Hybrid direct-current power transmission system for realizing offshore wind power transmission

Abstract

The utility model discloses a hybrid direct-current power transmission system capable of sending out offshore wind power. The hybrid direct-current power transmission system comprises an offshore wind power plant, an offshore converter station, a direct-current power transmission line and an onshore converter station, after being subjected to voltage transformation, electric energy output by a fan of the offshore wind plant is connected to the alternating current input end of the offshore converter station through a collection bus; the current converter of the offshore converter station adopts a voltage source current converter, and the current converter of the land converter station comprises a current source current converter and a voltage source current converter which are connected in parallel; the direct-current output end of the offshore converter station is connected with a direct-current power transmission line so as to transmit electric energy to the onshore converter station; the direct current input ends of the current source converter and the voltage source converter of the land converter station are respectively connected with the electric energy output end of the direct current transmission line, and the alternating current output ends of the current source converter and the voltage source converter of the land converter station are connected in parallel to an alternating current power grid. According to the utility model, the advantages of the voltage source converter and the current source converter are integrated, and the operation flexibility and reliability of the offshore wind power sending-out system can be improved.

Description

Technical Field

[0001] This utility model relates to the field of DC power transmission technology, and in particular to a hybrid DC power transmission system for transmitting offshore wind power. Background Technology

[0002] With the transformation and development of power energy, wind power and photovoltaic power generation will become the basic power sources in my country in the future. my country has a coastline of 18,000 kilometers, with abundant wind energy resources along the coast, which are located in economically developed areas. This promotes the development and utilization of near-shore wind power resources. However, as the available near-shore wind power resources become increasingly scarce, offshore wind power development will shift from near-shore to mid- and far-shore areas. At the same time, the capacity of wind farms and the distance from the shore will increase several times. The existing AC transmission system can no longer meet the needs of large-scale wind power transmission from mid- and far-shore areas.

[0003] Modular multilevel voltage source converters achieve AC / DC energy transfer through energy storage capacitors in bridge arm submodules. They have advantages such as low harmonic content in inverter output voltage, low switching frequency of submodules, and passive power supply capability, making them suitable for offshore wind power transmission scenarios. Based on controllable turn-off current source converters, DC-to-AC conversion is achieved by chopping the input DC current through two-phase short circuits in the connected AC system or the controllable turn-off capability of the bridge arm itself. After being filtered by an AC filter, the AC current is connected to the AC grid. This eliminates the need for capacitor energy storage components, semiconductor failure short circuits, low on-state voltage drop, strong surge resistance, and no explosion risk. The converters also have small footprint, light weight, and low losses, making them suitable for high-power DC transmission applications.

[0004] Currently, offshore wind power transmission uses flexible DC transmission technology, with modular multilevel voltage source converters used at both offshore and onshore stations to transmit offshore wind power. This technology suffers from drawbacks such as large and heavy converters, high losses, and high overall system investment costs. Summary of the Invention

[0005] The purpose of this invention is to provide a hybrid DC transmission system for offshore wind power transmission, which combines the advantages of voltage source converters and current source converters to achieve a hybrid DC transmission system and improve the operational flexibility and reliability of the offshore wind power transmission system.

[0006] To achieve the above objectives, the technical solution adopted by this utility model is as follows: a hybrid DC transmission system for transmitting offshore wind power, comprising an offshore wind farm, an offshore converter station, a DC transmission line, and an onshore converter station;

[0007] The electrical energy output from the wind turbines in the offshore wind farm is transformed and then collected by the collecting bus and connected to the AC input terminal of the offshore converter station;

[0008] The converter of the offshore converter station adopts a voltage source converter, and the converter of the onshore converter station includes a current source converter and a voltage source converter connected in parallel.

[0009] The DC output terminal of the offshore converter station is connected to a DC transmission line to transmit electrical energy to the onshore converter station.

[0010] The DC input terminals of the current source converter and voltage source converter of the onshore converter station are respectively connected to the power output terminals of the DC transmission line, and the AC output terminals of the current source converter and voltage source converter of the onshore converter station are connected in parallel to the AC power grid.

[0011] Optionally, the voltage source converter of the offshore converter station adopts a modular multilevel voltage source converter.

[0012] Optionally, the voltage source converter of the onshore converter station adopts a modular multilevel voltage source converter, and the current source converter adopts a 12-pulse converter. The two types of converters connected in parallel can switch operating states as needed at different stages of offshore wind power transmission, improving the operational flexibility of the offshore wind power transmission system.

