Receiving end current conversion device and direct current power transmission system

Abstract

The invention provides a receiving end converter device and a DC power transmission system, and the device comprises a modular multilevel converter of which the DC side is used for being electrically connected with a DC polar line, and the AC side is used for being electrically connected with a first AC power grid; the direct-current side of the current source type current converter is electrically connected with the direct-current polar line, the alternating-current side of the current source type current converter is electrically connected with a second alternating-current power grid, and the voltage of the first alternating-current power grid is different from that of the second alternating-current power grid; and the controller is in communication connection with the modular multilevel converter and the current source type converter, and the controller is used for controlling the modular multilevel converter and the current source type converter to operate in different working modes. The problem that in the prior art, the access mode of the receiving end converter device is poor in system stability is solved.

Description

Technical Field

[0001] This application relates to the field of power electronics technology, and more specifically, to a receiving-end converter and a DC transmission system. Background Technology

[0002] With the large-scale transmission of new energy sources and the increasing demand at the receiving end, the existing DC transmission system has problems with poor system stability. Summary of the Invention

[0003] The main objective of this application is to provide a receiving-end converter device and a DC transmission system to at least solve the problem of poor system stability in the existing receiving-end converter connection method.

[0004] To achieve the above objectives, according to one aspect of this application, a receiving-end converter device is provided, comprising: a modular multilevel converter, wherein the DC side of the modular multilevel converter is electrically connected to a DC pole line, and the AC side of the modular multilevel converter is electrically connected to a first AC power grid; a current source converter, wherein the DC side of the current source converter is electrically connected to the DC pole line, and the AC side of the current source converter is electrically connected to a second AC power grid, wherein the voltage of the first AC power grid is different from the voltage of the second AC power grid; and a controller, which is communicatively connected to both the modular multilevel converter and the current source converter, and the controller is used to control the modular multilevel converter and the current source converter to operate in different operating modes.

[0005] Optionally, the controller is used to control the modular multilevel converter and the current source converter to operate in different operating modes, including: the controller is used to control the voltage of the first converter to remain constant when the first converter is in normal operating mode, wherein the first converter is the converter in the current source converter and the modular multilevel converter with a larger absolute value of the potential at one end connected to the DC pole; the controller is used to control the power of the second converter to remain constant when the second converter is in normal operating mode, wherein the second converter is the converter in the current source converter and the modular multilevel converter with a smaller absolute value of the potential at one end connected to the DC pole, and the voltage of the AC grid connected to the first converter is less than the voltage of the AC grid connected to the second converter.

[0006] Optionally, the controller is further configured to: in the event of a fault in the first AC power grid or the second AC power grid, control the active power of the second converter to reduce, so that the DC current on the DC pole line is reduced, and the reduced DC current is not less than a preset DC current threshold.

[0007] Optionally, the controller is further configured to: determine whether the load of the total power grid is greater than a first predetermined load, the total power grid including at least the first AC power grid and the second AC power grid; if the load of the total power grid is determined to be greater than the first predetermined load, control the active power output of the second converter to increase, and control the total active power to remain unchanged, the total active power being the sum of the active power output of the first converter and the active power output of the second converter.

[0008] Optionally, the AC grid connected to the first converter is the target AC grid, and the AC grid connected to the second converter is the non-target AC grid. The controller is further configured to: determine the active power of the non-target AC grid based on the load of the non-target AC grid; when the active power output by the second converter is greater than the active power of the non-target AC grid, control the non-target AC grid to transmit active power to other AC grids and / or the target AC grid; when the non-target AC grid transmits the active power to other AC grids, the non-target AC grid is electrically connected to the other AC grids; when the non-target AC grid transmits the active power to the target AC grid, the non-target AC grid is electrically connected to the target AC grid.

[0009] Optionally, the controller is further configured to: determine whether the total load is less than a second predetermined load, wherein the total load is the sum of the load of the first AC grid and the load of the second AC grid; and, if the total load is less than the second predetermined load, control the active power output of the second converter to decrease, and control the total active power to remain unchanged, wherein the total active power is the sum of the active power output of the first converter and the active power output of the second converter.

[0010] Optionally, at least controlling the voltage of the first converter to remain constant includes: when the first converter is the modular multilevel converter and the second converter is the current source converter, controlling the DC voltage of the first converter to remain constant and controlling the voltage of the AC bus to remain constant, wherein the first converter is electrically connected to the first AC grid through the AC bus; when the first converter is the current source converter and the second converter is the modular multilevel converter, controlling the DC voltage of the first converter to remain constant and controlling the reactive power of the first converter to remain constant, and controlling the power of the second converter to remain constant includes: controlling the active power of the second converter to remain constant and controlling the reactive power of the second converter to remain constant.

[0011] According to another aspect of this application, a DC transmission system is provided, comprising: a DC pole; a receiving end, the receiving end including any of the receiving end converters described above; a first AC power grid; and a second AC power grid, wherein the voltage of the first AC power grid is different from the voltage of the second AC power grid.

