A direct current system ground fault real-time monitoring system and a control method thereof

Abstract

The application discloses a real-time monitoring system for DC system grounding fault, which comprises a voltage detection device, a DC current transformer, a differential protection device, a switch displacement collection module, an input module, a DSP module, a CPU controller, a memory, an output module, a communication module, a display device, a motor control module, a main switch controlled by the motor control, branch switches, sub-branch switches, bypass switches, side switches and sub-branch switches; the real-time monitoring system for DC system grounding fault and the control method thereof can quickly locate the interval where the grounding point is located through the differential protection device, control the motor to complete automatic switching of the switch, automatically complete the searching link of the DC system grounding fault, automatically isolate the fault point after the fault point is found, and restore power supply through the bypass; the real-time monitoring system for DC system grounding fault can quickly locate and complete fault searching, automatically isolate the fault, restore the power supply demand of the load on the fault line, and does not need to increase a standby power supply.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of direct current system grounding fault, in particular to a direct current system grounding fault real-time monitoring system and a control method thereof. BACKGROUND

[0002] The direct current system provides reliable direct current power for control devices, signal devices, relay protection, automatic devices and emergency lighting in a substation. It also provides reliable operating power for operation. The reliability of the direct current system plays a crucial role in the safe operation of the substation and is a guarantee for the safe operation of the substation. The main load of the direct current system is the control circuit in the electrical equipment and the electrical protection device. The excessive number of various control cables in the equipment and associated cables with other equipment inevitably leads to insulation damage of the cables, resulting in direct current grounding. When two-point grounding fault occurs in the direct current system, it may form a system grounding short circuit, which may cause misoperation or refusal of the relay protection device, signal and automatic device, and even cause the fusing of the direct current fuse, resulting in the complete loss of power supply of the protection and automatic device and control circuit, causing the interruption of the direct current system and making it unable to work. When two-point grounding fault occurs in the direct current system, maintenance personnel cannot arrive at the scene in time to handle it, which will adversely affect the direct current system and even develop into a fault of the direct current system, posing a great threat to the insulation of the direct current system. Insulation damage of the direct current system cannot be replaced in time, which may cause the risk of power loss of important direct current loads. Therefore, it is necessary to correctly and timely remove the grounding circuit from the system.

[0003] When the prior art direct current occurs grounding, the current will not change significantly as in alternating current grounding, so it is difficult to use overcurrent protection. If only relying on the grounding current as the grounding criterion, the system may be misjudged as grounding due to the increase of the load and the current. If leakage protection is used, the switch may be misoperated due to the excessive length of the cable line in the system, which produces a large zero sequence current. Therefore, neither overcurrent protection nor current leakage protection can be used in the direct current system. At present, there is no technical measure to directly determine the grounding point in the direct current system. When direct current grounding occurs, the traditional pull line method is usually used to find the fault, which needs to open and close each direct current branch one by one until the fault disappears. The processing time is long, the steps are complicated, and the risk of misoperation is high. Therefore, a method is needed to accurately locate the grounding position without affecting the normal operation of the branch.

[0004] In the prior art, patent applications with application number 201010229641.6, entitled "Method and Apparatus for Automatic Isolation and Location of DC System Ground Faults," and application number 201010594500.4, entitled "Method for Locating DC Ground Faults in Power Systems," both employ the method of adding power supply to switch DC branches to backup power. Adding backup power not only increases investment costs but also maintenance costs. Furthermore, both can only locate the fault on the entire main branch. During subsequent maintenance, it is still necessary to switch all branches and sub-branches under the main branch one by one until the ground fault point is found, making it impossible to accurately locate the specific ground fault point. Summary of the Invention

[0005] The purpose of this invention is to provide a real-time monitoring system and control method for grounding faults in DC systems. When a grounding fault occurs in a DC system, the differential protection device based on Kirchhoff's current law can quickly locate the grounding interval, solving the problem that current cannot be used as the basis for judging DC grounding. Furthermore, the bypass replacement method can accurately locate the specific grounding point without power loss. The DC bypass is used to disconnect the faulty DC line and switch the DC load to bypass power supply.

[0006] The technical solution to achieve the purpose of this invention is as follows:

[0007] A real-time monitoring system for grounding faults in a DC system is characterized by comprising a voltage detection device, a DC current transformer, a differential protection device, a switch position acquisition module, an input module, a DSP module, a CPU controller, a memory, an output module, a communication module, a display device, a motor control module, and a main switch, branch switches, side switches, bypass switches, side branch switches, and side side switches controlled by motors for opening and closing.

