Offshore wind power low-frequency networking power transmission system and control method thereof

Abstract

The invention discloses an offshore wind power low-frequency networking power transmission system and a control method thereof. The power transmission system comprises an onshore frequency conversion station, an onshore converter station, an offshore converter station, an island frequency conversion station, a centralized controller and n wind power clusters, each wind power cluster comprises h wind turbine generators, each wind power cluster outputs low-frequency alternating current, and the n wind power clusters are interconnected to form a low-frequency network; an onshore frequency conversion station and an offshore converter station which are connected with an onshore power frequency power grid are used as power supply nodes, an island frequency conversion station and n wind power clusters which are connected with an island power frequency load are used as load nodes, and measurement information and control instructions are transmitted between the integrated controller and all the power supply nodes through a communication network. According to the low-frequency network, low-frequency submarine cable sending out or direct-current submarine cable sending out can be flexibly adopted, the grid connection difficulty and the construction cost of the offshore wind power transmission system are reduced through a better sending out scheme, the voltage of the power source nodes is controlled through the integrated controller, and power steady-state control over all the nodes is achieved.

Description

Technical Field

[0001] The present invention belongs to the technical field of offshore wind power transmission, and particularly relates to an offshore wind power low-frequency networking transmission system and a control method thereof. Background Art

[0002] At present, the planning of offshore wind power projects is gradually advancing towards the deep sea, and the scale of the wind farms is increasing day by day. With the increase of the offshore distance, the influence of the submarine cable's capacitance to the ground becomes more severe, the charging reactive power is large, and the active power transmission capacity is limited. This problem has contributed to the birth of the offshore wind power low-frequency transmission system.

[0003] Due to the decrease in frequency, the cable charging power of the low-frequency transmission system decreases, the active power transmission capacity increases, and the transmission distance is significantly increased compared with the power-frequency transmission system. The existing nearshore wind power projects adopt the power-frequency transmission scheme, while most of the deep-sea wind power projects adopt the DC transmission scheme. The DC voltage frequency is regarded as 0 Hz, and there is no capacitance charging problem, which is very conducive to long-distance power transmission. However, the cost of the offshore converter platform is expensive, and the research and development of DC circuit breakers and DC transformers are difficult and costly. DC networking is difficult, and the technicality and economy of the DC transmission scheme still face great challenges. The low-frequency transmission scheme has small line charging reactive power and strong transmission capacity. The low-frequency current can be interrupted at zero crossing. The low-frequency transformer has the same principle as the power-frequency transformer. Low-frequency networking is easy, and the low-frequency electric energy can be directly output by the wind turbine without an offshore converter platform, saving construction costs.

[0004] Therefore, it is of great application value to study the offshore wind power networking system under low-frequency operation conditions. Summary of the Invention

[0005] Aiming at the problems existing in the above-mentioned prior art, the present invention provides an offshore wind power low-frequency networking transmission system, which can flexibly adopt low-frequency submarine cable transmission or DC submarine cable transmission according to the offshore distance, and uses a better transmission scheme to reduce the grid connection difficulty and construction cost of the offshore wind power system, so as to solve the problems of high technical difficulty of DC voltage transformation and high equipment cost of DC circuit breakers in the current offshore wind power DC networking system, and realize the power steady-state control of all nodes.

[0006] For this purpose, the present invention adopts the following technical solution: An offshore wind power low-frequency networking transmission system, which includes an onshore frequency conversion station, an offshore converter station, an onshore converter station, an island frequency conversion station, a centralized controller and n wind power clusters. Each wind power cluster includes h wind turbines. Each wind power cluster outputs low-frequency alternating current, and the n wind power clusters are interconnected to form a low-frequency network; The j-th wind power cluster among the n wind power clusters transmits the low-frequency alternating current to the onshore frequency conversion station through a low-frequency submarine cable, and the onshore frequency conversion station converts the low-frequency alternating current into power-frequency alternating current and then sends it to the onshore power-frequency power grid, 1≤j<n; The k-th wind power cluster among the n wind power clusters transmits low-frequency alternating current to the offshore converter station through a low-frequency submarine cable. The offshore converter station converts the low-frequency alternating current into direct current and then transmits it to the onshore converter station through a DC submarine cable. The onshore converter station converts the direct current into industrial-frequency alternating current and then transmits it to the onshore industrial-frequency power grid, where 1 ≤ k < n; The m-th wind power cluster among the n wind power clusters transmits low-frequency alternating current to the island frequency conversion station through a low-frequency submarine cable. The island frequency conversion station converts the low-frequency alternating current into industrial-frequency alternating current and then supplies power to the island industrial-frequency load, where 1 ≤ m < n; The onshore frequency conversion station and the offshore converter station connected to the onshore industrial-frequency power grid serve as power source nodes, and the island frequency conversion station and the n wind power clusters connected to the island industrial-frequency load serve as load nodes. The centralized controller transmits measurement information and control commands to all power source nodes through a communication network; The centralized controller receives the dispatching power command upward, obtains the active power command and reactive power command of the power source nodes, collects the voltage and current information of all power source nodes downward, and issues the voltage frequency command, voltage amplitude command, and voltage phase command of the power source nodes.

