A method and device for active support test of flexible direct current

Abstract

This invention discloses an active support test method and apparatus for flexible DC transmission, addressing the current technical problem of lacking verification of the active support function of grid-connected flexible DC transmission for the power grid based on actual flexible DC transmission. The flexible DC transmission includes a grid-commutated converter, a grid-simulated MMC, and a tested MMC. The method involves controlling and unlocking the grid-commutated converter to perform uncontrolled charging on both the grid-simulated MMC and the tested MMC. Once the capacitor voltages of both the grid-simulated MMC and the tested MMC reach the stable uncontrolled charging voltage, a first unlocking control strategy is implemented on the grid-simulated MMC, while a second unlocking control strategy is implemented on the tested MMC. When a preset rated voltage condition is detected, the tested MMC is switched to grid-connected operation mode. When the tested MMC enters steady-state operation in grid-connected operation mode, the power support capability of the tested MMC is verified through the grid-simulated MMC, obtaining the active support test results of the flexible DC transmission.

Description

Technical Field

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

[0002] Flexible DC transmission based on MMC (Modular Multilevel Converter) (MMC-based High Voltage DC, MMC-HVDC) and hybrid DC transmission have become one of the important ways to meet the demand for large-scale renewable energy transmission. Conventional MMC converters typically obtain the amplitude and phase of the grid voltage based on a phase-locked loop (PLL). The active and reactive power transmitted is then controlled by controlling the current injected into the grid. The reference values ​​for active and reactive power in the MMC are usually given by dispatch or aimed at transmitting all renewable energy generation power. This control method has a simple structure and high technological maturity, and is currently the mainstream control strategy for flexible DC transmission.

[0003] However, on the one hand, grid-based control strategies using phase-locked loops (PLLs) are entirely controlled by the grid, both in terms of voltage amplitude and phase. When the system experiences power deficits or temporary surpluses, MMCs and flexible DC systems struggle to proactively adjust their transmitted power to provide short-term inertia support, making it difficult to mitigate the rate of frequency changes. Furthermore, during system faults, MMC converters typically struggle to maintain the voltage amplitude at their point of common coupling (PCC) with the AC grid, potentially leading to further fault expansion. Moreover, existing research indicates that in weak grid scenarios, the nonlinear characteristics of the PLL itself may cause static instability in the converter under certain operating conditions, making grid-based MMCs unsuitable for operation in weak grid environments.

[0004] To date, while numerous studies have discussed solutions for how flexible DC (including MMC converters themselves and flexible DC systems) can support AC power grids, most of these approaches have validated their solutions through simulations or small-capacity experimental prototypes. However, real-world flexible DC systems are typically high-voltage, high-capacity systems. Their system complexity and control logic differ significantly from simulations and small-capacity experimental prototypes. Therefore, the currently employed active support testing schemes are difficult to apply to real-world flexible DC support capability verification scenarios. Summary of the Invention

[0005] This invention provides an active support test method for flexible DC, an active support test device for flexible DC, an electronic device, and a storage medium, which are used to solve or partially solve the current technical problem of lacking verification of the active support function of grid-type flexible DC for the power grid based on actual flexible DC.

[0006] This invention provides an active support test method for flexible DC transmission, wherein the flexible DC transmission includes a grid-commutated converter, a grid-simulated MMC, and the MMC under test; the method includes:

[0007] Control the unlocking of the grid phase-commutation converter to perform uncontrolled charging on the grid simulated MMC and the tested MMC;

[0008] Once the capacitor voltages of both the grid-simulated MMC and the tested MMC reach a stable uncontrolled charging voltage, a first unlocking control strategy is executed on the grid-simulated MMC, while a second unlocking control strategy is executed on the tested MMC.

[0009] When the preset rated voltage condition is met, the control switches the tested MMC to network operation mode;

[0010] When the tested MMC enters steady-state operation in the grid-connected operation mode, the power support capability of the tested MMC is verified by the grid-simulated MMC, and the active support test results of the flexible DC are obtained.

[0011] 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:

[0012] The analog DC controller is unlocked, and the capacitor voltage of the analog MMC of the power grid is gradually increased;

[0013] 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.

