A series DC transmission system and a power balance method

Abstract

This application discloses a series DC transmission system and a power balancing method. The solution provided by this application adds a balancing unit between the converter units of the converter stations at both ends of the DC system. The balancing unit is connected to the AC bus of the second converter station to balance the power on the AC bus of the second converter station, thereby achieving power balance of the entire system, avoiding power limitation, and realizing internal power balance of the series DC system. It is better suited for the isolated grid transmission of large-scale new energy systems in remote areas.

Description

Technical Field

[0001] This application relates to the field of DC power transmission technology, and in particular to a series DC power transmission system and a power balancing method. Background Technology

[0002] Currently, my country is gradually developing large-scale new energy systems in the Northwest and Southwest regions, especially photovoltaic systems in Tibet. The unique terrain of the western region, with most areas located at high altitudes, presents challenges in external insulation, making it unsuitable for the construction of DC converter stations. Furthermore, the weak power grid in the west means that large-scale transmission of new energy sources means that nearby AC systems cannot handle excessive loads, necessitating the use of islanded grids—transmissions that are not connected to the local power grid.

[0003] Currently, a series DC topology has been proposed for transmitting new energy, as shown in the following figure. Figure 1 As shown, this topology adopts a series DC structure, with converter station A and converter station B connected by a DC line. The operating voltage of converter station A is U. dcn Then the DC voltage of converter station B is 2×U dcn The DC voltage is increased by using a series connection. Because it is a series structure, the DC current of converter station A must be consistent with that of converter station B. At the same time, in order to keep the DC insulation level basically unchanged, the voltage of the two units must be kept basically consistent. Therefore, the DC power of converter station A and converter station B is basically the same.

[0004] However, in practical applications, the randomness and volatility of renewable energy systems must be considered, making it difficult to guarantee that the power output of two converter stations remains consistent in real time. Therefore, the two converter stations must be connected through the local AC power grid to ensure power balancing when renewable energy power fluctuates. However, for large-scale renewable energy systems transmitting power from isolated grids in remote areas, the local power grid is relatively weak and struggles to accept large-scale renewable energy power transfers, making it difficult to achieve effective power balancing. Summary of the Invention

[0005] This application provides a series DC transmission system and a power balancing method to solve the technical problem that existing technologies struggle to achieve effective power balancing when transmitting large-scale renewable energy power from isolated grids in remote areas, due to the relatively weak local power grid.

[0006] The first aspect of this application provides a series DC transmission system, including: a first converter station, a second converter station, and a balancing unit, wherein the first converter station and the second converter station are connected by a DC line;

[0007] The balancing unit is connected to the DC and AC lines on the side of the second converter station and is used to balance the power on the AC bus of the second converter station based on the power comparison results between the first converter station and the second converter station on the AC side.

[0008] Preferably, the balancing unit specifically includes a converter subunit and a transformer subunit, wherein the converter subunit is connected to the transformer subunit.

[0009] Preferably, the balancing unit is connected to the DC and AC lines on the second converter station side, specifically including:

[0010] The converter subunit is connected to the DC line on the side of the second converter station so as to be connected to the first converter station and the second converter station through the DC line;

[0011] The transformer subunit is connected to the AC bus on the second converter station side.

[0012] Preferably, when the AC power of the first converter station is greater than the AC power of the second converter station, the balancing unit is specifically used to invert the power input from the first converter station to the AC bus of the second converter station, and the converter of the second converter station then rectifies the AC bus power to the DC side.

[0013] Preferably, when the AC side power of the first converter station is less than the AC side power of the second converter station, the balancing unit is specifically used to rectify the AC bus power of the second converter station to the DC side.

[0014] A second aspect of this application provides a power balancing method for a series DC transmission system, the method being applied to the series DC transmission system provided in the first aspect of this application, the method comprising:

[0015] The balancing unit balances the power on the AC bus of the second converter station based on the comparison results of the AC side power of the first converter station and the second converter station.

