A marine cable reactive power optimization method for offshore wind power low-frequency AC power transmission M-G grid-connected system

Abstract

The present application relates to offshore wind power transmission technology field, especially to a kind of offshore wind power low-frequency AC transmission M-G grid-connected system sea cable reactive power optimization method, including the reactive power optimization target based on low-frequency sea cable operating current and sea cable active loss, reactive power optimization target includes sea cable current sub-target and sea cable active loss sub-target, and according to wind farm active power output adjusts the weight of sea cable current sub-target and sea cable active loss sub-target;Under multiple constraints, with wind farm reactive power and M-G system synchronous motor reactive power as optimization variable, obtain the reactive power configuration result corresponding to different wind farm active power;According to the current wind farm active power, determine the corresponding reactive power configuration result, and control wind farm side and M-G system synchronous motor side according to reactive power configuration result to execute reactive power regulation.The present application can automatically realize the coordination and balance between sea cable operating current control and sea cable active loss reduction under different operating conditions.

Description

Technical Field

[0001] This invention relates to the technical field of offshore wind power transmission, and in particular to a method for optimizing reactive power of submarine cables in an offshore wind power low-frequency AC transmission (MG) grid-connected system. Background Technology

[0002] With the expansion of offshore wind power development and the increasing demand for power transmission in mid-to-long-distance waters, offshore wind power transmission systems need to achieve power transmission over long distances via submarine cables. Low-frequency AC transmission schemes can reduce the impact of AC submarine cable capacitance effects on the transmission process.

[0003] In offshore wind power low-frequency AC transmission systems, the voltage, current, reactive power, and active power losses along the submarine cable are affected by factors such as changes in the active power output of the wind farm, submarine cable parameters, and the operating status of the grid connection system. When the active power output of the wind farm changes, the operating current and active power losses of the low-frequency submarine cable will also change accordingly. If the reactive power configuration is not adapted to the corresponding operating conditions, the operating current of the submarine cable may approach the current carrying capacity limit, or the active power loss of the submarine cable may be at a high level.

[0004] Therefore, in offshore wind power low-frequency AC transmission MG grid-connected systems, there is an urgent need for a submarine cable reactive power optimization method that can adapt to changes in the active power output of wind farms, so as to take into account both the low-frequency submarine cable operating current and the active power loss of submarine cables while meeting the system operation constraints. Summary of the Invention

[0005] This invention provides a method for optimizing reactive power of submarine cables in offshore wind power low-frequency AC transmission MG grid-connected systems, which can effectively solve the problems in the background technology.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A method for reactive power optimization of submarine cables in an offshore wind power low-frequency AC transmission MG grid-connected system, wherein the offshore wind power low-frequency AC transmission MG grid-connected system includes an offshore wind farm, a low-frequency submarine cable, an MG system, and an MG excitation system, and the method includes: A reactive power optimization target is established based on the low-frequency submarine cable operating current and active power loss of the submarine cable. The reactive power optimization target includes a submarine cable current sub-target and a submarine cable active power loss sub-target, and the weights of the submarine cable current sub-target and the submarine cable active power loss sub-target are adjusted according to the active power output of the wind farm. Under the constraints of reactive power output of wind farms, reactive power output of synchronous motors in MG systems, node power constraints, node voltage constraints, and submarine cable current carrying capacity constraints, reactive power output of wind farms and reactive power output of synchronous motors in MG systems are used as optimization variables to obtain reactive power configuration results corresponding to active power output of different wind farms. The reactive power configuration results include reactive power output values ​​of wind farms and reactive power output values ​​of synchronous motors in MG systems. The corresponding reactive power configuration result is determined based on the current active power output of the wind farm, and the reactive power regulation is performed on the wind farm side and the synchronous motor side of the MG system according to the reactive power configuration result.

[0007] Furthermore, the step of establishing a reactive power optimization target based on the low-frequency submarine cable operating current and active power loss of the submarine cable includes: constructing a reactive power optimization model for the submarine cable and setting the reactive power optimization target in the reactive power optimization model of the submarine cable.

[0008] Furthermore, the reactive power optimization model of the submarine cable is based on the T-type equivalent circuit per unit length. According to the actual engineering requirements, the sending submarine cable is divided into several segments, and the operating current and reactive power of the submarine cable are calculated segment by segment. The operating current of the submarine cable is used to determine the sub-target of the submarine cable current and the sub-target of the submarine cable active power loss.

[0009] Furthermore, the objective function of the reactive power optimization objective satisfies: ; Where F is the objective function, minF means minimizing F is the optimization objective, F1 is the sub-objective of submarine cable current, F2 is the sub-objective of submarine cable active power loss, and λ is the weighting coefficient of submarine cable current. 1_max F 2_max These are the maximum values ​​of optimization objectives F1 and F2, respectively.

[0010] Furthermore, the submarine cable current sub-target is obtained by comparing the current of each segment of the submarine cable to obtain the maximum submarine cable current, and the optimization target is to reduce the maximum submarine cable current.

[0011] Furthermore, the submarine cable current sub-target satisfies: F1=max{i n The optimization objective is to minimize F1. ; ; Among them, i n Let P be the current at the end of the nth segment of the submarine cable. S Q S These represent the active power and reactive power at the head end of the submarine cable, respectively. n Let ΔP be the distance from the end of the nth segment of the submarine cable to the beginning of the cable. R ΔQ represents the active power loss per unit length of the submarine cable. L ΔQ represents the reactive power loss per unit length of the submarine cable. C U0 is the charging power per unit length of the submarine cable, and R is the nominal voltage of the output submarine cable. S L is the equivalent resistance per unit length of the submarine cable. S To transmit the inductance per unit length of the submarine cable, C SThe capacitance per unit length of the transmitted submarine cable is given by f, where f is the operating frequency of the transmitted submarine cable.

[0012] Furthermore, the active power loss sub-objective of the submarine cable is obtained by summing the active power losses of each segment of the submarine cable, and the reduction of the active power loss of the submarine cable is taken as the optimization object.

[0013] Furthermore, the active power loss sub-target of the submarine cable satisfies: The optimization objective is to minimize F2. ; Among them, P loss For active power loss in submarine cables, i k-1 Let Δl be the current at the beginning of the k-th segment of the submarine cable. k Let R be the length of the k-th segment of the submarine cable. S This represents the equivalent resistance per unit length of the submarine cable.

[0014] Furthermore, the submarine cable current weighting coefficient satisfies: ; Among them, P WT P represents the per-unit value of the active power output of the wind farm. WTmax This is the per-unit value of the maximum active power output of the wind farm.

[0015] Furthermore, the node power constraint is used to determine the active and reactive power injected into each node based on the node voltage amplitude, node voltage phase angle difference, and node admittance, and to ensure that each node satisfies the power flow balance relationship. The reactive power output constraint of the wind farm is used to limit the reactive power output of the wind farm based on the current active power output and the current reactive power control capability limit of the wind farm. The reactive power constraint of the MG system synchronous motor is used to limit the reactive power of the MG system synchronous motor according to the upper and lower limits of the allowable reactive power of the MG system synchronous motor. The node voltage constraint is used to limit the node voltage at each node of the wind farm low-frequency power transmission system according to the preset node voltage limit. The submarine cable current carrying capacity constraint is used to limit the current of the entire submarine cable according to the current carrying capacity limit of the power transmission cable.

