Method and system for analyzing conveying capacity of offshore wind power through multi-terminal hybrid direct current

Abstract

The invention discloses a transmission capacity analysis method and system for offshore wind power through multi-terminal hybrid direct current, and the method comprises the steps: building a power flow model of an alternating current system-multi-terminal high-voltage direct current transmission system according to an alternating current system power flow equation and a converter direct current network equation; according to the power flow model, calculating a single-parameter continuous power flow-based single-wind-power-plant sending-out capability by using a continuous power flow algorithm to obtain a first conveying capability result; according to the power flow model, calculating the multi-wind-power-plant sending-out capacity based on the multi-parameter continuous power flow, and obtaining a second conveying capacity result; uncertain factors of a receiving end power grid are obtained, a power flow model containing parameters is constructed according to the uncertain factors, calculation is conducted according to the power flow model containing the parameters, and a wind power transmission capacity result is obtained. According to the method, the influence of the voltage source type current converter and the power grid commutation type current converter on direct current transmission is comprehensively considered, and the accuracy of transmission capacity evaluation is effectively improved.

Description

Technical Field

[0001] The present invention relates to the technical field of power systems, and in particular to a method and system for analyzing the transmission capacity of offshore wind power via multi-terminal hybrid direct current. Background Art

[0002] The large-scale development of offshore wind power is a major measure to support the country's energy goals. By the end of 2023, my country's offshore wind power installed capacity will reach 37.7 million kW, accounting for about 7.9% of the total wind power installed capacity. Compared with onshore wind power, offshore wind power has abundant wind energy resources, does not occupy land, and is closer to the load center of coastal economically developed areas, with huge development and construction potential.

[0003] At present, there are three main ways to deliver offshore wind power: 1) Power frequency AC delivery, but it is generally used in offshore due to the large charging capacitance of submarine cables; 2) Low frequency AC transmission, which can reduce line impedance and charging power, but it is not mature enough and many problems are still being explored; 3) Flexible DC (Voltage Source Converter based High Voltage Direct Current, VSC-HVDC) delivery, which is the main way to deliver offshore wind power because there is no charging and reactive power problem. The main disadvantage of VSC-HVDC is that the converter is expensive and has poor economic efficiency. Therefore, many schemes disclose a transmission model that is mixed with ordinary DC (Line Commutated Converter based High Voltage Direct Current, LCC-HVDC). The common scheme is to use VSC converter stations and LCC converter stations at the sending and receiving ends of the DC transmission line, respectively, to form a hybrid DC transmission model to improve economic efficiency while meeting the transmission requirements.

[0004] In the VSC-LCC hybrid transmission mode, the receiving-end LCC-HVDC needs to draw a large amount of reactive power from the receiving-end AC grid, which causes the receiving-end grid voltage to drop, affects the power supply quality, and may cause a series of static voltage stability problems, which in turn affects the safety and stability of the sending-end VSC-HVDC. Moreover, as the wind power transmission power continues to increase, the voltage drops further, which may induce static voltage collapse. Therefore, it is necessary to evaluate the power transmission capacity of offshore wind farms in the hybrid DC transmission mode.

[0005] In traditional AC power grids, continuous power flow (CPF) is a common method for evaluating load or power generation limits. However, there is still little research on power flow calculation of multi-terminal VSC-LCC hybrid DC systems. Most of the research literature on static voltage stability is conducted from the perspective of short-circuit ratio, and less based on accurate CPF quantitative analysis. Summary of the invention

[0006] In order to solve the above technical problems, an embodiment of the present invention provides a method and system for analyzing the transmission capacity of offshore wind power through multi-terminal hybrid DC, which effectively improves the accuracy of transmission capacity evaluation by comprehensively considering the impact of voltage source converters and grid-commutated converters on DC transmission.

[0007] A first aspect of an embodiment of the present invention provides a method for analyzing the transmission capacity of offshore wind power via multi-terminal hybrid direct current, the method comprising:

[0008] According to the AC system power flow equation and the converter DC network equation, the power flow model of AC system-multi-terminal high-voltage DC transmission system is established;

[0009] Determine a wind farm output as a parameter, and calculate the single wind farm delivery capacity based on a single parameter continuous power flow using a continuous power flow algorithm according to a power flow model to obtain a first delivery capacity result;

[0010] Determine the output of multiple wind farms as parameters, calculate the transmission capacity of multiple wind farms based on the multi-parameter continuous power flow according to the power flow model, and obtain a second transmission capacity result;

[0011] Acquire uncertainties of the receiving power grid, construct a power flow model with parameters according to the uncertainties, perform calculations according to the power flow model with parameters to obtain wind power transmission capacity results, and obtain transmission capacity analysis results according to the wind power transmission capacity results, the first transmission capacity results, and the second transmission capacity results.

[0012] In a possible implementation of the first aspect, a power flow model of an AC system-multi-terminal high-voltage direct current transmission system is established according to an AC system power flow equation and a converter DC network equation, including:

[0013] The power of the receiving grid node, offshore wind power node and voltage source AC side node is used to obtain the AC system power flow equation, where the AC system power flow equation is:

[0014]

[0015] Where P Gi is the active power output of the generator at node i, P Di is the load active power of node i, Q Gi is the reactive power output of the generator at node i, Q Di is the load reactive power of node i, V i is the voltage amplitude of node i, V j is the voltage amplitude at node j, θ ij is the voltage phase difference between node i and node j, G ijis the conductance between node i and node j, B ij is the susceptance between node i and node j, N is the total number of nodes, n PQ is the set of receiving grid nodes and offshore wind power nodes, n PV is a collection of nodes on the AC side of the voltage source converter;

[0016] According to the node set of the voltage source converter station and the grid commutation converter station, the converter DC network equation is obtained, where the converter DC network equation is:

[0017]

[0018] In the formula, I di is the DC current on the DC side of node i, with injection into the DC network as the positive direction, V dj is the DC voltage on the DC side of node j, G d,ij is the element of the node conductance matrix of the DC network. When there is no direct cable connecting DC nodes i and j, G d,ij is 0;

[0019] According to the AC system power flow equation, the converter DC network equation, the voltage source converter power flow model and the grid commutated converter power flow model, the power flow model of the AC system-multi-terminal high-voltage DC transmission system is obtained, where the power flow model is:

[0020]

[0021] In the formula, k Ti and X ci is the transformation ratio and connection reactance of the transformer between the converter and the AC node, θ di is the control angle of the converter, V i is the voltage amplitude of node i, V j is the voltage amplitude at node j, θ ij is the voltage phase difference between node i and node j, I di is the DC current on the DC side of node i, with injection into the DC network as the positive direction, V dj is the DC voltage on the DC side of node j, is the power factor angle of the inverter.

