Novel bipolar offshore direct current sending-out topological structure and control method

Abstract

The invention relates to a novel bipolar offshore direct current sending-out topological structure and a control method, the structure comprises a flexible direct current power transmission system, a high-voltage direct current power transmission line, an offshore wind power plant and an onshore power grid, the flexible direct current power transmission system is composed of a flexible direct current receiving end converter station, a flexible direct current sending end converter station and a wind power plant transformer bank, the flexible direct current receiving end converter station and the flexible direct current transmitting end converter station both adopt an M3C main circuit topological structure, and the flexible direct current receiving end converter station and the flexible direct current transmitting end converter station are connected through a direct current transmission line. Compared with the prior art, the method has the advantages of reducing the power transmission cost, improving the electric energy transmission reliability and the like.

Description

Technical Field

[0001] The present invention relates to the technical field of offshore direct current power transmission, and in particular to a novel bipolar offshore direct current transmission topology structure and a control method. Background Art

[0002] For large-scale long-distance bipolar power transmission at sea, the existing technology generally adopts the technology of connecting two groups of converter valves in series, one group of converter valves outputs positive voltage, and the other group of converter valves outputs negative voltage, and the two groups share a midpoint submarine cable. One of the implementation schemes is the dual converter valve transmission scheme. The offshore wind farm adopts the mainstream variable speed constant frequency AC wind turbines on the market (the output AC voltage of the wind turbine is usually 35kV or 66kV, and 110kV is currently being explored). After passing through the booster station, it is converted into high-voltage direct current through two flexible direct current sending-end converter stations, and reaches the two receiving-end converter stations through the high-voltage direct current transmission line. Finally, the electric energy is transmitted to the onshore power grid through the step-down station. The scheme adopts the MMC topology structure, which consists of six bridge arms. Each bridge arm is composed of N sub-modules and an inductor in series. Each sub-module includes 4 IGBTs and an intermediate capacitor C. The dual converter valve DC transmission scheme is as follows: Figure 1 As shown, the structure of MMC is as follows Figure 2 As shown. The second implementation scheme is the unipolar DC transmission scheme, which uses a set of converter valves to pass through the sending end converter station, and then transmits it to the step-down station through a DC transmission cable, and finally transmits it to the power grid. The main advantage of this technical solution is that it only requires a set of converter valves, which is more economical without considering the occurrence of failures, and can basically achieve the goal of long-distance power transmission. The unipolar DC transmission scheme is as follows Figure 3 shown.

[0003] The main disadvantage of the dual-valve DC transmission scheme is that two sets of converter valves are required to achieve bipolar voltage output, which is costly and difficult to maintain, restricting its application in engineering. The main disadvantage of the unipolar DC transmission scheme is that only one set of converter valves is used, which reduces construction and maintenance costs, but once a fault occurs on the DC side, the entire DC system will trip, unable to transmit power, and have low reliability. Summary of the invention

[0004] The purpose of the present invention is to provide a new topological structure and control method for bipolar offshore direct current transmission in order to reduce the transmission cost and improve the reliability of transmitted power.

[0005] A new type of bipolar offshore direct current transmission topology structure, the structure includes: a flexible direct current transmission system, a high voltage direct current transmission line, an offshore wind farm and an onshore power grid, the flexible direct current transmission system is composed of a flexible direct current receiving end converter station, a flexible direct current sending end converter station and a wind farm transformer group, the flexible direct current receiving end converter station and the flexible direct current sending end converter station both adopt an M3C main circuit topology structure, and the flexible direct current receiving end converter station and the flexible direct current sending end converter station are connected by a direct current transmission line;

[0006] The M3C main circuit topology structure is composed of nine bridge arms, each of which is composed of N sub-modules and an inductor connected in series.

[0007] Furthermore, the submodule includes 4 IGBTs and an intermediate capacitor C.

[0008] Furthermore, the four IGBTs are connected in series in pairs to form two series groups, the two series groups are connected in parallel, and the intermediate capacitor C is connected in parallel to the series groups.

[0009] Furthermore, one end of each bridge arm is connected to a series inductor, and the other end is connected to a load resistance inductor.

[0010] Further, the load resistance and inductance are a load resistance and a load inductance connected in series.

[0011] Furthermore, the converter station is provided with a bipolar high-voltage DC port and an AC port, and the bipolar high-voltage DC ports of the flexible DC receiving-end converter station and the flexible DC sending-end converter station are connected to each other.

[0012] Furthermore, the AC port of the flexible DC receiving end converter station is connected to the wind farm step-down transformer of the wind farm transformer group, and the wind farm step-down transformer is connected to the onshore power grid.

[0013] Furthermore, the AC port of the flexible direct current sending end converter station is connected to the wind farm step-up transformer of the wind farm transformer group, and the wind farm step-up transformer is connected to the offshore wind farm.

