CN112968470B - Topological optimization method for current collection system of offshore wind farm avoiding resonance area - Google Patents
Topological optimization method for current collection system of offshore wind farm avoiding resonance area Download PDFInfo
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J3/00—Circuit arrangements for ac mains or ac distribution networks
- H02J3/38—Arrangements for parallely feeding a single network by two or more generators, converters or transformers
- H02J3/381—Dispersed generators
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- H—ELECTRICITY
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- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J3/00—Circuit arrangements for ac mains or ac distribution networks
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
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- H02J2203/10—Power transmission or distribution systems management focussing at grid-level, e.g. load flow analysis, node profile computation, meshed network optimisation, active network management or spinning reserve management
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
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- H02J2300/00—Systems for supplying or distributing electric power characterised by decentralized, dispersed, or local generation
- H02J2300/20—The dispersed energy generation being of renewable origin
- H02J2300/28—The renewable source being wind energy
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Abstract
The invention relates to a method for analyzing the current collection topology of an offshore wind farm avoiding a resonance area, and discloses a method for evaluating the resonance risk of an all-offshore wind farm through an alternating current sea cable sending system (a current collection system, an offshore wind turbine generator set taking dynamic characteristics of a variable current control system into account, a power transformation system, a power transmission system and a grid connection system) and optimizing the topology structure of the current collection system. The method comprises the following steps: the method comprises the steps of deriving an impedance model of a current collecting system through an alternating current sea cable sending system aiming at an offshore wind farm, and establishing an admittance network model of a whole system sent out by the alternating current sea cable of the offshore wind farm by combining the derived impedance model of the offshore wind turbine, the impedance model of a power transformation system, a power transmission system and the impedance model of a grid-connected system; based on an admittance model of the whole system of the constructed offshore wind farm which is sent out of the system through an alternating current sea cable, performing singular value calculation and analysis, drawing a frequency-singular value curve, and judging a resonance frequency point in the system according to the amplitude-frequency characteristic of the singular value; the topology structure of the current collection system is changed, so that the frequency point corresponding to the singular value is changed or the singular value amplitude is reduced.
Description
Technical Field
The invention relates to topology structure optimization of an offshore wind farm alternating current sea cable sending system, in particular to topology structure optimization of a current collecting system of the offshore wind farm, and the topology structure of the current collecting system is optimized from the perspective of changing a system resonance area.
Technical Field
The offshore wind power has the advantages of abundant wind power resources, low noise and the like, is continuously developed and utilized, and the current collecting system of the offshore wind power plant is an important foundation for collecting electric energy and then carrying out centralized and external transmission, so that the development of the optimization evaluation of the topological structure of the current collecting system of the offshore wind power plant has important significance. However, the topology structure optimization of the current collection system of the offshore wind farm is mainly performed from the viewpoint of the manufacturing cost and reliability of equipment, but in recent years, the problem of overtemperature harmonic wave in the wind power transmission system caused by complex interaction among the offshore wind turbine generator, the current collection sea cable, the transmission sea cable and the grid-connected system exists, however, the topology structure optimization of the current collection system is developed from the viewpoint of full system resonance, and the current research focuses on reduction. Therefore, the topological structure optimization of the offshore wind power plant is developed from the perspective of system-level resonance, and has important theoretical significance and practical significance.
Disclosure of Invention
The invention aims to provide a topological structure optimization method of a current collecting system of an offshore wind farm, which calculates the influence of the topological structure of the current collecting system on resonance from the perspective of full-system resonance. The invention adopts the following technical scheme:
aiming at the topological structure of the alternating current sea cable sending system of the offshore wind farm, on the basis of constructing and obtaining an impedance model of each element in the alternating current sea cable sending system of the offshore wind farm, constructing and obtaining an impedance network circuit of the whole system of the alternating current sea cable sending system of the offshore wind farm according to the topological structure information of the alternating current sea cable sending system of the offshore wind farm; according to a circuit of an impedance network topological structure of the system sent out by the alternating current submarine cable of the offshore wind farm, constructing and obtaining an admittance network matrix of the system sent out by the alternating current submarine cable of the offshore wind farm by adopting a node admittance method; calculating singular values of an admittance network based on the admittance network matrix obtained by construction, judging the resonance risk of the system according to a singular value curve, drawing a frequency-singular value curve, and focusing on the condition that the singular value amplitude at a certain frequency is too high; and further, the singular value amplitude of the system is reduced or the frequency point corresponding to the singular value with high amplitude is far away from the frequency point corresponding to the harmonic source in the system by optimizing the sea cable topological structure of the current collecting system, so that the resonance risk in the system is reduced by optimizing the topological structure of the current collecting system.
