Offshore wind farm reactive power optimization configuration method and device

Abstract

The application discloses a kind of offshore wind farm reactive power optimization configuration method and device, the method comprises: according to the preset equivalent calculation model, using wind farm reactive power shortage determines first reactive power shortage surface and second reactive power shortage surface, and obtains first capacitive reactive power compensation and first inductive reactive power compensation;The wind power generator set is calculated under the condition of no load and full load wind farm reactive power shortage, determine high-voltage shunt reactor capacity, with harmonic resonance as constraint condition, after grouping high-voltage shunt reactor capacity, combined with the objective function constructed by the sum of dynamic compensation device investment cost annual value and annual active network loss, and with static voltage stability margin, first capacitive reactive power compensation, first inductive reactive power compensation and voltage deviation as constraint condition, the compensation capacity of dynamic reactive power compensation device is optimized and calculated.Reduces comprehensive cost and improves the security of reactive power configuration.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of reactive power compensation technology of wind farms in power systems, and in particular to a method and device for optimizing reactive power configuration of offshore wind farms. BACKGROUND

[0002] At present, offshore wind farms mostly use high-voltage AC submarine cables to collect wind power and transmit the wind power to land control stations after being boosted. Since the capacitance of submarine cables is much higher than that of overhead lines, the capacitive effect of submarine cables is likely to cause power frequency overvoltage. At the same time, harmonic resonance problems caused by power electronic devices such as wind turbine converters and reactive power compensation devices and line capacitance will seriously affect the power quality of the power grid. Therefore, overvoltage and harmonic resonance problems should be fully considered in the process of configuring reactive power of offshore wind farms. The existing technology mainly focuses on static optimization algorithms and reactive power control strategies, determines the capacity of high-voltage shunt reactors by calculating the capacitive reactive power of submarine cables, compensates for the reactive power fluctuations caused by wind speed changes by using dynamic reactive power compensation devices, and optimizes the reactive power output of different target reactive power sources, but the configuration and optimization process of this method cannot guarantee the voltage stability of the wind farm. SUMMARY

[0003] The present application aims to provide a method and device for optimizing reactive power configuration of offshore wind farms to solve the problem that the voltage stability of wind farms cannot be guaranteed in the prior art, optimize the reactive power configuration of offshore wind farms with voltage stability and resonance as constraints, and improve the safety of reactive power configuration.

[0004] To achieve the above-mentioned purpose, the present application provides a method for optimizing reactive power configuration of offshore wind farms, comprising:

[0005] According to a predetermined equivalent calculation model, the reactive power deficiency of the wind farm is calculated;

[0006] According to the reactive power deficiency of the wind farm, a first reactive power deficiency surface and a second reactive power deficiency surface are determined. If the surface between the first reactive power deficiency surface and the second reactive power deficiency surface containing the surface where the reactive power deficiency is zero is taken as the reactive power deficiency zero surface, the first capacitive reactive power compensation amount and the first inductive reactive power compensation amount are obtained by performing reactive power compensation on the points on the boundary of the first reactive power deficiency surface and the second reactive power deficiency surface that do not contain the reactive power deficiency zero surface;

[0007] The reactive power deficiency of the wind farm under the conditions of no-load and full-load of wind turbine generators is calculated, the reactive power demand is determined, the capacity of high-voltage shunt reactors is determined according to the reactive power demand, and the capacity of high-voltage shunt reactors is grouped as a constraint condition of harmonic resonance;

[0008] After determining the high-voltage shunt reactor capacity and grouping, a target function is constructed in combination with the sum of the annual value of the investment cost of the dynamic compensation device and the annual active network loss, and the static voltage stability margin, the first capacitive reactive compensation amount, the first inductive reactive compensation amount, and the voltage deviation are used as constraint conditions to optimize and calculate the compensation capacity of the dynamic reactive compensation device.

[0009] Preferably, the reactive power deficiency of the wind farm is calculated, in particular:

[0010] The reactive power deficiency of the wind farm is calculated from no-load to full-load and from 0.97pu to 1.07pu of the grid-connected point voltage according to the preset equivalent calculation model by traversing various power generation output levels and operating points of the grid-connected point voltage.

[0011] Preferably, the first reactive power deficiency surface and the second reactive power deficiency surface are determined according to the reactive power deficiency of the wind farm, in particular:

[0012] The first reactive power deficiency surface is drawn by calculating the wind turbine power factor equal to the first threshold value in the wind farm reactive power deficiency, and the second reactive power deficiency surface is drawn by calculating the wind turbine power factor equal to the second threshold value in the wind farm reactive power deficiency.

[0013] Preferably, the reactive power deficiency of the wind farm under no-load and full-load conditions of the wind turbine is calculated, the reactive power demand is determined, and the high-voltage shunt reactor capacity is determined according to the reactive power demand, in particular:

[0014] The reactive power demand includes a first reactive power demand and a second reactive power demand, and the high-voltage shunt reactor capacity Q SR , for example:

[0015] max(0, Q min )≤Q SR ≤Q max ;

[0016] In the formula, Q min represents the first reactive power demand, and Q max represents the second reactive power demand.

[0017] Preferably, the target function is constructed in combination with the sum of the annual value of the investment cost of the dynamic compensation device and the annual active network loss, and the static voltage stability margin, the first capacitive reactive compensation amount, the first inductive reactive compensation amount, and the voltage deviation are used as constraint conditions, in particular:

[0018] The target function F is as follows:

[0019]

[0020] In the formula, C Q This represents the annual value of the investment cost for the dynamic compensation device. Indicates annual active network loss, Q STATCOM C1 represents the compensation capacity of the dynamic compensation device, r represents the unit investment cost, m represents the discount rate, C2 represents the equipment service life, and P represents the unit electricity price. loss The active power loss is t, where t is the loss time.