[0016] Preferably, balancing the power on the AC bus of the second converter station based on the AC-side power comparison results specifically includes:

[0017] When the AC power of the first converter station is greater than the AC power of the second converter station, the power transmitted from the first converter station is inverted to the AC bus of the second converter station, and the converter of the second converter station then rectifies the AC bus power to the DC side.

[0018] Preferably, balancing the power on the AC bus of the second converter station based on the AC-side power comparison results specifically includes:

[0019] When the AC power of the first converter station is less than the AC power of the second converter station, the AC bus power of the second converter station is rectified to the DC side.

[0020] As can be seen from the above technical solutions, the embodiments of this application have the following advantages:

[0021] The solution provided in this application adds a balancing unit between the converter units of the converter stations at both ends of the DC system. This balancing unit is connected to the AC bus of the second converter station to balance the power on the AC bus of the second converter station, thereby achieving power balance of the entire system, avoiding power limitation, and realizing internal power balance of the series DC system. This solution is better suited for the isolated grid transmission of large-scale new energy systems in remote areas. Attached Figure Description

[0022] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0023] Figure 1 This is a topology diagram of an existing series DC transmission system.

[0024] Figure 2 A topology diagram of a series DC transmission system provided in this application. Detailed Implementation

[0025] This application provides a series DC transmission system and a power balancing method to address the technical problem that existing technologies struggle to achieve effective power balancing when transmitting large-scale renewable energy power from isolated grids in remote areas, where the local power grid is relatively weak and cannot accept the transfer of large-scale renewable energy power.

[0026] To make the inventive objectives, features, and advantages of this application more apparent and understandable, the technical solutions in the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the embodiments described below are only some embodiments of this application, and not all embodiments. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0027] First, an embodiment of a series DC transmission system provided in this application is described, as follows:

[0028] Please see Figure 2 This embodiment provides a series DC transmission system, including: a first converter station A, a second converter station B and a balancing unit C, wherein the first converter station A and the second converter station B are connected by a DC line;

[0029] The balancing unit C is connected to the DC and AC lines on the side of the second converter station B, and is used to balance the power on the AC bus of the second converter station B according to the comparison results of the AC side power of the first converter station A and the second converter station B.

[0030] More specifically, the balancing unit C specifically includes: a converter subunit and a transformer subunit, wherein the converter subunit is connected to the transformer subunit.

[0031] More specifically, the DC and AC lines connected to the balancing unit C on the side of the second converter station B specifically include:

[0032] The converter subunit is connected to the DC line on the side of the second converter station B, so as to be connected to the first converter station A and the second converter station B through the DC line;

[0033] The transformer subunit is connected to the AC bus on the B side of the second converter station.

[0034] It should be noted that the technical solution provided in this application adds a balancing unit C to the existing solution. The structure of this unit is exactly the same as that of a valve group in a DC converter station. This balancing unit C is connected to the AC bus of the second converter station B to balance the power on the AC bus of the second converter station B, thereby achieving power balance of the entire system, avoiding power limitation, and meeting the requirements of islanded grid transmission.

[0035] More specifically, when the AC power of the first converter station A is greater than the AC power of the second converter station B, the balancing unit C is specifically used to invert the power input from the first converter station A to the AC bus of the second converter station B, and the converter of the second converter station B then rectifies the AC bus power to the DC side.

[0036] More specifically, when the AC side power of the first converter station A is less than the AC side power of the second converter station B, the balancing unit C is specifically used to rectify the AC bus power of the second converter station B to the DC side.

[0037] Assume the AC power of the second converter station is P. b The AC side power of the first converter station is P. a Assume the converter's operating efficiency is η, and the transmission loss rate from station A to station B is δ.

[0038] 1) When the power of the first converter station A is greater than that of the second converter station B, the balancing unit C will act as an inverter unit to invert the power input from the first converter station A to the AC bus of the second converter station B. The converter of the second converter station B will then rectify the AC bus power to the DC side, thereby maintaining the basic balance between the two units and meeting the basic requirements of islanded grid series operation.