[0007] The offshore wind power low-frequency networking power transmission system of the present invention can be extended to the networking schemes for transmitting large-scale photovoltaic bases and interconnected multi-supply areas in cities in addition to being used for offshore wind power; and by controlling the voltage of the power source nodes through the centralized controller, the power steady-state control of all nodes is realized.

[0008] Further, the low-frequency alternating current output by the n wind power clusters is first stepped up by a low-frequency transformer, and the AC buses at the outlet of the low-frequency transformer are composed of the low-frequency network through low-frequency circuit breakers and low-frequency submarine cables.

[0009] Further, the low-frequency network transmits electric energy to the onshore industrial-frequency power grid with a short distance through a low-frequency submarine cable, transmits electric energy to the onshore industrial-frequency power grid with a long distance through the offshore converter station and the DC submarine cable, and transmits electric energy to the nearby island industrial-frequency load through a low-frequency submarine cable.

[0010] Further, both the onshore frequency conversion station and the island frequency conversion station adopt the modular multilevel matrix converter M3C topology.

[0011] Furthermore, both the onshore frequency conversion station M3C and the island frequency conversion station M3C are composed of multiple phase units interconnected. Each phase unit is composed of multiple sub-modules connected in series, and the sub-module structure of M3C is a full-bridge sub-module; the onshore frequency conversion station M3C adopts a control strategy of fixed AC voltage on the low-frequency side and a control strategy of stator module capacitor voltage and fixed reactive power on the industrial-frequency side; the island frequency conversion station M3C adopts a control strategy of stator module capacitor voltage and fixed reactive power on the low-frequency side and a control strategy of fixed AC voltage on the industrial-frequency side.

[0012] Furthermore, both the offshore converter station and the onshore converter station adopt the modular multilevel converter (MMC) topology.

[0013] Furthermore, both the MMC of the offshore converter station and the MMC of the onshore converter station are composed of multiple phase units interconnected. Each phase unit is composed of multiple sub-modules connected in series. The sub-module structure of the MMC is a half-bridge sub-module or a full-bridge sub-module; the MMC of the offshore converter station adopts a control strategy of constant AC voltage, and the MMC of the onshore converter station adopts control strategies of constant DC voltage and constant reactive power.

[0014] Furthermore, the frequency of the low-frequency alternating current is 10 - 20 Hz, and the frequency of the power-frequency alternating current is 50 Hz.

[0015] The present invention also provides a control method for the above-mentioned offshore wind power low-frequency networking power transmission system, including: selecting one of the power nodes as the balance node of the system, and its voltage amplitude command and phase command are constant constants; the voltages of the remaining power nodes are divided into a common-mode voltage and a differential-mode voltage. The common-mode voltage participates in the natural power distribution completed under the action of the network impedance between the power node and the load node, and the differential-mode voltage participates in the coordinated control power transfer completed under the action of the control algorithm between the power nodes.

[0016] Furthermore, the control method further includes: the voltage frequency commands of all power nodes are the same, equal to the reference voltage frequency of the power transmission system f L0 , and the voltage amplitude command and voltage phase command of the power node are coordinately controlled and adjusted according to the control algorithm. The steps of the control algorithm include: Construct the admittance matrix of the low-frequency network according to the low-frequency network topology and parameter information; Calculate the common-mode voltage and differential-mode voltage of the power node according to the measured voltage of the power node; calculate the differential-mode current of the power node according to the admittance matrix of the low-frequency network and the differential-mode voltage of the power node, and combine the measured current to obtain the common-mode current of the power node; calculate the common-mode power of the power node according to the common-mode voltage and common-mode current of the power node, and calculate the difference between the common-mode power of the power node and the dispatching power command; adjust the phase of the differential-mode voltage of the power node according to the active power difference, and adjust the amplitude of the differential-mode voltage of the power node according to the active power difference; Open-loop calculate the feed-forward quantity of the differential-mode voltage through the known low-frequency network topology information, and calculate the feedback quantity of the differential-mode voltage through the difference value and the proportional-integral controller; superimpose the feed-forward quantity and feedback quantity of the differential-mode voltage to obtain the differential-mode voltage amplitude command and differential-mode voltage phase command of the power node, and then superimpose the common-mode voltage amplitude and common-mode voltage phase to obtain the voltage amplitude command and voltage phase command of the power node; The power of the power supply node changes following the differential-mode voltage regulation, and the actual power of the power supply node is recalculated; it is determined whether the error between the actual power of the power supply node and the dispatching power instruction is less than the allowable value. If it is not less than the allowable value, the instruction for the voltage of the power supply node is continuously iteratively calculated; under the action of the control algorithm, the power interaction between the power supply node and the load node gradually reaches a steady state; when the load power changes, the voltage instructions of each power supply node are automatically adjusted; when the low-frequency network topology changes, it is necessary to recalculate the admittance matrix and then readjust the voltage instructions of each power supply node; when the power instruction of the dispatching layer changes, a new round of coordinated control adjustment process needs to be started.