[0021] The constraints are based on the static voltage stability margin, the first capacitive reactive power compensation, the first inductive reactive power compensation, and the voltage deviation. These constraints include equality constraints and inequality constraints. The equality constraints are as follows:

[0022]

[0023]

[0024] In the formula, N represents the set of nodes in an offshore wind farm, and P i Q i Let G represent the active power and reactive power injected at node i, respectively. ij B ij U represents the admittance of the line between nodes i and j, respectively. i U j Let θ represent the voltages at nodes i and j, respectively. ij This represents the phase difference between nodes i and j;

[0025] The inequality constraints are:

[0026] λ≥λ min ;

[0027] U i.min ≤U i ≤U i.max ;

[0028] Q STATCOM +Q SR ≥Q c.min ;

[0029] Q STATCOM ≥Q l.min ;

[0030] Q wg.min ≤Q wg ≤Q wg.max ;

[0031] In the formula, λ represents the static voltage stability margin, and U i Q represents the voltage at node i. SRrepresents the compensation capacity of the high-voltage shunt reactor, Q STATCOM represents the compensation capacity of the dynamic reactive power compensation device, Q c.min and Q l.min respectively represent the first capacitive reactive power compensation amount and the first inductive reactive power compensation amount, Q wg represents the reactive power output of the wind turbine, Q wg.min and Q wg.max respectively represent the upper and lower limits of the reactive power output of the wind turbine.

[0032] The application also provides an offshore wind farm reactive power optimization configuration device, comprising:

[0033] a first calculation module configured to calculate a wind farm reactive power deficiency according to a preset equivalent calculation model;

[0034] a second calculation module configured to determine a first reactive power deficiency surface and a second reactive power deficiency surface according to the wind farm reactive power deficiency, and to obtain a first capacitive reactive power compensation amount and a first inductive reactive power compensation amount by performing reactive power compensation on points on the boundary of the first reactive power deficiency surface and the second reactive power deficiency surface that do not contain a point on a reactive power deficiency zero surface, if the reactive power deficiency zero surface is a surface between the first reactive power deficiency surface and the second reactive power deficiency surface and contains a zero value of the reactive power deficiency;

[0035] a third calculation module configured to calculate the wind farm reactive power deficiency under no-load and full-load conditions of the wind turbine, to determine a reactive power demand, to determine a high-voltage shunt reactor capacity according to the reactive power demand, and to group the high-voltage shunt reactor capacity according to a harmonic resonance constraint condition;

[0036] an optimization module configured to determine the high-voltage shunt reactor capacity and the groups, to construct a target function by combining an investment cost annual value of a dynamic compensation device and an annual active network loss, and to optimize and calculate a compensation capacity of the dynamic reactive power compensation device according to a static voltage stability margin, the first capacitive reactive power compensation amount, the first inductive reactive power compensation amount, and a voltage deviation as constraint conditions.

[0037] Preferably, the first calculation module is further configured to:

[0038] traverse various power generation output levels and grid point voltage working points, and calculate the wind farm reactive power deficiency when the wind turbine changes from no-load to full-load and the grid point voltage changes from 0.97pu to 1.07pu according to the preset equivalent calculation model.

[0039] Preferably, the second calculation module is further configured to:

[0040] The work point of the reactive power deficiency of the wind farm is traversed, and the wind turbine power factor in the reactive power deficiency of the wind farm is equal to a first threshold value, and the first reactive power deficiency surface is drawn; the wind turbine power factor in the reactive power deficiency of the wind farm is equal to a second threshold value, and the second reactive power deficiency surface is drawn.

[0041] Preferably, the third calculation module is further configured to:

[0042] The reactive power demand includes a first reactive power demand and a second reactive power demand, and the high-voltage shunt reactor capacity Q is determined according to the first reactive power demand and the second reactive power demand. SR As follows:

[0043] max (0, Q min ) ≤ Q sR ≤ Q max ;

[0044] In the formula, Q min represents the first reactive power demand, and Q max represents the second reactive power demand.

[0045] Preferably, the optimization module is further configured to:

[0046] The objective function F is as follows:

[0047]

[0048] In the formula, C Q represents the annual value of the investment cost of the dynamic compensation device, represents the annual active network loss, Q STATCOM is the compensation capacity of the dynamic compensation device, C1 is the unit investment cost, r is the discount rate, m is the equipment service life, C2 is the unit electricity price, P loss is the active network loss, and t is the loss time.

[0049] According to the static voltage stability margin, the first capacitive reactive power compensation amount, the first inductive reactive power compensation amount, and the voltage deviation as the constraint condition, wherein the constraint condition includes an equality constraint and an inequality constraint, and the equality constraint is as follows:

[0050]

[0051]

[0052] In the formula, N represents a set of offshore wind farm nodes, P i and Q i respectively represent the active power and the reactive power injected at the i-th node, G ij and B ijrespectively represent admittance of line between i, j nodes, U i , U j respectively represent voltage of i, j nodes, θ ij represent phase difference of i, j nodes;

[0053] Inequality constraints are:

[0054] λ≥λ min ;

[0055] U i.min ≤U i ≤U i.max ;

[0056] Q STATCOM +Q SR ≥Q c.min ;

[0057] Q STATCOM ≥Q l.min ;

[0058] Q wg.min ≤Q wg ≤Q wg.max ;

[0059] In the formula, λ represents static voltage stability margin, U i represents voltage of i node, Q SR represents compensation capacity of high-voltage shunt reactor, Q STATCOM represents compensation capacity of dynamic reactive power compensation device, Q c.min and Q l.min respectively represent first capacitive reactive power compensation and first inductive reactive power compensation, Q wg represents wind turbine reactive power output, Q wg.min and Q wg.max respectively represent upper and lower limits of wind turbine reactive power output.

