Method and device for evaluating grounding clamp position of metal return line in flexible DC transmission system

Abstract

The present invention discloses a method and device for evaluating the grounding clamp position of a metal loop of a flexible DC transmission system, which fully utilizes the flexibility of the grounding clamp position design when the metal loop is in operation, performs a DC line end grounding fault simulation, and finally determines the grounding clamp position when the metal loop is in operation by comparing the voltage of the flexible DC sub-module capacitor at different clamp positions and determining whether there is an overvoltage phenomenon in the voltage of the flexible DC converter station sub-module capacitor at the selected grounding clamp position. This avoids the risk of overvoltage tripping of the converter station sub-module capacitor caused by continuous charging of the converter station sub-module capacitor by the DC side, and solves the technical problem that the DC side continuously charges the converter station sub-module capacitor and causes overvoltage tripping of the converter station sub-module capacitor when the existing metal loop is in operation.

Description

Technical Field

[0001] The present invention relates to the technical field of power systems, and in particular to a method and a device for evaluating a grounding clamp point of a metal return line of a flexible direct current transmission system. Background Art

[0002] Flexible DC transmission is flexible, controllable, and efficient, and is one of the important means of power transmission for renewable energy. For the sending end, the flexible DC converter can provide stable AC voltage for renewable energy power fields, can operate in an isolated island, and can quickly and dynamically compensate for reactive power, reduce the risk of renewable energy units being disconnected from the grid, and improve the utilization rate of renewable energy. Therefore, it is a feasible technical means to send the renewable energy isolated power grid through a true bipolar flexible DC overhead line system.

[0003] When true bipolar flexible DC is in operation, after one pole exits operation, in order to avoid continuous large current flowing through the earth and shortening the life of the grounding electrode, the remaining pole can use the uncharged opposite pole line as the current return path, and this wiring method is called the metal loop method. In actual projects, the grounding clamping point during metal loop operation can be on the rectifier side or the inverter side according to actual conditions. However, when the flexible DC with the ability to clear DC overhead line DC faults is used in large-scale transmission scenarios of new energy bases, when a grounding fault occurs in the DC overhead line, the flexible DC will output negative voltage to clear the DC fault. Before the DC fault is cleared, the AC side of the sending end will continue to charge the capacitor of the converter station submodule, and the DC side will also charge the capacitor of the converter station submodule during the process of the flexible DC outputting negative voltage to discharge the DC side energy. If the connection ground clamping point during the operation of the metal loop is improperly selected, it will cause the capacitor of the converter station submodule at the sending end to overvoltage, thereby causing the system to trip and stop operation. Therefore, how to determine the grounding clamp point when the metal loop is in operation to solve the technical problem that the DC side of the sending-end flexible DC continues to charge the capacitor of the sending-end converter station sub-module during the output negative voltage to clear the DC fault, resulting in overvoltage tripping of the converter station sub-module capacitor, is a technical problem that needs to be urgently solved by technical personnel in this field. Summary of the invention

[0004] The present invention provides a method and device for evaluating the grounding clamping point of a metal loop line in a flexible DC transmission system, which is used to solve the technical problem that the DC side continuously charges the capacitor of a converter station submodule, resulting in overvoltage tripping of the capacitor of the converter station submodule, due to improper selection of the connection ground clamping point during the operation of the existing metal loop line.

[0005] In view of this, the first aspect of the present invention provides a method for evaluating the grounding clamp position of a metal return line of a flexible DC transmission system, which is applied to a new energy bipolar flexible DC transmission system. The new energy bipolar flexible DC transmission system includes a new energy electric field, a sending-end flexible DC converter station, a receiving-end flexible DC converter station, a first DC overhead line, a second DC overhead line, an AC energy consumption device and a DC energy consumption device. The new energy electric field is connected to the sending-end flexible DC converter station through a three-phase AC line. The AC energy consumption device is configured between the new energy electric field and the sending-end flexible DC converter station through the three-phase AC line. An output terminal of the sending-end flexible DC converter station is connected to an input terminal of the receiving-end flexible DC converter station through the first DC overhead line. Another output terminal of the sending-end flexible DC converter station is connected to another input terminal of the receiving-end flexible DC converter station through the second DC overhead line. The DC energy consumption device is configured on the DC side of the receiving-end flexible DC converter station. The receiving-end flexible DC converter station is connected to the AC power grid.

