reversed, and the opposite power grid can send power through the same DC line to avoid large-scale power outage or reduce standby capacity demand; when there is a large amount of power generation in the flood season, and the load is high in the dry season and the water power output is insufficient, the power can be sent back through the power flow reversal to realize "bidirectional regulation"; in the process of black start of the partition power grid, in order to gradually charge the area without power and carry the load, the power can be supplied in reverse by the power side with power at the minimum power, and then the normal direction power transmission can be restored after the unit is connected to the grid.

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

[0074] Specifically, the change of the DC voltage of the first converter includes a first stage, a second stage and a third stage, the first stage includes a decrease from the first predetermined value to 0, the second stage includes that the DC voltage command is at 0, and the third stage includes a decrease from 0 to the second predetermined value. The decrease rate of the first stage is greater than the decrease rate of the third stage, for example, the decrease rate of the first stage is 600-800 kV / s, and the decrease rate of the third stage is 400-500 kV / s. Moreover, the duration of the first stage and the third stage is greater than the duration of the second stage, respectively, and the ratio of the duration of the first stage, the duration of the second stage and the duration of the third stage can be 8-12:1:8-12. For example, the duration of the first stage and the third stage can be 1-1.2 s, and the duration of the second stage can be 100-120 ms. The first stage is a voltage decrease stage, at the initial stage of power flow reversal, the first converter needs to slowly decrease the DC voltage from a positive value to near zero. The second stage is a buffer stage, when the DC voltage is close to zero, the system enters a sensitive area of voltage polarity switching, and the current is most likely to oscillate or overshoot. The third stage is a polarity reversal stage, the virtual impedance still needs to participate in the polarity reversal stage, but the decrease intensity thereof, i.e., the virtual impedance value, is smaller than that in the voltage decrease and buffer stages, so as to avoid hindering the reversal speed, while the current limiter remains full-time investment.

[0075] Specifically, the change of the DC voltage of the first converter includes a first stage, a second stage and a third stage, the first stage includes a decrease from the first predetermined value to 0, the second stage includes that the DC voltage command is at 0, and the third stage includes a decrease from 0 to the second predetermined value. The first stage is a voltage decrease stage, at the initial stage of power flow reversal, the first converter needs to slowly decrease the DC voltage from a positive value to near zero. The second stage is a buffer stage, when the DC voltage is close to zero, the system enters a sensitive area of voltage polarity switching, and the current is most likely to oscillate or overshoot. The third stage is a polarity reversal stage, the virtual impedance still needs to participate in the polarity reversal stage, but the decrease intensity thereof, i.e., the virtual impedance value, is smaller than that in the voltage decrease and buffer stages, so as to avoid hindering the reversal speed, while the current limiter remains full-time investment.

[0076] The second control unit 30 is configured 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 configured to control the DC current of the third converter to remain unchanged.

[0078] That is, the first converter adopts constant DC voltage control, constant reactive power control or constant AC voltage control. The above-mentioned step refers to gradually reducing the DC voltage from positive to negative value in stages, and the absolute values of the first predetermined value and the second predetermined value can be the same or different. That is, the active control link of the second converter adopts constant active power control, and the reactive control link adopts constant reactive power control, wherein the active control link includes constant DC voltage control and constant active power control, and the reactive control link includes constant AC voltage control and constant reactive power control. The third converter adopts constant DC current control.

[0079] Through the embodiment, a power flow reversal device of a DC power transmission system is provided, the DC power transmission system including a power sending end and a power receiving end, the power sending end including a first converter, and the power receiving end including a second converter and at least one third converter connected in series, the first converter being of the same type as the second converter, and the second converter being of a different type from the third converter. A determination unit determines whether the DC power transmission system receives a power flow reversal instruction. A first control unit obtains a DC voltage instruction value when the DC power transmission system receives the power flow reversal instruction, and controls the DC voltage of the first converter to change with the DC voltage instruction value, the DC voltage instruction value being reduced from a first predetermined value to a second predetermined value in steps. A second control unit controls the active power of the second converter according to the DC voltage of the first converter. A third control unit controls the DC current of the third converter to remain unchanged. The converters of the power receiving end include converters of the same type as the converters of the power sending end and converters of different types, and in the process of power supply, the DC voltage of the first converter is changed to replace the way of changing the DC current in the prior art, so as to realize online smooth conversion of power flow direction, and the power flow reversal can be realized without stopping the system, and the transient instability caused by control mode switching and the influence of current impact and power oscillation on equipment in the process of power flow reversal are avoided, so as to ensure stable operation of the system and solve the problem of how to overcome the complexity of power flow reversal operation and current impact.

