An offshore wind power low-frequency networking power transmission system and its control method

Abstract

The present invention discloses an offshore wind power low-frequency networking power transmission system and its control method. The power transmission system of the present invention 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 alternating current, and the n wind power clusters are interconnected to form a low-frequency network. The onshore frequency conversion station and the offshore converter station connected to the onshore power frequency grid serve as power source nodes, the island frequency conversion station connected to the island power frequency load and the n wind power clusters serve as load nodes, and the centralized controller transmits measurement information and control commands to all power source nodes through a communication network. The low-frequency network of the present invention can flexibly adopt low-frequency submarine cables or DC submarine cables for transmission, reduce the grid connection difficulty and construction cost of the offshore wind power transmission system with a better transmission scheme, and control the voltage of the power source nodes through the centralized controller to achieve the power steady-state control of all nodes.

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 in the distance from the shore, the influence of the submarine cable capacitance to the ground becomes more serious, 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 reduction of the frequency, the cable charging power of the low-frequency transmission system is reduced, the active power transmission capacity is improved, 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 0Hz, 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 distance from the shore, 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 such as the large technical difficulty of DC voltage conversion and the 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] To this end, the present invention adopts the following technical solution: An offshore wind power low-frequency networking transmission system 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;

[0007] 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, where 1 ≤ j < n;

[0008] The k-th wind power cluster among the n wind power clusters transmits low-frequency alternating current to an 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 an onshore converter station through a high-voltage direct current (HVDC) 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;

[0009] The m-th wind power cluster among the n wind power clusters transmits low-frequency alternating current to an 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;

[0010] 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;

[0011] The centralized controller receives the dispatching power command upward to obtain 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.

[0012] The offshore wind power low-frequency networking power transmission system of the present invention can be extended to the networking schemes for transmitting power from 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.

[0013] Further, the low-frequency alternating current output by the n wind power clusters is first stepped up by a low-frequency transformer, and the low-frequency network is formed by low-frequency circuit breakers and low-frequency submarine cables between the AC buses at the outlet of the low-frequency transformer.

[0014] 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 an offshore converter station and an HVDC submarine cable, and transmits electric energy to the nearby island industrial-frequency load through a low-frequency submarine cable.

[0015] Further, both the onshore frequency conversion station and the island frequency conversion station adopt the modular multilevel matrix converter (M3C) topology.

[0016] 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. 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 power 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 power frequency side.

[0017] Further, both the offshore converter station and the onshore converter station adopt a modular multi-level converter MMC topology.

[0018] Furthermore, 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 connected in series. The sub-module structure of 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.

[0019] Further, the frequency of the low-frequency alternating current is 10 - 20 Hz, and the frequency of the power frequency alternating current is 50 Hz.

[0020] 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 two parts: common-mode voltage and 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.

[0021] Further, the control method further includes: the voltage frequency commands of all power nodes are the same and 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:

[0022] Construct the admittance matrix of the low-frequency network according to the low-frequency network topology and parameter information;

[0023] Calculate the common-mode voltage and differential-mode voltage of the power supply node according to the measured voltage of the power supply node; calculate the differential-mode current of the power supply node based on the admittance matrix of the low-frequency network and the differential-mode voltage of the power supply node, and combine the measured current to obtain the common-mode current of the power supply node; calculate the common-mode power of the power supply node according to the common-mode voltage and common-mode current of the power supply node, and calculate the difference between the common-mode power of the power supply node and the dispatching power instruction; 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 active power difference.

[0024] Open-loop calculate the feedforward 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 said difference and the proportional-integral controller; superimpose the feedforward quantity and feedback quantity of the differential-mode voltage to obtain the differential-mode voltage amplitude instruction and differential-mode voltage phase instruction of the power supply node, and then superimpose the common-mode voltage amplitude and common-mode voltage phase to obtain the voltage amplitude instruction and voltage phase instruction of the power supply node.

[0025] After the differential-mode voltage is adjusted, the power of the power supply node changes accordingly, and the actual power of the power supply node is recalculated; determine 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, continuously iterate and calculate the instruction of the power supply node voltage; 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, automatically adjust the voltage instruction of each power supply node; when the topology of the low-frequency network changes, it is necessary to recalculate the admittance matrix and then readjust the voltage instruction 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.