Further, an impedance model of each element of the offshore wind farm through an alternating current sea cable sending system is built, the impedance model comprises a current collecting system, a wind turbine generator, a box transformer, a main transformer, an outgoing sea cable and a grid-connected system, wherein the impedance model of the wind turbine generator accounts for dynamic characteristics of a variable current control system closely related to harmonic waves, the dynamic characteristics comprise phase-locked loop dynamic characteristics, current inner loop dynamic characteristics and dynamic characteristics of a power outer loop, and on the basis, each impedance element in the system is connected according to a topological structure of the offshore wind farm through the alternating current sea cable sending system, so that an impedance circuit of the whole system is built.
Further, based on the impedance circuit of the alternating current sea cable sending system of the offshore wind farm, a full-system admittance network model of the alternating current sea cable sending system of the offshore wind farm is formed according to a node admittance method, the admittance network model comprises dynamic characteristics of single elements in the alternating current sea cable sending system of the offshore wind farm and topological structure information of the whole offshore wind farm and the alternating current sea cable sending system, and optimization of a current collecting topological structure can be carried out from the angle of reducing the full-system resonance risk of the alternating current sea cable sending system of the offshore wind farm based on the admittance network model.
Further, in the frequency-singular value characteristic curve, for a point with a larger singular value amplitude at a certain frequency point, the frequency corresponding to the point needs to be focused.
Further, the topological network structure of the current collecting system is adjusted, the frequency-singular value curve is recalculated, the resonant frequency point is shifted to be far away from the frequency of a harmonic source in the alternating current sea cable grid-connected system of the offshore wind power plant, or the singular value amplitude is reduced, namely the offshore wind power is changed to be sent out of a resonant area of the system through the alternating current sea cable.
According to the technical scheme, the invention has the following benefits: the topological structure evaluation and optimization method for the wind power plant collecting system at a system level is provided, and the collecting system is evaluated and optimized from the angle of full system resonance of the offshore wind power plant sending-out system through an alternating current sea cable. The derived model takes dynamic characteristics of a variable current control system of the offshore wind turbine into account, takes influences of various equipment elements of the box transformer, the current collection sea cable, the boost transformer, the outgoing sea cable and the grid-connected alternating current system into account, and simultaneously contains topological structure information of the offshore wind power plant outgoing system through the alternating current sea cable, so that topological structure optimization of the current collection system can be carried out from the angle of reducing resonance risk of the offshore wind power outgoing system through the alternating current sea cable.
Drawings
FIG. 1 is a topology of a full system of offshore wind power delivered via an alternating current sea cable.
Fig. 2 is an equivalent circuit of a 35kV combiner sea cable.
Fig. 3 is a topological structure block diagram of a grid-side converter of a wind turbine.
Fig. 4 is an equivalent circuit of a 220kV outgoing submarine cable.
Fig. 5 is a diagram of the impedance circuit of the offshore wind farm via an ac submarine cable delivery system.
Fig. 6 shows spectral characteristics of a singular value at different outgoing cable lengths.
Fig. 7 shows spectral characteristics of a certain singular value for different current collecting sea cable lengths.