[0022] In a possible implementation of the first aspect, according to the power flow model, a continuous power flow algorithm is used to calculate a single wind farm transmission capacity based on a single-parameter continuous power flow to obtain a first transmission capacity result, including:

[0023] The parameter is set as wind power, and combined with the power flow variables in the power flow model, the adjusted power flow model is obtained, where the adjusted power flow model is:

[0024] f(Pw1 ,x)=0

[0025] Where P w1 is the wind power, x is the power flow variable;

[0026] According to the adjusted power flow model, the tangent direction of the current operating point is calculated, the predicted value is calculated according to the tangent direction, and the next operating point is determined according to the predicted value. If the next operating point meets the constraint conditions, the first transmission capacity result is obtained. If the constraint conditions are not met, the latest operating point is predicted according to the next operating point. This step is repeated until the latest operating point meets the constraint conditions, and the first transmission capacity result is obtained, wherein the constraint conditions include fixed DC voltage constraint conditions and fixed arc extinction lead angle constraint conditions.

[0027] In a possible implementation of the first aspect, calculating the transmission capacity of multiple wind farms based on the multi-parameter continuous power flow according to the power flow model to obtain a second transmission capacity result includes:

[0028] Fix multiple parameters in the power flow model, make one parameter in the power flow model change in a preset direction, and obtain a new model, where the new model is:

[0029] (P w1 ,...,P wM )=(P w1 (0) ,...,P wM (0) )+P w ·q

[0030] Where P w1 ,...,P wM is the output of M independent wind farms, the superscript “0” represents the initial point, q is the search direction, P w It is the conveying capacity in the preset direction;

[0031] The new model is transformed into a single-parameter problem to obtain a single-parameter objective function. According to the single-parameter objective function, the continuous power flow algorithm is used to calculate and obtain the second transmission capacity result, wherein the second transmission capacity result includes multiple maximum values ​​of wind power transmission capacity. The single-parameter objective function is:

[0032]

[0033] Where P w1 ,...,P wM for M The output of an independent wind farm, q is the search direction, P w It is the conveying capacity in the preset direction.

[0034] In a possible implementation of the first aspect, a power flow model including parameters is constructed according to uncertain factors, and calculation is performed according to the power flow model including parameters to obtain the probability of wind power transmission capacity, including:

[0035] A power flow model with parameters is constructed based on uncertain factors, where the power flow model with parameters is:

[0036] g(ξ,P w ,...,q,x)=0

[0037] Where P w represents the transmission capacity of a single wind farm or the transmission capacity of multiple wind farms in direction q, ξ is a random vector;

[0038] According to the probability distribution of random vectors, a random sampling algorithm is used to generate multiple samples, each sample is substituted into the power flow model, and the continuous power flow algorithm is used to obtain the wind power transmission capacity value. The wind power transmission capacity value is used to calculate the various moments of the wind power transmission capacity value, and the wind power transmission capacity value and the various moments of the wind power transmission capacity value are determined as the wind power transmission capacity results.

[0039] A second aspect of an embodiment of the present invention provides a system for analyzing the transmission capacity of offshore wind power via multi-terminal hybrid direct current, the system comprising a power flow model building module, a first calculation module, a second calculation module and a third calculation module.

[0040] Among them, the power flow model building module is used to establish the power flow model of the AC system-multi-terminal high-voltage DC transmission system based on the AC system power flow equation and the converter DC network equation;

[0041] The first calculation module is used to determine a wind farm output as a parameter, and calculate the single wind farm delivery capacity based on a single parameter continuous power flow using a continuous power flow algorithm according to a power flow model to obtain a first delivery capacity result;

[0042] The second calculation module is used to determine the output of multiple wind farms as parameters, calculate the transmission capacity of multiple wind farms based on the multi-parameter continuous power flow according to the power flow model, and obtain a second transmission capacity result;

[0043] The third calculation module is used to obtain the uncertainties of the receiving power grid, construct a power flow model containing parameters according to the uncertainties, perform calculations according to the power flow model containing parameters to obtain the wind power transmission capacity results, and obtain the transmission capacity analysis results according to the wind power transmission capacity results, the first transmission capacity results, and the second transmission capacity results.

[0044] In a possible implementation of the second aspect, a power flow model of an AC system-multi-terminal high-voltage direct current transmission system is established according to an AC system power flow equation and a converter DC network equation, including:

[0045] The power of the receiving grid node, offshore wind power node and voltage source AC side node is used to obtain the AC system power flow equation, where the AC system power flow equation is:

[0046]

[0047] Where P Gi is the active power output of the generator at node i, P Di is the load active power of node i, Q Gi is the reactive power output of the generator at node i, Q Di is the load reactive power of node i, V i is the voltage amplitude of node i, V j is the voltage amplitude at node j, θ ij is the voltage phase angle difference between node i and node j, G ij is the conductance between node i and node j, B ij is the susceptance between node i and node j, N is the total number of nodes, n PQ is the set of receiving grid nodes and offshore wind power nodes, n PV is a collection of nodes on the AC side of the voltage source converter;

[0048] According to the node set of the voltage source converter station and the grid commutation converter station, the converter DC network equation is obtained, where the converter DC network equation is:

[0049]

[0050] In the formula, I di is the DC current on the DC side of node i, with injection into the DC network as the positive direction, V dj is the DC voltage on the DC side of node j, G d,ij is the element of the node conductance matrix of the DC network. When there is no direct cable connecting DC nodes i and j, G d,ij is 0;

[0051] According to the AC system power flow equation, the converter DC network equation, the voltage source converter power flow model and the grid commutated converter power flow model, the power flow model of the AC system-multi-terminal high-voltage DC transmission system is obtained, where the power flow model is:

[0052]

[0053] In the formula, k Ti and X ci is the transformation ratio and connection reactance of the transformer between the converter and the AC node, θ di is the control angle of the converter, V i is the voltage amplitude of node i, V jis the voltage amplitude at node j, θ ij is the voltage phase difference between node i and node j, I di is the DC current on the DC side of node i, with injection into the DC network as the positive direction, V dj is the DC voltage on the DC side of node j, is the power factor angle of the inverter.

