Receiving converter device and direct current power transmission system

Abstract

The application provides a kind of receiving end converter device and direct current transmission system, the receiving end converter device includes: modular multilevel converter, its direct current side is used to be electrically connected with direct current pole line, its alternating current side is used to be electrically connected with first alternating current network;Current source converter, its direct current side is used to be electrically connected with direct current pole line, its alternating current side is used to be electrically connected with second alternating current network, the voltage of first alternating current network is different from the voltage of second alternating current network;Controller is connected with modular multilevel converter and current source converter respectively, controller is used to control modular multilevel converter and current source converter to run in different working mode.The application solves the problem of poor system stability in the prior art of the access mode of the receiving end converter device.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of power electronics, in particular to a receiving-end converter device and a DC power transmission system. BACKGROUND

[0002] With large-scale sending of new energy, the demand for receiving-end access increases, and the existing access mode of the DC power transmission system has the problem of poor system stability. SUMMARY

[0003] The main purpose of the present application is to provide a receiving-end converter device and a DC power transmission system to at least solve the problem of poor system stability in the existing receiving-end converter access mode.

[0004] In order to achieve the above-mentioned purpose, according to one aspect of the present application, a receiving-end converter device is provided, comprising: a modular multilevel converter, a DC side of the modular multilevel converter being configured to be electrically connected with a DC pole line, and an AC side of the modular multilevel converter being configured to be electrically connected with a first AC power grid; a current source converter, a DC side of the current source converter being configured to be electrically connected with the DC pole line, and an AC side of the current source converter being configured to be electrically connected with a second AC power grid, a voltage of the first AC power grid being different from a voltage of the second AC power grid; and a controller, being in communication connection with the modular multilevel converter and the current source converter respectively, and being configured to control the modular multilevel converter and the current source converter to operate in different operation modes.

[0005] Optionally, the controller is configured to control the modular multilevel converter and the current source converter to operate in different operation modes, including: the controller is configured to control at least a voltage of a first converter to remain unchanged in a case that the first converter is in a normal operation mode, the first converter being a converter with a larger absolute value of potential of one end connected with the DC pole line among the current source converter and the modular multilevel converter; and the controller is configured to control a power of a second converter to remain unchanged in a case that the second converter is in the normal operation mode, the second converter being a converter with a smaller absolute value of potential of one end connected with the DC pole line among the current source converter and the modular multilevel converter, and a voltage of an AC power grid connected with the first converter being smaller than a voltage of an AC power grid connected with the second converter.

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

[0007] Optionally, the controller is further configured to determine whether a total load of a total power grid is greater than a first predetermined load, the total power grid comprising at least the first AC power grid and the second AC power grid; and control the active power output by the second converter to increase and control a total active power to be unchanged, the total active power being a sum of the active power output by the first converter and the active power output by the second converter, in a case where it is determined that the total load of the total power grid is greater than the first predetermined load.

[0008] Optionally, an AC power grid connected with the first converter is a target AC power grid, and an AC power grid connected with the second converter is a non-target AC power grid, and the controller is further configured to determine an active power of the non-target AC power grid according to a load of the non-target AC power grid; and control the non-target AC power grid to transmit an active power to other AC power grids and / or the target AC power grid in a case where the active power output by the second converter is greater than the active power of the non-target AC power grid, the non-target AC power grid being electrically connected with the other AC power grids in a case where the non-target AC power grid transmits the active power to the other AC power grids, and the non-target AC power grid being electrically connected with the target AC power grid in a case where the non-target AC power grid transmits the active power to the target AC power grid.

[0009] Optionally, the controller is further configured to determine whether a total load is less than a second predetermined load, the total load being a sum of a load of the first AC power grid and a load of the second AC power grid; and control the active power output by the second converter to decrease and control a total active power to be unchanged, the total active power being a sum of the active power output by the first converter and the active power output by the second converter, in a case where it is determined that the total load is less than the second predetermined load.

[0010] Optionally, the at least controlling the voltage of the first converter to remain unchanged comprises: in a case where the first converter is the modular multilevel converter and the second converter is the current source converter, controlling a direct current voltage of the first converter to remain unchanged and controlling a voltage of an AC bus to remain unchanged, the first converter being electrically connected with the first AC power grid through the AC bus; and in a case where the first converter is the current source converter and the second converter is the modular multilevel converter, controlling the direct current voltage of the first converter to remain unchanged and controlling a reactive power of the first converter to remain unchanged, and controlling a power of the second converter to remain unchanged, including controlling an active power of the second converter to remain unchanged and controlling a reactive power of the second converter to remain unchanged.