Detailed Description
The invention provides a topology structure optimization method of a current collection system of an offshore wind farm based on full-system resonance characteristic evaluation, which is implemented according to the following five steps:
1) On the basis of deducing impedance models of all elements of the offshore wind farm through the alternating current sea cable sending system, connecting the impedance models of all the elements according to topological structure information of the offshore wind farm through the alternating current sea cable sending system so as to construct an impedance circuit; 2) Based on the constructed impedance circuit, constructing an admittance network model of the offshore wind farm through an alternating current submarine cable sending system by adopting a node admittance method; 3) Calculating to obtain singular values of an admittance network by adopting a singular value calculation method based on the obtained admittance network model of the offshore wind farm through an alternating current submarine cable sending system; 4) Drawing a frequency-singular value curve, and focusing on points with larger singular values and corresponding frequencies thereof; 5) And adjusting the topological structure of the current collection system, and carrying out frequency point-singular value amplitude calculation again to enable the resonance frequency point of the system to be far away from the frequency point of a harmonic source in the system or directly reduce the harmonic amplitude in the system, so as to achieve the aim of reducing the resonance risk in the system.
1. Topology overview of offshore wind power delivery system via AC submarine cable
The current collecting topological structure of the offshore wind power transmission system through the alternating current submarine cable is shown in figure 1. As can be seen from the figure, after the wind turbine generators are boosted by the box transformer, the wind turbine generators are connected in series on a series of submarine cables, and 5-7 wind turbine generators are usually arranged on each submarine cable. Each string of submarine cables is collected at a bus, is concentrated and sent out through 220kV alternating current submarine cables after being boosted, and is finally connected with an alternating current system.
2. Topology structure optimization model derivation of offshore wind power transmission system through alternating current submarine cable
2.1 line impedance model
The wind power transformer groups are connected through 35kV converging sea cables, the lengths of the converging sea cables of each section are generally short, the converging sea cables can be directly replaced by a section of PI type equivalent circuit, and the topological structure of the PI type equivalent circuit is shown in figure 2:
for line impedance, its dynamic model is:
Z line1 ＝R l1 +L l1 s (1)
wherein R is l1 L and L l1 The line resistance and inductance, respectively.
For the impedance to ground, its dynamic model is:
Z c1 ＝1/(C 1 s) (2)
wherein C is 1 Is the capacitance to ground.
2.2 wind power impedance model
The offshore wind turbine mainly adopts a full-power grid-connected wind turbine, and the characteristics of the harmonic wave are closely related to a grid-side converter control system mainly due to the existence of a direct-current capacitor link, so that the influence of the grid-side converter is mainly considered when an impedance model of the wind turbine is deduced. The topological structure block diagram of the wind turbine generator grid-side converter is shown in fig. 3, and then the transfer function of the bus voltage of the offshore wind turbine generator with respect to the bus current, namely the impedance model of the offshore wind turbine generator, is deduced according to the propagation rule of harmonic signals in the converter control system:
Z W ＝ΔU(s)/ΔI(s) (3)
in the above formula, deltaU(s) is the bus voltage disturbance quantity of the offshore wind turbine, and DeltaI(s) is the bus current disturbance quantity of the offshore wind turbine. Impedance model Z of the fan W The dynamic response characteristics of a phase-locked loop, a current loop and a power outer loop of the grid-connected converter of the offshore wind turbine generator are considered, and the response characteristics of the converter control system to harmonic waves can be reflected. And a linear analytical model derivation process for each link of the wind turbine generator is as follows.
2.2.1 phase locked Loop model
For the voltage and current at the grid connection position of the fan, the positive sequence component disturbance quantity is considered, and the expression of the a-phase voltage and current is as follows:
wherein v is a (t)、i a (t) bus voltage and current, respectively; v (V) 1 、ω 1 The amplitude and the angular frequency of the bus voltage fundamental frequency component are respectively; deltaV s 、ω s Phi (phi) s The amplitude, the angular frequency and the initial phase of the bus voltage disturbance quantity are respectively; i 1 、ω 1 Phi (phi) 1 The amplitude, the angular frequency and the initial phase of the bus current are respectively; deltaI s 、ω s Phi (phi) is The amplitude, the angular frequency and the initial phase of the bus current disturbance quantity are respectively obtained.
The above formula is sorted to give the expression form of its vector:
the transfer function expression of the pll can be known from the pll transfer function block diagram in fig. 3:
wherein T is pll (θ) is Park equation, v c a,b,c Respectively three-phase components of bus voltage, v c d,q D/q axis voltages, respectively; h pll (s) is a phase-locked loop transfer function; θ pll (s) is the phase angle of the phase locked loop output.