[0012] Optionally, the receiving-end converter includes a modular multilevel converter and a current source converter. The DC pole is a bipolar-to-ground line, and the DC pole includes a positive DC pole and a negative DC pole. There are two modular multilevel converters, namely a first modular multilevel converter and a second modular multilevel converter connected in series. There are two current source converters, namely a first current source converter and a second current source converter connected in series. The positive DC pole is grounded through the first modular multilevel converter and the first current source converter connected in series, and the negative DC pole is grounded through the second modular multilevel converter and the second current source converter connected in series.

[0013] Optionally, the first terminal of the first current source converter is electrically connected to the positive DC pole line through the first modular multilevel converter; the first terminal of the second current source converter is electrically connected to the negative DC pole line through the second modular multilevel converter; the second terminal of the first current source converter is electrically connected to the second terminal of the second current source converter, and the second terminals of the first current source converter and the second current source converter are respectively grounded; the voltage of the first AC grid is less than the voltage of the second AC grid.

[0014] Optionally, the first terminal of the first modular multilevel converter is electrically connected to the positive DC pole line through the first current source converter; the first terminal of the second modular multilevel converter is electrically connected to the negative DC pole line through the second current source converter; the second terminal of the first modular multilevel converter is electrically connected to the second terminal of the second modular multilevel converter, and the second terminals of the first modular multilevel converter and the second terminal of the second modular multilevel converter are respectively grounded; the voltage of the first AC grid is greater than the voltage of the second AC grid.

[0015] Optionally, the DC transmission system further includes: a DC filter bank, the first end of which is electrically connected to the DC pole line, and the second end of which is grounded.

[0016] Applying the technical solution of this application, the receiving-end converter device of this application includes a modular multilevel converter and a current source converter, and also includes a controller that is communicatively connected to the modular multilevel converter and the current source converter respectively. The DC side of the modular multilevel converter is electrically connected to the DC pole line, and the AC side is electrically connected to the first AC grid. The DC side of the current source converter is electrically connected to the DC pole line, and the AC side is electrically connected to the second AC grid. The voltages of the first AC grid and the second AC grid are different. The controller is used to control the modular multilevel converter and the current source converter to operate in different working modes. Compared with the poor system stability of the receiving-end converter connection method in the prior art, this application connects the modular multilevel converter and the current source converter to AC power grids of different voltage levels respectively, and the controller controls the two converters to operate in different working modes. This realizes the hierarchical connection of the receiving-end converter device, reduces the mutual interference caused by the two converters responding to the same voltage level power grid disturbance, and can effectively reduce the dynamic mutual influence between the two converters. This allows the two converters to act independently and without affecting each other when facing different types of disturbances, thereby avoiding the chain reaction that may occur in a single working mode and improving the dynamic stability of the system. Attached Figure Description

[0017] The accompanying drawings, which form part of this application, are used to provide a further understanding of this application. The illustrative embodiments and descriptions of this application are used to explain this application and do not constitute an undue limitation of this application. In the drawings:

[0018] Figure 1 A schematic diagram of the topology of a DC transmission system provided in an embodiment of this application is shown;

[0019] Figure 2 A schematic diagram of the topology of a specific DC transmission system provided in an embodiment of this application is shown;

[0020] Figure 3 A schematic diagram of the topology of another specific DC transmission system provided in an embodiment of this application is shown.

[0021] The above figures include the following reference numerals:

[0022] 10. DC pole; 11. Modular multilevel converter; 12. First AC grid; 13. Current source converter; 14. Second AC grid; 101. Positive DC pole; 102. Negative DC pole; 111. First modular multilevel converter; 112. Second modular multilevel converter; 131. First current source converter; 132. Second current source converter. Detailed Implementation

[0023] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. This application will now be described in detail with reference to the accompanying drawings and embodiments.

[0024] To enable those skilled in the art to better understand the present application, the technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present application, and not all embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative effort should fall within the scope of protection of the present application.

[0025] It should be noted that the terms "first," "second," etc., in the specification, claims, and accompanying drawings of this application are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such data can be interchanged where appropriate for the embodiments of this application described herein. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover non-exclusive inclusion; for example, a process, method, system, product, or apparatus that comprises a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to such processes, methods, products, or apparatus.

[0026] As described in the background section, the existing connection methods for receiving-end converters have the problem of poor system stability. To solve the above problem, the embodiments of this application provide a receiving-end converter and a DC transmission system.

[0027] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention.

[0028] This application provides a receiving-end converter device, such as... Figures 1 to 3 As shown, it includes:

[0029] A modular multilevel converter 11, wherein the DC side of the modular multilevel converter 11 is used to be electrically connected to the DC pole line 10, and the AC side of the modular multilevel converter 11 is used to be electrically connected to the first AC power grid 12.

[0030] A current source converter 13, wherein the DC side of the current source converter 13 is used to be electrically connected to the DC pole line 10, and the AC side of the current source converter 13 is used to be electrically connected to the second AC grid 14, wherein the voltage of the first AC grid 12 is different from the voltage of the second AC grid 14.

[0031] Specifically, the modular multilevel converter 11 is connected in series with the current source converter 13.