[0008] The output terminal of the voltage detection device is connected to the input terminal of the input module. The voltage detection device transmits the real-time acquired signal to the input module. The voltage detection device is used to detect the voltage of the DC bus in real time.

[0009] The output terminal of the DC current transformer is connected to the input terminal of the input module. The DC current transformer transmits the real-time acquired signal to the input module. The DC current transformer is used to collect the current flowing through each main switch, each branch switch, and each sub-switch in real time.

[0010] The input terminal of the differential protection device is connected to the output terminal of the DC current transformer, and the output terminal of the differential protection device is connected to the input terminal of the CPU controller. The differential protection device is used to monitor the differential current within its protection range in real time. When the detected differential current is greater than the operating set value of the differential protection device, the differential protection device sends an abnormal command to the CPU controller.

[0011] The output end of the switch position acquisition module is connected with the input end of the incoming module, the switch position acquisition module transmits the real-time collected switch position signals to the incoming module, and the switch position acquisition module is used for monitoring the switch position information;

[0012] The output end of the incoming module is connected with the input end of the DSP module, the incoming module is used for receiving the signals collected by the voltage detection device, the direct current current transformer and the switch position acquisition module in real time, and transmitting the signals to the CPU controller through the DSP module;

[0013] The output end of the DSP module is connected with the input end of the CPU controller; the DSP module is used for converting the analog signals of the incoming module into digital signals, and transmitting the digital signals to the CPU controller;

[0014] The output end of the CPU controller is connected with the input end of the memory, the CPU controller judges whether the direct current system is in a normal or abnormal state by monitoring and analyzing the signals, marks the abnormal data, and stores the abnormal data in the memory;

[0015] The input end of the outgoing module is connected with the output end of the CPU controller, and the output end of the outgoing module is connected with the input end of the communication module; the outgoing module is used for executing the commands issued by the CPU controller and transmitting the commands through the communication module;

[0016] The output end of the communication module is connected with the input end of the display device and the motor control module respectively, and the communication module is used for receiving the signals transmitted by the outgoing module and displaying the alarm information on the display device;

[0017] The output end of the motor control module is connected with the control end of the main switch, the branch switch, the sub-branch switch, the bypass switch, the bypass branch switch and the bypass sub-branch switch respectively, the motor control module receives the commands transmitted by the communication module, and realizes the opening and closing of all switches by controlling the forward and reverse rotation of the motor.

[0018] Preferably, the direct current bus is connected with a plurality of main switches, each main switch is connected with a plurality of branch switches, and each branch switch is connected with a plurality of sub-branch switches; each branch switch is connected to the bypass switch through the bypass branch switch, each sub-branch switch is connected to the bypass switch through the bypass sub-branch switch, and the bypass switch is connected with the direct current bus.

[0019] Preferably, the action setting value of the differential protection device is that the differential current percentage is greater than or equal to 20%, that is, I differential ≥20%, and the differential current percentage = the absolute value of the sum of all branch current vectors / the absolute value of the total branch current*100%.

[0020] Preferably, the system further comprises a background control center and a smart mobile terminal, an input end of the background control center is connected with an output end of the communication module, and an output end of the background control center is connected with the smart mobile terminal; the communication module remotely transmits the signal to the background control center, and the background control center transmits the signal to the smart mobile terminal through wireless communication.

[0021] A control method of a direct current system ground fault real-time monitoring system, characterized in that the method comprises the following steps:

[0022] Step one: when the voltage mutation U1 collected by the voltage monitoring device is less than 20%, and the differential current I of the differential protection device is less than 20%, the voltage and the current are normal interval values, the system control display device displays a normal operation state of the system, and the system enters a normal operation program;

[0023] Step two: when the voltage mutation U1 collected by the voltage monitoring device is greater than or equal to 20%, and the differential current I of the differential protection device is less than 20%, the system detects that the voltage is obviously reduced, the system determines that the direct current voltage is abnormal, the system control display device issues an "abnormal direct current voltage" alarm, and uploads the alarm information to the background control center, and the system enters an abnormal direct current voltage operation program;

[0024] Step three: when the voltage mutation U1 collected by the voltage monitoring device is less than 20%, and the differential current I of the differential protection device is greater than or equal to 20%, the system determines that the direct current is abnormal, the system control display device issues an "abnormal direct current" alarm, and uploads the alarm information to the background control center, and the system enters an abnormal direct current operation program;

