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

Abstract

The present invention provides a control strategy switching method, device, and equipment for a flexible direct current (HVDC) transmission system. These methods belong to the field of flexible direct current (HVDC) transmission control technology and are applied to a flexible direct current (HVDC) transmission system connected to an external alternating current (AC) grid via a tie line. The method comprises: obtaining the operating status of the tie line; determining whether the grid state of the external AC grid has changed based on the operating status of the tie line, where the grid state indicates the grid strength of the external AC grid; if the grid state changes, determining a target control strategy for the flexible direct current transmission system based on the changed grid state, where the target control strategy indicates the control mode of a converter station in the flexible direct current transmission system; and switching the control strategy of the flexible direct current transmission system to the target control strategy. This method can adaptively switch the control strategy based on the external grid state, improving the sensitivity and reliability of the control strategy switching.

Description

Technical Field

[0001] The present application belongs to the field of flexible direct current transmission control technology, and specifically relates to a control strategy switching method, device and equipment for a flexible direct current transmission system. Background Art

[0002] Modern power systems are developing towards a high degree of power electronics. Many traditional power electronic converters use grid-following control, which lacks sufficient system inertia support. This change results in reduced system inertia, weakened damping characteristics, and challenges to system balancing capabilities. The grid structure is becoming more complex, and dynamic behavior is becoming more diverse. This can lead to broadband oscillations that affect system stability.

[0003] Currently, flexible DC systems utilize a variety of control strategies, such as grid-forming control and grid-following control, to control their operation. Grid-forming control simulates the behavior of synchronous generators to provide voltage and frequency support for the system, making it suitable for weak grids or islanded operation. However, it can cause system oscillations or even instability in strong grid environments. Grid-following control relies on a voltage reference signal from the external grid. While it offers good stability under strong grid conditions, its performance significantly degrades when grid strength is low or a fault occurs. With the diversification of flexible DC system application scenarios, a single control mode is no longer sufficient to meet the needs of complex operating conditions, necessitating the exploration of dynamic switching methods for multi-mode control strategies. Summary of the Invention

[0004] The present application provides a control strategy switching method, apparatus, and device for a flexible direct current transmission system, which adaptively switches the control strategy based on the external power grid state, thereby improving the sensitivity and reliability of the control strategy switching.

[0005] The present application provides a control strategy switching method for a flexible direct current (HVDC) transmission system, which is applied to the flexible direct current (HVDC) transmission system, wherein the flexible direct current (HVDC) transmission system is connected to an external alternating current (AC) grid via a tie line. The method includes:

[0006] Obtaining the working status of the contact line;

[0007] determining whether a grid state of the external AC grid has changed according to the working condition of the tie line, wherein the grid state is used to indicate the grid strength of the external AC grid;

[0008] In the event that the grid state changes, determining a target control strategy for the HVDC Flexible system according to the changed grid state, wherein the target control strategy is used to indicate a control mode of a converter station of the HVDC Flexible system;

[0009] The control strategy of the flexible direct current transmission system is switched to the target control strategy.

[0010] According to the control strategy switching method for a flexible direct current transmission system provided in the present application, the changed grid state is determined, where the grid state includes a grid-connected state and an off-grid state, and the grid-connected state includes a strong grid state and a weak grid state, with the grid strengths of the strong grid state, the weak grid state, and the off-grid state decreasing in sequence; determining the changed grid state includes: determining, based on the working condition of the tie line, whether the changed grid state is the grid-connected state or the off-grid state; when the changed grid state is the grid-connected state, obtaining changes in electrical parameters of a connection point between the flexible direct current transmission system and the external AC grid; and determining, based on the working condition of the tie line and changes in the electrical parameters, whether the changed grid state is the strong grid state or the weak grid state.

[0011] According to the control strategy switching method for a flexible direct current transmission system provided in the present application, the target control strategy includes a grid-following control strategy or a grid-forming control strategy, and determining the target control strategy of the flexible direct current transmission system according to the changed grid state includes: when the changed grid state is the strong grid state, determining the target control strategy to be 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 to be the grid-forming control strategy.

[0012] According to the control strategy switching method for a flexible direct current transmission system provided in the present application, when the changed grid state is a weak grid state or an off-grid state, determining the target control strategy of the flexible direct current 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 construction control algorithm in the grid construction 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 construction control algorithm in the grid construction control strategy to a second target virtual impedance and a second inertia time constant, respectively, to obtain the target control strategy, wherein 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.