[0011] According to another aspect of the present application, there is provided a direct current transmission system, comprising: a direct current pole line; a receiving end comprising any of the receiving end converter devices; a first alternating current power grid; and a second alternating current power grid, the voltage of the first alternating current power grid being different from the voltage of the second alternating current power grid.

[0012] Optionally, the receiving end converter device comprises a modular multilevel converter and a current source converter, the direct current pole line is a bipolar ground line, the direct current pole line comprises a positive direct current pole line and a negative direct current pole line; the modular multilevel converter comprises two modular multilevel converters connected in series, namely a first modular multilevel converter and a second modular multilevel converter; the current source converter comprises two current source converters connected in series, namely a first current source converter and a second current source converter; the positive direct current pole line is grounded through the first modular multilevel converter and the first current source converter connected in series, and the negative direct current pole line is grounded through the second modular multilevel converter and the second current source converter connected in series.

[0013] Optionally, a first end of the first current source converter is electrically connected to the positive direct current pole line through the first modular multilevel converter; a first end of the second current source converter is electrically connected to the negative direct current pole line through the second modular multilevel converter; a second end of the first current source converter is electrically connected to a second end of the second current source converter, and the second end of the first current source converter and the second end of the second current source converter are grounded respectively; the voltage of the first alternating current power grid is less than the voltage of the second alternating current power grid.

[0014] Optionally, a first end of the first modular multilevel converter is electrically connected to the positive direct current pole line through the first current source converter; a first end of the second modular multilevel converter is electrically connected to the negative direct current pole line through the second current source converter; a second end of the first modular multilevel converter is electrically connected to a second end of the second modular multilevel converter, and the second end of the first modular multilevel converter and the second end of the second modular multilevel converter are grounded respectively; the voltage of the first alternating current power grid is greater than the voltage of the second alternating current power grid.

[0015] Optionally, the direct current transmission system further comprises: a direct current filter group, a first end of the direct current filter group being electrically connected to the direct current pole line, and a second end of the direct current filter group being grounded.

[0016] With the technical solution of the application, the receiving end converter device includes a modular multilevel converter and a current source converter, further includes a controller in communication connection with the modular multilevel converter and the current source converter respectively, the DC side of the modular multilevel converter is electrically connected with the DC polar line, the AC side is electrically connected with the first AC power grid, the DC side of the current source converter is electrically connected with the DC polar line, the AC side is electrically connected with the second AC power grid, the voltage of the first AC power grid is different from that of the second AC power grid, and 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 access mode of the receiving end converter in the prior art, the modular multilevel converter and the current source converter are respectively connected to the AC power grids of different voltage levels in the application, and the controller controls the two converters to operate in different working modes, realizing the layered access of the receiving end converter device, reducing the mutual interference of the two converters caused by responding to the same voltage level grid disturbance, effectively reducing the dynamic mutual influence between the two converters, enabling the two converters to act independently when facing different types of disturbances, and not affecting each other, thereby avoiding the possible chain reaction in a single working mode, and improving the dynamic stability of the system. BRIEF DESCRIPTION OF DRAWINGS

[0017] The drawings accompanying the specification of the present application form a part thereof, serve to provide further understanding of the present application, and together with the specification explain the application. The use of these drawings in explaining the application does not imply that the application should be limited to the embodiments illustrated therein. In the drawings:

[0018] Figure 1 A topological structure schematic diagram of a DC power transmission system provided in an embodiment of the present application is shown;

[0019] Figure 2 A topological structure schematic diagram of a specific DC power transmission system provided in an embodiment of the present application is shown;

[0020] Figure 3 A topological structure schematic diagram of another specific DC power transmission system provided in an embodiment of the present application is shown.