[0017] The beneficial effects of the present invention are as follows: The low-frequency networked power transmission system for offshore wind power provided by the present invention can directly output low-frequency alternating current through wind turbines and then step up and collect it through low-frequency transformers, thus eliminating the offshore commutation platform and effectively reducing the system construction cost. Under this low-frequency networked structure, the offshore wind power cluster network can directly transmit electric energy to the onshore regional power grid and islands through low-frequency submarine cables, or can be connected to a flexible DC offshore commutation station for voltage boosting and commutation, and then transmit electric energy to the onshore regional power grid through DC submarine cables.

[0018] The present invention does not adopt a fully DC networking method, but uses a low-frequency AC networking method, overcoming the problems of high technical difficulty and high manufacturing cost of equipment such as DC transformers and DC circuit breakers, and significantly improving the technical economy of the networking system. In addition, according to the distance from the shore, the low-frequency network can flexibly adopt low-frequency submarine cable transmission or DC submarine cable transmission to reduce the grid connection difficulty and construction cost of the offshore wind power system with a better transmission scheme.

[0019] The steady-state control strategy of the low-frequency network provided by the present invention can realize the automatic adjustment of the active power between each frequency conversion station M3C or commutation station MMC without relying on communication; the transient process strategy of the low-frequency network provided by the present invention can automatically adjust the output of the wind power cluster when a disturbance occurs in the onshore power frequency power grid, realizing the active frequency support for the onshore power frequency power grid.

[0020] The present invention controls the voltage of the power supply node through a centralized controller, realizing the power steady-state control of all nodes. Description of the Drawings

[0021] Figure 1 is a schematic diagram of the low-frequency networked power transmission system for offshore wind power in an embodiment of the present invention; Figure 2 is a topology diagram of the M3C converter of the frequency conversion station in an embodiment of the present invention; Figure 3 is a topology diagram of the MMC converter of the commutation station in an embodiment of the present invention; Figure 4It is the control strategy block diagram of the onshore frequency conversion station M3C in the embodiment of the present invention; Figure 5 It is the control strategy block diagram of the island frequency conversion station M3C in the embodiment of the present invention; Figure 6 It is the control strategy block diagram of the offshore HVDC converter station MMC in the embodiment of the present invention; Figure 7 It is the control strategy block diagram of the onshore HVDC converter station MMC in the embodiment of the present invention; Figure 8 It is the control schematic diagram of the offshore wind power low-frequency network system in the embodiment of the present invention; Figure 9 It is the flowchart of the control algorithm in the control method of the offshore wind power low-frequency networking power transmission system in the embodiment of the present invention. Detailed implementation mode

[0022] In order to describe the present invention more specifically, the technical solution of the present invention will be described in detail below in conjunction with the accompanying drawings and specific implementation modes. This embodiment is implemented on the premise of the technical solution of the present invention, and detailed implementation modes and specific operation processes are given, but the protection scope of the present invention is not limited to the following embodiments.

[0023] Figure 1 It is the structural schematic diagram of the offshore wind power low-frequency networking power transmission system. n wind power clusters are interconnected through low-frequency submarine cables to form a low-frequency network. Each wind power cluster includes h wind turbines, and each wind turbine directly outputs low-frequency alternating current, which is connected to the low-frequency bus through a step-up transformer. The topology of the n wind power clusters for networking includes but is not limited to ring type, chain type, radial type, etc. Figure 1 In it is the ring-type networking topology. Low-frequency circuit breakers are configured at both ends of each line in the low-frequency network.

[0024] For the low-frequency network composed of the n wind power clusters, the power transmission method of electric energy is flexibly selected according to the offshore distance. When the transmission distance from the onshore power frequency power grid to the offshore wind power low-frequency network is less than 200 km, the electric energy is transmitted by the low-frequency power transmission method and then connected to the grid; when the transmission distance from the onshore power frequency power grid to the offshore wind power low-frequency network is higher than 200 km, the electric energy is transmitted by the DC power transmission method and then connected to the grid; when the transmission distance from the island power frequency power grid to the offshore wind power low-frequency network is less than 200 km, the electric energy is transmitted by the low-frequency power transmission method and then connected to the grid. The low-frequency alternating current frequency of the offshore wind power low-frequency networking power transmission system is 10 - 20 Hz, and the power frequency alternating current frequency is 50 Hz.

