A method for active support test of a flexible direct-current receiving converter

Abstract

This invention discloses an active support test method for flexible DC receiving-end converters, addressing the current problem that active support verification of flexible DC mainly relies on electromagnetic transient simulation and small-capacity experimental prototype testing, which makes it difficult to simulate the behavior of actual systems and results in different control levels compared to actual systems. The flexible DC system includes a grid-commutated converter, a grid-simulated MMC, and a receiving-end MMC. The method involves controlling and unlocking the grid-commutated converter to perform uncontrolled charging of the grid-simulated MMC. Once the capacitor voltage of the grid-simulated MMC reaches a stable uncontrolled charging voltage, a first unlocking control strategy is implemented on the grid-simulated MMC. When the grid-simulated MMC meets a first preset rated voltage condition, a second unlocking control strategy is implemented on the receiving-end MMC. When the receiving-end MMC meets a second preset rated voltage condition, the power support capability of the receiving-end MMC is verified through the grid-simulated MMC, obtaining the active support test results of the receiving-end MMC.

Description

Technical Field

[0001] This invention relates to the field of flexible DC transmission systems and their control technology, and in particular to an active support test method for a flexible DC receiving-end converter, an active support test device for a flexible DC receiving-end converter, an electronic device, and a storage medium. Background Technology

[0002] MMC (Modular Multi-Level Converter) has the advantages of easy scalability, low loss, and no need for direct series switching devices, making it the most mainstream topology in flexible DC transmission projects.

[0003] Traditional flexible DC grid-connected converters employ a grid-following control strategy based on phase-locked loops (PLLs), transmitting power by injecting current into the AC grid and exhibiting current source characteristics. However, increasing research indicates that grid-following control connected to weak AC grids is prone to synchronization instability, making it difficult to achieve the integration of high proportions of renewable energy in the future.

[0004] As the proportion of new energy sources in the power system gradually increases, the future power system will become a new type of power system dominated by new energy sources and featuring a high proportion of power electronics. Grid-based control is increasingly being recognized as a key technological path to achieve this transformation. Traditional research has explored the supporting role of flexible DC transmission systems in AC grid operation from multiple perspectives, covering various control strategies at the MMC converter level and system level. However, the relevant results mostly rely on simulation analysis or small-capacity experimental platforms for verification, and their conclusions are to some extent limited by model simplification and experimental scale.

[0005] In contrast, real-world flexible DC transmission systems typically feature high voltage levels, large transmission capacities, complex system structures, and multiple control levels, resulting in significant differences in their dynamic characteristics and operational behavior compared to simulations and experimental prototypes. The currently employed active support testing schemes are difficult to apply to actual flexible DC support capability verification scenarios. Summary of the Invention

[0006] This invention provides an active support test method for flexible DC receiving-end converters, an active support test device for flexible DC receiving-end converters, an electronic device, and a storage medium, which are used to solve or partially solve the problem that the current verification of active support of flexible DC mainly relies on electromagnetic transient simulation and small-capacity experimental prototype testing, which makes it difficult to simulate the behavior of actual systems and the control level is different from that of actual systems.

[0007] This invention provides an active support test method for a flexible DC receiving-end converter, wherein the flexible DC includes a grid-commutated converter, a grid-simulated MMC, and a receiving-end MMC; the method includes:

[0008] Control the unlocking of the grid commutation converter to perform uncontrolled charging of the grid simulated MMC;

[0009] Once the capacitor voltage of the grid-simulated MMC reaches the stable uncontrolled charging voltage, the first unlocking control strategy is executed on the grid-simulated MMC.

[0010] When the grid simulation MMC meets the first preset rated voltage condition, the second unlocking control strategy is executed on the receiving end MMC;

[0011] When the receiving-end MMC meets the second preset rated voltage condition, the power support capability of the receiving-end MMC is verified by the power grid simulation MMC, and the active support test results of the receiving-end MMC are obtained.

[0012] Optionally, the power grid simulation MMC includes an analog DC controller and an analog AC controller; the execution of the first unlocking control strategy on the power grid simulation MMC includes:

[0013] The analog DC controller is unlocked, and the capacitor voltage of the analog MMC of the power grid is gradually increased;

[0014] When the capacitor voltage of the simulated MMC reaches the preset rated capacitor voltage value, the simulated AC controller is unlocked, and the AC voltage of the simulated MMC is gradually increased until it reaches the preset rated AC voltage value.

[0015] Optionally, the receiving-end MMC includes a receiving-end DC controller and a receiving-end AC controller; the step of executing a second unlocking control strategy on the receiving-end MMC when the grid simulation MMC meets a first preset rated voltage condition includes:

[0016] When the capacitor voltage of the grid-simulated MMC reaches the preset capacitor voltage rating, the AC voltage of the grid-simulated MMC reaches the preset AC voltage rating, and the voltage at the grid coupling point of the flexible DC and AC grid reaches stability, the receiving-end AC controller is unlocked so that the receiving-end AC controller can charge the receiving-end MMC based on the capacitor voltage synchronous operation mode.

[0017] When the capacitor voltage of the receiving-end MMC reaches the preset rated capacitor voltage value, the receiving-end DC controller is unlocked so that the receiving-end DC controller controls the DC voltage of the receiving-end MMC until it reaches the preset rated DC voltage value.

[0018] Optionally, when the receiving-end MMC meets the second preset rated voltage condition, the power support capability of the receiving-end MMC is verified through the power grid simulated MMC to obtain the active support test results of the receiving-end MMC, including:

[0019] When the capacitor voltage of the receiving-end MMC reaches the preset capacitor voltage rating and the DC voltage of the receiving-end MMC is stable at the preset DC voltage rating, the power support capability of the receiving-end MMC is verified by the grid simulation MMC based on the step control strategy, and the active and reactive outputs of the receiving-end MMC, as well as the voltage amplitude of the grid coupling point of the flexible DC and AC grid connection are obtained.

[0020] The active power output, reactive power output, and voltage amplitude are integrated to form the active support test results of the receiving-end MMC.