[0013] According to the control strategy switching method of the flexible direct current transmission system provided in the present application, the method also includes: obtaining the AC voltage value and the AC current value of the external AC power grid within a target time window, the target time window being the time window before the time window corresponding to the current moment; obtaining a first amplitude and a second amplitude based on the AC voltage value and the AC current value, the first amplitude being the maximum amplitude of the oscillation power within the sub-supersynchronous frequency band within the target time window, and the second amplitude being the maximum amplitude of the oscillation power within the interharmonic frequency band within the target time window; determining a weighting coefficient based on the first amplitude and the second amplitude; and determining the first target virtual impedance or the second target virtual impedance based on the weighting coefficient and the power grid state.

[0014] According to the control strategy switching method for a flexible direct current transmission system provided in the present application, obtaining the first amplitude and the second amplitude based on the AC voltage value and the AC current value includes: obtaining instantaneous active power based on 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 a sub-supersynchronous frequency band and a second frequency component range corresponding to an interharmonic frequency band from the multiple frequency components; determining a first oscillation power for each frequency component within the first frequency component range and a second oscillation power for each frequency component within the second frequency component range; and determining that the amplitude of the frequency component corresponding to the maximum value in the first oscillation power is the first amplitude, and the amplitude of the frequency component corresponding to the maximum value in the second oscillation power is the second amplitude.

[0015] According to the control strategy switching method of the flexible direct current transmission system provided in the present application, the control strategy of the flexible direct current transmission system is switched to the target control strategy, including: when the grid state changes from the off-grid state to the grid-connected state, phase-locking the external AC grid to obtain a target voltage phasor; obtaining a reference voltage phasor of the flexible direct current transmission system; determining a phase difference angle based on the target voltage phasor and the reference voltage phasor; determining a target output voltage of the flexible direct current transmission system based on the phase difference angle, the target output voltage being phase-aligned and having the same amplitude as the target voltage phasor; updating the output voltage of the flexible direct current transmission system to the target output voltage, and switching the control strategy of the flexible direct current transmission system to the target control strategy.

[0016] The present application also provides a control strategy switching device for a flexible direct current transmission system, which is applied to the flexible direct current transmission system, wherein the flexible direct current transmission system is connected to an external alternating current grid via a tie line, and the device comprises:

[0017] an acquiring unit, configured to acquire the working condition of the tie line;

[0018] a first determining unit, configured to determine whether a grid state of the external AC grid has changed according to an operating condition of the tie line, wherein the grid state is used to indicate a grid strength of the external AC grid;

[0019] a second determining unit, configured to determine, when the grid state changes, a target control strategy of the flexible DC transmission system according to the changed grid state, wherein the target control strategy is used to indicate a control mode of a converter station of the flexible DC transmission system;

[0020] A switching unit is configured to switch the control strategy of the flexible direct current transmission system to the target control strategy.

[0021] The present application also provides an electronic device, including a memory, a processor, and a computer program stored in the memory and executable on the processor. When the processor executes the computer program, the control strategy switching method for a flexible direct current transmission system as described above is implemented.

[0022] The present application also provides a non-transitory computer-readable storage medium having a computer program stored thereon. When the computer program is executed by a processor, the control strategy switching method of any of the flexible direct current transmission systems described above is implemented.

[0023] The present application also provides a computer program product, including a computer program, which, when executed by a processor, implements any of the above-mentioned control strategy switching methods for a flexible direct current transmission system.

[0024] The control strategy switching method, apparatus, and device for a flexible direct current (HVDC) transmission system provided herein first obtain the operating status of the tie line, then determine whether the grid state of the external AC grid has changed based on the tie line operating status. If the grid state has changed, the method determines the target control strategy of the flexible direct current transmission system based on the changed grid state, and finally switches the control strategy of the flexible direct current transmission system to the target control strategy. This enables adaptive switching of control strategies based on the external grid state, improving the sensitivity and reliability of control strategy switching. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] In order to more clearly illustrate the technical solutions in the present invention or the prior art, a brief introduction is given below to the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0026] Figure 1This is a schematic diagram of the composition of a power system provided by this application.

[0027] Figure 2 This is one of the flow charts of a control strategy switching method for a flexible direct current transmission system provided in this application.

[0028] Figure 3 This is the second flow chart of a control strategy switching method for a flexible direct current transmission system provided in this application.

[0029] Figure 4 The present application provides a functional unit composition block diagram of a control strategy switching device for a flexible direct current transmission system.

[0030] Figure 5 It is a structural diagram of the electronic device provided in this application. DETAILED DESCRIPTION

[0031] To make the objectives, technical solutions, and advantages of the present invention more clear, the technical solutions of the present invention will be clearly and completely described below in conjunction with the accompanying drawings. Obviously, the embodiments described are only some of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts shall fall within the scope of protection of the present invention.