[0025] Step four: when the voltage mutation U1 collected by the voltage monitoring device is greater than or equal to 20%, and the differential current I of the differential protection device is greater than or equal to 20%, the system determines that there is a direct current ground, the system control display device issues a "direct current ground" alarm, and uploads the alarm information to the background control center, and the system enters a direct current ground operation program; the system locates the protection range corresponding to the differential protection device, and starts to search for a fault line in the protection range; if the system locates the protection range corresponding to the differential protection device as a branch, step five is executed; if the system locates the protection range corresponding to the differential protection device as a sub-circuit, step six is executed;

[0026] Step five: according to the system preset program, that is, according to the preset number of branch 1 to n, in turn, first close the bypass switch, then close the bypass switch 1 according to the number, open the branch switch 1, and so on until the voltage returns to normal, the system stops switching immediately, and determines that the branch k (k≤n) of the last switching exists a grounding point; at the same time, the DC branch that has completed switching is started from k-1, first close the corresponding branch switch k-1, separate the corresponding bypass switch k-1, close the corresponding branch switch k-2, separate the corresponding bypass switch k-2, and so on, to switch and restore the original operation mode; at this time, the grounding point of the branch k is isolated from the DC system between the branch switch k and the branch DC current transformer k, and the non-grounded part is continued to be powered by the bypass of the DC bus; then the system sends an alarm information of "DC fault isolation, bypass start";

[0027] Step six: according to the system preset program, that is, according to the preset number of branch 1 to n, in turn, first close the bypass switch, then close the bypass switch 1 according to the number, open the branch switch 1, and so on until the voltage returns to normal, the system stops switching immediately, and determines that the branch k (k≤n) of the last switching exists a grounding point; at the same time, the DC branch that has completed switching is started from k-1, first close the corresponding branch switch k-1, separate the corresponding bypass switch k-1, close the corresponding branch switch k-2, separate the corresponding bypass switch k-2, and so on, to switch and restore the original operation mode; at this time, the grounding point of the branch k is isolated from the DC system between the branch switch k and the branch DC current transformer k, and the non-grounded part is continued to be powered by the bypass of the DC bus; then the system sends an alarm information of "DC fault isolation, bypass start".

[0028] Compared with the prior art, the present application has the following advantages:

[0029] (1) The DC system grounding fault real-time monitoring system and control method can quickly locate the grounding point through the differential protection device, and only needs to find the line in the interval, greatly shortening the search time and realizing the quick positioning of the grounding fault point.

[0030] (2) The DC system grounding fault real-time monitoring system and control method controls the motor to complete the automatic switching of the switch, automatically completes the search link of the DC system grounding fault, automatically isolates the fault after finding the fault point, and restores the power supply by bypass; it can quickly locate and complete the fault search, and automatically isolate the fault.

[0031] (3) The DC system grounding fault real-time monitoring system and control method provides a bypass circuit and restores the power supply by bypass, which restores the power supply demand of the load of the fault line without increasing the standby power supply. BRIEF DESCRIPTION OF DRAWINGS

[0032] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the accompanying drawings required in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0033] Figure 1 This is a schematic diagram of the structure of the DC system grounding fault real-time monitoring system of the present invention.

[0034] Figure 2 This is a schematic diagram of the electrical connections for the real-time monitoring system for grounding faults in a DC system according to the present invention.

[0035] Figure 3 This is a schematic diagram of the switch connection of the DC system grounding fault real-time monitoring system of the present invention.

[0036] Figure 4 This is a wiring diagram of the differential protection device in the real-time monitoring system for ground faults in a DC system according to the present invention.

[0037] Figure 5 This is a flowchart illustrating the control method of the real-time monitoring system for grounding faults in a DC system according to the present invention. Detailed Implementation

[0038] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0039] Example 1:

[0040] like Figures 1-3 As shown, a real-time monitoring system for grounding faults in a DC system includes a voltage detection device, a DC current transformer, a differential protection device, a switch position acquisition module, an input module, a DSP module, a CPU controller, a memory, an output module, a communication module, a display device, a motor control module, and a main switch, branch switches, sectional switches, bypass switches, lateral switches, and lateral switches controlled by a motor for opening and closing. The DC bus is connected to several main switches, each main switch is connected to several branch switches, and each branch switch is connected to several sectional switches. Each branch switch is connected to a bypass switch via a lateral switch, and each sectional switch is connected to a bypass switch via a lateral switch. The bypass switches are connected to the DC bus.