Considering injection of voltage disturbance quantity, considering disturbance quantity of phase angle of phase-locked loop, and calculating to obtain an expression of bus voltage d/q axis component disturbance quantity:
it is calculated by sorting its vector form:
according to the equation, an expression of the phase-locked loop output phase angle disturbance Guan Muxian voltage disturbance is calculated and is arranged into a vector form:
according to the calculation expression of the phase-locked loop, calculating the expression of the d/q axis current component, and arranging the expression into a vector form:
2.2.2 DC capacitor link transfer function expression
The transfer function expression of the dc capacitance link is as follows:
wherein P is in For the power fed on the machine side, P out For the power output by the network side, the disturbance quantity of the DC capacitance link is mainly related to the disturbance quantity of the network side due to the action of the network side converter, and the network side outputs power P according to the reference directions of voltage and current in FIG. 3 out The calculated expression of (2) is as follows:
neglecting the high-order disturbance quantity, and sorting the vector form of the disturbance quantity expression of the obtained power as follows:
upper middle and upper mark * Represents conjugation;
vector of steady-state components of bus voltage; />
The bus voltage disturbance vector is;
consider the expression of bus voltage and converter outlet voltage:
u is set to C1 Substituting the expression into the power disturbance quantity, and calculating to obtain the expression of the power disturbance quantity of the converter outlet relative to the bus voltage and the bus current:
substituting the power disturbance quantity expression into the direct-current voltage disturbance quantity expression, and calculating to obtain the direct-current voltage disturbance quantity expression:
2.2.3 d/q-axis control System transfer function expression
The computational expression between the d/q axis voltage reference value and the three-phase voltage is as follows:
the expression of the bus voltage and the converter outlet voltage is as follows:
the impedance expressions of the combined wind turbine generator are calculated as follows:
2.3 boost variable impedance model derivation for Convergence station
Similar to the box transformer of a wind turbine generator, for the step-up transformer of the collection station, the inductance characteristics of the step-up transformer are mainly considered, and the impedance model is as follows:
Z T ＝L T s (22)
wherein L is T Is the equivalent inductance of the transformer.
Impedance model derivation of 2.4220kV outgoing submarine cable line
Fig. 4 is an equivalent circuit of a 220kV outgoing submarine cable, which may be represented by a PI-type equivalent circuit when the length is short, and may be represented by a multi-PI-type equivalent circuit when the length of the line is long.
For line impedance, its impedance model is as follows:
Z line30 ＝R l30 +L l30 s (23)
wherein R is l30 L and L l30 The line resistance and inductance, respectively.
For line-to-ground capacitance, its impedance model is as follows:
Z c30 ＝1/(C 30 s) (24)
wherein C is 30 Is the line-to-ground capacitance.
2.5 equivalent impedance of grid-connected System
For a grid-connected alternating current system, the equivalent inductance is used for representing the dynamic model, and the dynamic model is as follows:
Z s ＝L s s (25)
wherein L is s The equivalent inductance is the equivalent inductance of the grid-connected system.
3. Construction of full-system network admittance model sent out by offshore wind farm through alternating current submarine cable
And connecting all elements in the offshore wind farm through an alternating current submarine cable sending system according to a network topology structure, and constructing an impedance circuit shown in figure 5.
The names of the variables in the figure are described by taking node 1 in the figure as an example. Wherein Z is w1 A sequence impedance model of the fan at the node 1; z is Z Lt1 Equivalent impedance for the fan box transformer; g c1-1 G (G) c1-2 Admittance corresponding to the capacitance to ground of the 35kV converging submarine cable; z is Z H1 The impedance of the bus cable is 35 kV; z is Z T Is the transformer impedance; z is Z 30 The impedance of the outgoing submarine cable is 220 kV; g c30-1 G (G) c30-2 Is the earth admittance of the outgoing submarine cable; z is Z s Is the equivalent impedance of the grid-connected system.