[0021] Optionally, the step-control strategy-based verification of the power support capability of the receiving-end MMC through the grid-simulated MMC to obtain the active and reactive power outputs of the receiving-end MMC, and the voltage amplitude at the grid coupling point of the flexible DC and AC grid connection, includes:

[0022] The power balance variable step control is performed by the power grid simulation MMC to simulate the power imbalance inside the AC power grid, and the active power support capability of the receiving-end MMC is verified to obtain the active power output of the receiving-end MMC. In the process of verifying the active power support capability, the active power of the receiving-end MMC used to respond to the frequency change on the AC side comes entirely from its own capacitor energy storage.

[0023] After a preset time interval, the power balance variable is reset. When the system frequency no longer changes further, the voltage amplitude step control is performed through the grid simulation MMC to simulate the AC grid voltage sag. The reactive power support capability of the receiving-end MMC is verified, and the reactive power output of the receiving-end MMC and the voltage amplitude of the grid coupling point of the flexible DC and AC grid are obtained.

[0024] Optionally, the DC side of the grid-commutated converter is connected to the DC side of the grid-simulated MMC; the AC side of the grid-simulated MMC is connected to the AC side of the receiving-end MMC via a transformer; and the AC side of the grid-commutated converter is connected to the AC power grid.

[0025] Optionally, the grid-connected phase converter adopts a constant firing angle control mode; the AC side of the grid-simulated MMC adopts a constant voltage / frequency control mode and adds an inertia simulation stage; the DC side of the grid-simulated MMC adopts a capacitor voltage outer loop-DC current inner loop control mode; and the receiving-end MMC adopts a grid-connected control mode with capacitor voltage synchronization.

[0026] This invention also provides an active support test device for a flexible DC receiving-end converter, wherein the flexible DC includes a grid-commutated converter, a grid-simulated MMC, and a receiving-end MMC; the device includes:

[0027] An uncontrolled charging unit is used to control the unlocking of the grid commutation converter and perform uncontrolled charging on the grid simulated MMC;

[0028] The first unlocking control strategy execution unit is used to execute the first unlocking control strategy on the grid-simulated MMC after the capacitor voltage of the grid-simulated MMC reaches the stability of the uncontrolled charging voltage.

[0029] The second unlocking control strategy execution unit is used to execute the second unlocking control strategy on the receiving end MMC when the grid simulation MMC meets the first preset rated voltage condition.

[0030] The power support capability verification unit is used to verify the power support capability of the receiving-end MMC through the power grid simulation MMC when the receiving-end MMC meets the second preset rated voltage condition, and to obtain the active support test results of the receiving-end MMC.

[0031] The present invention also provides an electronic device, the device comprising a processor and a memory:

[0032] The memory is used to store program code and transmit the program code to the processor;

[0033] The processor is used to execute the active support test method for the flexible DC receiving-end converter as described above, according to the instructions in the program code.

[0034] The present invention also provides a computer-readable storage medium for storing program code for performing the active support test method for a flexible DC receiving-end converter as described in any of the preceding claims.

[0035] As can be seen from the above technical solutions, the present invention has the following advantages:

[0036] An active support test method for a flexible DC receiving-end converter is provided. The flexible DC system includes a grid-commutated converter, a grid-simulated MMC, and a receiving-end MMC. When verifying the active support function of the receiving-end MMC, the grid-commutated converter is unlocked, and the grid-simulated MMC is uncontrolled charged. Once the capacitor voltage of the grid-simulated MMC reaches a stable uncontrolled charging voltage, a first unlocking control strategy is executed on the grid-simulated MMC. When the grid-simulated MMC meets a first preset rated voltage condition, a second unlocking control strategy is executed on the receiving-end MMC. When the receiving-end MMC meets a second preset rated voltage condition, the power support capability of the receiving-end MMC is verified through the grid-simulated MMC, obtaining the active support test results of the receiving-end MMC. Thus, this invention, through a simple system topology and control structure, and a verification scheme with a clear process, concise steps, and convenient operation, completes the verification of the active support function of the receiving-end MMC on the flexible DC side. This scheme not only features concise steps and optimized process, but also convenient operation and low implementation cost, ensuring the efficiency of the verification process and the accuracy of the results. Attached Figure Description

[0037] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0038] Figure 1 This is a simplified connection diagram of the STATCOM wiring method used in the embodiments of the present invention;

[0039] Figure 2 A schematic diagram of the control strategy used for grid simulation MMC;

[0040] Figure 3 This is a schematic diagram of a receiving-end MMC grid-connected system;

[0041] Figure 4 A schematic diagram illustrating the similarity principle between the dynamic characteristics of MMC and synchronous generators;

[0042] Figure 5 A schematic diagram of the control strategy adopted by the receiving-end MMC;

[0043] Figure 6 A flowchart illustrating the steps of an active support test method for a flexible DC receiving-end converter;

[0044] Figure 7 A schematic diagram of the overall process for an active support test method for a flexible DC receiving-end converter;

[0045] Figure 8 This is a structural block diagram of an active support test device for a flexible DC receiving-end converter. Detailed Implementation

[0046] This invention provides an active support test method for a flexible DC receiving-end converter, an active support test device for a flexible DC receiving-end converter, an electronic device, and a storage medium. These methods address or partially address the current problem that active support verification of flexible DC converters mainly relies on electromagnetic transient simulation and small-capacity experimental prototype testing, which makes it difficult to simulate the behavior of actual systems and results in different control levels compared to actual systems.

[0047] To make the objectives, features, and advantages of this invention more apparent and understandable, the technical solutions of the embodiments of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the embodiments described below are only some embodiments of this invention, and not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0048] To enable those skilled in the art to better understand the technical solutions provided in the embodiments of the present invention, some of the technical features involved in the solutions are briefly described first:

[0049] MMC (Modular Multilevel Converter): An advanced power electronic topology primarily used in high-voltage direct current (HVDC) transmission and renewable energy grid connection. Its basic principle involves combining multiple low-voltage power electronic modules (sub-modules) in series or parallel to form a high-voltage, high-power converter.

[0050] Line commutated converter (LCC): A type of power electronic device based on thyristors, also known as a thyristor converter. It is mainly used to convert alternating current (AC) to direct current (DC) or vice versa. In high-voltage direct current (HVDC) transmission systems, LCCs are widely used in rectification and inversion processes.

