A neutral point grounding method for a three-phase AC power system

Abstract

The present invention discloses a neutral point grounding method for a three-phase AC power system, which includes a neutral point. A switching device is connected between the neutral point and the ground. When the three-phase AC power system operates normally, the switching device is in an open state. When a single-phase grounding occurs in the three-phase AC power system, the switching device changes from the open state to the conducting state and then changes back to the open state after a preset period of time. And the electric energy accumulated on the switching device and the three-phase AC power system respectively by the current passing through the switching device within the preset period does not exceed the tolerance limits of the switching device and the three-phase AC power system. In this grounding method, when a single-phase grounding occurs, it can generate an obvious instantaneous large current, or directly trip the system to cut off the fault, or can successfully detect the single-phase grounding fault point.

Description

Technical Field

[0001] The present invention relates to a three-phase AC power system, and more particularly to a neutral grounding method for a three-phase AC power system. Background Art

[0002] The electrical connection mode between the neutral point of a three-phase AC power system and the earth is called the grid neutral grounding method. For a distribution network below 35 kV, arc suppression coil grounding or small resistance grounding is commonly used. The advantage of grounding through an arc suppression coil is that it can operate with live wire after single-phase grounding. The disadvantage is that the grounding current is small, and it is not obvious from the load current during normal line operation, making it difficult to locate the fault point. The advantage of small resistance grounding is that it generates a large zero-sequence current leading to direct tripping and continued power supply from the standby power source. The disadvantage is that the small resistance needs to have high power, otherwise it is easily burned out, so it is difficult to manufacture and has a high cost. The invention patent application CN 109347074 A discloses a neutral grounding method switching method. When the power grid is operating normally, it is set to the neutral non-grounding operation mode. When a single-phase grounding fault occurs, the neutral non-grounding is switched to medium resistance grounding, so that the fault current is between 10 and 200 amperes, and the single-phase grounding fault point is located through the fault current. However, in practice, due to the random occurrence of grounding faults, the resistance value of the short-circuit loop resistance is unpredictable, so it is difficult to determine the resistance value of the medium resistance. Moreover, the current range of 10 to 200 amperes is large and the characteristics are not obvious enough, and it is difficult to distinguish from the load current during normal line operation in many cases. Therefore, the success rate of finding the fault point by this method is not high. Currently, there is no ideal grounding method to solve the above problems. Summary of the Invention

[0003] The object of the present invention is to provide a neutral grounding method for a three-phase AC power system. In this grounding mode, when a single-phase grounding occurs, it can generate an obvious instantaneous large current. This large current can either directly trip the system to cut off the fault when there is a standby power source, or can successfully detect the single-phase grounding fault point when operating with live wire without power interruption in the single-phase grounding fault mode, integrating the advantages of arc suppression coil grounding and small resistance grounding and making up for their deficiencies.

[0004] To achieve the above object, the present invention adopts the following technical solutions:

[0005] A neutral grounding method for a three-phase AC power system, including a neutral point, a switching device is connected between the neutral point and the ground. When the three-phase AC power system operates normally, the switching device is in an off state. When a single-phase grounding occurs in the three-phase AC power system, the switching device changes from the off state to the on state and then changes back to the off state after a preset period, and the electric energy accumulated by the current passing through the switching device or the three-phase AC power system within the preset period does not exceed the tolerance limits of the switching device and the three-phase AC power system respectively.

[0006] Preferably, the preset period is set as follows: Assuming that a metallic single-phase grounding occurs at the power supply outlet of the three-phase AC power system, the electric energy accumulated by the current passing through the switching device within the preset period does not exceed the tolerance limits of the switching device and the three-phase AC power system.

[0007] Preferably, it further includes a phase-selective closing device. The upper ports of the ABC three-phase independent circuit breakers of the phase-selective closing device are connected to the ABC three-phase of the bus, and the lower ports are connected to the ground. When a single-phase grounding occurs in a certain phase, the switching device operates. After the switching device disconnects, control the grounding-phase circuit breaker of the phase-selective closing device to close. After isolating the current single-phase grounding fault, then control the grounding-phase circuit breaker of the phase-selective closing device to trip.