[0014] Optionally, the MMC under test includes a DC controller under test and an AC controller under test; the execution of the second unlocking control strategy on the MMC under test includes:

[0015] The controller unlocks the DC controller under test and gradually increases the capacitor voltage of the MMC under test until it reaches the preset capacitor voltage rating.

[0016] While unlocking the DC controller under test, the AC controller under test is also unlocked, so that the AC controller under test gradually increases the output voltage of the MMC under test to the preset AC voltage rating value based on the grid-connected operation mode.

[0017] Optionally, the MMC under test includes a test AC controller; the step of controlling the MMC under test to switch to grid-connected operation mode when a preset rated voltage condition is detected includes:

[0018] When the AC voltage of the simulated MMC and the output voltage of the tested MMC both reach the preset AC voltage rating, the capacitor voltage of the tested MMC reaches the preset capacitor voltage rating, and the voltage at the grid coupling point of the flexible DC and AC grids reaches stability, the controller switches the tested AC controller from grid-following operation mode to grid-connected operation mode.

[0019] Optionally, the step of verifying the power support capability of the tested MMC through the grid-simulated MMC to obtain the active support test results of the flexible DC includes:

[0020] 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 MMC under test is verified to obtain the active power output of the MMC under test.

[0021] The voltage amplitude step control is performed by the grid simulation MMC to simulate the AC grid voltage sag, and the reactive power support capability of the tested MMC is verified to obtain the reactive power output of the tested MMC and the voltage amplitude of the grid coupling point of the flexible DC and AC grid connection.

[0022] The active power output, reactive power output, and voltage amplitude are integrated to form the active support test results of the flexible DC transmission.

[0023] Optionally, the DC side of the grid-commutated converter, the DC side of the grid-simulated MMC, and the DC side of the MMC under test are connected in parallel; the AC side of the grid-simulated MMC and the AC side of the MMC under test are connected through a transformer; and the AC side of the grid-commutated converter is connected to the AC power grid.

[0024] 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 MMC under test adopts a virtual synchronous machine control mode.

[0025] This invention also provides an active support test device for flexible DC transmission, the flexible DC transmission including a grid commutation converter, a grid simulated MMC, and the MMC under test; the device includes:

[0026] An uncontrolled charging unit is used to control the unlocking of the grid commutation converter and to perform uncontrolled charging on the grid simulated MMC and the tested MMC.

[0027] The unlocking control strategy execution unit is used to execute a first unlocking control strategy on the grid-simulated MMC and a second unlocking control strategy on the MMC under test after the capacitor voltages of both the grid-simulated MMC and the tested MMC have reached the stability of the uncontrolled charging voltage.

[0028] The network operation mode conversion unit is used to control the test MMC to switch to network operation mode when a preset rated voltage condition is detected.

[0029] The power support capability verification unit is used to verify the power support capability of the tested MMC through the grid-simulated MMC when the tested MMC enters steady-state operation in the grid-connected operation mode, and to obtain the active support test results of the flexible DC.

[0030] The present invention also provides an electronic device, the device comprising a processor and a memory:

[0031] The memory is used to store program code and transmit the program code to the processor;

[0032] The processor is used to execute the active support test method for flexible DC as described above, according to the instructions in the program code.

[0033] The present invention also provides a computer-readable storage medium for storing program code for performing the active support test method for flexible DC as described in any of the preceding claims.

[0034] As can be seen from the above technical solutions, the present invention has the following advantages:

[0035] A method for active support testing of flexible DC transmission is provided. The flexible DC transmission includes a grid-commutated converter, a grid-simulated MMC, and a tested MMC. When it is necessary to verify the active support function of the tested MMC, the grid-commutated converter is unlocked, and uncontrolled charging is performed on both the grid-simulated MMC and the tested MMC. After the capacitor voltages of both the grid-simulated MMC and the tested MMC reach the stable uncontrolled charging voltage, a first unlocking control strategy is executed on the grid-simulated MMC, and a second unlocking control strategy is executed on the tested MMC. When a preset rated voltage condition is detected, the tested MMC is switched to grid-connected operation mode. When the tested MMC enters steady-state operation in grid-connected operation mode, the power support capability of the tested MMC is verified through the grid-simulated MMC, and the active support test results of the flexible DC transmission are obtained. 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 flexible DC-side MMC over a relatively long time scale. This solution not only features simple steps and optimized processes, but also is easy to operate and has low implementation costs, ensuring the efficiency of the verification process and the accuracy of the results. Attached Figure Description

[0036] 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.

