Ground fault checking and positioning method for ungrounded direct current power supply system

Abstract

The invention discloses an ungrounded DC power supply system grounding fault troubleshooting and positioning method, and belongs to the field of fault detection. The method includes measuring voltage to ground of positive and negative buses; finding out a bus equipotential with the grounding busbar, and judging the bus as a bus where a grounding fault is located; a troubleshooting resistor is connected in series between the bus where the non-grounding fault is located and the grounding busbar; detecting the unbalanced current of each switch of each stage; gradually reducing the range until a tail end switch with unbalanced current is found; and the fault loop is cut off, then the voltage to ground of the positive bus and the negative bus is measured at the primary switch, and if the voltage to ground of the positive bus and the negative bus returns to normal, the ground fault point is isolated. The grounding fault of the ungrounded direct-current power supply system can be checked and positioned without cutting off power one by one for points where the grounding fault possibly occurs, the normal operation of a protection, control and monitoring system powered by the direct-current power supply system is not influenced, and the influence of the troubleshooting work on the normal production of enterprises and the normal life of residents is avoided.

Description

Technical Field

[0001] This invention belongs to the field of DC power supply fault detection, and particularly relates to a method for locating and diagnosing grounding faults in ungrounded DC power supply systems. Background Technology

[0002] In a DC power supply system (or DC system or DC power source), the direction of voltage and current is always constant and does not change over time. A storage battery is a typical DC power source, and the characteristics of DC voltage are often described by the "amplitude" parameter.

[0003] In an AC power supply system (AC system or AC power source), the direction of voltage and current changes periodically over time. The mains power that is relatively easy to obtain is AC power. The characteristics of AC voltage are often described by parameters such as "amplitude", "frequency" and "phase".

[0004] In practical production applications, readily available AC220 AC power is generally rectified into DC power through rectifier circuit modules. This DC power is then used to provide DC power supply (also known in the industry as DC control power supply) for secondary circuits or control circuits of high-voltage power distribution systems, PLCs (industrial control computers), and DCSs (distributed control systems). In some more critical applications, rechargeable batteries are installed in the DC system to improve the overall reliability of the AC power supply it serves.

[0005] When the AC power supply at the input of the DC system is working normally, the rechargeable battery is charged through the rectifier module in the DC system, and DC power is output to supply power to the load at the same time. When the AC power supply at the input of the DC system suddenly "loses power" due to an unpredictable fault, the battery in the DC system directly "supply power" to the load (this is called the "discharging" state of the battery). For a certain period of time, the DC system can still maintain normal power supply, giving sufficient time for the entire AC power supply system to handle faults and restore power.

[0006] For an AC power supply system, on the one hand, if it is to have the same function of continuous power supply when there is an external power outage, an inverter module needs to be configured to convert the DC power from the battery into AC power, which makes the operating cost of the AC system higher; on the other hand, because more electronic components are configured, the AC system has a higher probability of failure, and the power supply reliability is relatively lower.

[0007] At the same time, considering the characteristics of alternating current itself, the constant changes in current and voltage inevitably introduce electromagnetic interference, which in turn affects the operational stability of related electrical components.

[0008] Therefore, DC power supplies are widely used in high-voltage power distribution systems (including high-voltage power distribution equipment), PLC control systems, or DCS control systems due to their lower cost, higher power supply reliability, and anti-interference capabilities, to provide DC power supply for these systems (also referred to as DC control power supply or DC control system).

[0009] Taking the application of DC power in high-voltage power supply and distribution systems as an example, a failure in DC power can lead to serious consequences. In high-voltage power supply and distribution systems, all safety protection systems, signal indication systems, communication systems, and power supplies that provide power to operate high-voltage equipment (collectively referred to as protection, control, and monitoring systems) use DC power. If the DC power supply fails and loses power, all the functions of the above systems will fail, and the high-voltage power supply equipment will also be unable to operate normally, resulting in a power outage in the power supply and distribution system.