[0054] In a possible implementation of the second aspect, according to the power flow model, a continuous power flow algorithm is used to calculate a single wind farm transmission capacity based on a single-parameter continuous power flow to obtain a first transmission capacity result, including:

[0055] The parameter is set as wind power, and combined with the power flow variables in the power flow model, the adjusted power flow model is obtained, where the adjusted power flow model is:

[0056] f(P w1 ,x)=0

[0057] Where P w1 is the wind power, x is the power flow variable;

[0058] According to the adjusted power flow model, the tangent direction of the current operating point is calculated, the predicted value is calculated according to the tangent direction, and the next operating point is determined according to the predicted value. If the next operating point meets the constraint conditions, the first transmission capacity result is obtained. If the constraint conditions are not met, the latest operating point is predicted according to the next operating point. This step is repeated until the latest operating point meets the constraint conditions, and the first transmission capacity result is obtained, wherein the constraint conditions include fixed DC voltage constraint conditions and fixed arc extinction lead angle constraint conditions.

[0059] A third aspect of an embodiment of the present invention provides a computer device, including:

[0060] Memory for storing computer programs;

[0061] A processor is used to implement the method for analyzing the transmission capacity of offshore wind power via multi-terminal hybrid direct current as described in the first aspect when executing the computer program.

[0062] A fourth aspect of an embodiment of the present invention provides a storage medium having a computer program stored thereon, which, when executed by a processor, implements the steps of the method for analyzing the transmission capacity of offshore wind power via multi-terminal hybrid direct current as in the first aspect.

[0063] The method for analyzing the transmission capacity of offshore wind power through multi-terminal hybrid direct current provided by the embodiment of the present invention is to establish a flow model of an AC system-multi-terminal high-voltage direct current transmission system according to the AC system flow equation and the converter direct current network equation; determine the output of a wind farm as a parameter, and calculate the single wind farm delivery capacity based on a single-parameter continuous flow according to the flow model using a continuous flow algorithm to obtain a first transmission capacity result; determine the output of multiple wind farms as parameters, and calculate the delivery capacity of multiple wind farms based on a multi-parameter continuous flow according to the flow model to obtain a second transmission capacity result; obtain the uncertainty factors of the receiving-end power grid, construct a flow model containing parameters according to the uncertain factors, calculate according to the flow model containing parameters, and obtain the wind power transmission capacity result, and obtain the transmission capacity analysis result according to the wind power transmission capacity result, the first transmission capacity result and the second transmission capacity result. Through the above method, the influence of the voltage source converter and the grid commutation converter on direct current transmission is comprehensively considered, and the accuracy of the transmission capacity evaluation is effectively improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0064] Figure 1 : A schematic diagram of an analysis method flow chart of an embodiment of a method for analyzing the transmission capacity of offshore wind power via multi-terminal hybrid direct current provided by the present invention;

[0065] Figure 2 : A schematic diagram of the IEEE 30-node system structure of an embodiment of a method for analyzing the transmission capacity of offshore wind power via multi-terminal hybrid direct current provided by the present invention;

[0066] Figure 3 : A curve diagram showing the variation of the voltage amplitude of a node with the power of wind farm 1 in one embodiment of the method for analyzing the transmission capacity of offshore wind power via multi-terminal hybrid direct current provided by the present invention;

[0067] Figure 4 : A curve diagram showing changes in the LCC-HVDC control angle and power factor angle with the power of wind farm 1 in one embodiment of the offshore wind power transmission capacity analysis method via multi-terminal hybrid direct current provided by the present invention;

[0068] Figure 5 : A curve diagram showing the variation of the voltage amplitude of a node with the power of wind farm 2 in one embodiment of the method for analyzing the transmission capacity of offshore wind power via multi-terminal hybrid direct current provided by the present invention;

[0069] Figure 6 : A curve diagram showing the change of LCC-HVDC control angle and power factor angle with wind farm 2 power in one embodiment of the offshore wind power transmission capacity analysis method via multi-terminal hybrid direct current provided by the present invention;

[0070] Figure 7: A schematic diagram of the maximum power of wind farm 1 and wind farm 2 under different voltage limits in one embodiment of a method for analyzing the transmission capacity of offshore wind power via multi-terminal hybrid direct current provided by the present invention;

[0071] Figure 8 : A maximum power curve diagram of wind farm 1 and wind farm 2 under different DC voltage reference values ​​in one embodiment of the offshore wind power transmission capacity analysis method provided by the present invention via multi-terminal hybrid DC;

[0072] Fig. 9 : A probability distribution diagram of the maximum power when the power of wind farm 1 and wind farm 2 increases in different directions according to an embodiment of the method for analyzing the transmission capacity of offshore wind power via multi-terminal hybrid direct current provided by the present invention;

[0073] Fig.10 : A system block diagram of an embodiment of a method for analyzing the transmission capacity of offshore wind power via multi-terminal hybrid direct current provided by the present invention. DETAILED DESCRIPTION

[0074] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0075] Please refer to Figure 1 , which is a flow chart of an embodiment of a method for analyzing the transmission capacity of offshore wind power via multi-terminal hybrid direct current provided by an embodiment of the present invention, including steps S101 to S104, each of which is specifically as follows:

[0076] S101. Based on the AC system power flow equation and the converter DC network equation, a power flow model of the AC system-multi-terminal high voltage DC transmission system is established.

[0077] In this embodiment, an AC system-multi-terminal HVDC system power flow model taking into account a voltage source converter (VSC) and a line-commutated converter (LCC) is established.

[0078] It should be noted that HVDC refers to high voltage direct current transmission.

