Control strategy switching method, device and equipment for flexible direct current power transmission system

Abstract

The invention provides a control strategy switching method, device and equipment for a flexible direct-current power transmission system, belongs to the technical field of flexible direct-current power transmission control, is applied to the flexible direct-current power transmission system, and is characterized in that the flexible direct-current power transmission system is connected with an external alternating-current power grid through a tie line, and the method comprises the steps: obtaining the working condition of the tie line; determining whether the power grid state of the external AC power grid changes according to the working condition of the tie line, wherein the power grid state is used for indicating the power grid strength of the external AC power grid; under the condition that the power grid state changes, a target control strategy of the flexible direct-current power transmission system is determined according to the changed power grid state, and the target control strategy is used for indicating a control mode of a converter station of the flexible direct-current power transmission system; and switching the control strategy of the flexible DC power transmission system to the target control strategy. The self-adaptive switching control strategy based on the external power grid state can be realized, and the sensitivity and the reliability of the control strategy switching are improved.

Description

Technical Field

[0001] This application belongs to the technical field of flexible DC power transmission control, and specifically relates to a control strategy switching method, device, and equipment for a flexible DC power transmission system. Background Art

[0002] Modern new power systems are developing towards a highly power-electronic direction. A large number of traditional power electronic converter devices adopt grid-following control, lacking sufficient inertia support for the system. This change leads to a reduction in system inertia, a weakening of damping characteristics, and a challenge to the system's balancing ability. The grid structure is more complex, and the dynamic behavior is more diverse. It may trigger broadband oscillation problems and affect the stable operation of the system.

[0003] Currently, a flexible DC system uses a type of control strategy to control the operation of the flexible DC system, such as grid-forming control or grid-following control. Grid-forming control provides voltage and frequency support for the system by simulating the behavior of a synchronous generator and is suitable for weak grid or island operation scenarios; however, it may cause system oscillation or even instability in a strong grid environment. Grid-following control depends on the voltage reference signal of the external grid. Although it has good stability under strong grid conditions, its performance will significantly decline when the grid strength is low or a fault occurs. With the diversification of the application scenarios of flexible DC systems, a single control mode can no longer meet the requirements of complex working conditions, and it is necessary to explore a dynamic switching method for multi-mode control strategies. Summary of the Invention

[0004] This application provides a control strategy switching method, device, and equipment for a flexible DC power transmission system, which adaptively switches the control strategy based on the external grid state, improving the sensitivity and reliability of control strategy switching.

[0005] This application provides a control strategy switching method for a flexible DC power transmission system, which is applied to a flexible DC power transmission system. The flexible DC power transmission system is connected to an external AC grid through a tie line. The method includes: Obtain the operating conditions of the tie line; Determine whether the grid state of the external AC grid has changed according to the operating conditions of the tie line. The grid state is used to indicate the grid strength of the external AC grid; In the case where the grid state has changed, determine the target control strategy of the flexible DC power transmission system according to the changed grid state. The target control strategy is used to indicate the control mode of the converter station of the flexible DC power transmission system; Switch the control strategy of the flexible DC power transmission system to the target control strategy.

[0006] According to the control strategy switching method of the flexible DC power transmission system provided by the present application, determine the changed grid state. The grid state includes grid-connected state and off-grid state. The grid-connected state includes strong grid state and weak grid state. The grid strengths of the strong grid state, the weak grid state, and the off-grid state decrease in turn. The determination of the changed grid state includes: determining whether the changed grid state is the grid-connected state or the off-grid state according to the working condition of the tie line; when the changed grid state is the grid-connected state, obtaining the change of the electrical parameters at the connection point between the flexible DC power transmission system and the external AC grid; determining whether the changed grid state is the strong grid state or the weak grid state according to the working condition of the tie line and the change of the electrical parameters.

[0007] According to the control strategy switching method of the flexible DC power transmission system provided by the present application, the target control strategy includes a grid-following control strategy or a grid-forming control strategy. The determination of the target control strategy of the flexible DC power transmission system according to the changed grid state includes: when the changed grid state is the strong grid state, determining the target control strategy as the grid-following control strategy; when the changed grid state is the weak grid state or the off-grid state, determining the target control strategy as the grid-forming control strategy.

[0008] According to the control strategy switching method of the flexible DC power transmission system provided by the present application, when the changed grid state is the weak grid state or the off-grid state, the determination of the target control strategy of the flexible DC power transmission system according to the changed grid state includes: when the changed grid state is the weak grid state, updating the virtual impedance and the inertia time constant of the grid-forming control algorithm in the grid-forming control strategy to the first target virtual impedance and the first inertia time constant respectively to obtain the target control strategy; when the changed grid state is the off-grid state, updating the virtual impedance and the inertia time constant of the grid-forming control algorithm in the grid-forming control strategy to the second target virtual impedance and the second inertia time constant respectively to obtain the target control strategy. The first target virtual impedance is greater than the second target virtual impedance, and the first inertia time constant is less than the second inertia time constant.

