Power flow calculation method and device for power grid containing multi-terminal flexible DC power transmission system

Abstract

The invention discloses a power flow calculation method and device for a power grid containing a multi-terminal flexible DC power transmission system. The method comprises the steps of determining a target state variable value of a multi-terminal flexible DC power transmission system included in a power grid based on an AC side bus voltage value of a converter included in the power grid; based on the target state variable value, an initial power flow calculation result of the alternating current power transmission system included in the power grid is determined, the initial power flow calculation result is used for representing a voltage amplitude and a voltage phase angle of a node included in the alternating current power transmission system, and the node refers to a specific position where electrical attributes need to be displayed in the alternating current power transmission system; and under the condition that the initial load flow calculation result is smaller than a preset first judgment threshold value, determining the initial load flow calculation result as a target load flow calculation result of the power grid. According to the method and the device, the technical problem of inaccurate load flow calculation result of the power grid containing the multi-terminal flexible direct current power transmission system in the prior art is solved.

Description

Technical Field

[0001] This application relates to the field of power systems, and more specifically, to a method and apparatus for calculating power flow in a power grid containing a multi-terminal flexible DC transmission system. Background Technology

[0002] Against the backdrop of accelerating urbanization, increasing electricity demand, and the gradual improvement of power system and grid technology, urban power grids are becoming increasingly complex, placing higher demands on their safety, stability, and economic operation. Currently, the proportion of large-scale wind power, photovoltaic power, and other clean energy-generating power generation in the power system is increasing year by year, marking a new stage in the power system's shift from coal-based to clean energy-dominated systems. Simultaneously, the development of microgrid technology has facilitated the deployment of a large number of power electronic devices into the grid, profoundly impacting traditional AC power grids and forcing their structure to gradually transform towards hybrid AC / DC grids. Multi-Terminal High Voltage Direct Current system based on Voltage Source Converter (VSC-MTDC), as an emerging transmission technology, demonstrates great potential in solving many problems in urban power grids due to its flexibility, controllability, and high-efficiency transmission.

[0003] In related technologies, ill-conditioned power flow calculations for urban power grids with embedded multi-terminal flexible DC transmission systems typically employ unified iterative methods and ill-conditioned power flow methods. The unified iterative method suffers from low computational efficiency and slow convergence speed when the power grid is large, especially with an increased number of multi-terminal flexible DC systems, making it difficult to meet real-time calculation requirements. The ill-conditioned power flow method exhibits poor reliability and accuracy when the coupling between the DC and AC transmission systems is high. In summary, related technologies suffer from inaccurate power flow calculation results for power grids with embedded multi-terminal flexible DC transmission systems.

[0004] There is currently no effective solution to the above problems. Summary of the Invention

[0005] This application provides a method and apparatus for calculating power flow in a grid system containing a multi-terminal flexible DC transmission system, in order to at least solve the technical problem of inaccurate power flow calculation results for grid systems containing multi-terminal flexible DC transmission systems in related technologies.

[0006] According to one aspect of the embodiments of this application, a power flow calculation method for a power grid containing a multi-terminal flexible DC transmission system is provided, comprising: determining target state variable values ​​of the multi-terminal flexible DC transmission system in the power grid based on the AC bus voltage values ​​of the converters included in the power grid, wherein the target state variable values ​​are quantitative representations of state variables used to describe the dynamic behavior of the multi-terminal flexible DC transmission system; determining initial power flow calculation results of the AC transmission system included in the power grid based on the target state variable values, wherein the initial power flow calculation results are used to represent the voltage amplitude and voltage phase angle of the nodes included in the AC transmission system, and a node refers to a specific location in the AC transmission system where electrical attributes need to be displayed; and determining the initial power flow calculation results as the target power flow calculation results of the power grid if the initial power flow calculation results are less than a preset first determination threshold.

[0007] According to another aspect of the embodiments of this application, a power flow calculation device for a power grid including a multi-terminal flexible DC transmission system is provided, comprising: a first determining module, configured to determine the target state variable value of the multi-terminal flexible DC transmission system included in the power grid based on the AC bus voltage value of the converter included in the power grid, wherein the target state variable value is a quantitative representation of the state variable used to describe the dynamic behavior of the multi-terminal flexible DC transmission system; a second determining module, configured to determine the initial power flow calculation result of the AC transmission system included in the power grid based on the target state variable value, wherein the initial power flow calculation result is used to represent the voltage amplitude and voltage phase angle of the nodes included in the AC transmission system, and the node refers to a specific location in the AC transmission system where electrical attributes need to be displayed; and a target power flow calculation result determining module, configured to determine the initial power flow calculation result as the target power flow calculation result of the power grid if the initial power flow calculation result is less than a preset first determination threshold.

[0008] According to another aspect of the embodiments of this application, a non-volatile storage medium is provided, which stores multiple instructions, the instructions being adapted for a power flow calculation method for a multi-terminal flexible DC transmission system, any one of which can be loaded and executed by a processor.

[0009] According to another aspect of the embodiments of this application, an electronic device is provided, including: one or more processors and a memory, the memory being used to store one or more programs, wherein when the one or more programs are executed by the one or more processors, the one or more processors cause the one or more processors to implement any one of the following: a power flow calculation method for a multi-terminal flexible DC transmission system.

[0010] According to another aspect of the embodiments of this application, a computer program product is provided that, when executed on a data processing device, is adapted to perform the steps of a power flow calculation method for a multi-terminal flexible DC transmission system.

[0011] In this embodiment, the target state variable value of the multi-terminal flexible DC transmission system included in the power grid is determined based on the AC bus voltage value of the converter included in the power grid. The target state variable value is a quantitative representation of the state variable used to describe the dynamic behavior of the multi-terminal flexible DC transmission system. Based on the target state variable value, the initial power flow calculation result of the AC transmission system included in the power grid is determined. The initial power flow calculation result is used to represent the voltage amplitude and voltage phase angle of the nodes included in the AC transmission system. A node refers to a specific location in the AC transmission system where electrical attributes need to be displayed. If the initial power flow calculation result is less than a preset first judgment threshold, the initial power flow calculation result is determined as the target power flow calculation result of the power grid. The goal is to first determine the target state variables of a multi-terminal flexible DC transmission system based on the AC bus voltage value of the converter, then calculate the initial power flow results of the AC transmission system, and use a first judgment threshold to verify the feasibility of the results. When the first judgment result passes, the power flow calculation results of the power grid are accurately determined. This achieves the technical effect of improving the accuracy of power flow calculation results for power grids containing multi-terminal flexible DC transmission systems, thereby solving the technical problem of inaccurate power flow calculation results for power grids containing multi-terminal flexible DC transmission systems in related technologies. Attached Figure Description

[0012] The accompanying drawings, which are included to provide a further understanding of this application and form part of this application, illustrate exemplary embodiments and are used to explain this application, but do not constitute an undue limitation of this application. In the drawings:

[0013] Figure 1 This is a flowchart of an optional power flow calculation method for a multi-terminal flexible DC transmission system provided according to an embodiment of this application;

[0014] Figure 2 This is an iterative calculation flowchart of an optional DC transmission system provided according to an embodiment of this application;

[0015] Figure 3 This is an iterative calculation flowchart of an optional AC / DC hybrid power transmission system provided according to an embodiment of this application;

[0016] Figure 4 This is a structural block diagram of an optional power flow calculation method for a multi-terminal flexible DC transmission system provided according to an embodiment of this application;

[0017] Figure 5 This is a structural diagram of an optional three-terminal flexible DC transmission system provided according to an embodiment of this application;

[0018] Figure 6 This is a schematic diagram of an optional power flow calculation device for a multi-terminal flexible DC transmission system provided according to an embodiment of this application. Detailed Implementation

[0019] To enable those skilled in the art to better understand the present application, the technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present application, and not all embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative effort should fall within the scope of protection of the present application.

[0020] It should be noted that the terms "first," "second," etc., in the specification, claims, and accompanying drawings of this application are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such data can be interchanged where appropriate so that the embodiments of this application described herein can be implemented in orders other than those illustrated or described herein. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover non-exclusive inclusion; for example, a process, method, system, product, or apparatus that comprises a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to such processes, methods, products, or apparatus.