[0039] The power inverted to the AC side by the balancing unit C is as follows:

[0040]

[0041] In the formula,

[0042] 2) When the power of the first converter station A is less than that of the second converter station B, the balancing unit C will act as a rectifier unit to rectify the AC bus power of the second converter station B to the DC side, so that the power of the first converter station A and the power of the second converter station B are basically balanced, thereby meeting the basic requirements of islanded grid series operation.

[0043] The power rectified to the AC side by the balancing unit C is as follows:

[0044]

[0045] The above content is a detailed description of an embodiment of a series DC transmission system provided in this application. The following is a detailed description of an embodiment of a power balance method for a series DC transmission system provided in this application.

[0046] This embodiment provides a power balancing method for a series DC transmission system. The method is applied to the series DC transmission system mentioned in the first embodiment of this application, specifically the balancing unit C in the series DC transmission system. The method includes:

[0047] Based on the comparison results of the AC side power of the first converter station and the second converter station, the power on the AC bus of the second converter station is balanced according to the comparison results.

[0048] More specifically, balancing the power on the AC bus of the second converter station based on the AC side power comparison results specifically includes:

[0049] When the AC power of the first converter station is greater than the AC power of the second converter station, the power transmitted from the first converter station is inverted to the AC bus of the second converter station, and the converter of the second converter station then rectifies the AC bus power to the DC side.

[0050] More specifically, balancing the power on the AC bus of the second converter station based on the AC side power comparison results specifically includes:

[0051] When the AC power of the first converter station is less than the AC power of the second converter station, the AC bus power of the second converter station is rectified to the DC side.

[0052] In the description of this application, it should be noted that the terms "center," "upper," "lower," "left," "right," "vertical," "horizontal," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this application. Furthermore, the terms "first," "second," and "third" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0053] Unless otherwise expressly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to fixed connections, detachable connections, or integral connections; they can refer to mechanical connections or electrical connections; they can refer to direct connections or indirect connections through an intermediate medium; and they can refer to the internal connection between two components. Those skilled in the art can understand the specific meaning of the above terms in this application based on the specific circumstances.

[0054] The above-described embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of this application.

Claims

1. A series DC transmission system, characterized in that, include: The system comprises a first converter station, a second converter station, and a balancing unit, wherein the first converter station and the second converter station are connected by a DC line. The balancing unit is connected to the DC and AC lines on the second converter station side. When the AC power of the first converter station is greater than the AC power of the second converter station, the balancing unit is specifically used to invert the power input from the first converter station to the AC bus of the second converter station, and the converter of the second converter station then rectifies the AC bus power to the DC side. When the AC power of the first converter station is less than the AC power of the second converter station, the balancing unit is specifically used to rectify the AC bus power of the second converter station to the DC side.

2. The series DC transmission system according to claim 1, characterized in that, The balancing unit specifically includes a converter subunit and a transformer subunit, wherein the converter subunit is connected to the transformer subunit.

3. A series DC transmission system according to claim 2, characterized in that, The DC and AC lines connected to the balancing unit on the second converter station side specifically include: The converter subunit is connected to the DC line on the side of the second converter station so as to be connected to the first converter station and the second converter station through the DC line; The transformer subunit is connected to the AC bus on the second converter station side.

4. A power balancing method for a series DC transmission system, said method being applied to the series DC transmission system as described in any one of claims 1 to 3, characterized in that, The method includes: Based on the comparison of AC side power between the first converter station and the second converter station, the balancing unit inverts the power input from the first converter station to the AC bus of the second converter station when the AC side power of the first converter station is greater than that of the second converter station. The converter of the second converter station then rectifies the AC bus power to the DC side. When the AC side power of the first converter station is less than that of the second converter station, the AC bus power of the second converter station is rectified to the DC side.

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