Method for establishing and confirming target loop of multi-terminal parallel dc engineering metal ground conversion

Abstract

The application discloses a method for establishing and confirming a target loop of multi-terminal parallel DC engineering metal ground conversion, and specifically comprises the following steps: after a metal ground conversion command is issued, a parallel target loop is established by operating a DC field switch breaker; DC field data is acquired; firstly, the position of the DC field switch breaker is judged; if the condition is not met, it is confirmed that the target loop establishment fails; if the condition is met, the current of the target loop is continuously judged; if the condition is met, it is confirmed that the target loop establishment succeeds; if the condition is not met, the initial loop conversion switch is separated, and the current of the target loop is judged again; if the condition is met, it is confirmed that the target loop establishment succeeds; if the condition is not met, the initial loop switch is closed, and it is confirmed that the target loop establishment fails. The method can effectively overcome the defects of the prior art, has the advantages of simple and convenient process, no need for complex calculation, no need for power / current level adjustment, no need for dependence on inter-station communication, and the like, can be popularized to more terminals, effectively guarantees the safety of a power grid, and brings practical economic benefits.

Description

TECHNICAL FIELD

[0001] The application relates to a method for confirming establishment of a target loop of a multi-terminal parallel DC engineering metal ground conversion, and belongs to the technical field of DC power transmission. BACKGROUND

[0002] In a DC power transmission project, a bipolar ground return (GR) operation mode is the most commonly used operation mode. When one pole is out of service due to a fault, maintenance or other reasons, the other pole can continue to adopt a monopolar ground return operation mode to transmit power. At this time, the grounding pole current increases with the increase of the transmitted power. In order to avoid the negative impact of the grounding pole current on the nearby ecology, oil and gas pipelines and the like, the monopolar operation generally preferentially adopts a metallic return (MR) operation mode. When the other pole has power transmission conditions, the bipolar ground return operation mode is converted. In the above process, the conversion between the GR and the MR is the most basic sequence control operation of the DC project.

[0003] In a DC power transmission system, the conversion between the two-terminal DC and the multi-terminal DC GR and MR has some differences in the specific operation process, but the ideas are completely consistent. Generally, the conversion can be divided into the following steps: 1) establishing a parallel metallic return path (the initial GR loop and the target MR loop coexist); 2) confirming that the target MR loop is successfully established, separating the metallic return transfer breaker (MRTB), and disconnecting the current of the initial GR loop; 3) operating other switches so that the GR loop completely disappears, and the MR loop exists alone, and the conversion is successful. Similarly, the conversion from the MR to the GR: 1) establishing a parallel ground return path (the initial MR loop and the target GR loop coexist); 2) confirming that the target GR loop is successfully established, separating the earth return transfer breaker (ERTB), and disconnecting the current of the initial MR loop; 3) operating other switches so that the MR loop completely disappears, and the GR loop exists alone, and the conversion is successful.

[0004] In the above process, the target loop establishment confirmation is a very key step. If the target loop is not accurately confirmed to be established, the conversion switch is forcibly separated without taking other measures, which may cause damage to the primary equipment.

[0005] For two-terminal DC project, when GR and MR circuits coexist, the current of the parallel circuit and the equivalent resistance of the circuit are inversely proportional. The target circuit can be reliably confirmed by double criteria of switching quantity and analog quantity. The double criteria of switching quantity and analog quantity are described in detail as follows: the position of the switch related to the circuit (switching quantity criterion) is judged, and the current flowing through the circuit is judged. When the current of the newly established circuit reaches a stable value (analog quantity criterion), it is confirmed that the target circuit has been established.

[0006] For multi-terminal DC project, when GR and MR circuits coexist, the electrical characteristics of the parallel circuit are more complex due to the involvement of multiple stations. In special operating conditions, the current flowing through the target circuit may be very small, and in extreme cases, the current flowing through the target circuit may be close to 0, which may cause the analog quantity criterion to fail. The switching quantity criterion alone cannot accurately confirm the successful establishment of the target circuit.