[0032] A controller (not shown) is communicatively connected to the modular multilevel converter 11 and the current source converter 13, respectively. The controller is used to control the modular multilevel converter 11 and the current source converter 13 to operate in different working modes.

[0033] Through the above embodiments, the receiving-end converter of this application includes a modular multilevel converter and a current source converter, and also includes a controller that is communicatively connected to the modular multilevel converter and the current source converter respectively. The DC side of the modular multilevel converter is electrically connected to the DC pole line, and the AC side is electrically connected to the first AC grid. The DC side of the current source converter is electrically connected to the DC pole line, and the AC side is electrically connected to the second AC grid. The voltages of the first AC grid and the second AC grid are different. The controller is used to control the modular multilevel converter and the current source converter to operate in different working modes. Compared with the poor system stability of the receiving-end converter connection method in the prior art, this application connects the modular multilevel converter and the current source converter to AC power grids of different voltage levels respectively, and the controller controls the two converters to operate in different working modes. This realizes the hierarchical connection of the receiving-end converter device, reduces the mutual interference caused by the two converters responding to the same voltage level power grid disturbance, and can effectively reduce the dynamic mutual influence between the two converters. This allows the two converters to act independently and without affecting each other when facing different types of disturbances, thereby avoiding the chain reaction that may occur in a single working mode and improving the dynamic stability of the system.

[0034] Specifically, the aforementioned current source converters include fully controlled current source converters or hybrid grid-commutated converters.

[0035] Specifically, the Controlled Current Source Converter (CCSC) and the Hybrid Commutation Converter (HCC) use RB-IGCT (Robust Block-type Integrated Gate Commutated Thyristor) devices.

[0036] In one alternative embodiment, the controller is used to control the modular multilevel converter and the current source converter to operate in different operating modes, including: when the first converter is in normal operating mode, the controller controls at least the voltage of the first converter to remain constant, wherein the first converter is the converter in the current source converter and the modular multilevel converter whose voltage at the end connected to the DC pole has a larger absolute value; the controller is used to control the power of the second converter to remain constant when the second converter is in normal operating mode, wherein the second converter is the converter in the current source converter and the modular multilevel converter whose voltage at the end connected to the DC pole has a smaller absolute value, and the voltage of the AC grid connected to the first converter is less than the voltage of the AC grid connected to the second converter. In this embodiment, the high-voltage valve group (i.e., the converter with a higher ground potential) is connected to the AC side with a lower voltage, and the low-voltage valve group (i.e., the converter with a lower ground potential) is connected to the AC side with a higher voltage. This causes the AC voltage stress and DC ground stress to form a misaligned and complementary relationship on the valve-side winding of the converter. The combined AC + DC insulation stress borne by the valve-side winding of the converter is smaller. This can reduce the insulation level and creepage distance requirements of the converter and its bushing, and improve the manufacturability and reliability of the converter and its bushing.

[0037] Specifically, the first converter is one of a current source converter and a modular multilevel converter, and the second converter is the other of a current source converter and a modular multilevel converter. The absolute value of the potential at the end of the first converter connected to the DC pole line is larger, and the absolute value of the potential at the end of the second converter connected to the DC pole line is smaller.

[0038] Specifically, low-voltage grids require faster local reactive power and voltage support, while high-voltage grids require more stable power transmission. The first converter is responsible for voltage support and power quality, while the second converter is responsible for stabilizing DC current and preventing commutation failure (stabilizing power). This application uses a "layered access + differentiated control" approach to ensure that each of the two different AC grids receives the most suitable converter support, thereby improving the stability of the transmission system.

[0039] According to some exemplary embodiments of this application, the controller is further configured to: in the event of a fault in the first AC power grid or the second AC power grid, control the active power of the second converter to decrease, thereby reducing the DC current on the DC pole, and ensuring that the reduced DC current is not less than a preset DC current threshold. In this embodiment, in the event of an AC power grid fault, by reducing the active power of the second converter, the decrease in DC current is controlled, ensuring that the current flowing through the first converter is small, while ensuring that the DC current is not lower than the minimum threshold required for the safe operation of the converter, thus improving the fault ride-through capability of the system.

[0040] In practical applications, those skilled in the art can set the above-mentioned DC current threshold based on experience, or obtain it through multiple experiments. This application does not impose any specific restrictions on this.

[0041] Specifically, the active power of the second converter can be reduced by lowering the reference value of the active power of the second converter; the reduced DC current should not exceed 1.1-1.5 times the rated DC current of the first converter.

[0042] According to some further exemplary embodiments of this application, the controller is further configured to: determine whether the load of the total power grid is greater than a first predetermined load, wherein the total power grid includes at least the first AC power grid and the second AC power grid; and, if the load of the total power grid is determined to be greater than the first predetermined load, control the active power output of the second converter to increase while controlling the total active power to remain unchanged, wherein the total active power is the sum of the active power output of the first converter and the active power output of the second converter. In this embodiment, a dynamic power allocation strategy based on the total power grid load is proposed. The controller monitors the total power grid load in real time and automatically adjusts the active power output of the second converter to meet the power demand of the power grid. This strategy, based on accurate prediction of the power grid load and precise control of the converter power output, realizes intelligent power scheduling between power grids. Through dynamic power allocation, it improves the operating efficiency and transmission capacity of the power grid, especially during high load periods, fully utilizing the transmission potential of the ultra-high voltage AC power grid (i.e., the AC power grid connected to the second converter).

