Fault control method suitable for direct-current power transmission system and direct-current power transmission system

Abstract

The invention discloses a fault control method suitable for a direct-current power transmission system and the direct-current power transmission system, the direct-current power transmission system at least comprises a receiving end side and a sending end side, the receiving end side is provided with a receiving end alternating-current side suitable for being connected to an alternating-current system, and the method comprises the steps that the receiving end side monitors information of the receiving end alternating-current side in real time, reducing the DC voltage of the receiving end side to zero in response to the monitored information representing that a fault occurs; and the sending end side monitors the electrical parameter information of the polar line connected with the sending end side in real time, and outputs direct current to the polar line connected with the sending end side according to a constant direct current value in response to the electrical parameter information representing that a fault occurs, so that the safety and the economical efficiency of the direct current power transmission system can be improved.

Description

Technical Field

[0001] The present application relates to the technical field of flexible direct current transmission, and in particular to a fault control method and a direct current transmission system applicable to a direct current transmission system. Background Art

[0002] At present, since the distance between the two ends of the UHV flexible DC transmission technology is generally more than 2000km, for the UHV flexible DC system with ultra-long transmission distance, when the AC system on the receiving side has a transient fault, it is easy to cause significant surplus power on the receiving side. In particular, when the communication between the two sides of the UHV flexible DC transmission system is interrupted, the sending-end converter station cannot promptly know the operating status of the receiving-end converter station, which may cause more serious power surplus problems, and may further trigger protection lockout or even shutdown of the UHV flexible DC transmission system, causing serious economic losses and safety hazards. Summary of the invention

[0003] In order to solve the above technical problems, the embodiments of the present application propose a fault control method and a DC power transmission system applicable to a DC power transmission system, which can improve the safety and economy of the DC power transmission system.

[0004] The embodiment of the present application provides a fault control method applicable to a direct current transmission system, wherein the direct current transmission system has at least a receiving end side and a sending end side, and the receiving end side has a receiving end AC side suitable for accessing an AC system, and the method includes:

[0005] The receiving end side monitors the information of the receiving end AC side in real time, and in response to the monitored information indicating that a fault has occurred, reduces the DC voltage of the receiving end side to zero;

[0006] The sending end side monitors the electrical parameter information of the pole line connected to it in real time, and in response to the electrical parameter information indicating a fault, outputs direct current to the pole line connected to the sending end side according to a constant direct current value.

[0007] Optionally, the electrical parameter information includes pole line current and pole line voltage;

[0008] The electrical parameter information characterizes the conditions for the occurrence of a fault, including: the pole line current is higher than a first current threshold, and the pole line voltage is lower than a first voltage threshold.

[0009] Optionally, after the polar line connected to the sending end side outputs direct current according to a constant direct current value, the method further includes:

[0010] In response to the electrical parameter information indicating that the fault has been cleared, the sending end side outputs direct current to the pole line to which it is connected at a constant power value.

[0011] Optionally, the electrical parameter information characterizes a condition for fault clearing including: the pole line voltage is higher than a second voltage threshold, wherein the second voltage threshold is greater than the first voltage threshold.

[0012] Optionally, the constant power value includes: a rated power value of the sending end, or a DC output power value of the sending end at a moment before the electrical parameter information represents a fault.

[0013] Optionally, the constant DC value is 0.

[0014] Optionally, the monitored information includes a positive sequence component of a three-phase voltage on the receiving end AC side;

[0015] The monitored information represents the conditions under which the failure occurs, including any of the following:

[0016] The positive sequence component is lower than a third voltage threshold;

[0017] The value of the positive-sequence component at one monitoring moment is smaller than the value of the positive-sequence component at the previous monitoring moment, and within a time period after the monitoring moment that exceeds a first set time period, the value of the positive-sequence component is within an interval range between the value of the positive-sequence component at the previous monitoring moment and the third voltage threshold.

[0018] Optionally, reducing the DC voltage on the receiving end side to zero includes:

[0019] The DC voltage on the receiving end side is continuously reduced at a first rate until the DC voltage on the receiving end side is reduced to zero, wherein the first rate is a set constant rate.

[0020] Optionally, after reducing the DC voltage on the receiving end side to zero, the method further includes:

[0021] In response to the monitored information indicating that the fault is cleared, the receiving end side increases its DC voltage to a receiving end target voltage value.