[0051] Step: At a certain moment, a variable in the system (such as power) suddenly undergoes a fixed numerical change.

[0052] Power balance variable step: During the operation of a simulated AC power grid, the internal power balance variable ΔP... GA sudden, fixed-amplitude change is applied to study the behavior and response of the power grid after being subjected to such a disturbance.

[0053] Voltage amplitude step: During the operation of a simulated AC power grid, a sudden, step change is applied to the output voltage amplitude of the grid simulation MMC to test the dynamic response and stability of the receiving-end MMC (i.e., the MMC under test). In power grid simulation, this operation helps verify the rationality of the receiving-end MMC controller design and its ability to cope with sudden voltage change demands in actual operation, thereby ensuring the stability and reliability of the power grid.

[0054] As an example, traditional flexible DC grid-connected converters employ a grid-following control strategy based on phase-locked loops (PLLs), transmitting power by injecting current into the AC grid, exhibiting current source characteristics. However, increasing research indicates that grid-following control connected to weak AC grids is prone to synchronization instability, making it difficult to achieve the integration of a high proportion of renewable energy in the future.

[0055] As the proportion of new energy sources in the power system gradually increases, the future power system will become a new type of power system dominated by new energy sources and featuring a high proportion of power electronics. Grid-based control is increasingly being recognized as a key technological path to achieve this transformation. Traditional research has explored the supporting role of flexible DC transmission systems in AC grid operation from multiple perspectives, covering various control strategies at the MMC converter level and system level. However, the relevant results mostly rely on simulation analysis or small-capacity experimental platforms for verification, and their conclusions are to some extent limited by model simplification and experimental scale.

[0056] In contrast, real-world flexible DC transmission systems typically feature high voltage levels, large transmission capacities, complex system structures, and multiple control levels, resulting in significant differences in their dynamic characteristics and operational behavior compared to simulations and experimental prototypes. The currently employed active support testing schemes are difficult to apply to actual flexible DC support capability verification scenarios.

[0057] Therefore, one of the core inventive points of this invention is to provide an active support test method for a flexible DC receiving-end converter based on STATCOM (Static Var Compensator) wiring. The key to its implementation is that, through a simple system topology and control structure, and a verification scheme with a clear process, concise steps, and convenient operation, the active support function verification of the receiving-end MMC in a grid control mode using capacitor voltage synchronization is completed.

[0058] This invention proposes a verification scheme for flexible DC active support based on STATCOM wiring, which mainly includes three parts: wiring method, control strategy, and verification implementation scheme.

[0059] Reference Figure 1 The diagram shows a simplified connection diagram of the STATCOM wiring method used in an embodiment of the present invention.

[0060] like Figure 1 As shown, the flexible DC side mainly includes a thyristor-based line commutated converter (LCC), a grid simulation MMC, and a receiving-end MMC (MMC under test). The AC side of the grid simulation MMC is connected to the AC side of the receiving-end MMC via a transformer. Their DC sides are not connected (i.e., the DC side of the receiving-end MMC is floating). Furthermore, the DC side of the line commutated converter is connected to the DC side of the grid simulation MMC to compensate for power losses caused by the MMC's operation.

[0061] Specifically, resistance This is used to limit current during uncontrolled charging of the MMC from the DC side. After uncontrolled charging is complete, the isolating switch is closed. This can enable resistor bypass.

[0062] Specifically, in practice, the power offset wiring method is difficult to accurately simulate the control characteristics of the receiving-end MMC. In order to enable the tested receiving-end MMC to operate at U... dc In Q control mode, to more realistically simulate the control and operation characteristics of an actual MMC, this embodiment of the invention disconnects the DC line between the two MMCs, connecting them only on the AC side via a transformer. The DC side of the grid-commutated converter LCC remains connected to the DC side of the grid-simulated MMC to compensate for its power loss.

[0063] exist Figure 1 In the wiring configuration shown, the DC side of the MMC at the tested end lacks active power support, essentially operating in STATCOM mode. Therefore, this wiring configuration cannot verify the active power support capability of the MMC and flexible DC over a longer timescale (such as assisting primary frequency regulation). However, the submodule capacitors of the MMC at the tested end contain energy storage, which can provide a certain amount of active power briefly when the frequency changes. Therefore, this wiring configuration can still verify the MMC's own inertia support capability for the power grid.

[0064] Furthermore, the grid-commutated converter LCC is a current-source converter, typically requiring a DC current of no less than 1% of the rated current. However, due to the relatively low actual operating losses of the MMC, it may be difficult to meet the minimum DC current requirement during operation. In this scheme, the grid-commutated converter LCC mainly operates in constant DC voltage mode. Therefore, in some embodiments, the grid-commutated converter LCC can be replaced with a diode rectifier (DR) to achieve a similar function.

[0065] Therefore, in terms of wiring, this embodiment of the invention only requires two MMCs and one low-cost grid phase-commutation converter LCC to complete the active support function verification of the receiving-end MMC in the grid control mode using capacitor voltage synchronization. The system topology is simple, the test efficiency is high, and the calculation cost is low.

[0066] For the control strategy, the grid-commutated converter (LCC) is the power balancing node of the entire DC system. Therefore, the grid-commutated converter (LCC) can adopt a constant firing angle control mode.

[0067] Figure 2 A schematic diagram of the control strategy used in the power grid simulation MMC in an embodiment of the present invention is shown.

[0068] In practice, the grid simulation MMC primarily simulates the operating characteristics of a real AC power grid by controlling its own voltage and frequency. Specifically, the control objective of the grid simulation MMC is to control the amplitude and frequency of its own port AC voltage given a DC voltage. This simulates, to some extent, the inertial characteristics of an AC power grid and the transient characteristics that may occur, such as voltage dips and sags. Therefore, the AC side of the grid simulation MMC can operate directly in voltage / frequency (V / f) mode, with an additional inertia simulation stage. The AC voltage amplitude can be directly given based on the simulated voltage dips. Regarding the frequency, to simulate an inertial AC power grid, its frequency can be determined using the following formula:

[0069] (1)

[0070] in, This represents the equivalent moment of inertia of the simulated AC power grid. This indicates the power balance within the simulated AC power grid; This represents the active power flowing from the grid-simulated MMC to the receiving-end MMC.