[0006] The method for evaluating the grounding clamp position of the metal return line of the flexible DC transmission system includes the following steps:

[0007] S1. Build a new energy bipolar flexible DC transmission system in the simulation software, unlock the sending-end flexible DC converter station and the receiving-end flexible DC converter station in order according to the design requirements in a metal grounding manner, and adjust the power of the sending-end flexible DC converter station and the receiving-end flexible DC converter station to the rated value;

[0008] S2, respectively simulating the grounding fault at the end of the DC line when the grounding clamping point is set at the sending-end flexible DC converter station and the receiving-end flexible DC converter station during the operation of the metal loop;

[0009] S3, comparing the submodule capacitor voltages of the sending-end flexible DC converter station during the period when the output negative voltage clears the DC fault when the grounding clamping points are respectively set at the sending-end flexible DC converter station and the receiving-end flexible DC converter station, and selecting the grounding clamping point of the flexible DC converter station with the smallest submodule capacitor voltage as the system grounding clamping point;

[0010] S4. Check whether there is overvoltage in the voltage of the remaining sub-module capacitors of the flexible DC converter station at the sending end of the system grounding clamp point. If there is no overvoltage in the sub-module capacitors, the selected system grounding clamp point meets the design requirements. If there is overvoltage in the sub-module capacitors, remove the selected system grounding clamp point and return to step S3.

[0011] Optionally, the new energy bipolar flexible direct current transmission system is a true bipolar flexible direct current transmission system.

[0012] Optionally, the sending-end flexible DC converter station and the receiving-end flexible DC converter station adopt a single valve group structure or a double valve group series structure.

[0013] Optionally, the converter valves of the sending-end flexible DC converter station and the receiving-end flexible DC converter station adopt a full half-bridge submodule hybrid topology structure.

[0014] Optionally, the number of the sending-end flexible DC converter station and the receiving-end flexible DC converter station is one or more.

[0015] Optionally, when the number of sending-end flexible DC converter stations and receiving-end flexible DC converter stations is more than two, the first DC overhead line and the second DC overhead line connect the DC sides of each sending-end flexible DC converter station and receiving-end flexible DC converter station in a star structure or a triangle structure.

[0016] The second aspect of the present invention provides a device for evaluating the grounding clamping point of a metal return line of a flexible DC transmission system, which is applied to a new energy bipolar flexible DC transmission system. The new energy bipolar flexible DC transmission system includes a new energy electric field, a sending-end flexible DC converter station, a receiving-end flexible DC converter station, a first DC overhead line, a second DC overhead line, an AC energy consumption device, and a DC energy consumption device. The new energy electric field is connected to the sending-end flexible DC converter station through a three-phase AC line. The AC energy consumption device is configured between the new energy electric field and the sending-end flexible DC converter station through the three-phase AC line. One output end of the sending-end flexible DC converter station is connected to one input end of the receiving-end flexible DC converter station through the first DC overhead line. Another output end of the sending-end flexible DC converter station is connected to another input end of the receiving-end flexible DC converter station through the second DC overhead line. The DC energy consumption device is configured on the DC side of the receiving-end flexible DC converter station. The receiving-end flexible DC converter station is connected to the AC power grid.

[0017] The Flexible DC transmission system metal return line grounding clamp point assessment device is used to perform the following steps:

[0018] S1. Build a new energy bipolar flexible DC transmission system in the simulation software, unlock the sending-end flexible DC converter station and the receiving-end flexible DC converter station in order according to the design requirements in a metal grounding manner, and adjust the power of the sending-end flexible DC converter station and the receiving-end flexible DC converter station to the rated value;

[0019] S2, respectively simulating the grounding fault at the end of the DC line when the grounding clamping point is set at the sending-end flexible DC converter station and the receiving-end flexible DC converter station during the operation of the metal loop;

[0020] S3, comparing the submodule capacitor voltages of the sending-end flexible DC converter station during the period when the output negative voltage clears the DC fault when the grounding clamping points are respectively set at the sending-end flexible DC converter station and the receiving-end flexible DC converter station, and selecting the grounding clamping point of the flexible DC converter station with the smallest submodule capacitor voltage as the system grounding clamping point;

[0021] S4. Check whether there is overvoltage in the voltage of the remaining sub-module capacitors of the flexible DC converter station at the sending end of the system grounding clamp point. If there is no overvoltage in the sub-module capacitors, the selected system grounding clamp point meets the design requirements. If there is overvoltage in the sub-module capacitors, remove the selected system grounding clamp point and return to step S3.

[0022] Optionally, the new energy bipolar flexible direct current transmission system is a true bipolar flexible direct current transmission system.