[0009] According to the control strategy switching method of the flexible DC transmission system provided by the present application, the method further includes: obtaining the AC voltage value and the AC current value of the external AC power grid within a target time window, where the target time window is the previous time window of the time window corresponding to the current moment; obtaining a first amplitude and a second amplitude according to the AC voltage value and the AC current value, where the first amplitude is the amplitude with the largest oscillation power within the sub-supersynchronous frequency band range within the target time window, and the second amplitude is the amplitude with the largest oscillation power within the interharmonic frequency band range within the target time window; determining a weighting coefficient according to the first amplitude and the second amplitude; and determining the first target virtual impedance or the second target virtual impedance according to the weighting coefficient and the grid state.

[0010] According to the control strategy switching method of the flexible DC transmission system provided by the present application, the obtaining the first amplitude and the second amplitude according to the AC voltage value and the AC current value includes: obtaining the instantaneous active power according to the AC voltage value and the AC current value; performing a fast Fourier transform on the instantaneous active power to obtain the amplitudes of multiple frequency components; determining a first frequency component range corresponding to the sub-supersynchronous frequency band and a second frequency component range corresponding to the interharmonic frequency band from the multiple frequency components; determining a first oscillation power of each frequency component within the first frequency component range and a second oscillation power of each frequency component within the second frequency component range; and determining the amplitude of the frequency component corresponding to the maximum value in the first oscillation power as the first amplitude, and the amplitude of the frequency component corresponding to the maximum value in the second oscillation power as the second amplitude.

[0011] According to the control strategy switching method of the flexible DC transmission system provided by the present application, the switching the control strategy of the flexible DC transmission system to the target control strategy includes: when the grid state changes from the off-grid state to the on-grid state, performing phase locking on the external AC power grid to obtain a target voltage phasor; obtaining a reference voltage phasor of the flexible DC transmission system; determining a phase difference angle according to the target voltage phasor and the reference voltage phasor; determining a target output voltage of the flexible DC transmission system, where the target output voltage is in phase alignment and has the same amplitude as the target voltage phasor; updating the output voltage of the flexible DC transmission system to the target output voltage, and switching the control strategy of the flexible DC transmission system to the target control strategy.

[0012] The present application also provides a control strategy switching device for a flexible DC transmission system, which is applied to a flexible DC transmission system, and the flexible DC transmission system is connected to an external AC power grid through a tie line. The device includes: An obtaining unit, configured to obtain the operating condition of the tie line; A first determination unit, configured to determine whether the grid state of the external AC power grid has changed according to the operating condition of the tie line, where the grid state is used to indicate the grid strength of the external AC power grid; A second determination unit, configured to determine a target control strategy of the flexible DC transmission system according to the changed grid state when the grid state changes, where the target control strategy is used to indicate the control mode of the converter station of the flexible DC transmission system; A switching unit, configured to switch the control strategy of the flexible DC transmission system to the target control strategy.

[0013] The present application also provides an electronic device, including a memory, a processor, and a computer program stored on the memory and executable on the processor. When the processor executes the computer program, the control strategy switching method of any one of the above flexible DC transmission systems is implemented.

[0014] The present application also provides a non-transitory computer-readable storage medium, on which a computer program is stored. When the computer program is executed by a processor, the control strategy switching method of any one of the above flexible DC transmission systems is implemented.

[0015] The present application also provides a computer program product, including a computer program. When the computer program is executed by a processor, the control strategy switching method of any one of the above flexible DC transmission systems is implemented.

[0016] The control strategy switching method, device, and equipment of the flexible DC transmission system provided by the present application first obtain the operating condition of the tie line, then determine whether the grid state of the external AC power grid has changed according to the operating condition of the tie line, and then determine the target control strategy of the flexible DC transmission system according to the changed grid state when the grid state changes, and finally switch the control strategy of the flexible DC transmission system to the target control strategy. In this way, the function of adaptively switching the control strategy based on the external grid state can be realized, and the sensitivity and reliability of the control strategy switching can be improved. Description of the Drawings

[0017] In order to more clearly illustrate the technical solutions in the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the drawings in the following description are some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0018] Figure 1 It is a schematic diagram of the composition of a power system provided by the present application.

[0019] Figure 2 It is one of the schematic flowcharts of a control strategy switching method for a flexible DC transmission system provided by this application.

[0020] Figure 3 It is the second of the schematic flowcharts of a control strategy switching method for a flexible DC transmission system provided by this application.

[0021] Figure 4 The block diagram of the functional units of a control strategy switching device for a flexible DC transmission system provided by this application.

[0022] Figure 5 It is the schematic structural diagram of an electronic device provided by this application. Specific embodiments

[0023] To make the objectives, technical solutions and advantages of the present invention clearer, the technical solutions in the present invention will be clearly and completely described below with reference to the accompanying drawings in the present invention. Obviously, the described embodiments are some but not all of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art without making creative efforts based on the embodiments in the present invention belong to the scope of protection of the present invention.

[0024] The terms "first", "second", etc. in the specification and claims of this application and the above-mentioned drawings are used to distinguish different objects, rather than to describe a specific order. In addition, the terms "include" and "have" and any variations thereof are intended to cover non-exclusive inclusion. For example, a process, method, system, product or device that includes a series of steps or units is not limited to the listed steps or units, but optionally further includes steps or units not listed, or optionally further includes other steps or units inherent to these processes, methods, products or devices.