[0016] Furthermore, obtaining the reactive power configuration results corresponding to the active power output of different wind farms includes: setting an interval step size ΔP, and dividing the active power output of the wind farm [0, P] into intervals of [0, P]. WTmax [P] is divided into multiple intervals. i P i +ΔP], with P i+ΔP / 2 is used as the power output of the wind farm in the current interval. Reactive power optimization is performed on the low-frequency submarine cable to obtain the optimal reactive power value of the wind farm and the synchronous motor of the MG system in the current interval, and a lookup table of reactive power of the wind farm and the synchronous motor of the MG system under different active power outputs of the wind farm is generated.

[0017] Furthermore, reactive power optimization is performed on the low-frequency submarine cable, including: solving the objective function using the particle swarm optimization algorithm, with the reactive power output of the wind farm and the reactive power output of the synchronous motor of the MG system as the optimization variables x. i Particle position vector x i =[Q WT Q M Output the optimal reactive power optimization strategy x. best =[Q WT_best Q M_best ].

[0018] Furthermore, the control of the wind farm side and the synchronous motor side of the MG system performs reactive power regulation according to the reactive power configuration result, including: A coordinated control strategy for wind farm and MG system is adopted. The MG excitation system includes an M-side excitation device. The coordinated control strategy for wind farm and MG system includes reactive power control of wind turbine converter and excitation control of M-side excitation device based on terminal voltage command. The M-side excitation device adjusts the excitation current of the MG system synchronous motor according to the terminal voltage command value corresponding to the reactive power output value of the MG system synchronous motor, thereby adjusting the reactive power output of the MG system synchronous motor. The wind turbine converter changes the reactive power reference value according to the reactive power output value of the wind farm, and controls the reactive power output on the wind turbine side according to the reactive power reference value.

[0019] Furthermore, the method also includes hysteresis control, the hysteresis control comprising: Set the hysteresis width h, and set h to 10%ΔP; When the actual power output of the wind farm is in the range (P) i -h / 2,P i When +h / 2), the reactive power configuration result of the current interval remains unchanged; When the actual power output P of the wind farm WT ≥P i When +h / 2, update the reactive power configuration result to the reactive power configuration result of the (i+1)th interval; When the actual power output P of the wind farm WT ≤P i When -h / 2, the reactive power configuration result is updated to the reactive power configuration result of the (i-1)th interval.

[0020] The technical solution of this invention can achieve the following technical effects: by introducing the changes in active power output of wind farms into the reactive power optimization process of submarine cables, a dual-objective optimization mechanism combining the operating current and active power loss of submarine cables is established. The weights of each optimization objective are dynamically adjusted according to different active power output levels. At the same time, the reactive power regulation resources of the wind farm side and the synchronous motor side of the MG system are combined for collaborative optimization control, so that the system can determine the reactive power output values ​​of the wind farm and the synchronous motor of the MG system under different active power output ranges of wind farms, and perform dual-sided reactive power regulation accordingly.

[0021] The above description is only an overview of the technical solution of this application. In order to better understand the technical means of this application and to implement it in accordance with the contents of the specification, and to make the above and other objects, features and advantages of this application more obvious and understandable, the following are specific embodiments of this application. Attached Figure Description

[0022] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0023] Figure 1 This is a flowchart illustrating the reactive power optimization method for submarine cables in the offshore wind power low-frequency AC transmission MG grid-connected system of the present invention. Figure 2 This is a schematic diagram of the T-type equivalent circuit and segmented equivalent model per unit length of low-frequency submarine cable provided in an embodiment of the present invention; Figure 3 This is a schematic diagram of reactive power control on the synchronous motor side of the MG system provided in an embodiment of the present invention; Figure 4 This is a block diagram of the coordinated reactive power control of the wind turbine grid-side converter and MG system provided in an embodiment of the present invention; Figure 5 A schematic diagram of hysteresis control for switching active power output ranges in a wind farm, provided in an embodiment of the present invention; Figure 6 This is a simulation comparison diagram of a wind farm with an active power output of 150MW provided in an embodiment of the present invention. Figure 6 (a) is a comparison chart of active power transmitted along the submarine cable. Figure 6 (b) is a comparison diagram of current along the submarine cable; Figure 7 This is a simulation comparison diagram of a wind farm with an active power output of 200MW provided in an embodiment of the present invention. Figure 7 (a) is a comparison chart of active power transmitted along the submarine cable. Figure 7 (b) is a comparison diagram of current along the submarine cable; Figure 8 This is a simulation comparison diagram of a wind farm with an active power output of 250MW provided in an embodiment of the present invention. Figure 8 (a) is a comparison chart of active power transmitted along the submarine cable. Figure 8 (b) is a comparison diagram of the current along the submarine cable. Detailed Implementation

[0024] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0025] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used in this specification is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0026] like Figure 1 As shown, this embodiment provides a method for reactive power optimization of a submarine cable in an offshore wind power low-frequency AC transmission MG grid-connected system. The offshore wind power low-frequency AC transmission MG grid-connected system includes an offshore wind farm, a low-frequency submarine cable, an MG system, and an MG excitation system. The offshore wind farm is connected to the MG system via the low-frequency submarine cable, and the MG system is connected to the onshore power grid. The MG system includes a synchronous motor, and the MG excitation system is used to adjust the excitation state of the synchronous motor. This embodiment specifically includes the following steps: Step S100: Establish reactive power optimization targets based on low-frequency submarine cable operating current and active power loss of submarine cable. The reactive power optimization targets include submarine cable current sub-targets and submarine cable active power loss sub-targets, and adjust the weights of the submarine cable current sub-targets and the submarine cable active power loss sub-targets according to the active power output of the wind farm. Step S200: Under the constraints of reactive power output of wind farm, reactive power output of synchronous motor of MG system, node power constraint, node voltage constraint and submarine cable current carrying capacity, the reactive power output of wind farm and reactive power output of synchronous motor of MG system are used as optimization variables to obtain the reactive power configuration results corresponding to the active power output of different wind farms. The reactive power configuration results include the reactive power output value of wind farm and the reactive power output value of synchronous motor of MG system. Step S300: Determine the corresponding reactive power configuration result based on the current active power output of the wind farm, and control the wind farm side and the synchronous motor side of the MG system to perform reactive power adjustment according to the reactive power configuration result.

[0027] In this embodiment, the following technical effects can be achieved through the technical solution of the present invention: by introducing the changes in the active power output of the wind farm into the reactive power optimization process of the submarine cable, a dual-objective optimization mechanism combining the operating current of the submarine cable and the active power loss of the submarine cable is established, and the weights of each optimization objective are dynamically adjusted according to different active power output levels. At the same time, the reactive power regulation resources of the wind farm side and the synchronous motor side of the MG system are combined for collaborative optimization control, so that the system can determine the reactive power output value of the wind farm and the reactive power output value of the synchronous motor of the MG system under different active power output ranges of the wind farm, and perform dual-sided reactive power regulation accordingly.