[0014] Another aspect of the present invention provides a control method for the above-mentioned novel bipolar offshore direct current transmission topology structure, the method comprising:

[0015] By using the full rank theory in matrix theory, the capacitor-voltage equations of the M3C main circuit topology are listed, and then the coefficient matrix of the capacitor-voltage equation is guaranteed to be full rank, ensuring that each capacitor-voltage equation has only one solution, thus achieving capacitor-voltage self-balancing.

[0016] Furthermore, the capacitor-voltage equation of the M3C main circuit topology is:

[0017]

[0018] in V cxis the capacitor voltage of each bridge arm submodule, x is the submodule number, x=1, 2, ..., 6, V dc is the DC side voltage. When the DC bus voltage is V dc When the coefficient matrix Y 1 Contains all possible submodule switching states. When the DC bus voltage is 0, the coefficient matrix Y 2 Contains all possible submodule switching states. When the DC bus voltage is -V dc When the coefficient matrix Y 3 Contains all possible submodule switching states, and the coefficient matrix is ​​full rank. C1 =V C2 =V C3 =V C4 =V C5 =V C6 =V dc , then the three-level M3C is said to have self-voltage balancing characteristics.

[0019] The purpose of the present invention can be achieved by the following technical solutions:

[0020] Compared with the prior art, the present invention has the following beneficial effects:

[0021] The present invention proposes a converter station constructed with an M3C main circuit topology structure. The bridge arm multiplexing principle is utilized to save one set of converter valves compared with the prior art of two sets of converter valves, thereby reducing the transmission cost. At the same time, the switch matrix switching control scheme is simple to control, and there is no need to sample and sort the capacitor voltage. Only the switching matrix control switch tube needs to be designed, and no additional control is required, thereby improving the reliability of transmitted power. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 It is a double-converter-valve DC transmission scheme;

[0023] Figure 2 It is an MMC topology;

[0024] Figure 3 It is a unipolar DC transmission scheme;

[0025] Figure 4 The bipolar direct current transmission scheme of the present invention;

[0026] Figure 5 This is the M3C topology diagram;

[0027] Figure 6 It is a bipolar DC voltage waveform;

[0028] Figure 7 is the output current waveform;

[0029] Figure 8is a single capacitor voltage waveform;

[0030] Fig. 9 is the overall capacitor voltage waveform. DETAILED DESCRIPTION

[0031] The present invention is described in detail below in conjunction with the accompanying drawings and specific embodiments. This embodiment is implemented based on the technical solution of the present invention, and provides a detailed implementation method and specific operation process, but the protection scope of the present invention is not limited to the following embodiments.

[0032] The main disadvantage of the dual-valve DC transmission scheme is that two sets of converter valves are required to achieve bipolar voltage output, which is costly and difficult to maintain, restricting its application in engineering. The main disadvantage of the unipolar DC transmission scheme is that only one set of converter valves is used, which reduces construction and maintenance costs, but once a fault occurs on the DC side, the entire DC system will trip, unable to transmit power, and have low reliability.

[0033] In view of the above problems, the present invention proposes a new topological structure of bipolar offshore direct current transmission, the structure diagram of which is shown in FIG. Figure 4 As shown, it includes a multi-port DC system formed by a flexible DC sending-end converter station and a step-up transformer, wherein the step-up transformer is connected to an offshore wind farm; a multi-port DC system formed by a flexible DC receiving-end converter station and a step-down transformer, wherein the step-down transformer is connected to an onshore power grid, and the offshore wind farm is composed of several conventional variable-speed constant-frequency wind turbines. Its control method adopts a new switch matrix switching strategy, which controls the switch tubes in each bridge arm sub-module through the switching matrix, and uses the matrix theory principle to achieve the autonomous balance of the sub-module capacitor voltage to achieve the goal of stable voltage output. The specific structure includes: a flexible DC transmission system, a high-voltage DC transmission line, an offshore wind farm and an onshore power grid. The flexible DC transmission system consists of a flexible DC receiving-end converter station, a flexible DC sending-end converter station and a wind farm transformer group. The flexible DC receiving-end converter station and the flexible DC sending-end converter station both adopt an M3C main circuit topology structure, and the flexible DC receiving-end converter station and the flexible DC sending-end converter station are connected by a DC transmission line;

[0034] The M3C main circuit topology structure is composed of nine bridge arms, each of which is composed of N sub-modules and an inductor connected in series. Figure 5 The flexible DC receiving end converter station includes a bipolar high-voltage DC port and an AC port, wherein the AC port is connected to the onshore AC power grid, and the high-voltage DC port is connected to the DC transmission line.

[0035] The flexible direct current sending end converter station includes a bipolar high-voltage direct current port and an alternating current port. The alternating current port is connected to the offshore wind farm, and the high-voltage direct current port is connected to the direct current transmission line.

[0036] The wind farm transformer group includes a step-up transformer group and a step-down transformer group, wherein the step-up transformer group is connected to the offshore wind farm, and the step-down transformer group is connected to the onshore AC power grid.

[0037] M3C main circuit topology structure, the structure consists of nine bridge arms, each bridge arm is composed of N sub-modules and an inductor in series, and each sub-module includes 4 IGBTs and an intermediate capacitor C.