[0025] Referring to "embodiments" herein means that the specific features, structures or characteristics described in connection with the embodiments can be included in at least one embodiment of this application. The phrase appears in various places in the specification and does not necessarily refer to the same embodiment, nor is it an independent or alternative embodiment mutually exclusive with other embodiments. Those skilled in the art will explicitly and implicitly understand that the embodiments described herein can be combined with other embodiments.

[0026] Currently, the flexible DC system all adopts a single control mode and cannot adapt to various application scenarios of the flexible DC system.

[0027] In view of the above problems, the embodiments of this application provide a control strategy switching method, device and equipment for a flexible DC transmission system.

[0028] Please refer to Figure 1, the power system 10 includes a flexible DC transmission system 101 and an external AC power grid 102, and the flexible DC transmission system 101 is connected to the external AC power grid 102 through a tie line. The flexible DC transmission system (Flexible DC Transmission System, FDCT) is mainly used for long-distance power transmission and exchange, has dynamic regulation capabilities, can adjust the transmission current and voltage in real time to ensure stable power transmission. The flexible DC transmission system 101 in this solution may refer to a flexible DC converter. The tie line 103 is a transmission line used to connect different power grids. Its main function is to achieve power transmission and exchange between different regions or different power systems. Through the power transmission method, the tie line ensures the interconnection and coordination between different power grids, and improves the flexibility, stability and reliability of the power grid.

[0029] Please refer to Figure 2 , the control strategy switching method of the flexible DC transmission system is applied to the above-mentioned flexible DC transmission system 101, and specifically includes the following steps.

[0030] S201, obtain the operating conditions of the tie line.

[0031] Among them, the operating conditions of the tie line may include information such as the switch state, frequency, power fluctuation amount, power exchange amount of the tie line, direction and amount of the power flow of the tie line, etc.

[0032] S202, determine whether the grid state of the external AC power grid has changed according to the operating conditions of the tie line.

[0033] Among them, the grid state is used to indicate the grid strength of the external AC power grid. The grid state may include grid-connected state and off-grid state. The grid-connected state includes strong grid state and weak grid state. The grid strengths of the strong grid state, weak grid state and off-grid state decrease in turn. It is possible to determine whether the grid state has changed based on the switch state of the tie line. For example, when there are faults and maintenance, etc., the operating state of the tie line is the open state, and at this time, it can be determined that the grid state is the off-grid state. When the switch state of the tie line changes from the open state to the closed state, it is considered that the grid state has changed, changing from the off-grid state to the grid-connected state.

[0034] S203, in the case where the grid state has changed, determine the target control strategy of the flexible DC transmission system according to the changed grid state.

[0035] Among them, the target control strategy includes a grid-following control strategy or a grid-forming control strategy.

[0036] S204, switch the control strategy of the flexible DC transmission system to the target control strategy.

[0037] Among them, the target control strategy is used to indicate the control mode of the converter station of the flexible DC transmission system. The target control strategy may include a grid-following control strategy and a grid-forming control strategy, and the control mode may include a PQ control mode under grid-following or a virtual synchronous machine control mode under grid-forming. When switching the control strategy, it may include switching between the grid-following control strategy and the grid-forming control strategy, or switching between different control parameters corresponding to the same control strategy. In particular, in the debugging or experimental scenario, the control strategy can be manually switched to facilitate the analysis and verification of the fluctuations of key quantities during the control mode switching process. In actual engineering applications, the control strategy can be automatically switched based on the grid state.

[0038] In specific implementation, after switching the control strategy to the grid-following control strategy, the AC voltage of the external AC grid, the output current of the flexible DC transmission system, and the power control command are collected. The grid voltage is tracked through a phase-locked loop, and the output current is closed-loop controlled according to the power control command. After switching the control strategy to the grid-forming control strategy, the AC voltage, current, and frequency of the external AC grid are collected. The internal potential and power angle reference value of the grid-forming control voltage source are calculated through the virtual synchronous machine control algorithm according to the active power, reactive power, and frequency control commands, and the grid-forming control is carried out based on the internal potential and power angle reference value.

[0039] It can be seen that in this embodiment, first, the working condition of the tie line is obtained, then it is determined whether the grid state of the external AC grid has changed according to the working condition of the tie line. The grid state includes an off-grid state or a grid-connected state. Then, in the case where the grid state has changed, the target control strategy of the flexible DC transmission system is determined according to the changed grid state. The target control strategy includes a grid-following control strategy or a grid-forming control strategy. Finally, the control strategy of the flexible DC transmission system is switched to the target control strategy. In this way, the function of adaptively switching the control strategy based on the external grid state can be realized, and the sensitivity and reliability of the control strategy switching can be improved.

