An Asymmetric Current Source in a Small Current Grounding System

Abstract

The present invention discloses an asymmetric current source in a small current grounding system, which includes a high-voltage switch that is independent in two phases or three phases. One return line is selected as the first return line. One side of the high-voltage switch intersects with any two phases or three phases in the first return line at a connection point, and the other side is connected to the ground. The unidirectional wire impedance from the connection point position to the power supply outlet or the sum of the unidirectional wire impedance from the connection point position to the power supply outlet and the current-limiting impedance between the connection point position and the ground can make the magnitude of the short-circuit current not trigger the first-stage overcurrent protection of the first return line; the closing and opening time of the high-voltage switch can make the duration of the short-circuit current less than the current duration of the second-stage overcurrent protection of the first return line. The phase-to-phase short-circuit current manufactured by this device is significantly different from the line load current and is very easy to identify.

Description

Technical Field

[0001] The present invention relates to a detection technology for single-phase grounding faults occurring in lines of 10 kV or 35 kV small current grounding systems in power systems. Specifically, it relates to an asymmetrical current source for generating characteristic currents for detection in a small current grounding system. Background Art

[0002] Currently, most distribution networks in China adopt small current grounding systems (also known as indirectly grounded neutral systems). The distribution system has many branches and is complex. When a single-phase grounding fault occurs in such a system, due to the small fault current and complex fault characteristics, it is very difficult to locate the fault point.

[0003] Chinese Utility Model Patent No. 201020549446.7 discloses an asymmetrical current source, which includes a secondary control part, an electronic PT, a relay, and an AC high-voltage vacuum contactor. The electronic PT is connected to the input end of the secondary control part through a transmitter. The output end of the secondary control part is connected to the relay coil. The relay controls the coil of the AC high-voltage vacuum contactor. The primary sides of the three AC high-voltage vacuum contactors are respectively connected to the three phases A, B, and C, and the other sides are connected in series with a D high-voltage diode and an R current-limiting impedance, and the other end of the R current-limiting impedance is connected to the ground. Chinese Utility Model Patent No. 201220448758.8 simplifies the foregoing patent and can achieve the corresponding functions only by using two AC high-voltage vacuum contactors. In the above patents, when a grounding fault occurs in a certain phase, the voltages of the other two phases to the ground increase and are detected by the electronic PT. Then, the secondary control part controls one of the AC high-voltage vacuum contactors connected to the non-fault phase to close, so that this phase line is also connected to the ground, thus forming an inter-phase short circuit with the fault phase, generating a short-circuit current. After being rectified by the high-voltage diode, the short-circuit current can generate a half-wave current, which has obvious characteristics when the current is large and can be recognized by the fault indicators on the three-phase lines. According to the loop where the short-circuit current is located, the grounding fault point can be judged. For the above two methods of generating characteristic currents, a current-limiting impedance needs to be connected in series to avoid the short-circuit current triggering the over-current protection and causing a large-area power outage. However, because the grounding point of the single-phase grounding is unpredictable, the impedance value of the impedance in the short-circuit current loop cannot be determined. If the impedance connected in series is too small, it cannot play the role of current-limiting protection. If the impedance connected in series is too large, the short-circuit current will be too small to be detected, which limits the application of this method. So far, there is no suitable method to determine how to connect a suitable current-limiting impedance. Also due to the above reasons, the above two patents do not have ideal effects in actual use and are difficult to promote. Summary of the Invention

[0004] The object of the present invention is to provide an asymmetric current source in a small current grounding system. The phase - to - phase short - circuit current produced by this device is significantly different from the line load current, is extremely easy to identify, does not have the dilemma of choosing a current - limiting impedance, and will not cause the short - circuit current to be too large to trigger the line protection action.

[0005] In order to achieve the above object, the present invention adopts the following technical solutions:

[0006] An asymmetric current source in a small current grounding system includes a high - voltage switch that is independent in two phases or three phases. Select a circuit outgoing line in the small current grounding system as the first circuit outgoing line. One side of the high - voltage switch intersects with any two or three phases of the first circuit outgoing line at a connection point, and the other side is connected to the ground. When a ground fault occurs in one phase of a certain circuit outgoing line in the small current grounding system and the high - voltage switch of a non - fault phase of the first circuit outgoing line is closed and then opened to generate a short - circuit current, the one - way wire impedance from the connection point position to the power supply outlet of the small current grounding system or the sum of the one - way wire impedance from the connection point position to the power supply outlet of the small current grounding system and the current - limiting impedance between the connection point position and the ground can make the magnitude of the short - circuit current not trigger the first - stage over - current protection of the first circuit outgoing line; the closing and opening time of the high - voltage switch can make the duration of the short - circuit current less than the current duration of the second - stage over - current protection of the first circuit outgoing line.