Numbering the system nodes, and constructing and obtaining a system admittance network matrix:
in the above matrix, the solving process of each variable is as follows:
(1) Each chain end point
For nodes at one end of each chain, 1, 8, 15 and 22 have their self admittances G i,i Transadmittance G i,i+1 The following are respectively shown:
wherein i is the number of each node, G WTi For fan admittance at access node i, G ci,1 To ground for node iGuiding; g linei Line admittance between nodes i and i+1; z is Z Wi The equivalent impedance of the offshore wind turbine generator is the equivalent impedance of the offshore wind turbine generator connected with the node i; z is Z WTi Equivalent impedance for access node i box-section;
(2) Points within each chain
For nodes in the middle of each chain, 2-6,9-13, 16-21, 23-27, its self-admittance G i,i Transadmittance G i,i+1 The following are respectively shown:
wherein G is WTi G is admittance of fan at access node i linei-1 Line admittance between node i-1 and node i; g linei Is the admittance of the line between node i and node i + 1.
(3) Grid connection point of each chain
For the nodes connected to the bus bars in each chain, 7, 14, 21 and 28, the calculation formula giving the node admittance matrix thereof is as follows:
in the above, G WTi Fan admittance at the access node i; g linei-1 Line admittance between nodes i and i-1; g linei Is the line between node i and node 29; g ci-1,2 Admittance to ground for line i-1; g ci,1 For line i admittance to ground.
(4) Bus bar point
For bus bar node 29, its self and transadmittance is as follows:
G 29,29 ＝G line7 +G line14 +G line21 +G line28 +G c7-2 +G c14-2 +G c21-2 +G c28-2 +G T (30)
G 29,7 ＝-G line7 (31)
G 29,14 ＝-G line14 (32)
G 29,21 ＝-G line21 (33)
G 29,28 ＝-G line28 (34)
G 29,30 ＝-G T (35)
in the above formulae, G line7 、G line14 、G line21 G (G) line28 Line admittances of lines 7, 14, 21 and 28, respectively; g c7-2 、G c14-2 、G c21-2 G (G) c28-2 Line-to-ground admittances respectively; g T Is the equivalent admittance of the transformer.
(5) Boost-to-high voltage side node
For the boost-to-high side node 30, the expressions of self-admittance and transadmittance are as follows:
G 30,30 ＝G T +G c30 +G line30 (36)
G 30,31 ＝-G line30 (37)
G 30,29 ＝-G T (38)
wherein G is T Is the admittance of the transformer; g c30 The ground capacitor is used for sending the submarine cable; g line30 Is the admittance of the line 30.
(6) Grid-connected point
For the system point of fusion 31 of the outgoing submarine cable, the expressions of self-admittance and transadmittance are as follows:
G 31,31 ＝G line30 +G c30 +G s (39)
G 31,30 ＝-G line30 (40)
4. primary evaluation and optimization of resonance risk of offshore wind power transmission system through alternating current submarine cable
Based on the derived network admittance model, singular value analysis is carried out on the network admittance model, a curve of singular value amplitude frequency with respect to frequency is drawn, the amplitude of the corresponding singular value at a certain frequency point in the system is larger, the harmonic amplitude of the frequency point needs to be focused, and the risk of resonance exists at the point in the system.
And analyzing the amplitude characteristic of the harmonic wave based on the singular value of the network impedance, and further analyzing the influence of the change of the network topology structure on the amplitude-frequency characteristic of the harmonic wave. For a group of situations in which the harmonic risk is high, important attention is required, and the system structure should be adjusted to change the resonance frequency point of the system, so as to reduce the high-amplitude harmonic risk in the system.
Under the condition that the length of the outgoing submarine cable is changed, the calculated singular value 1 is shown in fig. 6, and as can be seen from the graph, the lengths of the outgoing submarine cables are different, the system resonance frequency points are 867.7Hz, 705.3Hz and 606.6Hz respectively, namely, the lengths of the outgoing submarine cables are changed, and the frequency point is obviously affected. Therefore, the frequency characteristic of the frequency point can be changed by selecting the submarine cable model, and further, a harmonic source with high risk is avoided.
Under the condition that the length of the sea cable of the current collecting system is changed, the calculated singular value 2 is shown in fig. 7, and as can be seen from the graph, the length of the sea cable of the current collecting system is increased, the system resonance frequency points are 2114Hz, 1855Hz and 1730Hz respectively, and the topology change of the current collecting system affects a high-frequency resonance point. Therefore, the frequency characteristic of the frequency point can be changed by selecting the submarine cable model, and further, a harmonic source with high risk is avoided.