[0040] In a possible embodiment, the grid-connected state includes a strong grid state and a weak grid state. Determine the changed power grid state, where the power grid state includes a grid-connected state and an off-grid state, the grid-connected state includes a strong grid state and a weak grid state, and the grid strengths of the strong grid state, the weak grid state, and the off-grid state decrease in sequence; determining the changed power grid state includes: determining whether the changed power grid state is the grid-connected state or the off-grid state according to the operating condition of the tie line; when the changed power grid state is the grid-connected state, obtaining the change condition of the electrical parameters at the connection point between the flexible DC transmission system and the external AC power grid; determining whether the changed power grid state is the strong grid state or the weak grid state according to the operating condition of the tie line and the change condition of the electrical parameters.

[0041] Among them, the tightness between the flexible DC transmission system and the external power grid can be judged based on the operating condition of the tie line. For example, a large and stable power exchange indicates a tight connection between the two. If the power exchange is small or fluctuates greatly, it may indicate a loose connection between them, thereby determining whether the power grid state is the off-grid state or the grid-connected state. The electrical parameters may include parameters such as the power and frequency at the connection point between the flexible DC transmission system and the external AC power grid. When the power grid state is the grid-connected state, the power grid state can be classified based on the change degree of electrical quantities such as power and frequency, thereby classifying the power grid state in the grid-connected state into the strong grid state or the weak grid state.

[0042] It can be seen that in this embodiment, by judging and classifying the grid strength level and using it as the input and basis for subsequent control mode selection, switching, and dynamic adjustment of control parameters, the sensitivity and reliability of the control mode switching are ensured, providing a guarantee for adapting to the control mode switching under complex working conditions.

[0043] In a possible embodiment, the target control strategy includes a grid-following control strategy or a grid-forming control strategy. Determining the target control strategy of the flexible DC transmission system according to the changed power grid state includes: when the changed power grid state is the strong grid state, determining the target control strategy as the grid-following control strategy; when the changed power grid state is the weak grid state or the off-grid state, determining the target control strategy as the grid-forming control strategy.

[0044] Among them, when switching the control strategy based on the power grid state, during the execution of the switching process, a ramp transition process can be performed on the target value of the control loop, such as setting the switching duration, and the control strategy switching can be achieved within this switching duration.

[0045] It can be seen that in this embodiment, the external grid state is classified, and a corresponding control mode switching mechanism is designed. The conditions for control mode switching and specific processing measures are clarified. In addition, flexible switching of control is achieved, avoiding simple weighted processing or direct switching between grid-following control and grid-forming control.

[0046] In a possible embodiment, when the changed grid state is a weak grid state or an off-grid state, determining the target control strategy of the flexible DC transmission system according to the changed grid state includes: when the changed grid state is the weak grid state, updating the virtual impedance and inertia time constant of the grid-forming control algorithm in the grid-forming control strategy to a first target virtual impedance and a first inertia time constant respectively to obtain the target control strategy; when the changed grid state is the off-grid state, updating the virtual impedance and inertia time constant of the grid-forming control algorithm in the grid-forming control strategy to a second target virtual impedance and a second inertia time constant respectively to obtain the target control strategy, where the first target virtual impedance is greater than the second target virtual impedance, and the first inertia time constant is less than the second inertia time constant.

[0047] Among them, the grid-forming control strategy is adopted both in the weak grid state and the off-grid state, but the control parameters of the grid-forming algorithm in the corresponding grid-forming control strategies in the weak grid state and the off-grid state are not completely the same. In particular, in the weak grid state, a larger virtual impedance and a smaller or moderate inertia time constant are set. While in the off-grid state, a smaller or moderate virtual impedance and a larger inertia time constant are set.

[0048] In specific implementation, when performing parameter update, ramp transition processing is executed during parameter update. For example, the control parameters are linearly changed, and the parameters are updated based on the linear change value to gradually reach the target value.

[0049] It can be seen that in this embodiment, the processing measures consider the switching of modes and the dynamic adaptive update of control parameters under the same control mode, achieving flexible switching of control and avoiding simple weighted processing or direct switching between grid-following control and grid-forming control.

[0050] In a possible embodiment, the method further includes: obtaining an AC voltage value and an AC current value of the external AC power grid within a target time window, where the target time window is the previous time window of the time window corresponding to the current moment; obtaining a first amplitude and a second amplitude according to the AC voltage value and the AC current value, where the first amplitude is the amplitude with the largest oscillation power within the sub-supersynchronous frequency band range within the target time window, and the second amplitude is the amplitude with the largest oscillation power within the interharmonic frequency band range within the target time window; determining a weighting coefficient according to the first amplitude and the second amplitude; and determining the first target virtual impedance or the second target virtual impedance according to the weighting coefficient and the grid state.

[0051] Wherein, in the case where the grid state is a weak grid state, the first target virtual impedance is determined based on the weighting coefficient, and in the case where the grid state is an off-grid state, the second target virtual impedance is determined based on the weighting coefficient. When obtaining the target virtual impedance, the target time window can be the time window corresponding to one second before the current moment. For fast Fourier transform (FFT) analysis, the power data per second can be saved in a buffer to form a time window (such as 5 seconds). Each time new data is collected, the new data is inserted into the buffer. In a specific implementation, the AC voltage and AC current signals of the system can be collected in real time, the three-phase real-time power can be calculated, and the three-phase power values can be buffered for 5-second time window data. The FFT analysis is performed on the real-time power data within the 5-second time window before the current moment, and the amplitudes with the largest oscillation power within the frequency band ranges are respectively counted according to the sub-supersynchronous frequency band and the interharmonic frequency band.