[0025] When transmitting electric energy from an offshore low-frequency network to an onshore power-frequency grid in a low-frequency power transmission mode, the j-th (1 ≤ j < n) wind power cluster in the low-frequency network directly transmits low-frequency alternating current to the onshore frequency conversion station through a low-frequency submarine cable at the AC bus at the outlet. The onshore frequency conversion station converts the low-frequency alternating current into power-frequency alternating current and then sends it to the onshore power-frequency grid.

[0026] When transmitting electric energy from an offshore low-frequency network to an onshore power-frequency grid in a DC power transmission mode, the k-th (1 ≤ k < n) wind power cluster in the low-frequency network transmits low-frequency alternating current to the offshore converter station through a low-frequency submarine cable at the AC bus at the outlet. The offshore converter station converts the low-frequency alternating current into direct current and then transmits it to the onshore converter station through a DC submarine cable. The onshore converter station converts the direct current into power-frequency alternating current and then transmits it to the onshore power-frequency grid.

[0027] When transmitting electric energy from an offshore low-frequency network to an island power-frequency grid in a DC power transmission mode, the m-th (1 ≤ m < n) wind power cluster in the low-frequency network directly transmits low-frequency alternating current to the island frequency conversion station through a low-frequency submarine cable at the AC bus at the outlet. The island frequency conversion station converts the low-frequency alternating current into power-frequency alternating current and then supplies power to the island power-frequency load.

[0028] The onshore frequency conversion station and the offshore converter station connected to the onshore power-frequency grid serve as power source nodes, and the island frequency conversion station and the n wind power clusters connected to the island power-frequency load serve as load nodes. The centralized controller transmits measurement information and control commands to all power source nodes through a communication network; the centralized controller receives dispatching commands upward to obtain active power commands and reactive power commands for the power source nodes, collects voltage and current information of all power source nodes downward, and issues voltage frequency commands, voltage amplitude commands, and voltage phase commands for the power source nodes.

[0029] The onshore frequency conversion station is used to convert low-frequency alternating current into power-frequency alternating current, and the island frequency conversion station is used to convert low-frequency alternating current into power-frequency alternating current. Both the onshore frequency conversion station and the island frequency conversion station adopt the modular multilevel matrix converter M3C topology, as Figure 2 shown. Both the onshore frequency conversion station M3C and the island frequency conversion station M3C are composed of multiple phase units interconnected. Each phase unit is composed of multiple sub-modules SM n connected in series, and the sub-module structure of M3C is a full-bridge sub-module.

[0030] The offshore converter station is used to convert low-frequency alternating current into direct current, and the onshore converter station is used to convert direct current into power-frequency alternating current. Both the offshore converter station and the onshore converter station adopt the modular multilevel converter MMC topology, as Figure 3 shown. Both the offshore converter station MMC and the onshore converter station MMC are composed of multiple phase units interconnected. Each phase unit is composed of multiple sub-modules SM nConnected in series, the sub-module structure of the MMC is a half-bridge sub-module or a full-bridge sub-module.

[0031] As Figure 4 shown, the onshore frequency conversion station M3C adopts a fixed AC voltage control strategy on the low-frequency side, and gives the voltage frequency command , voltage amplitude command and voltage phase command value of the low-frequency network; on the power frequency side, it adopts the control strategy of stator module capacitor voltage control and fixed reactive power, and gives the command of the sum of the capacitor voltages of the bridge arm sub-modules and the reactive power command of the M3C power frequency side port.

[0032] As Figure 5 shown, the island frequency conversion station M3C adopts the control strategy of stator module capacitor voltage control and fixed reactive power on the low-frequency side, and gives the command of the sum of the capacitor voltages of the bridge arm sub-modules and the reactive power command of the M3C low-frequency side port; on the power frequency side, it adopts a fixed AC voltage control strategy, and gives the voltage frequency command , voltage amplitude command and voltage phase command of the island power frequency network.

[0033] As Figure 6 shown, the offshore converter station MMC adopts a fixed AC voltage control strategy, and gives the voltage frequency command , voltage amplitude command and voltage phase command value of the low-frequency network.

[0034] As Figure 7 shown, the onshore converter station MMC adopts the control strategy of stator module capacitor voltage control and fixed reactive power. Give the DC voltage command of the power transmission system and the reactive power command of the MMC power frequency side port.