[0037] Figure 1 This is a simplified connection diagram of the power offset wiring method used in the embodiments of the present invention;

[0038] Figure 2 A schematic diagram of the control strategy used for grid simulation MMC;

[0039] Figure 3 A schematic diagram of the control strategy used by the tested MMC;

[0040] Figure 4 A flowchart illustrating the steps of an active support test method for flexible DC transmission.

[0041] Figure 5 This is a schematic diagram of the overall process of an active support test method for flexible DC.

[0042] Figure 6 This is a structural block diagram of a flexible DC active support test device. Detailed Implementation

[0043] This invention provides an active support test method for flexible DC, an active support test device for flexible DC, an electronic device, and a storage medium, which are used to solve or partially solve the current technical problem of lacking verification of the active support function of grid-type flexible DC for the power grid based on actual flexible DC.

[0044] 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.

[0045] 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:

[0046] 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.

[0047] 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.

[0048] Step: At a certain moment, a variable in the system (such as power) suddenly undergoes a fixed numerical change.

[0049] Power balance variable step: During the operation of a simulated AC power grid, the internal power balance variable ΔP... G A sudden, fixed-amplitude change is applied to study the behavior and response of the power grid after being subjected to such a disturbance.

[0050] 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 simulated MMC to test the dynamic response and stability of the MMC controller under test. In power grid simulation, this operation helps verify the rationality of the design of the MMC controller under test and whether it can cope with sudden voltage change demands in actual operation, thereby ensuring the stability and reliability of the power grid.

[0051] As an example, while numerous studies have discussed solutions for how flexible DC (including the MMC converter itself and the flexible DC system) can support the AC power grid, most of these methods have validated the proposed solutions through simulations or small-capacity experimental prototypes. However, actual flexible DC systems are typically high-voltage, high-capacity systems. Their system complexity and control logic differ significantly from simulations and small-capacity experimental prototypes. Therefore, the currently adopted active support testing schemes are difficult to apply to actual flexible DC support capability verification scenarios.

[0052] In recent years, grid-based control has gained widespread attention due to its significant advantages in improving grid stability and promoting renewable energy integration. To date, various grid-based control methods have been proposed, including Power Synchronous Control (PSC), Virtual Synchronous Generator (VSG) control, and Capacitor Voltage Synchronous Control (CVSC). In grid-based control, the AC side of the MMC can be equivalent to a voltage source with internal impedance. On one hand, it regulates active power by adjusting the phase difference between its internal electromotive force and the grid voltage. On the other hand, the MMC can use a voltage control outer loop for synchronization based on capacitor voltage or active power, and use a voltage outer loop-current inner loop control structure to support the PCC point voltage. Currently, a scientific and feasible experimental method is urgently needed to verify the active support function of grid-based flexible DC for the AC grid.

[0053] Therefore, one of the core inventive points of this invention is to provide a flexible DC active support test method based on a power offset wiring method. 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 MMC using the virtual synchronous machine VSG control mode is completed over a relatively long time scale.

[0054] This invention proposes a verification scheme for flexible DC active support based on a power offset wiring method, which mainly includes three parts: wiring method, control strategy, and verification implementation scheme.

[0055] Reference Figure 1 The diagram shows a simplified connection schematic of the power offset wiring method used in the embodiment of the present invention.

[0056] like Figure 1 As shown, the flexible DC side mainly includes a thyristor-based line commutated converter (LCC), a grid simulation MMC, and a test MMC. The AC side of the grid simulation MMC is connected to the AC side of the test MMC via a transformer. Their DC sides are directly connected. Furthermore, the DC side of the line commutated converter is connected to the DC sides of these two MMCs to compensate for power losses caused by MMC operation.

[0057] 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.

[0058] 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 MMC in the virtual synchronous machine (VSG) control mode. The system topology is simple, the test efficiency is high, and the computational cost is low.

[0059] For control strategies, the grid-commutated converter is the power balancing node of the entire DC system. Therefore, the grid-commutated converter can adopt a constant firing angle control mode.