[0021] According to an embodiment of this application, a method embodiment for calculating power flow in a power grid system containing a multi-terminal flexible DC transmission system is provided. It should be noted that the steps shown in the flowchart in the accompanying drawings can be executed in a computer system such as a set of computer-executable instructions. Furthermore, although a logical order is shown in the flowchart, in some cases, the steps shown or described may be executed in a different order than that shown here.

[0022] Figure 1 This is a flowchart of an optional power grid flow calculation method for a multi-terminal flexible DC transmission system provided according to an embodiment of this application, such as... Figure 1 As shown, the method includes the following steps:

[0023] Step S102: Based on the AC bus voltage values ​​of the converters included in the power grid, determine the target state variable values ​​of the multi-terminal flexible DC transmission system included in the power grid, wherein the target state variable values ​​are a quantitative representation of the state variables used to describe the dynamic behavior of the multi-terminal flexible DC transmission system.

[0024] It is understandable that, based on the AC bus voltage values ​​of the converters included in the power grid, the target state variable values ​​of the multi-terminal flexible DC transmission system included in the power grid are calculated. The target state variables (e.g., DC voltage, DC current, converter control angle) are used to describe the dynamic behavior of the multi-terminal flexible DC transmission system, and the target state variable values ​​are the quantitative representation of the target state variables. By accurately initializing the target state variable values ​​of the multi-terminal flexible DC transmission system using the AC bus voltage values ​​of the converters, the convergence and stability of the calculation can be effectively improved, and the accuracy of the power flow calculation results for power grids containing multi-terminal flexible DC transmission systems can be enhanced.

[0025] Optionally, when solving for a DC transmission system (i.e., multiple multi-terminal flexible DC transmission systems obtained after decomposing a DC transmission system, and treating these multiple multi-terminal flexible DC transmission systems as multiple DC transmission systems within the DC transmission system), the Newton-Raphson method, which has superlinear convergence characteristics, can be used for calculation. This ensures that each DC transmission system has fewer iterations and faster calculation speed, avoiding the problem of being unable to continue calculation due to the square root being less than zero. If an angle exceeding the limit occurs after the iteration of the DC transmission system, a new processing method is used to relax the converter transformer ratio, effectively combining tap adjustment and power flow calculation, and avoiding oscillations in the calculation process caused by tap adjustment.

[0026] In one optional embodiment, determining the target state variable value of the multi-terminal flexible DC transmission system included in the power grid based on the AC bus voltage value of the converters included in the power grid includes: determining the initial state variable value of the multi-terminal flexible DC transmission system based on the AC bus voltage value; determining the initial state variable value as the target state variable value if the initial state variable value is less than a preset second determination threshold; or iteratively updating the initial state variable value to obtain an updated initial state variable value if the initial state variable value is greater than or equal to the second determination threshold; ending the iterative update process if the updated initial state variable value is less than the second determination threshold, and determining the initial state variable value obtained from the last iterative update as the target state variable value.

[0027] It is understandable that the initial state variable values ​​of the multi-terminal flexible DC transmission system included in the power grid are calculated based on the AC bus voltage value. These initial state variable values ​​are compared with a pre-set second judgment threshold. If the initial state variable value is less than the pre-set second judgment threshold, it means that the result of the initial state variable value at this time meets the error requirement, and this initial state variable value is determined as the target state variable value. If the initial state variable value is greater than or equal to the pre-set second judgment threshold, it means that the result of the initial state variable value at this time does not meet the error requirement, and iterative updates are still needed to obtain updated initial state variable values. The update iteration stops when the updated initial state variable value is less than the second judgment threshold, and the initial state variable value obtained from the last iteration is determined as the target state variable value. By setting a second judgment threshold and performing necessary iterative updates based on this threshold, the accuracy of the target state variable values ​​of the multi-terminal flexible DC transmission system can be ensured, thereby improving the accuracy of the overall power flow calculation results.

[0028] Optionally, for the entire AC / DC hybrid transmission system, the power flow expression on the AC / DC connection bus needs to be corrected based on the calculation results of the multi-terminal flexible DC transmission system due to the existence of the multi-terminal flexible DC transmission system. The correction amount ΔP for active power and the correction amount ΔQ for reactive power can be determined in the following way:

[0029]

[0030] Among them, P spec and Q spec These are the sum of the given generator output and the load injection, respectively; U a θ and θ represent the amplitude and phase angle of the AC bus voltage, respectively; P ac (U a ,θ) and Q ac (U a x, θ) are the branch power flow equations; dc For the state variables of the multi-terminal flexible DC transmission system. This is because the active power P transmitted by the multi-terminal flexible DC transmission system... dc (U a ,x dc and the reactive power P exchanged dc (U a ,x dc The voltage amplitude U of the AC side bus a The state variable values ​​are related to those of the multi-terminal flexible DC transmission system, therefore P is used. dc (U a ,x dc ) and Q dc (U a ,x dc() represents the active and reactive power obtained by the converter from the AC system (i.e., the active and reactive power transmitted from the multi-terminal flexible DC transmission system to the AC transmission system), with the outflow from the AC bus as the positive direction.

[0031] Alternatively, because P in equation (1) spec and Q spec State variables U of AC power transmission system a Regarding the calculation using the Newton-Raphson method, the Jacobian matrix of the AC transmission system can be corrected by using... and Correct the corresponding partial derivatives.

[0032] Optionally, Figure 2 This is an optional iterative calculation flowchart of a DC transmission system according to an embodiment of this application. The process of solving the target state of the DC transmission system is as follows: Figure 2 As shown. For converter i, the state variable can be selected as DC voltage U. di DC current I di The cosine value of the converter control angle, cosθ i (abbreviated as A) i ), then we have For the k-th DC transmission system unit, taking a two-ended DC transmission system (i.e., including two converters) as an example, with the two converters numbered i and j respectively, the state variable x of this DC transmission system unit is... dck It can be determined in the following way:

[0033]

[0034] For each converter, the three balance equations are chosen as the expressions for DC voltage, DC current, and control equations, respectively. DC voltage equation f k1 (x dck ) and DC current equation f k2 (x dck It can be determined in the following way:

[0035]

[0036] Among them, B i U is the number of converter bridges for converter i; ai X represents the amplitude of the AC bus voltage on converter i; ci ζ is the equivalent commutation reactance of converter i; i The type of converter i ("1" indicates a rectifier, "-1" indicates an inverter); T iLet be the turns ratio of converter i. The positive direction of current is the direction of flow into the DC line, and the positive direction of power is the direction of flow from the AC bus to the converter. j∈i represents all converters connected to converter i via DC lines, and the resistance of the connected DC lines is R. dij .

[0037] The control methods that can be used in a converter include: constant power control (CP), constant current control (CC), constant voltage control (CV), and constant angle control (CA). The control equation for constant power control is f... k3 (x dck The control equation for constant current control is f k3 (x dck The control equation for constant voltage control is f k3 (x dck ) and the control equation f of constant angle control k3 (x dck It can be determined in the following way:

[0038]

[0039] The superscript "set" indicates a preset value.

[0040] For the k-th DC transmission system, converter i uses constant power control and converter j uses constant angle control. In this case, the three state variable equations of converter i and converter j can be determined as follows:

[0041]

[0042] The selection method for the state variables of other DC power transmission system units included in the DC power transmission system follows the same principle, and will not be elaborated here.

[0043] For the k-th DC transmission system unit, the unsolved state variable equation f under the corresponding state variables is... k (x dck It can be determined in the following way:

[0044] f k (x dck )=0 (6)

[0045] The above state variable equations are solved using the Newton-Raphson method. The state variables in the (l+1)th iteration are... It can be determined in the following way:

[0046]

[0047] in, This represents the state variable in the l-th iteration. This represents the state variable in the (l+1)th iteration. This represents the correction amount of the state variable value in the (l+1)th iteration. This represents the state variable equation in the l-th iteration, where l is the iteration number; J (l) Let be the Jacobian matrix of the l-th iteration (-1 represents the inverse matrix).