[0043] Specifically, the total power grid may also include other power grids, which may be AC ​​power grids, DC power grids, or AC / DC power grids. This application does not impose any specific restrictions on this.

[0044] In practical applications, those skilled in the art can set the first predetermined load based on experience or through multiple experiments; this application does not impose any specific restrictions on this.

[0045] In other embodiments, the AC grid connected to the first converter is the target AC grid, and the AC grid connected to the second converter is the non-target AC grid. The controller is further configured to: determine the active power of the non-target AC grid based on its load; control the non-target AC grid to transmit active power to other AC grids and / or the target AC grid when the active power output by the second converter is greater than the active power of the non-target AC grid; when the non-target AC grid transmits active power to other AC grids, the non-target AC grid is electrically connected to those other AC grids; and when the non-target AC grid transmits active power to the target AC grid, the non-target AC grid is electrically connected to the target AC grid. In this embodiment, when the active power output by the second converter exceeds the actual demand of the non-target AC grid, the system can automatically control the non-target AC grid to transmit excess active power to other AC grids and / or the target AC grid. This mechanism can promptly allocate power to where it is needed, enhancing the system's flexibility and reliability.

[0046] Specifically, after controlling the active power output of the second converter to increase, if the active power output of the second converter is greater than the active power of the non-target AC grid, the non-target AC grid can be controlled to transmit active power to other AC grids and / or the target AC grid.

[0047] According to some other exemplary embodiments of this application, the controller is further configured to: determine whether the total load is less than a second predetermined load, wherein the total load is the sum of the load of the first AC grid and the load of the second AC grid; and, if the total load is less than the second predetermined load, control the active power output of the second converter to decrease, and control the total active power to remain unchanged, wherein the total active power is the sum of the active power output of the first converter and the active power output of the second converter. In this embodiment, power distribution is optimized during low load periods by reducing the active power output of the second converter, thereby reducing unnecessary energy consumption. Since the total active power remains unchanged, the active power output of the first converter increases, allowing the first converter to better support voltage. By monitoring the total grid load in real time and automatically adjusting the power output of the converter, the system is ensured to operate economically during low load periods, reducing energy waste, lowering operating costs, and also reducing the burden on the converter and extending the service life of the equipment.

[0048] In practical applications, those skilled in the art can set the above-mentioned second predetermined load based on empirical values, or obtain it through multiple experiments. This application does not impose any specific restrictions on this.

[0049] According to another exemplary embodiment of this application, controlling the voltage of the first converter to remain constant includes: such as Figure 2 As shown, when the first converter (not shown) is the modular multilevel converter 11 and the second converter (not shown) is the current source converter 13, the DC voltage of the first converter is kept constant, and the voltage of the AC bus (not shown) is kept constant. The first converter is electrically connected to the first AC grid 12 via the AC bus. Figure 3 As shown, when the first converter (not shown) is the current source converter 13 and the second converter (not shown) is the modular multilevel converter 11, controlling the DC voltage of the first converter to remain constant, controlling the reactive power of the first converter to remain constant, and controlling the power of the second converter to remain constant include: controlling the active power of the second converter to remain constant, and controlling the reactive power of the second converter to remain constant. In this embodiment, the first converter maintains stable DC voltage and reactive power (or AC bus voltage), while the second converter maintains constant output of active and reactive power. By adjusting the control parameters, efficient operation of the converter is achieved. This control mode can effectively cope with power fluctuations in the power grid, ensure the stability of DC voltage, and also provide stable power output.

[0050] Specifically, when the first converter is the modular multilevel converter and the second converter is the current source converter, the control mode of the modular multilevel converter adopts a constant DC voltage control mode and a constant AC bus voltage control mode, and the control mode of the current source converter adopts a constant active power control mode and a constant reactive power control mode.

[0051] Specifically, when the first converter is the current source converter and the second converter is the modular multilevel converter, the control mode of the modular multilevel converter adopts a constant active power control mode and a constant reactive power control mode, and the control mode of the current source converter adopts a constant DC voltage control mode and a constant reactive power control mode.

[0052] Specifically, the modular multilevel converter (MMC) uses a hybrid half-bridge / full-bridge sub-module to improve DC fault isolation capability, and the MMC uses Nearest Level Modulation (NLM) to reduce harmonics and losses.

[0053] Specifically, the current source converter uses a specific carrier modulation method with a carrier frequency of 150Hz, such as PWM (Pulse Width Modulation).

[0054] Specifically, the power flow distribution strategy of the receiving-end converter includes: active power distribution according to a preset ratio and independent reactive power control. This enables the system to pre-set and automatically adjust the proportion of active power sent into the AC grid from different converter stations according to the real-time demand of the grid operation. This flexible power distribution strategy improves transmission efficiency while ensuring the supply and demand balance of the grid. Independent reactive power control means that modular multilevel converters and current source converters can provide reactive power support for their respective connected AC grids without relying on active power regulation. Independent reactive power control can respond more accurately to voltage fluctuations in the AC grid, provide rapid voltage support, avoid voltage collapse, and thus enhance the dynamic stability and power quality of the entire grid.