[0041] The output end of the voltage detection device is connected with the input end of the input module, the voltage detection device transmits the real-time collected signal to the input module, and the voltage detection device is used for real-time detection of the voltage of the DC bus.

[0042] The output end of the DC current transformer is connected with the input end of the input module, the DC current transformer transmits the real-time collected signal to the input module, and the DC current transformer is used for real-time collection of the current flowing through each total switch, each branch switch and each subbranch switch.

[0043] The input end of the differential protection device is connected with the output end of the DC current transformer, the output end of the differential protection device is connected with the input end of the CPU controller, and the differential protection device is used for real-time monitoring of the differential current in the protection range; according to Kirchhoff's current law, if there is no grounding current in the system, the current vector sum corresponding to the DC current transformer should be zero (the positive direction of the current is the direction of the bus), that is, the current vector sum measured by the differential protection device is zero, if the DC grounding occurs, the vector sum of the current sampled by the DC current transformer is not zero, that is, the current vector sum measured by the differential protection device is not zero. When the differential current greater than the action setting value of the differential protection device is detected, the differential protection device sends an abnormal instruction to the CPU controller; the action setting value of the differential protection device is that the differential current percentage is greater than or equal to 20%, that is, I differential ≥ 20%, the differential current percentage = the absolute value of the sum of the current vectors of all branches / the absolute value of the total branch current * 100%; that is, I differential = |i1+i2+i3+i4+iA| / |iA|*100%, wherein i1, i2, i3, i4 and iA are Figure Four current vectors in the formula.

[0044] The output end of the switch position acquisition module is connected with the input end of the input module, the switch position acquisition module transmits the real-time collected switch position signal to the input module, and the switch position acquisition module is used for monitoring the switch position information.

[0045] The output end of the input module is connected with the input end of the DSP module, and the input module is used for real-time receiving of the signals collected by the voltage detection device, the DC current transformer and the switch position acquisition module, and transmitting the signals to the CPU controller through the DSP module.

[0046] The output end of the DSP module is connected with the input end of the CPU controller; the DSP module is used for converting the analog signals of the input module into digital signals, and transmitting the digital signals to the CPU controller.

[0047] The output end of the CPU controller is connected with the input end of the memory, the CPU controller judges whether the DC system is in a normal or abnormal state by monitoring and analyzing the signals, marks the abnormal data, and stores the abnormal data in the memory.

[0048] The input end of the opening module is connected with the output end of the CPU controller, and the output end of the opening module is connected with the input end of the communication module; the opening module is used for executing the command issued by the CPU controller and transmitting through the communication module.

[0049] The output end of the communication module is connected with the input end of the display device and the motor control module respectively, and the communication module is used for receiving the signal transmitted by the opening module and displaying the alarm information on the display device.

[0050] The output end of the motor control module is connected with the control end of the main switch, the branch switch, the sub-branch switch, the bypass switch, the side branch switch and the side sub-branch switch respectively, and the motor control module receives the command transmitted by the communication module and realizes the opening and closing of all switches by controlling the forward and reverse rotation of the motor.

[0051] The DC system grounding fault real-time monitoring system further comprises a background control center and an intelligent mobile terminal, the input end of the background control center is connected with the output end of the communication module, and the output end of the background control center is connected with the intelligent mobile terminal; the communication module remotely transmits the signal to the background control center, and the background control center transmits to the intelligent mobile terminal through wireless communication.

[0052] As shown in Figure 5 A control method of the DC system grounding fault real-time monitoring system, comprising the following steps:

[0053] Step one: when the voltage mutation U1 collected by the voltage monitoring device is less than 20%, and the differential current I of the differential protection device is less than 20%, the voltage and the current are normal interval values, the system control display device displays the normal operation state of the system, and the system enters the normal operation program;

[0054] Step two: when the voltage mutation U1 collected by the voltage monitoring device is greater than or equal to 20%, and the differential current I of the differential protection device is less than 20%, the system detects that the voltage is obviously reduced, the system judges that the DC voltage is abnormal, the system control display device issues an "DC voltage abnormality" alarm, and uploads the alarm information to the background control center, and the system enters the DC voltage abnormality operation program;

[0055] Step three: when the voltage mutation U1 collected by the voltage monitoring device is less than 20%, and the differential current I of the differential protection device is greater than or equal to 20%, the system judges that the DC current is abnormal, the system control display device issues a "DC current abnormality" alarm, and uploads the alarm information to the background control center, and the system enters the DC current abnormality operation program;