[0007] In view of the above problems in the process of establishing and confirming the target circuit of the multi-terminal parallel DC metal ground conversion, the current research strategies mainly include: 1) predicting the risk of metal ground conversion failure based on line parameters and adjusting power / current in advance; 2) using different metal ground conversion operation sequences according to the power / current level; 3) first using double criteria of switching quantity and analog quantity, and then trying to adjust the power / current level if it fails. The above research strategies are currently mainly focused on three-terminal DC, and can effectively establish and confirm the circuit in some operating conditions, but still have the following shortcomings: 1) scheme 1 requires theoretical calculation, which requires high accuracy of line parameters. Considering that the line resistance value will change with the change of environmental temperature, it is difficult to accurately predict. Scheme 1 is effective in some operating conditions of three-terminal DC, but if it is extended to more terminals, the calculation amount will increase dramatically due to the increase of dimension, which makes it difficult to achieve accurate prediction. Scheme 1 requires adjusting the power / current level in advance, which has additional requirements for the operation mode of the power grid. 2) Scheme 2 requires pre-operation of different operating modes to adopt appropriate operation sequence, which is complex in logic implementation and not convenient for operation and maintenance. Scheme 2 is effective in some operating conditions of three-terminal DC, but if it is extended to more terminals, it is difficult to accurately confirm the change of operation sequence and the logic is difficult to implement. 3) Scheme 3 is effective in some operating conditions of three-terminal DC, but cannot be extended to more terminals. Scheme 3 requires adjusting the power / current level in advance, which temporarily affects the AC power grid. In addition, the above schemes all need to obtain data from other stations through inter-station communication, which is heavily dependent on inter-station communication. In the case of inter-station communication failure, the above methods are invalid.

[0008] In summary, the method for establishing and confirming the target circuit of the metal ground conversion of the multi-terminal parallel DC project is very important. The current research strategies have many shortcomings, and it is urgent to develop a metal ground conversion target circuit establishment and confirmation method that is suitable for parallel multi-terminal DC project, convenient and feasible, safe and reliable. SUMMARY

[0009] The technical problem solved by the present application is to provide a metal ground conversion target loop establishment confirmation method for multi-terminal parallel DC engineering, which is applicable to multi-terminal parallel DC engineering, convenient and feasible, safe and reliable.

[0010] The present application solves the above technical problems by using the following technical solutions:

[0011] A metal ground conversion target loop establishment confirmation method for multi-terminal parallel DC engineering (as shown in Figure 1 The method comprises the following steps:

[0012] After the metal ground conversion command is issued, the DC field switch knife switch is operated to establish the parallel target loop, and the DC field data (DC field switch knife switch position signal, current flowing through the MRTB and ERTB switches) is acquired, and the following steps are performed to confirm whether the target loop is established:

[0013] Step 1): Determine the position of the DC field switch knife switch. If the target loop condition is not met after the first delay (the delay considers the switch knife switch operation time and has a margin), it is confirmed that the target loop establishment fails, and step 4) is performed. If the target loop condition is met, step 2) is performed.

[0014] Step 2): Determine the target loop current. If the target loop switch current meets the first flow criterion (the switch current reaches a first threshold value and the duration reaches a second delay, the first threshold value considers zero drift and has a margin, and the second delay considers current confirmation delay and has a margin), the target loop establishment is confirmed to be successful, the initial loop conversion switch is separated, and step 5) is performed. If the target loop switch current does not meet the first flow criterion, the initial loop conversion switch is separated, and step 3) is performed.

[0015] Step 3): Determine the target loop current. If the target loop switch current meets the second flow criterion (the switch current reaches a second threshold value and the duration reaches a third delay, the second threshold value considers zero drift and measurement error, and the third delay considers anti-shake and can be fixed at 2-10 ms), the target loop establishment is confirmed to be successful, and step 5) is performed. If the target loop switch current does not meet the second flow criterion, the initial loop switch is closed, the target loop establishment is confirmed to fail, and step 4) is performed.

[0016] Step 4): If the target loop establishment fails, the conversion operation is interrupted.

[0017] Step 5): If the target loop establishment is successful, the initial loop other knife switch is separated, and the metal ground conversion is completed.

[0018] Specifically, if MR is converted to GR, the DC field switch knife switch is operated to establish the parallel GR loop, the DC field data is acquired, and the following steps are performed to confirm whether the GR loop is established (as shown in Figure 2 ).

[0019] Step 1): judging the position of the DC field switch breaker, if the GR loop condition is satisfied after the first delay, then performing step 2); if the GR loop condition is not satisfied after the first delay, then confirming that the GR loop establishment fails, and performing step 4).