[0010] In engineering applications, DC control systems generally operate in an ungrounded mode. When a single-point ground fault occurs, the fault current is zero amperes, which does not seriously affect the operation of the DC system, and the DC system can continue to operate. However, if a ground fault at a certain point is not cleared in time, a ground fault will occur at the non-faulty pole over time. In this case, the ground fault will evolve into a serious short-circuit fault. The short-circuit current can typically reach thousands or even tens of thousands of amperes. Excessive short-circuit current can cause DC switch tripping accidents, leading to a power outage of the DC system, and may also cause serious consequences such as damage to related equipment. After the DC system loses power, the protection, control, and monitoring systems that provide the power supply fail. Due to the loss of necessary safety protection functions, high-voltage power distribution equipment will also stop operating, and the high-voltage power supply will be interrupted, affecting normal production activities.

[0011] The characteristics of a DC control system are: a large number of output circuits (i.e., the number of output circuits that supply power to the outside, referred to as circuits) and a large number of power supply levels (referred to as levels or layers).

[0012] like Figure 1 The diagram shows an ungrounded DC control system. The red line represents the positive terminal of the DC power supply, the blue line represents the negative terminal, K1 represents the upstream switch (also called the primary switch), K11 to K15 represent the downstream switches (or secondary switches) of switch K1, the black line represents the grounding system, and F1 represents a negative grounding fault point (simply called the grounding point). Therefore, Figure 1 The given is a two-level (layer) 5-loop ungrounded DC control system.

[0013] For the ungrounded DC power supply system with the above network topology, when a negative ground fault occurs at load point F1 under the secondary switch K12 in this two-stage DC power supply system, the voltage to ground at the negative pole is zero. However, since no fault current loop is formed, the magnitude of the fault current is zero. Because the voltage to ground measured at any point on the negative pole (blue line) in the system shows zero voltage, and the ground fault current is also zero, the ground fault point cannot be identified due to the identical measurement results at all points.

[0014] For example Figure 1 The traditional troubleshooting method for the ungrounded DC control system shown is to individually "disconnect" the power supply switches from K11 to K15, while simultaneously monitoring the changes in the voltage to ground of the positive and negative busbars, and then troubleshooting each circuit one by one.

[0015] The above method for troubleshooting and locating grounding faults has the following problems: the DC system needs to be shut down (i.e., switches K11 to K15 need to be opened one by one to check each circuit by "powering off" separately). Because the power outage of a certain DC power supply circuit will cause the protection, control and monitoring systems powered by it to malfunction, and further cause the high-voltage power distribution equipment associated with these protection, control and monitoring systems to shut down, thus requiring a large-scale power outage.

[0016] This method of troubleshooting grounding faults is problematic for actual operating conditions requiring continuous production (such as long-process continuous production enterprises in steel companies). When a certain process stops (such as the steelmaking process), the upstream raw material, ironmaking, and downstream rolling processes must also stop simultaneously. The shutdown and recovery process of numerous devices is relatively long, wasting a lot of energy and causing huge economic losses due to downtime. In residential electricity use, power outages can also cause inconvenience to residents' lives.

[0017] The literature “Research and Analysis of a Method for Troubleshooting Grounding Faults in DC Systems” (Deng Ye, Modern Industrial Economy and Information Technology, No. 22, 2017, pp. 94-95) proposes a fault troubleshooting method that combines tree search theory and sequential search method. In this technical solution, according to the hierarchical structure of the DC power supply system, the circuits that may have grounding faults are disconnected one by one for troubleshooting until the faulty circuit is found.

[0018] The literature “Research and Analysis of a Method for Troubleshooting Grounding Faults in DC Systems” (Jin Yu et al., Electrical Engineering, No. 6, 2017, pp. 21-P) elaborates on the principles and methods of troubleshooting grounding faults in DC systems through the analysis of secondary drawings, analyzes the selection principle of the grounding fault selection device in DC systems, and makes a theoretical derivation.