[0023] Optionally, the sending-end flexible DC converter station and the receiving-end flexible DC converter station adopt a single valve group structure or a double valve group series structure.

[0024] Optionally, the converter valves of the sending-end flexible DC converter station and the receiving-end flexible DC converter station adopt a full half-bridge submodule hybrid topology structure.

[0025] Optionally, the number of the sending-end flexible DC converter station and the receiving-end flexible DC converter station is one or more.

[0026] Optionally, when the number of sending-end flexible DC converter stations and receiving-end flexible DC converter stations is more than two, the first DC overhead line and the second DC overhead line connect the DC sides of each sending-end flexible DC converter station and receiving-end flexible DC converter station in a star structure or a triangle structure.

[0027] It can be seen from the above technical solutions that the method for evaluating the grounding clamp position of the metal return line of the flexible DC transmission system provided by the present invention has the following advantages:

[0028] The method for evaluating the grounding clamp position of a metal loop of a flexible DC transmission system provided by the present invention makes full use of the flexibility of the grounding clamp position design when the metal loop is in operation to simulate the grounding fault at the end of the DC line, and finally determines the grounding clamp position when the metal loop is in operation by comparing the voltage of the capacitor of the flexible DC submodule at different clamp positions and whether there is an overvoltage phenomenon in the capacitor voltage of the flexible DC converter station submodule at the selected grounding clamp position. This avoids the risk of the converter station submodule capacitor being tripped due to overvoltage caused by continuous charging of the converter station submodule capacitor by the DC side, and solves the technical problem that the DC side continuously charges the converter station submodule capacitor and trips due to overvoltage caused by the converter station submodule capacitor when the existing metal loop is in operation. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other related drawings can be obtained based on these drawings without paying creative work.

[0030] Figure 1 A schematic flow chart of a method for evaluating the grounding clamping point of a metal return line of a flexible DC transmission system provided in an embodiment of the present invention;

[0031] Figure 2 A schematic diagram of the structure of a new energy bipolar flexible direct current transmission system provided in an embodiment of the present invention;

[0032] Figure 3 A schematic diagram of different grounding clamping points under the metal return line connection of the new energy bipolar flexible direct current transmission system provided in an embodiment of the present invention. DETAILED DESCRIPTION

[0033] In order to enable those skilled in the art to better understand the scheme of the present invention, the technical scheme in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0034] For easier understanding, see Figures 1 to 3 The present invention provides an embodiment of a method for evaluating the grounding clamping point of a metal return line of a flexible DC transmission system, which is applied to a new energy bipolar flexible DC transmission system. The new energy bipolar flexible DC transmission system includes a new energy electric field, a sending-end flexible DC converter station, a receiving-end flexible DC converter station, a first DC overhead line, a second DC overhead line, an AC energy consumption device, and a DC energy consumption device. The new energy electric field includes a wind farm, a photovoltaic and other renewable energy electric field. The new energy electric field is connected to the sending-end flexible DC converter station through a three-phase AC line. The AC energy consumption device is configured between the new energy electric field and the sending-end flexible DC converter station through a three-phase AC line. The AC energy consumption device is used to dissipate the continuous active output of the new energy electric field when the active power transmission channel of the DC line is blocked, so as to avoid overvoltage caused by energy accumulation in the sending-end AC feeder. One output terminal of the sending-end flexible DC converter station is connected to one input terminal of the receiving-end flexible DC converter station through a first DC overhead line, and the other output terminal of the sending-end flexible DC converter station is connected to the other input terminal of the receiving-end flexible DC converter station through a second DC overhead line. A DC energy dissipation device is configured on the DC side of the receiving-end flexible DC converter station. The DC energy dissipation device is used to dissipate the continuous active power output of the wind farm that is difficult to be transmitted through the DC transmission line during the AC fault of the AC power grid, so as to avoid overvoltage of the DC line due to energy accumulation. The receiving-end flexible DC converter station is connected to the AC power grid.

[0035] The new energy bipolar flexible DC transmission system is a true bipolar flexible DC transmission system with completely symmetrical bipolar poles. The bipolar operation greatly improves the stable operation capability of the DC transmission system. The grounding method is simple in design. In the event of a single-pole fault or even a final lockout, the normal pole can maintain the stable operation of the sending-end island system.