[0071] This indicates that the generator power in the simulated power grid is greater than the load power within the power grid. It can simulate that the power of generators inside the power grid is equal to the power of loads inside the power grid; The simulation assumes that the generator power is less than the load power within the power grid.

[0072] Based on the above analysis, the control of the power grid simulation MMC is as follows: Figure 2 As shown in (a) of the diagram. Wherein, , These are the AC voltage amplitude and angular frequency, respectively. These are the reference values ​​for three-phase AC voltage.

[0073] The primary control objective of the DC controller in a grid simulation MMC is to maintain the capacitor voltage near its rated value by adjusting the DC port voltage. Figure 2 (b) is the control block diagram of the DC side of the grid simulation MMC.

[0074] in, This represents the average voltage across the capacitor. It is direct current; It is a DC voltage; This represents the DC internal potential; the superscript "*" indicates the corresponding reference value. The outer loop is the capacitor voltage control loop, and its output is the DC current reference value of the MMC. The inner loop is a DC current control loop, and the output of the control is the DC internal potential. The MMC can maintain the capacitor voltage near its rated value by controlling its own DC current. The reference values ​​for the six bridge arm voltages are as follows:

[0075] (2)

[0076] Among them, subscript and These represent the positive bridge arm and the negative bridge arm, respectively. It represents three phases.

[0077] For the tested receiving-end MMC side, the tested receiving-end MMC actually simulates the receiving-end converter in DC transmission, with U dc In Q mode operation, a grid control strategy based on capacitor voltage synchronization can be adopted. The generator's rotor motion equation is as follows:

[0078] (3)

[0079] In the formula, Indicates the moment of inertia; This represents the rotor speed of the synchronous generator SG; Indicates mechanical power; Indicates electromagnetic power.

[0080] For a synchronous generator SG, the output frequency can be calculated as follows:

[0081] (4)

[0082] in, Indicates the electric angular frequency of the generator; This indicates the number of pole pairs of the generator.

[0083] Figure 3 A schematic diagram of a receiving-end MMC grid-connected system is shown.

[0084] For example Figure 3 The receiving-end MMC grid-connected system shown has each bridge arm of the MMC consisting of N sub-modules and one bridge arm inductor. This indicates the capacitance value of the submodule. This indicates the inductance value of the bridge arm. The MMC is connected to the AC power grid via a transformer and transmission lines. This indicates the transformer leakage reactance. This represents the equivalent connection reactance between the transformer and the AC power grid.

[0085] Figure 3 In this context, the AC grid voltage is represented as... .in, This represents the rated RMS value of the AC phase voltage. The MMC output voltage phasor is expressed as... To simplify the analysis, we ignore the fluctuation component of the submodule capacitor voltage and assume that all submodule capacitors have the same voltage. The total energy stored in the MMC submodule can then be calculated as follows:

[0086] (5)

[0087] To maintain power balance between the AC and DC sides of the MMC, the energy dynamic equation of the MMC can be expressed as:

[0088] (6)

[0089] in, This indicates the input power of the receiving-end MMC; This indicates the output power transmitted from the receiving end MMC to the power grid.

[0090] Equation (6) shows that the capacitor voltage of the MMC submodule It varies with the power difference between the AC and DC sides. The rotor motion equations show that the rotor speed... The changes are related to the power imbalance of the synchronous generator.

[0091] Figure 4 A schematic diagram illustrating the similarity principle between the dynamic characteristics of MMC and synchronous generator is shown.

[0092] like Figure 4 As shown, there is a certain similarity between the dynamic equations of MMC and synchronous generator, which allows for further analogy of the relationship between the submodule capacitor voltage and the rotor speed.

[0093] The combined expression (4) reflects and The relationship between the submodule capacitor voltage and the output voltage frequency can be constructed using the similarity of equations (3) and (6). Therefore, the frequency and phase angle of the MMC output voltage can be obtained by controlling the square of the submodule capacitor voltage (representing the energy of the submodule capacitor). Based on this, the principle of achieving grid synchronization by constructing the virtual inertial response of the submodule capacitor voltage can be obtained.

[0094] like Figure 3 As shown, the output power transmitted from the MMC to the grid It can be calculated using the following formula:

[0095] (7)

[0096] in, This represents the output voltage phasor of the MMC; This indicates the phase difference between the output voltage of the MMC and the mains voltage.

[0097] When the system reaches steady state, the MMC output voltage frequency equals the grid frequency, and the MMC output voltage phase remains unchanged. However, during system transients, the grid frequency changes. If the MMC output voltage frequency remains constant, then the phase difference between the two voltages will increase. The phase difference will change. As shown in equation (7), with the increase of the power grid frequency, the phase difference... Decrease The power also decreases accordingly. According to equation (6), the decrease in AC power leads to an increase in the total energy of the submodule, i.e., the voltage of the submodule capacitor. This will increase. This deviation can be used to control the frequency rise of the MMC's output voltage, eventually aligning it with the grid frequency. Similarly, when the grid frequency decreases, It will increase, and through control Due to the deviation, the output voltage frequency of the MMC will decrease until it matches the grid frequency.

[0098] By establishing a control relationship between the submodule capacitor voltage and the output voltage frequency, the output voltage frequency can spontaneously synchronize with the grid frequency. As long as the grid frequency remains near its rated value, the submodule capacitor voltage of the receiving-end MMC can remain balanced. Therefore, based on this control principle of virtual inertia synchronization of submodule capacitor voltage, the DC-side power and AC-side power of the receiving-end MMC can automatically maintain a balance, and the output voltage can automatically synchronize with the grid voltage. Thus, the MMC does not have to bear the negative impact that a phase-locked loop might bring. Since this connection is established based on the submodule capacitor voltage, and the DC voltage of the MMC is achieved by controlling the switching of submodules, this is also the premise of DC voltage decoupling control in a DC controller.

[0099] Therefore, the frequency of the AC output voltage can be obtained through the synchronous control of the capacitor voltage in the submodule. and phase angle As mentioned earlier, the submodule capacitor voltage control determines the power balance of the MMC and is the basis for grid synchronization. Based on the above analysis, the proposed submodule voltage synchronization control is as follows:

[0100] (8)

[0101] in, Indicates the AC voltage frequency of the MMC; Indicates its reference value; This indicates the rated value of the capacitor voltage in the submodule; Indicates the inertial simulation coefficient; Indicates the damping simulation coefficient; Indicates the coefficient of inertia; This represents the damping coefficient.