[0019] The technical solutions disclosed in the two documents mentioned above both require disconnection while the circuit is energized. During the disconnection process, the circuit being disconnected (equivalent to a secondary switch) needs to be de-energized, which is not suitable for production scenarios that require continuous production.

[0020] The paper "Research on a Method for Troubleshooting Ground Faults in DC Systems" (Xiong Lixiang, *Technology & Market*, Vol. 25, No. 12, 2018, pp. 84-85) uses historical field data from 340 inverters in a large 170MW installation to study a method for troubleshooting ground faults in DC systems. This paper measures the leakage current in the field and uses electrical components such as overcurrent relays, fuses, and RCDs (Resettable Circuit Breakers) to disconnect the fault circuit. However, the method described in this paper has the following problems: 1) When a ground fault occurs in an ungrounded DC power supply, the fault current is not significantly different from the leakage current during normal operation, making it impossible to activate the aforementioned protective devices; 2) With a large number of DC distribution switches in the field, installing an RCD on each switch is impractical in engineering applications; 3) This method only studies the troubleshooting method for photovoltaic systems and lacks universality.

[0021] The literature “Analysis of DC System Ground Fault Detection Methods” (Zhan Jialin, Electrical Engineering Technology, No. 8, 2010, pp. 70-71) introduces the ZJD portable DC system ground fault location device. After the device is connected to the system, it sends ultra-low frequency pulse signals to the positive and negative buses and the ground wire through a signal generator. These signals will cause certain interference to the normal operation of the DC system, especially the high-precision instrument control system. The fault diagnosis mode it uses requires the use of special instruments, the method of use is complicated, and it cannot identify AC and DC mixed grounding.

[0022] In summary, without using dedicated fault location devices and without shutting down each possible grounding circuit one by one, how to simply and quickly investigate and locate grounding faults in an ungrounded DC power supply system is a technical problem that urgently needs to be solved in actual operation and maintenance. Summary of the Invention

[0023] The technical problem to be solved by this invention is to provide a method for locating and troubleshooting ground faults in an ungrounded DC power supply system. Using only conventional electrical tools and components, this method can quickly and easily locate and troubleshoot ground faults in an ungrounded DC power supply system by detecting the voltage to ground of the positive and negative busbars (referred to as the positive pole) and the negative busbar (referred to as the negative pole). It eliminates the need to shut down power at each potential ground fault point, does not affect the normal operation of protection, control, and monitoring systems powered by the DC power supply system, and avoids the impact of fault diagnosis on normal production and daily life.

[0024] The technical solution of this invention is: to provide a method for locating and diagnosing grounding faults in an ungrounded DC power supply system, characterized in that:

[0025] 1) Measure the voltage to ground between the positive busbar, negative busbar and grounding busbar;

[0026] 2) Locate the busbar in the positive and negative busbars that is at the same potential as the grounded busbar, and determine which busbar is the one where the ground fault is located;

[0027] 3) Connect a fault-finding resistor R in series between the busbar where the non-grounded fault is located and the grounded busbar;

[0028] 4) Starting from the primary switch, detect the unbalanced current of each switch in each stage in order from top to bottom;

[0029] 5) If an unbalanced current is detected at the output terminal of a certain level switch, it is determined that the load of a certain power supply circuit corresponding to the output terminal of that level switch has a grounding fault.

[0030] 6) For each downstream power supply circuit corresponding to the switch output terminal with unbalanced current, measure whether there is unbalanced current, gradually narrowing down the range until the end switch with unbalanced current is found.

[0031] 7) If an unbalanced current is detected in the power supply circuit of one of the end switches in each of the next-level power supply circuits, it is determined that there is a grounding fault in that power supply circuit.