[0032] The terms "first," "second," and the like in the specification and claims of this application and the accompanying drawings are used to distinguish between different objects, not to describe a particular order. Furthermore, the terms "including," "having," and any variations thereof, are intended to cover non-exclusive inclusions. For example, a process, method, system, product, or apparatus comprising a series of steps or elements is not limited to the listed steps or elements but may optionally include steps or elements not listed, or may optionally include other steps or elements inherent to the process, method, product, or apparatus.

[0033] References herein to "embodiments" mean that a particular feature, structure, or characteristic described in connection with the embodiments may be included in at least one embodiment of the present application. The appearance of this phrase in various places in the specification does not necessarily refer to the same embodiment, nor does it constitute an independent or alternative embodiment that is mutually exclusive of other embodiments. It is understood, both explicitly and implicitly, by those skilled in the art that the embodiments described herein may be combined with other embodiments.

[0034] Currently, flexible DC systems all adopt a single control mode, which cannot adapt to the various application scenarios of flexible DC systems.

[0035] In response to the above problems, embodiments of the present application provide a control strategy switching method, apparatus, and device for a flexible direct current transmission system.

[0036] See also Figure 1 The power system 10 includes a flexible direct current transmission system 101 and an external alternating current grid 102, and the flexible direct current transmission system 101 is connected to the external alternating current grid 102 via an interconnection line. The flexible direct current transmission system (FDCT) is mainly used for long-distance transmission and exchange of electric power. It has dynamic adjustment capabilities and can adjust the transmission current and voltage in real time to ensure stable power transmission. The flexible direct current transmission system 101 in this solution may refer to a flexible direct current converter. The interconnection line 103 is a transmission line used to connect different power grids. Its main function is to realize power transmission and exchange between different regions or different power systems. The interconnection line ensures the interconnection and coordination between different power grids through power transmission, thereby improving the flexibility, stability and reliability of the power grid.

[0037] See also Figure 2 The control strategy switching method of the flexible direct current transmission system is applied to the above-mentioned flexible direct current transmission system 101, and specifically includes the following steps.

[0038] S201: Obtain the working status of the tie line.

[0039] The working condition 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 power flow of the tie line, etc.

[0040] S202: Determine whether a grid state of the external AC grid changes according to the working condition of the tie line.

[0041] The grid status indicates the strength of the external AC grid. The grid status can include a connected state and an off-grid state. The connected state includes a strong grid state and a weak grid state, with the strong grid state, weak grid state, and off-grid state each having decreasing grid strength. A change in the grid status can be determined based on the switch status of the tie line. For example, in the event of a fault or maintenance, the tie line is disconnected, and the grid status can be determined to be off-grid. When the tie line switch status changes from an open state to a closed state, the grid status is considered to have changed from an off-grid state to a connected state.

[0042] S203 : When the grid state changes, determine a target control strategy for the flexible DC transmission system according to the changed grid state.

[0043] The target control strategy includes a network following control strategy or a network building control strategy.

[0044] S204: Switching the control strategy of the flexible HVDC system to the target control strategy.

[0045] Among them, the target control strategy is used to indicate the control mode of the converter station of the flexible direct current 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 debugging or experimental situations, the control strategy can be switched manually, which facilitates the analysis and verification of the fluctuation of key quantities during the control mode switching process. In actual engineering applications, the control strategy can be automatically switched based on the grid status.

[0046] In specific implementation, after switching the control strategy to the grid-following control strategy, the system collects the AC voltage of the external AC grid, the output current of the flexible DC transmission system, and the power control command. The system then tracks the grid voltage through a phase-locked loop (PLL), and performs closed-loop control of the output current based on the power control command. After switching the control strategy to the networking control strategy, the system collects the AC voltage, current, and frequency of the external AC grid. Based on the active power, reactive power, and frequency control commands, the virtual synchronous machine control algorithm calculates the internal potential and power angle reference value of the networking control voltage source. Networking control is then performed based on these internal potential and power angle reference values.

[0047] As can be seen, in this embodiment, the operating status of the tie line is first obtained. Then, based on the operating status of the tie line, it is determined whether the grid state of the external AC power grid has changed. The grid state includes an off-grid state or a grid-connected state. Then, if the grid state changes, the target control strategy of the flexible DC transmission system is determined based on 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. This enables the function of adaptively switching the control strategy based on the external grid state, improving the sensitivity and reliability of control strategy switching.