[0028] As a specific embodiment of the present invention, in the offshore wind power low-frequency AC transmission MG grid-connected system, the offshore wind farm is connected to the MG system via a low-frequency submarine cable, and the MG system is connected to the onshore power grid. The MG system includes a synchronous motor, and the MG excitation system is used to adjust the excitation state of the synchronous motor. The transmission distance of this system is relatively long, and the voltage, current, reactive power, and active power loss along the low-frequency submarine cable change with the active power output of the wind farm, the submarine cable parameters, and the operating status of the grid connection side. In particular, when the active power output of the wind farm increases, the operating current and active power loss of the low-frequency submarine cable change accordingly. If the reactive power configuration is not matched with the operating conditions, the operating current of the submarine cable may approach the current carrying capacity limit, and the active power loss of the submarine cable may also remain at a high level.

[0029] When determining the reactive power optimization target, this embodiment does not use a single reactive power compensation amount as the evaluation object, but instead incorporates both the low-frequency submarine cable operating current and the active power loss of the submarine cable into the optimization objective. This approach stems from the fact that the submarine cable operating constraints and line losses are not the same type of indicator. The former mainly corresponds to the submarine cable current-carrying safety boundary, while the latter mainly corresponds to the active power loss during the transmission process. The response direction and weight requirements of the two to reactive power configuration change with the wind farm's power output conditions, so they need to be characterized separately in the same objective function.

[0030] In step S100, a reactive power optimization model for the submarine cable is first constructed, and a reactive power optimization objective is set in the reactive power optimization model; for example... Figure 2 As shown, the reactive power optimization model is based on a unit length T-type equivalent circuit. According to the actual engineering requirements of the submarine cable, the submarine cable is divided into several segments, and the operating current and reactive power of the submarine cable are calculated segment by segment. After adopting the segmented calculation method, the current change at different positions of the submarine cable can be included in the optimization calculation, avoiding the use of the electrical quantity at a single point at the beginning or end of the submarine cable to represent the operating state of the entire line.

[0031] Specifically, the reactive power optimization objectives include a submarine cable current sub-objective F1 and a submarine cable active power loss sub-objective F2. Since the submarine cable current sub-objective corresponds to current quantity and the submarine cable active power loss sub-objective corresponds to power quantity, directly adding the two does not meet the requirement of dimensional consistency. Therefore, in this embodiment, F1 and F2 are used respectively. 1_max and F2_max F1 and F2 are normalized and then weighted according to the submarine cable current weighting coefficient λ; the objective function of the reactive power optimization objective satisfies: ; Where F is the objective function, minF means minimizing F is the optimization objective, F1 is the sub-objective of submarine cable current, F2 is the sub-objective of submarine cable active power loss, and λ is the weighting coefficient of submarine cable current. 1_max F 2_max These are the maximum values ​​of the optimization objectives F1 and F2, respectively. After this normalization and weighting process, the submarine cable operating current index and the submarine cable active power loss index can be evaluated in the same objective function.

[0032] More specifically, for the submarine cable current sub-objective, the current at the end of each segment of the submarine cable is first calculated separately, and then the currents of each segment are compared. The maximum submarine cable current is taken as the submarine cable current sub-objective. The submarine cable current carrying capacity constraint is limited by the location of the maximum current along the entire line. Taking the maximum submarine cable current as the optimization object allows the optimization calculation to directly correspond to the submarine cable current constraint. The submarine cable current sub-objective satisfies: F1=max{i n The optimization objective is to minimize F1. ; ; Among them, i n Let P be the current at the end of the nth segment of the submarine cable. S Q S These represent the active power and reactive power at the head end of the submarine cable, respectively. n Let ΔP be the distance from the end of the nth segment of the submarine cable to the beginning of the cable. R ΔQ represents the active power loss per unit length of the submarine cable. L ΔQ represents the reactive power loss per unit length of the submarine cable. C U0 is the charging power per unit length of the submarine cable, and R is the nominal voltage of the output submarine cable. S L is the equivalent resistance per unit length of the submarine cable. S To transmit the inductance per unit length of the submarine cable, C S The capacitance per unit length of the transmitted submarine cable is given by f, where f is the operating frequency of the transmitted submarine cable.

[0033] For the active power loss sub-objective of submarine cable, the active power loss of the submarine cable is obtained by summing the active power losses of each segment, and the optimization target is to reduce the active power loss of the submarine cable. This loss calculation method corresponds to the aforementioned segmented modeling of submarine cable, and the current at the beginning of each segment and the segment length are both included in the loss calculation. The active power loss sub-objective of submarine cable satisfies: The optimization objective is to minimize F2. ; Among them, P loss For active power loss in submarine cables, i k-1 Let Δl be the current at the beginning of the k-th segment of the submarine cable. k Let R be the length of the k-th segment of the submarine cable. S This represents the equivalent resistance per unit length of the submarine cable.

[0034] After determining the above two sub-objectives, the weights of the submarine cable current sub-objective and the submarine cable active power loss sub-objective are adjusted according to the active power output of the wind farm. When the active power output of the wind farm is high, the submarine cable current is more likely to approach the current carrying capacity constraint, so the proportion of the submarine cable current sub-objective in the objective function is increased. When the active power output of the wind farm is low, the submarine cable current has a relative margin from the current carrying capacity limit, so the proportion of the submarine cable active power loss sub-objective in the objective function is increased. The submarine cable current weighting coefficient satisfies the following: ; Among them, P WT P represents the per-unit value of the active power output of the wind farm. WTmax This is the per-unit value of the maximum active power output of the wind farm. According to this weighting relationship, when the active power output of the wind farm changes, the proportions of the submarine cable current sub-objective and the submarine cable active power loss sub-objective in the objective function are adjusted accordingly, so that the reactive power optimization objective corresponds to the submarine cable operating state under the current active power output condition.

[0035] In this embodiment, after processing in step S100, the maximum operating current of the submarine cable and the active power loss of the submarine cable are included in the same reactive power optimization model for evaluation, and their weights are adjusted according to the change of the active power output of the wind farm. In subsequent optimization solutions, the objective function can be used as the evaluation function, and combined with the reactive power output of the wind farm, the reactive power output of the synchronous motor of the MG system and the corresponding operating constraints, the reactive power configuration results under different active power outputs of the wind farm can be determined. Thus, the obtained reactive power configuration results can simultaneously correspond to the submarine cable current carrying capacity constraints and the submarine cable active power loss control requirements.

[0036] In a specific implementation, as one example, after establishing the reactive power optimization target involving both the low-frequency submarine cable operating current and active power loss in step S100, step S200 is executed. The reactive power configuration in the offshore wind power low-frequency AC transmission MG grid-connected system is not only controlled by the optimization target, but also simultaneously constrained by the wind farm's reactive power regulation capability, the MG system synchronous motor excitation regulation range, node power flow balance, node voltage limits, and submarine cable current carrying capacity limits. If only the minimum objective function value is used as the judgment criterion without constraining the above operating boundaries, the resulting reactive power output value may exceed the adjustable range of the wind turbine converter or the MG system synchronous motor, and may also cause node voltage or submarine cable current to exceed limits. Therefore, in this embodiment, in step S200, the reactive power output of the wind farm and the reactive power output of the MG system synchronous motor are used as joint optimization variables to obtain the reactive power configuration results corresponding to different wind farm active power outputs under the joint constraints of multiple operating constraints, as shown below: Step S210: Set the active power output range of the wind farm; using the maximum active power output P of the wind farm. WTmax As the upper limit of active power output, a step size ΔP is set, defining the active power output range of the wind farm as [0, P]. WTmax [P] is divided into multiple continuous intervals. i P i +ΔP]; For any current interval, take P i +ΔP / 2 is used as the representative value of the active power output of the wind farm within this interval, and this representative value is used as the input of the active power output of the wind farm in the current interval reactive power optimization calculation. By using the interval representative value for optimization, the same active power output interval corresponds to a set of determined reactive power configuration results. In subsequent operation, the corresponding configuration is called according to the interval where the active power output of the current wind farm is located, without having to re-perform the complete optimization calculation for each real-time sample value.