[0048] J (l) It can be determined in the following way:

[0049]

[0050] Optionally, for multi-terminal flexible DC transmission systems, two-terminal flexible DC transmission systems are the most common, while five-terminal flexible DC transmission systems have the most terminals. Therefore, for each DC transmission system unit, the Jacobian matrix J... (l) The order is not large (3n for an n-terminal DC transmission system), making inversion easy. Matrix J (l) It neither satisfies the diagonal dominance property (diagonal elements may even be zero) nor possesses symmetry; therefore, a Jacobian matrix J can be used. (l) Equation (7) is solved by directly finding the inverse. The Jacobian matrix J (l) All possible control methods are reversible, and if constant power control is not used, the Jacobian matrix is ​​a constant matrix, requiring only one inversion calculation. Due to the superlinear convergence of the Newton-Raphson method, for DC transmission system units, the number of iterations required for solving according to equations (7) and (8) is small, resulting in fast calculation speed. For dual-ended flexible DC transmission systems, a stable solution definitely exists, and divergence due to "illness" will not occur.

[0051] Optionally, a second decision threshold can be determined based on the initial state variable value and the error value that meets the requirements. If the initial state variable value is less than the second decision threshold, the sum of the initial state variable value and the error value is used as the second decision threshold; if the initial state variable value is greater than or equal to the second decision threshold, the difference between the initial state variable value and the error value is used as the second decision threshold.

[0052] Using the above iterative calculation method, the initial state variable values ​​corresponding to the k DC transmission system units included in the DC transmission system are determined to satisfy the error requirement x. dcki <ε x The target state variable value. dcki ε represents the initial state variable value. xThis represents the second decision threshold. The aforementioned ε x It is a three-dimensional vector, with the three dimensions being the thresholds of the three state variables. Only when the initial state variable values ​​of the three state variables are all less than the thresholds of the three state variables is it said that the initial state variable value is less than the pre-set second judgment threshold.

[0053] Optionally, the calculation process for the multi-terminal flexible DC transmission system described above can be extended to the calculation of the target state variable values ​​for a DC transmission system containing multiple multi-terminal flexible DC transmission systems. The DC transmission system is decomposed into multiple multi-terminal flexible DC transmission systems. Each of these systems is treated as a DC transmission system unit, and the sub-target state variable values ​​for each unit are solved using the method described above. The sub-target state variable values ​​corresponding to each multi-terminal flexible DC transmission system are then used as the target state variable values ​​for the entire DC transmission system.

[0054] Optionally, in a DC transmission system containing multiple multi-terminal flexible DC transmission systems, a single multi-terminal flexible DC transmission system can be treated as a single DC transmission system unit within the DC transmission system. The entire DC transmission system can then be decomposed into multiple DC transmission system units for calculation, instead of solving all DC transmission system units together. This reduces the computational complexity for DC transmission systems with multiple DC transmission system units. For each DC transmission system unit's converter, three state variables and three imbalance equations are selected. Thus, the dimension of the equations for solving a single DC transmission system unit is three times the number of converters in that DC transmission system. By dividing the entire DC transmission system into multiple low-dimensional equations for calculation on a per-DC transmission system basis, computational speed is ensured, and the equation dimension is prevented from increasing excessively as the number of DC transmission systems grows. This improvement avoids singularities in the coefficient matrix of the DC transmission system's equations. Furthermore, if the operating parameters of one DC transmission system are unreasonable, it will not affect the normal calculation of other DC transmission systems.

[0055] In one optional embodiment, when the state variables include DC voltage, DC current, and the control angle of the converter, and the initial state variable values ​​include initial DC voltage value, initial DC current value, and initial control angle cosine value, determining the initial state variable values ​​of the multi-terminal flexible DC transmission system based on the AC side bus voltage value includes: determining the initial DC voltage value based on the AC side bus voltage value; determining the initial DC current value based on the DC resistance and the initial DC voltage value; and determining the initial control angle cosine value based on the preset control angle cosine value.

[0056] It is understandable that if the state variables of a multi-terminal flexible DC transmission system include DC voltage, DC current, and the converter control angle, and the corresponding initial state variable values ​​include initial DC voltage, initial DC current, and initial control angle cosine, then the initial DC voltage value of the multi-terminal flexible DC transmission system can be calculated based on the AC bus voltage value of the converter; the initial DC current value can be calculated based on the DC resistance and the initial DC voltage value; and the initial control angle cosine value of the converter can be calculated based on the preset control angle cosine value. By accurately initializing the state variables of the multi-terminal flexible DC transmission system, including DC voltage, DC current, and converter control angle, the accuracy of power flow calculation results for power grids containing multi-terminal flexible DC transmission can be effectively improved, ensuring robust convergence of the calculation process and providing solid data support for the efficient operation and planning of power systems.

[0057] In an optional embodiment, when the state variables include DC voltage, DC current, and the control angle of the converter, and the initial state variable values ​​include initial DC voltage values, initial DC current values, and initial control angle cosine values, the method further includes: during the iterative update process, if the control angle cosine value is greater than a preset control angle cosine value threshold, determining the target iterative update number corresponding to the control angle cosine value being greater than the control angle cosine value threshold, and the control angle cosine value corresponding to the previous iterative update number of the target iterative update number; determining the control angle cosine value corresponding to the previous iterative update number as the target control angle cosine value; replacing the control angle included in the state variables with the converter turns ratio to obtain the updated state variables and the updated initial state variable values, wherein the turns ratio is used to describe the proportional relationship between the voltages on both sides of the converter; and determining the target state variable value based on the updated state variables and the updated initial state variable values.

[0058] It is understandable that if the state variables of a multi-terminal flexible DC transmission system include DC voltage, DC current, and the control angle of the converter, and the corresponding initial state variable values ​​include the initial DC voltage value, the initial DC current value, and the initial control angle cosine value, then if, during the iterative update of the initial state variable values, the control angle cosine value of the converter exceeds a pre-set control angle cosine value threshold, the target iteration update number when the control angle cosine value exceeds the threshold, and the control angle cosine value corresponding to the previous iteration update number, are determined. The control angle cosine value obtained at the previous iteration update number is used as the target control angle cosine value, and the converter control angle included in the state variables is replaced with the converter turns ratio (used to describe the proportional relationship between the voltages on both sides of the converter), resulting in the updated state variables and the updated initial state variable values. Based on the updated state variables and the updated initial state variable values, it is determined that the target state variable value only needs to be updated iteratively until the target state variable value that meets the error requirements is obtained. By using the transformer ratio as a substitute state variable to replace the control angle, the rapid convergence of the multi-terminal flexible DC transmission system under various control combinations can be ensured, and effective convergence can be achieved even under extreme conditions, thereby improving the accuracy of power flow calculation results.

[0059] Optionally, the Newton-Raphson method is used to solve for the target state variable value. After each iteration, the absolute value of the cosine of the firing angle or extinction angle (i.e., the control angle) is verified to be less than 1. If the absolute value of the cosine is greater than 1, it indicates that the operating parameters of the DC transmission system (i.e., the multi-terminal flexible DC transmission system) are contradictory. This problem can be solved by modifying the state variable. The converter transformer turns ratio is relaxed and used as the state variable, replacing the cosine value A of the control angle in the original state variable. Therefore, after the iteration, the obtained turns ratio T is the most suitable converter transformer turns ratio (i.e., the target turns ratio value of the state variable turns ratio).

