Offshore wind power transmission system and control method thereof

Abstract

The present invention discloses an offshore wind power transmission system and control method thereof. The converter at the offshore sending-end converter station adopts a six-pulse bridge topology. One half-bridge of the converter is composed entirely of thyristors, and the other half-bridge is composed entirely of diodes. When a DC-side fault occurs in the system, the trigger angle of the thyristors is adjusted to switch the thyristors to an inverter state. Combined with the forward conduction characteristics of the diodes, the rectifier converter does not output the fault current. The DC voltage is then rapidly reduced through control of the inverter at the onshore receiving-end converter station, thereby reducing the DC current at the fault point and clearing the fault. This solves the technical problem that offshore wind power transmission systems using unidirectional current-type uncontrolled rectifier converters based on diode elements at the sending end cannot clear DC-side faults in a timely manner.

Description

Technical Field

[0001] The present invention relates to the technical field of offshore wind power, and in particular to an offshore wind power transmission system and a control method thereof. Background Art

[0002] At present, offshore wind power mainly adopts the transmission mode of nearshore AC and offshore DC, and the capacity of a single project is about one million kilowatts. The AC transmission scheme cannot meet the needs of large-scale deep-sea offshore wind power development, while the traditional DC transmission scheme is high in cost and cannot meet the needs of large-scale offshore wind power economic development. It is necessary to explore new low-cost transmission schemes to provide new technical options for deep-sea offshore wind power development. At present, in practical applications, the mainstream scheme for DC transmission is the flexible DC transmission technology based on the MMC (Modular Multilevel Converter) topology, but its platform is large in size and weight, and the cost is high. The use of a unidirectional current-type uncontrolled rectifier converter based on diode elements at the sending end helps to achieve lightweight offshore platforms. Its topology is as follows: Figure 1 However, due to the unidirectional current characteristics of this solution, when a fault occurs on the DC side, the fault current continues to be injected into the fault point, resulting in the DC fault not being cleared in a timely manner, thus limiting the practical application of this topology. Therefore, how to solve the technical defect of offshore wind power transmission systems that use unidirectional current-type uncontrolled rectifier converters based on diode elements at the sending end and cannot clear DC side faults in a timely manner, while maintaining low cost and small size, is a technical problem that technicians in this field urgently need to solve. Summary of the Invention

[0003] The present invention provides an offshore wind power transmission system and a control method thereof, which are used to solve the technical problem that an offshore wind power transmission system using a unidirectional current-type uncontrolled rectifier converter based on diode elements at the sending end cannot clear DC side faults in a timely manner on the basis of low cost and small size.

[0004] In view of this, the first aspect of the present invention provides an offshore wind power transmission system, including an offshore sending-end converter station and an onshore receiving-end converter station, wherein the offshore sending-end converter station is connected to the onshore receiving-end converter station via a DC line, and the converter of the offshore sending-end converter station is a six-pulse bridge topology structure, wherein one half-bridge of the converter is entirely composed of thyristors, and the other half-bridge of the converter is entirely composed of diodes.

[0005] Optionally, the upper half bridge of the converter is entirely composed of thyristors, and the lower half bridge of the converter is entirely composed of diodes.

[0006] Optionally, the lower half bridge of the converter is entirely composed of thyristors, and the upper half bridge of the converter is entirely composed of diodes.

[0007] Optionally, a normal operation trigger angle of the thyristor is 0-90°, and a trigger angle of the thyristor when a DC side fault occurs in the offshore wind power transmission system is 140-160°.

[0008] Optionally, a trigger angle of the thyristor when a DC side fault occurs in the offshore wind power transmission system is 150°.

[0009] A second aspect of the present invention provides a method for controlling an offshore wind power transmission system according to any one of the first aspects, comprising:

[0010] When the offshore wind power transmission system is operating normally, adjusting the trigger angle of the thyristor within the normal operating trigger angle range of the thyristor according to the operating requirements of the offshore wind power transmission system;

[0011] When a DC side fault occurs in the offshore wind power transmission system, adjusting the trigger angle of the thyristor so that the thyristor is converted to an inverter state, and transmitting the DC voltage signal to the onshore receiving converter station;

[0012] After receiving the DC voltage signal, the onshore receiving converter station controls the DC voltage signal to And after maintaining the second preset time, the DC voltage signal is controlled to zero, wherein K is the control voltage coefficient, is the DC rated voltage.