[0008] Preferably, the preset period is less than one cycle of the current.

[0009] Preferably, the preset period is less than half a cycle of the current.

[0010] Preferably, the voltage closing phase angle when the switching device conducts is within 60 - 90 degrees.

[0011] Preferably, after a single-phase grounding occurs, signals of the voltage rise of two phases to the ground and the voltage drop of one phase to the ground in the three-phase AC power system are obtained through a voltage transformer to instruct the switching device to change from off to on.

[0012] Preferably, the switching device includes an arc extinguishing chamber, in which a static contact and a moving contact are provided. The static contact is connected to the neutral point / ground, and the moving contact is connected to the ground / neutral point. The moving contact can approach the static contact under the drive of the moving contact drive mechanism and can move away from the static contact under the drive of the moving contact reset mechanism.

[0013] Preferably, the power source of the moving contact drive mechanism is a first electromagnetic coil or a first spring, and the power source of the moving contact reset mechanism is a second spring or a second electromagnetic coil. The preset period of the switching device conducting is achieved by adjusting the distance between the moving contact and the static contact or adjusting the driving forces of the moving contact drive mechanism and the moving contact reset mechanism on the moving contact.

[0014] Preferably, the neutral point is the neutral point of the star connection of the power supply in the three-phase AC power system or the neutral point generated by the delta connection through a zigzag transformer.

[0015] The beneficial effects of the present invention are as follows: when a single-phase grounding fault occurs, the neutral point is connected to the ground through the on-off device within a sufficiently short time, and an instantaneous large current can be generated. This instantaneous large current can directly cause the protection action of the system to trip, thereby removing the fault. At this time, compared with the small-resistance grounding method, there is no need to manufacture a high-power small resistance, which has the advantages of easy manufacturing, low cost, and convenient maintenance; compared with the arc suppression coil or non-grounding method, this instantaneous large current moves in the grounding fault loop, and its characteristics are very obvious and easier to detect, so as to quickly and accurately respond to indicate the fault loop and locate the grounding fault point. After the on-off device is disconnected, the fault phase is grounded at the bus through the phase selection closing device, so that it can be energized and operated before the fault is isolated, which not only has the advantages of arc suppression coil grounding, but also overcomes the disadvantages of small fault current and difficult fault point search when the arc suppression coil is grounded. Description of the Drawings

[0016] Figure 1 is the wiring schematic diagram of the present invention;

[0017] Figure 2 is the external structure schematic diagram of the on-off device used in the present invention;

[0018] Figure 3 is the sectional structure schematic diagram of the on-off device used in the present invention;

[0019] Figure 4 The oscillogram curves of each electrical parameter during the simulation of the method of the present invention at 10 kV;

[0020] Figure 5 is Figure 4 the schematic diagram of the relevant circuit in the experiment. Detailed Embodiments

[0021] The following further illustrates the present invention through specific embodiments in conjunction with the drawings:

[0022] Embodiment 1 Neutral Point Grounding Method

[0023] The present invention is directed to a three-phase AC power system, where the power supply end is a transformer or a generator. In the system, the ABC three-phase coils are connected in a star connection to form a neutral point, or are connected in a delta connection and a neutral point is obtained through a Z transformer. The power supply is connected to a busbar, the busbar is connected to outgoing lines, and further lower-level outgoing lines can be branched from the outgoing lines, and so on. Switches are provided on the outgoing lines to drive loads and the like. A plurality of fault indicators can be provided on the outgoing lines to capture fault currents and indicate the fault circuits. A three-phase line-to-ground voltage inductor 2 is also provided on the busbar to determine that when a single-phase ground fault occurs, the line-to-ground voltages of two phases in the three-phase line increase, and the line-to-ground voltage of one phase decreases. The phase with the decreased voltage is the grounded phase. A phase-selection closing device 3 is provided on the busbar to ground the grounded phase, eliminate the arc grounding current, and increase the safety of live operation during single-phase grounding.