[0080] The first control unit of the present application comprises a first acquisition module and a first calculation module. The first acquisition module is configured to acquire 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. The DC voltage reference value is a reference value of a DC voltage of the first converter. The target virtual impedance value is used to increase a virtual impedance of the first converter to suppress current mutation of the first converter. The target current limiting value is used to suppress a DC current of the first converter being greater than a first current threshold. The first calculation module is configured to calculate a difference between a sum of the DC voltage reference value and the target virtual impedance value and the target current limiting value to obtain the DC voltage instruction value. The method combines the target virtual impedance value and the target current limiting value, i.e., the virtual impedance control and the current limiting control are synergistically used. The virtual impedance can provide "soft damping", and the current limiting control serves as "hard constraint", forming double-layer protection of "soft damping + hard constraint". Meanwhile, the method further avoids current overshoot and power oscillation in the power flow reversal process, so that the system power flow can be smoothly reversed.

[0081] The first acquisition module comprises a first acquisition submodule, a first calculation submodule, and a second calculation submodule. The first acquisition submodule is configured to acquire a predetermined slope and a predetermined time length. The predetermined slope is a rate at which the first predetermined value decreases to 0. The predetermined time length is a time length during which the first predetermined value decreases to 0. The first calculation submodule is configured to calculate a product of the predetermined slope and the predetermined time length to obtain a first product. The second calculation submodule is configured to calculate a difference between the first predetermined value and the first product to obtain the DC voltage reference value. The device can further calculate an accurate DC voltage reference value.

[0082] Specifically, the calculation formula of the DC voltage reference value is as follows: wherein, is the DC voltage reference value, is the first predetermined value, is the predetermined slope, is the predetermined time length.

[0083] The first obtaining module further includes a second obtaining submodule, a third calculating submodule, a fourth calculating submodule and a fifth calculating submodule. The second obtaining submodule is configured to obtain a resistance value of a virtual resistor of the first converter, an inductance value of a virtual inductor of the first converter and an output current of the first converter. The resistance value of the virtual resistor is used to simulate impedance characteristics of an actual resistor. The inductance value of the virtual inductor is used to simulate impedance characteristics of an actual inductor. The third calculating submodule is configured to calculate a product of a derivative of the output current with respect to time and the inductance value of the virtual inductor to obtain a first product. The fourth calculating submodule is configured to calculate a product of the resistance value of the virtual resistor and the output current to obtain a second product. The fifth calculating submodule is configured to calculate a sum of the first product and the second product to obtain the target virtual impedance value. The device can further quickly obtain the target virtual impedance value.

[0084] Specifically, a calculation formula of the target virtual impedance value is as follows: wherein, is the target virtual impedance value, is the resistance value of the virtual resistor, is the output current, is the inductance value of the virtual inductor. The resistance value of the virtual resistor and the inductance value of the virtual inductor can be different in the first stage, the second stage and the third stage.

[0085] The second obtaining submodule includes a first determining submodule, a second determining submodule and a third determining submodule. The first determining submodule is configured to determine a size relationship between an absolute value of the output current and a second current threshold. The second current threshold is smaller than the first current threshold. The second determining submodule is configured to determine that the resistance value of the virtual resistor is a maximum virtual resistor value corresponding to a case where the direct current voltage of the first converter is 0, when the absolute value of the output current is greater than the second current threshold. The third determining submodule is configured to determine that the resistance value of the virtual resistor is a minimum virtual resistor value corresponding to a case where the direct current voltage of the first converter is 0, when the absolute value of the output current is less than or equal to the second current threshold. The device can further obtain an accurate resistance value of the virtual resistor.

[0086] Specifically, a calculation formula of the resistance value of the virtual resistor in the second stage is as follows: wherein, is the resistance value of the virtual resistor, is the maximum virtual resistor value, is the minimum virtual resistor value, is the output current, is the second current threshold, 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 second control unit includes a second acquisition module, a second calculation module, and a third calculation module. The second acquisition module is configured to acquire the active power instruction value of the second converter. The second calculation module is configured to calculate the ratio of the DC voltage instruction value to the first predetermined value in real time to obtain a second ratio. The third calculation module is configured to calculate the product of the active power instruction value of the second converter and the second ratio in real time to obtain a target active power, and control the second converter to operate at the target active power obtained in real time. The device can further achieve accurate control of the second converter.

[0092] Specifically, the active power instruction value of the second converter is calculated according to the following formula: , wherein, is the active power instruction value when the DC voltage is , and is the DC voltage instruction value, is the first predetermined value.