[0048] In a possible embodiment, the grid-connected state includes a strong grid state and a weak grid state, and the changed grid state is determined. The grid state includes a grid-connected state and an off-grid state, and the grid-connected state includes a strong grid state and a weak grid state. 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 working condition of the interconnection line; when the changed grid state is the grid-connected state, obtaining changes in electrical parameters of the connection point between the flexible direct current transmission system and the external alternating current grid; and determining that the changed grid state is the strong grid state or the weak grid state according to the working condition of the interconnection line and the changes in the electrical parameters.

[0049] The closeness of the flexible DC transmission system to the external grid can be determined based on the operating conditions of the tie lines. For example, a large and stable power exchange indicates a close connection between the two. A small or fluctuating power exchange may indicate a loose connection, thereby determining whether the grid is off-grid or on-grid. Electrical parameters may include parameters such as power and frequency at the connection point between the flexible DC transmission system and the external AC grid. When the grid is on-grid, the grid status can be graded based on the degree of change in electrical quantities such as power and frequency, thereby classifying the grid status in the on-grid state as strong or weak.

[0050] As can be seen, in this embodiment, by determining and classifying the grid strength level, and using it as input and basis for subsequent control mode selection and switching, as well as dynamic adjustment of control parameters, the sensitivity and reliability of control mode switching are ensured, providing a guarantee for adapting to control mode switching under complex working conditions.

[0051] In a possible embodiment, the target control strategy includes a grid-following control strategy or a grid-forming control strategy, and determining the target control strategy of the flexible direct current transmission system according to the changed grid state includes: when the changed grid state is the strong grid state, determining the target control strategy to be 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 to be the grid-forming control strategy.

[0052] When switching the control strategy based on the grid status, the target value of the control loop may be subjected to ramp transition processing during the switching process, such as setting a switching duration, and implementing control strategy switching within the switching duration.

[0053] As can be seen, this embodiment implements hierarchical processing of external grid conditions and designs a corresponding control mode switching mechanism. It also clarifies the conditions and specific handling measures for control mode switching. It also enables flexible control switching, avoiding simple weighted processing or direct switching between grid-following control and grid-forming control.

[0054] 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 direct current 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 control algorithm in the grid 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 the inertia time constant of the grid control algorithm in the grid control strategy to a second target virtual impedance and a second inertia time constant, respectively, to obtain the target control strategy, wherein 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.

[0055] The network formation control strategy is used in both weak and off-grid conditions. However, the control parameters of the network formation algorithm in the corresponding weak and off-grid control strategies are different. Specifically, in weak grid conditions, a larger virtual impedance and a smaller or moderate inertia time constant are set. In off-grid conditions, a smaller or moderate virtual impedance and a larger inertia time constant are set.

[0056] In a specific implementation, when updating parameters, a ramp transition process is performed during parameter update, for example, a control parameter is linearly changed, and the parameter is updated based on the linear change value so that it gradually reaches the target value.

[0057] It can be seen that in this embodiment, the processing measures take into account the mode switching and the dynamic adaptive update of control parameters under the same control mode, realizing flexible switching of control and avoiding simple weighted processing or direct switching of network following control and network building control.

[0058] 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 a time window before the time window corresponding to the current moment; obtaining a first amplitude and a second amplitude based on the AC voltage value and the AC current value, where the first amplitude is the maximum amplitude of the oscillation power within the sub-supersynchronous frequency band within the target time window, and the second amplitude is the maximum amplitude of the oscillation power within the interharmonic frequency band within the target time window; determining a weighting coefficient based on the first amplitude and the second amplitude; and determining the first target virtual impedance or the second target virtual impedance based on the weighting coefficient and the power grid state.

[0059] When the grid is in a weak state, a first target virtual impedance is determined based on a weighting factor. When the grid is in an off-grid state, a second target virtual impedance is determined based on a weighting factor. When obtaining the target virtual impedance, the target time window can be the time window corresponding to one second before the current moment. To perform Fast Fourier Transform (FFT) analysis, power data per second can be stored in a buffer to form a time window (e.g., 5 seconds). Each time new data is collected, the new data is inserted into the buffer. In a specific implementation, the system AC voltage and AC current signals can be collected in real time to calculate the three-phase real-time power. The three-phase power values ​​are buffered for a 5-second time window. FFT analysis is performed on the real-time power data within the 5-second time window before the current moment. The maximum amplitude of the oscillation power within the sub-supersynchronous frequency band and the interharmonic frequency band is calculated.

[0060] Sub-synchronous frequency band: This generally refers to a frequency band below the system fundamental frequency, such as the 2.5-47.5 Hz sub-synchronous frequency band and the 52.5-97.5 Hz super-synchronous frequency band. Interharmonic frequency band: This generally refers to a collection of non-integer harmonic frequencies between 100 Hz and 2500 Hz.