[0035] Since the onshore frequency conversion station M3C, the offshore converter station MMC, and the island frequency conversion station M3C all have excellent decoupling control performance, when designing the control method of the offshore wind power low-frequency networking power transmission system, only the low-frequency sides of the onshore frequency conversion station M3C, the offshore converter station MMC, and the island frequency conversion station M3C need to be concerned, and there is no need to pay attention to the power frequency sides of the onshore frequency conversion station M3C, the onshore converter station MMC, and the island frequency conversion station M3C. The frequency conversion station M3C and the converter station MMC connected to the onshore power frequency grid are regarded as the power source nodes of the system. There are X power source nodes in the whole system, among which there are X M3CThere are X MMC power nodes of the MMC type in the converter station. X = X M3C + X MMC . Both M3C connected to the island power frequency load and the wind power cluster are regarded as load nodes of the system.

[0036] As Figure 8 shown is the control schematic diagram of the offshore wind power low-frequency networking power transmission system provided by the present invention. The centralized controller transmits measurement information and control commands to and from X power nodes through the communication network. The centralized controller receives the dispatching commands from the dispatching layer upwards and obtains the active power command and reactive power command of the power nodes; it collects the voltage and current information (i.e., measurement information) of all power nodes downwards, and after calculation by the control algorithm, issues the voltage frequency command, voltage amplitude command and voltage phase command of the power nodes.

[0037] The voltage frequency commands of all power nodes are the same and equal to the reference voltage frequency f L0 of the system, and the voltage amplitude command and voltage phase command are coordinately controlled and adjusted according to the control algorithm. X The voltage frequency commands received by f L0 power nodes are the same and equal to the reference voltage frequency X of the system. Select one power node from U L0 power nodes as the balancing node, and its voltage amplitude command and voltage phase command are constant constants. The value of the voltage amplitude command of the balancing node is the reference voltage amplitude θ L0 of the system, and the voltage phase command of the balancing node (1) During the operation of the offshore wind power low-frequency networking power transmission system, the topological structure of the low-frequency network is basically fixed. Therefore, store the low-frequency network topological structure information, low-frequency submarine cable parameter information, and connection transformer information in the centralized controller. The submarine cable is modeled by a segmented π-type equivalent circuit to construct the admittance matrix Y net of the low-frequency network.

[0038] (2) Among them, b is the number of branches, is the branch admittance, is the node-branch incidence vector, is the transpose of.

[0039] The voltage of the remaining ( X -1) power nodes except the slack node is decomposed into two parts: the common-mode voltage and the differential-mode voltage.

[0040] (3) Among them, U is the measured voltage vector of the power node, U comm is the decomposed common-mode voltage vector, U diff is the decomposed differential-mode voltage vector. Each element in the common-mode voltage vector U comm is equal to the voltage vector of the slack node .

[0041] The common-mode voltage participates in the natural power distribution completed under the action of the network impedance between the power node and the load node, and the differential-mode voltage participates in the coordinated control power transfer completed under the action of the control algorithm between the power nodes. In the initial operating state, the voltages of all power nodes are equal to the voltage of the slack node, that is, only the common-mode voltage exists and the differential-mode voltage does not exist. With the action of the control algorithm, the differential-mode voltage is no longer 0.

[0042] The steps of the control algorithm include: First, according to the measured voltage vector U of the power node, calculate the common-mode voltage vector and the differential-mode voltage vector: (4) Among them, E is the unit vector.

[0043] According to the admittance matrix Y net of the low-frequency network and the differential-mode voltage vector U diff calculate the differential-mode current vector I diff of the power node, and then combined with the measured current vector I the differential-mode current vector I comm can be obtained.

[0044] (5) Then, calculate the common-mode power according to the common-mode voltage and the common-mode current of the power node. The power difference between the power command dispatched and the common-mode power is the part that needs to change the differential-mode voltage for adjustment.

[0045] (6) (7) Among them, P comm is the common - mode active power, Q comm is the common - mode reactive power, diag means constructing a diagonal matrix by taking the elements in all column vectors, represents the conjugate of the common - mode current. Δ P is the active - power difference to be regulated by the differential - mode voltage, Δ Q is the reactive - power difference to be regulated by the differential - mode voltage.

[0046] Adjust the phase of the differential - mode voltage of the power supply node according to the active - power difference, and adjust the amplitude of the differential - mode voltage of the power supply node according to the reactive - power difference. The mapping control algorithm from the power difference to the voltage command consists of two parts: one is the feed - forward quantity, which is calculated by open - loop through the known low - frequency network topology information; the other is the feedback quantity, which is obtained by combining the measured power difference with a proportional - integral controller.