[0102] The control strategy adopted by the receiving-end MMC is shown in the diagram below. Figure 5 As shown. Among them, Figure 5 In the middle (a), the phase controller of the receiving end MMC is represented. Figure 5 In the middle (b), the voltage amplitude controller of the receiving end MMC is represented.

[0103] In terms of control, since the control structure of the grid phase-changing converter LCC and the two MMCs is simple, the active support function verification of the receiving-end MMC can be completed.

[0104] Based on the content described in the foregoing embodiments, for verifying the implementation scheme, please refer to... Figure 6 This document illustrates a flowchart of the active support test method for a flexible DC receiving-end converter according to an embodiment of the present invention. The flexible DC converter includes a grid-commutated converter, a grid-simulated MMC, and a receiving-end MMC; the method specifically includes the following steps:

[0105] Step 601: Control the unlocking of the grid phase-commutation converter to perform uncontrolled charging of the grid simulated MMC;

[0106] Based on the preceding discussion, on the flexible DC side, the DC side of the grid-commutated converter is connected to the DC side of the grid-simulated MMC. The AC side of the grid-simulated MMC is connected to the AC side of the receiving-end MMC via a transformer. The AC side of the grid-commutated converter is connected to the AC power grid.

[0107] The grid-connected converter employs a constant firing angle control mode. The AC side of the grid-simulated MMC uses a constant voltage / frequency control mode, with an added inertia simulation stage. The DC side of the grid-simulated MMC uses a capacitor voltage outer loop-DC current inner loop control mode. The receiving-end MMC uses a grid-connected control mode with capacitor voltage synchronization.

[0108] Therefore, in the specific implementation, when it is necessary to verify the active support function of the receiving-end MMC, the grid commutator is first unlocked and the grid simulated MMC is charged uncontrolled.

[0109] Step 602: After the capacitor voltage of the grid-simulated MMC reaches the stable uncontrolled charging voltage, the first unlocking control strategy is executed on the grid-simulated MMC.

[0110] Once the capacitor voltage of the grid-simulated MMC reaches the stable uncontrolled charging voltage, the first unlocking control strategy can then be executed on the grid-simulated MMC.

[0111] In some embodiments, the grid simulation MMC includes a DC controller and an AC controller. To distinguish it from the controller of the receiving-end MMC, the DC controller of the grid simulation MMC is defined as an analog DC controller, and the AC controller is defined as an analog AC controller.

[0112] In a specific implementation, the process of executing the first unlocking control strategy on the grid-simulated MMC may include: controlling the unlocking of the analog DC controller and gradually increasing the capacitor voltage of the grid-simulated MMC; when the capacitor voltage of the grid-simulated MMC reaches the preset capacitor voltage rating, controlling the unlocking of the analog AC controller and gradually increasing the AC voltage of the grid-simulated MMC until it reaches the preset AC voltage rating.

[0113] Step 603: When the grid simulation MMC meets the first preset rated voltage condition, the second unlocking control strategy is executed on the receiving end MMC;

[0114] Based on the aforementioned steps, when the grid simulation MMC meets the first preset rated voltage condition, the second unlocking control strategy is executed on the receiving-end MMC.

[0115] In some embodiments, the receiving-end MMC may also include a DC controller and an AC controller. To distinguish it from the controller of the grid simulation MMC, the DC controller of the receiving-end MMC is defined as the DC controller under test, and the AC controller is defined as the AC controller under test.

[0116] In a specific implementation, when the grid-simulated MMC meets the first preset rated voltage condition, the implementation process of executing the second unlocking control strategy on the receiving-end MMC may include: when the capacitor voltage of the grid-simulated MMC reaches the preset rated capacitor voltage value, the AC voltage of the grid-simulated MMC reaches the preset rated AC voltage value, and the voltage at the grid coupling point of the flexible DC and AC grids reaches stability, the receiving-end AC controller is unlocked so that the receiving-end AC controller charges the receiving-end MMC based on the capacitor voltage synchronous operation mode; when the capacitor voltage of the receiving-end MMC reaches the preset rated capacitor voltage value, the receiving-end DC controller is unlocked so that the receiving-end DC controller controls the DC voltage of the receiving-end MMC until it reaches the preset rated DC voltage value.

[0117] Step 604: When the receiving-end MMC meets the second preset rated voltage condition, the power support capability of the receiving-end MMC is verified by the power grid simulation MMC to obtain the active support test results of the receiving-end MMC.

[0118] Based on the aforementioned steps, when the receiving-end MMC meets the second preset rated voltage condition, the power support capability of the receiving-end MMC is verified by the grid simulation MMC, and the active support test results of the receiving-end MMC are obtained.

[0119] In specific implementation, when the receiving-end MMC meets the second preset rated voltage condition, the process of verifying the power support capability of the receiving-end MMC through the grid-simulated MMC and obtaining the active support test results of the receiving-end MMC can include: when the capacitor voltage of the receiving-end MMC reaches the preset rated capacitor voltage value and the DC voltage of the receiving-end MMC is stable at the preset rated DC voltage value, the power support capability of the receiving-end MMC is verified through the grid-simulated MMC based on a step control strategy, and the active and reactive power outputs of the receiving-end MMC, as well as the voltage amplitude of the grid coupling point of the flexible DC and AC grid connection are obtained; the active power output, reactive power output, and voltage amplitude are integrated as the active support test results of the receiving-end MMC.

[0120] Furthermore, the power support capability of the receiving-end MMC is verified through a step control strategy using a grid-simulated MMC. The specific implementation process for obtaining the active and reactive power outputs of the receiving-end MMC, as well as the voltage amplitude at the grid coupling point of flexible DC and AC grid connections, can include:

[0121] First, the active power support capability is verified: the power balance variable step control is performed by the grid simulation MMC to simulate the power imbalance inside the AC grid, and the active power support capability of the receiving-end MMC is verified to obtain the active power output of the receiving-end MMC; in the process of verifying the active power support capability, the active power used by the receiving-end MMC to respond to the frequency change on the AC side comes entirely from its own capacitor energy storage.