[0055] Specifically, the priority of hierarchical coordinated control of the receiving-end converter is as follows: the first converter prioritizes voltage support, and the second converter prioritizes power stability.

[0056] In other embodiments, the controller is further configured to: collect historical load data, historical renewable energy generation data, historical weather forecast information, and historical power flow trends, wherein the historical power flow trends characterize the changes in power flow in the DC transmission system within a certain time range in the past, and the historical load data characterizes the historical load data of the first AC grid and the second AC grid; train a neural network model using the historical load data, historical renewable energy generation data, historical weather forecast information, and historical power flow trends to obtain a target neural network model; acquire current load data, current renewable energy generation data, and current weather forecast information, and then combine the current load data with the current renewable energy generation data. New energy power generation data and current weather forecast information are input into the target neural network model to obtain the current power flow trend. When the current power flow trend indicates that future new energy power generation and load fluctuations are less than a predetermined fluctuation threshold, the power sharing ratio of the first and second converters is pre-allocated to 1:1. When the current power flow trend indicates that the future increase in new energy power generation is greater than the predetermined increase in power generation, the power sharing ratio of the second converter is increased, for example, the power sharing ratio of the first and second converters is allocated to 1:2. When the current power flow trend indicates that the future increase in load is greater than the predetermined increase in load, the power sharing ratio of the second converter is further increased. Through advance prediction and adjustment, system instability caused by sudden power flow changes is effectively prevented, improving the overall reliability and safety of the DC transmission system.

[0057] In other embodiments, controlling the transmission of active power from the non-target AC grid to other AC grids and / or the target AC grid includes: determining the difference between the active power output by the second converter and the active power of the non-target AC grid to obtain residual power; determining the active power of the target AC grid based on the load of the target AC grid, and determining the active power of other AC grids based on the load of other AC grids; obtaining the ratio of the active power of the target AC grid to the active power of other AC grids; and controlling the transmission of active power from the non-target AC grid to the other AC grids and / or the target AC grid based at least on the ratio of the active power of the target AC grid to the active power of other AC grids. Based at least on the ratio of the active power of the target AC grid to the active power of other AC grids, control the transmission of active power from the non-target AC grid to the other AC grids and / or the target AC grid, including: when the ratio of the active power of the target AC grid to the active power of other AC grids is greater than a first preset ratio (the first preset ratio is greater than 1.5), and the active power of the target AC grid is greater than the remaining power, control the transmission of active power from the non-target AC grid to the target AC grid; when the ratio of the active power of the target AC grid to the active power of other AC grids is greater than the first preset ratio, and the active power of the target AC grid is not greater than the remaining power, control the transmission of active power from the non-target AC grid to the target AC grid. The system transmits active power to the target AC grid and other AC grids, and controls the active power transmitted from non-target AC grids to the target AC grid to be equal to the active power of the target AC grid. When the ratio of the active power of the target AC grid to the active power of other AC grids is not greater than a first preset ratio (the first preset ratio is greater than 1.5) and is greater than a second preset ratio (the second preset ratio is greater than 1 and less than 1.5), the system controls the transmission of active power from non-target AC grids to the target AC grid and other AC grids, and controls the ratio of the active power transmitted to the target AC grid to the active power transmitted to other AC grids to be equal to the ratio of the active power of the target AC grid to the active power of other AC grids. In this embodiment, efficient and flexible power allocation and power flow management are achieved, effectively avoiding overload situations and realizing intelligent load balancing during active power transmission between grids. This improves the power transmission efficiency and stability of the entire system and enhances the grid's ability to respond to sudden load changes.

[0058] In practical applications, those skilled in the art can set the first and second preset ratios based on experience, or obtain them through multiple experiments. This application does not impose any specific restrictions on this.

[0059] This application also provides a DC power transmission system, such as... Figures 1 to 3 As shown, it includes:

[0060] DC pole 10;

[0061] Receiving end (not shown), the aforementioned receiving end includes any of the aforementioned receiving end converter devices (not shown).

[0062] A first AC power grid 12; a second AC power grid 14, wherein the voltage of the first AC power grid 12 is different from the voltage of the second AC power grid 14.

[0063] Through the above embodiments, the DC transmission system of this application includes a DC pole line, a receiving end, a first AC grid, and a second AC grid. The receiving end includes any of the above-mentioned receiving-end converter devices. Compared with the problems of commutation failure, inflexible power flow control, weak support for AC grids, and poor system stability in the connection method of receiving-end converters in the prior art, this application connects modular multilevel converters and current source converters to AC grids of different voltage levels respectively, and the controller controls the two converters to operate in different working modes. This realizes the hierarchical connection of receiving-end converter devices, reduces the mutual interference caused by the two converters responding to the same voltage level grid disturbance, and can effectively reduce the dynamic mutual influence between the two converters. This allows the two converters to act independently and without affecting each other when facing different types of disturbances, thereby avoiding the chain reaction that may occur in a single working mode and improving the dynamic stability of the system.