[0020] Step 2): judging whether the GR loop has flow, if the MRTB current satisfies the first flow criterion (the MRTB switch current reaches a first threshold value and the duration reaches a second delay, the first threshold value considering zero drift and leaving a margin, and the second delay considering current confirmation delay and leaving a margin), then confirming that the GR loop is successfully established, and separating the initial loop transfer switch ERTB, and performing step 5); if the MRTB current does not satisfy the first flow criterion, then separating the ERTB, and performing step 3).

[0021] Step 3): judging whether the GR loop has flow, if the MRTB current satisfies the second flow criterion (the MRTB switch current reaches a second threshold value and the duration reaches a third delay, the second threshold value considering zero drift and measurement error, and the third delay considering anti-shake delay, and the value is 2-10 ms), then confirming that the GR loop is successfully established, and performing step 5); if the MRTB current does not satisfy the second flow criterion, then closing the ERTB switch, confirming that the GR loop establishment fails, and performing step 4).

[0022] Step 4): confirming that the GR loop establishment fails, then interrupting the transfer operation.

[0023] Step 5): confirming that the GR loop is successfully established, and continuing to operate to separate other breakers, and completing the MR to GR transfer.

[0024] Specifically, if it is a GR transfer to MR, operating the DC field switch breaker to establish a parallel MR loop, obtaining the DC field data, and confirming whether the target loop is established according to the following steps (as shown in Figure 3

[0025] Step 1): judging the position of the DC field switch breaker, if the MR loop condition is satisfied after the first delay, then performing step 2); if the MR loop condition is not satisfied after the first delay, then confirming that the MR loop establishment fails, and performing step 4).

[0026] Step 2): judging whether the MR loop has flow, if the ERTB current satisfies the first flow criterion (the ERTB switch current reaches a first threshold value and the duration reaches a second delay, the first threshold value considering zero drift and leaving a margin, and the second delay considering current confirmation delay and leaving a margin), then confirming that the MR loop is successfully established, and separating the initial loop transfer switch MRTB, and performing step 5); if the ERTB current does not satisfy the first flow criterion, then separating the MRTB, and performing step 3).

[0027] ​Step 3) judging whether the MR loop has flow, if the ERTB current meets the second flow criterion (the ERTB switch current reaches the second threshold value and the duration reaches the third delay, the second threshold value considers zero drift and measurement error, and the third delay considers anti-shake delay and is valued at 2-10 ms), confirming that the MR loop is successfully established, and performing step 5); if the ERTB current does not meet the second flow criterion, quickly recombining the MRTB switch, confirming that the MR loop is unsuccessfully established, and performing step 4).

[0028] Step 4) confirming that the MR loop is unsuccessfully established, then interrupting the conversion operation.

[0029] Step 5) confirming that the MR loop is successfully established, then continuing to operate to separate other switches, and completing the GR to MR conversion.

[0030] Compared with the prior art, the technical scheme of the present application has the following technical effects:

[0031] 1. The present application can effectively overcome the defects of the existing metal ground conversion loop establishment confirmation method and provide a new loop establishment confirmation method. Compared with the existing method, the method proposed in the present application avoids adjusting the direct current power in advance or temporarily from the design principle, reduces the influence of metal ground conversion on the system operation mode, reduces the theoretical calculation workload, does not depend on inter-station communication, improves the metal ground conversion reliability, effectively guarantees the power grid safety and brings tangible economic benefits.

[0032] 2. The present application is not only suitable for three-terminal parallel DC projects, but also suitable for four-terminal, five-terminal and other multi-terminal DC projects, and of course, with slight adjustment, it is also suitable for two-terminal DC projects. BRIEF DESCRIPTION OF DRAWINGS

[0033] Figure 1 is a flow chart of the metal ground conversion target loop establishment confirmation method of the multi-terminal parallel DC project of the present application.

[0034] Figure 2 is a flow chart of the MR conversion to GR loop confirmation method in the present application.

[0035] Figure 3 is a flow chart of the GR conversion to MR loop confirmation method in the present application.

[0036] Fig. 4(a)-(h) is a schematic diagram of the MR to GR loop confirmation method in the three-terminal parallel DC project of the embodiment 1 of the present application, wherein:

[0037] Fig. 4(a) is a schematic diagram of a DC field.

[0038] Fig. 4(b) is a schematic diagram of an initial loop MR.