[0079] In some embodiments, step S101 of "establishing a power flow model of an AC system-multi-terminal high-voltage direct current transmission system according to the AC system power flow equation and the converter DC network equation" includes but is not limited to the following steps:

[0080] The power of the receiving grid node, offshore wind power node and voltage source AC side node is used to obtain the AC system power flow equation, where the AC system power flow equation is:

[0081]

[0082] Where P Gi is the active power output of the generator at node i, P Di is the load active power of node i, Q Gi is the reactive power output of the generator at node i, Q Di is the load reactive power of node i, V i is the voltage amplitude of node i, V j is the voltage amplitude at node j, θ ij is the voltage phase difference between node i and node j, G ij is the conductance between node i and node j, B ij is the susceptance between node i and node j, N is the total number of nodes, n PQ is the set of receiving grid nodes and offshore wind power nodes, n PV is a collection of nodes on the AC side of the voltage source converter;

[0083] According to the node set of the voltage source converter station and the grid commutation converter station, the converter DC network equation is obtained, where the converter DC network equation is:

[0084]

[0085] In the formula, I di is the DC current on the DC side of node i, with injection into the DC network as the positive direction, V dj is the DC voltage on the DC side of node j, G d,ij is the element of the node conductance matrix of the DC network. When there is no direct cable connecting DC nodes i and j, G d,ij is 0;

[0086] According to the AC system power flow equation, the converter DC network equation, the voltage source converter power flow model and the grid commutated converter power flow model, the power flow model of the AC system-multi-terminal high-voltage DC transmission system is obtained, where the power flow model is:

[0087]

[0088] In the formula, k Ti and X ci is the transformation ratio and connection reactance of the transformer between the converter and the AC node, θ di is the control angle of the converter, V i is the voltage amplitude of node i, V jis the voltage amplitude at node j, θ ij is the voltage phase difference between node i and node j, I di is the DC current on the DC side of node i, with injection into the DC network as the positive direction, V dj is the DC voltage on the DC side of node j, is the power factor angle of the inverter.

[0089] In this embodiment, AC and DC network equations are established, and the AC system includes receiving grid nodes, offshore wind power nodes (regarded as PQ nodes) and VSC-HVDC AC side nodes (PV nodes, phase reference points). Its equation is a power flow equation in polar coordinate form:

[0090]

[0091] Where P Gi is the active power output of the generator at node i, P Di is the load active power of node i, Q Gi is the reactive power output of the generator at node i, Q Di is the load reactive power of node i, V i is the voltage amplitude of node i, V j is the voltage amplitude at node j, θ ij is the voltage phase difference between node i and node j, G ij is the conductance between node i and node j, B ij is the susceptance between node i and node j, N is the total number of nodes, n PQ is the set of receiving grid nodes and offshore wind power nodes, n PV is a collection of nodes on the AC side of the voltage source converter;

[0092] The set of nodes where the VSC-HVDC and LCC-HVDC converter stations are connected is denoted as n VSC and n LCC , so the DC network equation is:

[0093]

[0094] In the formula, I di is the DC current on the DC side of node i, with injection into the DC network as the positive direction, V dj is the DC voltage on the DC side of node j, G d,ij is the element of the node conductance matrix of the DC network. When there is no direct cable connecting DC nodes i and j, G d,ij is 0;

[0095] The HDVC component equations are established. The VSC-HVDC converter station is a PV node with no additional equations. Its connection with the sending-end AC grid is to provide active power injection, that is:

[0096]

[0097] The LCC-HVDC converter station is a PQ node. Its connection with the receiving AC grid is to provide active and reactive power injection, namely:

[0098]

[0099] In the formula, is the power factor angle of the inverter, which is determined by the inverter control method, B i0 is the susceptance value of the reactive power compensation capacitor on the AC side. Since reactive power is always absorbed from the AC power grid,

[0100] Under the appropriate per-unit system, the basic equation of LCC-HVDC converter is:

[0101]

[0102] In the formula, k Ti and X ci is the transformation ratio and connection reactance of the transformer between the converter and the AC node, θ di is the control angle of the converter, V i is the voltage amplitude of node i, V j is the voltage amplitude at node j, θ ij is the voltage phase difference between node i and node j, I di is the DC current on the DC side of node i, with injection into the DC network as the positive direction, V dj is the DC voltage on the DC side of node j, is the power factor angle of the inverter, k γ It is generally approximated as a constant of 0.995.

[0103] The above LCC-HVDC basic equation also needs to be combined with a specific control method. One site uses constant DC voltage control, while other sites use fixed arc extinction advance angle control, that is:

[0104]

[0105] Where V dj is the DC voltage on the DC side of node j, θ di is the control angle of the converter.

[0106] S102: Determine a wind farm output as a parameter, and calculate the single wind farm transmission capacity based on the single-parameter continuous power flow using a continuous power flow algorithm according to the power flow model to obtain a first transmission capacity result.

[0107] In this embodiment, the transmission capacity of a single wind farm based on a single parameter continuous power flow is calculated.

[0108] It should be noted that

[0109] In some embodiments, in step S102, "calculating the single wind farm transmission capacity based on the single parameter continuous power flow using the continuous power flow algorithm according to the power flow model to obtain a first transmission capacity result" includes but is not limited to the following steps:

[0110] The parameter is set as wind power, and combined with the power flow variables in the power flow model, the adjusted power flow model is obtained, where the adjusted power flow model is:

[0111] f(P w1 ,x)=0

[0112] Where P w1 is the wind power, x is the power flow variable;

[0113] According to the adjusted power flow model, the tangent direction of the current operating point is calculated, the predicted value is calculated according to the tangent direction, and the next operating point is determined according to the predicted value. If the next operating point meets the constraint conditions, the first transmission capacity result is obtained. If the constraint conditions are not met, the latest operating point is predicted according to the next operating point. This step is repeated until the latest operating point meets the constraint conditions, and the first transmission capacity result is obtained, wherein the constraint conditions include fixed DC voltage constraint conditions and fixed arc extinction lead angle constraint conditions.