[0060] Compared with the prior art, the present application has the beneficial effects that:

[0061] The present application analyzes the reactive power characteristics of offshore wind farms, and calculates the minimum compensation capacity after fully considering the reactive power regulation capacity of wind turbines under various power generation output levels and grid connection point voltages in the steady state, i.e. the first capacitive reactive power compensation and the first inductive reactive power compensation, so that the wind farm meets the basic requirements of technical specifications for reactive power configuration. The reactive power compensation capacity of high-voltage shunt reactors is maximally utilized, and harmonic resonance restraint is added when optimizing grouping, while considering the influence of compensation capacity and wind speed on resonance points, to avoid system resonance at characteristic harmonics and background harmonics. The compensation capacity of dynamic reactive power compensation devices is optimized with static voltage stability margin and minimum compensation as constraints, taking into account economy and safety. Attached Figure Description

[0062] To more clearly illustrate the technical solution of the present invention, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0063] Figure 1 This is a flowchart illustrating a method for optimizing reactive power allocation in offshore wind farms according to a certain embodiment of the present invention.

[0064] Figure 2 This is the structural design of an offshore wind farm;

[0065] Figure 3 It is the reactive power deficit surface of offshore wind farms;

[0066] Figure 4 This is a graph showing the relationship between the speed and output power of a doubly-fed asynchronous wind turbine.

[0067] Figure 5 This is the modal impedance diagram for a 35Mvar reactor;

[0068] Figure 6 This is a schematic diagram of the reactive power optimization configuration device for offshore wind farms provided in a certain embodiment of the present invention. Detailed Implementation

[0069] The technical solutions of the embodiments 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. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0070] It should be understood that the step numbers used in the text are for ease of description only and are not intended to limit the order in which the steps are performed.

[0071] It should be understood that the terminology used in this specification is for the purpose of describing particular embodiments only and is not intended to limit the invention. As used in this specification and the appended claims, the singular forms “a,” “an,” and “the” are intended to include the plural forms unless the context clearly indicates otherwise.

[0072] The terms "comprises", "comprising", "includes", "including" and "contains", "containing" mean "including but not limited to" when used in this specification, and for describing the presence of features, integers, steps, operations, elements, and / or components. The terms "consisting of" and "consisting essentially of" mean "including and limited to".

[0073] The term "and / or", "and / or" means any combination of one or more of the associated listed terms and all possible combinations, and includes these combinations.

[0074] Please refer to Figure 1 , the present application provides a kind of offshore wind farm reactive power optimization configuration method. As shown in Figure Figure 1 , the offshore wind farm reactive power optimization configuration method includes step S101 to step S104. Each step is as follows specifically:

[0075] Step S101: according to the preset equivalent calculation model, the wind farm reactive power shortage is calculated.

[0076] Establish the equivalent calculation model of offshore wind farm for reactive voltage calculation, including access public grid model, offshore booster station model, box transformer model, wind turbine model and collection line, submarine cable, overhead line model. Access public grid model is composed of equivalent impedance and infinite bus, and infinite bus is used as the balance node in offshore wind farm power flow calculation. The doubly-fed asynchronous wind turbine and permanent magnet direct-drive synchronous wind turbine commonly used in current offshore wind farm can realize active and reactive decoupling control, and the node power factor is set to be dynamically adjustable within the range of 0.95 ahead to 0.95 lag.

[0077] Traverse various power generation output levels and grid connection point voltage operating points, and calculate the wind farm reactive power shortage of wind turbine from no load to full load and grid connection point voltage from 0.97pu to 1.07pu according to the preset equivalent calculation model.

[0078] Step S102: determine the first reactive power shortage surface and the second reactive power shortage surface according to the wind farm reactive power shortage. If the surface containing zero reactive power shortage between the first reactive power shortage surface and the second reactive power shortage surface is taken as the reactive power shortage zero surface, the first capacitive reactive power compensation amount and the first inductive reactive power compensation amount are obtained by performing reactive power compensation on the points on the boundary of the first reactive power shortage surface and the second reactive power shortage surface without containing the reactive power shortage zero surface.

[0079] Analyze the influence of various factors on the reactive power of offshore wind farm, and calculate the wind farm reactive power shortage under various power generation output levels and grid connection point voltage. According to the technical requirements of reactive power configuration of offshore wind farm, the minimum capacitive reactive power compensation amount considering the reactive power regulation capacity of wind turbine is calculated as the first capacitive reactive power compensation amount and the inductive reactive power compensation amount is calculated as the first inductive reactive power compensation amount.

[0080] The working point of the reactive power shortage of the wind farm is traversed, the power factor of the wind turbine in the reactive power shortage of the wind farm is calculated to be equal to the first threshold value, and a first reactive power shortage surface is drawn, and the power factor of the wind turbine in the reactive power shortage of the wind farm is calculated to be equal to the second threshold value, and a second reactive power shortage surface is drawn. The first threshold value is leading 0.95, and the second threshold value is lagging 0.95.