[0039] Fig. 4(c) is a schematic diagram of S1 MR and GR in parallel.

[0040] Fig. 4(d) is a schematic diagram of S2 MR and GR in parallel.

[0041] Fig. 4(e) is a schematic diagram after S2 ERTB is separated.

[0042] Fig. 4(f) is a schematic diagram of S3 MR and GR in parallel.

[0043] Fig. 4(g) is a schematic diagram after S3 ERTB is separated.

[0044] Fig. 4(h) is a schematic diagram of target loop GR.

[0045] Figs. 5(a)-(f) are schematic diagrams of a two-terminal DC engineering GR to MR loop confirmation method in Embodiment 2 of the present application, wherein:

[0046] Fig. 5(a) is a schematic diagram of a DC field.

[0047] Fig. 5(b) is a schematic diagram of an initial loop GR.

[0048] Fig. 5(c) is a schematic diagram of S2 GR and MR in parallel.

[0049] Fig. 5(d) is a schematic diagram of S1 GR and MR in parallel.

[0050] Fig. 5(e) is a schematic diagram after S2 MRTB is separated.

[0051] Fig. 5(f) is a schematic diagram of target loop MR. DETAILED DESCRIPTION

[0052] Embodiments of the present application are described in detail below, examples of which are shown in the accompanying drawings. The embodiments described below by reference to the drawings are exemplary only, and are used merely for the purpose of explanation of the present application, and are not to be construed as limiting the present application.

[0053] Embodiment 1

[0054] This embodiment is a target loop establishment confirmation method when three-terminal parallel DC MR is converted to GR. A schematic diagram of three-terminal parallel DC is shown in Fig. 4(a): three stations are station 1 (S1), station 2 (S2), and station 3 (S3); two poles are pole 1 (P1) and pole 2 (P2); each pole can be a single valve group (high voltage DC) or a double valve group (extra-high voltage DC); it can be a conventional DC (LCC), a flexible DC (VSC), or a hybrid DC (LCC+VSC); R1, R2, and R3 are line equivalent resistances of the three stations, and R4, R5, and R6 are ground pole line equivalent resistances of the three stations.

[0055] The position of the DC transfer switch (MRTB / ERTB) of the three-terminal parallel DC is configured according to the specific engineering characteristics, and the order of the metal ground conversion operation is also different, and the method provided by the application can be applied. In the embodiment, the DC transfer switch is configured at S2 and S3, and the MR is converted to GR in the mode of S2 first conversion and S3 later conversion.

[0056] Considering that the target loop establishment confirmation method is interspersed in the metal ground conversion process, the conversion process will be described below in combination with the drawings.

[0057] 1. Initial MR state, as shown in Fig. 4(b), the MRTB of S2 and S3 is in the split state. Id1, Id2 and Id3 are the DC currents of the three stations, respectively. MRTB2 , MRTB3 is the current flowing through the S2 and S3 MRTB switch, I ERTB2 , ERTB3 is the current flowing through the S2 and S3 ERTB switch, and the switch current in the MR state is as follows:

[0058] I MRTB2 = 0

[0059] I ERTB2 = I d2

[0060] I MRTB3 = 0

[0061] I ERTB3 = I d3

[0062] 2. S1 MR and GR parallel state, as shown in Fig. 4(c), the switch current is:

[0063] I MRTB2 = 0

[0064] I ERTB2 = I d2

[0065] I MRTB3 = 0

[0066] I ERTB3 = I d3

[0067] 3. S2 MR and GR parallel state, as shown in Fig. 4(d), at this time, the MRTB and ERTB of S2 are in the closed state, and the switch current is:

[0068]

[0069]

[0070] IMRTB3 = 0

[0071] I ERTB3 = I d3

[0072] The target loop establishment confirmation method for the three-terminal parallel DC MR to GR provided by the application comprises: obtaining DC field data, and confirming whether the GR loop is established according to the following steps:

[0073] Step 1): judging the position of the DC field switch breaker, if the delay meets the GR loop condition, step 2) is executed; if the delay does not meet the GR loop condition, it is confirmed that the GR loop establishment fails, and step 4) is executed.

[0074] Step 2): judging whether the GR loop has flow, if the MRTB current meets the first flow criterion, it is confirmed that the GR loop establishment succeeds; if the MRTB current does not meet the first flow criterion, the ERTB is separated, and step 3) is executed.