[0114] In this embodiment, the operation limit is that the node voltage does not exceed its upper and lower limits. For LCC-HVDC, it is also necessary to satisfy that the control angle is greater than the threshold to avoid commutation failure. Therefore, there are the following constraints:

[0115]

[0116] Where V i is the voltage amplitude at node i, θ di is the control angle of the converter, θ dmin is the minimum control angle of the converter, V min is the minimum voltage amplitude of node i, V max is the maximum voltage amplitude of node i.

[0117] In each iteration of the continuous power flow, first, the current operating point (P w1 ,x) of the tangent direction r, which satisfies

[0118]

[0119] The left side of the above equation is an N×(N+1)-dimensional matrix, where N is the dimension of x, so r is not unique. Add the length condition ||r||2=1 and let the direction of r be the same as P w1 The growth direction of is the same as that of , and a unique solution for r can be obtained.

[0120] Then, the predicted value is calculated based on the tangent direction:

[0121] (P w1 (1) ,x (1) )=(P w1 ,x)+τr

[0122] Where τ is the prediction step size. The smaller its value is, the better the tracking accuracy and effect of the continuous power flow will be.

[0123] Finally, based on the above predicted values, the next operating point is determined by accurately solving the following equation:

[0124]

[0125] And update the operating point to (P w1 ,x):=(P w1 (2) ,x (2) ). If the operating point still meets the operating constraints at this time, repeat the above process; otherwise, the maximum output power point has been found and the algorithm ends.

[0126] S103, determining the outputs of multiple wind farms as parameters, calculating the transmission capacity of multiple wind farms based on the multi-parameter continuous power flow according to the power flow model, and obtaining a second transmission capacity result.

[0127] In this embodiment,

[0128] In some embodiments, the step S103 of "calculating the transmission capacity of multiple wind farms based on the multi-parameter continuous power flow according to the power flow model to obtain a second transmission capacity result" includes but is not limited to the following steps:

[0129] Fix multiple parameters in the power flow model, make one parameter in the power flow model change in a preset direction, and obtain a new model, where the new model is:

[0130] (P w1 ,...,P wM )=(P w1 (0) ,...,P wM (0) )+P w ·q

[0131] Where P w1 ,...,P wM is the output of M independent wind farms, the superscript “0” represents the initial point, q is the search direction, P w It is the conveying capacity in the preset direction;

[0132] The new model is transformed into a single-parameter problem to obtain a single-parameter objective function. According to the single-parameter objective function, the continuous power flow algorithm is used to calculate and obtain the second transmission capacity result, wherein the second transmission capacity result includes multiple maximum values ​​of wind power transmission capacity. The single-parameter objective function is:

[0133]

[0134] Where P w1 ,...,P wM is the output of M independent wind farms, q is the search direction, P w It is the conveying capacity in the preset direction.

[0135] In this embodiment, the transmission capacity of multiple wind farms based on multi-parameter continuous power flow is calculated. The above continuous power flow algorithm is applicable to the case where the output of a single wind farm is used as a parameter. When considering multiple offshore wind farms supplying power to multiple loads through a DC network, the maximum transmission capacity of these wind farms is interrelated with each other, similar to the Pareto front. So the problem becomes finding a critical point that meets the operation constraints in the multi-parameter space with multiple wind power outputs as parameters.

[0136] To solve this problem, we can fix other parameters and change only one parameter each time. More generally, we can specify the direction of change of multi-dimensional parameters and search for critical points along this direction. By continuously changing the search direction, we can obtain all critical operating points that characterize the transmission capacity of multiple wind power. Its mathematical expression is:

[0137] (P w1 ,...,P wM )=(P w1 (0) ,...,P wM (0) )+P w ·q

[0138] Where P w1 ,...,P wM is the output of M independent wind farms (i.e., multiple parameters), the superscript “0” represents the initial point, q is the search direction, P w is a single parameter in this direction. In addition, parameters related to DC control, such as the voltage setpoint or control angle of LCC-HVDC, can also be used as parameters.

[0139] The original multi-parameter problem can be transformed into w The single parameter problem:

[0140]

[0141] By constantly changing the direction q and tracking the equation using a continuous power flow algorithm, multiple maximum points of wind power transmission capacity can be obtained.

[0142] S104, obtaining uncertainties of the receiving-end power grid, constructing a power flow model containing parameters according to the uncertainties, performing calculations according to the power flow model containing parameters to obtain wind power transmission capacity results, and obtaining transmission capacity analysis results according to the wind power transmission capacity results, the first transmission capacity results, and the second transmission capacity results.

[0143] In this embodiment, uncertainties of the receiving power grid are obtained, a power flow model containing parameters is constructed based on the uncertainties, calculations are performed based on the power flow model containing parameters to obtain wind power transmission capacity results, and transmission capacity analysis results are obtained based on the wind power transmission capacity results, the first transmission capacity results, and the second transmission capacity results.

[0144] In some embodiments, in step S104, "constructing a power flow model with parameters according to the uncertain factors, and performing calculations according to the power flow model with parameters to obtain the probability of wind power transmission capacity" includes but is not limited to the following steps:

[0145] A power flow model with parameters is constructed based on uncertain factors, where the power flow model with parameters is:

[0146] g(ξ,P w ,...,q,x)=0

[0147] Where P w represents the transmission capacity of a single wind farm or the transmission capacity of multiple wind farms in direction q, ξ is a random vector;

[0148] According to the probability distribution of random vectors, a random sampling algorithm is used to generate multiple samples, each sample is substituted into the power flow model, and the continuous power flow algorithm is used to obtain the wind power transmission capacity value. The wind power transmission capacity value is used to calculate the various moments of the wind power transmission capacity value, and the wind power transmission capacity value and the various moments of the wind power transmission capacity value are determined as the wind power transmission capacity results.

[0149] In this embodiment, the power flow distribution of the receiving-end power grid is not only affected by the offshore wind power transmission power, but also by factors such as the load size of local users. Therefore, the uncertainty factors of the receiving-end power grid, such as random fluctuations in load, are also factors that restrict the offshore wind power transmission capacity.

[0150] Assuming that the uncertainty factor can be represented by a random vector ξ, which obeys a certain probability distribution, the power flow model with parameters becomes:

[0151] g(ξ,P w ,...,q,x)=0

[0152] Where P w It represents the transmission capacity of a single wind farm or the transmission capacity of multiple wind farms in direction q.