[0052] Sub-synchronous frequency band: Generally refers to the frequency band lower than the system fundamental frequency, such as the sub-synchronous frequency band of 2.5 - 47.5 Hz and the super-synchronous frequency band of 52.5 - 97.5 Hz. Interharmonic frequency band generally refers to the set of non-integer harmonic frequencies above 100 Hz and below 2500 Hz.

[0053] It can be seen that in this embodiment, during the dynamic adjustment process of the grid-forming control parameters, the current wide-band harmonic level is specifically considered as the basis for calculating the weighting coefficient for adjustment, realizing the differential setting and dynamic adjustment of the grid-forming control parameters under different wide-band oscillation harmonic conditions.

[0054] In a possible embodiment, obtaining the first amplitude and the second amplitude according to the AC voltage value and the AC current value includes: obtaining the instantaneous active power according to the AC voltage value and the AC current value; performing a fast Fourier transform on the instantaneous active power to obtain the amplitudes of a plurality of frequency components; determining a first frequency component range corresponding to the sub-supersynchronous frequency band and a second frequency component range corresponding to the interharmonic frequency band from the plurality of frequency components; determining a first oscillation power of each frequency component within the first frequency component range and a second oscillation power of each frequency component within the second frequency component range; determining the amplitude of the frequency component corresponding to the maximum value in the first oscillation power as the first amplitude, and the amplitude of the frequency component corresponding to the maximum value in the second oscillation power as the second amplitude.

[0055] Wherein, assuming that the time interval of each sampling point is Δt (such as 1 millisecond, 10 ms, etc.), then within 1 second, we will obtain a power data sequence with a length of N, where N = 1 second / Δt. Assuming the sampling frequency is fs = 1 / Δt, and N data points are collected within a 1-second time window. Then, the fast Fourier transform is used to perform frequency-domain analysis on the power data within 1 second, realizing the conversion of the time-domain signal to the frequency-domain signal. The output of the FFT is a complex number array, and its amplitude represents the energy of the signal at each frequency. The FFT result will return a series of frequency components, each frequency component corresponding to a specific amplitude. Finally, find all the frequency components whose frequencies are within the sub-supersynchronous frequency band range, calculate the amplitudes corresponding to these frequency components, and select the maximum value among these amplitudes as the first amplitude. Find all the frequency components whose frequencies are within the interharmonic frequency band range, calculate the amplitudes corresponding to these frequency components, and select the maximum value among these amplitudes as the second amplitude.

[0056] It can be seen that in this embodiment, under the grid-forming control strategy, the current broadband harmonic level of the system is considered, and the virtual impedance size within the frequency band is dynamically adjusted in different frequency bands accordingly, realizing the dynamic adjustment of the control parameters and enhancing the suppression effect of the system on broadband oscillations and the stability of switching.

[0057] In a possible embodiment, switching the control strategy of the flexible DC transmission system to the target control strategy includes: when the grid state changes from the off-grid state to the on-grid state, performing phase locking on the external AC grid to obtain the target voltage phasor; obtaining the reference voltage phasor of the flexible DC transmission system; determining the phase difference angle according to the target voltage phasor and the reference voltage phasor; determining the target output voltage of the flexible DC transmission system, where the target output voltage is in phase alignment and has the same amplitude as the target voltage phasor; updating the output voltage of the flexible DC transmission system to the target output voltage, and switching the control strategy of the flexible DC transmission system to the target control strategy.

[0058] Among them, when switching the control strategy, it is also possible to determine whether it is necessary to perform synchronization processing for grid connection switching, that is, whether the grid state switches between the off-grid state and the grid-connected state. If the grid state changes from the off-grid state to the grid-connected state, synchronization processing is required. Specifically, by phase-locking to the external AC grid, calculating the phase angle difference between the voltage of the external AC grid and the voltage of the flexible DC system, and adjusting the output voltage of the flexible DC system based on the phase angle, synchronous grid connection can be achieved.

[0059] It can be seen that in this embodiment, when the flexible DC system switches from off-grid to grid-connected state, synchronization processing is performed, which can reduce the impact caused by differences in amplitude, phase, etc. between the voltages of the flexible DC converter and the external grid before and after switching, making the control strategy switching smoother and safer.

[0060] Please refer to Figure 3 , and the following will combine Figure 3 to elaborate on this solution in detail.