[0037] Step S220: Determine the reactive power optimization objective corresponding to the current interval; set P i Substituting +ΔP / 2 into the calculation relationship of the submarine cable current weight coefficient λ in step S100, the weights of the submarine cable current sub-target F1 and the submarine cable active power loss sub-target F2 in the current interval are determined. Subsequently, these weights are written into the reactive power optimization objective function, so that the optimization direction of the current interval corresponds to the active power output of the wind farm in that interval. When the active power output of the wind farm is high, the proportion of the submarine cable current sub-target in the objective function increases accordingly; when the active power output of the wind farm is low, the proportion of the submarine cable active power loss sub-target in the objective function increases accordingly. This process enables different weight relationships to be used for different active power output intervals, avoiding the problem that fixed weights cannot simultaneously take into account the current carrying capacity limitation in the high output interval and the loss control in the low output interval.

[0038] Step S230: Use the particle swarm optimization algorithm to solve the objective function for the current interval, and calculate the reactive power output Q of the wind farm. WTReactive power Q of synchronous motor in MG system M As an optimization variable, the particle position vector of the i-th particle is represented as: x i =[Q WT Q M ]; Among them, Q WT For reactive power output from wind farms, Q M The reactive power output of the synchronous motor in the MG system; each particle position corresponds to a set of dual-sided reactive power configuration schemes to be evaluated; this set of Q... WT and Q M Write the power flow calculation model of the low-frequency transmission system and combine it with the representative value of active power output P in the current interval. i +ΔP / 2, calculate the voltage at each node, the phase angle at each node, the injected power at each node, the segment current of the submarine cable, and the active power loss of the submarine cable.

[0039] Step S240: Set node power constraints; node power constraints are used to determine the active and reactive power injected into each node based on the node voltage magnitude, node voltage phase angle difference, and node admittance, and to ensure that each node satisfies the power flow balance relationship; for the i-th node, its injected active power P i and injected reactive power Q i It can be determined by the following formula: ; Among them, U i U j Let θ represent the voltage magnitudes of the i-th and j-th nodes, respectively. ij G represents the voltage phase angle difference between the i-th node and the j-th node. ij B ij These are the conductance and susceptance components of the element in the i-th row and j-th column of the nodal admittance matrix, respectively, N. bus The total number of nodes; for each node connected to the wind farm, low-frequency submarine cable, MG system, and onshore power grid, the corresponding active and reactive power boundaries are determined according to its node type to match the node injection power with the power flow calculation results; through this constraint, Q WT and Q M It is not an isolated value, but is evaluated within the power flow balance relationship of the low-frequency AC transmission system.

[0040] Step S250: Set reactive power output constraints for the wind farm and reactive power output constraints for the synchronous motors of the MG system; for the representative value of active power output P in the current interval i +ΔP / 2, the reactive power output of the wind farm is limited by the current reactive power regulation capability limit of the wind farm, which satisfies: ; ; Among them, S WTmax This refers to the maximum apparent power of the direct-drive wind turbine inverter, typically 110% or more of the rated value; P WT Q represents the active power output of the wind turbine. WT The reactive power output of the wind turbine; this constraint makes Q in the particle position vector... WT Within the current reactive power control capability of the wind farm, avoid exceeding the executable boundary of the wind turbine converter in terms of reactive power configuration value.

[0041] The reactive power output of the synchronous motor in the MG system is limited by the upper and lower limits of the allowable reactive power output of the synchronous motor, which satisfies the following: ; in, This is the lower limit of the permissible reactive power output of the synchronous motor in the MG system. This is the upper limit of the allowed reactive power output of the synchronous motor in the MG system; this constraint makes Q in the particle position vector... M Within the adjustable range of the synchronous motor and MG excitation system, the subsequent excitation control can be executed according to the corresponding reactive power output value.

[0042] Step S260: Set node voltage constraints and submarine cable current carrying capacity constraints; the node voltage constraint is used to limit the node voltage at each point in the wind farm low-frequency transmission system according to the preset node voltage limit; for the i-th node, its node voltage amplitude U i satisfy: ; in, This is the preset lower limit of the node voltage for the i-th node. The preset upper limit of the node voltage for the i-th node is set; voltage constraint judgments are performed on the wind farm grid-connected nodes, the equivalent segment nodes of the low-frequency submarine cable, the MG system connection nodes, and the onshore power grid connection nodes; if the voltage of any node exceeds the corresponding limit, the reactive power configuration corresponding to that particle is not considered as the optimal configuration result for the current interval.

[0043] Submarine cable current carrying capacity constraint is used to limit the current of the entire submarine cable according to the current carrying capacity limit of the power transmission cable; the current i of each segment of the submarine cable is obtained according to the segmented calculation method in step S100. n And make it satisfy: i n ≤I lim n=1, 2, ..., N cab ; Among them, I lim For the current carrying capacity limit of power transmission cables, N cabThe number of submarine cable segments is defined. This constraint corresponds to the setting in step S100 where the maximum submarine cable current is used as the sub-objective of the submarine cable current, so that the optimization process incorporates the maximum submarine cable current into the objective function evaluation and uses the submarine cable current carrying capacity limit as the operating boundary.

[0044] Step S270: Use the particle swarm optimization algorithm to find the optimal reactive power configuration for the current interval; when initializing the particle swarm, set the position x of each particle... i =[Q WT Q M The particles fall within the limits of the reactive power output constraints of the wind farm and the reactive power output constraints of the synchronous motor in the MG system, respectively, and the particle velocity vector v is set. i In each iteration, based on the Q corresponding to the particle position... WT and Q M Perform power flow calculation, submarine cable segment current calculation, and submarine cable active power loss calculation to determine whether node power constraints, node voltage constraints, and submarine cable current carrying capacity constraints are satisfied. For particles that satisfy the constraints, calculate the fitness according to the objective function of step S100. For particles that do not satisfy the constraints, do not use them as the update objects for individual optimal position and global optimal position, or include them in the ranking after a penalty value is calculated according to the degree of constraint violation.

[0045] Particle velocity and position can be updated using the following formula: ; ; Where t is the number of iterations, ω is the inertia weight, c1 and c2 are learning factors, r1 and r2 are random numbers, and p best,i (t) represents the optimal position of the i-th particle before the t-th iteration, g best (t) represents the globally optimal position of the particle swarm before the t-th iteration; after the particle position is updated, if Q WT Exceeding to If Q is within the range, then it is corrected according to the corresponding boundary value; if Q M Exceeding to If the range is defined, it is corrected according to the corresponding boundary values; after judging by the preset number of iterations or convergence conditions, the optimal reactive power optimization strategy within the current interval is output: x best =[Q WT_best Q M_best ]; Among them, Q WT_best Q represents the optimal reactive power output of the wind farm within the current interval. M_best This represents the optimal reactive power output of the synchronous motor in the MG system within the current interval.