[0021] Among the above drawings, the following reference signs are used:

[0022] 10, DC polar line; 11, modular multilevel converter; 12, first AC power grid; 13, current source converter; 14, second AC power grid; 101, positive DC polar line; 102, negative DC polar line; 111, first modular multilevel converter; 112, second modular multilevel converter; 131, first current source converter; 132, second current source converter. DETAILED DESCRIPTION

[0023] It should be noted that the embodiments in the present application and the features in the embodiments can be combined with each other without conflict. The technical solutions in the embodiments of the present application will be described in detail below with reference to the drawings and in combination with the embodiments.

[0024] In order for those skilled in the art to better understand the present application, the technical solutions in the embodiments of the present application will be described clearly and completely below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, not all. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor should belong to the scope of protection of the present application.

[0025] It should be noted that the terms "first", "second" and the like in the specification and claims of the present application and the above-described drawings are used to distinguish similar objects, and do not necessarily indicate a specific order or a chronological sequence. It should be understood that the data thus used can be interchanged under appropriate circumstances, so that the embodiments of the present application described herein can be implemented. In addition, the terms "include" and "have" and any variations thereof are intended to cover non-exclusive inclusion, for example, a process, method, system, product or device including a series of steps or units does not have to be limited to those steps or units clearly listed, but can include other steps or units not clearly listed or inherent to these processes, methods, products or devices.

[0026] As introduced in the background, the access mode of the receiving end converter device in the prior art has the problem of poor system stability. To solve the above problem, the embodiments of the present application provide a receiving end converter device and a DC power transmission system.

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

[0028] The embodiments of the present application provide a receiving end converter device, as shown in Figures 1 to 3 The receiving end converter device comprises:

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

[0030] A current source converter 13, a DC side of the current source converter 13 is used to be electrically connected with the DC pole line 10, and an AC side of the current source converter 13 is used to be electrically connected with a second AC power grid 14, and a voltage of the first AC power grid 12 is different from a voltage of the second AC power 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 with the modular multilevel converter 11 and the current source converter 13 respectively, and is configured to control the modular multilevel converter 11 and the current source converter 13 to operate in different operation modes.

[0033] According to the above embodiment, the receiving end converter device includes a modular multilevel converter and a current source converter, and further includes a controller communicatively connected with the modular multilevel converter and the current source converter respectively. The DC side of the modular multilevel converter is electrically connected with the DC polar line, and the AC side is electrically connected with the first AC power grid. The DC side of the current source converter is electrically connected with the DC polar line, and the AC side is electrically connected with the second AC power grid. The voltage of the first AC power grid is different from that of the second AC power grid. The controller is configured to control the modular multilevel converter and the current source converter to operate in different operation modes. Compared with the poor system stability of the existing receiving end converter access mode, the modular multilevel converter and the current source converter are connected to AC power grids of different voltage levels respectively, and the controller controls the two converters to operate in different operation modes, thereby realizing the hierarchical access of the receiving end converter device, reducing the mutual interference of the two converters caused by responding to the same voltage level grid disturbance, effectively reducing the dynamic mutual influence between the two converters, enabling the two converters to act independently when facing different types of disturbances, and avoiding the chain reaction that may occur in a single operation mode, thereby improving the dynamic stability of the system.

[0034] Specifically, the current source converter includes a controlled current source converter or a hybrid grid commutation converter.

[0035] Specifically, the controlled current source converter (CCSC) and the hybrid grid commutation converter (HCC) adopt RB-IGCT (Robust Block-type Integrated Gate Commutated Thyristor) devices.

[0036] In an alternative, the controller is configured to control the modular multilevel converter and the current source converter to operate in different modes, including: the controller is configured to control the voltage of the first converter to remain unchanged when the first converter is in a normal operation mode, the first converter being the converter with a larger absolute value of the potential of one end of the DC pole line among the current source converter and the modular multilevel converter; the controller is configured to control the power of the second converter to remain unchanged when the second converter is in a normal operation mode, the second converter being the converter with a smaller absolute value of the potential of one end of the DC pole line among the current source converter and the modular multilevel converter, and the voltage of the AC power grid connected to the first converter being smaller than the voltage of the AC power grid connected to the second converter. In this embodiment, the high-voltage valve group (i.e., the converter with a higher potential to ground) is connected to the AC side with a lower voltage, and the low-voltage valve group (i.e., the converter with a lower potential to ground) is connected to the AC side with a higher voltage, so that the AC voltage stress and the DC-to-ground stress are complementary to each other on the valve-side winding of the converter, and the valve-side winding of the converter bears a smaller AC+DC comprehensive insulation stress, thereby reducing the insulation level and creepage distance requirements of the converter and its bushing, and improving the manufacturability and reliability of the converter and its bushing.