[0013] Optionally, when a DC side fault occurs in the offshore wind power transmission system, adjusting the trigger angle of the thyristor so that the thyristor is converted to an inverter state and transmits a DC voltage signal to the onshore receiving converter station includes:

[0014] When a DC side fault occurs in the offshore wind power transmission system, the trigger angle of the thyristor is smoothly changed to 150° within a third preset time, so that the thyristor is converted to an inverter state and transmits a DC voltage signal to the onshore receiving converter station.

[0015] Optionally, when a DC side fault occurs in the offshore wind power transmission system, the trigger angle of the thyristor is adjusted so that the thyristor is switched to an inverter state, and a DC voltage signal is transmitted to the onshore receiving converter station, and the method further includes:

[0016] The DC side current of the offshore wind power transmission system is detected, and when the rising rate of the DC side current is greater than or equal to 1 kA / ms, it is determined that a DC side fault occurs in the offshore wind power transmission system.

[0017] Optionally, the control voltage coefficient is 0.2~0.5.

[0018] Optionally, the first preset time is 10ms and the second preset time is 20ms.

[0019] Optionally, the third preset time is 10ms.

[0020] From the above technical solutions, it can be seen that the offshore wind power transmission system provided by the present invention has the following advantages:

[0021] The offshore wind power transmission system provided by the present invention has a converter at the offshore sending-end converter station with a six-pulse bridge topology. One half-bridge of the converter is entirely composed of thyristors, and the other half-bridge is entirely composed of diodes. When a DC side fault occurs in the system, the trigger angle of the thyristor is adjusted to convert the thyristor into an inverter state. Combined with the forward conduction characteristics of the diode, the rectifier converter does not output the fault current to the outside. Then, through the control of the inverter at the onshore receiving-end converter station, the DC voltage is rapidly reduced, thereby reducing the DC current at the fault point to clear the fault. By controlling the fault current through thyristors, the problem of the inability of a pure diode system to clear DC side faults is solved, while also maintaining the technical characteristics of the diode system in terms of low cost and small size. On the basis of low cost and small size, the technical problem of the inability of offshore wind power transmission systems using unidirectional current-type uncontrolled rectifier converters based on diode elements at the sending end to clear DC side faults in a timely manner is solved. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other related drawings can be obtained based on these drawings without paying any creative work.

[0023] Figure 1 A schematic diagram of the topology of an offshore wind power transmission system using a unidirectional current-type uncontrolled rectifier converter based on diode elements at the sending end in the prior art;

[0024] Figure 2 A schematic diagram of the topological structure of an offshore wind power transmission system provided in an embodiment of the present invention;

[0025] Figure 3 The present invention provides a flow chart of a method for controlling an offshore wind power transmission system. DETAILED DESCRIPTION

[0026] In order to enable those skilled in the art to better understand the solutions of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts shall fall within the scope of protection of the present invention.

[0027] For easier understanding, see Figure 2 The present invention provides an embodiment of an offshore wind power transmission system, including an offshore sending-end converter station and an onshore receiving-end converter station. The offshore sending-end converter station is connected to the onshore receiving-end converter station through a DC line. The converter of the offshore sending-end converter station has a six-pulse bridge topology structure. One half-bridge of the converter is entirely composed of thyristors, and the other half-bridge of the converter is entirely composed of diodes.

[0028] It should be noted that if Figure 2 As shown, converters A and B are converters for the offshore sending-end converter station. Converters A and B adopt a six-pulse bridge topology, with one half-bridge consisting entirely of thyristors and the other half-bridge consisting entirely of diodes. Specifically, the converter for the offshore sending-end converter station can have the upper half-bridge consisting entirely of thyristors and the lower half-bridge consisting entirely of diodes. The converter for the offshore sending-end converter station can also have the lower half-bridge consisting entirely of thyristors, with the upper half-bridge of the converter consisting entirely of diodes. The cost of diodes is much lower than that of thyristors, and the volume is much smaller than that of MMCs. If the six-pulse bridge is composed entirely of diodes, it will not be possible to quickly clear the fault on the DC side. In the present invention, one half-bridge of the six-pulse bridge is composed entirely of thyristors, and thyristors are used to control the fault current and clear the DC side fault. The other half-bridge is composed entirely of diodes, maintaining the technical characteristics of the diode system's low cost and small volume.