[0007] Preferably, define the presettable impedance as the impedance value of the single - phase wire impedance from the connection point position to the power supply outlet of the small current grounding system or the sum of the impedance value of the single - phase wire impedance from the connection point position to the power supply outlet of the small current grounding system and the series - connected current - limiting impedance. Then, the connection point position is determined as follows:

[0008] (a) Assume that a metallic two - phase direct short - circuit occurs at the power supply outlet of the small current grounding system for the first circuit outgoing line, and define the short - circuit current at this time as the limit short - circuit current;

[0009] (b) Insert a hypothetical impedance into the short - circuit loop in step (a) to reduce the magnitude of the short - circuit current from the limit short - circuit current to less than or equal to 0.9 times the current setting value of the first - stage over - current protection of the first circuit outgoing line. Then, the impedance value of the hypothetical impedance is the impedance value of the presettable impedance. Then, calculate the wire length to be inserted according to the impedance value of the presettable impedance and the impedance of the wire per unit length, and thus determine the connection point position, or determine the connection point position according to the difference between the impedance value of the presettable impedance minus the impedance value of the current - limiting impedance and the impedance value of the wire per unit length.

[0010] Preferably, among the various circuit outgoing lines of the small current grounding system, select the outgoing line with the smallest first - stage over - current protection current setting value as the first circuit outgoing line.

[0011] Preferably, the closing and opening time of the high-voltage switch can ensure that the duration of the short-circuit current does not exceed one cycle.

[0012] Preferably, the duration of the short-circuit current is shorter than half a cycle, or equal to half a cycle, or longer than half a cycle and shorter than one cycle.

[0013] Preferably, the high-voltage switch includes a vacuum interrupter, in which a static contact and a moving contact are provided. Two-phase or three-phase static contacts are connected to two-phase or three-phase of the low-current grounding system, and the moving contact is connected to the ground. The moving contact can approach the static contact under the drive of the moving contact driving mechanism and can move away from the static contact under the drive of the moving contact reset mechanism.

[0014] Preferably, 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 reset mechanism is a second spring or a second electromagnetic coil.

[0015] Preferably, a voltage transformer is provided to monitor the voltage of the first outgoing line. The voltage transformer is connected to the control unit, and the control unit controls the actions of the moving contact driving mechanism and the moving contact reset mechanism.

[0016] Preferably, each high-voltage switch is composed of a first high-voltage switch and a second high-voltage switch connected in series. The first high-voltage switch is in an open state, and the second high-voltage switch is in a closed state.

[0017] The beneficial effect of the present invention is that the device starts from the coordination of the magnitude and duration of the short-circuit current to generate a characteristic current as large as possible for easy detection. Since the location of the contact fault point is unpredictable, the wire impedance and grounding impedance of the faulty line connected in series to the short-circuit loop become impedances with unpredictable values. However, the preset impedance connected in series in the present invention can ensure that the first-stage overcurrent protection of the line is not triggered. Adding the above-mentioned unpredictable impedance, it is certain that the first-stage overcurrent protection will not be triggered. At the same time, the short-circuit current generated by the present invention is also the largest safe current that can be controlled artificially ( "safe" means not triggering the first-stage overcurrent protection). At the same time, the closing and opening speed of the high-voltage switch in the present invention needs to meet the requirement that the duration of the short-circuit current does not trigger the second-stage overcurrent protection. Compared with the prior art that does not solve the problem in terms of the magnitude and duration of the current, the present invention specifically solves the problem of generating the characteristic current from the magnitude and duration of the short-circuit current. The generated characteristic current is generally dozens or even hundreds of times larger than the normal load current, which is very easy to detect, providing a precondition for the promotion and practical application of the grounding fault point detection. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 is a wiring schematic diagram of the method of the present invention;

[0019] Figure 2It is a schematic diagram of the external structure of the high-voltage switch used in the present invention;

[0020] Figure 3 It is a schematic cross-sectional structure diagram of the high-voltage switch used in the present invention;

[0021] Figure 4 Oscillogram curves of various electrical parameters during simulation at 10 KV using the present invention.

[0022] Figure 5 is Figure 4 Schematic diagram of the relevant circuit in the experiment. Specific embodiments

[0023] The following further illustrates the present invention with reference to the accompanying drawings through specific embodiments. The small current grounding system in the present invention includes a three-phase system with an unearthed neutral point or grounded through an arc suppression coil and a high impedance.