[0081] Specifically, the reactive power characteristics of the offshore wind farm are analyzed, and various power generation output levels and grid connection point voltages are traversed. Based on the equivalent model established in step 101, the reactive power shortage of the wind farm is calculated when the wind turbine changes from no load to full load and the grid connection point voltage changes from 0.97pu to 1.07pu, and the reactive power shortage surface is drawn when the power factor of the wind turbine is leading 0.95 and lagging 0.95. The surface containing the zero reactive power shortage between the first reactive power shortage surface and the second reactive power shortage surface is taken as the zero reactive power shortage surface, and if the zero reactive power shortage surface is contained between the two surfaces, it means that the wind farm can realize zero reactive power exchange with the public grid through the reactive power regulation capability of the wind turbine itself. Due to the large charging power of the submarine cable, high-voltage shunt reactors and dynamic reactive power compensation devices STATCOM are often configured to compensate for reactive power in offshore wind farms. Through the points on the boundary of the two surfaces that do not contain the zero reactive power shortage, the first capacitive reactive power compensation amount Q c.min and the first inductive reactive power compensation amount Q l.min are obtained. The high-voltage shunt reactor can compensate for capacitive reactive power, and the STATCOM can compensate for both capacitive and inductive reactive power. Therefore, the minimum compensation capacity constraint considering the reactive power regulation capability of the wind turbine can be represented as follows:

[0082]

[0083] In the formula: Q SR is the compensation capacity of the high-voltage shunt reactor, and Q STATCOM is the compensation capacity of the STATCOM.

[0084] Step S103: Calculate the reactive power shortage of the wind farm under the conditions of no load and full load of the wind turbine, determine the reactive power demand, determine the capacity of the high-voltage shunt reactor according to the reactive power demand, and group the capacity of the high-voltage shunt reactor according to the harmonic resonance constraint condition.

[0085] The harmonic model of offshore wind farm is established, and the harmonic modal impedance of offshore wind farm under different operating conditions is scanned by modal analysis method to analyze the influence of reactive power configuration and operating conditions on resonance. The harmonic model of offshore wind farm includes wind turbine, transformer, cable, shunt reactor and power grid harmonic model. Specifically, doubly-fed induction generator and permanent magnet direct-driven synchronous generator are usually used in wind turbine, and the harmonic model of doubly-fed induction generator is composed of back-to-back converter and induction motor. The back-to-back converter can be decoupled into rotor-side converter RSC and grid-side converter GSC, and it is equivalent to two harmonic voltage sources. The equivalent harmonic source on the grid side injects harmonic into the wind farm through the filter inductance, and the equivalent harmonic source on the rotor side injects harmonic into the grid through the induction motor excitation system. The harmonic model of induction motor is affected by the slip, and its harmonic equivalent circuit can be expressed as:

[0086]

[0087] In the formula: h is the harmonic number; X' r is the rotor leakage resistance reduced to the stator side; R' r is the rotor resistance reduced to the stator side; R s is the stator resistance; X s is the stator leakage resistance; X m is the excitation reactance; s h is the harmonic slip.

[0088] According to the MPPT control of doubly-fed wind turbine, the relationship between wind speed and slip can be obtained, and the harmonic model of wind turbine considering wind speed change is derived.

[0089] Due to the existence of DC capacitor, the harmonic of permanent magnet direct-driven synchronous wind turbine is mainly generated by the grid-side inverter, which can be equivalent to a harmonic voltage source. At the same time, LCL type filter is installed at the outlet of the inverter of both kinds of wind turbines.

[0090] The transformer harmonic model adopts CIGRE model, and its expression is:

[0091]

[0092]

[0093]

[0094] In the formula: R s and R p are equivalent resistances, S is the rated capacity of transformer, X T is the leakage reactance of transformer.

[0095] The cable harmonic model, shunt reactor and public grid harmonic model all adopt common harmonic models. Since the STATCOM has no harmonic pollution, the resonance point can be analyzed without adding the model. The modal analysis method is used to scan the harmonic modal impedance of the offshore wind farm under different operating conditions, and the resonance point and harmonic modal impedance are calculated. The modal analysis method determines whether resonance occurs through the eigenvalue diagonal matrix elements of the system node admittance matrix. When the modal impedance is very large, a very small modal current injection will generate a very large modal voltage, so the resonance point can be determined by judging the size of the modal impedance, and the resonance position can be identified. Then, the influence of reactive power configuration capacity and operating conditions on resonance is analyzed.

[0096] Based on the harmonic model, the modal analysis method is used to scan the harmonic modal impedance of the offshore wind farm under different operating conditions, and the resonance point and harmonic modal impedance are calculated. The modal analysis method determines whether resonance occurs through the eigenvalue diagonal matrix elements of the system node admittance matrix. When the modal impedance is very large, a very small modal current injection will generate a very large modal voltage, so the resonance point can be determined by judging the size of the modal impedance, and the resonance position can be identified. Then, the influence of reactive power configuration capacity and operating conditions on resonance is analyzed.

[0097] According to the reactive power deficiency corresponding to different power generation output levels of the grid connection point at rated voltage, the total compensation capacity of the shunt high-voltage reactor is determined. The grouping capacity of the high-voltage shunt reactor is optimized and calculated with the resonance point avoiding characteristic harmonics and background harmonics as constraints, and the number of switching groups corresponding to different power generation output levels is determined.

[0098] The reactive power demand includes a first reactive power demand and a second reactive power demand, and the high-voltage shunt reactor capacity Q SR is determined according to the first reactive power demand and the second reactive power demand.