[0029] The working principle of the offshore wind power transmission system provided in the present invention is:

[0030] When the offshore wind power transmission system is operating normally, the thyristor trigger angle can be adjusted within a range of 0-90° (i.e., the normal operating angle of the thyristor is 0-90°) based on the offshore wind power transmission system's operating requirements, thereby meeting the requirements of the thyristor converter. When a DC-side fault occurs in the offshore wind power transmission system, the thyristor trigger angle is adjusted to 140-160°, preferably 150°, causing the thyristor to switch to an inverter state. This, combined with the forward conduction characteristics of the diode, prevents the rectifier converter from outputting fault current. When the inverter at the onshore receiving converter station (i.e., a flexible DC inverter composed of an MMC structure) receives the DC voltage signal transmitted from the rectifier converter, it controls the DC voltage signal to drop rapidly, thereby reducing the DC current at the fault point and clearing the fault.

[0031] The offshore wind power transmission system provided by the present invention has a converter at the offshore sending-end converter station with a six-pulse bridge topology. One half-bridge of the converter is entirely composed of thyristors, and the other half-bridge is entirely composed of diodes. When a DC side fault occurs in the system, the trigger angle of the thyristor is adjusted to convert the thyristor into an inverter state. Combined with the forward conduction characteristics of the diode, the rectifier converter does not output the fault current to the outside. Then, through the control of the inverter at the onshore receiving-end converter station, the DC voltage is rapidly reduced, thereby reducing the DC current at the fault point to clear the fault. By controlling the fault current through thyristors, the problem of the inability of a pure diode system to clear DC side faults is solved, while also maintaining the technical characteristics of the diode system in terms of low cost and small size. On the basis of low cost and small size, the technical problem of the inability of offshore wind power transmission systems using unidirectional current-type uncontrolled rectifier converters based on diode elements at the sending end to clear DC side faults in a timely manner is solved.

[0032] For easier understanding, see Figure 3 The present invention provides an embodiment of a control method applied to any offshore wind power transmission system provided in the present invention, comprising:

[0033] Step 101: When the offshore wind power transmission system is operating normally, adjust the trigger angle of the thyristor within the normal operating trigger angle range of the thyristor according to the operating requirements of the offshore wind power transmission system.

[0034] It should be noted that when the offshore wind power transmission system is operating normally, the thyristor can adjust the trigger angle within the angle range of 0~90° according to the operating requirements of the offshore wind power transmission system to meet the commutation requirements of the thyristor.

[0035] Step 102: When a DC side fault occurs in the offshore wind power transmission system, the trigger angle of the thyristor is adjusted so that the thyristor is switched to the inverter state, and the DC voltage signal is transmitted to the onshore receiving converter station.

[0036] It should be noted that the DC side current of the offshore wind power transmission system is detected. When the DC side current rises rapidly, that is, when the DC side current rise rate is greater than or equal to 1kA / ms, a DC side fault is determined to have occurred in the offshore wind power transmission system. The fault clearing procedure is initiated, and the trigger angle of the thyristors is smoothly changed from the current operating angle to 140° to 160°, preferably 150°, within a third preset time, so that the thyristors enter the inverter state and transmit the DC voltage signal to the onshore receiving converter station. The third preset time is 10ms.

[0037] Step 103: After receiving the DC voltage signal, the onshore receiving converter station controls the DC voltage signal to And after maintaining the second preset time, the DC voltage signal is controlled to zero, wherein K is the control voltage coefficient, is the DC rated voltage.

[0038] It should be noted that when the flexible DC inverter of the onshore receiving terminal receives the DC voltage signal, it controls the DC voltage signal to ,in, is the DC rated voltage, K is the control voltage coefficient, and the value of the control voltage coefficient K is 0.2~0.5. After the DC voltage signal is controlled to zero after the state is maintained for a second preset time, the flexible DC inverter at the onshore receiving converter station controls the rapid drop of the DC voltage, thereby reducing the DC current at the fault point and clearing the fault. In one embodiment, the first preset time is 10ms and the second preset time is 20ms.