[0032] 8) Disconnect the faulty circuit, and then measure the voltage to ground between the positive and negative poles and the grounding busbar at the primary switch. If the voltage to ground between the positive busbar, the negative busbar and the grounding busbar has returned to normal, it indicates that the grounding fault point has been isolated.

[0033] 9) Locate and handle the grounding fault of the load equipment corresponding to the power supply circuit with grounding fault; after handling, test the insulation resistance to ground of the load equipment, and put the equipment into operation after the insulation resistance to ground meets the requirements; at this point, the entire investigation and handling process is completed.

[0034] Specifically, in step 1), use a voltmeter or multimeter in voltage measurement mode to measure the voltage to ground between the positive and negative busbars and the grounding busbar.

[0035] Specifically, in step 2), if the voltage to ground between the positive busbar and the grounded busbar is 0V, and the voltage to ground between the negative busbar and the grounded busbar is the absolute value of the rated operating voltage of the DC power supply system, then the ground fault is determined to be located on the positive busbar side, and the negative busbar at this time is called the busbar where the ground fault is not located.

[0036] If the voltage to ground between the positive busbar and the grounded busbar is the absolute value of the rated operating voltage of the DC power supply system, and the voltage to ground between the negative busbar and the grounded busbar is 0V, then the ground fault is determined to be located on the negative busbar side, and the positive busbar at this time is called the busbar where the ground fault is not located.

[0037] Specifically, in step 3), the resistance value of the fault-checking resistor is calculated according to the following formula:

[0038] R≈U / 10KΩ

[0039] Where U is the rated voltage of the DC system in volts, and R is the estimated resistance value of the fault diagnosis resistor in kiloohms.

[0040] Furthermore, in step 3), using insulated clamps, the fault diagnosis resistor R is connected between the busbar where the non-grounded fault is located and the grounded busbar.

[0041] Furthermore, in step 4), a clamp meter is used to detect the unbalanced current of each switch in order from top to bottom.

[0042] Specifically, the unbalanced current is in the milliampere or tens of milliamperes range.

[0043] Furthermore, the wires of the "+" and "-" terminals on the output side of each switch are all connected to the detection coil of a clamp meter.

[0044] Specifically, in step 8), a voltmeter is used to measure the voltage to ground between the positive and negative terminals and the grounding busbar at the primary switch.

[0045] Furthermore, for a rated operating voltage of V e In an ungrounded DC power supply system, under normal operating conditions without grounding faults, the absolute value of the voltage to ground between its positive busbar and the grounded busbar is half the absolute value of its rated operating voltage Ve; the absolute value of the voltage to ground between its negative busbar and the grounded busbar is half the absolute value of its rated operating voltage Ve.

[0046] Compared with the prior art, the advantages of the present invention are:

[0047] 1. By adopting the technical solution of the present invention, during the investigation and location of grounding fault points, it is not necessary to shut down the power to each possible grounding fault point one by one, and it does not affect the normal operation of the protection, control and monitoring system powered by it. As a result, the high-voltage power supply system associated with it does not need to be shut down, thus avoiding the impact of fault investigation work on the normal production of enterprises and the normal life of residents.

[0048] 2. By adopting the technical solution of the present invention, the investigation and location of grounding fault points can be completed using only conventional electrical tools (voltmeters, clamp meters) and conventional electrical components (resistors), without the need for other special instruments and meters, resulting in low implementation costs;

[0049] 3. The technical solution of this invention provides a simple method for troubleshooting and locating grounding faults, which can be implemented by operators with basic electrical knowledge;

[0050] 4. The grounding fault point investigation and location method of this technical solution is highly versatile and applicable to various occasions using DC power sources, such as photovoltaic systems, DC control systems, and DC power supplies. It is also applicable to AC power systems that operate without grounding.