[0022] Optionally, the receiving end target voltage value includes any one of the following:

[0023] The rated DC voltage value of the receiving end side;

[0024] The monitored information at the receiving end represents a DC voltage value at a moment before a fault occurs.

[0025] Optionally, the receiving end side increases its DC voltage to a receiving end target voltage value, including:

[0026] The receiving end side continuously increases its DC voltage at a second rate until the DC voltage of the receiving end side increases to the receiving end target voltage value, wherein the second rate is a set constant rate.

[0027] The embodiment of the present application further provides a direct current transmission system, wherein the direct current transmission system has at least a receiving end side and a sending end side, wherein the receiving end side has a receiving end AC side suitable for accessing an AC system:

[0028] The receiving end side is configured to monitor information of the receiving end AC side in real time, and in response to the monitored information indicating a fault, reduce the DC voltage of the receiving end side to zero;

[0029] The sending end side is configured to monitor the electrical parameter information of the pole line connected thereto in real time, and in response to the electrical parameter information indicating a fault, output direct current to the pole line connected to the sending end side at a constant direct current value.

[0030] Optionally, the electrical parameter information includes pole line current and pole line voltage;

[0031] The electrical parameter information characterizes the conditions for the occurrence of a fault, including: the pole line current is higher than a first current threshold, and the pole line voltage is lower than a first voltage threshold.

[0032] Optionally, after the pole line connected to the sending end side outputs direct current according to a constant direct current value;

[0033] In response to the electrical parameter information indicating that the fault has been cleared, the sending end side is further configured to output direct current to the pole line connected thereto at a constant power value.

[0034] Optionally, the electrical parameter information characterizes a condition for fault clearing including: the pole line voltage is higher than a second voltage threshold, wherein the second voltage threshold is greater than the first voltage threshold.

[0035] Optionally, the constant power value includes: a rated power value of the sending end, or a DC output power value of the sending end at a moment before the electrical parameter information represents a fault.

[0036] Optionally, the constant DC value is 0.

[0037] Optionally, the monitored information includes a positive sequence component of a three-phase voltage on the receiving end AC side;

[0038] The monitored information represents the conditions under which the failure occurs, including any of the following:

[0039] The positive sequence component is lower than a third voltage threshold;

[0040] The value of the positive-sequence component at one monitoring moment is smaller than the value of the positive-sequence component at the previous monitoring moment, and within a time period after the monitoring moment that exceeds a first set time period, the value of the positive-sequence component is within an interval range between the value of the positive-sequence component at the previous monitoring moment and the third voltage threshold.

[0041] Optionally, reducing the DC voltage on the receiving end side to zero includes:

[0042] The DC voltage on the receiving end side is continuously reduced at a first rate until the DC voltage on the receiving end side is reduced to zero, wherein the first rate is a set constant rate.

[0043] Optionally, after the DC voltage on the receiving end side is reduced to zero;

[0044] In response to the monitored information indicating that the fault is cleared, the receiving end side is further configured to increase its DC voltage to a receiving end target voltage value.

[0045] Optionally, the receiving end target voltage value includes any one of the following:

[0046] The rated DC voltage value of the receiving end side;

[0047] The monitored information at the receiving end represents a DC voltage value at a moment before a fault occurs.

[0048] Optionally, the step of increasing the DC voltage to a target voltage value at the receiving end includes:

[0049] The DC voltage is continuously increased at a second rate until the DC voltage at the receiving end side is increased to the receiving end target voltage value, wherein the second rate is a set constant rate.

[0050] In summary, the embodiments of the present application have at least the following beneficial effects:

[0051] According to the embodiment of the present application, a DC power transmission system has at least a receiving side and a sending side, wherein the receiving side has a receiving AC side suitable for accessing an AC system, and the method comprises: the receiving side monitors information of the receiving AC side in real time, and in response to the monitored information indicating a fault, reduces the DC voltage of the receiving side to zero; the sending side monitors electrical parameter information of the pole line connected thereto in real time, and in response to the electrical parameter information indicating a fault, outputs DC power to the pole line connected to the sending side according to a constant DC value, thereby improving the safety and economy of the DC power transmission system. BRIEF DESCRIPTION OF THE DRAWINGS