[0047] The feed - forward quantity of the differential - mode voltage U diff_fwd satisfies the non - linear power - flow equation of the low - frequency network under the injection of Δ P and Δ Q , and is solved by the Newton - Raphson method to obtain U diff_fwd , and then the amplitude feed - forward quantity and phase feed - forward quantity of the differential - mode voltage are obtained.

[0048] (8) (9) Among them, U diff_fwdx represents U diff_fwd the modulus of the x th element of θ diff_fwdx represents U diff_fwd the phase angle of the x th element of represents the conjugate of the differential - mode current.

[0049] Taking the differences Δ P 、Δ Q between the measured power and the common - mode power as the input quantities, after passing through a proportional - integral controller, the amplitude feedback quantity U diff_bckx and the phase feedback quantity θ diff_bckx of the differential - mode voltage can be obtained.

[0050] (10) Among them, K PP is the proportional coefficient of the phase feedback quantity operation, K IP is the integral coefficient of the phase feedback quantity operation, K PQ is the proportional coefficient of the amplitude feedback quantity operation, K IQ is the integral coefficient of the amplitude feedback quantity operation.

[0051] Finally, the feedforward quantity and feedback quantity of the differential-mode voltage are superimposed respectively, and the differential-mode voltage amplitude command and differential-mode voltage phase command of the power supply node can be obtained; then the common-mode voltage amplitude and common-mode voltage phase are superimposed, and the voltage amplitude command and voltage phase command can be obtained.

[0052] (11) (12) Among them, θ diffx is the differential-mode voltage phase command of the x th power supply node, U diffx is the differential-mode voltage amplitude command of the x th power supply node, is the voltage phase command of the x th power supply node, is the voltage amplitude command of the x th power supply node. The of all power supply nodes constitutes the voltage phase command vector , and the of all power supply nodes constitutes the voltage amplitude command vector .

[0053] After the differential-mode voltage is adjusted, the power of the power supply node changes accordingly. It is judged whether the error between the power of the power supply node and the scheduling command is less than the allowable value. If the error is not less than the allowable value, the voltage command of the power supply node is continuously iteratively calculated. Under the action of the control algorithm, the power interaction between the power supply nodes and load nodes in the system gradually reaches a steady state. When the load power changes, the system automatically adjusts the voltage commands of each power supply node. When the low-frequency network topology of the system changes, the admittance matrix needs to be recalculated, and then the voltage commands of each power supply node are readjusted. When the power command of the dispatching layer changes, a new round of cooperative control adjustment process needs to be started.

[0054] Application Example For the offshore wind power low-frequency networking power transmission system, the number of wind power clusters n = 4, the number of wind turbines in each wind power cluster h = 30, j = 3, k = 4, m = 4. That is, the 3rd wind power cluster (the third wind power cluster) uses the low-frequency power transmission method to transmit electric energy to the onshore power frequency grid, the 4th wind power cluster (the fourth wind power cluster) uses the DC power transmission method to transmit electric energy to the onshore power frequency grid, and the 4th wind power cluster (the fourth wind power cluster) uses the low-frequency power transmission method to transmit electric energy to the island low-frequency grid.

[0055] The low-frequency frequency of the offshore wind power low-frequency networking power transmission system is 20 Hz. The active power P sent by each wind power cluster is 300 MW, and the reactive power Q is 0 Var. The parameters of each transformer are shown in Table 1. The voltage level after the wind power clusters are aggregated through the low-frequency AC line is 66 kV, and then it is stepped up to 220 kV through transformers T1 - T4. The rated voltage of the entire offshore wind power low-frequency network is 220 kV.

[0056] Table 1 Parameters of each transformer in the offshore wind power low-frequency networking power transmission system

[0057] The distance from the first onshore power frequency grid to the 220 kV low-frequency busbar of the third wind power cluster is 150 km. The low-frequency power transmission method is used to transmit electric energy, and the voltage level of the low-frequency submarine cable is 220 kV. The turns ratios of the low-frequency transformer T5 and the power frequency transformer T6 are both 220 kV / 64 kV. The rated voltages of the low-frequency side and the power frequency side of the onshore frequency conversion station are both 64 kV, and the rated voltage of the first onshore power frequency grid is 220 kV.

[0058] The distance from the second onshore power frequency grid to the 220 kV low-frequency busbar of the fourth wind power cluster is 230 km. The DC power transmission method is used to transmit electric energy, and the voltage level of the DC submarine cable is 800 kV. The turns ratios of the low-frequency transformer T7 and the power frequency transformer T8 are both 420 kV / 220 kV. The rated voltages of the AC sides of the offshore converter station and the onshore converter station are both 420 kV, and the rated voltage of the second onshore power frequency grid is 220 kV.