[0061] First, real-time electrical quantity data is collected. The electrical quantity data includes AC voltage, current, frequency, and the output current and voltage of the flexible DC transmission system, etc. Then, the broadband harmonic level of the flexible DC transmission system is calculated to respectively obtain the maximum amplitudes of the oscillating power in the sub-super-synchronous frequency band and the inter-harmonic frequency band. Under the grid-following control strategy, closed-loop control of the flexible DC grid-following current is achieved based on the AC voltage, the output current of the flexible DC transmission system, and the power control command. Under the grid-forming control strategy, closed-loop control of the active and reactive power of the flexible DC grid-forming is achieved based on the AC voltage, current, and frequency. Then, the external AC grid state monitoring classification is obtained, including three levels: strong grid state, weak grid state, and off-grid state. Based on the grid state, the flexible DC control strategy is judged and switched. That is, under the strong grid state, the grid-following control strategy is adopted, and under the weak grid state and the off-grid state, the grid-forming control strategy is adopted. At the same time, when the grid-forming control strategy is adopted, the grid-forming control parameters are updated. The control parameters include virtual impedance and inertia time constant. At the same time, when the grid state changes from off-grid to grid-connected, the off-grid / grid-connected switching synchronization process is also carried out, and finally, the flexible DC control strategy switching execution is performed based on the determined target control strategy.

[0062] The following describes a control strategy switching device for a flexible DC transmission system provided by the present application. The control strategy switching device for the flexible DC transmission system described below corresponds to and refers to the control strategy switching method for the flexible DC transmission system described above.

[0063] Please refer to Figure 4, the control strategy switching device 400 of the flexible DC transmission system is applied to the flexible DC transmission system. The flexible DC transmission system is connected to an external AC power grid through a tie line. The control strategy switching device 400 of the flexible DC transmission system includes: an acquisition unit 401 for acquiring the operating condition of the tie line; a first determination unit 402 for determining whether the grid state of the external AC power grid has changed according to the operating condition of the tie line, where the grid state is used to indicate the grid strength of the external AC power grid; a second determination unit 403 for determining the target control strategy of the flexible DC transmission system according to the changed grid state when the grid state changes, where the target control strategy is used to indicate the control mode of the converter station of the flexible DC transmission system; and a switching unit 404 for switching the control strategy of the flexible DC transmission system to the target control strategy.

[0064] In a possible embodiment, before determining the target control strategy of the flexible DC transmission system according to the changed grid state, the first determination unit 402 is specifically configured to: determine the changed grid state, where the grid state includes a grid-connected state and an off-grid state, the grid-connected state includes a strong grid state and a weak grid state, and the grid strengths of the strong grid state, the weak grid state, and the off-grid state decrease in sequence; determining the changed grid state includes: determining that the changed grid state is the grid-connected state or the off-grid state according to the operating condition of the tie line; when the changed grid state is the grid-connected state, acquiring the change condition of the electrical parameters at the connection point between the flexible DC transmission system and the external AC power grid; and determining that the changed grid state is the strong grid state or the weak grid state according to the operating condition of the tie line and the change condition of the electrical parameters.

[0065] In a possible embodiment, the target control strategy includes a grid-following control strategy or a grid-forming control strategy. In terms of determining the target control strategy of the flexible DC transmission system according to the changed grid state, the second determination unit 403 is specifically configured to: when the changed grid state is the strong grid state, determine that the target control strategy is the grid-following control strategy; and when the changed grid state is the weak grid state or the off-grid state, determine that the target control strategy is the grid-forming control strategy.

[0066] In a possible embodiment, when the changed grid state is a weak grid state or an off-grid state, in terms of determining the target control strategy of the flexible DC transmission system according to the changed grid state, the second determination unit 403 is specifically configured to: when the changed grid state is the weak grid state, update the virtual impedance and the inertia time constant of the grid-forming control algorithm in the grid-forming control strategy to a first target virtual impedance and a first inertia time constant respectively, so as to obtain the target control strategy; when the changed grid state is the off-grid state, update the virtual impedance and the inertia time constant of the grid-forming control algorithm in the grid-forming control strategy to a second target virtual impedance and a second inertia time constant respectively, so as to obtain the target control strategy, where the first target virtual impedance is greater than the second target virtual impedance, and the first inertia time constant is less than the second inertia time constant.

[0067] In a possible embodiment, the second determination unit 403 is further configured to: obtain the AC voltage value and the AC current value of the external AC grid within a target time window, where the target time window is the previous time window corresponding to the current moment; obtain a first amplitude and a second amplitude according to the AC voltage value and the AC current value, where the first amplitude is the amplitude with the largest oscillation power within the sub-supersynchronous frequency band range within the target time window, and the second amplitude is the amplitude with the largest oscillation power within the interharmonic frequency band range within the target time window; determine a weighting coefficient according to the first amplitude and the second amplitude; and determine the first target virtual impedance or the second target virtual impedance according to the weighting coefficient and the grid state.

[0068] In a possible embodiment, in terms of obtaining the first amplitude and the second amplitude according to the AC voltage value and the AC current value, the second determination unit 403 is specifically configured to: obtain the instantaneous active power according to the AC voltage value and the AC current value; perform a fast Fourier transform on the instantaneous active power to obtain the amplitudes of a plurality of frequency components; determine a first frequency component range corresponding to the sub-supersynchronous frequency band and a second frequency component range corresponding to the interharmonic frequency band from the plurality of frequency components; determine a first oscillation power of each frequency component within the first frequency component range and a second oscillation power of each frequency component within the second frequency component range; and determine the amplitude of the frequency component corresponding to the maximum value in the first oscillation power as the first amplitude, and the amplitude of the frequency component corresponding to the maximum value in the second oscillation power as the second amplitude.