[0052] Figure 1 is a flow chart of a fault control method applicable to a DC power transmission system provided in an embodiment of the present application;

[0053] Figure 2 is a schematic diagram of electrical parameter information on a DC pole line during a fault provided by an embodiment of the present application;

[0054] Figure 3 It is a schematic diagram of the maximum voltage of the receiving-end converter valve submodule when a fault occurs, provided in an embodiment of the present application;

[0055] Figure 4 It is a schematic diagram of the structure of a direct current transmission system provided in an embodiment of the present application. DETAILED DESCRIPTION

[0056] The following will be combined with the drawings in the embodiments of the present application to clearly and completely describe the technical solutions in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of this application.

[0057] In the description of the present application, the terms "first", "second", "third", etc. are used for descriptive purposes only and are not to be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Thus, features defined as "first", "second", "third", etc. may explicitly or implicitly include one or more of the features. In the description of the present application, unless otherwise specified, the meaning of "multiple" is two or more. In the description of the present application, the term "including" and its variations are open inclusions, i.e., "including but not limited to". The term "based on" means "at least partially based on". The term "according to" means "at least partially based on". The term "one embodiment" means "at least one embodiment"; the term "another embodiment" means "at least one other embodiment"; the term "some embodiments" means "at least some embodiments".

[0058] In the description of this application, it should be noted that, unless otherwise clearly specified and limited, the terms "installed", "connected", and "connected" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or it can be indirectly connected through an intermediate medium, or it can be the internal communication of two components. For ordinary technicians in this field, the specific meanings of the above terms in this application can be understood according to specific circumstances.

[0059] In the description of this application, it should be noted that, unless otherwise defined, all technical and scientific terms used in this application have the same meaning as those commonly understood by those skilled in the art. The terms used in the specification of this application are only for the purpose of describing specific embodiments and are not intended to limit this application. For those of ordinary skill in the art, the specific meanings of the above terms in this application can be understood by specific circumstances.

[0060] See also Figure 1 , a flow chart of a fault control method applicable to a DC power transmission system provided by an embodiment of the present application is shown, wherein the DC power transmission system has at least a receiving side and a sending side, the communication between the receiving side and the sending side is interrupted, and a pole line suitable for transmitting DC power is connected between the receiving side and the sending side, the sending side is configured to transmit DC power to the receiving side via the pole line connected thereto, and the receiving side has a receiving AC side suitable for accessing an AC system, the method comprising steps S101-S102, which are specifically as follows:

[0061] S101, the receiving end side monitors the information of the receiving end AC side in real time, and in response to the monitored information indicating that a fault has occurred, reduces the DC voltage of the receiving end side to zero;

[0062] S102, the sending end side monitors the electrical parameter information of the pole line connected thereto in real time, and in response to the electrical parameter information indicating a fault, outputs direct current to the pole line connected to the sending end side according to a constant direct current value.

[0063] In one example, the DC transmission system in this embodiment is a typical ±800kV / 8GW ultra-high voltage flexible DC transmission system, and its main circuit parameters are as follows: submodule rated working voltage 2.0kV, submodule capacitance 24mF, bridge arm reactor 25mH, current limiting reactor 300mH. The DC transmission line is 2400km long, using 6 split conductors, and the line resistance is about 10Ω. The ultra-high voltage flexible DC transmission system adopts a true bipolar wiring scheme, with two valve groups cascaded per pole, and the DC voltage of each valve group is 400kV. The valve group adopts a full half-bridge hybrid module, and the full bridge ratio is 60%. The AC side of the sending end converter station is in a networking mode, the sending end adopts a fixed active power and fixed reactive power control strategy, and the receiving end adopts a fixed DC voltage and fixed reactive power control strategy.

[0064] It can be understood that the information that the sending side and the receiving side need to detect for determining whether a fault has occurred is different in this embodiment, and each detects independently. Therefore, there is no need to rely on communication interaction between the sending side and the receiving side to inform each other whether a fault has occurred. This is more effective for addressing serious power surplus problems caused by communication interruption on both sides.

[0065] In an optional implementation, the electrical parameter information includes pole line current and pole line voltage;

[0066] The electrical parameter information characterizes the conditions for the occurrence of a fault, including: the pole line current is higher than a first current threshold, and the pole line voltage is lower than a first voltage threshold.