[0059] The distance from the island power frequency grid to the 220 kV low-frequency busbar of the fourth wind power cluster is 30 km. The low-frequency power transmission method is used to transmit electric energy, and the voltage level of the low-frequency submarine cable is 220 kV. The turns ratio of the low-frequency transformer T9 is 220 kV / 35 kV, and the turns ratio of the power frequency transformer T10 is 35 kV / 35 kV. The rated voltages of the low-frequency side and the power frequency side of the island frequency conversion station are both 35 kV, and the rated voltage of the island power frequency grid is 35 kV.

[0060] The entire system has a total of 2 power nodes, including 1 power node of the M3C type in the frequency conversion station and 1 power node of the MMC type in the converter station; the entire system has a total of 5 load nodes, including 4 load nodes of the wind farm type and 1 load node of the island type. According to Figure 1 In the shown offshore wind power low-frequency networking system, wind power clusters 1 to 4 are nodes 1 to 4 respectively, the island frequency conversion station is node 5, the onshore frequency conversion station is node 6, and the onshore converter station is node 7. Select the power node of the M3C type, that is, node 7 as the balancing node for system control, with a voltage reference value of 220 kV and a phase reference value of 0°.

[0061] Set the resistance per unit length of the submarine cable to 0.0286 Ω / km, the inductance per unit length of the submarine cable to 0.6189 mH / km, and the capacitance per unit length of the submarine cable to 0.154 uF / km. The distance between node 1 and node 2 is 100 km, the distance between node 2 and node 4 is 120 km, the distance between node 4 and node 3 is 100 km, the distance between node 3 and node 1 is 120 km, the distance between node 5 and node 4 is 30 km, the distance between node 6 and node 3 is 60 km, and the distance between node 7 and node 4 is 50 km.

[0062] Set the active power of wind power clusters 1 to 4 to -100 MW and the reactive power to 0 MVar respectively. Set the active power of the island load to 30 MW and the reactive power to 0 Mvar. Set the active power command value of node 6 to 180 MW and the reactive power command value to 0 MVar. Before the control algorithm takes effect, the power of each node is naturally distributed according to the low-frequency network. The active power of node 6 is 167 MW and the reactive power is 6.4 Mvar, and the active power of node 7 is 197 MW and the reactive power is 6.9 Mvar. After 3 iterations of calculation, the active power of node 6 is 180 MW and the reactive power is 0 MVar, and the active power of node 7 is 184 MW and the reactive power is -2.2 MVar.

[0063] The above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that: they can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements for some of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present invention, and should all be included in the protection scope of the present invention.

Claims

1. An offshore wind power low-frequency networking transmission system, characterized in that: It includes an onshore frequency conversion station, an onshore converter station, an offshore converter station, an island frequency conversion station, a centralized controller and n wind power clusters. Each wind power cluster includes h wind turbines. Each wind power cluster outputs low-frequency AC power. The n wind power clusters are interconnected to form a low-frequency network. The j-th wind power cluster among the n wind power clusters transmits low-frequency AC power to an onshore frequency conversion station through a low-frequency submarine cable. The onshore frequency conversion station converts the low-frequency AC power into industrial frequency AC power and then transmits it to the onshore industrial frequency power grid. 1≤j <n; The kth wind power cluster among the n wind power clusters transmits low-frequency AC power to an offshore converter station via a low-frequency submarine cable. The offshore converter station converts the low-frequency AC power into DC power, which is then transmitted to an onshore converter station via a DC submarine cable. The onshore converter station converts the DC power into industrial frequency AC power and then transmits it to an onshore industrial frequency power grid. 1≤k <n; The mth wind power cluster among the n wind power clusters transmits low-frequency AC power to the island frequency conversion station through a low-frequency submarine cable. The island frequency conversion station converts the low-frequency AC power into industrial frequency AC power and supplies power to the island industrial frequency load. 1≤m <n; The onshore frequency conversion station and the offshore frequency conversion station connected to the onshore power frequency power grid are used as power supply nodes, and the island frequency conversion station and n wind power clusters connected to the island power frequency load are used as load nodes. The centralized controller transmits measurement information and control instructions to all power supply nodes through the communication network. The centralized controller receives the dispatching power instructions upward, obtains the active power instructions and reactive power instructions of the power supply nodes, collects the voltage and current information of all power supply nodes downward, and sends the voltage frequency instructions, voltage amplitude instructions and voltage phase instructions of the power supply nodes.

2. The offshore wind power low-frequency networking transmission system according to claim 1, characterized in that: The low-frequency AC power output by the n wind power clusters is first boosted by a low-frequency transformer, and the AC busbars at the outlets of the low-frequency transformers are connected through low-frequency circuit breakers and low-frequency submarine cables to form the low-frequency network.