According to the given strength of the system, the length of the sent submarine cable is different, the resonance frequency point of the system is changed, and further, the proper submarine cable model is selected, parameters are different, and the resonance frequency is changed to a certain extent.
Similarly, for the sea cable of the current collecting system, the length of the sea cable of the current collecting system is changed, and the resonance frequency point of the system is also obviously changed, so that the resonance frequency of the system can be changed and specific harmonic source points can be avoided by selecting different sea cable types of the current collecting system.
The foregoing description of the embodiments is provided to facilitate the understanding and application of the invention to those skilled in the art. It will be apparent to those having ordinary skill in the art that various modifications to the above-described embodiments may be readily made and the generic principles described herein may be applied to other embodiments without the use of inventive faculty. Therefore, the present invention is not limited to the above-described embodiments, and those skilled in the art, based on the present disclosure, should make improvements and modifications within the scope of the present invention.
Claims (5)
1. The topological analysis method for the current collection of the offshore wind farm avoiding the resonance area is characterized in that the topological structure optimization of a wind farm current collection system is carried out from the perspective of changing the resonance characteristic of the system sent out by an alternating current sea cable of the offshore wind farm, and the influence of the dynamic characteristic, a power transformation system, a power transmission system and a grid-connected system of the current collection system of the offshore wind turbine is considered when the resonance risk assessment of the current collection system is carried out; on the basis of establishing impedance models of all elements of a current collecting system, a wind turbine, a box transformer, a main transformer, an outgoing sea cable and a grid-connected system, according to the topological structure characteristics of the alternating current sea cable outgoing system of the offshore wind farm, connecting all the elements according to the topological structure of the alternating current sea cable outgoing system of the offshore wind farm, and further constructing an impedance circuit of the alternating current sea cable outgoing system of the offshore wind farm; based on an impedance circuit of the whole system of the alternating current sea cable sending system of the offshore wind farm, a node admittance method is adopted to construct and obtain a whole system admittance network model of the alternating current sea cable sending system of the offshore wind farm; based on the obtained admittance network model of the offshore wind farm sent out of the system through the alternating current sea cable, singular value calculation is carried out, a frequency-singular value amplitude curve is drawn, and points with larger singular value amplitude are selected; the topological structure of the current collecting system is adjusted, the singular value amplitude is reduced or the frequency point is inconsistent with the harmonic source frequency point, and the purpose of reducing the harmonic amplitude in the system sent out by the offshore wind farm through the alternating current sea cable is achieved; therefore, the resonance area of the system is avoided by optimizing the network topology structure of the current collecting system;
the network admittance model of the whole system considering the network topology structure of the current collecting system and the dynamic characteristics of the wind turbine generator is shown as follows:
in the above matrix, the solving process of each variable is as follows:
for nodes at one end of each chain, 1, 8, 15 and 22 have their self admittances G i,i Transadmittance G i,i+1 The following are respectively shown:
wherein i is the number of each node, G WTi For fan admittance at access node i, G ci,1 Ground conductance for node i; g linei Line admittance between nodes i and i+1; z is Z Wi The equivalent impedance of the offshore wind turbine generator is the equivalent impedance of the offshore wind turbine generator connected with the node i; z is Z WTi Equivalent impedance for access node i box-section;
for nodes in the middle of each chain, 2-6,9-13, 16-21, 23-27, its self-admittance G i,i Transadmittance G i,i+1 The following are respectively shown:
G ii ＝G WTi +G linei-1 +G linei +G ci-1,2 +G ci,1
G i,i-1 ＝-G linei-1
G i,i+1 ＝-G linei
i＝2,3,4,5,6,9，10,11,12,13,16，17,18,19,20,21,23,24,25,26,27
wherein G is WTi G is admittance of fan at access node i linei-1 Line admittance between node i-1 and node i; g linei Admittance of the line between node i and node i+1;