[0069] In a possible embodiment, in terms of switching the control strategy of the flexible DC transmission system to the target control strategy, the switching unit 404 is specifically configured to: when the grid state changes from the off-grid state to the on-grid state, perform phase locking on the external AC grid to obtain a target voltage phasor; acquire the reference voltage phasor of the flexible DC transmission system; determine the phase difference angle according to the target voltage phasor and the reference voltage phasor; determine the target output voltage of the flexible DC transmission system, where the target output voltage is in phase alignment and has the same amplitude as the target voltage phasor; update the output voltage of the flexible DC transmission system to the target output voltage, and switch the control strategy of the flexible DC transmission system to the target control strategy.

[0070] Please refer to Figure 5 , Figure 5 is a schematic structural diagram of an electronic device provided by the present application. As Figure 5 shown, the electronic device may include: a processor 510, a communication interface 520, a memory 530, and a communication bus 540. Among them, the processor 510, the communication interface 520, and the memory 530 communicate with each other through the communication bus 540. The processor 510 can call the logical instructions in the memory 530 to execute the control strategy switching method of the flexible DC transmission system. The method includes: acquiring the working condition of the tie line; determining whether the grid state of the external AC grid has changed according to the working condition of the tie line, where the grid state is used to indicate the grid strength of the external AC grid; when the grid state changes, determining the target control strategy of the flexible DC transmission system according to the changed grid state, where the target control strategy is used to indicate the control mode of the converter station of the flexible DC transmission system; switching the control strategy of the flexible DC transmission system to the target control strategy.

[0071] In addition, when the logical instructions in the above-mentioned memory 530 are implemented in the form of software functional units and sold or used as independent products, they can be stored in a computer-readable storage medium. Based on such an understanding, the technical solution of the present invention, in essence, or the part that contributes to the prior art, or a 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 for causing a computer device (which may be a personal computer, a server, or a network device, etc.) to execute all or part of the steps of the methods described in various embodiments of the present invention. The aforementioned storage medium includes: various media such as USB flash drives, mobile hard disks, read-only memories (ROM, Read-Only Memory), random access memories (RAM, Random Access Memory), magnetic disks, or optical discs that can store program codes.

[0072] On the other hand, the present invention also provides a non-transitory computer-readable storage medium, on which a computer program is stored. When the computer program is executed by a processor, it is used to implement the control strategy switching method of the flexible DC transmission system provided by the above-mentioned various methods. The method includes: obtaining the working condition of the tie line; determining whether the grid state of the external AC grid has changed according to the working condition of the tie line, where the grid state is used to indicate the grid strength of the external AC grid; in the case where the grid state has changed, determining the target control strategy of the flexible DC transmission system according to the changed grid state, where the target control strategy is used to indicate the control mode of the converter station of the flexible DC transmission system; and switching the control strategy of the flexible DC transmission system to the target control strategy.

[0073] On another aspect, the present application also provides a computer program product, including a computer program. When the computer program is executed by a processor, it is used to implement the control strategy switching method of any one of the flexible DC transmission systems as described above. The method includes: obtaining the working condition of the tie line; determining whether the grid state of the external AC grid has changed according to the working condition of the tie line, where the grid state is used to indicate the grid strength of the external AC grid; in the case where the grid state has changed, determining the target control strategy of the flexible DC transmission system according to the changed grid state, where the target control strategy is used to indicate the control mode of the converter station of the flexible DC transmission system; and switching the control strategy of the flexible DC transmission system to the target control strategy.

[0074] The device embodiments described above are merely illustrative. The units described as separate components may or may not be physically separated, and the components shown as units may or may not be physical units, that is, they may be located in one place or distributed to multiple network units. Some or all of the modules can be selected according to actual needs to achieve the purpose of the solution of this embodiment. A person of ordinary skill in the art can understand and implement it without creative labor.

[0075] Through the description of the above embodiments, those skilled in the art can clearly understand that each embodiment can be implemented by means of software plus a necessary general hardware platform, and of course, it can also be implemented by hardware. Based on such an understanding, the essence of the above technical solution, or the part that contributes to the prior art, can be embodied in the form of a software product. This computer software product can be stored in a computer-readable storage medium, such as ROM / RAM, magnetic disk, optical disk, etc., and includes several instructions for causing a computer device (which can be a personal computer, a server, or a network device, etc.) to execute the methods described in each embodiment or some parts of the embodiments.

[0076] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements on some of the technical features; and these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of each embodiment of the present invention.

Claims

1. A control strategy switching method for a flexible DC transmission system, characterized in that Applied to a flexible DC power transmission system, the flexible DC power transmission system is connected to an external AC power grid through a tie line, and the method includes: Obtain the operating condition of the tie line; Determine whether the grid state of the external AC power grid has changed according to the operating condition of the tie line, where the grid state is used to indicate the grid strength of the external AC power grid; When the grid state changes, determine the target control strategy of the flexible DC power transmission system according to the changed grid state, where the target control strategy is used to indicate the control mode of the converter station of the flexible DC power transmission system; Switch the control strategy of the flexible DC power transmission system to the target control strategy.