[0046] Step S280: Generate a reactive power lookup table for different active power outputs of the wind farm; for each active power output interval [P] i P i +ΔP] Perform steps S220 to S270 one by one to obtain Q for each interval. WT_best and Q M_best And generate a lookup table of reactive power of the wind farm and the synchronous motor of the MG system under different active power outputs of the wind farm; the lookup table includes at least the active power output range [P i P i +ΔP], Representative value of active power output in the interval P i +ΔP / 2, reactive power output of the wind farm Q WT_best and the reactive power output Q of the synchronous motor in the MG system M_best During subsequent operation, when the active power output of the wind farm falls within a certain active power output range, the corresponding Q value in the lookup table for that range will be used. WT_best and Q M_best Determine the reactive power configuration result.

[0047] In this embodiment, step S200 does not involve separately calculating the reactive power output of the wind farm side, nor does it involve separately adjusting the reactive power output of the synchronous motor of the MG system. Instead, under the dual objectives of submarine cable current and active power loss determined in step S100, Q is... WT and Q M As a joint optimization variable, the reactive power output constraints of the wind farm, the reactive power output constraints of the MG system synchronous motor, the node power constraints, the node voltage constraints, and the submarine cable current carrying capacity constraints are applied simultaneously. Through this combination, the resulting reactive power configuration corresponds to both the submarine cable current control requirements and the submarine cable active power loss control requirements under the current active power output range, and is also within the allowable operating range of the wind farm, the MG system synchronous motor, and the low-frequency transmission system. Compared with the fixed reactive power configuration method, this method can make different active power output ranges of the wind farm correspond to executable dual-sided reactive power configuration results. Compared with the method of relying solely on single-sided reactive power regulation, this method can utilize the reactive power regulation capabilities of the wind farm side and the MG system synchronous motor side to jointly change the power flow distribution along the low-frequency submarine cable, reducing the situation where the submarine cable current approaches the current carrying capacity limit or the submarine cable active power loss is too high due to the limitation of single-sided reactive power capability.

[0048] In a specific implementation, as one example, after step S200 has generated a reactive power lookup table for the wind farm and the synchronous motor of the MG system under different active power outputs, step S300 is executed. Before this, the reactive power configuration result obtained in step S200 needs to be transferred to the equipment control layer. The wind turbine converter typically uses the reactive power reference value as the control input, such as... Figure 3As shown, the synchronous motor side of the MG system changes the excitation current through the excitation device on the M side, and the reactive power output of the synchronous motor is affected by the terminal voltage regulation; the controlled objects, actuators, and regulation paths of the two are different; if only a unified reactive power optimization result is given, without distinguishing the execution methods of the wind farm side and the synchronous motor side of the MG system, then the Q obtained in step S200 will be... WT_best and Q M_best It is difficult to directly correspond to the equipment control quantity; therefore, in step S300 of this embodiment, the corresponding reactive power configuration result is called according to the current active power output of the wind farm, and the reactive power reference value of the wind turbine converter and the voltage command value of the M-side excitation device are formed respectively, so that the wind farm side and the synchronous motor side of the MG system coordinate to operate according to the same reactive power configuration result, as shown below: Step S310: Obtain the current active power output of the wind farm; collect the active power measurement value at the grid-connected side or main bus of the wind farm and use it as the current active power output of the wind farm; the current active power output of the wind farm has the same per-unit value or power reference as the active power output range used when generating the lookup table in step S200; if the lookup table uses per-unit values, then the collected active power of the wind farm is set according to the maximum active power output P of the wind farm. WTmax The conversion is performed before participating in the interval judgment; if the actual power value is used in the lookup table, then the actual power value is directly used in the interval judgment.

[0049] Step S320: Determine the reactive power configuration result corresponding to the current active power output of the wind farm; determine the active power output range [P] based on the current active power output of the wind farm. i P i +ΔP], and read the reactive power output value Q of the wind farm corresponding to this interval from the reactive power lookup table generated in step S200. WT_best The reactive power output value Q of the synchronous motor in the MG system M_best The Q WT_best As the reactive power regulation target on the wind farm side, the Q M_best As the target for reactive power regulation on the synchronous motor side of the MG system; since this lookup table is generated in step S200 under the combined constraints of wind farm reactive power output, MG system synchronous motor reactive power output, node power constraints, node voltage constraints, and submarine cable current carrying capacity constraints, the obtained Q is read. WT_best and Q M_best This corresponds to the current control requirements of submarine cables, the active power loss control requirements of submarine cables, and the system operation constraints under the current active power output range.

[0050] Step S330: Generate coordinated control commands between the wind farm and the MG system. The read Q... WT_best and Q M_bestThe reactive power is fed into the reactive power control channel on the wind farm side and the excitation control channel on the synchronous motor side of the MG system, respectively. The coordinated control strategy between the wind farm and the MG system includes reactive power control of the wind turbine converter and excitation control of the M-side excitation device based on terminal voltage commands. Through this control strategy, Q... WT_best Instead of being calculated in isolation, it is transformed into an executable reactive power reference value for the wind turbine converter; Q M_best Instead of being a value directly applied to the excitation current, it is converted into a terminal voltage command value corresponding to the reactive power output of the synchronous motor. Therefore, the wind farm side and the synchronous motor side of the MG system execute the same set of reactive power configuration results according to their respective control interfaces.

[0051] Step S340: Control the M-side excitation device to adjust the reactive power output of the synchronous motor in the MG system; for the synchronous motor side of the MG system, according to Q M_best The corresponding terminal voltage command value is determined; the terminal voltage command value can be determined by a preset correspondence between the reactive power output value of the synchronous motor and the terminal voltage command value; after receiving the terminal voltage command value, the M-side excitation device compares the terminal voltage command value with the measured terminal voltage value of the synchronous motor in the MG system, and adjusts the excitation current of the synchronous motor according to the comparison result; after the excitation current changes, the terminal voltage and reactive power exchange state of the synchronous motor change accordingly, causing the reactive power output of the synchronous motor in the MG system to shift towards Q. M_best The corresponding operating status was adjusted.

[0052] During this process, the M-side excitation device does not directly use the submarine cable current or submarine cable loss as the control input, but instead executes the Q determined in step S200. M_best The corresponding terminal voltage command; the reason for adopting this method is that the reactive power output of the synchronous motor is related to the terminal voltage, excitation state, and system power flow, and the terminal voltage command is a control quantity that can be directly received and executed by the M-side excitation control; Q M_best After being converted to the terminal voltage command value, the synchronous motor side of the MG system can participate in the reactive power optimization of the submarine cable through the existing excitation regulation path.

[0053] Step S350: Control the wind turbine converter to adjust the reactive power output on the wind farm side; for the wind farm side, Q... WT_best As the target for reactive power output of the wind farm, the reactive power reference value of the wind turbine converter is adjusted accordingly. The wind turbine converter compares the reactive power reference value with the actual reactive power output on the wind farm side or the reactive power feedback from each wind turbine converter to form a reactive power adjustment deviation. The converter control loop generates a corresponding current command based on the reactive power adjustment deviation and changes the reactive power output on the wind turbine side through the converter inner loop control, so that the reactive power output of the wind farm is directed towards Q. WT_best Corresponding operational status adjustments; When a wind farm includes multiple wind turbines, the target reactive power output Q of the wind farm is... WT_bestThe reactive power can be allocated to each wind turbine converter according to the current available reactive power regulation capacity, wind turbine operating status, or preset allocation coefficient. After receiving the allocated reactive power reference value, each wind turbine converter adjusts its own reactive power output. This allocation method does not change the total reactive power output target of the wind farm determined in step S200. Its function is to make the total reactive power output of the wind farm consistent with Q. WT_best Correspondingly.