[0024] In the neutral point grounding method of the present invention, the neutral point is connected to a switching device KZ, and the switching device is connected to the ground. When the three-phase AC power system is operating normally, the switching device KZ is disconnected; when a single-phase ground fault occurs, the switching device closes after receiving an action instruction, making the neutral point conduct with the ground, and then disconnects after a very short duration. This duration can be set artificially and is defined as a preset period. The characteristics and requirements that the preset period should satisfy are as follows: 1. This period starts from the moment when the switching device starts to conduct the current between the neutral point and the ground and ends at the moment when the switching device cuts off the current between the neutral point and the ground; 2. During this period, the current will pass through a grounding fault loop composed of the system neutral point, the ground, the single-phase ground fault point, and the corresponding fault-phase conductor. This current is the fault current. By adjusting the preset period (and the control of the voltage closing phase), the current can reach a peak value and the energy accumulated during this period does not exceed the tolerance index of the impact electrical energy in the design parameters of the power supply and the switching device, thus avoiding burnout; in a preferred method for calculating the preset period, it is assumed that a metallic single-phase ground occurs at the power supply outlet. At this time, the energy accumulated during the preset period does not exceed the tolerance limits of the switching device and the power supply, so that the power supply and the switching device will not be burned out when a single-phase ground occurs at other positions. 3. The adjustment of the length of the preset period is achieved by effectively controlling the closing and opening processes of the switching device.

[0025] In one embodiment, the switching device adopts the following structure: As Figure 2 , Figure 3As shown in the figure, it includes an arc extinguishing chamber 50. Inside the arc extinguishing chamber 50, there are a static contact 51 and a moving contact 52. One end of the static contact 51 extends out of the arc extinguishing chamber 50 and is connected to the neutral point. The moving contact 52 extends out of the arc extinguishing chamber 50 and is connected to the moving contact driving mechanism and the moving contact reset mechanism, and is simultaneously connected to the ground through a grounding wire. The moving contact driving mechanism can drive the moving contact to move towards the static contact, and the moving contact reset mechanism can drive the moving contact away from the static contact. In a specific embodiment, the moving contact driving mechanism includes a connecting rod 53 with one end connected to the moving contact. The other end of the connecting rod 53 is connected to a side panel 541 of a metal angle plate 54 hinged on the base. The other side panel 542 of the metal angle plate 54 is substantially perpendicular to the panel 541 and can be attracted by an electromagnetic coil 55. There are pin shafts 543 at both ends of the connection between the panel 541 and the panel 542, and it is hinged on the base 59 through this pin shaft. When the electromagnetic coil 55 attracts the panel 542, the metal angle plate 54 rotates, and the panel 541 pushes the connecting rod 53 upward and then pushes the moving contact 52 closer to the static contact. A spring 56 is also sleeved on the connecting rod 53. When the connecting rod 53 pushes the moving contact 52 towards the static contact 51, the spring 56 is compressed simultaneously. When the electromagnetic coil 55 loses power, the magnetic force of the electromagnetic coil disappears, then the spring 56 presses the panel 541 downward, causing the connecting rod 53 to move downward and separating the moving contact 52 from the static contact 51. The power source of the moving contact driving mechanism can be an electromagnetic coil or a spring. The power source of the moving contact reset mechanism can be a spring or can also be set as an electromagnetic coil.