[0093] In some other embodiments, the first converter is connected in parallel with the second converter and the third converter, respectively. The second acquisition module further includes a sixth acquisition submodule configured to acquire a target DC voltage value of the second converter to obtain the DC voltage instruction value. By acquiring only the DC voltage value of the second converter at the receiving end, the DC voltage instruction values of the first converter and the second converter can be obtained simultaneously, and it is not necessary to acquire the DC instruction values of the first converter and the second converter respectively. Therefore, the process of acquiring the DC voltage instruction value can be further simplified.

[0094] The first converter is connected in parallel with the second converter and the third converter, respectively. The voltages are equal in parallel connection. Therefore, when the DC voltage of the first converter changes, the voltage of the second converter will change accordingly. Without considering the line voltage drop between the second converter and the third converter, 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, and when the line voltage drop is ignored, Udc=Udc(n-1), that is, the DC voltage of the first converter is equal to the DC voltage of the second converter.

[0095] The power flow inversion device of the DC power transmission system includes a processor and a memory. The determination unit, the first control unit, the second control unit, and the third control unit are stored in the memory as program units, and the corresponding functions are realized by the processor executing the program units stored in the memory. The modules are located in the same processor; or the modules are located in different processors in any combination.

[0096] The processor comprises a core, and the core retrieves corresponding program units in the memory.

[0097] The memory can comprise a non-permanent memory in a computer readable medium, random access memory (RAM) and / or non-volatile memory such as read-only memory (ROM) or flash memory (flash RAM), and the memory comprises at least one memory chip.

[0098] The embodiment of the present application provides a computer readable storage medium, and the computer readable storage medium comprises a stored program, wherein the program controls a device where the computer readable storage medium is located to perform the power flow inversion method of the DC power transmission system when the program is running.

[0099] Specifically, the power flow inversion method of the DC power transmission system comprises:

[0100] Step S201, determining whether the DC power transmission system receives a power flow inversion instruction;

[0101] Step S202, in the case that the DC power transmission system receives the power flow inversion instruction, obtaining a DC voltage instruction value, and controlling a DC voltage of the first converter to change along with the DC voltage instruction value, wherein the DC voltage instruction value is lowered from a first predetermined value to a second predetermined value in steps, the first predetermined value is greater than 0, the second predetermined value is less than 0, and an absolute value of a difference between the DC voltage of the first converter at each moment and the DC voltage instruction value is less than or equal to a threshold value;

[0102] Step S203, controlling active power of the second converter according to the DC voltage of the first converter;

[0103] Step S204, controlling a DC current of the third converter to remain unchanged.

[0104] Optionally, the DC voltage of the first converter is lowered from the first predetermined value to the second predetermined value in steps, comprising: obtaining a DC voltage instruction value, wherein the DC voltage instruction value is a target DC voltage value of the first converter, and a change of the DC voltage instruction value comprises a first stage, a second stage and a third stage, the first stage comprises lowering from the first predetermined value to 0, the second stage comprises that an absolute value of a difference between the DC voltage instruction value and 0 is less than a first absolute value threshold, and the third stage comprises lowering from 0 to the second predetermined value; and controlling an absolute value of a difference between the DC voltage of the first converter and the DC voltage instruction value to be less than a second absolute value threshold.

[0105] Optionally, the DC voltage instruction value is obtained by: 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 a DC voltage of the first converter, the target virtual impedance value is used to increase a virtual impedance of the first converter to suppress a current mutation of the first converter, and the target current limiting value is used to suppress the DC current of the first converter being greater than a first current threshold; and calculating a difference between a sum of the DC voltage reference value and the target virtual impedance value and the target current limiting value to obtain the DC voltage instruction value.

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

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

[0108] Optionally, the resistance value of the virtual resistor of the first converter is obtained by: determining a size relationship between an absolute value of the output current and a second current threshold, wherein the second current threshold is less than the first current threshold; in a case where the absolute value of the output current is greater than the second current threshold, determining that a maximum virtual resistor value corresponding to a case where the DC voltage of the first converter is 0 is the resistance value of the virtual resistor; and in a case where the absolute value of the output current is less than or equal to the second current threshold, determining that a minimum virtual resistor value corresponding to a case where the DC voltage of the first converter is 0 is the resistance value of the virtual resistor.

[0109] Optionally, the method further comprises: obtaining the inductance value of the virtual inductor of the first converter, including: obtaining a maximum inductance value of the first converter; calculating a ratio of an absolute value of a derivative of the output current with respect to time to the second current threshold value to obtain a first ratio; and calculating a 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 0 of the first converter.