[0075] Step 3): judging whether the GR loop has flow, if the MRTB current meets the second flow criterion, it is confirmed that the GR loop establishment succeeds, and step 5) is executed; if the MRTB current does not meet the second flow criterion, the ERTB switch is closed, it is confirmed that the GR loop establishment fails, and step 4) is executed.

[0076] Step 4): confirming that the GR loop establishment fails, and interrupting the conversion operation.

[0077] Step 5): confirming that the GR loop establishment succeeds, continuing to operate other breakers, and completing the MR to GR conversion.

[0078] 4, the state after the S2 ERTB is successfully separated, as shown in Fig. 4(e), the switch current is:

[0079] I MRTB2 = I d2

[0080] I ERTB2 = 0

[0081] I MRTB3 = 0

[0082] I ERTB3 = I d3

[0083] 5, the S3 MR and GR parallel state, as shown in Fig. 4(f), at this time, the MRTB and ERTB of S3 are both closed, and the switch current is:

[0084] I MRTB2 = I d2

[0085] I ERTB2= 0

[0086]

[0087]

[0088] Different from the parallel state of S2 MR and GR, when S3 MR and GR are in parallel, the current flowing through the S3 MR TB can be 0, if the operating condition satisfies the following equation:

[0089]

[0090] I MRTB3 = 0, when in this condition, even if the GR loop is established, the current flowing through the loop is still 0, at this time, the analog quantity criterion is invalid, and the switch knife gap position contact abnormality cannot be effectively distinguished when the GR loop is established.

[0091] The target loop establishment confirmation method for three-terminal parallel DC MR to GR provided by the application is adopted, DC field data is acquired, and whether the GR loop is established is confirmed according to the following steps:

[0092] Step 1): The DC field switch knife gap position is judged, if the delay satisfies the GR loop condition, step 2) is executed; if the delay does not satisfy the GR loop condition, it is confirmed that the GR loop establishment fails, and step 4) is executed.

[0093] Step 2): It is judged whether the GR loop has flow, if the MRTB current satisfies the first flow criterion, it is confirmed that the GR loop is successfully established; if the MRTB current does not satisfy the first flow criterion, the ERTB is separated, and step 3) is executed.

[0094] Step 3): It is judged whether the GR loop has flow, if the MRTB current satisfies the second flow criterion, it is confirmed that the GR loop is successfully established, and step 5) is executed; if the MRTB current does not satisfy the second flow criterion, the ERTB switch is closed, it is confirmed that the GR loop establishment fails, and step 4) is executed.

[0095] Step 4): It is confirmed that the GR loop establishment fails, and the conversion operation is interrupted.

[0096] Step 5): It is confirmed that the GR loop is successfully established, other switches are continued to be operated, and the MR to GR conversion is completed.

[0097] 6, the state after the S3 ERTB is successfully separated, as shown in Fig. 4(g), the switch current is:

[0098] I MRTB2 = I d2

[0099] I ERTB2 = 0

[0100] I MRTB3 = I d3

[0101] I ERTB3 = 0

[0102] 7、Target loop GR, as shown in Figure 4 (h), the switch current is:

[0103] I MRTB2 = I d2

[0104] I ERTB2 = 0

[0105] I MRTB3 = I d3

[0106] I ERTB3 = 0

[0107] In summary, in some operating conditions, the existing three-terminal metal ground conversion target loop establishment confirmation method can also be effective, but there are calculation complexity, the need to adjust the power / current level, dependent on inter-station communication and other shortcomings described above. The confirmation method mentioned in the present application is simple and convenient, without complex calculation, without adjusting the power / current level, without relying on inter-station communication, and the method is completely consistent when S2 conversion and S3 conversion, so it can be effectively promoted to four-terminal, five-terminal and more terminals.

[0108] Example 2

[0109] This embodiment is a target loop establishment confirmation method for two-terminal DC GR to MR, and the schematic diagram of two-terminal DC is shown in Figure 5 (a): two stations are station 1 (S1) and station 2 (S2); two poles are pole 1 (P1) and pole 2 (P2); each pole can be a single valve group (high voltage DC), or a double valve group (extra-high voltage DC); it can be a conventional DC (LCC), or a flexible DC (VSC), or a hybrid DC (LCC+VSC), R1 is the equivalent resistance of the DC line, and R2 and R3 are the equivalent resistances of the grounding electrode lines of the two stations. In this embodiment, the DC conversion switch is arranged at S1.