[0026] The beneficial effects of the present invention are as follows:

[0027] The low-frequency networking 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 converter platform and effectively reducing the system construction cost. Under this low-frequency networking 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 converter station for step-up conversion, and then transmit electric energy to the onshore regional power grid through DC submarine cables.

[0028] The present invention does not adopt a fully DC networking method, but uses a low-frequency AC networking method, which overcomes the problems of high technical difficulty and high manufacturing cost of equipment such as DC transformers and DC circuit breakers, and significantly improves 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.

[0029] The low-frequency network steady-state control strategy provided by the present invention can achieve automatic regulation of active power between M3Cs of each frequency conversion station or MMCs of converter stations without relying on communication; the low-frequency network transient process strategy provided by the present invention can automatically regulate the output of a wind power cluster when a disturbance occurs in the onshore power frequency grid, realizing active frequency support for the onshore power frequency grid.

[0030] The present invention realizes the steady-state power control of all nodes by controlling the voltage of power supply nodes through a centralized controller. Description of the Drawings

[0031] Figure 1 is a schematic diagram of the offshore wind power low-frequency networking power transmission system according to an embodiment of the present invention;

[0032] Figure 2 is a topology diagram of the M3C converter of the frequency conversion station according to an embodiment of the present invention;

[0033] Figure 3 is a topology diagram of the MMC converter of the converter station according to an embodiment of the present invention;

[0034] Figure 4 is a control strategy block diagram of the onshore frequency conversion station M3C according to an embodiment of the present invention;

[0035] Figure 5 is a control strategy block diagram of the island frequency conversion station M3C according to an embodiment of the present invention;

[0036] Figure 6 is a control strategy block diagram of the offshore converter station MMC according to an embodiment of the present invention;

[0037] Figure 7 is a control strategy block diagram of the onshore converter station MMC according to an embodiment of the present invention;

[0038] Figure 8 is a control principle diagram of the offshore wind power low-frequency network system according to an embodiment of the present invention;

[0039] Figure 9 is a flowchart of the control algorithm in the control method of the offshore wind power low-frequency networking power transmission system according to an embodiment of the present invention. Detailed Embodiments

[0040] In order to describe the present invention more specifically, the technical solutions of the present invention will be described in detail below in conjunction with the drawings and specific embodiments. This embodiment is implemented on the premise of the technical solution of the present invention, and gives detailed implementation manners and specific operation processes, but the protection scope of the present invention is not limited to the following embodiments.

[0041] Figure 1It is a schematic structural diagram of a low-frequency power grid connection and transmission system for offshore wind power. 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 power grid connection includes but is not limited to ring type, chain type, radial type, etc., Figure 1 in which is the ring-type power grid connection topology. Low-frequency circuit breakers are configured at both ends of each line in the low-frequency network.

[0042] For the low-frequency network composed of the n wind power clusters, the power transmission method is flexibly selected according to the offshore distance. When the transmission distance from the onshore power frequency grid to the offshore wind power low-frequency network is less than 200 km, the power is transmitted and grid-connected by the low-frequency power transmission method; when the transmission distance from the onshore power frequency grid to the offshore wind power low-frequency network is more than 200 km, the power is transmitted and grid-connected by the DC power transmission method; when the transmission distance from the island power frequency grid to the offshore wind power low-frequency network is less than 200 km, the power is transmitted and grid-connected by the low-frequency power transmission method. The frequency of the low-frequency alternating current in the offshore wind power low-frequency power grid connection and transmission system is 10 - 20 Hz, and the frequency of the power frequency alternating current is 50 Hz.

[0043] When using the low-frequency power transmission method to transmit power from the offshore low-frequency network to the onshore power frequency grid, the jth (1 ≤ j < n) wind power cluster in the low-frequency network directly transmits the 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.

[0044] When using the DC power transmission method to transmit power from the offshore low-frequency network to the onshore power frequency grid, the kth (1 ≤ k < n) wind power cluster in the low-frequency network transmits the 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.

[0045] When using the DC power transmission method to transmit power from the offshore low-frequency network to the island power frequency grid, the mth (1 ≤ m < n) wind power cluster in the low-frequency network directly transmits the 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.