[0060] Optionally, if after the iterative calculation of the DC transmission system unit is completed, it is found that the obtained state variables correspond to A i If the value is greater than 1, then for A i The over-limit converter i will change the state variable. Change to By relaxing the converter transformer turns ratio using the above method, the control angle can be calculated as a known quantity, A. i Take the iteration result before the limit is exceeded. Using the above method, it is possible to modify only the partial derivatives of the voltage expression with respect to the state variables in equation (3), while keeping the rest unchanged. Therefore, the modification to the Jacobian matrix is ​​very small, which is convenient to implement. Since a converter using fixed-angle control cannot have the cosine value A of the control angle. iFor cases greater than 1, the partial derivatives of the governing equations with respect to the state variables cannot all be zero. Using the above processing method, the correction amount calculated by equation (7) is used to correct the corresponding state variables. Since the correction amount must be an integer multiple of the tap position step size, for state variables with relaxed turns ratios, the correction amount needs to be discretized before correction. Assume the corresponding converter transformer tap position step size is T. step The corresponding converter transformer turns ratio correction amount calculated according to equation (7) is ΔT. i ′, actual correction amount ΔT i It can be determined in the following way:

[0061] ΔT i =R OUND (ΔT i ′ / T step )·T step (10)

[0062] Where ΔT represents rounding, that is, taking the nearest integer. Equation (10) ensures that the correction amount of the transformer ratio meets the requirements of tap position discreteness. Since the transformer ratio is relaxed and discretized by step size, the iteration result can ensure the rationality of the transformer ratio, and the operating parameters of the DC transmission system unit fully meet the external characteristics of the converter, the DC network equation and its control equation. The above tap adjustment method can ensure the rapid convergence of the DC transmission system unit, and also ensure that it will not be repeatedly calculated due to the adjustment of the tap position, avoiding the problem of numerical oscillation during the calculation process due to unreasonable adjustment.

[0063] Step S104: Based on the target state variable values, determine the initial power flow calculation results of the AC transmission system included in the power grid. The initial power flow calculation results are used to represent the voltage amplitude and voltage phase angle of the nodes included in the AC transmission system. A node refers to a specific location in the AC transmission system where electrical attributes need to be displayed.

[0064] It is understandable that the initial power flow calculation results of the AC transmission system included in the power grid are calculated based on the target state variable values ​​of the multi-terminal flexible DC transmission system. These initial power flow calculation results include the voltage magnitude and voltage phase angle of the nodes included in the AC transmission system; each node represents a specific location in the AC transmission system where electrical attributes need to be displayed. Since the target state variable values ​​of the multi-terminal flexible DC transmission system have been precisely determined, they can be used as boundary conditions for the AC transmission system iteration, thereby improving the accuracy of the AC power flow calculation. Furthermore, by dividing the calculation process into DC and AC parts, computational resources can be allocated more effectively, improving resource utilization efficiency.

[0065] In one optional embodiment, determining the initial power flow calculation results of the AC transmission system included in the power grid based on the target state variable values ​​includes: determining the active power and reactive power transmitted from the multi-terminal flexible DC transmission system to the AC transmission system based on the target state variable values; and determining the initial power flow calculation results based on the active power and reactive power.

[0066] It is understandable that, based on the obtained target state variable values ​​of the multi-terminal flexible DC transmission system, the active and reactive power transmitted from the multi-terminal flexible DC transmission system to the AC transmission system are determined. Based on these active and reactive power values, the initial power flow calculation results for the power grid are determined. By accurately calculating the power injected into the AC transmission system from the multi-terminal flexible DC transmission system and using this calculation as input for the AC transmission system's power flow calculation, the accuracy of the starting point for the AC transmission system's calculation can be ensured, thereby improving the accuracy of the power flow calculation results for the entire power grid system.

[0067] Step S106: If the initial power flow calculation result is less than the preset first judgment threshold, the initial power flow calculation result is determined as the target power flow calculation result of the power grid.

[0068] It is understandable that by comparing the initial power flow calculation result with a pre-set first judgment threshold, if the initial power flow calculation result is less than the pre-set first judgment threshold, it indicates that the power flow calculation result of the power grid meets the error requirements, and the initial power flow calculation result at this time is determined as the target power flow calculation result of the power grid. The setting of the first judgment threshold can effectively ensure the accuracy of the power flow calculation result, ensure high consistency between the calculated results of node voltage and phase angle of AC transmission system and the actual system state, and provide strong data support for improving the safe operation of the entire power grid.

[0069] In one optional embodiment, when the initial power flow calculation result includes voltage amplitude and voltage phase angle, and the corresponding first determination threshold includes voltage amplitude threshold and voltage phase angle threshold, if the initial power flow calculation result is less than the preset first determination threshold, the initial power flow calculation result is determined as the target power flow calculation result of the power grid, including: if the voltage amplitude is less than the voltage amplitude threshold and the voltage phase angle is less than the voltage phase angle threshold, the initial power flow calculation result is determined as the target power flow calculation result.

[0070] It is understood that in the initial power flow calculation results of an AC transmission system, which include voltage amplitude and voltage phase angle, and where the corresponding first judgment thresholds include voltage amplitude thresholds and voltage phase angle thresholds, an initial power flow calculation result less than the pre-set first judgment threshold specifically means that the voltage amplitude included in the initial power flow calculation result is less than the voltage amplitude threshold, and the voltage phase angle included in the initial power flow calculation result is less than the voltage phase angle threshold. The initial power flow calculation result that meets the above conditions is determined as the target power flow calculation result for the power grid. By strictly controlling the deviations in voltage amplitude and voltage phase angle, it can be ensured that the power flow calculation results conform to the actual standards of power grid operation, thereby improving the accuracy of power flow calculation results for urban power grids with multi-terminal flexible DC embedded components, and providing a solid data foundation for the stable operation of the power grid.

[0071] In an optional embodiment, the method further includes: if the initial power flow calculation result is greater than or equal to a first determination threshold, iteratively updating the target state variable value based on the voltage amplitude included in the initial power flow calculation result to obtain an updated target state variable value; iteratively updating the initial power flow calculation result based on the updated target state variable value to obtain an updated initial power flow calculation result; if the updated initial power flow calculation result is less than the first determination threshold, stopping the iterative update, and determining the initial power flow calculation result obtained from the last iterative update as the target power flow calculation result.

[0072] It is understandable that if the initial power flow calculation result of the power grid is greater than or equal to the first judgment threshold, it indicates that the power flow calculation result of the power grid meets the error requirements. At this point, it is necessary to iteratively update the target state variable values ​​of the multi-terminal flexible DC transmission system based on the voltage amplitude included in the initial power flow calculation result, obtaining updated target state variable values. Then, based on the updated target state variable values, the initial power flow calculation result of the AC transmission system is updated iteratively to obtain updated initial power flow calculation results. This iterative update is repeated until the obtained initial power flow calculation result is less than the first judgment threshold, at which point the iterative update stops, and the initial power flow calculation result obtained from the last iterative update is determined as the target power flow calculation result of the power grid. The iterative update process ensures that the initial power flow calculation result gradually approaches the actual power grid state, ensuring that the final power flow calculation result is more accurate and can truly reflect the actual operating conditions of the power grid.

[0073] Optionally, the above-mentioned "iterative update of the target state variable values ​​of the multi-terminal flexible DC transmission system based on the voltage amplitude included in the initial power flow calculation results to obtain the updated target state variable values" is a process of updating and iterating the target state variable values ​​of the multi-terminal flexible DC transmission system in the power grid based on the voltage amplitude included in the calculated initial power flow calculation results. First, the initial state variable values ​​of the multi-terminal flexible DC transmission system are updated according to the voltage amplitude. The initial state variable values ​​are compared with the second judgment threshold. If the initial state variable value is less than the second judgment threshold, the initial state variable value is determined to be the updated target state variable value. If the initial state variable value is greater than or equal to the second judgment threshold, the initial state variable value is updated iteratively to obtain the updated initial state variable value. The iteration stops when an initial state variable value that meets the error requirement is obtained, and the initial state variable value obtained in the last iteration is determined as the target state variable value.

[0074] Optionally, the above-mentioned "updating and iterating the initial power flow calculation results of the AC transmission system based on the updated target state variable values ​​to obtain updated initial power flow calculation results" is a process of updating and iterating the initial power flow calculation results of the AC transmission system based on the updated target state variable values ​​of the multi-terminal flexible DC transmission system. The voltage amplitude included in the initial power flow calculation results is used as a known quantity to update the target state variable values ​​of the multi-terminal flexible DC transmission system, resulting in updated target state variable values. Based on the updated target state variable values, the active power and reactive power transmitted from the multi-terminal flexible DC transmission system to the AC transmission system are updated, resulting in updated active power and reactive power. Based on the updated active power and reactive power, the updated initial power flow calculation results of the AC transmission system are obtained. It is then determined whether the updated initial power flow calculation results meet the error requirements. If they do, the updated initial power flow calculation results are determined as the target power flow calculation results of the power grid. If they do not meet the requirements, the initial power flow calculation results are updated and iterated according to the above process until an initial power flow calculation result that meets the error requirements is obtained.