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

[0038] Specifically, the low-voltage power grid needs more local fast reactive power and voltage support, and the high-voltage power grid needs more stable power transmission; the first converter is responsible for voltage support and power quality, and the second converter is responsible for stabilizing the DC current and preventing commutation failure (stabilizing power). The application provides the "layered access + differential control" mode to enable each of the two different AC power grids to obtain the most suitable converter support, thereby improving the stability of the power transmission system.

[0039] According to some example embodiments of the 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, so that the DC current on the DC pole line decreases, and the decreased DC current is not less than a preset DC current threshold. In this embodiment, in the event of a fault in the AC power grid, the active power of the second converter is decreased to control the decrease of the DC current, so as to ensure that the current flowing through the first converter is small, and at the same time ensure that the DC current is not lower than the minimum threshold required for safe operation of the converter, thereby improving the fault ride-through capability of the system.

[0040] In practical applications, the direct current threshold value can be set by experience value or obtained through multiple experiments, and the application does not make specific limitations.

[0041] Specifically, the active power of the second converter can be reduced by reducing the active power reference value of the second converter; and the reduced direct current is not more than 1.1-1.5 times the direct current rating of the first converter.

[0042] According to some example embodiments of the application, the controller is further configured to determine whether the total power grid load 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 in the case that the total power grid load is greater than the first predetermined load, control the active power output by the second converter to increase, and control the total active power to remain unchanged, wherein the total active power is the sum of the active power output by the first converter and the active power output by the second converter. In this embodiment, a dynamic power distribution 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 by the second converter to meet the power demand of the power grid. This strategy is based on accurate prediction of the power grid load and precise control of the power output of the converter, and realizes intelligent scheduling of power between power grids. Through dynamic power distribution, the operation efficiency and power transmission capacity of the power grid are improved, and especially during high load periods, the transmission potential of the ultra-high voltage AC power grid (i.e., the AC power grid connected to the second converter) can be fully utilized.

[0043] Specifically, the total power grid can further include other power grids, which can be AC power grids, DC power grids, or AC / DC power grids, and the application does not make specific limitations.

[0044] In practical applications, the first predetermined load can be set by experience value or obtained through multiple experiments, and the application does not make specific limitations.

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

[0046] Specifically, the controller can also control the non-target AC power grid to transmit active power to other AC power grids and / or the target AC power grid when the active power output by the second converter is greater than the active power of the non-target AC power grid after the active power output by the second converter is increased.

[0047] According to some example embodiments of the present application, the controller is further configured to: determine whether a total load is less than a second predetermined load, the total load being a sum of a load of the first AC power grid and a load of the second AC power grid; and control the active power output by the second converter to decrease and control a total active power to be unchanged when the total load is less than the second predetermined load, the total active power being a sum of the active power output by the first converter and the active power output by the second converter. In this embodiment, power distribution is optimized during low load. By decreasing the active power output by the second converter, unnecessary energy consumption is reduced. Since the total active power is unchanged, the active power output by the first converter is increased, so that the first converter can better support voltage. By monitoring the total load of the power grid in real time, the power output of the converter is automatically adjusted, thereby ensuring economic operation of the system during low load, reducing energy waste, reducing operating costs, and reducing the burden on the converter, thereby prolonging the service life of the equipment.

[0048] In actual applications, the second predetermined load can be set by experience or obtained through multiple experiments, and the present application does not make a specific limitation in this regard.

[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 device includes: active power is distributed according to a preset proportion, and reactive power is independently controlled. This enables the system to pre-set and automatically adjust the proportion of active power sent from different converter stations into the AC power grid according to the real-time demand of the power grid operation, and this flexible power distribution strategy improves the power transmission efficiency while ensuring the balance between supply and demand of the power grid; the independent control of reactive power means that the modular multilevel converter and the current source converter can provide reactive power support for the AC power grid connected respectively, and this does not need to rely on the adjustment of active power. The independent reactive power control can more accurately respond to the voltage fluctuation of the AC power grid, provide fast voltage support, avoid voltage collapse, and thus enhance the dynamic stability and power quality of the entire power grid.