[0026] During the process of the above on-off device conducting and cutting off the neutral point and the ground, the moving contact first approaches the static contact and conducts the circuit, and then leaves the static contact and cuts off the circuit, specifically including the following five stages: 1. The moving contact starts to move, that is, under the drive of the moving contact driving mechanism, the moving contact obtains an acceleration from a state with an initial velocity of zero and moves towards the static contact. At this time, the moving contact only makes a mechanical movement and does not conduct the circuit; 2. When the moving contact approaches the static contact to a certain extent, the circuit conducts. At this time, the moving contact may just physically contact the static contact, or there may still be a certain distance between the moving contact and the static contact, but this distance is small enough that it is no longer an insulating distance under the current voltage conditions; 3. The electromagnetic coil of the moving contact driving mechanism loses power, that is, it loses the acceleration towards the static contact, but at this time it still retains the initial velocity of approaching the static contact (if it has already contacted the static contact when losing power, then it no longer has an initial velocity). At the same time, the restoring force of the spring starts to act alone, generating an acceleration away from the static contact. Because at this time the moving contact is still approaching the static contact or has contacted and not separated yet, the circuit is still conducting; 4. The moving contact moves in the direction away from the static contact until it reaches the insulating distance from the static contact, and at this time the circuit is cut off; 5. When the moving contact reaches the insulating distance from the static contact, it further moves away until it returns to the initial position.

[0027] The key to the above process lies in the second to the fourth stages, because it is during this stage that the entire process of the circuit conducting to being cut off occurs, that is, the process during which the short-circuit current exists. The time of this process is also the time of the short-circuit current, which is the period that needs to be adjusted according to expectations, namely the aforementioned preset period. The present invention first needs to ensure that the electrical energy accumulated by the current passing through within the preset period does not damage the switching device and the power system. On this premise, if the short-circuit current reaches or exceeds the operating threshold of the power system protection device, the system trips, thereby isolating the fault (usually there is a standby power supply and it is enabled). At this time, the function of this grounding method is the same as that of the small-resistance grounding method. However, for the small resistance to withstand the impact of a large current, it needs to have a sufficiently large power, resulting in a high manufacturing cost, and sometimes it is easily burned out due to excessive current. As long as the present patent's grounding method controls an appropriate preset period, it can ensure tripping and cause no damage to the switching device. For the existing arc suppression coil grounding system method, the instantaneous large current generated by using the grounding method of the present invention can be used as a characteristic current for the fault indicator to identify and detect. This current is much larger than the fault current generated by grounding through the arc suppression coil and is a short-time large pulse, which is significantly different from the normal load current of the line and is extremely easy to be identified by the fault indicator. This not only helps to simplify the logical judgment process of the fault indicator but also can increase the probability of accurate detection. When specifically adjusting the preset period, since the grounding fault occurs randomly, the resistance of the fault loop formed after the switching device closes is unpredictable. As mentioned above, the present invention can assume a metallic single-phase ground fault occurs at the power supply outlet, calculate the short-circuit current based on this, and then control the duration of the short-circuit current so that the energy accumulated by it does not burn out devices such as the power supply, wires, and switching devices. In this way, when a single-phase ground fault occurs elsewhere, it is even less likely to burn out devices such as the power supply, wires, and switching devices. Based on the designed power-carrying capacity of each participating component such as the power supply, switching device, and wire, the current within the preset period can be controlled not to exceed one cycle (the AC cycle of the power system in our country is 20 ms), more preferably not to exceed half a cycle (10 ms), and even can be considered to be controlled within a few milliseconds. For example, the voltage closing phase of the switching device during conduction can be controlled within the range of 60 to 90 degrees, which can cause the current to peak and is more convenient for detection. As long as the power of the electromagnetic coil of the moving contact driving mechanism of the AC switching device and the elastic force of the spring of the moving contact reset mechanism (such as adjusting the spring constant) are adjusted, as well as the respective action times of the two, the speed of the moving contact approaching the static contact and the speed of moving away from the static contact can be controlled, thereby controlling the circuit on-off time. By adjusting the combination of the power of the electromagnetic coil of the moving contact driving mechanism and the restoring force of the moving contact reset mechanism, the short-circuit current can be made not to exceed half a cycle. If the power of the electromagnetic coil and the restoring force of the spring are given, the duration of the current can also be adjusted by adjusting the initial distance between the moving contact and the static contact.For example, through test adjustment, before the moving contact moves upward to conduct the circuit, the driving mechanism of the moving contact is stopped, and the reset mechanism of the moving contact acts alone. The moving contact can continue to move forward to the critical position of conduction relying on the initial velocity at this time. At this time, the velocity becomes zero, and then it moves in the reverse direction to cut off the circuit again. Then the whole process of conducting the circuit will be shorter than half a cycle. The above function can also be realized by using high-power high-voltage IGBTs.