[0056] Step four: when the voltage mutation U1 collected by the voltage monitoring device is greater than or equal to 20%, and the differential current I of the differential protection device is greater than or equal to 20%, the system determines that there is a DC grounding, the display device controlled by the system issues an "DC current grounding" alarm, and the alarm information is uploaded to the background control center, and the system enters the DC grounding operation program; the system locates the protection range corresponding to the differential protection device, and starts to search for the fault line in the protection range; if the system locates the protection range corresponding to the differential protection device as a branch, step five is executed; if the system locates the protection range corresponding to the differential protection device as a sub-branch, step six is executed;

[0057] Step five: according to the system preset program, that is, according to the preset numbers of branch 1 to n in sequence, first close the bypass switch, then close the bypass switch 1 according to the number, open the branch switch 1, and so on until the voltage returns to normal, the system immediately stops switching, and determines that the branch k (k≤n) exists at the grounding point; at the same time, the DC branch that has completed switching is started from k-1, the corresponding branch switch k-1 is closed, the corresponding bypass switch k-1 is opened, the corresponding branch switch k-2 is closed, the corresponding bypass switch k-2 is opened, and so on, the switching is performed, and the original operation mode is restored; at this time, the branch k grounding point is isolated from the DC system between the branch switch k and the branch DC current transformer k, and the non-grounding part is continued to be powered by the DC bus through the bypass; the system issues an "DC fault isolation, bypass start" alarm information;

[0058] Step six: according to the system preset program, that is, according to the preset numbers of sub-branch 1 to n in sequence, first close the bypass switch, then close the bypass switch 1 according to the number, open the branch switch 1, and so on until the voltage returns to normal, the system immediately stops switching, and determines that the branch k (k≤n) exists at the grounding point; at the same time, the DC branch that has completed switching is started from k-1, the corresponding branch switch k-1 is closed, the corresponding bypass switch k-1 is opened, the corresponding branch switch k-2 is closed, the corresponding bypass switch k-2 is opened, and so on, the switching is performed, and the original operation mode is restored; at this time, the branch k grounding point is isolated from the DC system between the branch switch k and the branch DC current transformer k, and the non-grounding part is continued to be powered by the DC bus through the bypass; the system issues an "DC fault isolation, bypass start" alarm information.

[0059] In summary, the DC system grounding fault real-time monitoring system and control method thereof can quickly locate the grounding point through the differential protection device, and only needs to search the line in the interval, greatly shortens the search time, and realizes the quick positioning of the grounding fault point. The DC system grounding fault real-time monitoring system and control method thereof controls the motor to complete the automatic switching of the switch, automatically completes the search link of the DC system grounding fault, automatically isolates the fault after the fault point is found, and restores power supply by bypass; both the quick positioning and the fault search can be completed, and the fault can be automatically isolated. The DC system grounding fault real-time monitoring system and control method thereof provides a bypass loop, restores power supply by bypass, restores the power supply demand of the load of the fault line, and does not need to increase a standby power supply.

[0060] Finally, it should be noted that: the above only for the preferred embodiments of the present application, and not for limiting the present application, although the foregoing embodiments of the present application are described in detail, for those skilled in the art, it still can be modified, or equivalent replacement of part of the technical features recorded in the foregoing embodiments. Any modification, equivalent replacement, improvement, etc. within the spirit and principles of the present application shall be included in the protection scope of the present application.

Claims

1. A real-time monitoring system for DC system ground faults, characterized in that, The device comprises a voltage detection device, a direct current transformer, a differential protection device, a switch position acquisition module, an input module, a DSP module, a CPU controller, a memory, an output module, a communication module, a display device, a motor control module, and a general switch controlled by the motor control, branch switches, sub-branch switches, bypass switches, side switches, and sub-branch switches. The output end of the voltage detection device is connected with the input end of the input module, and the voltage detection device transmits the real-time collected signals to the input module. The output end of the direct current transformer is connected with the input end of the input module, and the direct current transformer transmits the real-time collected signals to the input module. The input end of the differential protection device is connected with the output end of the direct current transformer, and the output end of the differential protection device is connected with the input end of the CPU controller. The output end of the switch position acquisition module is connected with the input end of the input module, and the switch position acquisition module transmits the real-time collected switch position signals to the input module. The output end of the DSP module is connected with the input end of the CPU controller. The output end of the CPU controller is connected with the input end of the memory. The input end of the output module is connected with the output end of the CPU controller, and the output end of the output module is connected with the input end of the communication module. The output end of the communication module is connected with the input end of the display device and the motor control module. The output end of the motor control module is connected with the control end of the general switch, the branch switch, the sub-branch switch, the bypass switch, the side switch, and the sub-branch switch. ​ 2. The real-time monitoring system for DC system ground faults according to claim 1, characterized in that, The direct current bus is connected with several total switches, each of which is connected with several branch switches, each of which is connected with several sub-branch switches; each branch switch is connected to the bypass switch through a bypass switch, and each sub-branch switch is connected to the bypass switch through a bypass sub-switch, and the bypass switch is connected with the direct current bus.