[0153] It can be seen that under the action of the random vector ξ, the wind power transmission capacity (denoted as P w,max ) also becomes a random variable.

[0154] Then, a random sampling method such as Monte Carlo or Latin Hypercube is used to evaluate the random variable P w,max The statistical information and probability distribution of wind power transmission capacity are quantitatively analyzed. The calculation process is as follows:

[0155] Step 1: Generate a large number of samples ξ using a random sampling algorithm based on the probability distribution of the random vector ξ (1) ,...,ξ (L) ;

[0156] Step 2: For each sample ξ (k) , k=1,...,L, substitute its value into the power flow model, use the continuous power flow algorithm of claim 3 to obtain the critical point that satisfies the operation constraints, and obtain the corresponding wind power transmission capacity value, which is recorded as P w,max (k) ,k=1,...,L;

[0157] Step 3: The moments of wind power transmission capacity can be calculated based on the above sample points:

[0158]

[0159] Among them, P w,max The probability density function and cumulative probability distribution function can be obtained by using the Gram-Charlier series expansion based on the above moments; or directly obtained by using the kernel density fitting method based on the obtained sample points.

[0160] The present application also implements the technical solution provided in the embodiment of the present application, using the IEEE 30 node system as the implementation object, such as Figure 2 As shown. Set up two wind farms, their active output P w1 and P w2The reactive power output is set to 0 for the transmission capacity to be studied. The two wind farms are connected to the two offshore HVDC stations through AC lines and transformers, and fed into the IEEE30 node system through the four-terminal DC network from nodes 13 and 22. The DC sending end is controlled by fixed Vf, and the receiving end LCC-HVDC converter station of node 13 adopts fixed DC voltage control, and its set value is V d13 =0.93, the converter station at node 22 adopts a fixed control angle mode, and its setting value is θ d22 =18°.

[0161] The IEEE30-node system data is changed as follows: the system base power is 1000MW, nodes 13 and 22 are changed from generator nodes to DC feed-in points, and 300MVar reactive compensation capacitors are connected respectively, and the active power of node 2 is set to 0. The total system load is 1892MW, generators 23 and 27 bear 461MW in total, and the remaining 1431MW and network losses are borne by the balancing node (and generator 1) and the offshore wind farm, as shown in Figure 2. Figure 2 As shown, the specific steps are as follows:

[0162] (1) According to the method of S101, an AC system-multi-terminal HVDC system power flow model taking into account voltage source converters (VSCs) and line-commutated converters (LCCs) is established.

[0163] (2) According to the method of S102-S103, first, calculate the transmission capacity of a single wind farm, assuming that the transmission power of wind farm 2 is 0 (i.e., P w2 =0), and calculate the transmission capacity of wind farm 1 alone. The power flow variables and wind farm 1 power P can be obtained w1 The change curve of Figure 3 and Figure 4 As shown, for the transmission capacity analysis of wind farm 2, we can also set P w1 = 0, using the continuous power flow algorithm, the power flow variables and wind farm 2 power P are obtained w2 The change curve of Figure 5 and Figure 6 Then, the combined transmission capacity of the two wind farms and the LCC-HVDC control angle threshold θ are calculated. dmin =15°, the lower limits of the voltages at the important load nodes are V min =0.95, 0.90, 0.85, the curve of the combined transmission capacity of the two wind farms is as follows Figure 7 As shown, when the DC voltage at node 13 takes different set values, the load voltage and the control are maintained at V min =0.90,θ dmin =15°, the wind farm power transmission capacity curve is as follows Figure 8 shown.

[0164] According to the method of S104, the transmission capacity considering the randomness of the load is calculated. Assuming that the power of the important load nodes 8, 12, 17, 19, 21, and 24 of the receiving power grid fluctuates randomly, its randomness obeys the normal distribution with an expectation of 0 and a standard deviation of 3% of the rated power. Based on the smooth kernel density estimation, its probability density curve is obtained. The results are as follows: Fig. 9 As shown in the figure, α represents the growth direction of the two wind powers.

[0165] It should be understood that although Figure 1 The steps in the flowchart are shown in sequence as indicated by the arrows, but these steps are not necessarily executed in the order indicated by the arrows. Unless otherwise specified in this document, there is no strict order restriction for the execution of these steps, and these steps can be executed in other orders. Moreover, Figure 1 At least part of the steps may include multiple steps or multiple stages. These steps or stages are not necessarily performed at the same time, but can be performed at different times. The execution order of these steps or stages is not necessarily sequential, but can be performed in turn or alternately with other steps or at least part of the steps or stages in other steps.

[0166] In some embodiments, Fig.10 As shown, it shows a block diagram of an offshore wind power transmission capacity analysis system 1000 through multi-terminal hybrid direct current provided in an embodiment of the present application, including: a power flow model construction module 1001, a first calculation module 1002, a second calculation module 1003 and a third calculation module 1004, wherein:

[0167] The power flow model building module 1001 is used to establish a power flow model of an AC system-multi-terminal high-voltage direct current transmission system according to the AC system power flow equation and the converter DC network equation;

[0168] The first calculation module 1002 is used to determine a wind farm output as a parameter, and calculate the single wind farm transmission capacity based on a single parameter continuous power flow using a continuous power flow algorithm according to a power flow model to obtain a first transmission capacity result;

[0169] The second calculation module 1003 is used to determine the output of multiple wind farms as parameters, calculate the transmission capacity of multiple wind farms based on the multi-parameter continuous power flow according to the power flow model, and obtain a second transmission capacity result;

[0170] The third calculation module 1004 is used to obtain uncertainties of the receiving power grid, construct a power flow model containing parameters according to the uncertainties, perform calculations according to the power flow model containing parameters to obtain wind power transmission capacity results, and obtain transmission capacity analysis results according to the wind power transmission capacity results, the first transmission capacity results, and the second transmission capacity results.

[0171] The specific implementation of the offshore wind power transmission capacity analysis system via multi-terminal hybrid direct current is basically the same as the specific implementation of the offshore wind power transmission capacity analysis method via multi-terminal hybrid direct current described above, and will not be repeated here.