[0013] Optionally, the 12-pulse converter is composed of two 6-pulse converters connected in series, wherein the series connection point of the two 6-pulse converters is grounded.

[0014] Optionally, each arm of the 6-pulse converter is connected in series with a set number of semi-controlled thyristors and fully controlled IGCTs, so that the arm has controllable turn-off capability.

[0015] Optionally, the ratio of semi-controlled thyristors and fully controlled IGCTs connected in series on each arm of the 6-pulse converter is 2:1.

[0016] Optionally, the DC transmission line is a high-voltage DC submarine cable.

[0017] Beneficial effects

[0018] In the power transmission system of this utility model, the offshore converter station adopts a modular multilevel converter, and the onshore converter station adopts a parallel structure of voltage source converter and current source converter. By switching the converter at different operating stages, the flexible DC power transmission system can be operated economically.

[0019] Each arm of the current source converter in the land-based converter station adopts a thyristor and IGCT series structure design. Under normal operating conditions, it can rely on the grid voltage phase-to-phase short circuit for natural commutation, which can reduce converter losses. Under abnormal operating conditions of the AC system, it can avoid commutation failure through forced commutation, thereby improving the operational stability of the AC / DC transmission system.

[0020] The voltage source converter of the onshore converter station can charge and start the offshore converter station during the startup phase, and switch to a STATCOM-like operation mode during the power transmission phase, which can support the stability of the onshore AC power grid and improve the operational stability of the entire AC / DC power transmission system. Attached Figure Description

[0021] Figure 1 The diagram shown is a schematic diagram of the topology of the hybrid DC transmission system of this utility model;

[0022] Figure 2 The diagram shows the circuit structure of the MMC used in the voltage source converter.

[0023] Figure 3 The diagram shows the circuit structure of a submodule in MMC.

[0024] Figure 4 The diagram shows the circuit structure of the 12-pulse converter used in the current source converter. Detailed Implementation

[0025] The following description, in conjunction with the accompanying drawings and specific embodiments, provides further details.

[0026] The technical concept of this utility model is as follows: a voltage source converter (VSC) based on MMC structure is designed for the offshore converter station, and a controllable current source converter (CSC) and a voltage source converter based on MMC structure are designed for the onshore station in parallel combination to form a hybrid DC transmission system, so as to realize the transmission of offshore wind power. This system inherits the stable advantages of flexible DC transmission while reducing investment costs and converter losses.

[0027] Example 1

[0028] refer to Figure 1 This embodiment is used to realize a hybrid DC transmission system for offshore wind power, including an offshore wind farm, an offshore converter station, a DC transmission line, and an onshore converter station. The electrical energy output from the offshore wind farm turbines is transformed by the tower step-up transformer, collected by the collecting bus, and connected to the AC input terminal of the offshore converter station. The offshore converter station converts the AC power into DC power, which is then connected to the onshore converter station via a submarine high-voltage DC cable. The onshore converter station converts the DC power back into AC power and then connects it to the AC power grid.

[0029] Among them, the converters of the offshore converter station are voltage source converters, while the converters of the onshore converter station include current source converters and voltage source converters connected in parallel.

[0030] like Figure 2 As shown, the voltage source converter adopts a modular multilevel voltage source converter. The bridge arm submodule adopts a half-bridge structure, and its core fully controlled devices (such as IGBTs or IGCTs) are connected in anti-parallel with freewheeling diodes. Figure 3 As shown.

[0031] The voltage source converter of the onshore converter station adopts, for example Figure 2 and Figure 3The modular multilevel voltage source converter shown employs a 12-pulse converter for the current source converter. For example... Figure 4 As shown, the 12-pulse converter is composed of two 6-pulse converters connected in series. The series connection point of the two 6-pulse converters is grounded. Each arm of the 6-pulse converter has a set number of semi-controlled thyristors and fully controlled IGCTs connected in series, enabling the arm to have controllable turn-off capability. Figure 4 In the 6-pulse converter, the ratio of semi-controlled thyristors and fully controlled IGCTs connected in series on each bridge arm is 2:1.

[0032] In this embodiment, the operation of the offshore wind power transmission system can include two stages: the startup stage and the offshore wind power stable transmission stage. The onshore station voltage source converter performs different functions in different operating stages. In the system startup stage, it completes the charging of the offshore station converter and system startup; in the offshore wind power transmission stage, it completes STATCOM-like functions. The control strategies for each stage can be designed as follows.