[0061] It can be seen that in this embodiment, the dynamic adjustment process of the networking control parameters takes into detailed consideration the current broadband harmonic level as the basis for calculating the adjustment weighting coefficient, thereby realizing differentiated setting and dynamic adjustment of the networking control parameters under different broadband oscillation harmonics.

[0062] In a possible embodiment, obtaining the first amplitude and the second amplitude based on the AC voltage value and the AC current value includes: obtaining instantaneous active power based on the AC voltage value and the AC current value; performing a fast Fourier transform on the instantaneous active power to obtain amplitudes of multiple frequency components; determining, from the multiple frequency components, a first frequency component range corresponding to a sub-supersynchronous frequency band and a second frequency component range corresponding to an interharmonic frequency band; determining a first oscillation power for each frequency component within the first frequency component range and a second oscillation power for each frequency component within the second frequency component range; and determining that the amplitude of the frequency component corresponding to the maximum value in the first oscillation power is the first amplitude, and the amplitude of the frequency component corresponding to the maximum value in the second oscillation power is the second amplitude.

[0063] Assuming the time interval between each sampling point is Δt (e.g., 1 millisecond, 10 milliseconds, etc.), we will obtain a power data sequence of length N within 1 second, where N = 1 second / Δt. Assuming the sampling frequency is fs = 1 / Δt, and collecting N data points within a 1-second time window, we then use a fast Fourier transform (FFT) to perform frequency domain analysis on the power data within 1 second, converting the time-domain signal into the frequency domain. The FFT output is an array of complex numbers, whose amplitudes represent the signal energy at each frequency. The FFT result returns a series of frequency components, each corresponding to a specific amplitude. Finally, we find all frequency components with frequencies within the sub-supersynchronous frequency band, calculate their corresponding amplitudes, and select the maximum of these amplitudes as the first amplitude. We find all frequency components with frequencies within the interharmonic frequency band, calculate their corresponding amplitudes, and select the maximum of these amplitudes as the second amplitude.

[0064] It can be seen that in this embodiment, when adopting the network control strategy, the current broadband harmonic level of the system is taken into account, and the virtual impedance size within the frequency band is dynamically adjusted according to this, so as to realize dynamic adjustment of control parameters and enhance the system's suppression effect on broadband oscillation and switching stability.

[0065] In a possible embodiment, the control strategy for switching the flexible direct current transmission system is the target control strategy, including: when the grid state changes from the off-grid state to the grid-connected state, phase-locking the external AC grid to obtain a target voltage phasor; obtaining a reference voltage phasor of the flexible direct current transmission system; determining a phase difference angle based on the target voltage phasor and the reference voltage phasor; determining a target output voltage of the flexible direct current transmission system based on the phase difference angle, the target output voltage being phase-aligned and having the same amplitude as the target voltage phasor; updating the output voltage of the flexible direct current transmission system to the target output voltage, and switching the control strategy of the flexible direct current transmission system to the target control strategy.

[0066] When switching control strategies, it is also possible to determine whether synchronization with grid connection is required, specifically whether the grid state is switching between off-grid and on-grid states. If the grid state changes from off-grid to on-grid, synchronization is required. This can be achieved by phase-locking the external AC grid, calculating the phase angle difference between the external AC grid voltage and the Flexible DC system voltage, and adjusting the Flexible DC system's output voltage based on this phase angle.

[0067] It can be seen that in this embodiment, when the flexible DC system is converted from an off-grid state to a grid-connected state, synchronous processing is performed, which can reduce the impact caused by the differences in amplitude, phase, etc. between the voltage of the flexible DC converter and the external grid before and after switching, making the control strategy switching smoother and safer.

[0068] See also Figure 3 , combined with Figure 3 Describe this plan in detail.

[0069] First, real-time electrical quantity data is collected, including AC voltage, current, frequency, and the output current and voltage of the Flexible HVDC system. Then, the broadband harmonic level of the Flexible HVDC system is calculated to obtain the maximum amplitude of the oscillation power within the sub-supersynchronous frequency band and the interharmonic frequency band. Under the grid-following control strategy, closed-loop current control of the Flexible HVDC grid-following current is implemented based on the AC voltage, the output current of the Flexible HVDC system, and the power control instructions. Under the grid-forming control strategy, closed-loop control of the active and reactive power of the Flexible HVDC grid-forming is implemented based on the AC voltage, current, and frequency. Then, the external AC grid status monitoring classification is obtained, including strong grid state, weak grid state, and off-grid state. Based on the grid state, the Flexible HVDC control strategy is determined and switched. Specifically, in a strong grid state, the grid-following control strategy is adopted, while in weak grid state and off-grid state, the grid-forming control strategy is adopted. Furthermore, when adopting the grid-forming control strategy, the grid-forming control parameters are updated. These control parameters include the virtual impedance and the inertia time constant. At the same time, when the grid state changes from off-grid to on-grid state, on-grid and off-grid switching synchronization processing is performed, and finally the flexible DC control strategy switching is executed based on the determined target control strategy.