[0099] max(0,Q min )≤Q SR ≤Q max ;

[0100] In the formula, Q min represents the first reactive power demand, and Q max represents the second reactive power demand. That is, [max(0,Q min ),Q max ].

[0101] According to the reactive power deficiency corresponding to different generation output levels of the point of common coupling at the rated voltage, the total compensation capacity of the shunt high-voltage reactor is determined. Considering that the reactive power demand is different when the wind turbine generates different levels of output, according to the equivalent model of the offshore wind farm in step S101, the reactive power demand is taken as a control variable, the internal node voltage deviation is taken as a target, and the genetic algorithm is used to calculate the second reactive power demand Q max and the first reactive power demand Q min , and the objective function is as follows:

[0102]

[0103] In the formula, Q is the reactive power demand of the wind farm, U i is the voltage of node i, U i.ref is the reference voltage of the node, and N is the node set.

[0104] According to the reactive power loss calculation formula of the transmission line:

[0105]

[0106] In the formula, ΔQ is the reactive power loss of the line, P and Q are the transmitted active power and reactive power, U1 and U2 are the voltages at the beginning and end of the line, and X and B are the line reactance and susceptance.

[0107] The more active output P the wind turbine generates, the greater the reactive power loss during transmission. Therefore, when the wind turbine is at no load, the wind farm has capacitive reactive power demand due to the large cable susceptance and large charging power, and the demand is the largest. When the wind turbine is at full load, the capacitive reactive power demand is the smallest, and even inductive due to excessive reactive power loss of the line. The compensation capacity Q SR of the shunt high-voltage reactor should be in the range of [max(0, Q min ), Q max ]. Then the shunt high-voltage reactor is grouped in the compensation capacity range. If the capacity of the shunt high-voltage reactor is not properly selected, the reactor and the line capacitance will be in parallel resonance, and adjusting the compensation capacity can change the system resonance point. Therefore, when grouping the shunt high-voltage reactor, the harmonic resonance is selected as a constraint to avoid the resonance point coinciding with the characteristic harmonic or the background harmonic at the compensation capacity corresponding to a certain generation output level. The background harmonics of the power grid are mainly the 3rd, 5th, 7th, 11th and 13th harmonics, and the characteristic harmonics of the wind turbine are 6n±1. Therefore, the harmonic resonance constraint is:

[0108]

[0109] In the formula: f h is the resonance frequency, P out is the active output, and n is a positive integer.

[0110] Step S104: After determining the high-voltage shunt reactor capacity and grouping, the dynamic reactive power compensation device compensation capacity is calculated by optimizing the target function of the sum of the annual value of the dynamic compensation device investment cost and the annual active network loss, and the static voltage stability margin, the first capacitive reactive power compensation amount, the first inductive reactive power compensation amount, and the voltage deviation as the constraint conditions.

[0111] The dynamic compensation device compensation capacity is calculated by optimizing the target function of the sum of the annual value of the dynamic compensation device investment cost and the annual active network loss, and the static voltage stability margin, the voltage deviation, etc. as the constraint, to reduce the comprehensive operation cost of the system.

[0112] After determining the high-voltage shunt reactor capacity and grouping, the dynamic compensation device compensation capacity is calculated by optimizing the target function of the sum of the annual value of the dynamic compensation device investment cost and the annual active network loss, and the static voltage stability margin, the minimum compensation capacity, the voltage deviation, etc. as the constraint conditions, to reduce the comprehensive cost of the wind farm. The target function is:

[0113]

[0114] In the formula, Q STATCOM is the compensation capacity of the dynamic compensation device STATCOM, C1 is the unit investment cost, r is the discount rate, m is the equipment service life, C2 is the unit price, P loss is the active network loss, and t is the loss time.

[0115] The optimization is constrained by the static voltage stability margin, the minimum compensation capacity, the voltage deviation, etc. The equality constraint is the power balance constraint:

[0116]

[0117]

[0118] In the formula, N represents the set of offshore wind farm nodes, P i and Q i represent the active power and reactive power injected at the i-th node, respectively, G ij and B ij represent the admittance between i and j nodes, and θ ij represents the phase difference between i and j nodes.

[0119] The inequality constraint is:

[0120] λ≥λ min ;

[0121] U i.min ≤U i ≤U i.max ;

[0122] Q STATCOM +Q SR ≥Q c.min ;

[0123] Q STATCOM ≥Q l.min ;

[0124] Q wg.min ≤Q wg ≤Q wg.max ;

[0125] In the formula, λ represents static voltage stability margin, U i represents i-node voltage, Q SR represents high-voltage shunt reactor compensation capacity, Q STATCOM represents compensation capacity of dynamic reactive power compensation device STATCOM, Q c.min and Q l . min respectively represent first capacitive reactive power compensation and first inductive reactive power compensation, Q wg represents wind turbine reactive power output, Q wg.min and Q wg.max respectively represent wind turbine reactive power output upper and lower limits.

[0126] The application reduces wind turbine reactive power output and system reactive power flow through dynamic reactive power compensation device, thereby reducing network loss, but since high-voltage shunt reactor capacity is sufficient and grouping is not large, additional dynamic reactive power compensation device capacity is not needed to replace wind turbine reactive power output, and dynamic reactive power compensation device capacity is equal to the minimum value meeting compensation capacity constraint.

[0127] In an embodiment, an offshore wind farm with 25 single units of 3.6MW double-fed asynchronous wind turbines is used, and a structure diagram thereof is shown in Figure 2 , wherein the length of submarine cable is 20km, and other specific parameters are shown in Table 1 and Table 2.