[0033] 1) Start-up phase

[0034] During this phase, the onshore voltage source converter is put into operation, forming a dual-ended flexible DC transmission system with the offshore voltage source converter. The onshore voltage source converter charges and unlocks the offshore converter, providing a constant frequency and amplitude AC voltage to the wind farm for grid connection of the offshore wind turbines. The offshore station control strategy during this phase employs constant frequency and AC voltage amplitude control, while the onshore voltage source converter uses constant DC voltage control for active power control and constant AC voltage or constant reactive power control for reactive power control.

[0035] 2) Stable transmission phase of offshore wind power

[0036] During this phase, the onshore current source converter is put into operation, forming a hybrid DC transmission system with the offshore voltage source converter. The offshore station's control strategy during this phase involves constant frequency and AC voltage amplitude control, while the onshore current source converter employs a constant DC voltage control strategy. Upon detecting the onshore current source converter's operation, the onshore voltage source converter automatically switches to a STATCOM-like operating mode, using either constant AC voltage or constant reactive power control.

[0037] In summary, the parallel structure of the voltage source converter and current source converter at the onshore station in this embodiment can support the economical operation of the flexible DC transmission system through converter switching at different operating stages. Each arm of the current source converter adopts a thyristor and IGCT series structure design. Under normal operating mode, it relies on the natural commutation of the grid voltage phase-to-phase short circuit to reduce converter losses. Under abnormal operating conditions of the AC system, forced commutation is used to avoid commutation failure and improve the operational stability of the AC / DC transmission system. The voltage source converter can charge and start the offshore converter station during the startup phase, and switch to a STATCOM-like operating mode during the power transmission phase to support the stability of the onshore AC grid and improve the operational stability of the entire AC / DC transmission system.

[0038] The embodiments of the present utility model have been described above with reference to the accompanying drawings. However, the present utility model is not limited to the specific embodiments described above. The specific embodiments described above are merely illustrative and not restrictive. Those skilled in the art can make many other forms under the guidance of the present utility model without departing from the spirit and scope of the claims. All of these forms are within the protection scope of the present utility model.

Claims

1. A hybrid DC transmission system for transmitting offshore wind power, comprising an offshore wind farm, an offshore converter station, a DC transmission line, and an onshore converter station; characterized in that: The electrical energy output from the wind turbines in the offshore wind farm is transformed and then collected by the collecting bus and connected to the AC input terminal of the offshore converter station; The converters in the offshore converter station are voltage source converters, while the converters in the onshore converter station include current source converters and voltage source converters connected in parallel. The voltage source converters are used during the system startup phase and the stable transmission phase of offshore wind power: during startup, they complete the charging of the offshore converter station converters and system startup, forming a dual-ended flexible DC transmission system with the voltage source converters of the offshore converter station; during the stable transmission phase of offshore wind power, they perform STATCOM-like functions. The current source converters are used during the stable transmission phase of offshore wind power, forming a hybrid DC transmission system with the voltage source converters of the offshore converter station. The DC output terminal of the offshore converter station is connected to a DC transmission line to transmit electrical energy to the onshore converter station. The DC input terminals of the current source converter and voltage source converter of the onshore converter station are respectively connected to the power output terminals of the DC transmission line, and the AC output terminals of the current source converter and voltage source converter of the onshore converter station are connected in parallel to the AC power grid.

2. The hybrid DC transmission system for transmitting offshore wind power according to claim 1, characterized in that: The voltage source converter of the offshore converter station adopts a modular multilevel voltage source converter.

3. The hybrid DC transmission system for transmitting offshore wind power according to claim 1, characterized in that: The voltage source converter of the onshore converter station adopts a modular multilevel voltage source converter.

4. The hybrid DC transmission system for transmitting offshore wind power according to claim 3, characterized in that: The current source converter adopts a 12-pulse converter.

5. The hybrid DC transmission system for transmitting offshore wind power according to claim 4, characterized in that: The 12-pulse converter is composed of two 6-pulse converters connected in series, wherein the series connection point of the two 6-pulse converters is grounded.

6. The hybrid DC transmission system for transmitting offshore wind power according to claim 5, characterized in that: Each arm of the 6-pulse converter is connected in series with a set number of semi-controlled thyristors and fully controlled IGCTs.

7. The hybrid DC transmission system for transmitting offshore wind power according to claim 6, characterized in that: The ratio of semi-controlled thyristors to fully controlled IGCTs connected in series on each arm of the 6-pulse converter is 2:

1.

8. The hybrid DC transmission system for transmitting offshore wind power according to claim 1, characterized in that: The DC transmission line is a high-voltage DC submarine cable.

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