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

[0111] Optionally, the method further comprises: controlling the active power of the second converter according to the DC voltage of the first converter, including: obtaining the active power instruction value of the second converter; calculating a ratio of the DC voltage instruction value to the first predetermined value in real time to obtain a second ratio; and calculating a product of the active power instruction value of the second converter and the second ratio in real time to obtain a target active power, and controlling the second converter to operate at the target active power obtained in real time.

[0112] The embodiment of the present application provides a processor, which is used for running a program, wherein the program performs the power flow reversal method of the DC power transmission system when running.

[0113] Specifically, the power flow reversal method of the DC power transmission system comprises:

[0114] Step S201, determining whether the DC power transmission system receives a power flow reversal instruction;

[0115] Step S202, in the case that the DC power transmission system receives the power flow reversal instruction, obtaining a DC voltage instruction value, and controlling the DC voltage of the first converter to change along with the DC voltage instruction value, wherein the DC voltage instruction value is lowered from a first predetermined value to a second predetermined value in steps, the first predetermined value is greater than 0, the second predetermined value is less than 0, and an absolute value of a difference between the DC voltage of the first converter at each moment and the DC voltage instruction value is less than or equal to a preset threshold value.

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

[0117] Step S204, controlling the DC current of the third converter to remain unchanged.

[0118] Optionally, the DC voltage instruction value is obtained by: 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 a DC voltage of the first converter, the target virtual impedance value is used to increase a virtual impedance of the first converter to suppress a current mutation of the first converter, and the target current limiting value is used to suppress the DC current of the first converter being greater than a first current threshold; and calculating a difference between a sum of the DC voltage reference value and the target virtual impedance value and the target current limiting value to obtain the DC voltage instruction value.

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

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

[0121] Optionally, the resistance value of the virtual resistor of the first converter is obtained by: determining a size relationship between an absolute value of the output current and a second current threshold, wherein the second current threshold is less than the first current threshold; in a case where the absolute value of the output current is greater than the second current threshold, determining that a maximum virtual resistor value corresponding to a case where the DC voltage of the first converter is 0 is the resistance value of the virtual resistor; and in a case where the absolute value of the output current is less than or equal to the second current threshold, determining that a minimum virtual resistor value corresponding to a case where the DC voltage of the first converter is 0 is the resistance value of the virtual resistor.

[0122] Optionally, the method further comprises: obtaining the maximum inductance value of the first converter; calculating a ratio of an absolute value of a derivative of the output current with respect to time and the second current threshold to obtain a first ratio; and calculating a 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 0 of the first converter.

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

[0124] Optionally, the method further comprises: obtaining the active power instruction value of the second converter; calculating a ratio of the DC voltage instruction value and the first predetermined value in real time to obtain a second ratio; calculating a product of the active power instruction value of the second converter and the second ratio in real time to obtain a target active power; and controlling the second converter to operate at the target active power obtained in real time.

[0125] An apparatus is provided, and the apparatus includes a processor, a memory, and a program stored on the memory and executable on the processor, and the processor implements at least the following steps when executing the program:

[0126] Step S201, determining whether the DC power transmission system receives a power flow reversal instruction;

[0127] Step S202, in a case where the DC power transmission system receives the power flow reversal instruction, obtaining a DC voltage instruction value, and controlling a DC voltage of the first converter to follow a change of the DC voltage instruction value, wherein the DC voltage instruction value is stepped down 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 an absolute value of a difference between the DC voltage of the first converter at each moment and the DC voltage instruction value is less than or equal to a preset threshold value;

[0128] Step S203, controlling an active power of the second converter according to the DC voltage of the first converter;

[0129] Step S204, controlling a DC current of the third converter to remain unchanged.

[0130] The apparatus herein can be a server, a PC, a PAD, a mobile phone, or the like.

[0131] The application also provides a computer program product, which is suitable for executing the program of the following method steps when executed on a data processing device:

[0132] Step S201, determining whether the DC power transmission system receives a power flow reversal instruction;

[0133] Step S202, in the case that the DC power transmission system receives the power flow reversal instruction, obtaining a DC voltage instruction value, and controlling the DC voltage of the first converter to follow the change of the DC voltage instruction value, wherein the DC voltage instruction value is lowered from a first predetermined value to a second predetermined value in steps, 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 at each moment and the DC voltage instruction value is less than or equal to a preset threshold value;

[0134] Step S203, controlling the active power of the second converter according to the DC voltage of the first converter;

[0135] Step S204, controlling the DC current of the third converter to remain unchanged.