[0128] Table 1 related parameter values

[0129] Parameter Value Wind farm rated power 90 MW Wind turbine rated voltage 0.69 kV Wind turbine rated power 3.6 MW Box transformer 4 MVA, Uk% = 7, Pk= 28.8 kw, I0% = 1, P0= 4.55 kw Main transformer 100 MVA, Uk% = 12, Pk= 290 kw, I0% = 0.56, P0= 92 kw

[0130] Table 2 related parameter values

[0131] Resistance Ω / km Inductance mH / km Capacitance μF / km 35 kV submarine cable 0.268 0.483 0.134 220 kV submarine cable 0.019 0.255 0.124

[0132] 1. An equivalent calculation model is established to calculate reactive power deficiency.

[0133] To analyze the reactive power characteristics of offshore wind farms, the operating points for various power generation levels and grid connection voltages were traversed. An equivalent model was established to calculate the reactive power deficit of the wind farm when the wind turbines operate from zero to full capacity and when the grid connection voltage changes from 0.97 pu to 1.07 pu. The results were plotted as follows: Figure 3 The power factor of the wind turbine shown represents the reactive power deficit surface when it leads by 0.95 and lags by 0.95. Due to the large charging power of the submarine cable, the minimum inductive reactive power compensation Q is... l.min =0. When the grid connection point voltage is 1.07 pu and the wind farm output power is 0, the capacitive reactive power demand is at its maximum, and the minimum capacitive reactive power compensation is Q. c.min = 45.95 Mvar. Therefore, the minimum compensation capacity constraint considering the reactive power regulation capability of the wind turbine can be expressed as:

[0134]

[0135] 2. Establish a harmonic model, analyze harmonic characteristics, and determine the capacity and grouping of high-voltage parallel reactors.

[0136] Please see Figure 4 , Figure 4 The relationship between wind turbine output power and rotational speed is shown, allowing for the calculation of wind turbine slip and the establishment of a doubly-fed induction generator (DFIG) harmonic model. Based on the analysis in step 3 above, a harmonic model of the wind farm in the embodiment is established, and modal analysis is used to scan the resonant points and harmonic mode impedances of the wind farm. Analysis shows that the resonant frequency increases with the increase of the high-voltage shunt reactor capacity and rotational speed.

[0137] Based on the reactive power deficit corresponding to different power generation levels at the grid connection point at rated voltage, the total compensation capacity of the parallel high-voltage reactors is determined. Considering the different reactive power demands when the wind turbines generate power at different levels, based on the equivalent model of the offshore wind farm in step 1, reactive power demand is used as a control variable. With the goal of minimizing the voltage deviation of internal nodes, a genetic algorithm is used to calculate the reactive power demand Q for both no-load and full-load conditions of the wind turbines. max = 40.0 Mvar, Q min = 22.6 Mvar. The total capacity of the high-voltage shunt reactors is selected to be 40 Mvar. Then, the high-voltage shunt reactors are grouped according to harmonic resonance constraints. The harmonic resonance constraint conditions are:

[0138]

[0139] Please see Figure 5 When 5 Mvar is selected as the unit capacity for grouping, and 7 groups of reactors are connected, i.e., 35 Mvar, the modal impedance diagram of the offshore wind farm is as follows. Figure 5As shown in the figure, three key modes with high modal impedances, close to background or characteristic harmonics, are illustrated. It can be seen that when the wind turbine output power decreases and the slip decreases, the resonant frequency of mode 1 coincides with the 7th harmonic of the grid background, causing severe harmonic resonance. The figure also shows the modal impedances when connecting 3 groups (24 Mvar), 4 groups (32 Mvar), and 5 groups (40 Mvar) with a unit group capacity of 8 Mvar. When a 24 Mvar reactor is connected, the corresponding wind turbine output power is relatively high. When the output power decreases, the resonant point shifts to the left, and modes 1 and 2 will move away from the background 3rd and 13th harmonics, respectively. The resonant frequency of mode 3 has low sensitivity to changes in reactor capacity and output power, basically does not shift, and will not coincide with the 43rd characteristic harmonic. When a 32 Mvar reactor is connected, because the resonant points of modes 1 and 2 are far from the adjacent background harmonics, changes in output power will not cause harmonic resonance. When a 40Mvar reactor is connected, the corresponding wind turbine output power is relatively small. As the output power increases, the resonant points of modes 1 and 2 shift to the right, but remain far from the 9th and 15th background harmonics, thus avoiding harmonic resonance. Using 8Mvar as the unit group capacity satisfies the harmonic resonance constraint, and its output power versus number of switching groups is shown in Table 3.

[0140] Table 3 Output Power - Number of Switching Groups

[0141] Output level Number of groups 0-46％ 5 47-80％ 4 81-100％ 3

[0142] 3. After determining the capacity and grouping of the high-voltage shunt reactors, the compensation capacity of the dynamic compensation device is optimized by using the sum of the annual investment cost of the dynamic compensation device and the annual active power loss as the objective. Table 4 shows the optimization process parameters and results, using an average annual wind power utilization of 2082 hours to calculate the annual active power loss. The dynamic reactive power compensation device can reduce the reactive power output of the wind turbine and reduce reactive power flow within the system, thereby reducing network losses. However, since the capacity of the high-voltage shunt reactors is sufficient and the grouping is not large, it can be seen from the optimization results that no additional dynamic reactive power compensation device capacity is needed to replace the reactive power output of the wind turbine. Therefore, the capacity of the dynamic reactive power compensation device STATCOM is equal to the minimum value of 5.95 Mvar that satisfies the compensation capacity constraint.