[0172] In one embodiment of the present application, a computer device is provided, which includes a memory and a processor, wherein a computer program is stored in the memory, and the above steps are implemented when the processor executes the computer program; the computer device provided in this embodiment has an implementation principle and technical effects similar to those of the above method embodiments, and will not be repeated here.

[0173] In one embodiment of the present application, a computer-readable storage medium is provided, on which a computer program is stored, and the above steps are implemented when the computer program is executed by a processor; the computer-readable storage medium provided in this embodiment has an implementation principle and technical effects similar to those of the above method embodiment, and will not be repeated here.

[0174] Those skilled in the art can understand that all or part of the processes in the above-mentioned embodiment methods can be completed by instructing the relevant hardware through a computer program, and the computer program can be stored in a non-volatile computer-readable storage medium. When the computer program is executed, it can include the processes of the embodiments of the above-mentioned methods. Among them, any reference to memory, storage, database or other media used in the embodiments provided in this application can include non-volatile and / or volatile memory. Non-volatile memory can include read-only memory (ROM), programmable ROM (PROM), electrically programmable ROM (EPROM), electrically erasable programmable ROM (EEPROM) or flash memory. Volatile memory can include random access memory (RAM) or external cache memory. As an illustration and not limitation, RAM is available in many forms, such as static RAM (SRAM), dynamic RAM (DRAM), synchronous DRAM (SDRAM), double data rate SDRAM (DDRSDRAM), enhanced SDRAM (ESDRAM), synchronous link (Synchlink) DRAM (SLDRAM), memory bus (Rambus) direct RAM (RDRAM), direct memory bus dynamic RAM (DRDRAM), and memory bus dynamic RAM (RDRAM).

[0175] The technical features of the above embodiments may be combined arbitrarily. To make the description concise, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0176] The specific embodiments described above further illustrate the purpose, technical solutions and beneficial effects of the present invention. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the scope of protection of the present invention. It is particularly pointed out that for those skilled in the art, any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for analyzing the transmission capacity of offshore wind power via multi-terminal hybrid direct current, characterized in that: include: According to the AC system power flow equation and the converter DC network equation, the power flow model of AC system-multi-terminal high-voltage DC transmission system is established; Determine a wind farm output as a parameter, and calculate the single wind farm delivery capacity based on the single parameter continuous power flow using a continuous power flow algorithm according to the power flow model to obtain a first delivery capacity result; Determine the output of multiple wind farms as parameters, calculate the transmission capacity of multiple wind farms based on the multi-parameter continuous power flow according to the power flow model, and obtain a second transmission capacity result; Acquire uncertainties of the receiving power grid, construct a power flow model containing parameters according to the uncertainties, perform calculations according to the power flow model containing parameters to obtain wind power transmission capacity results, and obtain transmission capacity analysis results according to the wind power transmission capacity results, the first transmission capacity results, and the second transmission capacity results.

2. The method for analyzing the transmission capacity of offshore wind power via multi-terminal hybrid direct current according to claim 1, characterized in that: The power flow model of the AC system-multi-terminal high-voltage direct current transmission system is established according to the AC system power flow equation and the converter DC network equation, including: The power of the receiving grid node, the offshore wind power node and the voltage source type AC side node is used to obtain the AC system power flow equation, where the AC system power flow equation is: Where P Gi is the active power output of the generator at node i, P Di is the load active power of node i, Q Gi is the reactive power output of the generator at node i, Q Di is the load reactive power of node i, V i is the voltage amplitude of node i, V j is the voltage amplitude at node j, θ ij is the voltage phase difference between node i and node j, G ij is the conductance between node i and node j, B ij is the susceptance between node i and node j, N is the total number of nodes, n PQ is the set of receiving grid nodes and offshore wind power nodes, n PV is a collection of nodes on the AC side of the voltage source converter; According to the node set connected to the voltage source converter station and the grid commutation converter station, the converter DC network equation is obtained, wherein the converter DC network equation is: In the formula, I di is the DC current on the DC side of node i, with injection into the DC network as the positive direction, V dj is the DC voltage on the DC side of node j, G d,ij is the element of the node conductance matrix of the DC network. When there is no cable directly connecting DC nodes i and j, G d,ij is 0; According to the AC system power flow equation, the converter DC network equation, the voltage source converter power flow model and the grid commutation converter power flow model, the power flow model of the AC system-multi-terminal high-voltage DC transmission system is obtained, wherein the power flow model is: In the formula, k Ti and X ci is the transformation ratio and connection reactance of the transformer between the converter and the AC node, θ di is the control angle of the converter, V i is the voltage amplitude of node i, V j is the voltage amplitude at node j, θ ij is the voltage phase difference between node i and node j, I di is the DC current on the DC side of node i, with injection into the DC network as the positive direction, V dj is the DC voltage on the DC side of node j, is the power factor angle of the inverter.

3. The method for analyzing the transmission capacity of offshore wind power via multi-terminal hybrid direct current according to claim 1, characterized in that: The method of calculating the transmission capacity of a single wind farm based on a single-parameter continuous power flow using a continuous power flow algorithm according to the power flow model to obtain a first transmission capacity result includes: The parameter is set as wind power, and combined with the power flow variables in the power flow model, an adjusted power flow model is obtained, wherein the adjusted power flow model is: f(P w1 ,x)=0 Where P w1 is the wind power, x is the power flow variable; According to the adjusted power flow model, the tangent direction of the current operating point is calculated, the predicted value is calculated according to the tangent direction, and the next operating point is determined according to the predicted value. If the next operating point meets the constraint conditions, the first transmission capacity result is obtained. If the constraint conditions are not met, the latest operating point is predicted according to the next operating point. This step is repeated until the latest operating point meets the constraint conditions, and the first transmission capacity result is obtained, wherein the constraint conditions include fixed DC voltage constraint conditions and fixed arc extinction lead angle constraint conditions.