[0028] An important application of the neutral point grounding method of the present invention is to solve the single-phase grounding problem, including providing a large and short characteristic current to make the fault indicator on the line turn over to indicate the faulty line and the single-phase grounding fault point; or accurately judging and isolating the fault by providing a large and short fault current to the differential device.

[0029] As Figure 1 shown, when a single-phase grounding fault occurs, the on-off device quickly turns on and off. Under the condition of ensuring the tolerance of the power system (within the design allowance), a huge (possibly hundreds of amperes) and short current will still pass through the fault loop. The fault indicator on the fault loop can smoothly and accurately detect this current and thus make a reaction, while the fault indicators on other lines have no reaction. As Figure 1 described, the fault indicator 1 on one side of the single-phase grounding fault point F is in the fault loop, so it acts, while the fault indicator 4 on the other side is not in the fault loop, so it has no reaction. In this way, the different reactions of the fault function indicator 1 and the fault indicator 4 can determine the single-phase grounding point interval. After the fault point is judged, the fault can be isolated for maintenance.

[0030] Based on the above-mentioned huge and short fault current, the differential device on the line will also automatically analyze the position of the fault point according to the fault current, so as to control the action of the controlled switch at the upper end of the faulty line to automatically isolate the faulty line.

[0031] Embodiment 2 Simulation example of controlling the on-off device

[0032] The control of neutral point grounding by means of a make-and-break device can be carried out as follows: When a single-phase grounding fault occurs, the voltage transformer detects that the voltage of the faulty phase decreases and the voltages of the non-faulty phases increase, and transmits the signal to the control unit. The control unit controls the moving contact of the make-and-break device to drive the electromagnetic coil to act through a relay, and starts to drive the moving contact to approach the static contact. When approaching until the circuit is conducted, the alternating voltage across the make-and-break device at this moment is the voltage closing phase angle. The magnitude of the voltage closing phase angle determines the magnitude of the current at conduction. When the voltage closing phase angle is between 60° and 90°, it is the stage when the positive voltage rises to a relatively high voltage value, which will make the current larger and easier to detect. Taking the control of the voltage closing phase angle to be zero as an example, the following illustrates how to control the magnitude of the voltage closing phase angle: It is necessary to coordinate the movement of the moving contact with the change of the alternating voltage. Generally, it can be designed according to the following principle, that is, when a grounding fault occurs and is detected by the control unit, the control unit first obtains the moment when the alternating voltage between the neutral point and the ground will not pass through zero (which can be obtained through a zero-crossing and voltage cycle detection circuit, which is an existing technology), and can also determine the time △t required for the make-and-break device to drive the moving contact to move until the circuit is conducted through pre-tests. a , so that the control unit issues an order at a moment △t a before the voltage passes through zero to make the electromagnetic coil of the make-and-break device start to act, and it can be achieved that when the moving contact and the static contact approach each other until the circuit is conducted, the alternating voltage across the high-voltage switch just passes through zero, that is, the voltage closing phase angle is zero.

[0033] The methods for controlling the voltage closing phase angle to be 60°, 90°, etc. are similar to this, and are determined in combination with the specific mechanical and electrical properties of the make-and-break device.