[0054] Step S360: Perform dual-side coordinated reactive power regulation; the wind farm side and the MG system synchronous motor side receive corresponding control commands within the same control cycle; the wind farm side adjusts the reactive power output of the wind turbine converter according to the reactive power reference value, and the MG system synchronous motor side adjusts the excitation current according to the terminal voltage command value; the two-side regulation actions jointly change the reactive power distribution, node voltage state and submarine cable current distribution along the low-frequency submarine cable, so that the reactive power configuration result corresponding to the current active power output range enters the low-frequency transmission system operation process.

[0055] In this embodiment, step S300 does not involve resolving the objective function, nor does it involve a single device handling all reactive power regulation. Instead, Q has already been determined in step S200. WT_best and Q M_best Under the premise of this, the two are converted into the reactive power reference value of the wind turbine converter and the terminal voltage command value of the M-side excitation device, respectively. The wind turbine converter changes the reactive power output of the wind farm side through reactive power control, and the M-side excitation device changes the excitation current of the synchronous motor and adjusts the reactive power output of the synchronous motor through the terminal voltage command. This process enables the reactive power configuration result obtained by offline optimization to be executed according to the actual control interface of the equipment, avoiding the situation where only the optimal reactive power value is obtained but cannot be corresponding to the control quantities of the wind turbine converter and excitation device. Through step S300 above, the current active power output of the wind farm, the reactive power configuration results in the lookup table, the reactive power control of the wind turbine converter, and the excitation control on the M side are connected in series into the same execution link; this link forms a continuous relationship with steps S100 and S200: step S100 determines the evaluation method of submarine cable current and submarine cable active power loss, and step S200 obtains Q corresponding to different active power output ranges under multiple operating constraints. WT_best and Q M_best Step S300 then sets the Q corresponding to the current interval. WT_best and Q M_best The reactive power regulation is implemented on both the wind farm side and the synchronous motor side of the MG system. As a result, the system does not need to re-execute particle swarm iteration at every sampling moment during operation, and the reactive power regulation results will not remain at the lookup table level, but will be applied to the reactive power operation state of the low-frequency submarine cable through the joint action of the wind turbine converter and the M-side excitation device. This combination method can reduce the situation where the fixed reactive power configuration is mismatched with the current active power output of the wind farm, and can also reduce the problem of insufficient regulation caused by the limitation of the reactive power capacity boundary of the single-side equipment when relying solely on single-side reactive power regulation.

[0056] Since the power output of a wind farm changes constantly, to avoid small fluctuations in the active power output of the wind farm at the interval boundary, which would cause the reactive power of the wind farm and the synchronous motor of the MG system to switch back and forth between two optimized values, resulting in the reactive power command switching repeatedly between adjacent optimized values ​​and increasing control command fluctuations, this embodiment further sets up hysteresis control. The hysteresis control is used to limit the switching process of the interval to which the actual power output of the wind farm belongs, so that when the actual power output of the wind farm changes within the preset hysteresis range, the controller output remains unchanged; when the actual power output of the wind farm crosses the hysteresis boundary, the corresponding reactive power configuration result is updated.

[0057] Specifically, the interval corresponding to the currently executing reactive power configuration result is denoted as the i-th interval, and the reactive power output value of the wind farm corresponding to the i-th interval is denoted as Q. WT_best,i The reactive power output of the synchronous motor in the MG system is denoted as Q. M_best,i Let P be the output judgment value corresponding to the i-th interval. i The actual power output of the wind farm is denoted as P. WT The output judgment value P i Used to determine whether the current reactive power configuration result should be maintained or switched to an adjacent interval; like Figure 5 As shown, a hysteresis width h is set, and h is taken as 10%ΔP, where ΔP is the interval step size used when dividing the active power output interval of the wind farm; after adopting this hysteresis width, in P i Both sides form a holding interval of h / 2; when the actual output P of the wind farm WT Satisfy: P i -h / 2<P WT <P i When +h / 2, it is determined that the actual power output of the wind farm is still within the hysteresis range corresponding to the current interval; at this time, the reactive power configuration result is not updated, and the Q corresponding to the i-th interval is continued to be used. WT_best,i and Q M_best,i The reactive power reference value on the wind farm side remains consistent with Q. WT_best,i Correspondingly, the terminal voltage command value on the synchronous motor side of the MG system remains the same as Q. M_best,i Correspondingly, the slight fluctuations in the actual power output of the wind farm near the boundary of the interval will not directly cause the switching of reactive power commands between the wind farm side and the synchronous motor side of the MG system.

[0058] When the actual power output P of the wind farm WT Satisfy: P WT ≥P iWhen +h / 2, it is determined that the actual power output of the wind farm has exceeded the upper hysteresis boundary of the current interval; at this time, the reactive power configuration result is updated to the reactive power configuration result of the (i+1)th interval, that is, the reactive power output value Q of the wind farm corresponding to the (i+1)th interval is adopted. WT_best,i+1 The reactive power output value Q of the synchronous motor in the MG system M_best,i+1 According to Q on the wind farm side WT_best,i+1 Update the reactive power reference value; the synchronous motor side of the MG system is based on Q. M_best,i+1 The excitation current is adjusted according to the corresponding terminal voltage command value; if the current i-th interval is already the highest output interval, the reactive power configuration result corresponding to the highest output interval is maintained.

[0059] When the actual power output P of the wind farm WT Satisfy: P WT ≤P i When -h / 2, it is determined that the actual power output of the wind farm has exceeded the lower hysteresis boundary of the current interval; at this time, the reactive power configuration result is updated to the reactive power configuration result of the (i-1)th interval, that is, the reactive power output value Q of the wind farm corresponding to the (i-1)th interval is adopted. WT_best,i-1 The reactive power output value Q of the synchronous motor in the MG system M_best,i-1 According to Q on the wind farm side WT_best,i-1 Update the reactive power reference value; the synchronous motor side of the MG system is based on Q. M_best,i-1 The excitation current is adjusted according to the corresponding terminal voltage command value; if the current i-th interval is already the lowest output interval, the reactive power configuration result corresponding to the lowest output interval is maintained.

[0060] Through the aforementioned hysteresis control, the actual power output of the wind farm only occurs after exceeding P. i +h / 2 or P i The reactive power configuration result is only switched to the adjacent interval when the hysteresis boundary defined by -h / 2 is reached; in P i -h / 2 to P i When the reactive power fluctuates within the range of +h / 2, the wind farm side and the synchronous motor side of the MG system continue to execute the reactive power configuration results of the current interval. This process can reduce the back-and-forth switching of reactive power commands caused by the repeated changes of the active power output of the wind farm near the interval boundary, so that the update process of the reactive power reference value of the wind turbine converter and the voltage command value of the synchronous motor terminal of the MG system has a maintenance range, thereby making the reactive power regulation process on both sides more continuous.