[0110] Considering that the two-terminal and three-terminal metal ground conversion ideas mentioned above are the same, while the operation process is different, for detailed comparison and illustration, the conversion process is described as follows in combination with the drawings:

[0111] 1、Initial GR state, as shown in Figure 5 (b), ERTB is divided, Id is the DC current, I MRTB , I ERTB are the currents flowing through the MRTB and ERTB switches respectively, and the switch current in the GR state is as follows:

[0112] I MRTB =I d

[0113] I ERTB =0

[0114] 2. S2, MR, and GR are connected in parallel, as shown in Figure 5(c). The switching current is:

[0115] I MRTB =I d

[0116] I ERTB =0

[0117] 3. S1 MR and GR are in parallel, as shown in Figure 5(d). At this time, MRTB and ERTB are both closed, and the switching current is:

[0118]

[0119]

[0120] Unlike three-terminal (multi-terminal) circuits, the switching current in the above formula is always greater than 0, and there is no condition where it equals 0. Using the target circuit establishment confirmation method for parallel DC GR to MR circuits provided by this invention, with slight adjustments and removing the original step 3), DC field data is obtained, and the MR circuit is confirmed as follows:

[0121] Step 1): Determine the position of the MR circuit switch. If the MR circuit conditions are met after a delay, proceed to Step 2); if the MR circuit conditions are not met after a delay, confirm that the MR circuit establishment has failed and proceed to Step 3.

[0122] Step 2): Determine if there is current in the MR loop. If the ERTB current meets the current criterion, the MR loop is successfully established, and proceed to step 4. If the ERTB current does not meet the current criterion, the MR loop establishment fails, and proceed to step 3.

[0123] Step 3): If the MR loop establishment fails, interrupt the conversion operation.

[0124] Step 4): Once the MR circuit is successfully established, continue operating other disconnect switches to complete the GR to MR conversion.

[0125] 4. The state after MRTB successfully separates is shown in Figure 5(e). The switching current is:

[0126] I MRTB =0

[0127] I ERTB =I d

[0128] 5. Target loop MR, as shown in Fig. 5(f), the switch current is:

[0129] I MRTB = 0

[0130] I ERTB = I d

[0131] The above examples are only to illustrate the technical idea of the present application, and cannot limit the protection scope of the present application. Any modification made according to the technical idea of the present application on the basis of the technical scheme falls within the protection scope of the present application.

Claims

1. A method for establishing and confirming a target loop of a multi-terminal parallel DC engineering metal ground conversion, characterized in that, After the command of the metal ground conversion is issued, the parallel target loop is established by operating the DC field switch knife switch, the DC field data is acquired, and whether the target loop is established is confirmed according to the following steps: Step 1, the position of the DC field switch knife switch is judged, if the target loop position criterion is met, step 2 is executed, if the target loop position criterion is not met, it is confirmed that the target loop is not established successfully, and step 4 is executed; Step 2, the current of the target loop is judged, if the first current criterion is met, the target loop is established successfully, the initial loop conversion switch is separated, and step 5 is executed, if the first current criterion is not met, the initial loop conversion switch is separated, and step 3 is executed; Step 3, the current of the target loop is judged, if the second current criterion is met, the target loop is established successfully, and step 5 is executed, if the second current criterion is not met, the initial loop conversion switch is closed, it is confirmed that the target loop is not established successfully, and step 4 is executed; Step 4, it is confirmed that the target loop is not established successfully, and the conversion operation is interrupted; Step 5, it is confirmed that the target loop is established successfully, the other knife switches of the initial loop are separated, and the metal ground conversion is completed.

2. The method according to claim 1, wherein the method is characterized by: The command of the metal ground conversion is specifically that when the metal loop MR is converted to the ground loop GR, the parallel GR loop is established by operating the DC field switch knife switch, the DC field data is acquired, and whether the GR loop is established is confirmed according to the following steps: Step 1, the position of the DC field switch knife switch is judged, if the GR loop position criterion is met, step 2 is executed, if the GR loop position criterion is not met, it is confirmed that the GR loop is not established successfully, and step 4 is executed; Step 2, the current of the GR loop is judged, if the first current criterion is met, the GR loop is established successfully, the ground loop conversion switch ERTB is separated, and step 5 is executed, if the first current criterion is not met, ERTB is separated, and step 3 is executed; Step 3, the current of the GR loop is judged, if the second current criterion is met, the GR loop is established successfully, and step 5 is executed, if the second current criterion is not met, ERTB is closed, it is confirmed that the GR loop is not established successfully, and step 4 is executed; Step 4, it is confirmed that the GR loop is not established successfully, and the conversion operation is interrupted; Step 5, it is confirmed that the GR loop is established successfully, the other knife switches of the MR loop are separated, and the conversion from MR to GR is completed.