[0070] A control strategy switching device for a flexible direct current transmission system provided in the present application is described below. The control strategy switching device for a flexible direct current transmission system described below corresponds to the control strategy switching method for a flexible direct current transmission system described above.

[0071] See also Figure 4A control strategy switching device 400 for a flexible direct current transmission system is applied to a flexible direct current transmission system, wherein the flexible direct current transmission system is connected to an external alternating current grid via a tie line. The control strategy switching device 400 for the flexible direct current transmission system includes: an acquisition unit 401 for acquiring a working condition of the tie line; a first determination unit 402 for determining whether a grid state of the external alternating current grid has changed according to the working condition of the tie line, wherein the grid state is used to indicate the grid strength of the external alternating current grid; a second determination unit 403 for determining a target control strategy of the flexible direct current transmission system according to the changed grid state when the grid state changes, wherein the target control strategy is used to indicate a control mode of a converter station of the flexible direct current transmission system; and a switching unit 404 for switching the control strategy of the flexible direct current transmission system to the target control strategy.

[0072] In a possible embodiment, before determining the target control strategy of the flexible direct current transmission system according to the changed grid state, the first determining unit 402 is specifically used to: determine the changed grid state, where the grid state includes a grid-connected state and an off-grid state, and 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 working condition of the tie line; when the changed grid state is the grid-connected state, obtaining changes in electrical parameters of the connection point between the flexible direct current transmission system and the external AC grid; and determining that 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 changes in the electrical parameters.

[0073] In a possible embodiment, the target control strategy includes a grid-following control strategy or a grid-forming control strategy. In determining the target control strategy of the flexible direct current transmission system according to the changed grid state, the second determination unit 403 is specifically used to: 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.

[0074] In a possible embodiment, when the changed grid state is a weak grid state or an off-grid state, in determining the target control strategy of the flexible direct current transmission system according to the changed grid state, the second determining 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 control algorithm in the grid 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 control algorithm in the grid 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.

[0075] In a possible embodiment, the second determination unit 403 is further used to: obtain 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 a time window before the time window corresponding to the current moment; 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 maximum amplitude of the oscillation power within the sub-supersynchronous frequency band within the target time window, and the second amplitude is the maximum amplitude of the oscillation power within the interharmonic frequency band within the target time window; determine a weighting coefficient based on the first amplitude and the second amplitude; and determine the first target virtual impedance or the second target virtual impedance based on the weighting coefficient and the power grid state.

[0076] In one possible embodiment, in terms of obtaining the first amplitude and the second amplitude based on the AC voltage value and the AC current value, the second determination unit 403 is specifically configured to: obtain 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 amplitudes of multiple frequency components; determine, from the multiple frequency components, a first frequency component range corresponding to a sub-supersynchronous frequency band and a second frequency component range corresponding to an interharmonic frequency band; determine a first oscillation power for each frequency component within the first frequency component range and a second oscillation power for each frequency component within the second frequency component range; and determine that the amplitude of the frequency component corresponding to the maximum value in the first oscillation power is the first amplitude, and the amplitude of the frequency component corresponding to the maximum value in the second oscillation power is the second amplitude.

[0077] In a possible embodiment, in terms of switching the control strategy of the flexible direct current transmission system to the target control strategy, the switching unit 404 is specifically used to: when the grid state changes from the off-grid state to the grid-connected state, phase-lock the external AC grid to obtain a target voltage phasor; obtain a reference voltage phasor of the flexible direct current transmission system; determine a phase difference angle based on the target voltage phasor and the reference voltage phasor; determine a target output voltage of the flexible direct current transmission system based on the phase difference angle, the target output voltage being aligned in phase and having the same amplitude as the target voltage phasor; update the output voltage of the flexible direct current transmission system to the target output voltage, and switch the control strategy of the flexible direct current transmission system to the target control strategy.

[0078] See also Figure 5 , Figure 5 This is a schematic diagram of the structure of the electronic device provided by this application. Figure 5 As shown, the electronic device may include: a processor 510, a communications interface 520, a memory 530, and a communications bus 540, wherein the processor 510, the communications interface 520, and the memory 530 communicate with each other via the communications bus 540. The processor 510 may call logic instructions in the memory 530 to execute a control strategy switching method for a flexible DC transmission system, the method comprising: obtaining the operating status of the tie line; determining whether the grid state of the external AC power grid has changed based on the operating status of the tie line, the grid state being used to indicate the grid strength of the external AC power grid; if the grid state has changed, determining a target control strategy of the flexible DC transmission system based on the changed grid state, the target control strategy being used to indicate a control mode of a converter station of the flexible DC transmission system; and switching the control strategy of the flexible DC transmission system to the target control strategy.