[0061] To verify the effectiveness of the reactive power optimization method for submarine cables in offshore wind power low-frequency AC transmission MG grid-connected systems under different active power output conditions in different wind farms, PSCAD / EMTDC simulation software was used to construct a simulation system for offshore wind power. Figure 4The electromagnetic transient simulation model of an offshore wind power low-frequency AC transmission (MG) grid-connected system is shown below. The simulation model includes an offshore wind farm, a low-frequency submarine cable, an MG system, and an onshore power grid. The offshore wind farm is connected to the MG system via the low-frequency submarine cable, and the MG system is connected to the onshore power grid. The simulation parameters are configured as shown in the table below.

[0062] In the above simulation model, three operating conditions are set as follows: active power output of the wind farm is 150MW, active power output of the wind farm is 200MW, and active power output of the wind farm is 250MW. Step S200 of the method of this invention is executed for each of the three operating conditions to obtain the optimal reactive power value under the current active power output of the wind farm. Then, step S300 is executed, where the synchronous motor and wind turbine converter respond to commands, and the submarine cable current and transmitted active power of the system are observed. The simulation results obtained after simulation are as follows: Figures 6 to 8 As shown; like Figure 6 As shown, under the operating condition of a wind farm with an active power output of 150MW, Figure 6 (a) shows the comparison results before and after optimization of the active power transmitted along the submarine cable as a function of the cable length. Figure 6 (b) shows the comparison results before and after optimization of the change in current along the submarine cable with the length of the submarine cable; Figure 6 (a) and Figure 6 (b) shows the simulation curves before and after optimization, respectively, before and after implementing the reactive power optimization method. Before optimization, the active power transmitted at the end of the submarine cable was 144.32MW, and after optimization, the active power transmitted at the end of the submarine cable was 145.59MW. Correspondingly, the active power loss of the submarine cable changed from 3.79% to 2.94%. It can be seen that under this operating condition, after adopting the reactive power optimization method, the active power transmitted at the end of the submarine cable increased by 1.27MW, and the active power loss of the submarine cable decreased by 0.85 percentage points. This result shows that under the 150MW output condition, the combined configuration of reactive power output on the wind farm side and reactive power output on the synchronous motor side of the MG system can change the reactive power distribution along the submarine cable, thereby reducing the active power loss of the submarine cable accordingly.

[0063] like Figure 7 As shown, under the operating condition of a wind farm with an active power output of 200MW, Figure 7 (a) shows the comparison results before and after optimization of the active power transmitted along the submarine cable as a function of the cable length. Figure 7 (b) shows the comparison results before and after optimization of the change in current along the submarine cable with the length of the submarine cable; Figure 7 (a) and Figure 7(b) shows the simulation curves before and after optimization, respectively, when the reactive power optimization method was implemented. Before optimization, the active power transmitted at the end of the submarine cable was 191.58MW, and after optimization, it was 192.84MW. Correspondingly, the active power loss of the submarine cable changed from 4.21% to 3.58%. It can be seen that under this operating condition, after adopting the reactive power optimization method, the active power transmitted at the end of the submarine cable increased by 1.26MW, and the active power loss of the submarine cable decreased by 0.63 percentage points. At the same time, before optimization, the current at the end of the submarine cable exceeded the current carrying capacity limit, while after optimization, the current along the submarine cable was within the current carrying capacity limit constraint range. This result shows that under the 200MW output condition, if the original reactive power configuration is simply maintained, the current at the end of the submarine cable may exceed the limit. However, after incorporating the maximum current of the submarine cable and the active power loss of the submarine cable into the reactive power optimization target, the resulting dual-sided reactive power configuration can simultaneously meet the current carrying capacity constraint and active power loss control requirements of the submarine cable.

[0064] like Figure 8 As shown, under the operating condition of a wind farm with an active power output of 250MW, Figure 8 (a) shows the comparison results before and after optimization of the active power transmitted along the submarine cable as a function of the cable length. Figure 8 (b) shows the comparison results before and after optimization of the change in current along the submarine cable with the length of the submarine cable; Figure 8 (a) and Figure 8 (b) shows the simulation curves before and after optimization, respectively, before and after implementing the reactive power optimization method. Before optimization, the active power transmitted at the end of the submarine cable was 238.80MW, and after optimization, the active power transmitted at the end of the submarine cable was 239.58MW. Correspondingly, the active power loss of the submarine cable changed from 4.48% to 4.17%. It can be seen that under this operating condition, after adopting the reactive power optimization method, the active power transmitted at the end of the submarine cable increased by 0.78MW, and the active power loss of the submarine cable decreased by 0.31 percentage points. At the same time, before optimization, the current at the end of the submarine cable exceeded the current carrying capacity limit, and after optimization, the current along the submarine cable was within the current carrying capacity limit constraint range. This result shows that under the operating condition of higher wind farm active power output, the impact of submarine cable current constraint on reactive power configuration results increases. By adjusting the weights of the submarine cable current sub-objective and the submarine cable active power loss sub-objective according to the active power output of the wind farm, the optimization calculation can make the reactive power configuration more biased towards limiting the maximum current of the submarine cable, while taking into account the active power loss of the submarine cable under this constraint.

[0065] Combination Figures 6 to 8Based on the above three simulation conditions, it can be seen that as the active power output of the wind farm increases from 150MW to 250MW, the active power loss of the submarine cable increases with the increase in output, both before and after optimization. However, under each condition, after adopting the submarine cable reactive power optimization method, the active power transmitted at the end of the submarine cable is higher than the corresponding value before optimization, and the active power loss of the submarine cable is lower than the corresponding value before optimization. Under the 200MW and 250MW conditions, before optimization, there was a situation where the current at the end of the submarine cable exceeded the current carrying capacity limit. After adopting the method, the current along the submarine cable is limited by the current carrying capacity constraint. This simulation result corresponds to the technical concept of this method: the submarine cable current sub-objective and the submarine cable active power loss sub-objective are introduced into the low-frequency submarine cable reactive power optimization objective, and the weights of the two are changed according to the active power output of the wind farm. Then, the reactive power regulation is jointly performed by the wind farm side and the synchronous motor side of the MG system, so that the reactive power configuration results under different wind farm output conditions can correspond to the submarine cable current distribution and submarine cable active power loss status.

[0066] Although this application has been described in conjunction with specific features and embodiments, it is obvious that various modifications and combinations can be made thereto without departing from the spirit and scope of this application. Accordingly, this specification and drawings are merely exemplary illustrations of the application as defined herein, and are to be considered as covering any and all modifications, variations, combinations, or equivalents within the scope of this application. Clearly, those skilled in the art can make various alterations and modifications to this application without departing from its scope. Thus, if such modifications and modifications fall within the scope of this application and its equivalents, this application intends to include such modifications and modifications.