[0034] The above illustrates how to set the time when the moving contact driving mechanism starts to act, and it is also equally important when the moving contact driving mechanism stops acting. From this moment on, the spring starts to act alone and makes the moving contact finally leave the static contact to cut off the circuit. The time elapsed from when the moving contact driving mechanism stops acting on the moving contact after driving the moving contact to conduct the circuit, and then the moving contact reset mechanism acts alone and makes the moving contact leave the static contact to cut off the circuit determines the duration of the short-circuit current. It is possible to set in the control unit that the electromagnetic coil of the moving contact driving mechanism loses power after a time △t b after the closing command is issued, and then the spring immediately acts alone. This △t b time can be determined through off-line tests to control an alternating current of half a cycle or less than half a cycle.

[0035] Figure 4The oscillogram curve shows that when the on-off device KZ conducts and cuts off the line under the conditions of small current and 10 kV voltage, the recorder records multiple electrical parameters. This test has been adjusted so that the voltage closing phase angle is zero and the current passing through does not exceed one cycle. Among them, curve a is the voltage change curve at both ends of the on-off device, and curve c is the zero-crossing detection signal of the on-off device voltage. The vertex of each wave crest on curve c is the voltage zero-crossing point. It can be seen that the on-off device shows periodic changes before closing. When the on-off device closes and the line just starts to conduct, the voltage at both ends of the on-off device is zero (curve a), and as can be seen from the position of the dashed line L, at this time it coincides with the time point of the periodic voltage zero-crossing point (this conclusion can be obtained by comparing curves a and c), indicating that the voltage closing phase angle is zero, and the voltage of the on-off device remains zero for some time thereafter, indicating that this process is the process in which the on-off device starts to conduct continuously with current from the conduction of the line. Curve e is the current curve passing through the on-off device. The abscissa time of curve e corresponding to the dashed line M is the time when the current is zero. At this time, the line is cut off, and the voltage at both ends of the on-off device starts to change in waveform again (curve a). The part of curve e between the dashed line L and the dashed line M is the alternating current not exceeding one cycle, specifically half a cycle of alternating current plus the decaying current after the zero-crossing point of this half-cycle of alternating current. If the spring restoring force increases, causing the moving contact to just leave the static contact and cut off the circuit during the half-cycle time of conduction, the current can be exactly half a cycle or even shorter than half a cycle. The alternating current with a half-cycle or shorter than half a cycle has a short time but can have a large current value, with very obvious characteristics and is easy to detect. Figure 4 Curve b in it is the voltage curve of the electromagnetic coil of the moving contact driving mechanism of the on-off device, and curve d is the voltage curve of the DC solid-state relay that controls the electromagnetic coil of the moving contact driving mechanism. Figure 5 The relevant circuit of this test is shown. Among them, the 5 channels of the acquisition card of the recorder 81 are connected to the wire 811, wire 812, wire 813, wire 814, and wire 815, which are used to collect the electrical quantities corresponding to the above five curves a, b, c, d, and e. The circuit also includes the electromagnetic coil 87 of the moving contact driving mechanism, the current transformer 88, the voltage transformer 1, the secondary PT 83, the zero-crossing detection circuit 86, the DC fixed relay 84 for controlling the electromagnetic coil 87, the DC Hall element 82, and the control device 85. Among them, the secondary PT 83 collects the on-off device voltage, corresponding to curve a; the electromagnetic coil 87 of the moving contact driving mechanism corresponds to curve b, and the zero-crossing detection corresponds to curve c; the DC solid-state relay corresponds to curve d; the current transformer 88 is used to obtain the load current, that is, curve e. The on-off device can be flexibly controlled in a similar way to implement the neutral point grounding method of the three-phase AC power system of the present invention, and further solve the problem of finding the single-phase grounding fault point, or isolate the faulty line by using differential technology, etc.

[0036] The above embodiments are merely illustrative of several aspects of the inventive concept and its implementation, and do not limit it. Under the inventive concept, technical solutions without substantial transformation are still within the scope of protection.