[0060] 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.

[0061] In practice, the control objective of a power grid simulation MMC is to control the amplitude and frequency of its port AC voltage under a given DC voltage. This simulates, to some extent, the inertia of an AC power grid and the transient characteristics that may occur, such as voltage dips and sags. Therefore, the AC side of the power grid simulation MMC can directly operate in voltage / frequency (V / f) mode. The AC voltage amplitude can be directly given based on the simulated voltage dips. Regarding the frequency, to simulate an inertial AC power grid, the frequency can be determined using the following formula:

[0062] (1)

[0063] 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 simulated MMC in the power grid to the measured MMC. This indicates that the generator power in the simulated power grid is greater than the load power within the power grid.

[0064] 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.

[0065] 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.

[0066] 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:

[0067] (2)

[0068] Among them, subscript and These represent the positive bridge arm and the negative bridge arm, respectively. It represents three phases.

[0069] Figure 3 A schematic diagram of the control strategy adopted by the tested MMC in an embodiment of the present invention is shown.

[0070] The tested MMC can be controlled using a Virtual Synchronizer (VSG). The frequency is generated according to the following formula:

[0071] (3)

[0072] in, and These are the set values ​​(reference values) for frequency and active power, respectively. It is the moment of inertia; The damping coefficient; The frequency of the VSG control output.

[0073] In terms of control, the control structure of the grid phase-changing converter LCC and the two MMCs is simple, and the active support function verification can be completed over a longer time scale.

[0074] Based on the content described in the foregoing embodiments, for verifying the implementation scheme, please refer to... Figure 4 This document illustrates a flowchart of an active support test method for flexible DC transmission provided by an embodiment of the present invention. The flexible DC transmission includes a grid-commutated converter, a grid-simulated MMC, and the MMC under test; the method specifically includes the following steps:

[0075] Step 401: Control the unlocking of the grid phase-commutation converter to perform uncontrolled charging on the grid simulated MMC and the tested MMC;

[0076] Based on the preceding discussion, on the flexible DC side, the DC side of the grid-commutated converter, the DC side of the grid-simulated MMC, and the DC side of the MMC under test are connected in parallel. The AC side of the grid-simulated MMC and the AC side of the MMC under test are connected through a transformer. The AC side of the grid-commutated converter is connected to the AC power grid.

[0077] 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 MMC under test uses a virtual synchronous machine control mode.

[0078] Therefore, in the specific implementation, when it is necessary to verify the active support function of the MMC under test, the grid commutator is first unlocked to perform uncontrolled charging on the grid simulated MMC and the MMC under test.

[0079] Step 402: When the capacitor voltages of both the grid-simulated MMC and the tested MMC reach the uncontrolled charging voltage stabilization, the first unlocking control strategy is executed on the grid-simulated MMC, and the second unlocking control strategy is executed on the tested MMC.

[0080] Once the capacitor voltages of both the grid-simulated MMC and the tested MMC have reached the uncontrolled charging voltage stabilization point, the first unlocking control strategy can be executed on the grid-simulated MMC, while the second unlocking control strategy can be executed on the tested MMC.

[0081] In some embodiments, the power grid simulation MMC includes a DC controller and an AC controller. To distinguish it from the controller of the MMC under test, the DC controller of the power grid simulation MMC is defined as an analog DC controller, and the AC controller is defined as an analog AC controller.

[0082] 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.

[0083] In some embodiments, the MMC under test 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 MMC under test is defined as the DC controller under test, and the AC controller is defined as the AC controller under test.

[0084] In a specific implementation, the process of executing the second unlocking control strategy on the MMC under test may include: controlling the unlocking of the DC controller under test and gradually increasing the capacitor voltage of the MMC under test until it reaches the preset capacitor voltage rating; while unlocking the DC controller under test, controlling the unlocking of the AC controller under test, so that the AC controller under test gradually increases the output voltage of the MMC under test to the preset AC voltage rating based on the grid-connected operation mode.

[0085] Step 403: When the preset rated voltage condition is detected, the control switches the tested MMC to network operation mode.