3. The real-time monitoring system for DC system ground faults according to claim 1, characterized in that, The action setting value of the differential protection device is that the differential current percentage is greater than or equal to 20%, that is, I differential ≥ 20%, and the differential current percentage = absolute value of sum of all branch current vectors / absolute value of total branch current*100%.

4. The real-time monitoring system for DC system ground faults according to claim 1, characterized in that, The background control center and the intelligent mobile terminal are further included, the input end of the background control center is connected with the output end of the communication module, the output end of the background control center is connected with the intelligent mobile terminal; the communication module remotely transmits signals to the background control center, and the background control center transmits to the intelligent mobile terminal through wireless communication.

5. A control method of the direct current system ground fault real-time monitoring system according to claim 1, characterized by, The method comprises the following steps: Step one: when the voltage mutation U1 collected by the voltage monitoring device is less than 20%, and the differential current I differential of the differential protection device is less than 20%, the voltage and the current are normal interval values, the system control display device displays the normal operation state of the system, and the system enters the normal operation program; Step two: when the voltage mutation U1 collected by the voltage monitoring device is greater than or equal to 20%, and the differential current I differential of the differential protection device is less than 20%, the system detects that the voltage is obviously reduced, the system judges that the direct current voltage is abnormal, the system control display device issues an "abnormal direct current voltage" alarm, and uploads the alarm information to the background control center, and the system enters the direct current voltage abnormal operation program; Step three: when the voltage mutation U1 collected by the voltage monitoring device is less than 20%, and the differential current I differential of the differential protection device is greater than or equal to 20%, the system judges that the direct current is abnormal, the system control display device issues an "abnormal direct current" alarm, and uploads the alarm information to the background control center, and the system enters the direct current abnormal operation program; Step four: when the voltage mutation U1 collected by the voltage monitoring device is greater than or equal to 20%, and the differential current I differential of the differential protection device is greater than or equal to 20%, the system judges that there is a direct current grounding, the system control display device issues an "direct current grounding" alarm, and uploads the alarm information to the background control center, and the system enters the direct current grounding operation program; the system locates the protection range corresponding to the differential protection device, and starts to search for the fault line in the protection range; If the system locates the protection range corresponding to the differential protection device as a branch, step five is executed; If the system locates the protection range corresponding to the differential protection device as a sub-branch, step six is executed; Step five: according to the system preset program, that is, according to the preset number of branch 1 to n in turn, first close the bypass switch, then close the bypass switch 1 according to the number, open the branch switch 1, and so on until the voltage returns to normal, the system stops switching immediately, and determines that the branch k (k≤n) exists grounding point of the last switching; At the same time, the DC branch that has completed switching starts from k-1, first closes the corresponding branch switch k-1, separates the corresponding bypass switch k-1, closes the corresponding branch switch k-2, separates the corresponding bypass switch k-2, and so on, to switch and restore the original operation mode; At this time, the grounding point of branch k is isolated from the DC system between branch switch k and branch DC current transformer k, and the non-grounding part is continued to be powered by the bypass of the DC bus; Then the system sends the "DC fault isolation, bypass start" alarm information; Step six: according to the system preset program, that is, according to the preset number of branch 1 to n in turn, first close the bypass switch, then close the bypass switch 1 according to the number, open the branch switch 1, and so on until the voltage returns to normal, the system stops switching immediately, and determines that the branch k (k≤n) exists grounding point of the last switching; At the same time, the DC branch that has completed switching starts from k-1, first closes the corresponding branch switch k-1, separates the corresponding bypass switch k-1, closes the corresponding branch switch k-2, separates the corresponding bypass switch k-2, and so on, to switch and restore the original operation mode; At this time, the grounding point of branch k is isolated from the DC system between branch switch k and branch DC current transformer k, and the non-grounding part is continued to be powered by the bypass of the DC bus; Then the system sends the "DC fault isolation, bypass start" alarm information.

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