[0051] 5. Throughout the entire process of troubleshooting and locating grounding faults, operators do not need to touch the system (high-voltage system) once, making the entire testing process safe. Attached Figure Description

[0052] Figure 1 This is a schematic diagram of the traditional fault location and troubleshooting mode when a single-point grounding fault occurs in an ungrounded DC control system.

[0053] Figure 2 This is a schematic diagram of the fault location and troubleshooting method of the present invention;

[0054] Figure 3 This is a schematic diagram of the DC grounding fault location and troubleshooting steps in this invention;

[0055] Figure 4 This is a detailed schematic diagram of the grounding electrode determination step of the present invention;

[0056] Figure 5 This is a schematic diagram of an embodiment of the present invention for locating and investigating DC grounding faults. Detailed Implementation

[0057] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0058] Specifically, such as Figure 2 As shown, the specific implementation process of the technical solution of the present invention is as follows:

[0059] When a fault occurs at point F1 on the negative busbar of K12, a fault location resistor R (referred to as resistor) is connected between the positive terminal of K1 and the grounding point. The resistance value is determined according to the system voltage level.

[0060] After resistor R is connected, the current flowing through it is approximately 10mA. The resistance value can be estimated using the following formula:

[0061] R≈U / 10KΩ (Equation 1)

[0062] In the above formula, U is the rated voltage of the DC system in volts, and R is the estimated resistance in kiloohms.

[0063] The ground fault detection and location principle of this technical solution is as follows: After connecting the resistor according to the above formula, about 10mA of current flows through both the ground fault circuit and the resistor R; for the fault circuit, this current only flows through the negative pole, and there is an unbalanced current between the positive and negative poles, while the normal operating circuit will not have this fault current, and the currents of the two poles are balanced.

[0064] Therefore, by utilizing this feature, an unbalanced current can be detected using a clamp meter (the meter's measurement range can cover DC currents up to 10mA), gradually narrowing down the range until the grounding loop is finally found.

[0065] See Figure 3 , Figure 4 As shown, the grounding fault detection and location method of this technical solution is as follows (taking a negative grounding fault in a DC 110V system as an example; when no grounding fault occurs, the positive voltage to ground is 55V, and the negative voltage to ground is -55V; the method for troubleshooting positive grounding faults is similar):

[0066] The first step is to use a voltmeter to measure the voltage to ground of the positive and negative busbars respectively. Figure 2 During a fault, the negative terminal voltage to ground was detected to be 0V, while the positive bus voltage to ground changed to the system rated voltage of 110V, confirming that the negative terminal was grounded.

[0067] The second step is to connect a resistor R between the non-faulty positive terminal and the grounding terminal, and the resistance value is calculated according to the above "Equation 1".

[0068] To avoid affecting system operation during connection, insulated clamps can be used to connect the resistor.

[0069] The third step is to use a clamp meter to test the first-level switch K1 and the second-level switches K11-K15 in order from top to bottom. Note that the wires of the "+" and "-" terminals of each switch must be tested through the clamp meter coil.

[0070] When K1 and K12 are detected, there is a current of 10mA, while the current of other switches is 0A, indicating that the ground fault point occurs in the power supply circuit of K1 and K12.

[0071] Fourth, if K12 has a downstream switch, repeat the method in the third step to gradually narrow down the scope until the end switch is found, and the fault point can be confirmed.

[0072] Example:

[0073] During on-site production, a ground fault occurred in a 110V DC system supplying power to the control circuit of a high-voltage switch in the power distribution system. This system employs a three-level power supply structure, such as... Figure 5 As shown in the image.

[0074] The grounding fault circuit needs to be investigated and identified. In the diagram, K1 is the main power switch (primary switch) of the DC system, K11-K15 are the secondary switches of the system, and K121-K124 are the tertiary switches of the system. Point A is the output side of switch K1, point B is the output side of switch K12, point C is the output side of switch K122, E is the electrical equipment powered by switch K122 (the equipment that has experienced a grounding fault), R is the resistor connected during the grounding fault investigation, and the green arrow indicates the unbalanced current flow path after connecting the fault investigation resistor R.