3. The method of claim 1, wherein the method further comprises: determining whether the target circuit is connected to the plurality of DC power sources. The command of the metal ground conversion is specifically that when the ground loop GR is converted to the metal loop MR, the parallel MR loop is established by operating the DC field switch knife switch, the DC field data is acquired, and whether the MR loop is established is confirmed according to the following steps: Step 1, the position of the DC field switch knife switch is judged, if the MR loop position criterion is met, step 2 is executed, if the MR loop position criterion is not met, it is confirmed that the MR loop is not established successfully, and step 4 is executed; Step 2, judging the MR loop current, if the earth return transfer switch (ERTB) current meets the first current criterion, confirming that the MR loop is successfully established, separating the metal return transfer switch (MRTB), and executing step 5; if the ERTB current does not meet the first current criterion, separating the MRTB, and executing step 3; Step 3, judging the MR loop current, if the ERTB current meets the second current criterion, confirming that the MR loop is successfully established, and executing step 5; if the ERTB current does not meet the second current criterion, closing the MRTB, confirming that the MR loop is unsuccessfully established, and executing step 4; Step 4, confirming that the MR loop is unsuccessfully established, and interrupting the transfer operation; Step 5, confirming that the MR loop is successfully established, and continuing to operate other breakers of the GR loop to complete the GR-to-MR transfer.

4. The method of claim 1, wherein the method further comprises: determining whether the target circuit is connected to the plurality of DC power sources. The DC field data includes: DC field breaker position signals, currents flowing through the MRTB and the ERTB.

5. The method of claim 1, wherein the method further comprises: determining whether the target circuit is connected to the plurality of DC power sources. The target loop position criterion in step 1 is that the breaker position signals meet the target loop state after a first delay, and the value of the first delay considers the operation time of the breakers of the target loop and has a margin.

6. The method of claim 1, wherein the method further comprises: The first current criterion in step 2 is that the target loop current reaches a first threshold value and lasts for a second delay; the value of the first threshold value considers zero drift and has a margin, and the value of the second delay considers current confirmation delay and has a margin.

7. The method of claim 1, wherein the method further comprises: determining whether the target circuit is connected to the plurality of DC power sources. The second current criterion in step 3 is that the target loop current reaches a second threshold value and lasts for a third delay; the value of the second threshold value considers zero drift and measurement error, and the value of the third delay considers anti-shake delay and is 2-10 ms.

8. The method of claim 2, wherein the method further comprises: determining whether the target circuit is connected to the plurality of DC power sources. The GR loop position criterion in step 1 is that the breaker position signals meet the GR loop state after a first delay, and the value of the first delay considers the operation time of the breakers of the GR loop and has a margin; the first current criterion in step 2 is that the MRTB switch current reaches a first threshold value and lasts for a second delay, the value of the first threshold value considers zero drift and has a margin, and the value of the second delay considers current confirmation delay and has a margin; the second current criterion in step 3 is that the MRTB switch current reaches a second threshold value and lasts for a third delay, the value of the second threshold value considers zero drift and measurement error, and the value of the third delay considers anti-shake delay and is 2-10 ms.

9. The method of claim 3, wherein the method further comprises: determining whether the target circuit is connected to the plurality of DC power sources. The MR loop position criterion in step 1 is that the breaker position signals meet the MR loop state after a first delay, and the value of the first delay considers the operation time of the breakers of the MR loop and has a margin; the first current criterion in step 2 is that the ERTB switch current reaches a first threshold value and lasts for a second delay, the value of the first threshold value considers zero drift and has a margin, and the value of the second delay considers current confirmation delay and has a margin; the second current criterion in step 3 is that the ERTB switch current reaches a second threshold value and lasts for a third delay, the value of the second threshold value considers zero drift and measurement error, and the value of the third delay considers anti-shake delay and is 2-10 ms.

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