for the nodes connected to the bus bars in each chain, 7, 14, 21 and 28, the calculation formula giving the node admittance matrix thereof is as follows:
G i,i ＝G WTi +G linei-1 +G linei +G ci-1,2 +G ci,1
G i,i-1 ＝-G linei-1
G i,29 ＝-G linei
i＝7,14,21,28
in the above, G WTi Fan admittance at the access node i; g linei-1 Line admittance between nodes i and i-1; g linei Is the line between node i and node 29; g ci-1,2 Admittance to ground for line i-1; g ci,1 Admittance to ground for line i;
for bus bar node 29, its self and transadmittance is as follows:
G 29,29 ＝G line7 +G line14 +G line21 +G line28 +G c7-2 +G c14-2 +G c21-2 +G c28-2 +G T
G 29,7 ＝-G line7
G 29,14 ＝-G line14
G 29,21 ＝-G line21
G 29,28 ＝-G line28
G 29,30 ＝-G T
in the above formulae, G line7 、G line14 、G line21 G (G) line28 Line admittances of lines 7, 14, 21 and 28, respectively; g c7-2 、G c14-2 、G c21-2 G (G) c28-2 Line-to-ground admittances respectively; g T Is the equivalent admittance of the transformer;
for the boost-to-high side node 30, the expressions of self-admittance and transadmittance are as follows:
G 30,30 ＝G T +G c30 +G line30
G 30,31 ＝-G line30
G 30,29 ＝-G T
wherein G is T Is the admittance of the transformer; g c30 The ground capacitor is used for sending the submarine cable; g line30 Is the admittance of the line 30;
for the system point of fusion 31 of the outgoing submarine cable, the expressions of self-admittance and transadmittance are as follows:
G 31,31 ＝G line30 +G c30 +G s
G 31,30 ＝-G line30
the impedance expression of the wind turbine generator is as follows:
wherein, fan impedance model Z W ；V 1 The bus voltage fundamental frequency component amplitude is given; i 1 Is the magnitude of the bus current; superscript * Representing conjugation.
2. The method for analyzing the current collection topology of the offshore wind farm avoiding the resonance area according to claim 1 is characterized by constructing an impedance model of each element of the offshore wind farm through an alternating current sea cable transmission system, comprising each element of the current collection system, a wind turbine, a box transformer, a main transformer, an outgoing sea cable and a grid-connected system, wherein the impedance model of the wind turbine takes dynamic characteristics of a current transformation control system closely related to harmonic waves into account, the dynamic characteristics comprise phase-locked loop dynamic characteristics, current inner loop dynamic characteristics and dynamic characteristics of a power outer loop, and on the basis, the impedance model is connected with each impedance element of the offshore wind farm through the alternating current sea cable transmission system according to the topology structure of the entire system of the offshore wind farm through the alternating current sea cable transmission system, so that an impedance circuit of the entire system is constructed.
3. The method for analyzing the current collecting topology of the offshore wind farm avoiding the resonance area according to claim 1, wherein a full-system admittance network model of the offshore wind farm via the alternating current submarine cable delivery system is formed according to a node admittance method based on the constructed impedance circuit of the offshore wind farm via the alternating current submarine cable delivery system, the admittance network model comprises dynamic characteristics of single elements in the offshore wind farm via the alternating current submarine cable delivery system and topology structure information of the whole offshore wind farm via the alternating current submarine cable delivery system, and the current collecting topology structure optimization can be performed from the angle of reducing the full-system resonance risk of the offshore wind farm via the alternating current submarine cable delivery system based on the admittance network model.
4. The method for analyzing the current collecting topology of the offshore wind farm avoiding the resonance area according to claim 1, wherein in the frequency-singular value characteristic curve, for a point with a larger singular value amplitude at a certain frequency point, the frequency corresponding to the point needs to be focused.
5. The method for analyzing the current collection topology of the offshore wind farm avoiding the resonance area according to claim 1, wherein the topology network structure of the current collection system is adjusted, the frequency-singular value curve is recalculated, the frequency point of resonance is shifted to be far away from the frequency of the harmonic source in the offshore wind farm through the alternating current sea cable grid-connected system or the singular value amplitude is reduced, namely the resonance area of the offshore wind power sent out of the system through the alternating current sea cable is changed.
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