[0086] Based on the aforementioned steps, when the preset rated voltage condition is met, the control switches the tested MMC to grid-connected operation mode. Specifically, when both the AC voltage of the grid-simulated MMC and the output voltage of the tested MMC reach the preset AC voltage rated value, the capacitor voltage of the tested MMC reaches the preset capacitor voltage rated value, and the voltage at the grid coupling point of the flexible DC and AC grids reaches stability, the control switches the tested AC controller from grid-connected operation mode to grid-connected operation mode.

[0087] Step 404: When the tested MMC enters steady-state operation in the grid-connected operation mode, the power support capability of the tested MMC is verified by the grid-simulated MMC to obtain the active support test results of the flexible DC.

[0088] Based on the aforementioned steps, when the tested MMC enters steady-state operation under grid-connected operation mode, the power support capability of the tested MMC is verified by simulating the grid MMC, and the active support test results of flexible DC are obtained.

[0089] In specific implementation, the process of verifying the power support capability of the tested MMC through grid-simulated MMC and obtaining the active support test results of flexible DC can include:

[0090] On the one hand, it verifies the active power support capability, that is: by performing step control of the power balance variable through the grid simulation MMC to simulate the power imbalance inside the AC grid, the active power support capability of the MMC under test is verified and the active power output of the MMC under test is obtained.

[0091] On the other hand, the reactive power support capability is verified, namely: by performing voltage amplitude step control through the grid simulated MMC to simulate the AC grid voltage sag, the reactive power support capability of the tested MMC is verified, and the reactive power output of the tested MMC and the voltage amplitude of the grid coupling point of the flexible DC and AC grid are obtained.

[0092] Finally, the active power output, reactive power output, and voltage amplitude can be integrated as the results of the active support test for flexible DC.

[0093] It should be noted that the active power support capability verification and the reactive power support capability verification are completely decoupled, and their order can be reversed. That is, if the execution order is set, the active power support capability verification can be performed first, followed by the reactive power support capability verification. Alternatively, the reactive power support capability verification can be performed first, followed by the active power support capability verification. If no special execution order is required, the two can also be executed independently in parallel. It is understood that this invention does not impose any limitations on this.

[0094] 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 flexible DC transmission. 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.

[0095] This invention proposes an active support testing method for flexible DC transmission. Based on previous embodiments, in terms of wiring, this invention requires only two Multi-Mode Converters (MMCs) and one low-cost grid-commutated converter (LCC) to verify the active support function of the MMCs under Virtual Synchronous Generator (VSG) control mode. The system topology is simple, the testing efficiency is high, and the computational cost is low. In terms of control, due to the simple control structure of the grid-commutated converter (LCC) and the two MMCs, active support function verification over a longer timescale can be completed. Regarding the verification implementation scheme, this invention provides a logically clear and well-organized verification scheme for the active support function of flexible DC transmission. 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.

[0096] For better explanation, refer to Figure 5This diagram illustrates the overall flow of an active support test method for flexible DC based on power offset wiring, according to 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 flexible DC. 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.

[0097] 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 flow limiter; the two MMCs pass through Uncontrolled charging is complete. Once the capacitor voltages of both MMCs have reached and stabilized at the uncontrolled charging voltage, the bypass switch is closed. Bypass the current-limiting resistor and proceed to step 2.

[0098] Step 2: Control the grid-simulated MMC to unlock its DC controller and gradually increase its capacitor voltage. At this time, the grid-simulated MMC's AC controller remains locked, and its AC voltage output is 0. Simultaneously, control the tested MMC to unlock its DC controller and gradually increase its capacitor voltage. At the same time, control the tested MMC to unlock its AC controller, but the AC side operates in grid-connected mode. Since the grid-simulated MMC's AC voltage output is 0 at this time, the tested MMC's AC voltage is also 0. After both MMCs' capacitors are charged to their rated voltage, proceed to Step 3.

[0099] Step 3: Unlock the AC controller of the grid-simulated MMC and gradually increase the amplitude of the AC voltage. Since the AC controller of the MMC under test is in grid-connected operation mode at this time, the output voltage of the MMC under test will also increase to the rated value. However, the power transmitted between the two is 0. After the grid-simulated MMC increases the AC voltage amplitude to the rated value and the voltage of the PCC at the grid coupling point between the flexible DC side and the AC grid stabilizes, proceed to step 4.