[0064] In one exemplary embodiment, such as Figure 2 and Figure 3 As shown, the receiving-end converter includes a modular multilevel converter 11 and a current source converter 13. The DC pole 10 is a bipolar-to-ground line, and the DC pole 10 includes a positive DC pole 101 and a negative DC pole 102. There are two modular multilevel converters 11, namely a first modular multilevel converter 111 and a second modular multilevel converter 112 connected in series. There are two current source converters 13, namely a first current source converter 131 and a second current source converter 132 connected in series. The positive DC pole 101 is grounded through the first modular multilevel converter 111 and the first current source converter 131 connected in series, and the negative DC pole 102 is grounded through the second modular multilevel converter 112 and the second current source converter 132 connected in series. In this embodiment, by adopting a bipolar-to-ground DC pole line design, even if a single pole line fails, the other pole line can still continue to work, thereby further improving the reliability and stability of the entire DC transmission system. Furthermore, the bipolar-to-ground line combined with the series design of two modular multilevel converters and two current source converters also improves the transmission capacity of the system.

[0065] According to some exemplary embodiments of this application, such as Figure 2As shown, the first terminal of the first current source converter 131 is electrically connected to the positive DC line 101 through the first modular multilevel converter 111; the first terminal of the second current source converter 132 is electrically connected to the negative DC line 102 through the second modular multilevel converter 112; the second terminal of the first current source converter 131 is electrically connected to the second terminal of the second current source converter 132, and the second terminals of the first current source converter 131 and the second current source converter 132 are respectively grounded; the voltage of the first AC grid 12 is less than the voltage of the second AC grid 14. In this embodiment, the first current source converter and the second current source converter are connected in series with the first modular multilevel converter and the second modular multilevel converter, respectively, forming two independent DC current paths. These paths allow for independent control of the positive and negative DC currents, enabling faster power flow adjustment and preventing system instability when handling AC grid faults of different voltage levels.

[0066] In other embodiments, such as Figure 3 As shown, the first terminal of the first modular multilevel converter 111 is electrically connected to the positive DC line 101 through the first current source converter 131; the first terminal of the second modular multilevel converter 112 is electrically connected to the negative DC line 102 through the second current source converter 132; the second terminal of the first modular multilevel converter 111 is electrically connected to the second terminal of the second modular multilevel converter 112, and the second terminals of the first modular multilevel converter 111 and the second modular multilevel converter 112 are respectively grounded; the voltage of the first AC grid 12 is greater than the voltage of the second AC grid 14. In this embodiment, the first current source converter and the second current source converter are connected in series with the first modular multilevel converter and the second modular multilevel converter, respectively, forming two independent DC current paths. These paths allow for independent control of the positive and negative DC currents, enabling faster power flow adjustment and preventing system instability when dealing with AC grid faults of different voltages.

[0067] In some alternative embodiments of this application, the aforementioned DC transmission system further includes: a DC filter bank, wherein a first terminal of the DC filter bank is electrically connected to the aforementioned DC pole line, and a second terminal of the DC filter bank is grounded. In this embodiment, the DC filter bank can filter out harmonics in the DC transmission system, thereby improving the power quality of the DC transmission.

[0068] Specifically, there can be two DC filter banks.

[0069] Specifically, the DC transmission system also includes multiple AC filters, some of which are located in the first AC grid, and the remaining AC filters are located in the second AC grid. Particularly when the current source converter is an HCC, corresponding AC filters need to be configured in the second AC grid.

[0070] In summary, the DC transmission system of this application adopts a "layered access" approach, connecting the MMC converter to the first AC grid and the current source converter to the second AC grid. This layered operation at both high and low ends allows each AC grid at different voltage levels to be supported by its most suitable converter technology. In multi-infeed DC systems, this effectively reduces dynamic interactions between converters, minimizes voltage oscillations and circulating currents, and improves system dynamic stability. The two control systems for the MMC and current source converters are coordinated and divided through control logic. The first converter prioritizes voltage support, while the second converter prioritizes current stability. When a DC line fault or power flow change occurs, the two converters can quickly switch control targets, achieving millisecond-level power flow adjustment and preventing large-scale system instability. The layered access structure allows for separate adjustment of the power sharing ratio between the MMC and current source converters in the scheduling strategy, through coordinated control. By achieving hierarchical power flow optimization, power allocation can be flexibly adjusted according to the grid operation requirements. For example, during peak periods, the proportion of power transmission from the high-voltage grid can be increased, while during off-peak periods, the low-voltage grid can undertake more reactive power support, thereby improving the utilization rate of transmission channels. The MMC and current source converter operate at their respective voltage levels and control modes. The first converter mainly undertakes dynamic support and part of the active power flow, while the second converter mainly undertakes high-power DC transmission and reception tasks, reducing the overvoltage and overcurrent impacts on the devices, extending the converter life, and reducing operating losses. Adapting to future multi-terminal DC and flexible DC expansion, the DC transmission system of this application naturally supports multi-terminal DC access. By adjusting the controller parameters and coordination strategies, it can be expanded to three-terminal or higher systems, meeting the needs of future large-scale access of new energy sources and cross-regional multi-channel DC interconnection, and providing a technical foundation for flexible grid dispatch and safe and stable grid operation.