[0055] Specifically, the priority of the hierarchical coordinated control of the receiving-end converter device is: 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 new energy power generation data, historical weather forecast information, and historical power flow change trend, the historical power flow change trend representing the change of the flow of electric energy in the DC power transmission system within a certain time range in the past, and the historical load data representing historical load data of the first AC power grid and the second AC power grid; train the neural network model using the historical load data, the historical new energy power generation data, the historical weather forecast information, and the historical power flow change trend to obtain a target neural network model; obtain current load data, current new energy power generation data, and current weather forecast information, and input the current load data, the current new energy power generation data, and the current weather forecast information into the target neural network model to obtain a current power flow change trend; when the current power flow change trend represents that the future new energy power generation and load fluctuation are less than a predetermined fluctuation threshold, the power bearing proportion of the first converter and the second converter is allocated in advance as 1:1; when the current power flow change trend represents that the amount of future new energy power generation is greater than a predetermined power generation increase, the power bearing proportion of the second converter is increased, for example, the power bearing proportion of the first converter and the second converter is allocated as 1:2; when the current power flow change trend represents that the amount of future load increase is greater than a predetermined load increase, the power bearing proportion of the second converter is increased. Through early prediction and adjustment, the system instability caused by sudden changes in power flow is effectively prevented, and the overall reliability and safety of the DC power transmission system are improved.

[0057] In other embodiments, the control of the transmission of active power from the non-target AC power grid to the other AC power grid and / or the target AC power grid comprises: determining the difference between the active power output by the second converter and the active power of the non-target AC power grid to obtain residual power; determining the active power of the target AC power grid according to the load of the target AC power grid, and determining the active power of the other AC power grid according to the load of the other AC power grid; obtaining the ratio of the active power of the target AC power grid to the active power of the other AC power grid; and controlling the transmission of active power from the non-target AC power grid to the other AC power grid and / or the target AC power grid according to at least the ratio of the active power of the target AC power grid to the active power of the other AC power grid. The control of the transmission of active power from the non-target AC power grid to the other AC power grid and / or the target AC power grid according to at least the ratio of the active power of the target AC power grid to the active power of the other AC power grid comprises: when the ratio of the active power of the target AC power grid to the active power of the other AC power grid is greater than a first preset ratio (the first preset ratio is greater than 1.5) and the active power of the target AC power grid is greater than the residual power, controlling the transmission of active power from the non-target AC power grid to the target AC power grid; when the ratio of the active power of the target AC power grid to the active power of the other AC power grid is greater than the first preset ratio and the active power of the target AC power grid is not greater than the residual power, controlling the transmission of active power from the non-target AC power grid to the target AC power grid and the other AC power grid, and controlling the active power transmitted to the target AC power grid to be equal to the active power of the target AC power grid; and when the ratio of the active power of the target AC power grid to the active power of the other AC power grid is not greater than the 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), controlling the transmission of active power from the non-target AC power grid to the target AC power grid and the other AC power grid, and controlling the ratio of the active power transmitted to the target AC power grid to the active power transmitted to the other AC power grid to be equal to the ratio of the active power of the target AC power grid to the active power of the other AC power grid. In this embodiment, efficient and flexible power distribution and flow management are achieved, which not only effectively avoids the occurrence of overload, but also realizes intelligent load balancing during the transmission of active power between power grids, improves the power transmission efficiency and stability of the entire system, and enhances the response capability of the power grid to sudden load changes.

[0058] In actual application, the first preset ratio and the second preset ratio can be set according to experience values or obtained through multiple experiments, and the present application does not make a specific limitation in this regard.

[0059] The present application also provides a DC power transmission system, as shown in Figures 1 to 3 The DC pole line 10 comprises:

[0060] The DC pole line 10

[0061] a receiving end (not shown) comprising any of the receiving end converter devices (not shown) described above;

[0062] a first AC power grid 12 and a second AC power grid 14, the voltage of the first AC power grid 12 being different from the voltage of the second AC power grid 14.