[0122] Next, the reactive power support capability is verified: after a preset time interval, the power balance variable is reset. When the system frequency no longer changes further, the voltage amplitude step control is executed through the grid simulation MMC to simulate the AC grid voltage sag. The reactive power support capability of the receiving-end MMC is verified to obtain the reactive power output of the receiving-end MMC and the voltage amplitude of the grid coupling point of the flexible DC and AC grid connection.

[0123] Based on the above implementation steps, this embodiment of the invention provides a logically clear and well-organized verification scheme for the active support function of a flexible DC receiving-end converter. This scheme not only features simple steps and optimized processes, but also convenient operation and low implementation cost, ensuring the efficiency of the verification process and the accuracy of the results.

[0124] This invention proposes an active support test method for flexible DC receiving-end converters. Based on previous embodiments, in terms of wiring, this invention requires only two MMCs and one low-cost grid-commutated converter LCC to verify the active support function of the receiving-end MMC under a grid control mode using capacitor voltage synchronization. The system topology is simple, the test efficiency is high, and the computational cost is low. In terms of control, the control structure of the grid-commutated converter LCC and the two MMCs is simple, enabling the verification of the active support function of the receiving-end MMC. Regarding the verification implementation scheme, this invention provides a logically clear and well-organized verification scheme for the active support function of flexible DC. This scheme not only features simple steps and optimized processes but also convenient operation and low implementation cost, ensuring the efficiency of the verification process and the accuracy of the results.

[0125] For better explanation, refer to Figure 7 This diagram illustrates the overall flow of an active support test method for a flexible DC receiving-end converter under a STATCOM connection configuration, as provided in an embodiment of the present invention. It should be noted that this embodiment only provides a brief overview of the general flow of the active support test for the flexible DC receiving-end converter. The specific implementation process of each step can be understood by referring to the relevant content in the foregoing embodiments, and will not be elaborated upon here. It is understood that the present invention does not impose any limitations on this.

[0126] Step 1: Close the LCC AC circuit breaker, unlock the LCC, and power on the system. Unlocking the LCC provides DC voltage to the entire system, acting as a DC power balancing node. DC resistance. It serves as a current limiter; the power grid simulation MMC passes through... Complete uncontrolled charging. Once the capacitor voltage of the grid-simulated MMC reaches and stabilizes at the uncontrolled charging voltage, close the bypass switch. Bypass the current-limiting resistor and proceed to step 2.

[0127] Step 2: Unlock the DC controller of the grid simulation MMC and gradually increase the capacitor voltage. At this time, the AC controller of the grid simulation MMC remains locked, and the AC voltage output is 0. After the capacitor of the grid simulation MMC is charged to its rated voltage, proceed to Step 3.

[0128] Step 3: Control the grid simulation MMC to unlock its AC controller and gradually increase the AC voltage amplitude. After the grid simulation MMC increases the AC voltage amplitude to the rated value and the voltage at the PCC coupling point between the flexible DC side and the AC grid stabilizes, proceed to Step 4.

[0129] Step 4: Control the receiving-end MMC to unlock its AC controller. The receiving-end MMC operates in capacitor voltage synchronization mode, so the energy flowing into the receiving-end MMC can be controlled by adjusting the phase of its own output voltage to charge the capacitor voltage. After the capacitor voltage of the receiving-end MMC is charged to the rated value, proceed to step 5.

[0130] Step 5: Unlock the DC controller of the receiving-end MMC. The control objective of the DC-side controller of the receiving-end MMC is to control the DC voltage. After the DC voltage controlled by the receiving-end MMC reaches the rated value and stabilizes, proceed to step 6.

[0131] Step 6: In the controller of the power grid simulation MMC, the power balance variable inside the simulated AC power grid is... A step test is used to simulate power imbalance within the AC power grid and to measure the active power output of the receiving-end MMC. However, the receiving-end MMC has no active power support on the DC side. Therefore, the active power used by the receiving-end MMC to respond to frequency changes on the AC side comes entirely from its own capacitor energy storage. Since the capacitor energy storage of the receiving-end MMC is relatively limited, it is difficult to absorb or release power for an extended period. Therefore, it must be reset after a short time. To prevent the system frequency from changing further, proceed to step 7.

[0132] Step 7: In the controller of the power grid simulation MMC, the voltage amplitude is... A step test is performed to simulate a voltage dip in the AC power grid, and the reactive power output of the receiving-end MMC and the voltage amplitude at the PCC point are measured.

[0133] Reference Figure 8 This diagram illustrates a structural block diagram of an active support test device for a flexible DC receiving-end converter according to an embodiment of the present invention. The flexible DC converter includes a grid-commutated converter, a grid-simulated MMC, and a receiving-end MMC; the device specifically may include:

[0134] The uncontrolled charging unit 801 is used to control the unlocking of the grid phase-commutation converter and perform uncontrolled charging on the grid simulated MMC.

[0135] The first unlocking control strategy execution unit 802 is used to execute the first unlocking control strategy on the grid-simulated MMC after the capacitor voltage of the grid-simulated MMC reaches the stability of the uncontrolled charging voltage.

[0136] The second unlocking control strategy execution unit 803 is used to execute the second unlocking control strategy on the receiving end MMC when the grid simulation MMC meets the first preset rated voltage condition.

[0137] The power support capability verification unit 804 is used to verify the power support capability of the receiving-end MMC through the power grid simulation MMC when the receiving-end MMC meets the second preset rated voltage condition, and obtain the active support test results of the receiving-end MMC.

[0138] In one optional embodiment, the power grid simulation MMC includes an analog DC controller and an analog AC controller; the first unlocking control strategy execution unit 802 includes:

[0139] The analog DC controller unlock control unit is used to control the unlocking of the analog DC controller and gradually increase the capacitor voltage of the grid analog MMC;

[0140] The analog AC controller unlock control unit is used to unlock the analog AC controller when the capacitor voltage of the analog MMC reaches the preset capacitor voltage rating value, and gradually increase the AC voltage of the analog MMC until it reaches the preset AC voltage rating value.