[0075] The specific investigation steps are as follows:

[0076] 1) Using a voltmeter, measure the voltage to ground of the positive and negative busbars at the output terminals of switch K1. The results show that the voltage to ground of the positive pole is 110V and the voltage to ground of the negative pole is 0V. The voltage of the negative pole changed from -55V to ground during normal operation to being at the same potential as the grounding system. The voltage of the positive pole increased from 55V to ground during normal operation to an abnormal 110V. The measurement results indicate that a grounding fault has occurred on the negative pole.

[0077] 2) Using insulated clamps, connect a fault-finding resistor R between the positive busbar and the grounding busbar. The resistor value is 110 / 10 = 11KΩ.

[0078] 3) Use a clamp meter to measure the fault current at point A. The current is 10mA, indicating that there is a grounding fault in the load powered by switch K1.

[0079] 4) Using a clamp meter, check the load switches K11-K15 under switch K1 one circuit at a time. It was found that only switch K12 at point B had a current of 10mA, indicating that the grounding fault point was in the power supply load corresponding to switch K12.

[0080] 5) Using a clamp meter, check the loads K121-K124 under switch K12 one circuit at a time. It was found that only switch K122 at point C had a current of 10mA, indicating that the grounding fault point was in the power supply load "Electrical Equipment E" corresponding to switch K122.

[0081] 6) Disconnect switch K122 to cut off the fault circuit. Then, use a voltmeter to measure the voltage to ground between the positive and negative terminals and the grounding busbar at the primary switch K1. The measured results show that the absolute value of the voltage to ground of the positive terminal is 55V and the absolute value of the voltage to ground of the negative terminal is also 55V. The voltages to ground of both the positive and negative terminals have returned to normal, indicating that the grounding fault point has been isolated.

[0082] 7. Locate and handle the grounding fault of the load "Electrical Equipment E" corresponding to switch K122. After handling, perform a ground insulation resistance test. Once the ground insulation resistance meets the requirements, put the equipment into operation.

[0083] This concludes the entire investigation and handling process.

[0084] The technical solution of this invention first measures the voltage to ground between the positive busbar, negative busbar, and grounding busbar to determine the polarity of the busbar where the ground fault occurs (i.e., whether the positive busbar or the negative busbar is grounded). Then, a fault-finding resistor is connected in series between the ungrounded phase busbar and the grounding busbar. By measuring the unbalanced current on the outgoing side of each switch in each stage, the investigation range is narrowed down step by step until the circuit where the ground fault occurs is finally found. The faulty circuit is then disconnected. A voltmeter is used to measure the voltage to ground between the positive and negative poles and the grounding busbar at the first-stage switch. If the voltage to ground of both the positive and negative poles returns to normal, it indicates that the ground fault point has been isolated. The ground fault of the power supply load corresponding to the switch where the ground fault point is located is located and handled. After handling, the insulation resistance to ground is tested. Once the insulation resistance to ground meets the requirements, the equipment can be put into operation. During the investigation and location of ground fault points, it is not necessary to shut down the power to each possible ground fault point one by one, and it does not affect the normal operation of the protection, control, and monitoring systems powered by it, thus avoiding the impact of ground fault investigation work on the normal production of enterprises and the normal life of residents.

[0085] This invention can be widely used in the operation and maintenance management of ungrounded DC power supply systems and in the field of grounding fault investigation.