[0063] With the above embodiments, the DC power transmission system of the present application comprises a DC pole line, a receiving end, a first AC power grid and a second AC power grid, the receiving end comprising any of the receiving end converter devices described above. Compared with the connection mode of the receiving end converter in the prior art, which has the problems of commutation failure, inflexible power flow control, weak support for the AC power grid and poor system stability, the present application connects the modular multilevel converter and the current source converter to different voltage level AC power grids respectively, and the controller controls the two types of converters to operate in different modes, realizing the layered connection of the receiving end converter device, reducing the mutual interference of the two types of converters due to the response to the same voltage level grid disturbance, effectively reducing the dynamic mutual influence between the two types of converters, enabling the two types of converters to act independently when facing different types of disturbances, and thus avoiding the chain reaction that may occur in a single mode of operation, and improving the dynamic stability of the system.

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

[0065] According to some exemplary embodiments of the present application, as shown in 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 direct current transmission system further comprises: a plurality of AC filters, part of the AC filters being located in the first AC power grid and the rest of the AC filters being located in the second AC power grid. Especially when the current source converter is HCC, corresponding AC filters need to be configured in the second AC power grid.

[0070] In summary, the direct current transmission system of the present application adopts a "layered access" mode, with the MMC converter being connected to the first AC power grid and the current source converter being connected to the second AC power grid, and the high and low ends being operated in layers, so that the AC power grids of different voltage levels are supported by the most suitable converter technology, which can effectively reduce the dynamic interaction between the converters in the multi-infeed direct current system, reduce voltage oscillation and circulating current, and improve the dynamic stability of the system; the two sets of control systems of the MMC and the current source converter are divided by the coordination control logic, the first converter prioritizes voltage support, and the second converter prioritizes current stability, and when a fault occurs in the direct current line or the power flow changes suddenly, the two types of converters can quickly switch control targets to achieve millisecond-level power flow adjustment and avoid large-scale instability of the system; the layered access structure can adjust the power sharing ratio of the MMC and the current source converter in the dispatching strategy, and realize layered power flow optimization through coordination control, so that the power distribution can be flexibly adjusted according to the operation requirements of the power grid, such as increasing the proportion of power transmission from the high-voltage power grid side during peak hours and bearing more reactive power support from the low-voltage power grid side during off-peak hours, thereby improving the utilization rate of the power transmission channel; the MMC and the current source converter are operated in the adaptive voltage level and control mode, the first converter mainly undertakes dynamic support and part of the active power flow, and the second converter mainly undertakes the task of large-power direct current transmission and reception, which reduces the overvoltage and overcurrent impact on the devices, prolongs the service life of the converter, and reduces the operating loss; the direct current transmission system of the present application naturally supports multi-terminal direct current access, and can be expanded to a three-terminal or more system by adjusting the controller parameters and coordination strategy, meeting the needs of large-scale new energy access and cross-regional multi-channel direct current interconnection in the future, and providing a technical foundation for flexible dispatching and safety and stability of the power grid.

[0071] Obviously, those skilled in the art should understand that the modules or steps of the present application described above can be realized by general computing devices, which can be concentrated on a single computing device or distributed on a network composed of multiple computing devices, and they can be realized by program codes executable by the computing devices, so that they can be stored in storage devices and executed by the computing devices, and in some cases, the steps shown or described can be executed in different order, or they can be manufactured into individual integrated circuit modules, or multiple modules or steps can be manufactured into a single integrated circuit module. Therefore, the present application is not limited to any specific combination of hardware and software.

[0072] Those skilled in the art will appreciate that embodiments of the application can be readily used as software, hardware, or a combination of software and hardware. In a typical configuration, a computing device includes one or more processors (CPUs), input / output interfaces, network interfaces, and memory.

[0073] The computer program instructions can also be loaded onto a computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer-implemented process such that the instructions which execute on the computer or other programmable apparatus provide steps for implementing the functions specified in the flowchart block or blocks. Figure 1 one or more functions specified in the flowchart block or blocks. Figure 1 means for performing each of the functions specified in the flowchart block or blocks.

[0074] These computer program instructions can also be stored in a computer- readable memory that can direct a computer or other programmable data processing apparatus to function in a particular manner, such that the instructions stored in the computer-readable memory produce an article of manufacture including instructions which implement the functions specified in the flowchart block or blocks. Figure 1 one or more functions specified in the flowchart block or blocks. Figure 1 means for performing each of the functions specified in the flowchart block or blocks.