[0067] In some cases, the first current threshold in this embodiment may be 1.1 pu, wherein pu in each embodiment of this application refers to a unitary value.

[0068] In some cases, the first voltage threshold in this embodiment may be 0.4 pu.

[0069] In an optional implementation manner, after the polar line connected to the sending end side outputs direct current according to a constant direct current value, the method further includes:

[0070] In response to the electrical parameter information indicating that the fault has been cleared, the sending end side outputs direct current to the pole line to which it is connected at a constant power value.

[0071] In an optional implementation, the electrical parameter information characterizes the condition for fault clearing including: the pole line voltage is higher than a second voltage threshold, wherein the second voltage threshold is greater than the first voltage threshold.

[0072] In some cases, the second voltage threshold in this embodiment may be 0.8 pu.

[0073] In an optional implementation, the constant power value includes: a rated power value of the sending end, or a DC output power value of the sending end at a moment before the electrical parameter information represents a fault.

[0074] In an optional implementation, the constant DC value is 0.

[0075] In an optional implementation manner, the monitored information includes a positive sequence component of a three-phase voltage on the AC side of the receiving end;

[0076] The monitored information represents the conditions under which the failure occurs, including any of the following:

[0077] The positive sequence component is lower than a third voltage threshold;

[0078] The value of the positive sequence component at one monitoring moment is smaller than the value of the positive sequence component at the previous monitoring moment, and within a time period after the monitoring moment that exceeds a first set time period, the value of the positive sequence component is within an interval range between the value of the positive sequence component at the previous monitoring moment and the third voltage threshold.

[0079] In some cases, the third voltage threshold in this embodiment may be 0.7 pu.

[0080] In some cases, the first set time length in this embodiment may be 100 ms.

[0081] In an optional implementation manner, reducing the DC voltage on the receiving end side to zero includes:

[0082] The DC voltage on the receiving end side is continuously reduced at a first rate until the DC voltage on the receiving end side is reduced to zero, wherein the first rate is a set constant rate.

[0083] In some cases, the first rate in this embodiment may be 800 V / μs.

[0084] In an optional implementation manner, after reducing the DC voltage on the receiving end side to zero, the method further includes:

[0085] In response to the monitored information indicating that the fault is cleared, the receiving end side increases its DC voltage to a receiving end target voltage value.

[0086] In an optional implementation manner, the receiving end target voltage value includes any one of the following:

[0087] The rated DC voltage value of the receiving end side;

[0088] The monitored information at the receiving end represents a DC voltage value at a moment before a fault occurs.

[0089] In an optional implementation manner, the receiving end side increases its DC voltage to a receiving end target voltage value, including:

[0090] The receiving end side continuously increases its DC voltage at a second rate until the DC voltage of the receiving end side increases to the receiving end target voltage value, wherein the second rate is a set constant rate.

[0091] In some cases, the second rate in this embodiment may be 100 V / μs.

[0092] A specific example is given below.

[0093] Based on a typical ±800kV / 8GW ultra-high voltage flexible direct current transmission system, its main circuit parameters are as follows: submodule rated working voltage 2.0kV, submodule capacitance 24mF, bridge arm reactor 25mH, current limiting reactor 300mH. The length of the direct current transmission line is 2400km, using 6 split conductors, and the line resistance is about 10Ω. The ultra-high voltage flexible direct current transmission system adopts a true bipolar wiring scheme, with two valve groups cascaded per pole, and the DC voltage of each valve group is 400kV. The valve group adopts a full half-bridge hybrid module, and the full bridge ratio is 60%. The AC side of the sending end converter station is in networking mode, the sending end adopts a fixed active power and fixed reactive power control strategy, and the receiving end adopts a fixed DC voltage and fixed reactive power control strategy.