[0141] In one optional embodiment, the receiving-end MMC includes a receiving-end DC controller and a receiving-end AC controller; the second unlocking control strategy execution unit 803 includes:

[0142] The receiving-end AC controller unlocking control unit is used to unlock the receiving-end AC controller when the capacitor voltage of the grid simulated MMC reaches a preset capacitor voltage rating, the AC voltage of the grid simulated MMC reaches a preset AC voltage rating, and the voltage at the grid coupling point of the flexible DC and AC grid reaches stability, so that the receiving-end AC controller can charge the receiving-end MMC based on the capacitor voltage synchronous operation mode.

[0143] The receiving-end DC controller unlocking control unit is used to unlock the receiving-end DC controller when the capacitor voltage of the receiving-end MMC reaches a preset rated capacitor voltage value, so that the receiving-end DC controller controls the DC voltage of the receiving-end MMC until it reaches the preset rated DC voltage value.

[0144] In one optional embodiment, the power support capability verification unit 804 includes:

[0145] The step control strategy execution unit is used to verify the power support capability of the receiving-end MMC based on the step control strategy through the grid simulation MMC when the capacitor voltage of the receiving-end MMC reaches the preset capacitor voltage rating value and the DC voltage of the receiving-end MMC is stable at the preset DC voltage rating value, thereby obtaining the active and reactive power output of the receiving-end MMC and the voltage amplitude of the grid coupling point of the flexible DC and AC grid connection.

[0146] The active support test result integration unit is used to integrate the active output, the reactive output, and the voltage amplitude as the active support test result of the receiving-end MMC.

[0147] In one optional embodiment, the step control strategy execution unit includes:

[0148] The active power support capability verification unit is used to perform power balance variable step control through the power grid simulation MMC to simulate the power imbalance inside the AC power grid, and to verify the active power support capability of the receiving-end MMC to obtain the active power output of the receiving-end MMC; wherein, when performing active power support capability verification, the active power of the receiving-end MMC in response to AC side frequency changes comes entirely from its own capacitor energy storage.

[0149] The reactive power support capability verification unit is used to reset the power balance variable after a preset time interval. When the system frequency no longer changes further, it performs voltage amplitude step control through the grid simulation MMC to simulate the AC grid voltage sag and verify the reactive power support capability of the receiving-end MMC. It obtains the reactive power output of the receiving-end MMC and the voltage amplitude of the grid coupling point of the flexible DC and AC grid connection.

[0150] In one optional embodiment, the DC side of the grid-commutated converter is connected to the DC side of the grid-simulated MMC; the AC side of the grid-simulated MMC is connected to the AC side of the receiving-end MMC via a transformer; and the AC side of the grid-commutated converter is connected to the AC power grid.

[0151] In one optional embodiment, the grid-connected phase converter adopts a constant firing angle control mode; the AC side of the grid-simulated MMC adopts a constant voltage / frequency control mode and adds an inertia simulation stage; the DC side of the grid-simulated MMC adopts a capacitor voltage outer loop-DC current inner loop control mode; and the receiving-end MMC adopts a grid-connected control mode with capacitor voltage synchronization.

[0152] As the device embodiment is basically similar to the method embodiment, it is described in a relatively simple way. For relevant details, please refer to the description of the method embodiment above.

[0153] It should be noted that, in order to enable those skilled in the art to better distinguish data of the same type but with different actual meanings, the embodiments of the present invention use "first" and "second" to distinguish and describe some technical features. "First" and "second" are only used to distinguish data and have no other special meaning. It is understood that the present invention does not impose any limitations on them.

[0154] This invention also provides an electronic device, which includes a processor and a memory:

[0155] The memory is used to store program code and transfer the program code to the processor;

[0156] The processor is used to execute the active support test method for the flexible DC receiving-end converter according to the instructions in the program code of any embodiment of the present invention.

[0157] This invention also provides a computer-readable storage medium for storing program code for executing the active support test method for a flexible DC receiving-end converter according to any embodiment of this invention.

[0158] Those skilled in the art will clearly understand that, for the sake of convenience and brevity, the specific working processes of the systems, devices, and units described above can be referred to the corresponding processes in the foregoing method embodiments, and will not be repeated here.

[0159] It should be noted that the user information (including but not limited to user device information, user personal information, etc.) and data (including but not limited to data used for analysis, data stored, data displayed, etc.) involved in this invention are all information and data authorized by the user or fully authorized by all parties. Furthermore, the collection, use and processing of related data must comply with the relevant laws, regulations and standards of the relevant countries and regions, and corresponding operation entry points are provided for users to choose to authorize or refuse.

[0160] In the embodiments provided by this invention, it should be understood that the disclosed systems, apparatuses, and methods can be implemented in other ways. For example, the apparatus embodiments described above are merely illustrative; for instance, the division of units is only a logical functional division, and in actual implementation, there may be other division methods. For example, multiple units or components may be combined or integrated into another system, or some features may be ignored or not executed. Furthermore, the coupling or direct coupling or communication connection shown or discussed may be an indirect coupling or communication connection between devices or units through some interfaces, and may be electrical, mechanical, or other forms.

[0161] The units described as separate components may or may not be physically separate. The components shown as units may or may not be physical units; that is, they may be located in one place or distributed across multiple network units. Some or all of the units can be selected to achieve the purpose of this embodiment according to actual needs.

[0162] Furthermore, the functional units in the various embodiments of the present invention can be integrated into one processing unit, or each unit can exist physically separately, or two or more units can be integrated into one unit. The integrated unit can be implemented in hardware or as a software functional unit.

[0163] If the integrated unit is implemented as a software functional unit and sold or used as an independent product, it can be stored in a computer-readable storage medium. Based on this understanding, the technical solution of the present invention, in essence, or the part that contributes to the prior art, or all or part of the technical solution, can be embodied in the form of a software product. This computer software product is stored in a storage medium and includes several instructions to cause a computer device (which may be a personal computer, server, or network device, etc.) to execute all or part of the steps of the methods described in the various embodiments of the present invention. The aforementioned storage medium includes various media capable of storing program code, such as USB flash drives, portable hard drives, read-only memory (ROM), random access memory (RAM), magnetic disks, or optical disks.