[0100] Step 4: Control the tested MMC to switch from follow-the-network operation mode to build-the-network operation mode. The network construction control strategy is as follows: Figure 3 As shown. To test the MMC's ability to support frequency variations as much as possible, the active power reference value is shown at this time. The initial value is 0. After entering steady-state operation, proceed to step 5.

[0101] Step 5: In the controller of the power grid simulation MMC, the power balance variable inside the simulated AC power grid is... A step test is performed to simulate power imbalance within the AC power grid, and the active power output of the tested MMC is measured. After the system returns to steady-state operation, step 6 is executed.

[0102] Step 6: 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 tested MMC and the voltage amplitude at the PCC point are measured.

[0103] Step 5 corresponds to verifying the active power support capability, and step 6 corresponds to verifying the reactive power support capability (i.e., voltage support capability). These two steps are completely decoupled, and their execution order can be interchanged.

[0104] Reference Figure 6 This diagram illustrates a structural block diagram of an active support test device for flexible DC transmission according to an embodiment of the present invention. The flexible DC transmission includes a grid commutation converter, a grid simulation MMC, and the MMC under test; the device may specifically include:

[0105] The uncontrolled charging unit 601 is used to control the unlocking of the grid phase-commutation converter and to perform uncontrolled charging on the grid simulated MMC and the tested MMC.

[0106] The unlocking control strategy execution unit 602 is used to execute a first unlocking control strategy on the grid-simulated MMC and a second unlocking control strategy on the MMC under test after the capacitor voltages of both the grid-simulated MMC and the tested MMC have reached the stability of the uncontrolled charging voltage.

[0107] The network operation mode conversion unit 603 is used to control the tested MMC to switch to network operation mode when a preset rated voltage condition is detected.

[0108] The power support capability verification unit 604 is used to verify the power support capability of the MMC under test through the grid simulation MMC when the MMC under test enters steady-state operation in the grid operation mode, and obtain the active support test results of the flexible DC.

[0109] In one embodiment, the power grid simulation MMC includes an analog DC controller and an analog AC controller; the unlocking control strategy execution unit 602 includes:

[0110] 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;

[0111] 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.

[0112] In one embodiment, the MMC under test includes a DC controller under test and an AC controller under test; the unlocking control strategy execution unit 602 includes:

[0113] The DC controller under test unlock control unit is used to control the unlocking of the DC controller under test and gradually increase the capacitor voltage of the MMC under test until it reaches the preset capacitor voltage rating.

[0114] The AC controller unlocking control unit is used to unlock the DC controller under test while simultaneously unlocking the AC controller under test, so that the AC controller under test gradually increases the output voltage of the MMC under test to a preset AC voltage rating based on the grid-connected operation mode.

[0115] In one embodiment, the MMC under test includes an AC controller under test; the network operation mode conversion unit 603 is specifically used for:

[0116] When the AC voltage of the simulated MMC and the output voltage of the tested MMC both reach the preset AC voltage rating, the capacitor voltage of the tested MMC reaches the preset capacitor voltage rating, and the voltage at the grid coupling point of the flexible DC and AC grids reaches stability, the controller switches the tested AC controller from grid-following operation mode to grid-connected operation mode.

[0117] In one embodiment, the power support capability verification unit 604 includes:

[0118] 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, verify the active power support capability of the MMC under test, and obtain the active power output of the MMC under test.

[0119] The reactive power support capability verification unit is used to perform voltage amplitude step control through the grid simulated MMC to simulate AC grid voltage sag, verify the reactive power support capability of the MMC under test, obtain the reactive power output of the MMC under test, and the voltage amplitude of the grid coupling point of the flexible DC and AC grid connection.

[0120] The test result integration unit is used to integrate the active power output, the reactive power output, and the voltage amplitude as the active support test results of the flexible DC.

[0121] In one embodiment, the DC side of the grid-commutated converter, the DC side of the grid-simulated MMC, and the DC side of the MMC under test are connected in parallel; the AC side of the grid-simulated MMC and the AC side of the MMC under test are connected through a transformer; and the AC side of the grid-commutated converter is connected to the AC power grid.

[0122] In one embodiment, the grid-commutated 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 MMC under test adopts a virtual synchronous machine control mode.

[0123] 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.

[0124] 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.