[0094] Before the fault occurs, the UHV flexible DC transmission system operates at full capacity. Figure 2 As shown in Figure 5.1s, U_P1, I_P1, P_P1 are the voltage, current, and power on the positive polarity transmission line, respectively, and U_P2, I_P2, P_P2 are the voltage, current, and power on the negative polarity transmission line, respectively). A two-phase metallic grounding fault occurs on the AC side of the receiving-end converter, and the positive sequence component of the three-phase voltage on the AC side drops to 0.33pu. About 10ms after the fault, the receiving-end converter station detects the fault and immediately reduces the DC voltage to zero at a rate of 800V / μs. About 20ms after the fault, the pole line current (i.e., I_P1 or I_P2, it should be understood that I_P1=I_P2 in the same loop) of the sending-end converter station is higher than 1.1pu, and the pole line voltage (i.e., the voltage difference between U_P1 and U_P2) is lower than 0.4pu. The sending-end converter station will immediately change the fixed active power control strategy to the fixed DC current control strategy, and set the DC current command value to zero. About 100ms after the fault, the AC side fault of the receiving-end converter is cleared. About 110ms after the fault, after the receiving-end converter station detects that the AC side fault has been cleared, it immediately increases the DC voltage to the previous value at a rate of 100V / μs. About 170ms after the fault, the pole line voltage of the sending-end converter station is higher than 0.8pu, and the sending-end converter station immediately changes the fixed DC current control strategy to the fixed active power control strategy, and the power command is 1pu. At this time, the DC power (that is, the difference between P_P1 and P_P2) is restored to 1pu and remains stable. Figure 2 As shown in the figure, about 500ms after the fault, the voltage, current and power on the DC pole line basically return to the operating state before the fault. Figure 3The maximum voltage of the converter valve submodule of the receiving station during the fault ride-through period is a three-phase voltage and includes: Va1, Va3, Vb1, Vb3, Vc1, Vc3, which are respectively the maximum voltage of the submodule of the upper bridge arm of phase a, the maximum voltage of the submodule of the lower bridge arm of phase a, the maximum voltage of the submodule of the upper bridge arm of phase b, the maximum voltage of the submodule of the lower bridge arm of phase b, the maximum voltage of the submodule of the upper bridge arm of phase c, and the maximum voltage of the submodule of the lower bridge arm of phase c, which do not exceed 2.7kV, which is lower than the overvoltage protection setting of the submodule. It can be seen that the method provided in the embodiment of the present application can realize system fault ride-through after a fault occurs on the AC side of the receiving converter station.

[0095] The present application embodiment also provides a direct current transmission system 400, see Figure 4 The DC power transmission system 400 at least has a receiving side 402 and a sending side 401, and the receiving side 402 has a receiving AC side suitable for accessing an AC system:

[0096] The receiving side 402 is configured to monitor the information of the receiving AC side in real time, and in response to the monitored information indicating a fault, reduce the DC voltage of the receiving side 402 to zero;

[0097] The sending end side 401 is configured to monitor the electrical parameter information of the pole line connected thereto in real time, and in response to the electrical parameter information indicating a fault, output direct current to the pole line connected to the sending end side 401 according to a constant direct current value.

[0098] In an optional implementation, the electrical parameter information includes pole line current and pole line voltage;

[0099] The electrical parameter information characterizes the conditions for the occurrence of a fault, including: the pole line current is higher than a first current threshold, and the pole line voltage is lower than a first voltage threshold.

[0100] In an optional implementation, after the polar line connected to the sending end side 401 outputs direct current according to a constant direct current value;

[0101] In response to the electrical parameter information indicating that the fault has been cleared, the sending end side 401 is further configured to output direct current to the pole line connected thereto at a constant power value.

[0102] In an optional implementation, the electrical parameter information characterizes the condition for fault clearing including: the pole line voltage is higher than a second voltage threshold, wherein the second voltage threshold is greater than the first voltage threshold.

[0103] In an optional implementation, the constant power value includes: a rated power value of the sending end 401, or a DC output power value of the sending end 401 at a moment before the electrical parameter information represents a fault.

[0104] In an optional implementation, the constant DC value is 0.

[0105] In an optional implementation manner, the monitored information includes a positive sequence component of a three-phase voltage on the AC side of the receiving end;

[0106] The monitored information represents the conditions under which the failure occurs, including any of the following:

[0107] The positive sequence component is lower than a third voltage threshold;

[0108] The value of the positive-sequence component at one monitoring moment is smaller than the value of the positive-sequence component at the previous monitoring moment, and within a time period after the monitoring moment that exceeds a first set time period, the value of the positive-sequence component is within an interval range between the value of the positive-sequence component at the previous monitoring moment and the third voltage threshold.