[0075] The computer program instructions can also be loaded onto a computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer-implemented process such that the instructions which execute on the computer or other programmable apparatus provide steps for implementing the functions specified in the flowchart block or blocks. Figure 1 one or more functions specified in the flowchart block or blocks. Figure 1 means for performing each of the functions specified in the flowchart block or blocks.

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

[0077] The memory can include non-persistent memory and / or volatile memory, such as random access memory (RAM) about which the processor can execute instructions. The memory can also include non-volatile memory, such as read only memory (ROM) or flash RAM, about which permanent information can be stored; such information can not change much and / or can only change slowly.

[0078] Computer-readable media includes permanent and non-permanent, movable and non-movable media that can be implemented by any method or technology to store information. The information can be computer-readable instructions, data structures, program modules 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, compact disc read-only memory (CD-ROM), digital versatile disc (DVD) or other optical storage, magnetic cassette, magnetic disk storage or other magnetic storage devices, or any other non-transmission medium that can be used to store information accessible by a computing device. According to the definition herein, computer-readable media does not include transitory media such as modulated data signals and carriers.

[0079] The technical features of the above-described embodiments can be combined in any manner. In order to make the description concise, not all possible combinations of the technical features in the above-described embodiments are described, however, as long as the combinations of the technical features do not exist contradictions, it should be considered that they are within the scope of the present disclosure.

[0080] It should also be noted that the terms "comprising", "including", or any other variant thereof are intended to cover non-exclusive inclusion, so that processes, methods, articles or devices that include a series of elements not only include those elements, but also include other elements not explicitly listed, or inherent to such processes, methods, articles or devices. Without more limitations, the element defined by the statement "comprising a" does not exclude the presence of other identical elements in the process, method, article or device that includes the element.

[0081] From the above description, it can be seen that the above-described embodiments of the present application achieve the following technical effects:

[0082] In the direct current power transmission system of the present application, the receiving end converter device of the present application comprises a modular multilevel converter and a current source converter, and further comprises a controller in communication connection with the modular multilevel converter and the current source converter respectively, the direct current side of the modular multilevel converter is electrically connected with a direct current pole line, and the alternating current side is electrically connected with a first alternating current power grid, the direct current side of the current source converter is electrically connected with a direct current pole line, and the alternating current side is electrically connected with a second alternating current power grid, the voltage of the first alternating current power grid is different from that of the second alternating current power grid, and 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 problem existing in the access mode of the receiving end converter in the prior art, the modular multilevel converter and the current source converter are respectively connected to alternating current power grids of different voltage levels in the present application, and the controller controls the two converters to operate in different working modes, thereby realizing the layered access of the receiving end converter device, reducing the mutual interference of the two converters due to the response to the same voltage level grid disturbance, effectively reducing the dynamic mutual influence between the two converters, enabling the two converters to act independently when facing different types of disturbances, and not affecting each other, thereby avoiding the possible chain reaction in a single working mode, and improving the dynamic stability of the system.

[0083] The above only describes the preferred embodiments of the present application and is not intended to limit the present application. The present application can have various modifications and changes for those skilled in the art. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A receiving end converter device, characterized by The application relates to a power transmission system comprising: a modular multilevel converter, a direct current side of the modular multilevel converter being configured to be electrically connected with a direct current pole line, an alternating current side of the modular multilevel converter being configured to be electrically connected with a first alternating current power grid; a current source converter, a direct current side of the current source converter being configured to be electrically connected with the direct current pole line, an alternating current side of the current source converter being configured to be electrically connected with a second alternating current power grid, a voltage of the first alternating current power grid being different from a voltage of the second alternating current power grid; a controller, the controller being configured to communicate with the modular multilevel converter and the current source converter respectively, the controller being configured to: control at least a voltage of a first converter to remain unchanged when the first converter is in a normal operation mode, the first converter being a converter with a larger absolute value of a potential of one end connected with the direct current pole line between the current source converter and the modular multilevel converter; control a power of a second converter to remain unchanged when the second converter is in the normal operation mode, the second converter being a converter with a smaller absolute value of the potential of the one end connected with the direct current pole line between the current source converter and the modular multilevel converter, a voltage of an alternating current power grid connected with the first converter being smaller than a voltage of an alternating current power grid connected with the second converter; determine whether a total load of a total power grid is greater than a first predetermined load, the total power grid at least comprising the first alternating current power grid and the second alternating current power grid, control an active power output by the second converter to increase and control a total active power to remain unchanged when it is determined that the total load of the total power grid is greater than the first predetermined load, the total active power being a sum of the active power output by the first converter and the active power output by the second converter.