[0071] It is obvious to those skilled in the art that the modules or steps of the present invention described above can be implemented using general-purpose computing devices. They can be centralized on a single computing device or distributed across a network of multiple computing devices. They can be implemented using computer-executable program code, and thus can be stored in a storage device for execution by a computing device. In some cases, the steps shown or described can be performed in a different order than those described herein, or they can be fabricated as separate integrated circuit modules, or multiple modules or steps can be fabricated as a single integrated circuit module. Thus, the present invention is not limited to any particular combination of hardware and software.

[0072] Those skilled in the art will understand that embodiments of this application can be provided as methods, systems, or computer program products. Therefore, this application can take the form of a completely hardware embodiment, a completely software embodiment, or an embodiment combining software and hardware aspects. Furthermore, this application can take the form of a computer program product embodied on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code.

[0073] This application is described with reference to flowchart illustrations and / or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of this application. It will be understood that each block of the flowchart illustrations and / or block diagrams, and combinations of blocks in the flowchart illustrations and / or block diagrams, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, special-purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, generate instructions for implementing the flowchart... Figure 1 One or more processes and / or boxes Figure 1 A device that provides the functions specified in one or more boxes.

[0074] These computer program instructions may also be stored in a computer-readable storage medium that can direct a computer or other programmable data processing device to function in a particular manner, such that the instructions stored in the computer-readable storage medium produce an article of manufacture including instruction means, which are implemented in a process Figure 1 One or more processes and / or boxes Figure 1 The function specified in one or more boxes.

[0075] These computer program instructions may also be loaded onto a computer or other programmable data processing equipment to cause a series of operational steps to be performed on the computer or other programmable equipment to produce a computer-implemented process, thereby providing instructions that execute on the computer or other programmable equipment for implementing the process. Figure 1 One or more processes and / or boxes Figure 1 The steps of the function specified in one or more boxes.

[0076] In a typical configuration, a computing device includes one or more processors (CPU), input / output interfaces, network interfaces, and memory.

[0077] Memory may include non-persistent memory in computer-readable media, such as random access memory (RAM) and / or non-volatile memory, like read-only memory (ROM) or flash RAM. Memory is an example of computer-readable media.

[0078] Computer-readable media includes both permanent and non-permanent, removable and non-removable media that can store information using any method or technology. Information can be computer-readable instructions, data structures, modules of programs, or other data. Examples of computer storage media include, but are not limited to, phase-change memory (PRAM), static random access memory (SRAM), dynamic random access memory (DRAM), other types of random access memory (RAM), read-only memory (ROM), electrically erasable programmable read-only memory (EEPROM), flash memory or other memory technologies, CD-ROM, digital versatile optical disc (DVD) or other optical storage, magnetic tape, disk storage or other magnetic storage devices, or any other non-transferable medium that can be used to store information accessible by a computing device. As defined herein, computer-readable media does not include transient computer-readable media, such as modulated data signals and carrier waves.

[0079] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0080] It should also be noted that the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such process, method, article, or apparatus. Unless otherwise specified, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes that element.

[0081] As can be seen from the above description, the embodiments of this application achieve the following technical effects:

[0082] In the DC transmission system of this application, the receiving-end converter device includes a modular multilevel converter and a current source converter, and also includes a controller that is communicatively connected to the modular multilevel converter and the current source converter respectively. The DC side of the modular multilevel converter is electrically connected to the DC pole line, and the AC side is electrically connected to the first AC grid. The DC side of the current source converter is electrically connected to the DC pole line, and the AC side is electrically connected to the second AC grid. The voltages of the first AC grid and the second AC grid are different. The controller is used to control the modular multilevel converter and the current source converter to operate in different working modes. Compared with the poor system stability of the receiving-end converter connection method in the prior art, this application connects the modular multilevel converter and the current source converter to AC power grids of different voltage levels respectively, and the controller controls the two converters to operate in different working modes. This realizes the hierarchical connection of the receiving-end converter device, reduces the mutual interference caused by the two converters responding to the same voltage level power grid disturbance, and can effectively reduce the dynamic mutual influence between the two converters. This allows the two converters to act independently and without affecting each other when facing different types of disturbances, thereby avoiding the chain reaction that may occur in a single working mode and improving the dynamic stability of the system.

[0083] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A receiving-end converter device, characterized in that, include: A modular multilevel converter, wherein the DC side of the modular multilevel converter is used for electrical connection with a DC pole line, and the AC side of the modular multilevel converter is used for electrical connection with a first AC power grid; A current source converter, wherein the DC side of the current source converter is used to be electrically connected to the DC pole line, and the AC side of the current source converter is used to be electrically connected to a second AC grid, wherein the voltage of the first AC grid is different from the voltage of the second AC grid; The controller is communicatively connected to both the modular multilevel converter and the current source converter, and is used to control the modular multilevel converter and the current source converter to operate in different working modes.