[0136] Obviously, those skilled in the art should understand that the modules or steps of the application can be realized by general computing devices, which can be concentrated on a single computing device or distributed on a network composed of multiple computing devices, and they can be realized by program codes executable by the computing devices, so that they can be stored in storage devices and executed by the computing devices, and in some cases, the steps shown or described can be executed in different order, or they can be manufactured into individual integrated circuit modules, or multiple modules or steps can be manufactured into a single integrated circuit module. Therefore, the application is not limited to any specific combination of hardware and software.

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

[0138] The computer program instructions can also be loaded onto a computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer-implemented process such that the instructions which execute on the computer or other programmable apparatus provide steps for implementing the functions specified in the flowchart block or blocks. Figure 1 one or more flow or blocks Figure 1 means for functionally implementing the steps in the flowchart block or blocks

[0139] These computer program instructions can also be stored in a computer readable memory that can direct a computer or other programmable data processing apparatus to function in a particular manner, such that the instructions stored in the computer readable memory produce an article of manufacture including instructions which implement the function specified in the flowchart block or blocks. Figure 1 one or more flow or blocks Figure 1 means for functionally implementing the steps in the flowchart block or blocks

[0140] The computer program instructions can also be loaded onto a computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer-implemented process such that the instructions which execute on the computer or other programmable apparatus provide steps for implementing the functions specified in the flowchart block or blocks. Figure 1 one or more flow or blocks Figure 1 means for functionally implementing the steps in the flowchart block or blocks

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

[0142] The memory can include non-persistent memory and / or persistent memory, for example, read only memory (ROM) and / or flash memory, for example, in the form of a computer readable medium. The memory is an example of computer readable media.

[0143] Computer-readable media includes permanent and non-permanent, movable and non-movable media that can be implemented by any method or technology to store information. The information can be computer-readable instructions, data structures, program modules 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, compact disc read-only memory (CD-ROM), digital versatile disc (DVD) or other optical storage, magnetic cassette, magnetic disk storage or other magnetic storage devices, or any other non-transmission medium that can be used to store information accessible by a computing device. According to the definition herein, computer-readable media does not include transitory media such as modulated data signals and carriers.

[0144] The technical features of the above-described embodiments can be combined in any manner. In order to make the description concise, not all possible combinations of the technical features in the above-described embodiments are described, but as long as the combinations of the technical features do not exist contradictions, it should be considered as the scope of the present disclosure.

[0145] It should also be noted that the terms "comprising", "including", or any other variant thereof are intended to cover non-exclusive inclusion, so that processes, methods, articles or devices including a series of elements not only include those elements, but also include other elements not explicitly listed, or inherent to such processes, methods, articles or devices. Without more limitations, the element defined by the statement "comprising a" does not exclude the presence of other identical elements in the process, method, article or device including the element.

[0146] From the above description, it can be seen that the above-described embodiments of the present 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 descriptions are only the preferred embodiments of the present application, and are not intended to limit the present application. The present application can have various modifications and changes for those skilled in the art. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present 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 command value in real time 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 first predetermined value is greater than 0 and the second predetermined value is less than 0. 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 active power of the second converter is controlled based on the DC voltage of the first converter. The DC current of the third converter is kept constant.

2. The method according to claim 1, characterized in that, Obtain DC voltage command values, including: The DC voltage reference value, the target virtual impedance value, and the target current limit value of the first converter are obtained. 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. The target current limit value is used to suppress the DC current of the first converter from exceeding a first current threshold. The difference between the sum of the DC voltage reference value and the target virtual impedance value and the target current limiting value is calculated to obtain the DC voltage command value.

3. The method according to claim 2, 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.

4. The method according to claim 2, characterized in that, Obtaining the target virtual impedance value of the first converter includes: Obtain 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; The first product is obtained by multiplying the derivative of the output current with respect to time by the inductance value of the virtual inductor. Calculate the product of the resistance value of the virtual resistor and the output current to obtain the second product; The target virtual impedance value is obtained by calculating the sum of the first product and the second product.

5. The method according to claim 4, 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.

6. The method according to claim 4, 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.

7. The method according to claim 2, characterized in that, Obtaining the target current limiting value for the first converter includes: Obtain the output current and current limiting coefficient of the first converter; Calculate the difference between the absolute value of the output current and the first current threshold to obtain the first difference; The maximum value between the first difference and 0 is obtained to get the third predetermined value; The target current limiting value is obtained by multiplying the current limiting coefficient by the third predetermined value.

8. 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.

9. 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; The first control unit is 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 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. 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.

10. A direct current 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 8.

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