[0143] Table 4. Optimization process parameters and results

[0144]

[0145]

[0146] Please see Figure 6 Another embodiment of the present invention provides a reactive power optimization configuration device for offshore wind farms, comprising:

[0147] The first calculation module 11 is configured to calculate the reactive power deficiency of the wind farm according to a preset equivalent calculation model.

[0148] The second calculation module 12 is configured to determine a first reactive power deficiency surface and a second reactive power deficiency surface according to the reactive power deficiency of the wind farm, to obtain a first capacitive reactive power compensation amount and a first inductive reactive power compensation amount by performing reactive power compensation on points on the boundary of the first reactive power deficiency surface and the second reactive power deficiency surface that do not contain a zero-value surface of the reactive power deficiency, if the zero-value surface of the reactive power deficiency is taken as the zero-value surface.

[0149] The third calculation module 13 is configured to calculate the reactive power deficiency of the wind farm under the conditions of no-load and full-load of the wind turbine generator, to determine a reactive power demand amount, to determine a high-voltage shunt reactor capacity according to the reactive power demand amount, and to group the high-voltage shunt reactor capacity with a harmonic resonance as a constraint condition.

[0150] The optimization module 14 is configured to determine the high-voltage shunt reactor capacity and the grouping, to construct a target function by combining the sum of the annual value of the investment cost of the dynamic compensation device and the annual active network loss, and to optimize the compensation capacity of the dynamic reactive power compensation device with the static voltage stability margin, the first capacitive reactive power compensation amount, the first inductive reactive power compensation amount, and the voltage deviation as constraint conditions.

[0151] The specific limitations of the offshore wind farm reactive power optimization configuration device can refer to the limitations of the offshore wind farm reactive power optimization configuration method described above, and will not be repeated here. Each module in the offshore wind farm reactive power optimization configuration device described above can be realized by software, hardware, and combinations thereof, in whole or in part. The above-mentioned modules can be embedded in or independent of the processor in the computer device in hardware form, or can be stored in the memory in the computer device in software form, so as to be called and executed by the processor to perform the operations corresponding to the above-mentioned modules.

[0152] The above is the preferred embodiment of the present application. It should be noted that for ordinary skilled persons in the technical field, without departing from the principles of the present application, a number of improvements and refinements can be made, which are also considered within the scope of protection of the present application.

Claims

1. A method for optimizing reactive power allocation in offshore wind farms, characterized in that, include: By iterating through various power generation output levels and grid connection voltages, and based on a pre-set equivalent calculation model, the reactive power deficit of the wind farm is calculated when the wind turbine is operating from no load to full load and when the grid connection voltage changes from 0.97 pu to 1.07 pu. Based on the reactive power deficit of the wind farm, a first reactive power deficit surface and a second reactive power deficit surface are determined. If the surface containing the zero reactive power deficit between the first and second reactive power deficit surfaces is taken as the zero reactive power deficit surface, reactive power compensation is performed on the points on the boundary between the first and second reactive power deficit surfaces that do not contain the zero reactive power deficit surface to obtain a first capacitive reactive power compensation amount and a first inductive reactive power compensation amount. Specifically, determining the first and second reactive power deficit surfaces based on the reactive power deficit of the wind farm involves: traversing to obtain the operating points of the reactive power deficit of the wind farm, calculating that the power factor of the wind turbine generator in the reactive power deficit of the wind farm is equal to a first threshold and drawing the first reactive power deficit surface, and calculating that the power factor of the wind turbine generator in the reactive power deficit of the wind farm is equal to a second threshold and drawing the second reactive power deficit surface. The reactive power deficit of the wind farm is calculated under no-load and full-load conditions of the wind turbine generator. The reactive power demand is determined by a genetic algorithm. The capacity of the high-voltage parallel reactor is determined based on the reactive power demand. The capacity of the high-voltage parallel reactor is grouped with harmonic resonance as a constraint condition. After determining the capacity and grouping of the high-voltage parallel reactors, the objective function is constructed by combining the annual value of the investment cost of the dynamic compensation device with the sum of the annual active power network loss. The compensation capacity of the dynamic reactive power compensation device is optimized by using the static voltage stability margin, the first capacitive reactive power compensation amount, the first inductive reactive power compensation amount, and the voltage deviation as constraints.

2. The reactive power optimization configuration method for offshore wind farms according to claim 1, characterized in that, The calculation of the reactive power deficit of the wind farm under no-load and full-load conditions of the wind turbine generators, the determination of reactive power demand, and the determination of the capacity of the high-voltage shunt reactor based on the reactive power demand are as follows: The reactive power demand includes a first reactive power demand and a second reactive power demand. The capacity of the high-voltage shunt reactor is determined based on the first reactive power demand and the second reactive power demand. ,as follows: ; In the formula, This represents the first reactive power demand. The second reactive power demand is represented, wherein the first reactive power demand and the second reactive power demand are the reactive power demands of the wind turbine unit under no-load and full-load conditions, respectively.