[0075] Optionally, Figure 3 This is an iterative calculation flowchart of an optional AC / DC hybrid transmission system provided according to an embodiment of this application. The iterative calculation process of the AC / DC hybrid transmission system is as follows: Figure 3 As shown. For the alternating iterative process of an AC / DC hybrid transmission system, it involves embedding a complete calculation process for solving a DC transmission system within the iteration of the AC transmission system. The power flow calculation results of the AC / DC hybrid transmission system can be calculated as follows: First, based on the initial value of the entire network voltage (i.e., the AC bus voltage value of the converter at the first iteration), calculate the target state variable value of the DC transmission system. If the AC bus voltage of the converter has no initial value, then take a per-unit value of 1 for calculation. The calculated P...dc and Q dc Solve for the voltage amplitude and voltage phase angle. If the correction values ​​for the voltage amplitude and voltage phase angle do not meet the threshold requirements, substitute the obtained voltage amplitude into the equation. Figure 2 The target state variable values ​​of the DC transmission system are updated and iterated continuously until the iteration converges, i.e., the error requirement {U} is met. a , θ}<ε y , ε y This represents the first judgment threshold. The aforementioned ε y It is a two-dimensional vector with the voltage amplitude threshold and the voltage phase angle threshold as the two dimensions, respectively. Only when the voltage amplitude is less than the voltage amplitude threshold and the voltage phase angle is less than the voltage phase angle threshold is it considered that the initial power flow calculation result is less than the first judgment threshold.

[0076] Through the above steps S102 to S106, the target state variables of the multi-terminal flexible DC system can be determined based on the AC bus voltage value of the converter, and then the initial power flow result of the AC transmission system can be calculated. The feasibility of the result can be verified by using the first judgment threshold, and the power flow calculation result of the power grid can be determined when the first judgment result passes. This achieves the technical effect of improving the accuracy of the power flow calculation result of the power grid containing the multi-terminal flexible DC transmission system, and solves the technical problem of inaccurate power flow calculation result of the power grid containing the multi-terminal flexible DC transmission system in the related technology.

[0077] Based on the above embodiments and optional embodiments, this application proposes an optional implementation method for power flow calculation of a power grid containing a multi-terminal flexible DC transmission system. This method can be understood as a power flow calculation method considering a multi-terminal flexible DC embedded medium-voltage distribution network. First, this method analyzes the mechanism of power flow calculation in AC / DC hybrid transmission systems and the characteristics of multi-terminal flexible DC transmission systems, studying power flow calculation methods under AC grids containing multi-terminal flexible DC embedded systems. Second, by modifying the alternating iteration method, one of the power flow calculation methods for AC / DC hybrid transmission systems, it adopts the robust and convergent Newton-Raphson method to solve the multi-terminal flexible DC transmission system. Simultaneously, it proposes a method to flexibly adjust the tap position when the control angle cosine value is greater than 1 (i.e., the control angle cosine value threshold). Through the above method, it is possible to ensure that, while meeting a certain convergence accuracy, the computational efficiency and accuracy of power flow calculation results for AC / DC hybrid transmission systems containing multi-terminal flexible DC are improved, solving the problem of non-convergence in power flow calculation (i.e., ill-conditioned power flow) caused by the singularity of the Jacobian matrix in the context of multi-terminal flexible DC transmission system embedding. Figure 4 This is a structural block diagram of an optional power flow calculation method for a multi-terminal flexible DC transmission system provided according to an embodiment of this application, such as... Figure 4 As shown, the steps of the power flow calculation method considering multi-terminal flexible DC embedded medium-voltage distribution networks include:

[0078] Step S1: Calculation and analysis of power flow in the AC / DC hybrid transmission system.

[0079] Currently, there are two main methods for calculating power flow in AC / DC hybrid transmission systems that include HVDC transmission systems: the unified iterative method and the alternating iterative method. The unified iterative method, also known as the joint solution method, works by simultaneously solving the constraint equations of the AC / DC hybrid transmission system. This method has good convergence; however, as the number of converters in the AC / DC hybrid transmission system increases, the size of the Jacobian matrix expands, and the calculation speed decreases exponentially due to the increased computational load. While the unified iterative algorithm has good convergence, its poor compatibility necessitates significant modifications when improving it, leading to substantial differences between the final algorithm and the unified iterative method.

[0080] The alternating iteration method, also known as the alternating solution method, is based on the idea of ​​weakening the connection between the AC transmission system and the multi-terminal flexible DC transmission system, and achieving the final solution by calculating the AC transmission system and the multi-terminal flexible DC transmission system separately. When solving a multi-terminal flexible DC transmission system, the previous iteration result of the AC transmission system is used as a known quantity. The most crucial aspect is that the voltage magnitude (i.e., the AC bus voltage of the converter) of the AC-DC connection node (where the converter is located) after the previous AC transmission system iteration is used as a known quantity and applied to calculate the state variables and power distribution of the multi-terminal flexible DC transmission system. When solving the AC transmission system, the previous iteration result of the multi-terminal flexible DC transmission system is again used as a known quantity. That is, the active and reactive power of the AC-DC connection node (i.e., the active and reactive power transmitted from the multi-terminal flexible DC transmission system to the AC transmission system) is injected as a known quantity and applied to calculate the voltage magnitude and phase angle of the AC transmission system node. This process is repeated, alternating between using the previous iteration results of both systems and obtaining the current iteration results from each system, until the iteration results of both the AC transmission system and the multi-terminal flexible DC transmission system converge. The advantages of the alternating iterative method are that different algorithms can be used for the DC and AC components, which facilitates programming. Furthermore, it requires less computer memory during program execution. In addition, the algorithm has good inheritance; only minor modifications are needed to solve problems such as ill-conditioned power flow, making it easy to utilize existing power flow calculation programs for hybrid AC / DC transmission systems during modifications.

[0081] In summary, analysis and comparison reveal that alternating solutions are commonly used to calculate power flow results in Energy Management Systems (EMS). However, for multi-terminal flexible DC transmission systems, this method does not adequately address the issues of rapid control and proper coordination of tap positions. During voltage reduction and power reduction operation of multi-terminal flexible DC transmission systems, power flow calculations encounter several problems under different operating modes: First, directly deriving calculation formulas from the converter external characteristics and network equations can lead to the taking of square roots of negative numbers during calculations; second, the cosine value of the converter's firing angle (extinguishing angle) becomes unrecoverable during iterative calculations due to its greater than 1; third, the inability to correctly determine the adjustment amount when adjusting tap positions can cause oscillations between the AC transmission system and the multi-terminal flexible DC transmission system, resulting in non-convergence of power flow calculations.

[0082] Step S2, Analysis of the control mode of multi-terminal flexible DC transmission system.

[0083] Stable operation of multi-terminal flexible DC transmission systems requires coordinated control of multiple converters to achieve power balance and DC voltage stability. Different combinations of converter control methods directly affect the convergence and divergence of power flow calculation results. For example, in a three-terminal flexible DC transmission system, with two converters using constant-angle control and one using constant-power control, the system will exhibit unstable operation. Even if the Newton-Raphson method is used to correct the excessive correction caused by the "illness" of the Jacobian matrix, it can only guarantee the convergence of DC calculation results for each iteration, not the convergence of calculation results obtained when the entire AC / DC hybrid transmission system is iteratively calculated.

[0084] Step S3: Improved alternating iterative algorithm.

[0085] For the entire AC / DC hybrid transmission system, the power flow expression on the AC / DC connection bus needs to be corrected based on the calculation results of the multi-terminal flexible DC transmission system due to the existence of the multi-terminal flexible DC transmission system. The determination methods for the correction amount ΔP of active power and the correction amount ΔQ of reactive power are the same as those in the above embodiments, and will not be repeated here.

[0086] Because P in equation (1) spec and Q spec State variables U of AC power transmission system a Regarding this, when using the Newton-Raphson method for calculation, it is necessary to correct the Jacobian matrix of the AC transmission system, that is, to use... and Correct the corresponding partial derivatives.