[0036] The sending-end flexible DC converter station and the receiving-end flexible DC converter station can adopt a single valve group structure or a double valve group series structure. The converter valves of the sending-end flexible DC converter station and the receiving-end flexible DC converter station adopt a full half-bridge submodule hybrid topology structure. The number of the sending-end flexible DC converter station is one or more, realizing single power supply or multiple power supply. The receiving-end flexible DC converter station can be one or more, realizing single-point or multi-point power supply. When the number of the sending-end flexible DC converter station and the receiving-end flexible DC converter station is more than two, the first DC overhead line and the second DC overhead line connect the DC sides of each sending-end flexible DC converter station and the receiving-end flexible DC converter station in a star structure or a triangle structure.

[0037] The method for evaluating the grounding clamp position of the metal return line of the flexible DC transmission system includes the following steps:

[0038] Step S1, build a new energy bipolar flexible direct current transmission system in the simulation software, unlock the sending-end flexible direct current converter station and the receiving-end flexible direct current converter station in order according to the design requirements in a metal grounding manner, and adjust the power of the sending-end flexible direct current converter station and the receiving-end flexible direct current converter station to the rated value;

[0039] Step S2, respectively simulating the grounding fault at the end of the DC line when the grounding clamping point is set at the sending-end flexible DC converter station and the receiving-end flexible DC converter station when the metal loop is running (such as Figure 3 Position A, B in the middle);

[0040] Step S3, comparing the submodule capacitor voltages of the sending-end flexible DC converter station during the period when the output negative voltage clears the DC fault when the grounding clamping points are respectively set at the sending-end flexible DC converter station and the receiving-end flexible DC converter station, and selecting the grounding clamping point of the flexible DC converter station with the smallest submodule capacitor voltage as the system grounding clamping point;

[0041] Step S4, check whether there is overvoltage in the voltage of the remaining sub-module capacitors of the sending-end flexible DC converter station at the system grounding clamping point. If there is no overvoltage in the sub-module capacitors, the selected system grounding clamping point meets the design requirements. If there is overvoltage in the sub-module capacitors, remove the selected system grounding clamping point and return to step S3.

[0042] The method for evaluating the grounding clamp position of a metal loop of a flexible DC transmission system provided by the present invention makes full use of the flexibility of the grounding clamp position design when the metal loop is in operation to simulate the grounding fault at the end of the DC line, and finally determines the grounding clamp position when the metal loop is in operation by comparing the voltage of the capacitor of the flexible DC submodule at different clamp positions and whether there is an overvoltage phenomenon in the capacitor voltage of the flexible DC converter station submodule at the selected grounding clamp position. This avoids the risk of the converter station submodule capacitor being tripped due to overvoltage caused by continuous charging of the converter station submodule capacitor by the DC side, and solves the technical problem that the DC side continuously charges the converter station submodule capacitor and trips due to overvoltage caused by the converter station submodule capacitor when the existing metal loop is in operation.

[0043] The present invention provides an embodiment of a device for evaluating the grounding clamping point of a metal return line of a flexible DC transmission system, which is applied to a new energy bipolar flexible DC transmission system. The new energy bipolar flexible DC transmission system includes a new energy electric field, a sending-end flexible DC converter station, a receiving-end flexible DC converter station, a first DC overhead line, a second DC overhead line, an AC energy consumption device, and a DC energy consumption device. The new energy electric field includes a wind farm, a photovoltaic and other renewable energy electric field. The new energy electric field is connected to the sending-end flexible DC converter station through a three-phase AC line. The AC energy consumption device is configured between the new energy electric field and the sending-end flexible DC converter station through a three-phase AC line. The AC energy consumption device is used to dissipate the continuous active output of the new energy electric field when the active power transmission channel of the DC line is blocked, so as to avoid overvoltage caused by energy accumulation in the sending-end AC feeder. One output terminal of the sending-end flexible DC converter station is connected to one input terminal of the receiving-end flexible DC converter station through a first DC overhead line, and the other output terminal of the sending-end flexible DC converter station is connected to the other input terminal of the receiving-end flexible DC converter station through a second DC overhead line. A DC energy dissipation device is configured on the DC side of the receiving-end flexible DC converter station. The DC energy dissipation device is used to dissipate the continuous active power output of the wind farm that is difficult to be transmitted through the DC transmission line during the AC fault of the AC power grid, so as to avoid overvoltage of the DC line due to energy accumulation. The receiving-end flexible DC converter station is connected to the AC power grid.

[0044] The new energy bipolar flexible DC transmission system is a true bipolar flexible DC transmission system with completely symmetrical bipolar poles. The bipolar operation greatly improves the stable operation capability of the DC transmission system. The grounding method is simple in design. In the event of a single-pole fault or even a final lockout, the normal pole can maintain the stable operation of the sending-end island system.