[0125] This invention also provides an electronic device, which includes a processor and a memory:

[0126] The memory is used to store program code and transfer the program code to the processor;

[0127] The processor is used to execute the active support test method for flexible DC according to the instructions in the program code of any embodiment of the present invention.

[0128] This invention also provides a computer-readable storage medium for storing program code for executing the active support test method for flexible DC in any embodiment of this invention.

[0129] 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.

[0130] 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.

[0131] 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.

[0132] 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.

[0133] 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.

[0134] 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.

[0135] 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 method of actively supporting a test of a flexible DC, characterized by, Flexible DC transmission includes a grid-commutated converter, a grid-simulated MMC, and a tested MMC; the method includes: Control the unlocking of the grid phase-commutation converter to perform uncontrolled charging on the grid simulated MMC and the tested MMC; Once the capacitor voltages of both the grid-simulated MMC and the tested MMC reach a stable uncontrolled charging voltage, a first unlocking control strategy is executed on the grid-simulated MMC, while a second unlocking control strategy is executed on the tested MMC. When the preset rated voltage condition is met, the control switches the tested MMC to network operation mode; When the tested MMC enters steady-state operation in the grid-connected operation mode, the power support capability of the tested MMC is verified by the grid-simulated MMC, and the active support test results of the flexible DC are obtained.

2. The active support test method of flexible DC according to claim 1, characterized by, 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 of flexible DC according to claim 1, characterized by, The tested MMC includes a tested DC controller and a tested AC controller; the execution of the second unlocking control strategy on the tested MMC includes: The controller unlocks the DC controller under test and gradually increases the capacitor voltage of the MMC under test until it reaches the preset capacitor voltage rating. While unlocking the DC controller under test, the AC controller under test is also unlocked, so that the AC controller under test gradually increases the output voltage of the MMC under test to the preset AC voltage rating value based on the grid-connected operation mode.

4. The active support test method of flexible DC of claim 1, wherein, The tested MMC includes a tested AC controller; the step of controlling the tested MMC to switch to network operation mode when a preset rated voltage condition is met includes: When the AC voltage of the simulated MMC and the output voltage of the tested MMC both reach the preset AC voltage rating, the capacitor voltage of the tested MMC reaches the preset capacitor voltage rating, and the voltage at the grid coupling point of the flexible DC and AC grids reaches stability, the controller switches the tested AC controller from grid-following operation mode to grid-connected operation mode.

5. The active support test method for flexible DC of claim 1, wherein, The process of verifying the power support capability of the tested MMC through the grid-simulated MMC, and obtaining the active support test results of the flexible DC transmission, 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 MMC under test is verified to obtain the active power output of the MMC under test. The voltage amplitude step control is performed by the grid simulation MMC to simulate the AC grid voltage sag, and the reactive power support capability of the tested MMC is verified to obtain the reactive power output of the tested MMC and the voltage amplitude of the grid coupling point of the flexible DC and AC grid connection. The active power output, reactive power output, and voltage amplitude are integrated to form the active support test results of the flexible DC transmission.

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

7. The active support test method of flexible DC according to claim 6, wherein The grid-connected 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 MMC under test adopts a virtual synchronous machine control mode.

8. A flexible HVDC active support test device, characterized by Flexible DC transmission includes a grid-commutated converter, a grid-simulated MMC, and a tested MMC; the device includes: An uncontrolled charging unit is used to control the unlocking of the grid commutation converter and to perform uncontrolled charging on the grid simulated MMC and the tested MMC. The unlocking control strategy execution unit is used to execute a first unlocking control strategy on the grid-simulated MMC and a second unlocking control strategy on the MMC under test after the capacitor voltages of both the grid-simulated MMC and the tested MMC have reached the stability of the uncontrolled charging voltage. The network operation mode conversion unit is used to control the test MMC to switch to network operation mode when a preset rated voltage condition is detected. The power support capability verification unit is used to verify the power support capability of the tested MMC through the grid-simulated MMC when the tested MMC enters steady-state operation in the grid-connected operation mode, and to obtain the active support test results of the flexible DC.

9. An electronic device, comprising: 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 flexible DC according to any one of claims 1-7 according to 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 flexible DC as described in any one of claims 1-7.

Images