[0079] Furthermore, the logic instructions in the aforementioned memory 530 can be implemented as software functional units and, when sold or used as independent products, can be stored in a computer-readable storage medium. Based on this understanding, the technical solution of the present invention, or the portion that contributes to the prior art, or a portion 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 can be a personal computer, server, or 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 capable of storing program code, such as a USB flash drive, a mobile hard drive, a read-only memory (ROM), a random access memory (RAM), a magnetic disk, or an optical disk.

[0080] On the other hand, the present invention also provides a non-transitory computer-readable storage medium having a computer program stored thereon, which, when executed by a processor, is implemented to execute the control strategy switching method of the flexible direct current transmission system provided by the above-mentioned methods, the method comprising: obtaining the working condition of the tie line; determining whether the grid state of the external AC power grid has changed based on the working condition of the tie line, the grid state being used to indicate the grid strength of the external AC power grid; in the event that the grid state changes, determining the target control strategy of the flexible direct current transmission system based on the changed grid state, the target control strategy being used to indicate the control mode of the converter station of the flexible direct current transmission system; and switching the control strategy of the flexible direct current transmission system to the target control strategy.

[0081] On the other hand, the present application also provides a computer program product, including a computer program, which, when executed by a processor, implements a control strategy switching method for a flexible direct current transmission system as described above, the method including: obtaining the working condition of the tie line; determining whether the grid state of the external AC power grid has changed based on the working condition of the tie line, and the grid state is used to indicate the grid strength of the external AC power grid; when the grid state changes, determining a target control strategy of the flexible direct current transmission system based on the changed grid state, and the target control strategy is used to indicate the control mode of the converter station of the flexible direct current transmission system; switching the control strategy of the flexible direct current transmission system to the target control strategy.

[0082] The device embodiments described above are merely illustrative. The units described as separate components may or may not be physically separate, and the components shown as units may or may not be physical units, i.e., they may be located in one location or distributed across multiple network units. Some or all of the modules may be selected based on actual needs to achieve the objectives of the present embodiment. Persons of ordinary skill in the art will be able to understand and implement the present invention without inventive effort.

[0083] Through the above description of the embodiments, those skilled in the art will clearly understand that each embodiment can be implemented using software plus a necessary general-purpose hardware platform, or of course, hardware. Based on this understanding, the essence of the above technical solution, or the portion 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, a magnetic disk, or an optical disk, and includes a number of instructions for causing a computer device (such as a personal computer, server, or network device) to execute the methods described in each embodiment or certain portions of the embodiments.

[0084] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the various embodiments of the present invention.

Claims

1. A control strategy switching method for a flexible direct current transmission system, characterized in that: Applied to a flexible direct current transmission system, the flexible direct current transmission system is connected to an external alternating current grid via a tie line, and the method comprises: Obtaining the working status of the contact line; determining whether a grid state of the external AC grid has changed according to the working condition of the tie line, wherein the grid state is used to indicate the grid strength of the external AC grid; In the case where the grid state changes, determining the changed grid state, where the grid state includes a grid-connected state and an off-grid state, where the grid-connected state includes a strong grid state and a weak grid state, and where the grid strengths of the strong grid state, the weak grid state, and the off-grid state decrease in sequence; When the changed power grid state is the weak grid state, determining the target control strategy to be a network building control strategy; Acquire 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 a time window before a time window corresponding to a current moment; Obtaining a first amplitude and a second amplitude according to the AC voltage value and the AC current value, wherein the first amplitude is an amplitude at which the oscillation power within the sub-supersynchronous frequency band within the target time window is the maximum, and the second amplitude is an amplitude at which the oscillation power within the interharmonic frequency band within the target time window is the maximum; determining a weighting coefficient according to the first amplitude and the second amplitude; determining a first target virtual impedance according to the weighting coefficient and the weak network state; Updating the virtual impedance and the inertia time constant of the network construction control algorithm in the network construction control strategy to the first target virtual impedance and the first inertia time constant, respectively, to obtain the target control strategy, wherein the target control strategy is used to indicate a control mode of the converter station of the flexible HVDC transmission system; The control strategy of the flexible direct current transmission system is switched to the target control strategy.