[0087] The following three improvements were made to the power flow calculation equations for multi-terminal flexible DC transmission systems:

[0088] 1) In DC transmission systems containing multiple multi-terminal flexible DC transmission systems, each multi-terminal flexible DC transmission system is treated as a single DC transmission system unit, and the entire DC transmission system is decomposed into multiple DC transmission system units for calculation, rather than solving all DC transmission system units together. This reduces the computational burden for DC transmission systems with multiple DC transmission system units. For each DC transmission system unit's converter, three state variables and three unbalance equations are selected. Thus, the dimension of the equations for solving a single DC transmission system unit is three times the number of converters in that DC transmission system. By dividing the entire DC transmission system into multiple low-dimensional equations for calculation, the computational speed is ensured, and the equation dimension does not increase excessively as the number of DC transmission systems increases. This improvement avoids singularities in the coefficient matrix of the DC transmission system's equations. Furthermore, if the operating parameters of one DC transmission system are unreasonable, it will not affect the normal calculation of other DC transmission systems.

[0089] 2) When solving the DC power transmission system, the Newton-Raphson method, which has superlinear convergence characteristics, is used to ensure that the number of iterations for each DC power transmission system is small and the calculation speed is fast. This avoids the problem of being unable to continue the calculation due to the square root being less than zero. If the angle exceeds the limit after the iteration of the DC power transmission system, a new processing method is used to relax the converter transformer ratio, so that the tap adjustment and power flow calculation are effectively combined, avoiding the problem of oscillation in the calculation process caused by tap adjustment.

[0090] 3) Flexible handling of angle exceeding limits and tap adjustment issues during power flow calculations. The Newton-Raphson method is used for solving the problem. After each iteration, the absolute value of the cosine of the firing angle or extinction angle (i.e., the control angle) is verified to be less than 1. If the absolute value of the cosine is greater than 1, it indicates a contradiction between the operating parameters of the DC power transmission system. This problem is addressed by modifying the state variables. The converter transformer turns ratio is relaxed and used as a state variable to replace the cosine value A of the control angle in the original state variables. Therefore, after the iteration, the obtained turns ratio T is the most suitable converter transformer turns ratio (i.e., the target turns ratio value of the state variable turns ratio).

[0091] The process of solving the target state of a DC transmission system is as follows: Figure 2 As shown. For converter i, the state variable is chosen as DC voltage U. di DC current I diThe cosine value of the converter control angle, cosθ i Abbreviated as A i Then there is For the k-th DC transmission system unit, taking a two-ended DC transmission system (i.e., including two converters) as an example, with the two converters numbered i and j respectively, the state variable x of this DC transmission system unit is... dck The method for determining the value is the same as in the above embodiments, and will not be repeated here.

[0092] For each converter, the three balance equations are chosen as the expressions for DC voltage, DC current, and control equations, respectively. DC voltage equation f k1 (x dck ) and DC current equation f k2 (x dck The method for determining ) is the same as in the above embodiments, and will not be repeated here.

[0093] The control methods that can be used in a converter include: constant power control (CP), constant current control (CC), constant voltage control (CV), and constant angle control (CA). The control equation for constant power control is f... k3 (x dck The control equation for constant current control is f k3 (x dck The control equation for constant voltage control is f k3 (x dck ) and the control equation f of constant angle control k3 (x dck The method for determining ) is the same as in the above embodiments, and will not be repeated here.

[0094] For the k-th DC transmission system, converter i adopts constant power control and converter j adopts constant angle control. In this case, the determination method of the three state variable equations of converter i and converter j can be expressed in the following way. The method is the same as that in the above embodiment, and will not be repeated here.

[0095] The selection method for state variables of other DC power transmission system units included in the DC power transmission system follows the same principle.

[0096] For the k-th DC transmission system unit, the unsolved state variable equation f under the corresponding state variables is... k (x dck The method for determining ) is the same as in the above embodiments, and will not be repeated here.

[0097] The above state variable equations are solved using the Newton-Raphson method. The state variables in the (l+1)th iteration are... With J (l) The method for determining the value is the same as in the above embodiments, and will not be repeated here.

[0098] For multi-terminal flexible DC transmission systems, two-terminal flexible DC transmission systems are the most common, while five-terminal flexible DC transmission systems have the most terminals. Therefore, for each DC transmission system unit, the Jacobian matrix J... (l) The order is not large (3n for an n-terminal DC transmission system), making inversion easy. Matrix J (l) It neither satisfies the diagonal dominance property (diagonal elements may even be zero) nor possesses symmetry; therefore, a Jacobian matrix J is used. (l) Equation (7) is solved by directly finding the inverse. The Jacobian matrix J (l) All possible control methods are reversible, and if constant power control is not used, the Jacobian matrix is ​​a constant matrix, requiring only one inversion calculation. Due to the superlinear convergence of the Newton-Raphson method, for DC transmission system units, the number of iterations required to solve according to equations (7) and (8) is small, resulting in fast calculation speed. For dual-ended flexible DC transmission systems, a stable solution definitely exists, and there will be no divergence due to "illness".

[0099] If, after the iterative calculation of the DC transmission system unit is completed, it is found that the obtained state variables correspond to A... i If the value is greater than 1, then for A i The over-limit converter i will change the state variable. Change to By relaxing the converter transformer turns ratio using the above method, and calculating the control angle as a known quantity, A i Take the iteration result before the limit is exceeded. Using the above method, it is possible to modify only the partial derivatives of the voltage expression with respect to the state variables in equation (3), while keeping the rest unchanged. Therefore, the modification to the Jacobian matrix is ​​very small, which is convenient to implement. Since a converter using fixed-angle control cannot have the cosine value A of the control angle. i For cases greater than 1, the partial derivatives of the governing equations with respect to the state variables cannot all be zero. Using the above processing method, the correction amount calculated by equation (7) is used to correct the corresponding state variables. Since the correction amount must be an integer multiple of the tap position step size, for state variables with relaxed turns ratios, the correction amount needs to be discretized before correction. Assume the corresponding converter transformer tap position step size is T. step The corresponding converter transformer turns ratio correction amount calculated according to equation (7) is ΔT. i ′, actual correction amount ΔT i The method for determining the value is the same as in the above embodiments, and will not be repeated here.

[0100] Equation (10) ensures that the correction amount of the transformer ratio meets the requirements of tap position discreteness. Since the transformer ratio is relaxed and discretized by step size, the iteration result can ensure the rationality of the transformer ratio, and the operating parameters of the DC transmission system unit fully satisfy the external characteristics of the converter, the DC network equations and their control equations. The above tap adjustment method can ensure the rapid convergence of the DC transmission system unit, and also ensure that it will not be repeatedly calculated due to the adjustment of the tap position, thus avoiding the problem of numerical oscillation during the calculation process due to unreasonable adjustment.

[0101] Using the iterative calculation method described above, the error requirement x for the k DC transmission system units included in the DC transmission system is determined. dcki <ε x The target state variable value.

[0102] The iterative calculation process of the AC / DC hybrid transmission system is as follows: Figure 3 As shown. For the alternating iterative process of an AC / DC hybrid transmission system, a complete calculation process for solving a DC transmission system is embedded within the iterative process of the AC transmission system. First, based on the initial value of the entire network voltage (i.e., the AC bus voltage value of the converter at the first iteration), the target state variable value of the DC transmission system is calculated. If the AC bus voltage of the converter has no initial value, a per-unit value of 1 is used for calculation. The calculated P... dc and Q dc Solve for the voltage amplitude and voltage phase angle. If the correction values ​​for the voltage amplitude and voltage phase angle do not meet the threshold requirements, substitute the obtained voltage amplitude into the equation. Figure 2 The target state variable values ​​of the DC transmission system are updated and iterated continuously until the iteration converges, i.e., the error requirement {U} is met. a , θ}<ε y .

[0103] Step S4: Simulation to verify the effectiveness of the algorithm.