3. The method for optimizing reactive power allocation in offshore wind farms according to claim 1, characterized in that, The objective function, constructed by combining the annual investment cost of the dynamic compensation device with the annual active power loss, and constrained by the static voltage stability margin, the first capacitive reactive power compensation amount, the first inductive reactive power compensation amount, and the voltage deviation, is as follows: The objective function F is as follows: ; In the formula, This represents the annual value of the investment cost for the dynamic compensation device. Indicates annual network loss. Q STATCOM The compensation capacity of the dynamic compensation device, C 1 represents the unit investment cost. r For the discount rate, m For the service life of the equipment, C 2 represents the unit electricity price. P loss For active network loss, t This is to save time; The constraints are based on the static voltage stability margin, the first capacitive reactive power compensation, the first inductive reactive power compensation, and the voltage deviation. These constraints include equality constraints and inequality constraints. The equality constraints are as follows: ; ; In the formula, N This represents the node set of an offshore wind farm. P i 、Q i They represent the first i Active power and reactive power injected into the nodes G ij 、 B ij They represent i, j Admittance of the line between nodes. U i 、U j They represent i, j Node voltage, express i, j Node phase difference; The inequality constraints are: ; ; ; ; ; In the formula, This indicates the static voltage stability margin. U i Indicates the voltage at node i. Q SR Indicates the compensation capacity of the high-voltage shunt reactor. Q STATCOM This indicates the compensation capacity of the dynamic reactive power compensation device. Q c.min and Q l.min These represent the first capacitive reactive power compensation amount and the first inductive reactive power compensation amount, respectively. Q wg This indicates the reactive power output of the wind turbine. Q wg.min and Q wg.max These represent the upper and lower limits of reactive power output of the wind turbine generator set, respectively.

4. A reactive power optimization configuration device for offshore wind farms, characterized in that, include: The first calculation module is used to traverse the operating points of various power generation output levels and grid connection point voltages. Based on the preset equivalent calculation model, it calculates the reactive power deficit of the wind farm when the wind turbine unit changes from no load to full load and when the grid connection point voltage changes from 0.97pu to 1.07pu. The second calculation module is used to determine the first reactive power deficit surface and the second reactive power deficit surface based on the reactive power deficit of the wind farm. If the surface between the first reactive power deficit surface and the second reactive power deficit surface containing the reactive power deficit of zero value is taken as the reactive power deficit zero surface, reactive power compensation is performed on the points on the boundary between the first reactive power deficit surface and the second reactive power deficit surface that do not contain the reactive power deficit zero surface to obtain the first capacitive reactive power compensation amount and the first inductive reactive power compensation amount. Specifically, determining the first reactive power deficit surface and the second reactive power deficit surface based on the reactive power deficit of the wind farm involves: traversing to obtain the operating points of the reactive power deficit of the wind farm, calculating that the power factor of the wind turbine in the reactive power deficit of the wind farm is equal to the first threshold and drawing the first reactive power deficit surface, and calculating that the power factor of the wind turbine in the reactive power deficit of the wind farm is equal to the second threshold and drawing the second reactive power deficit surface. The third calculation module is used to calculate the reactive power deficit of the wind farm under no-load and full-load conditions of the wind turbine generator, use a genetic algorithm to determine the reactive power demand, determine the capacity of the high-voltage parallel reactor based on the reactive power demand, and group the capacity of the high-voltage parallel reactor with harmonic resonance as a constraint condition. The optimization module is used to determine the capacity and grouping of the high-voltage parallel reactors, and then combine the objective function constructed by the sum of the annual investment cost of the dynamic compensation device and the annual active power network loss, and optimize the calculation of the compensation capacity of the dynamic reactive power compensation device with static voltage stability margin, the first capacitive reactive power compensation amount, the first inductive reactive power compensation amount and voltage deviation as constraints.

5. The reactive power optimization configuration device for offshore wind farms according to claim 4, characterized in that, The third calculation module is also used for: The reactive power demand includes a first reactive power demand and a second reactive power demand. The capacity of the high-voltage shunt reactor is determined based on the first reactive power demand and the second reactive power demand. ,as follows: ; In the formula, This represents the first reactive power demand. The second reactive power demand is represented, wherein the first reactive power demand and the second reactive power demand are the reactive power demands of the wind turbine unit under no-load and full-load conditions, respectively.

6. The reactive power optimization configuration device for offshore wind farms according to claim 4, characterized in that, The optimization module is also used for: The objective function F is as follows: ; In the formula, This represents the annual value of the investment cost for the dynamic compensation device. Indicates annual network loss. Q STATCOM The compensation capacity of the dynamic compensation device, C 1 represents the unit investment cost. r For the discount rate, m For the service life of the equipment, C 2 represents the unit electricity price. P loss For active network loss, t This is to save time; The constraints are based on the static voltage stability margin, the first capacitive reactive power compensation, the first inductive reactive power compensation, and the voltage deviation. These constraints include equality constraints and inequality constraints. The equality constraints are as follows: ; ; In the formula, N This represents the node set of an offshore wind farm. P i 、Q i They represent the first i Active power and reactive power injected into the nodes G ij 、 B ij They represent i, j Admittance of the line between nodes. U i 、U j They represent i, j Node voltage, express i, j Node phase difference; The inequality constraints are: ; ; ; ; ; In the formula, This indicates the static voltage stability margin. U i Indicates the voltage at node i. Q SR Indicates the compensation capacity of the high-voltage shunt reactor. Q STATCOM This indicates the compensation capacity of the dynamic reactive power compensation device. Q c.min and Q l.min These represent the first capacitive reactive power compensation amount and the first inductive reactive power compensation amount, respectively. Q wg This indicates the reactive power output of the wind turbine. Q wg.min and Q wg.max These represent the upper and lower limits of reactive power output of the wind turbine generator set, respectively.

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