[0045] The sending-end flexible DC converter station and the receiving-end flexible DC converter station can adopt a single valve group structure or a double valve group series structure. The converter valves of the sending-end flexible DC converter station and the receiving-end flexible DC converter station adopt a full half-bridge submodule hybrid topology structure. The number of the sending-end flexible DC converter station is one or more, realizing single power supply or multiple power supply. The receiving-end flexible DC converter station can be one or more, realizing single-point or multi-point power supply. When the number of the sending-end flexible DC converter station and the receiving-end flexible DC converter station is more than two, the first DC overhead line and the second DC overhead line connect the DC sides of each sending-end flexible DC converter station and the receiving-end flexible DC converter station in a star structure or a triangle structure.

[0046] The method for evaluating the grounding clamp position of the metal return line of the flexible DC transmission system includes the following steps:

[0047] Step S1, build a new energy bipolar flexible direct current transmission system in the simulation software, unlock the sending-end flexible direct current converter station and the receiving-end flexible direct current converter station in order according to the design requirements in a metal grounding manner, and adjust the power of the sending-end flexible direct current converter station and the receiving-end flexible direct current converter station to the rated value;

[0048] Step S2, respectively simulating the grounding fault at the end of the DC line when the grounding clamping point is set at the sending-end flexible DC converter station and the receiving-end flexible DC converter station when the metal loop is running (such as Figure 3 Position A, B in the middle);

[0049] Step S3, comparing the submodule capacitor voltages of the sending-end flexible DC converter station during the period when the output negative voltage clears the DC fault when the grounding clamping points are respectively set at the sending-end flexible DC converter station and the receiving-end flexible DC converter station, and selecting the grounding clamping point of the flexible DC converter station with the smallest submodule capacitor voltage as the system grounding clamping point;

[0050] Step S4, check whether there is overvoltage in the voltage of the remaining sub-module capacitors of the sending-end flexible DC converter station at the system grounding clamping point. If there is no overvoltage in the sub-module capacitors, the selected system grounding clamping point meets the design requirements. If there is overvoltage in the sub-module capacitors, remove the selected system grounding clamping point and return to step S3.

[0051] As described above, the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit the same. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that the technical solutions described in the aforementioned embodiments may still be modified, or some of the technical features thereof may be replaced by equivalents. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for evaluating the grounding clamp position of a metal return line in a flexible DC transmission system, characterized in that: Applied to a new energy bipolar flexible direct current transmission system, the new energy bipolar flexible direct current transmission system includes a new energy electric field, a sending-end flexible direct current converter station, a receiving-end flexible direct current converter station, a first direct current overhead line, a second direct current overhead line, an AC energy consumption device and a DC energy consumption device. The new energy electric field is connected to the sending-end flexible direct current converter station through a three-phase AC line. The AC energy consumption device is configured between the new energy electric field and the sending-end flexible direct current converter station through a three-phase AC line. An output terminal of the sending-end flexible direct current converter station is connected to an input terminal of the receiving-end flexible direct current converter station through a first direct current overhead line. Another output terminal of the sending-end flexible direct current converter station is connected to another input terminal of the receiving-end flexible direct current converter station through a second direct current overhead line. The DC energy consumption device is configured on the DC side of the receiving-end flexible direct current converter station. The receiving-end flexible direct current converter station is connected to the AC power grid. The method for evaluating the grounding clamp position of the metal return line of the flexible DC transmission system includes the following steps: S1. Build a new energy bipolar flexible DC transmission system in the simulation software, unlock the sending-end flexible DC converter station and the receiving-end flexible DC converter station in order according to the design requirements in a metal grounding manner, and adjust the power of the sending-end flexible DC converter station and the receiving-end flexible DC converter station to the rated value; S2, respectively simulating the grounding fault at the end of the DC line when the grounding clamping point is set at the sending-end flexible DC converter station and the receiving-end flexible DC converter station during the operation of the metal loop; S3, comparing the submodule capacitor voltages of the sending-end flexible DC converter station during the period when the output negative voltage clears the DC fault when the grounding clamping points are respectively set at the sending-end flexible DC converter station and the receiving-end flexible DC converter station, and selecting the grounding clamping point of the flexible DC converter station with the smallest submodule capacitor voltage as the system grounding clamping point; S4. Check whether there is overvoltage in the voltage of the remaining sub-module capacitors of the flexible DC converter station at the sending end of the system grounding clamp point. If there is no overvoltage in the sub-module capacitors, the selected system grounding clamp point meets the design requirements. If there is overvoltage in the sub-module capacitors, remove the selected system grounding clamp point and return to step S3.