[0109] In an optional implementation manner, reducing the DC voltage of the receiving end side 402 to zero includes:

[0110] The DC voltage of the receiving end side 402 is continuously reduced at a first rate until the DC voltage of the receiving end side 402 is reduced to zero, wherein the first rate is a set constant rate.

[0111] In an optional implementation, after the DC voltage of the receiving end side 402 is reduced to zero;

[0112] In response to the monitored information indicating that the fault is cleared, the receiving side 402 is further configured to increase its DC voltage to a receiving side target voltage value.

[0113] In an optional implementation manner, the receiving end target voltage value includes any one of the following:

[0114] The rated DC voltage value of the receiving end side 402;

[0115] The monitored information of the receiving end 402 represents the DC voltage value at a moment before the fault occurs.

[0116] In an optional implementation manner, the DC voltage is increased to a target voltage value at the receiving end, including:

[0117] The DC voltage is continuously increased at a second rate until the DC voltage of the receiving end side 402 is increased to the receiving end target voltage value, wherein the second rate is a set constant rate.

[0118] In summary, the embodiments of the present application have at least the following beneficial effects:

[0119] According to the embodiment of the present application, a DC power transmission system has at least a receiving side and a sending side, wherein the receiving side has a receiving AC side suitable for accessing an AC system, and the method comprises: the receiving side monitors information of the receiving AC side in real time, and in response to the monitored information indicating a fault, reduces the DC voltage of the receiving side to zero; the sending side monitors electrical parameter information of the pole line connected thereto in real time, and in response to the electrical parameter information indicating a fault, outputs DC power to the pole line connected to the sending side according to a constant DC value, thereby improving the safety and economy of the DC power transmission system.

[0120] Through the description of the above implementation methods, those skilled in the art can clearly understand that the present application can be implemented by means of software plus the necessary hardware platform, and of course, it can also be implemented entirely by hardware. Based on such an understanding, all or part of the contribution of the technical solution of the present application to the background technology can be embodied in the form of a software product, and the computer software product can be stored in a storage medium, such as ROM / RAM, a disk, an optical disk, etc., including a number of instructions for a computer device (which can be a personal computer, a server, or a network device, etc.) to execute the methods described in each embodiment of the present application or some parts of the embodiments.

[0121] The above is a preferred embodiment of the present application. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present application. These improvements and modifications are also considered to be within the scope of protection of the present application.

Claims

1. A fault control method applicable to a DC power transmission system, characterized in that: The DC power transmission system has at least a receiving side and a sending side, the receiving side has a receiving AC side suitable for accessing an AC system, and the method includes: The receiving end side monitors the information of the receiving end AC side in real time, and in response to the monitored information indicating that a fault has occurred, reduces the DC voltage of the receiving end side to zero; The sending end side monitors the electrical parameter information of the pole line connected to it in real time, and in response to the electrical parameter information indicating a fault, outputs direct current to the pole line connected to the sending end side according to a constant direct current value.

2. The method according to claim 1, characterized in that The electrical parameter information includes pole line current and pole line voltage; The electrical parameter information characterizes the conditions for the occurrence of a fault, including: the pole line current is higher than a first current threshold, and the pole line voltage is lower than a first voltage threshold.

3. The method according to claim 2, characterized in that After the polar line connected to the sending end side outputs direct current according to a constant direct current value, the method further includes: In response to the electrical parameter information indicating that the fault has been cleared, the sending end side outputs direct current to the pole line to which it is connected at a constant power value.

4. The method according to claim 3, characterized in that The electrical parameter information indicates that the condition for clearing the fault includes: the pole line voltage is higher than a second voltage threshold, wherein the second voltage threshold is greater than the first voltage threshold.

5. The method according to claim 3, characterized in that The constant power value includes: a rated power value of the sending end, or a DC output power value of the sending end at a moment before the electrical parameter information represents a fault.

6. The method according to claim 1, characterized in that The constant DC value is 0.

7. The method according to claim 1, characterized in that The monitored information includes the positive sequence component of the three-phase voltage on the receiving end AC side; The monitored information represents the conditions under which the failure occurs, including any of the following: The positive sequence component is lower than a third voltage threshold; The value of the positive sequence component at one monitoring moment is smaller than the value of the positive sequence component at the previous monitoring moment, and within a time period after the monitoring moment that exceeds a first set time period, the value of the positive sequence component is within an interval range between the value of the positive sequence component at the previous monitoring moment and the third voltage threshold.