Claims

1. A neutral point grounding method for a three-phase AC power system, including a neutral point, characterized in that, A switching device is connected between the neutral point and the earth, and the switching device includes an arc extinguishing chamber, in which a static contact and a moving contact are arranged, one end of the static contact extends out of the arc extinguishing chamber and is connected to the neutral point, the moving contact extends out of the arc extinguishing chamber and is connected to a moving contact driving mechanism and a moving contact resetting mechanism, and is simultaneously connected to the earth through a grounding wire, the moving contact driving mechanism includes a connecting rod connected to the moving contact at one end, the other end of the connecting rod is connected to a first side panel of a metal angle plate hinged on the base, the second side panel of the metal angle plate is perpendicular to the first side panel and can be attracted by an electromagnetic coil, pins are provided at both ends of the connection between the first side panel and the second side panel, and are hinged to the base through the pins, when the electromagnetic coil attracts the second side panel, the metal angle plate rotates, and the first side panel moves upward The connecting rod is pushed and then the moving contact is pushed closer to the static contact. A spring is also sleeved on the connecting rod. When the connecting rod pushes the moving contact toward the static contact, the spring is compressed at the same time. When the electromagnetic coil loses power, the spring presses the first side panel downward, causing the connecting rod to move downward and causing the moving contact to separate from the static contact. When the three-phase AC power system operates normally, the on-off device is in an off state. When a single-phase grounding occurs in the three-phase AC power system, the on-off device changes from an off state to an on state and changes to an off state again after a preset period of time. Moreover, the electric energy accumulated in the on-off device and the three-phase AC power system by the current passing through the on-off device during the preset period does not exceed the bearing limits of the on-off device and the three-phase AC power system, respectively.

2. The neutral point grounding method of the three-phase AC power system according to claim 1, characterized in that, The preset time period is set as follows: assuming that a metallic single-phase grounding occurs at the power outlet of the three-phase AC power system, the electric energy accumulated by the current passing through the switching device during the preset time period does not exceed the tolerance limit of the switching device and the three-phase AC power system.

3. The neutral point grounding method of the three-phase AC power system according to claim 1, characterized in that, It also includes a phase-selective closing device, wherein the upper port of the three-phase independent circuit breaker ABC of the phase-selective closing device is connected to the three phases of the busbar ABC, and the lower port is connected to the ground. When single-phase grounding occurs in one phase, the on-off device is actuated, and after the on-off device is disconnected, the grounding phase circuit breaker of the phase-selective closing device is controlled to close. After the single-phase grounding fault is isolated, the grounding phase circuit breaker of the phase-selective closing device is controlled to trip.

4. The neutral point grounding method of the three-phase AC power system according to claim 3, characterized in that, The preset time period is smaller than one cycle of the current.

5. The neutral grounding method of the three-phase AC power system according to claim 4, characterized in that, The preset time period is smaller than a half cycle of the current.

6. The neutral point grounding method of the three-phase AC power system according to claim 1, characterized in that, The voltage closing phase angle when the on-off device is turned on is within a range of 60-90 degrees.

7. The neutral point grounding method of the three-phase AC power system according to claim 1, 2 or 3, characterized in that, After a single-phase ground fault occurs, a voltage sensor is used to obtain signals of two-phase ground voltage increase and one-phase ground voltage decrease in the three-phase AC power system to instruct the on-off device to change from disconnection to conduction.

8. The neutral point grounding method of the three-phase AC power system according to claim 1, characterized in that The power source of the moving contact driving mechanism is a first electromagnetic coil or a first spring, and the power source of the moving contact resetting mechanism is a second spring or a second electromagnetic coil. The on-off device is turned on for the preset time period by adjusting the distance between the moving contact and the static contact or adjusting the driving force of the moving contact driving mechanism and the moving contact resetting mechanism on the moving contact.

9. The neutral point grounding method of the three-phase AC power system according to claim 1, 2 or 3, characterized in that, The neutral point is the neutral point of the star connection of the power supply in the three-phase AC power system or the neutral point generated by the delta connection through a zigzag transformer.

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