Claims

1. A method for optimizing reactive power in submarine cables of an offshore wind power low-frequency AC transmission MG grid-connected system, characterized in that, The offshore wind power low-frequency AC transmission MG grid-connected system includes an offshore wind farm, a low-frequency submarine cable, an MG system, and an MG excitation system. The method includes: The reactive power optimization objective is established based on the low-frequency submarine cable operating current and active power loss, including: constructing a reactive power optimization model for the submarine cable and setting the reactive power optimization objective in the model; the reactive power optimization objective includes a submarine cable current sub-objective and a submarine cable active power loss sub-objective, and adjusting the weights of the submarine cable current sub-objective and the submarine cable active power loss sub-objective according to the active power output of the wind farm; the reactive power optimization model for the submarine cable is based on a unit-length T-type equivalent circuit, and according to actual engineering requirements, the transmitting submarine cable is divided into several segments, and the submarine cable operating current and reactive power are calculated segment by segment; the submarine cable operating current is used to determine the submarine cable current sub-objective and the submarine cable active power loss sub-objective; the objective function of the reactive power optimization objective satisfies: ； Where F is the objective function, minF means minimizing F is the optimization objective, F1 is the sub-objective of submarine cable current, F2 is the sub-objective of submarine cable active power loss, and λ is the weighting coefficient of submarine cable current. 1_max F 2_max These represent the maximum values ​​of optimization objectives F1 and F2, respectively. The submarine cable current sub-target is obtained by comparing the currents of each segment of the submarine cable to determine the maximum submarine cable current, and the optimization objective is to reduce the maximum submarine cable current; the submarine cable current sub-target satisfies: F1=max{i n The optimization objective is to minimize F1. ； ； Among them, i n Let P be the current at the end of the nth segment of the submarine cable. S Q S These represent the active power and reactive power at the head end of the submarine cable, respectively. n Let ΔP be the distance from the end of the nth segment of the submarine cable to the beginning of the cable. R ΔQ represents the active power loss per unit length of the submarine cable. L ΔQ represents the reactive power loss per unit length of the submarine cable. C U0 is the charging power per unit length of the submarine cable, and R is the nominal voltage of the output submarine cable. S L is the equivalent resistance per unit length of the submarine cable. S To transmit the inductance per unit length of the submarine cable, C S The capacitance per unit length of the transmitted submarine cable is given by f, where f is the operating frequency of the transmitted submarine cable. Under the constraints of reactive power output of wind farms, reactive power output of synchronous motors in MG systems, node power constraints, node voltage constraints, and submarine cable current carrying capacity constraints, reactive power output of wind farms and reactive power output of synchronous motors in MG systems are used as optimization variables to obtain reactive power configuration results corresponding to active power output of different wind farms. The reactive power configuration results include reactive power output values ​​of wind farms and reactive power output values ​​of synchronous motors in MG systems. The corresponding reactive power configuration result is determined based on the current active power output of the wind farm, and the reactive power regulation is performed on the wind farm side and the synchronous motor side of the MG system according to the reactive power configuration result.

2. The reactive power optimization method for submarine cables in offshore wind power low-frequency AC transmission MG grid-connected systems according to claim 1, characterized in that, The active power loss sub-objective of the submarine cable is obtained by summing the active power loss of each segment of the submarine cable, and the optimization object is to reduce the active power loss of the submarine cable.

3. The reactive power optimization method for submarine cables in offshore wind power low-frequency AC transmission MG grid-connected systems according to claim 2, characterized in that, The active power loss sub-target of the submarine cable satisfies: The optimization objective is to minimize F2. ； Among them, P loss For active power loss in submarine cables, i k-1 Let Δl be the current at the beginning of the k-th segment of the submarine cable. k Let R be the length of the k-th segment of the submarine cable. S This represents the equivalent resistance per unit length of the submarine cable.

4. The reactive power optimization method for submarine cables in offshore wind power low-frequency AC transmission MG grid-connected systems according to claim 1, characterized in that, The submarine cable current weighting coefficient satisfies: ； Among them, P WT P represents the per-unit value of the active power output of the wind farm. WTmax This is the per-unit value of the maximum active power output of the wind farm.

5. The reactive power optimization method for submarine cables in offshore wind power low-frequency AC transmission MG grid-connected systems according to claim 1, characterized in that, The node power constraint is used to determine the active and reactive power injected into each node based on the node voltage amplitude, node voltage phase angle difference, and node admittance, and to ensure that each node satisfies the power flow balance relationship. The reactive power output constraint of the wind farm is used to limit the reactive power output of the wind farm based on the current active power output and the current reactive power control capability limit of the wind farm. The reactive power constraint of the MG system synchronous motor is used to limit the reactive power of the MG system synchronous motor according to the upper and lower limits of the allowable reactive power of the MG system synchronous motor. The node voltage constraint is used to limit the node voltage at each node of the wind farm low-frequency power transmission system according to the preset node voltage limit. The submarine cable current carrying capacity constraint is used to limit the current of the entire submarine cable according to the current carrying capacity limit of the power transmission cable.

6. The reactive power optimization method for submarine cables in offshore wind power low-frequency AC transmission MG grid-connected systems according to claim 1 or 5, characterized in that, The process of obtaining the reactive power configuration results corresponding to the active power output of different wind farms includes: setting an interval step size ΔP, and dividing the active power output of the wind farm [0, P... WTmax [P] is divided into multiple intervals. i P i +ΔP], with P i +ΔP / 2 is used as the power output of the wind farm in the current interval. Reactive power optimization is performed on the low-frequency submarine cable to obtain the optimal reactive power value of the wind farm and the synchronous motor of the MG system in the current interval, and a lookup table of reactive power of the wind farm and the synchronous motor of the MG system under different active power outputs of the wind farm is generated.

7. The reactive power optimization method for submarine cables in offshore wind power low-frequency AC transmission MG grid-connected systems according to claim 6, characterized in that, Reactive power optimization for low-frequency submarine cables includes: solving the objective function using a particle swarm optimization algorithm, with the reactive power output of wind farms and the reactive power output of synchronous motors in the MG system as optimization variables x. i Particle position vector x i =[Q WT Q M Output the optimal reactive power optimization strategy x. best =[Q WT_best Q M_best ].

8. The reactive power optimization method for submarine cables in offshore wind power low-frequency AC transmission MG grid-connected systems according to claim 7, characterized in that, The control wind farm side and the MG system synchronous motor side perform reactive power regulation according to the reactive power configuration results, including: A coordinated control strategy for wind farm and MG system is adopted. The MG excitation system includes an M-side excitation device. The coordinated control strategy for wind farm and MG system includes reactive power control of wind turbine converter and excitation control of M-side excitation device based on terminal voltage command. The M-side excitation device adjusts the excitation current of the MG system synchronous motor according to the terminal voltage command value corresponding to the reactive power output value of the MG system synchronous motor, thereby adjusting the reactive power output of the MG system synchronous motor. The wind turbine converter changes the reactive power reference value according to the reactive power output value of the wind farm, and controls the reactive power output on the wind turbine side according to the reactive power reference value.

9. The reactive power optimization method for submarine cables in offshore wind power low-frequency AC transmission MG grid-connected systems according to claim 6, characterized in that, The method further includes hysteresis control, wherein the hysteresis control includes: Set the hysteresis width h, and set h to 10%ΔP; When the actual power output of the wind farm is in the range (P) i -h / 2,P i When +h / 2), the reactive power configuration result of the current interval remains unchanged; When the actual power output P of the wind farm WT ≥P i When +h / 2, update the reactive power configuration result to the reactive power configuration result of the (i+1)th interval; When the actual power output P of the wind farm WT ≤P i When -h / 2, the reactive power configuration result is updated to the reactive power configuration result of the (i-1)th interval.

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