[0104] Figure 5 This is a structural diagram of an optional three-terminal flexible DC transmission system provided according to an embodiment of this application, such as... Figure 5 The figure shows a hybrid AC / DC transmission system that includes a three-terminal flexible DC transmission system. Figure 5The nodes numbered 1 to 14 represent different components of the power system, such as generators, loads, converter substations, and nodes used for reactive power compensation or energy storage. These are key connection points in the power grid. VSC (Voltage Source Converter) represents the converter, G represents the generator, the downward arrow represents the load, and the two intersecting circles represent the transformer. The three-terminal flexible DC transmission system replaces the original three AC lines connected between nodes 2, 4, and 5. The control method of the DC transmission system is shown in Table 1. In Table 1, except for the control angle which is an angle value, all other values ​​are per-unit values. In the table, R represents the rectifier, and I represents the inverter.

[0105] Table 1 Parameters of Three-Terminal Flexible DC Transmission System

[0106]

[0107] For example Figure 5 The AC / DC hybrid power transmission system shown was subjected to nine iterations, taking 3.87 seconds. The simulation results are shown in Table 2.

[0108] To demonstrate the robustness of the above method, power flow calculations were performed under a flat-start mode, where the initial voltage and current values ​​of the DC transmission system were both 1, and the voltage amplitude of the AC bus was also 1. The iterative convergence accuracy for both the AC and DC transmission systems was set to 10. -5 At this point, the AC / DC hybrid power transmission system typically converges around 10 times.

[0109] Table 2 Simulation Results

[0110]

[0111]

[0112] The above optional implementation methods achieve at least the following effects: By accurately initializing the state variables of a multi-terminal flexible DC transmission system, including DC voltage, DC current, and converter control angle, the accuracy of power flow calculation results for power grids with multi-terminal flexible DC transmission can be effectively improved, ensuring robust convergence of the calculation process and providing solid data support for the efficient operation and planning of the power system; by using the transformer ratio as a substitute state variable to replace the control angle, the rapid convergence of the multi-terminal flexible DC transmission system under various control mode combinations can be ensured, and effective convergence can be achieved even under extreme operating conditions, improving the accuracy of power flow calculation results.

[0113] It should be noted that the steps shown in the flowchart in the accompanying drawings can be executed in a computer system such as a set of computer-executable instructions, and although a logical order is shown in the flowchart, in some cases the steps shown or described may be executed in a different order than that shown here.

[0114] This embodiment also provides a power flow calculation device for a multi-terminal flexible DC transmission system. This device is used to implement the above embodiments and preferred embodiments, and details already described will not be repeated. As used below, the terms "module" and "device" can refer to a combination of software and / or hardware that performs a predetermined function. Although the device described in the following embodiments is preferably implemented in software, hardware implementation, or a combination of software and hardware, is also possible and contemplated.

[0115] According to an embodiment of this application, an apparatus embodiment for implementing a power flow calculation method for a power grid containing a multi-terminal flexible DC transmission system is also provided. Figure 6 This is a schematic diagram of a power grid flow calculation device for a multi-terminal flexible DC transmission system according to an embodiment of this application, as shown below. Figure 6 As shown, the above-mentioned power flow calculation device for a power grid containing a multi-terminal flexible DC transmission system includes a first determining module 602, a second determining module 604, and a target power flow calculation result determining module 606. The device will be described below.

[0116] The first determining module 602 is used to determine the target state variable value of the multi-terminal flexible DC transmission system included in the power grid based on the AC side bus voltage value of the converter included in the power grid, wherein the target state variable value is a quantitative representation of the state variable used to describe the dynamic behavior of the multi-terminal flexible DC transmission system;

[0117] The second determining module 604, connected to the first determining module 602, is used to determine the initial power flow calculation results of the AC transmission system included in the power grid based on the target state variable values. The initial power flow calculation results are used to represent the voltage amplitude and voltage phase angle of the nodes included in the AC transmission system. A node refers to a specific location in the AC transmission system where electrical attributes need to be displayed.

[0118] The target power flow calculation result determination module 606 is connected to the second determination module 604 and is used to determine the initial power flow calculation result as the target power flow calculation result of the power grid when the initial power flow calculation result is less than a preset first judgment threshold.

[0119] In a power flow calculation device for a multi-terminal flexible DC transmission system provided in this application embodiment, a first determining module 602 is set up to determine the target state variable value of the multi-terminal flexible DC transmission system in the power grid based on the AC bus voltage value of the converters included in the power grid. The target state variable value is a quantitative representation of the state variable used to describe the dynamic behavior of the multi-terminal flexible DC transmission system. A second determining module 604, connected to the first determining module 602, is used to determine the initial power flow calculation result of the AC transmission system included in the power grid based on the target state variable value. The initial power flow calculation result is used to represent the voltage amplitude and voltage phase angle of the nodes included in the AC transmission system. A node refers to a specific location in the AC transmission system that needs to display electrical attributes. A target power flow calculation result determining module 606, connected to the second determining module 604, is used to determine the initial power flow calculation result as the target power flow calculation result of the power grid if the initial power flow calculation result is less than a preset first judgment threshold. The goal is to first determine the target state variables of a multi-terminal flexible DC transmission system based on the AC bus voltage value of the converter, then calculate the initial power flow results of the AC transmission system, verify the feasibility of the results using a first judgment threshold, and determine the power flow calculation results of the power grid when the first judgment result passes. This achieves the technical effect of improving the accuracy of power flow calculation results for power grids containing multi-terminal flexible DC transmission systems, thereby solving the technical problem of inaccurate power flow calculation results for power grids containing multi-terminal flexible DC transmission systems in related technologies.

[0120] It should be noted that the above modules can be implemented by software or hardware. For example, for the latter, it can be implemented in the following ways: the above modules can be located in the same processor; or the above modules can be located in different processors in any combination.

[0121] It should be noted that the first determining module 602, the second determining module 604, and the target power flow calculation result determining module 606 mentioned above correspond to steps S102 to S106 in the embodiments. The instances and application scenarios implemented by the above modules and corresponding steps are the same, but are not limited to the content disclosed in the above embodiments. It should be noted that the above modules, as part of the device, can run in a computer terminal.

[0122] It should be noted that the optional or preferred implementation methods of this embodiment can be found in the relevant descriptions in the embodiments, and will not be repeated here.

[0123] The aforementioned power flow calculation device for a multi-terminal flexible DC transmission system may also include a processor and a memory. The first determination module 602, the second determination module 604, the target power flow calculation result determination module 606, etc., are all stored in the memory as program units, and the processor executes the aforementioned program units stored in the memory to realize the corresponding functions.

[0124] The processor contains a core that retrieves the corresponding program unit from memory. One or more cores may be configured. Memory may include non-persistent memory in computer-readable media, random access memory (RAM), and / or non-volatile memory, such as read-only memory (ROM) or flash RAM. Memory includes at least one memory chip.

[0125] This application provides a non-volatile storage medium storing a program that, when executed by a processor, implements a power flow calculation method for a multi-terminal flexible DC transmission system containing any of the above-mentioned features.

[0126] This application provides an electronic device, which includes a processor, a memory, and a program stored in the memory and executable on the processor. When the processor executes the program, it implements any of the above-described steps of the power flow calculation method for a multi-terminal flexible DC transmission system. The device described herein may be a server, PC, etc.

[0127] This application also provides a computer program product, which, when executed on a data processing device, is suitable for executing an initialization program with the following method steps: determining the target state variable value of the multi-terminal flexible DC transmission system included in the power grid based on the AC bus voltage value of the converter included in the power grid, wherein the target state variable value is a quantitative representation of the state variable used to describe the dynamic behavior of the multi-terminal flexible DC transmission system; determining the initial power flow calculation result of the AC transmission system included in the power grid based on the target state variable value, wherein the initial power flow calculation result is used to represent the voltage amplitude and voltage phase angle of the nodes included in the AC transmission system, and a node refers to a specific location in the AC transmission system where electrical attributes need to be displayed; if the initial power flow calculation result is less than a preset first judgment threshold, determining the initial power flow calculation result as the target power flow calculation result of the power grid.

[0128] Those skilled in the art will understand that embodiments of this application can be provided as methods, systems, or computer program products. Therefore, this application can take the form of a completely hardware embodiment, a completely software embodiment, or an embodiment combining software and hardware aspects. Furthermore, this application can take the form of a computer program product embodied on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code.