[0046] 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 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 and from all power source nodes through a communication network. The centralized controller receives dispatching commands upward, obtains 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.

[0047] 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.

[0048] 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 n connected in series, and the sub-module structure of MMC is a half-bridge sub-module or a full-bridge sub-module.

[0049] As Figure 4 shown, the onshore frequency conversion station M3C adopts a constant 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 a control strategy of stator module capacitor voltage control and constant reactive power, and gives the sum command of the capacitor voltages of the sub-modules of the bridge arm and the reactive power command of the power frequency side port of M3C.

[0050] As Figure 5 shown, the island frequency conversion station M3C adopts a control strategy of stator module capacitor voltage control and constant reactive power on the low-frequency side, and gives the sum command of the capacitor voltages of the sub-modules of the bridge arm and the reactive power command of the low-frequency side port of M3C; on the power frequency side, it adopts a constant AC voltage control strategy, and gives the voltage frequency command , voltage amplitude command and voltage phase command 。

[0051] As Figure 6 shown, the MMC of the offshore converter station adopts a control strategy of constant AC voltage, and gives the voltage frequency command , voltage amplitude command and voltage phase command value 。

[0052] As Figure 7 shown, the MMC of the onshore converter station adopts a control strategy of stator module capacitor voltage control and constant reactive power. Given the DC voltage command of the transmission system and the reactive power command of the MMC power frequency side port.

[0053] Since the M3C of the onshore frequency conversion station, the MMC of the offshore converter station, and the M3C of the island frequency conversion station all have excellent decoupling control performance, when designing the control method of the offshore wind power low-frequency networking transmission system, only the low-frequency sides of the M3C of the onshore frequency conversion station, the MMC of the offshore converter station, and the M3C of the island frequency conversion station need to be concerned, and there is no need to pay attention to the power frequency sides of the M3C of the onshore frequency conversion station, the MMC of the onshore converter station, and the M3C of the island frequency conversion station. The frequency conversion stations M3C and converter stations MMC connected to the onshore power frequency grid are regarded as the power source nodes of the system. There are a total of X power source nodes in the whole system, among which there are a total of X M3C power source nodes of the M3C type of frequency conversion station, and there are a total of X MMC power source nodes of the MMC type of converter station, X = X M3C + X MMC 。The M3C and wind power clusters connected to the island power frequency load are regarded as the load nodes of the system.

[0054] As Figure 8 shown is the control schematic diagram of the offshore wind power low-frequency networking transmission system provided by the present invention. The centralized controller transmits measurement information and control instructions to X power source nodes through a communication network. The centralized controller receives the dispatching instructions from the dispatching layer upward, and obtains the active power command and reactive power command of the power source nodes; downward, it collects the voltage and current information (i.e., measurement information) of all power source nodes, and after calculation by the control algorithm, it issues the voltage frequency command, voltage amplitude command, and voltage phase command of the power source nodes.

[0055] The voltage frequency commands of all power source nodes are the same and equal to the reference voltage frequency fL0 The voltage amplitude command and the voltage phase command are then co - controlled and adjusted according to the control algorithm. X The voltage frequency commands received by each power supply node are the same and equal to the reference voltage frequency of the system. f L0 . From X One power supply node is selected from the U L0 power supply nodes as the balancing node. 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 of the system. θ L0 The voltage phase command of the balancing node can take the value of 0. The voltage vector of the balancing node

[0056] (1)

[0057] 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, the low - frequency network topological structure information, low - frequency submarine cable parameter information, and connection transformer information are stored in the centralized controller. The submarine cable is modeled using a segmented π - type equivalent circuit to construct the admittance matrix of the low - frequency network. Y net .

[0058] (2)

[0059] Among them, b is the number of branches, is the branch admittance, is the node - branch incidence vector, is the transpose of.

[0060] The voltages of the remaining ( X - 1) power supply nodes except the balancing node are decomposed into a common - mode voltage and a differential - mode voltage.

[0061] (3)

[0062] Among them, U is the measured voltage vector of the power supply 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 balancing node.

[0063] The common-mode voltage participates in the natural power distribution completed under the action of the 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. In the initial operating state, the voltages of all power supply nodes are equal to the voltage of the balanced 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 zero.