2. The method according to claim 1, characterized in that The determining of the changed power grid state includes: Determining, according to the working condition of the tie line, whether the changed grid state is the grid-connected state or the off-grid state; When the changed grid state is the grid-connected state, obtaining changes in electrical parameters of a connection point between the flexible direct current transmission system and the external alternating current grid; The changed power grid state is determined to be the strong network state or the weak network state according to the working condition of the tie line and the change of the electrical parameters.

3. The method according to claim 2, characterized in that The target control strategy includes a network following control strategy or a network building control strategy, and the method further includes: When the changed power grid state is the strong grid state, determining the target control strategy to be the grid-following control strategy; When the changed grid state is the off-grid state, the target control strategy is determined to be the grid-building control strategy.

4. The method according to claim 3, characterized in that In a case where the changed grid state is an off-grid state, the method further includes: When the changed grid state is the off-grid state, the virtual impedance and the inertia time constant of the grid construction control algorithm in the grid construction control strategy are updated to the second target virtual impedance and the second inertia time constant, respectively, to obtain the target control strategy, wherein 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, characterized in that The method further comprises: Acquire 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 a time window before a time window corresponding to a current moment; Obtaining a first amplitude and a second amplitude according to the AC voltage value and the AC current value, wherein the first amplitude is an amplitude at which the oscillation power within the sub-supersynchronous frequency band within the target time window is the maximum, and the second amplitude is an amplitude at which the oscillation power within the interharmonic frequency band within the target time window is the maximum; determining a weighting coefficient according to the first amplitude and the second amplitude; The second target virtual impedance is determined according to the weighting coefficient and the off-grid state.

6. The method according to claim 1, characterized in that The obtaining of the first amplitude and the second amplitude according to the AC voltage value and the AC current value includes: Obtaining 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 amplitudes of multiple frequency components; Determine, from the plurality of frequency components, a first frequency component range corresponding to a sub-supersynchronous frequency band and a second frequency component range corresponding to an interharmonic frequency band; determining a first oscillation power for each frequency component within the first frequency component range and a second oscillation power for each frequency component within the second frequency component range; The amplitude of the frequency component corresponding to the maximum value of the first oscillation power is determined to be the first amplitude, and the amplitude of the frequency component corresponding to the maximum value of the second oscillation power is determined to be the second amplitude.

7. The method according to any one of claims 2 to 6, characterized in that: The control strategy for switching the flexible direct current transmission system is the target control strategy, including: When the grid state changes from the off-grid state to the grid-connected state, performing phase locking on the external AC grid to obtain a target voltage phasor; Obtaining a reference voltage phasor of the flexible direct current 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 HVDC system according to the phase difference angle, wherein the target output voltage is aligned in phase with the target voltage phasor and has the same amplitude; The output voltage of the flexible direct current transmission system is updated to the target output voltage, and the control strategy of the flexible direct current transmission system is switched to the target control strategy.

8. A control strategy switching device for a flexible direct current transmission system, characterized in that: Applied to a flexible direct current transmission system, the flexible direct current transmission system is connected to an external alternating current grid via a tie line, and the device comprises: an acquiring unit, configured to acquire the working condition of the tie line; a first determining unit, configured to determine whether a grid state of the external AC grid has changed according to an operating condition of the tie line, wherein the grid state is used to indicate a grid strength of the external AC grid; The second determining unit is used to determine the changed power grid state, the power grid state includes the grid-connected state and the off-grid state, the grid-connected state includes the strong grid state and the weak grid state, and the grid strengths of the strong grid state, the weak grid state and the off-grid state decrease in sequence; and is used to determine that the target control strategy is the network control strategy when the changed power grid state is the weak grid state; and is used to obtain the AC voltage value and the AC current value of the external AC power grid within the target time window, the target time window is the previous time window of the time window corresponding to the current moment; and is used to obtain the first amplitude and the second amplitude according to the AC voltage value and the AC current value, The first amplitude is the amplitude of the maximum oscillation power within the sub-supersynchronous frequency band within the target time window, and the second amplitude is the amplitude of the maximum oscillation power within the interharmonic frequency band within the target time window; and is used to determine a weighting coefficient according to the first amplitude and the second amplitude; and is used to determine a first target virtual impedance according to the weighting coefficient and the weak network state; and is used to update the virtual impedance and inertia time constant of the network control algorithm in the network control strategy to the first target virtual impedance and the first inertia time constant, respectively, to obtain the target control strategy, and the target control strategy is used to indicate the control mode of the converter station of the flexible DC transmission system; A switching unit is configured to switch the control strategy of the flexible direct current transmission system to the target control strategy.

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

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, the control strategy switching method for a flexible direct current transmission system according to any one of claims 1 to 7 is implemented.

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