8. The method according to claim 1, characterized in that The step of reducing the DC voltage on the receiving end side to zero includes: The DC voltage on the receiving end side is continuously reduced at a first rate until the DC voltage on the receiving end side is reduced to zero, wherein the first rate is a set constant rate.

9. The method according to any one of claims 1 to 8, characterized in that: After reducing the DC voltage on the receiving end side to zero, the method further includes: In response to the monitored information indicating that the fault is cleared, the receiving end side increases its DC voltage to a receiving end target voltage value.

10. The method according to claim 9, characterized in that The receiving end target voltage value includes any of the following: The rated DC voltage value of the receiving end side; The monitored information at the receiving end represents a DC voltage value at a moment before a fault occurs.

11. The method according to claim 9, characterized in that The receiving end side increases its DC voltage to a receiving end target voltage value, including: The receiving end side continuously increases its DC voltage at a second rate until the DC voltage of the receiving end side increases to the receiving end target voltage value, wherein the second rate is a set constant rate.

12. A direct current transmission system, characterized in that: The DC power transmission system at least has a receiving side and a sending side, and the receiving side has a receiving AC side suitable for accessing an AC system: The receiving end side is configured to monitor information of the receiving end AC side in real time, and in response to the monitored information indicating a fault, reduce the DC voltage of the receiving end side to zero; The sending end side is configured to monitor the electrical parameter information of the pole line connected thereto in real time, and in response to the electrical parameter information indicating a fault, output direct current to the pole line connected to the sending end side at a constant direct current value.

13. The system of claim 12, wherein: The electrical parameter information includes pole line current and pole line voltage; The electrical parameter information characterizes the conditions for the occurrence of a fault, including: the pole line current is higher than a first current threshold, and the pole line voltage is lower than a first voltage threshold.

14. The system of claim 13, wherein: After the pole line connected to the sending end side outputs direct current according to a constant direct current value; In response to the electrical parameter information indicating that the fault has been cleared, the sending end side is further configured to output direct current to the pole line connected thereto at a constant power value.

15. The system of claim 14, wherein: The electrical parameter information indicates that the condition for clearing the fault includes: the pole line voltage is higher than a second voltage threshold, wherein the second voltage threshold is greater than the first voltage threshold.

16. The system of claim 14, wherein: The constant power value includes: a rated power value of the sending end, or a DC output power value of the sending end at a moment before the electrical parameter information represents a fault.

17. The system of claim 12, wherein: The constant DC value is 0.

18. The system of claim 12, wherein: The monitored information includes the positive sequence component of the three-phase voltage on the receiving end AC side; The monitored information represents the conditions under which the failure occurs, including any of the following: The positive sequence component is lower than a third voltage threshold; The value of the positive sequence component at one monitoring moment is smaller than the value of the positive sequence component at the previous monitoring moment, and within a time period after the monitoring moment that exceeds a first set time period, the value of the positive sequence component is within an interval range between the value of the positive sequence component at the previous monitoring moment and the third voltage threshold.

19. The system of claim 12, wherein: The step of reducing the DC voltage on the receiving end side to zero includes: The DC voltage on the receiving end side is continuously reduced at a first rate until the DC voltage on the receiving end side is reduced to zero, wherein the first rate is a set constant rate.

20. The system according to any one of claims 12 to 19, characterized in that: After the DC voltage on the receiving end side is reduced to zero; In response to the monitored information indicating that the fault is cleared, the receiving end side is further configured to increase its DC voltage to a receiving end target voltage value.

21. The system of claim 20, wherein: The receiving end target voltage value includes any of the following: The rated DC voltage value of the receiving end side; The monitored information at the receiving end represents a DC voltage value at a moment before a fault occurs.

22. The system of claim 20, wherein: The step of increasing the DC voltage to a target voltage value at the receiving end includes: The DC voltage is continuously increased at a second rate until the DC voltage at the receiving end side is increased to the receiving end target voltage value, wherein the second rate is a set constant rate.

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