2. The receiving converter device according to claim 1, characterized in that The controller is further configured to: control the active power of the second converter to decrease when a fault occurs in the first alternating current power grid or the second alternating current power grid, so that a direct current in the direct current pole line decreases, and the decreased direct current is not less than a preset direct current threshold.

3. The receiving-end converter device according to claim 1, characterized in that, The alternating current power grid connected with the first converter is a target alternating current power grid, and the alternating current power grid connected with the second converter is a non-target alternating current power grid, the controller is further configured to: determine an active power of the non-target alternating current power grid according to a load of the non-target alternating current power grid; control the non-target alternating current power grid to transmit an active power to other alternating current power grids and / or the target alternating current power grid when the active power output by the second converter is greater than the active power of the non-target alternating current power grid, the non-target alternating current power grid being electrically connected with the other alternating current power grids when the non-target alternating current power grid transmits the active power to the other alternating current power grids, and the non-target alternating current power grid being electrically connected with the target alternating current power grid when the non-target alternating current power grid transmits the active power to the target alternating current power grid.

4. The receiving-end converter device according to claim 1, characterized in that, The controller is further configured to: determine whether a total load is less than a second predetermined load, the total load being a sum of a load of the first alternating current power grid and a load of the second alternating current power grid. In a case where the total load is less than the second predetermined load, the active power output by the second converter is controlled to decrease, and the total active power, which is the sum of the active power output by the first converter and the active power output by the second converter, is controlled to be unchanged.

5. The receiving-end converter device according to claim 1, wherein controlling at least the voltage of the first converter to remain unchanged includes, in a case where 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 unchanged and controlling the voltage of the AC bus to remain unchanged, wherein the first converter is electrically connected to the first AC power grid through the AC bus, and in a case where 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 unchanged and controlling the reactive power of the first converter to remain unchanged, controlling the power of the second converter to remain unchanged includes controlling the active power of the second converter to remain unchanged and controlling the reactive power of the second converter to remain unchanged.

6. A direct current power transmission system, characterized by comprises a DC pole line; a receiving end comprising the receiving-end converter device according to any one of claims 1 to 5; a first AC power grid; a second AC power grid, the voltage of the first AC power grid being different from the voltage of the second AC power grid.

7. The direct current power transmission system of claim 6, wherein, the receiving-end converter device comprises a modular multilevel converter and a current-source converter, the DC pole line is a bipolar ground line, and the DC pole line comprises a positive DC pole line and a negative DC pole line; the modular multilevel converter includes two modular multilevel converters connected in series, which are a first modular multilevel converter and a second modular multilevel converter; the current-source converter includes two current-source converters connected in series, which are a first current-source converter and a second current-source converter; the positive DC pole line is grounded through the first modular multilevel converter and the first current-source converter connected in series, and the negative DC pole line is grounded through the second modular multilevel converter and the second current-source converter connected in series.

8. The DC power transmission system according to claim 7, wherein a first end of the first current-source converter is electrically connected to the positive DC pole line through the first modular multilevel converter; a first end of the second current-source converter is electrically connected to the negative DC pole line through the second modular multilevel converter; a second end of the first current-source converter is electrically connected to a second end of the second current-source converter, and the second end of the first current-source converter and the second end of the second current-source converter are grounded respectively; the voltage of the first AC power grid is less than the voltage of the second AC power grid.

9. The DC power transmission system according to claim 7, wherein a first end of the first modular multilevel converter is electrically connected to the positive DC pole line through the first current-source converter. The first end of the second modular multilevel converter is electrically connected with the negative direct current pole line through the second current source type converter; The second end of the first modular multilevel converter is electrically connected with the second end of the second modular multilevel converter, and the second end of the first modular multilevel converter and the second end of the second modular multilevel converter are grounded respectively. The voltage of the first alternating current power grid is greater than the voltage of the second alternating current power grid.

10. The direct current power transmission system of claim 6, wherein, The direct current power transmission system further comprises: A direct current filter group, a first end of the direct current filter group is electrically connected with the direct current pole line, and a second end of the direct current filter group is grounded.

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