2. The method according to claim 1, wherein Before determining the target control strategy of the flexible DC power transmission system according to the changed grid state, the method further includes: Determine the changed grid state, where the grid state includes a grid-connected state and an off-grid state, the grid-connected state includes a strong grid state and a weak grid state, and the grid strengths of the strong grid state, the weak grid state, and the off-grid state decrease in sequence; The determining the changed grid state includes: Determine that the changed grid state is the grid-connected state or the off-grid state according to the operating condition of the tie line; When the changed grid state is the grid-connected state, obtain the change condition of the electrical parameters at the connection point between the flexible DC power transmission system and the external AC power grid; Determine that the changed grid state is the strong grid state or the weak grid state according to the operating condition of the tie line and the change condition of the electrical parameters.

3. The method according to claim 2, wherein The target control strategy includes a grid-following control strategy or a grid-forming control strategy. Determining the target control strategy of the flexible DC power transmission system according to the changed grid state includes: When the changed grid state is the strong grid state, determine that the target control strategy is the grid-following control strategy; When the changed grid state is the weak grid state or the off-grid state, determine that the target control strategy is the grid-forming control strategy.

4. The method according to claim 3, characterized in that, When the changed grid state is the weak grid state or the off-grid state, determining the target control strategy of the flexible DC power transmission system according to the changed grid state includes: When the changed grid state is the weak grid state, update the virtual impedance and the inertia time constant of the grid-forming control algorithm in the grid-forming control strategy to a first target virtual impedance and a first inertia time constant respectively to obtain the target control strategy; When the changed grid state is the off-grid state, update the virtual impedance and the inertia time constant of the grid-forming control algorithm in the grid-forming control strategy to a second target virtual impedance and a second inertia time constant respectively to obtain the target control strategy, where the first target virtual impedance is greater than the second target virtual impedance, and the first inertia time constant is less than the second inertia time constant.

5. The method according to claim 4, wherein The method further includes: Obtain the AC voltage value and the AC current value of the external AC power grid within a target time window, where the target time window is the previous time window corresponding to the current time window; Obtain a first amplitude and a second amplitude based on the AC voltage value and the AC current value, where the first amplitude is the amplitude with the largest oscillation power within the sub-supersynchronous frequency band range in the target time window, and the second amplitude is the amplitude with the largest oscillation power within the interharmonic frequency band range in the target time window; Determine a weighting coefficient based on the first amplitude and the second amplitude; Determine the first target virtual impedance or the second target virtual impedance based on the weighting coefficient and the grid state.

6. The method according to claim 5, wherein The obtaining the first amplitude and the second amplitude based on the AC voltage value and the AC current value includes: Obtain the instantaneous active power based on the AC voltage value and the AC current value; Perform a fast Fourier transform on the instantaneous active power to obtain the amplitudes of multiple frequency components; Determine a first frequency component range corresponding to the sub-supersynchronous frequency band and a second frequency component range corresponding to the interharmonic frequency band from the multiple frequency components; Determine the first oscillation power of each frequency component within the first frequency component range and the second oscillation power of each frequency component within the second frequency component range; Determine the amplitude of the frequency component corresponding to the maximum value in the first oscillation power as the first amplitude, and the amplitude of the frequency component corresponding to the maximum value in the second oscillation power as the second amplitude.

7. The method according to any one of claims 2-6, characterized in that, The switching the control strategy of the flexible DC transmission system to the target control strategy includes: In the case where the grid state changes from the off-grid state to the on-grid state, perform phase locking on the external AC grid to obtain a target voltage phasor; Obtain the reference voltage phasor of the flexible DC transmission system; Determine the phase difference angle based on the target voltage phasor and the reference voltage phasor; Determine the target output voltage of the flexible DC transmission system based on the phase difference angle, where the target output voltage is in phase alignment and has the same amplitude as the target voltage phasor; Update the output voltage of the flexible DC transmission system to the target output voltage, and switch the control strategy of the flexible DC transmission system to the target control strategy.

8. A control strategy switching device for a flexible DC power transmission system, characterized in that, Applied to a flexible DC transmission system, the flexible DC transmission system is connected to an external AC grid through a tie line, and the device includes: An acquisition unit for acquiring the operating condition of the tie line; A first determination unit for determining whether the grid state of the external AC grid has changed according to the operating condition of the tie line, where the grid state is used to indicate the grid strength of the external AC grid; A second determination unit for determining the target control strategy of the flexible DC transmission system according to the changed grid state in the case where the grid state has changed, where the target control strategy is used to indicate the control mode of the converter station of the flexible DC transmission system; A switching unit for switching the control strategy of the flexible DC transmission system to the target control strategy.

9. An electronic device, comprising a memory, a processor, and a computer program stored on the memory and running on the processor, characterized in that, When the processor executes the computer program, it implements the control strategy switching method of the flexible DC transmission system according to any one of claims 1 to 7.

10. A non-transitory computer-readable storage medium having a computer program stored thereon, characterized in that, When the computer program is executed by a processor, it implements the control strategy switching method of the flexible DC transmission system according to any one of claims 1 to 7.

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