[0038] For the above structure, the present invention also proposes a control method for the structure, using the full rank theory in matrix theory, by designing the on and off of each switch tube on the bridge arm of M3C, listing its capacitor voltage equation, and then ensuring that its coefficient matrix is ​​full rank, that is, ensuring that each capacitor voltage equation has and only has a unique solution, so as to achieve the purpose of capacitor voltage self-balancing.

[0039] Traditional bipolar power transmission usually requires the construction of two sets of converter valves at sea. The cost of construction, installation and subsequent maintenance doubles, which restricts its application in actual projects. The present invention saves a set of converter valves by using the bridge arm reuse principle, greatly improving the economic efficiency. The present invention is simple to control and does not require sampling and sorting of capacitor voltages; it only needs to design a switching matrix to control the switch tube, without additional control.

[0040] After simulation verification, the bipolar voltage and output current waveforms of the present invention are as follows: Figure 6 , 7 The single capacitor voltage waveform and the total capacitor voltage waveform are as follows Figure 8 , 9 As shown in FIG. 1 , under the condition that the switching matrix is ​​full rank, the submodule capacitor voltage can achieve self-balancing.

[0041] It can be verified by simulation that this invention can realize capacitor voltage self-balancing and has high reliability.

[0042] The preferred specific embodiments of the present invention are described in detail above. It should be understood that a person skilled in the art can make many modifications and changes based on the concept of the present invention without creative work. Therefore, any technical solution that can be obtained by a person skilled in the art through logical analysis, reasoning or limited experiments based on the concept of the present invention on the basis of the prior art should be within the scope of protection determined by the claims.

Claims

1. A new topological structure of bipolar offshore direct current transmission, characterized in that: The structure includes: A flexible direct current transmission system, a high-voltage direct current transmission line, an offshore wind farm and an onshore power grid, wherein the flexible direct current transmission system is composed of a flexible direct current receiving-end converter station, a flexible direct current sending-end converter station and a wind farm transformer group, wherein the flexible direct current receiving-end converter station and the flexible direct current sending-end converter station both adopt an M3C main circuit topology structure, and the flexible direct current receiving-end converter station and the flexible direct current sending-end converter station are connected by a direct current transmission line; The M3C main circuit topology structure is composed of nine bridge arms, each of which is composed of N sub-modules and an inductor connected in series.

2. A novel bipolar offshore direct current transmission topology structure according to claim 1, characterized in that: The submodule includes four IGBTs and an intermediate capacitor C.

3. A novel bipolar offshore direct current transmission topology structure according to claim 2, characterized in that: The four IGBTs are connected in series in pairs to form two series groups, the two series groups are connected in parallel, and the intermediate capacitor C is connected in parallel with the series groups.

4. A novel bipolar offshore direct current transmission topology structure according to claim 1, characterized in that: One end of each bridge arm is connected to a series inductor, and the other end is connected to a load resistor.

5. A novel bipolar offshore direct current transmission topology structure according to claim 4, characterized in that: The load resistance and inductance are the load resistance and load inductance connected in series.

6. A novel bipolar offshore direct current transmission topology structure according to claim 5, characterized in that: The converter station is provided with a bipolar high-voltage DC port and an AC port, and the bipolar high-voltage DC ports of the flexible DC receiving-end converter station and the flexible DC sending-end converter station are connected to each other.

7. A novel bipolar offshore direct current transmission topology structure according to claim 6, characterized in that: The AC port of the flexible DC receiving-end converter station is connected to the wind farm step-down transformer of the wind farm transformer group, and the wind farm step-down transformer is connected to the onshore power grid.

8. A novel bipolar offshore direct current transmission topology structure according to claim 7, characterized in that: The AC port of the flexible direct current sending end converter station is connected to the wind farm step-up transformer of the wind farm transformer group, and the wind farm step-up transformer is connected to the offshore wind farm.

9. A control method for a novel bipolar offshore direct current transmission topology structure according to any one of claims 1 to 8, characterized in that the method include: By using the full rank theory in matrix theory, the capacitor-voltage equations of the M3C main circuit topology are listed, and then the coefficient matrix of the capacitor-voltage equation is guaranteed to be full rank, ensuring that each capacitor-voltage equation has only one solution, thus achieving capacitor-voltage self-balancing.

10. The control method of a novel bipolar offshore direct current transmission topology structure according to claim 9, characterized in that: The capacitor voltage equation of the M3C main circuit topology is: in V cx is the capacitor voltage of each bridge arm submodule, x is the submodule number, x=1, 2, ..., 6, V dc is the DC side voltage. When the DC bus voltage is V dc When the DC bus voltage is 0, the coefficient matrix Y1 contains all possible submodule switching states. When the DC bus voltage is -V dc When V C1 =V C2 =V C3 =V C4 =V C5 =V C6 =V dc , then the three-level M3C is said to have self-voltage balancing characteristics.

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