3. The offshore wind power low-frequency networking power transmission system according to claim 1, characterized in that: The low-frequency network transmits electric energy to the nearby onshore industrial frequency power grid through the low-frequency submarine cable, transmits electric energy to the distant onshore industrial frequency power grid through the offshore converter station and the DC submarine cable, and transmits electric energy to the nearby island industrial frequency load through the low-frequency submarine cable.

4. The offshore wind power low-frequency networking transmission system according to claim 1, characterized in that: The onshore frequency conversion station and the island frequency conversion station both adopt a modular multi-level matrix converter M3C topology.

5. The offshore wind power low-frequency networking power transmission system according to claim 4, characterized in that: The onshore frequency conversion station M3C and the island frequency conversion station M3C are both composed of multiple interconnected phase units, each phase unit is composed of multiple sub-modules connected in series, and the sub-module structure of M3C is a full-bridge sub-module; the onshore frequency conversion station M3C adopts a constant AC voltage control strategy on the low-frequency side, and a stator module capacitor voltage and constant reactive power control strategy on the power frequency side; the island frequency conversion station M3C adopts a stator module capacitor voltage and constant reactive power control strategy on the low-frequency side, and a constant AC voltage control strategy on the power frequency side.

6. The offshore wind power low-frequency networking transmission system according to claim 1, characterized in that: The offshore converter station and the onshore converter station both adopt modular multi-level converter MMC topology.

7. The offshore wind power low-frequency networking power transmission system according to claim 6, characterized in that: The offshore converter station MMC and the onshore converter station MMC are both composed of multiple interconnected phase units, each phase unit is composed of multiple sub-modules connected in series, and the sub-module structure of the MMC is a half-bridge sub-module or a full-bridge sub-module; the offshore converter station MMC adopts a control strategy of fixed AC voltage, and the onshore converter station MMC adopts a control strategy of fixed DC voltage and fixed reactive power.

8. An offshore wind power low-frequency networking transmission system according to any one of claims 1 to 7, characterized in that: The frequency of the low-frequency alternating current is 10-20 Hz, and the frequency of the industrial-frequency alternating current is 50 Hz.

9. The control method of the offshore wind power low-frequency networking power transmission system according to any one of claims 1 to 8, characterized in that: One of the power supply nodes is selected as the balancing node of the system, and its voltage amplitude command and phase command are constant. The voltages of the remaining power supply nodes are divided into common-mode voltage and differential-mode voltage. The common-mode voltage participates in the natural power distribution completed under the action of network impedance between the power supply node and the load node, and the differential-mode voltage participates in the coordinated control power transfer completed under the action of the control algorithm between the power supply nodes.

10. The control method of the offshore wind power low-frequency networking power transmission system according to claim 9, characterized in that: The voltage frequency command of all power nodes is the same, which is equal to the reference voltage frequency of the transmission system. f L0 , the voltage amplitude command and the voltage phase command of the power supply node are coordinated and controlled according to the control algorithm, and the steps of the control algorithm include: Construct the admittance matrix of the low-frequency network according to the low-frequency network topology and parameter information; The common-mode voltage and differential-mode voltage of the power supply node are calculated according to the measured voltage of the power supply node; the differential-mode current of the power supply node is calculated according to the admittance matrix of the low-frequency network and the differential-mode voltage of the power supply node, and the common-mode current of the power supply node is obtained by combining the measured current; the common-mode power of the power supply node is calculated according to the common-mode voltage and common-mode current of the power supply node, and the difference between the common-mode power of the power supply node and the dispatching power instruction is calculated; the differential-mode voltage phase of the power supply node is adjusted according to the active power difference, and the differential-mode voltage amplitude of the power supply node is adjusted according to the active power difference; The feedforward of the differential mode voltage is calculated in an open loop by using the known low-frequency network topology information, and the feedback of the differential mode voltage is calculated by using the difference and proportional integral controller; the feedforward and feedback of the differential mode voltage are superimposed to obtain the differential mode voltage amplitude instruction and the differential mode voltage phase instruction of the power supply node, and then the common mode voltage amplitude and the common mode voltage phase are superimposed to obtain the voltage amplitude instruction and the voltage phase instruction of the power supply node; After the differential mode voltage is adjusted, the power of the power node changes accordingly, and the actual power of the power node is recalculated; it is determined whether the error between the actual power of the power node and the scheduling power instruction is less than the allowable value. If it is not less than the allowable value, the voltage instruction of the power node is continuously iterated and calculated; under the action of the control algorithm, the power interaction between the power node and the load node gradually reaches a steady state; when the load power changes, the voltage instruction of each power node is automatically adjusted; when the low-frequency network topology changes, it is necessary to recalculate the admittance matrix and then readjust the voltage instruction of each power node; when the power instruction of the scheduling layer changes, it is necessary to start a new round of collaborative control and adjustment process.

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