[0064] The steps of the control algorithm include:

[0065] First, according to the measured voltage vector of the power supply node U Calculate the common-mode voltage vector and the differential-mode voltage vector:

[0066] (4)

[0067] Among them, E is a unit vector.

[0068] According to the admittance matrix of the low-frequency network Y net and the differential-mode voltage vector U diff Calculate the differential-mode current vector of the power supply node I diff , and then combined with the measured current vector I The differential-mode current vector I comm can be obtained.

[0069] (5)

[0070] Then, calculate the common-mode power according to the common-mode voltage and common-mode current of the power supply 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 to adjust.

[0071] (6)

[0072] (7)

[0073] 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 adjusted by the differential-mode voltage, and Δ Q is the reactive power difference to be adjusted by the differential-mode voltage.

[0074] 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 feedforward 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.

[0075] Feedforward quantity of differential-mode voltage U diff_fwd Satisfy Δ P and Δ Q Under the injection, solve the non-linear power flow equation of the low-frequency network through the Newton-Raphson method to calculate U diff_fwd , and then obtain the amplitude feedforward quantity and phase feedforward quantity of the differential-mode voltage.

[0076] (8)

[0077] (9)

[0078] 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 x th element of the phase angle of, Represents the conjugate of the differential-mode current.

[0079] Taking the difference Δ P 、Δ Q between the measured power and the common-mode power as the input quantity, the amplitude feedback quantity U diff_bckx and the phase feedback quantity θ diff_bckx of the differential-mode voltage can be obtained after passing through the proportional-integral controller.

[0080] (10)

[0081] 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.

[0082] Finally, the feedforward and feedback amounts of the differential-mode voltage are respectively superimposed to obtain the differential-mode voltage amplitude command and differential-mode voltage phase command of the power supply node; then, the common-mode voltage amplitude and common-mode voltage phase are superimposed to obtain the voltage amplitude command and the voltage phase command .

[0083] (11)

[0084] (12)

[0085] 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, and is the voltage phase command of the x th power supply node, is the x th voltage amplitude command of the power supply node. The of all power supply nodes constitute the voltage phase command vector , and the of all power supply nodes constitute the voltage amplitude command vector .

[0086] 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 dispatching instruction is less than the allowable value. If the error is not less than the allowable value, the voltage instruction 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 instructions of each power supply node. When the low-frequency network topology of the system 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 cooperative control adjustment process needs to be started.

[0087] Application example

[0088] In the low-frequency networking power transmission system of offshore wind power, 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.

[0089] The low-frequency of the offshore wind power low-frequency networking transmission system is 20Hz. The active power P sent out by each wind power cluster is 300MW, and the reactive power Q is 0Var. The parameters of each transformer are shown in Table 1. The voltage level after the wind power clusters are gathered through low-frequency AC lines is 66kV, and then it is stepped up to 220kV through transformers T1 - T4. The rated voltage of the entire offshore wind power low-frequency network is 220kV.

[0090] Table 1 Parameters of each transformer in the offshore wind power low-frequency networking transmission system

[0091]

[0092] The distance from the first onshore power frequency grid to the 220kV low-frequency bus of the third wind power cluster is 150km. Electric energy is transmitted by low-frequency power transmission mode, and the voltage level of the low-frequency submarine cable is 220kV. The turns ratios of low-frequency transformer T5 and power frequency transformer T6 are both 220kV / 64kV. The rated voltages of the low-frequency side and the power frequency side of the onshore frequency conversion station are both 64kV, and the rated voltage of the first onshore power frequency grid is 220kV.

[0093] The distance from the second onshore power frequency grid to the 220kV low-frequency bus of the fourth wind power cluster is 230km. Electric energy is transmitted by DC power transmission mode, and the voltage level of the DC submarine cable is 800kV. The turns ratios of low-frequency transformer T7 and power frequency transformer T8 are both 420kV / 220kV. The rated voltages of the AC sides of the offshore converter station and the onshore converter station are both 420kV, and the rated voltage of the second onshore power frequency grid is 220kV.