2. The receiving-end converter device according to claim 1, characterized in that, The controller is used to control the modular multilevel converter and the current source converter to operate in different operating modes, including: The controller is used to control the voltage of the first converter to remain constant when the first converter is in normal operating mode. The first converter is the converter with a larger absolute value of the potential at one end of the DC pole line in the current source converter and the modular multilevel converter. The controller is used to maintain the power of the second converter at a constant level when the second converter is in normal operation mode. The second converter is the converter with a smaller absolute value of the potential at one end of the DC pole line in the current source converter and the modular multilevel converter. The voltage of the AC grid connected to the first converter is less than the voltage of the AC grid connected to the second converter.

3. The receiving-end converter according to claim 2, characterized in that, The controller is also used for: In the event of a fault in the first AC power grid or the second AC power grid, the active power of the second converter is reduced to reduce the DC current on the DC pole line, and the reduced DC current is not less than a preset DC current threshold.

4. The receiving-end converter according to claim 2, characterized in that, The controller is also used for: Determine whether the load of the total power grid is greater than a first predetermined load, wherein the total power grid includes at least the first AC power grid and the second AC power grid; If it is determined that the load of the total power grid is greater than the first predetermined load, the active power output of the second converter is increased while the total active power remains unchanged. The total active power is the sum of the active power output of the first converter and the active power output of the second converter.

5. The receiving-end converter according to claim 2, characterized in that, The AC grid connected to the first converter is the target AC grid, and the AC grid connected to the second converter is a non-target AC grid. The controller is further configured to: Determine the active power of the non-target AC grid based on its load; When the active power output by the second converter is greater than the active power of the non-target AC grid, the non-target AC grid is controlled to transmit active power to other AC grids and / or the target AC grid. When the non-target AC grid transmits the active power to other AC grids, the non-target AC grid is electrically connected to the other AC grids. When the non-target AC grid transmits the active power to the target AC grid, the non-target AC grid is electrically connected to the target AC grid.

6. The receiving-end converter according to claim 2, characterized in that, The controller is also used for: Determine whether the total load is less than the second predetermined load, wherein the total load is the sum of the load of the first AC power grid and the load of the second AC power grid; When the total load is less than the second predetermined load, the active power output of the second converter is reduced, while the total active power remains unchanged. The total active power is the sum of the active power output of the first converter and the active power output of the second converter.

7. The receiving-end converter according to claim 2, characterized in that, Controlling the voltage of the first converter to remain constant includes: when the first converter is the modular multilevel converter and the second converter is the current source converter, controlling the DC voltage of the first converter to remain constant and controlling the voltage of the AC bus to remain constant, wherein the first converter is electrically connected to the first AC grid via the AC bus; when the first converter is the current source converter and the second converter is the modular multilevel converter, controlling the DC voltage of the first converter to remain constant and controlling the reactive power of the first converter to remain constant. Controlling the power of the second converter to remain constant includes: controlling the active power of the second converter to remain constant, and controlling the reactive power of the second converter to remain constant.

8. A DC transmission system, characterized in that, include: DC poles; The receiving end includes the receiving end converter according to any one of claims 1 to 7; First AC power grid; A second AC power grid, wherein the voltage of the first AC power grid is different from the voltage of the second AC power grid.

9. The DC transmission system according to claim 8, characterized in that, The receiving-end converter includes a modular multilevel converter and a current source converter. The DC pole line is a bipolar ground line, and the DC pole line includes a positive DC pole line and a negative DC pole line; There are two modular multilevel converters, namely a first modular multilevel converter and a second modular multilevel converter connected in series; There are two current source converters, namely a first current source converter and a second current source converter connected in series; The positive DC pole is grounded through the first modular multilevel converter and the first current source converter connected in series, and the negative DC pole is grounded through the second modular multilevel converter and the second current source converter connected in series.

10. The DC transmission system according to claim 9, characterized in that, The first terminal of the first current source converter is electrically connected to the positive DC pole line through the first modular multilevel converter. The first terminal of the second current source converter is electrically connected to the negative DC pole line through the second modular multilevel converter; The second terminal of the first current source converter is electrically connected to the second terminal of the second current source converter, and the second terminals of the first current source converter and the second current source converter are respectively grounded; The voltage of the first AC power grid is less than the voltage of the second AC power grid.

11. The DC transmission system according to claim 9, characterized in that, The first terminal of the first modular multilevel converter is electrically connected to the positive DC pole line through the first current source converter. The first terminal of the second modular multilevel converter is electrically connected to the negative DC pole line through the second current source converter; The second terminal of the first modular multilevel converter is electrically connected to the second terminal of the second modular multilevel converter, and the second terminals of the first modular multilevel converter and the second modular multilevel converter are respectively grounded; The voltage of the first AC power grid is greater than the voltage of the second AC power grid.

12. The DC transmission system according to claim 8, characterized in that, The DC transmission system also includes: A DC filter bank, wherein the first end of the DC filter bank is electrically connected to the DC pole line, and the second end of the DC filter bank is grounded.

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