2. The method for evaluating the grounding clamp position of the metal return line of the flexible DC transmission system according to claim 1 is characterized in that: The new energy bipolar flexible direct current transmission system is a true bipolar flexible direct current transmission system.

3. The method for evaluating the grounding clamp position of the metal return line of the flexible DC transmission system according to claim 1 is characterized in that: The sending-end flexible DC converter station and the receiving-end flexible DC converter station adopt a single valve group structure or a double valve group series structure.

4. The method for evaluating the grounding clamp position of the metal return line of the flexible DC transmission system according to claim 1 is characterized in that: The converter valves of the sending-end flexible DC converter station and the receiving-end flexible DC converter station adopt a full half-bridge sub-module hybrid topology structure.

5. The method for evaluating the grounding clamp position of the metal return line of the flexible DC transmission system according to claim 1 is characterized in that: The number of the sending-end flexible DC converter station and the receiving-end flexible DC converter station is one or more.

6. The method for evaluating the grounding clamp position of the metal return line of the flexible DC transmission system according to claim 5 is characterized in that: When the number of the sending-end flexible DC converter station and the receiving-end flexible DC converter station is more than two, the first DC overhead line and the second DC overhead line connect the DC sides of each sending-end flexible DC converter station and the receiving-end flexible DC converter station in a star structure or a delta structure.

7. A device for evaluating the grounding clamping position of a metal return line in a flexible DC transmission system, characterized in that: Applied to a new energy bipolar flexible DC transmission system, the new energy bipolar flexible DC transmission system includes a new energy electric field, a sending-end flexible DC converter station, a receiving-end flexible DC converter station, a first DC overhead line, a second DC overhead line, an AC energy consumption device and a DC energy consumption device. The new energy electric field is connected to the sending-end flexible DC converter station through a three-phase AC line. The AC energy consumption device is configured between the new energy electric field and the sending-end flexible DC converter station through a three-phase AC line. An output terminal of the sending-end flexible DC converter station is connected to an input terminal of the receiving-end flexible DC converter station through a first DC overhead line. Another output terminal of the sending-end flexible DC converter station is connected to another input terminal of the receiving-end flexible DC converter station through a second DC overhead line. The DC energy consumption device is configured on the DC side of the receiving-end flexible DC converter station. The receiving-end flexible DC converter station is connected to the AC power grid. The Flexible DC transmission system metal return line grounding clamp point assessment device is used to perform the following steps: S1. Build a new energy bipolar flexible DC transmission system in the simulation software, unlock the sending-end flexible DC converter station and the receiving-end flexible DC converter station in order according to the design requirements in a metal grounding manner, and adjust the power of the sending-end flexible DC converter station and the receiving-end flexible DC converter station to the rated value; S2, respectively simulating the grounding fault at the end of the DC line when the grounding clamping point is set at the sending-end flexible DC converter station and the receiving-end flexible DC converter station during the operation of the metal loop; S3, comparing the submodule capacitor voltages of the sending-end flexible DC converter station during the period when the output negative voltage clears the DC fault when the grounding clamping points are respectively set at the sending-end flexible DC converter station and the receiving-end flexible DC converter station, and selecting the grounding clamping point of the flexible DC converter station with the smallest submodule capacitor voltage as the system grounding clamping point; S4. Check whether there is overvoltage in the voltage of the remaining sub-module capacitors of the flexible DC converter station at the sending end of the system grounding clamp point. If there is no overvoltage in the sub-module capacitors, the selected system grounding clamp point meets the design requirements. If there is overvoltage in the sub-module capacitors, remove the selected system grounding clamp point and return to step S3.

8. The device for evaluating the grounding clamping position of a metal return line of a flexible DC transmission system according to claim 7, characterized in that: The new energy bipolar flexible direct current transmission system is a true bipolar flexible direct current transmission system.

9. The device for evaluating the grounding clamping position of a metal return line of a flexible DC transmission system according to claim 7, characterized in that: The sending-end flexible DC converter station and the receiving-end flexible DC converter station adopt a single valve group structure or a double valve group series structure.

10. The device for evaluating the grounding clamping position of a metal return line of a flexible DC transmission system according to claim 7, characterized in that: The converter valves of the sending-end flexible DC converter station and the receiving-end flexible DC converter station adopt a full half-bridge sub-module hybrid topology structure.

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