[0094] The distance from the island power frequency grid to the 220kV low-frequency bus of the fourth wind power cluster is 30km. Electric energy is transmitted by low-frequency power transmission mode, and the voltage level of the low-frequency submarine cable is 220kV. The turns ratio of low-frequency transformer T9 is 220kV / 35kV, and the turns ratio of power frequency transformer T10 is 35kV / 35kV. The rated voltages of the low-frequency side and the power frequency side of the island frequency conversion station are both 35kV, and the rated voltage of the island power frequency grid is 35kV.

[0095] The entire system has 2 power source nodes, including 1 power source node of the M3C type at the frequency conversion station M3C and 1 power source node of the MMC type at the converter station; the entire system has 5 load nodes, including 4 load nodes of the wind farm type and 1 load node of the island type. According to Figure 1 The shown offshore wind power low-frequency networking system, wind power clusters 1 - 4 are nodes 1 - 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 source node of the M3C type, that is, node 7 as the balancing node for system control, with its voltage reference value of 220kV and phase reference value of 0°.

[0096] 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 μF / 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.

[0097] Set the active powers of Wind Power Clusters 1 to 4 to -100 MW respectively, and the reactive powers to 0 MVar. 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. 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. The active power of Node 7 is 184 MW and the reactive power is -2.2 MVar.

[0098] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than 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 on 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. A low-frequency networking power transmission system for offshore wind power, 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 alternating current, and the n wind power clusters are interconnected to form a low-frequency network; For the j-th wind power cluster among the n wind power clusters, the low-frequency alternating current is transmitted to the onshore frequency conversion station through a low-frequency submarine cable. The onshore frequency conversion station converts the low-frequency alternating current into industrial-frequency alternating current and then sends it to the onshore industrial-frequency power grid, where 1 ≤ j < n; For the k-th wind power cluster among the n wind power clusters, the low-frequency alternating current is transmitted 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 high-voltage direct current (HVDC) 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; For the m-th wind power cluster among the n wind power clusters, the low-frequency alternating current is transmitted 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 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 connected to the island industrial-frequency load and the n wind power clusters 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 to obtain the active power command and reactive power command of the power source node, 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 node.

2. The offshore wind power low-frequency networking power transmission system according to claim 1, wherein, The low-frequency alternating current output by the n wind power clusters is first stepped up by a low-frequency transformer. The AC buses at the outlet of the low-frequency transformer are connected by 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 a low-frequency submarine cable, transmits electric energy to the onshore industrial-frequency power grid at a long distance through the offshore converter station and the HVDC submarine cable, and transmits electric energy to the nearby island industrial-frequency load through a low-frequency submarine cable.

4. The offshore wind power low-frequency networking power transmission system according to claim 1, wherein Both the onshore frequency conversion station and the island frequency conversion station adopt the modular multilevel matrix converter (M3C) topology.

5. The offshore wind power low-frequency networking power transmission system according to claim 4, characterized in that, Both the onshore M3C frequency conversion station and the island M3C frequency conversion 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 M3C is a full-bridge sub-module. The onshore M3C frequency conversion station 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 M3C frequency conversion station 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.

6. The offshore wind power low-frequency networked power transmission system according to claim 1, wherein Both the offshore converter station and the onshore converter station adopt the modular multilevel converter (MMC) topology.

7. The offshore wind power low-frequency networking power transmission system according to claim 6, wherein, The MMC of the offshore converter station and the MMC of the onshore converter station are both composed of multiple interconnected phase units. 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 a control strategy of constant DC voltage and constant reactive power.

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

9. The control method of the offshore wind power low-frequency networking power transmission system according to any one of claims 1-8, characterized in that, One of the power nodes is selected 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 two parts: common-mode voltage and 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.

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 commands of all power nodes are the same and 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 jointly 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 feedforward 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 said difference and the proportional-integral controller; superimpose the feedforward 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; 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; judge whether the error between the actual power of the power node and the dispatching power command is less than the allowable value. If it is not less than the allowable value, continuously iterate to calculate the command of the power node voltage; 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, automatically adjust the voltage commands of each power node; when the low-frequency network topology changes, it is necessary to recalculate the admittance matrix and then readjust the voltage commands of each power node; when the power command of the dispatching layer changes, a new round of coordinated control adjustment process needs to be started.

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