[0164] The above-described embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A test method for active support of a flexible DC receiving-end converter, characterized in that, Flexible DC transmission includes a grid-commutated converter, a grid-simulated MMC, and a receiving-end MMC; the method includes: Control the unlocking of the grid commutation converter to perform uncontrolled charging of the grid simulated MMC; Once the capacitor voltage of the grid-simulated MMC reaches the stable uncontrolled charging voltage, the first unlocking control strategy is executed on the grid-simulated MMC. When the grid simulation MMC meets the first preset rated voltage condition, the second unlocking control strategy is executed on the receiving end MMC; When the receiving-end MMC meets the second preset rated voltage condition, the power support capability of the receiving-end MMC is verified by the power grid simulation MMC, and the active support test results of the receiving-end MMC are obtained.

2. The active support test method for the flexible DC receiving-end converter according to claim 1, characterized in that, The power grid simulation MMC includes an analog DC controller and an analog AC controller; the execution of the first unlocking control strategy on the power grid simulation MMC includes: The analog DC controller is unlocked, and the capacitor voltage of the analog MMC of the power grid is gradually increased; When the capacitor voltage of the simulated MMC reaches the preset rated capacitor voltage value, the simulated AC controller is unlocked, and the AC voltage of the simulated MMC is gradually increased until it reaches the preset rated AC voltage value.

3. The active support test method for the flexible DC receiving-end converter according to claim 1, characterized in that, The receiving-end MMC includes a receiving-end DC controller and a receiving-end AC controller; when the grid simulation MMC meets the first preset rated voltage condition, the second unlocking control strategy is executed on the receiving-end MMC, including: When the capacitor voltage of the grid-simulated MMC reaches the preset capacitor voltage rating, the AC voltage of the grid-simulated MMC reaches the preset AC voltage rating, and the voltage at the grid coupling point of the flexible DC and AC grid reaches stability, the receiving-end AC controller is unlocked so that the receiving-end AC controller can charge the receiving-end MMC based on the capacitor voltage synchronous operation mode. When the capacitor voltage of the receiving-end MMC reaches the preset rated capacitor voltage value, the receiving-end DC controller is unlocked so that the receiving-end DC controller controls the DC voltage of the receiving-end MMC until it reaches the preset rated DC voltage value.

4. The active support test method for the flexible DC receiving-end converter according to claim 1, characterized in that, When the receiving-end MMC meets the second preset rated voltage condition, the power support capability of the receiving-end MMC is verified through the power grid simulated MMC to obtain the active support test results of the receiving-end MMC, including: When the capacitor voltage of the receiving-end MMC reaches the preset capacitor voltage rating and the DC voltage of the receiving-end MMC is stable at the preset DC voltage rating, the power support capability of the receiving-end MMC is verified by the grid simulation MMC based on the step control strategy, and the active and reactive outputs of the receiving-end MMC, as well as the voltage amplitude of the grid coupling point of the flexible DC and AC grid connection are obtained. The active power output, reactive power output, and voltage amplitude are integrated to form the active support test results of the receiving-end MMC.

5. The active support test method for the flexible DC receiving-end converter according to claim 4, characterized in that, The process of verifying the power support capability of the receiving-end MMC using the grid-simulated MMC based on a step control strategy, obtaining the active and reactive power outputs of the receiving-end MMC, and the voltage amplitude at the grid coupling point of the flexible DC and AC grid connection, includes: The power balance variable step control is performed by the power grid simulation MMC to simulate the power imbalance inside the AC power grid, and the active power support capability of the receiving-end MMC is verified to obtain the active power output of the receiving-end MMC. In the process of verifying the active power support capability, the active power of the receiving-end MMC used to respond to the frequency change on the AC side comes entirely from its own capacitor energy storage. After a preset time interval, the power balance variable is reset. When the system frequency no longer changes further, the voltage amplitude step control is performed through the grid simulation MMC to simulate the AC grid voltage sag. The reactive power support capability of the receiving-end MMC is verified, and the reactive power output of the receiving-end MMC and the voltage amplitude of the grid coupling point of the flexible DC and AC grid are obtained.

6. The active support test method for a flexible DC receiving-end converter according to any one of claims 1 to 5, characterized in that, The DC side of the grid-commutated converter is connected to the DC side of the grid-simulated MMC; the AC side of the grid-simulated MMC is connected to the AC side of the receiving-end MMC via a transformer; the AC side of the grid-commutated converter is connected to the AC power grid.

7. The active support test method for a flexible DC receiving-end converter according to claim 6, characterized in that, The grid-connected phase converter adopts a constant firing angle control mode; the AC side of the grid-simulated MMC adopts a constant voltage / frequency control mode and adds an inertia simulation stage; the DC side of the grid-simulated MMC adopts a capacitor voltage outer loop-DC current inner loop control mode; and the receiving-end MMC adopts a grid-connected control mode with capacitor voltage synchronization.

8. An active support test device for a flexible DC receiving-end converter, characterized in that, Flexible DC transmission includes a grid-commutated converter, a grid-simulated MMC, and a receiving-end MMC; the device includes: An uncontrolled charging unit is used to control the unlocking of the grid commutation converter and perform uncontrolled charging on the grid simulated MMC; The first unlocking control strategy execution unit is used to execute the first unlocking control strategy on the grid-simulated MMC after the capacitor voltage of the grid-simulated MMC reaches the stability of the uncontrolled charging voltage. The second unlocking control strategy execution unit is used to execute the second unlocking control strategy on the receiving end MMC when the grid simulation MMC meets the first preset rated voltage condition. The power support capability verification unit is used to verify the power support capability of the receiving-end MMC through the power grid simulation MMC when the receiving-end MMC meets the second preset rated voltage condition, and to obtain the active support test results of the receiving-end MMC.

9. An electronic device, characterized in that, The device includes a processor and a memory: The memory is used to store program code and transmit the program code to the processor; The processor is used to execute the active support test method for the flexible DC receiving-end converter according to any one of the claims 1-7, based on the instructions in the program code.

10. A computer-readable storage medium, characterized in that, The computer-readable storage medium is used to store program code for executing the active support test method for the flexible DC receiving-end converter according to any one of claims 1-7.

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