[0039] The control method for an offshore wind power transmission system provided by the present invention is applicable to any offshore wind power transmission system provided by the present invention. When a DC side fault occurs in the system, the trigger angle of the thyristor is adjusted to convert the thyristor to an inverter state. Combined with the forward conduction characteristics of the diode, the rectifier converter is prevented from outputting the fault current. Then, the DC voltage is rapidly reduced by controlling the inverter at the onshore receiving converter station, thereby reducing the DC current at the fault point and clearing the fault. By controlling the fault current through the thyristor, the problem of the inability of a pure diode system to clear DC side faults is solved, while maintaining the technical characteristics of the diode system in terms of low cost and small size. On the basis of low cost and small size, the technical problem of the inability of offshore wind power transmission systems using unidirectional current-type uncontrolled rectifier converters based on diode elements at the sending end to clear DC side faults in a timely manner is solved.

[0040] The terms "first," "second," "third," and the like in the present invention are used to distinguish similar objects and are not necessarily used to describe a particular order or precedence. It should be understood that the terms used in this manner are interchangeable where appropriate, so that the embodiments of the present invention described herein can, for example, be implemented in an order other than those illustrated or described herein. In addition, the terms "including" and "having," as well as any variations thereof, are intended to cover non-exclusive inclusions. For example, a process, method, system, product, or apparatus comprising a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units that are not explicitly listed or that are inherent to such processes, methods, products, or apparatus.

[0041] As described above, the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit the same. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that the technical solutions described in the above embodiments can still be modified, or some of the technical features thereof can be replaced by equivalents. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. An offshore wind power transmission system, characterized in that: The invention comprises an offshore sending-end converter station and an onshore receiving-end converter station, wherein the offshore sending-end converter station is connected to the onshore receiving-end converter station via a DC line, and the converter of the offshore sending-end converter station has a six-pulse bridge topology structure, wherein one half-bridge of the converter is entirely composed of thyristors, and the other half-bridge of the converter is entirely composed of diodes; When the offshore wind power transmission system is in operation, it operates using the following control method: When the offshore wind power transmission system is operating normally, adjusting the trigger angle of the thyristor within the normal operating trigger angle range of the thyristor according to the operating requirements of the offshore wind power transmission system; When a DC side fault occurs in the offshore wind power transmission system, adjusting the trigger angle of the thyristor so that the thyristor is converted to an inverter state, and transmitting the DC voltage signal to the onshore receiving converter station; After receiving the DC voltage signal, the onshore receiving converter station controls the DC voltage signal to And after maintaining the second preset time, the DC voltage signal is controlled to zero, wherein K is the control voltage coefficient, is the DC rated voltage.

2. The offshore wind power transmission system according to claim 1, characterized in that: The upper half bridge of the converter is entirely composed of thyristors, and the lower half bridge of the converter is entirely composed of diodes.

3. The offshore wind power transmission system according to claim 1, characterized in that: The lower half bridge of the converter is entirely composed of thyristors, and the upper half bridge of the converter is entirely composed of diodes.

4. The offshore wind power transmission system according to any one of claims 1 to 3, characterized in that: The normal operation trigger angle of the thyristor is 0-90 degrees, and the trigger angle of the thyristor when a DC side fault occurs in the offshore wind power transmission system is 140-160 degrees.

5. The offshore wind power transmission system according to claim 4, characterized in that: The trigger angle of the thyristor when a DC side fault occurs in the offshore wind power transmission system is 150°.

6. The offshore wind power transmission system according to claim 1, characterized in that: When a DC side fault occurs in the offshore wind power transmission system, adjusting the trigger angle of the thyristor so that the thyristor is converted to an inverter state and transmitting a DC voltage signal to the onshore receiving-end converter station includes: When a DC side fault occurs in the offshore wind power transmission system, the trigger angle of the thyristor is smoothly changed to 150° within a third preset time, so that the thyristor is converted to an inverter state and transmits a DC voltage signal to the onshore receiving converter station.

7. The offshore wind power transmission system according to claim 6, characterized in that: When a DC side fault occurs in the offshore wind power transmission system, the trigger angle of the thyristor is adjusted so that the thyristor is converted to an inverter state, and a DC voltage signal is transmitted to the onshore receiving converter station, and the method further includes: The DC side current of the offshore wind power transmission system is detected, and when the rising rate of the DC side current is greater than or equal to 1 kA / ms, it is determined that a DC side fault occurs in the offshore wind power transmission system.

8. The offshore wind power transmission system according to claim 1, characterized in that: The control voltage coefficient is 0.2~0.

5.

9. The offshore wind power transmission system according to claim 1, characterized in that: The first preset time is 10ms, and the second preset time is 20ms.

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