[0129] This application is described with reference to flowchart illustrations and / or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of this application. It will be understood that each block of the flowchart illustrations and / or block diagrams, and combinations of blocks in the flowchart illustrations and / or block diagrams, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, special-purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, generate instructions for implementing the flowchart... Figure 1 One or more processes and / or boxes Figure 1 A device that provides the functions specified in one or more boxes.

[0130] These computer program instructions may also be stored in a computer-readable storage medium that can direct a computer or other programmable data processing device to function in a particular manner, such that the instructions stored in the computer-readable storage medium produce an article of manufacture including instruction means, which are implemented in a process Figure 1 One or more processes and / or boxes Figure 1 The function specified in one or more boxes.

[0131] These computer program instructions may also be loaded onto a computer or other programmable data processing equipment to cause a series of operational steps to be performed on the computer or other programmable equipment to produce a computer-implemented process, thereby providing instructions that execute on the computer or other programmable equipment for implementing the process. Figure 1 One or more processes and / or boxes Figure 1 The steps of the function specified in one or more boxes.

[0132] In a typical configuration, a computing device includes one or more processors (CPU), input / output interfaces, network interfaces, and memory.

[0133] Memory may include non-persistent memory in computer-readable media, such as random access memory (RAM) and / or non-volatile memory, such as read-only memory (ROM) or flash RAM. Memory is an example of computer-readable media.

[0134] Computer-readable media includes both permanent and non-permanent, removable and non-removable media that can store information using any method or technology. Information can be computer-readable instructions, data structures, modules of programs, or other data. Examples of computer storage media include, but are not limited to, phase-change memory (PRAM), static random access memory (SRAM), dynamic random access memory (DRAM), other types of random access memory (RAM), read-only memory (ROM), electrically erasable programmable read-only memory (EEPROM), flash memory or other memory technologies, CD-ROM, digital versatile optical disc (DVD) or other optical storage, magnetic tape, magnetic disk storage or other magnetic storage devices, or any other non-transferable medium that can be used to store information accessible by a computing device. As defined herein, computer-readable media does not include transient computer-readable media, such as modulated data signals and carrier waves.

[0135] It should also be noted that the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such process, method, article, or apparatus. Unless otherwise specified, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes that element.

[0136] Those skilled in the art will understand that embodiments of this application can be provided as methods, systems, or computer program products. Therefore, this application can take the form of a completely hardware embodiment, a completely software embodiment, or an embodiment combining software and hardware aspects. Furthermore, this application can take the form of a computer program product embodied on one or more computer-usable storage media (including, but not limited to, disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code.

[0137] The above are merely embodiments of this application and are not intended to limit the scope of this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the scope of the claims of this application.

Claims

1. A power flow calculation method for a power grid containing a multi-terminal flexible DC transmission system, characterized in that, include: Based on the AC bus voltage values ​​of the converters included in the power grid, the target state variable values ​​of the multi-terminal flexible DC transmission system included in the power grid are determined, wherein the target state variable values ​​are a quantitative representation of the state variables used to describe the dynamic behavior of the multi-terminal flexible DC transmission system; Based on the target state variable values, the initial power flow calculation results of the AC transmission system included in the power grid are determined, wherein the initial power flow calculation results are used to represent the voltage amplitude and voltage phase angle of the nodes included in the AC transmission system, and a node refers to a specific location in the AC transmission system where electrical attributes need to be displayed; If the initial power flow calculation result is less than a preset first determination threshold, the initial power flow calculation result is determined as the target power flow calculation result of the power grid.

2. The method according to claim 1, characterized in that, The determination of target state variable values ​​for the multi-terminal flexible DC transmission system within the power grid, based on the AC bus voltage values ​​of the converters included in the power grid, includes: Based on the AC bus voltage value, determine the initial state variable values ​​of the multi-terminal flexible DC transmission system; If the initial state variable value is less than a preset second determination threshold, the initial state variable value is determined as the target state variable value; or If the initial state variable value is greater than or equal to the second determination threshold, the initial state variable value is iteratively updated to obtain the updated initial state variable value. If the updated initial state variable value is less than the second determination threshold, the iterative update process ends, and the initial state variable value obtained from the last iterative update is determined as the target state variable value.

3. The method according to claim 2, characterized in that, When the state variables include DC voltage, DC current, and the converter control angle, and the initial state variable values ​​include initial DC voltage values, initial DC current values, and the cosine value of the initial control angle, determining the initial state variable values ​​of the multi-terminal flexible DC transmission system based on the AC side bus voltage value includes: The initial DC voltage value is determined based on the AC bus voltage value. The initial DC current value is determined based on the DC resistance and the initial DC voltage value; The initial control angle cosine value is determined based on the preset control angle cosine value.

4. The method according to claim 2, characterized in that, When the state variables include DC voltage, DC current, and the converter control angle, and the initial state variable values ​​include initial DC voltage values, initial DC current values, and initial control angle cosine values, the method further includes: During the iterative update process, if the control angle cosine value is greater than a preset control angle cosine value threshold, the target iterative update number corresponding to the control angle cosine value being greater than the control angle cosine value threshold, and the control angle cosine value corresponding to the previous iterative update number of the target iterative update number are determined. The cosine value of the control angle corresponding to the previous iteration update number is determined as the target cosine value of the control angle. The control angle included in the state variables is replaced with the turns ratio of the converter to obtain the updated state variables and the updated initial state variable values, wherein the turns ratio is used to describe the proportional relationship between the voltages on both sides of the converter. The target state variable value is determined based on the updated state variable and the updated initial state variable value.

5. The method according to claim 1, characterized in that, The step of determining the initial power flow calculation results of the AC transmission system included in the power grid based on the target state variable values ​​includes: Based on the target state variable values, determine the active power and reactive power transmitted from the multi-terminal flexible DC transmission system to the AC transmission system; Based on the active power and the reactive power, the initial power flow calculation result is determined.

6. The method according to claim 1, characterized in that, When the initial power flow calculation result includes voltage amplitude and voltage phase angle, and the corresponding first determination threshold includes a voltage amplitude threshold and a voltage phase angle threshold, then when the initial power flow calculation result is less than the preset first determination threshold... Determining the initial power flow calculation result as the target power flow calculation result of the power grid includes: If the voltage amplitude is less than the voltage amplitude threshold and the voltage phase angle is less than the voltage phase angle threshold, the initial power flow calculation result is determined as the target power flow calculation result.

7. The method according to any one of claims 1 to 6, characterized in that, The method further includes: If the initial power flow calculation result is greater than or equal to the first determination threshold, the target state variable value is iteratively updated based on the voltage amplitude included in the initial power flow calculation result to obtain the updated target state variable value. Based on the updated target state variable values, the initial power flow calculation results are iteratively updated to obtain the updated initial power flow calculation results. If the updated initial power flow calculation result is less than the first determination threshold, the iterative update is stopped, and the initial power flow calculation result obtained from the last iterative update is determined as the target power flow calculation result.

8. A power flow calculation device for a power grid containing a multi-terminal flexible DC transmission system, characterized in that, include: The first determining module is used to determine the target state variable value of the multi-terminal flexible DC transmission system included in the power grid based on the AC bus voltage value of the converter included in the power grid, wherein the target state variable value is a quantitative representation of the state variable used to describe the dynamic behavior of the multi-terminal flexible DC transmission system; The second determining module is used to determine the initial power flow calculation results of the AC transmission system included in the power grid based on the target state variable value, wherein the initial power flow calculation results are used to represent the voltage amplitude and voltage phase angle of the nodes included in the AC transmission system, and the node refers to a specific location in the AC transmission system where electrical attributes need to be displayed; The target power flow calculation result determination module is used to determine the initial power flow calculation result as the target power flow calculation result of the power grid when the initial power flow calculation result is less than a preset first determination threshold.

9. A non-volatile storage medium, characterized in that, The non-volatile storage medium stores multiple instructions, which are adapted to be loaded by a processor and executed by the power flow calculation method for a multi-terminal flexible DC transmission system as described in any one of claims 1 to 7.

10. An electronic device, characterized in that, include: One or more processors and a memory, the memory being used to store one or more programs, wherein when the one or more programs are executed by the one or more processors, the one or more processors cause the one or more processors to implement the power flow calculation method for a multi-terminal flexible DC transmission system as described in any one of claims 1 to 7.

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