Claims

1. A method for locating ground fault in an ungrounded DC power supply system, characterized in that: 1) the voltage between the positive busbar, the negative busbar and the ground busbar is measured; 2) the phase busbar in the positive busbar and the negative busbar which is at the same potential as the ground busbar is found, and the busbar where the ground fault is located is determined; 3) a fault locating resistor R is connected between the busbar where the ground fault is not located and the ground busbar; 4) starting from the first switch, the unbalanced current of each switch at each level is detected in sequence from top to bottom; 5) if the unbalanced current is detected at the outlet end of a switch, it is determined that the load of a certain power supply circuit corresponding to the outlet end of the switch has a ground fault; 6) the unbalanced current of each lower level power supply circuit corresponding to the outlet end of the switch where the unbalanced current exists is measured, and the range is gradually narrowed until the end switch where the unbalanced current exists is found; 7) if the unbalanced current is detected in the power supply circuit of the end switch in one of the lower level power supply circuits, it is determined that the power supply circuit has a ground fault; 8) the fault circuit is removed, and the voltage between the positive busbar, the negative busbar and the ground busbar at the first switch is measured, if the voltage between the positive busbar, the negative busbar and the ground busbar returns to normal, it indicates that the ground fault point has been isolated; 9) the ground fault of the load equipment corresponding to the power supply circuit where the ground fault exists is found and treated, and after the treatment is completed, the ground insulation resistance of the load equipment is detected, and after the ground insulation resistance meets the requirements, the equipment is put into operation, and thus the whole process of locating and treating is completed. In step 1), the voltage between the positive busbar, the negative busbar and the ground busbar is measured by using the voltage measurement range of a voltmeter or a multimeter. In step 2), if the voltage between the positive busbar and the ground busbar is 0V, and the voltage between the negative busbar and the ground busbar is the absolute value of the rated working voltage of the DC power supply system, it is determined that the ground fault is located at the positive busbar side, and the negative busbar at this time is called the busbar where the ground fault is not located; If the voltage between the positive busbar and the ground busbar is the absolute value of the rated working voltage of the DC power supply system, and the voltage between the negative busbar and the ground busbar is 0V, it is determined that the ground fault is located at the negative busbar side, and the positive busbar at this time is called the busbar where the ground fault is not located. In step 3), the resistance value of the fault locating resistor is calculated according to the following formula: R≈U / 10KΩ wherein U is the rated voltage of the DC system, in volts, and R is the estimated resistance value of the fault locating resistor, in kilo-ohms. In step 3), the fault locating resistor R is connected between the busbar where the ground fault is not located and the ground busbar by using an insulation clamp. In step 4), the unbalanced current of each switch is detected in sequence from top to bottom by using a clamp-on ammeter. The unbalanced current is in the order of milliampere or ten milliampere.

2. The method of claim 1, wherein the step of identifying the ground fault is performed by: The leads of the "+" and "-" poles of the outlet side of each switch pass through the detection coil of the clamp-on ammeter. ​ 3. The method of claim 1, wherein the step of identifying the ground fault is performed by: In step 8), the voltage between the positive busbar, the negative busbar and the ground busbar at the first switch is measured by using a voltmeter. ​ ​ 4. The method of claim 1, wherein the method further comprises: determining whether the ground fault is in the DC power supply system or in the load. ​ ​ ​ 5. The method for locating and diagnosing grounding faults in an ungrounded DC power supply system according to claim 1, characterized in that: ​ 6. The method of claim 1, wherein the method further comprises: determining whether the ground fault is in the DC power supply system or in the load. ​ 7. The method of claim 1, wherein the method further comprises: determining the location of the ground fault by using a ground fault locating method. ​ 8. The method of claim 1, wherein the method further comprises: determining the location of the ground fault by using a ground fault locating method. ​ 9. The method of claim 1, wherein the step of identifying the ground fault is further defined as: ​ ​ 10. The method of claim 1, wherein the method further comprises: determining whether the ground fault is in the DC power supply system or in the load. For the ungrounded DC power supply system with a rated working voltage Ve, in the normal operation state without ground fault, the absolute value of the voltage between the positive bus and the ground busbar is half of the absolute value of the rated working voltage Ve; the absolute value of the voltage between the negative bus and the ground busbar is also half of the absolute value of the rated working voltage Ve.

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