4. The method for analyzing the transmission capacity of offshore wind power via multi-terminal hybrid direct current according to claim 1, characterized in that: The calculating the transmission capacity of multiple wind farms based on the multi-parameter continuous power flow according to the power flow model to obtain a second transmission capacity result includes: Fix multiple parameters in the power flow model, make one parameter in the power flow model change according to a preset direction, and obtain a new model, wherein the new model is: Where P w1 ,...,P wM is the output of M independent wind farms, the superscript "0" represents the initial point, q is the search direction, P w It is the conveying capacity in the preset direction; The new model is converted into a single-parameter problem to obtain a single-parameter objective function, and the continuous power flow algorithm is used to calculate according to the single-parameter objective function to obtain a second transmission capacity result, wherein the second transmission capacity result includes multiple maximum values ​​of wind power transmission capacity, and the single-parameter objective function is: Where P w1 ,...,P wM is the output of M independent wind farms, q is the search direction, P w It is the conveying capacity in the preset direction.

5. The method for analyzing the transmission capacity of offshore wind power via multi-terminal hybrid direct current according to claim 1, characterized in that: The step of constructing a power flow model containing parameters according to the uncertain factors, and performing calculations according to the power flow model containing parameters to obtain the probability of wind power transmission capacity includes: A power flow model containing parameters is constructed according to the uncertain factors, wherein the power flow model containing parameters is: g(ξ,P w ,...,q,x)=0 Where P w represents the transmission capacity of a single wind farm or the transmission capacity of multiple wind farms in direction q, ξ is a random vector; According to the probability distribution of the random vector, a plurality of samples are generated by using a random sampling algorithm, each of the samples is substituted into the power flow model, a wind power transmission capacity value is obtained by using a continuous power flow algorithm, and each order moment of the wind power transmission capacity value is calculated using the wind power transmission capacity value, and the wind power transmission capacity value and each order moment of the wind power transmission capacity value are determined as the wind power transmission capacity result.

6. A transmission capacity analysis system for offshore wind power via multi-terminal hybrid direct current, characterized in that: It includes a power flow model building module, a first calculation module, a second calculation module and a third calculation module. The power flow model building module is used to establish a power flow model of an AC system-multi-terminal high-voltage direct current transmission system according to the AC system power flow equation and the converter DC network equation; The first calculation module is used to determine a wind farm output as a parameter, and calculate the single wind farm delivery capacity based on a single parameter continuous power flow using a continuous power flow algorithm according to the power flow model to obtain a first delivery capacity result; The second calculation module is used to determine the output of multiple wind farms as parameters, calculate the transmission capacity of multiple wind farms based on the multi-parameter continuous power flow according to the power flow model, and obtain a second transmission capacity result; The third calculation module is used to obtain uncertainties of the receiving power grid, construct a power flow model containing parameters according to the uncertainties, perform calculations according to the power flow model containing parameters to obtain wind power transmission capacity results, and obtain transmission capacity analysis results according to the wind power transmission capacity results, the first transmission capacity results and the second transmission capacity results.

7. The offshore wind power transmission capacity analysis system via multi-terminal hybrid direct current according to claim 6, characterized in that: The power flow model of the AC system-multi-terminal high-voltage direct current transmission system is established according to the AC system power flow equation and the converter DC network equation, including: The power of the receiving grid node, the offshore wind power node and the voltage source type AC side node is used to obtain the AC system power flow equation, where the AC system power flow equation is: Where P Gi is the active power output of the generator at node i, P Di is the load active power of node i, Q Gi is the reactive power output of the generator at node i, Q Di is the load reactive power of node i, V i is the voltage amplitude of node i, V j is the voltage amplitude at node j, θ ij is the voltage phase difference between node i and node j, G ij is the conductance between node i and node j, B ij is the susceptance between node i and node j, N is the total number of nodes, n PQ is the set of receiving grid nodes and offshore wind power nodes, n PV is a collection of nodes on the AC side of the voltage source converter; According to the node set connected to the voltage source converter station and the grid commutation converter station, the converter DC network equation is obtained, wherein the converter DC network equation is: In the formula, I di is the DC current on the DC side of node i, with injection into the DC network as the positive direction, V dj is the DC voltage on the DC side of node j, G d,ij is the element of the node conductance matrix of the DC network. When there is no direct cable connecting DC nodes i and j, G d,ij is 0; According to the AC system power flow equation, the converter DC network equation, the voltage source converter power flow model and the grid commutation converter power flow model, the power flow model of the AC system-multi-terminal high-voltage DC transmission system is obtained, wherein the power flow model is: In the formula, k Ti and X ci is the transformation ratio and connection reactance of the transformer between the converter and the AC node, θ di is the control angle of the converter, V i is the voltage amplitude of node i, V j is the voltage amplitude at node j, θ ij is the voltage phase difference between node i and node j, I di is the DC current on the DC side of node i, with injection into the DC network as the positive direction, V dj is the DC voltage on the DC side of node j, is the power factor angle of the inverter.

8. The offshore wind power transmission capacity analysis system via multi-terminal hybrid direct current according to claim 6, characterized in that: The method of calculating the transmission capacity of a single wind farm based on a single-parameter continuous power flow using a continuous power flow algorithm according to the power flow model to obtain a first transmission capacity result includes: The parameter is set as wind power, and combined with the power flow variables in the power flow model, an adjusted power flow model is obtained, wherein the adjusted power flow model is: f(P w1 ,x)=0 Where P w1 is the wind power, x is the power flow variable; According to the adjusted power flow model, the tangent direction of the current operating point is calculated, the predicted value is calculated according to the tangent direction, and the next operating point is determined according to the predicted value. If the next operating point meets the constraint conditions, the first transmission capacity result is obtained. If the constraint conditions are not met, the latest operating point is predicted according to the next operating point. This step is repeated until the latest operating point meets the constraint conditions, and the first transmission capacity result is obtained, wherein the constraint conditions include fixed DC voltage constraint conditions and fixed arc extinction lead angle constraint conditions.

9. A computer device, characterized in that: include: Memory for storing computer programs; A processor, configured to implement the method for analyzing the transmission capacity of offshore wind power via multi-terminal hybrid direct current as claimed in any one of claims 1 to 5 when executing the computer program.

10. A storage medium, characterized in that: The storage medium stores a computer program, which, when executed by a processor, implements the steps of the method for analyzing the transmission capacity of offshore wind power via multi-terminal hybrid direct current as described in any one of claims 1 to 5.

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