[0024] As Figure 1 shown, in the small current grounding system, there are multiple outgoing lines at the power supply. One of them is selected as the first outgoing line. When a grounding fault occurs in one phase of a certain outgoing line among the multiple outgoing lines (such as phase A), then the other two phases (phase B and phase C) of the first outgoing line except phase A are defined as non-fault phases. The present invention relates to the following wiring method: One end of two 10 KV AC high-voltage vacuum contactors (i.e., high-voltage switches) K1 and K2 is respectively connected to any two phases of the A, B, and C phases of the 10 KV first outgoing line (or three independent AC high-voltage vacuum contactors can also be used to be respectively connected to the three phases). For example, connect to phases A and B (hereinafter, connecting to phases A and B will be used as an example for illustration). The connection points of the two AC high-voltage vacuum contactors to phases A and B are points O and P. The other end is directly connected to the ground or a current-limiting impedance is connected in series between it and the ground. By setting the connection point positions (i.e., points O and P), the length of the wire connected in series between the high-voltage switch and the power supply can be determined. The impedance of this length of wire is equal to the impedance per unit length of the wire multiplied by the wire length. When a grounding fault occurs in one phase (suppose it is phase A) of a certain outgoing line, the high-voltage switch of phase B in the first outgoing line is made to act and conduct with the ground, generating a short-circuit current. The impedance in this short-circuit loop includes the impedance of the single-phase wire between the connection point position and the power supply, the impedance of the wire between the grounding point and the power supply, and the grounding impedance (these two are impedance values of unpredictable impedances, and their magnitudes cannot be determined artificially and can only be determined after each grounding fault actually occurs), and also includes the current-limiting impedance connected in series between the high-voltage switch and the ground (a current-limiting impedance can be connected in series or not). The impedance of the single-phase wire between the connection point position and the power supply and the current-limiting impedance are collectively called the presettable impedance. Thus, it can be seen that by adjusting the connection point position, the magnitude of the presettable impedance can be adjusted, and the magnitude of the short-circuit current can be affected. The method of the present invention requires that the presettable impedance should not cause the magnitude of the short-circuit current to trigger the first-stage overcurrent protection under various grounding faults. It can be specifically realized by the following method:

[0025] (a) Assume that a metallic two-phase direct short circuit occurs at the power outlet of the first outgoing line, and define the short-circuit current at this time as the ultimate short-circuit current (because the maximum short-circuit current is generated at this time);

[0026] (b) Insert a hypothetical impedance into the short-circuit loop in step (a) to reduce the magnitude of the short-circuit current from the ultimate short-circuit current to less than or equal to 0.9 times the current setting value of the first section overcurrent protection of the first outgoing line (when it is equal to 0.9 times, it is the most ideal choice, which will generate the required maximum short-circuit current. When it is less than 0.9 times, the current will become smaller, but there will also be a clear distinction from the normal load current of the line, and the purpose of smooth detection can also be achieved; for a substation equipped with a protection device that can still reliably not operate when the first section overcurrent protection setting value is 0.95 times, this number can be adjusted to 0.95). Then, the impedance value of the hypothetical impedance is the impedance value of the presettable impedance. Then, calculate the length of the wire to be inserted according to the impedance value of the presettable impedance and the impedance of the wire per unit length, and thus determine the position of the connection point (at this time, no current-limiting impedance is inserted, the high-voltage switch is directly connected to the ground, and the impedance between the connection point position and the ground can generally be ignored. This operation is simpler and saves the cost of the current-limiting impedance, which is the best choice), or determine the position of the connection point according to the difference between the impedance value of the presettable impedance minus the impedance value of the current-limiting impedance and the impedance of the wire per unit length (at this time, the current-limiting impedance is inserted. The impedance value of the current-limiting impedance is generally used to offset the impedance of the single-phase wire from the connection point position to the power supply. That is, compared with the case of not inserting the current-limiting impedance, if the connection point position is closer to the power supply, the reduced impedance of the wire can be replaced by the current-limiting impedance, so as to avoid the current exceeding 0.9 times the ultimate short-circuit current; of course, there are other combination methods for the current-limiting impedance and the single-phase wire impedance, and the purpose is to control the short-circuit current).

[0027] In the above scheme, for the case where the cable thickness, material, and line length of each outgoing line are basically the same, the current setting values of the first section overcurrent protection of each outgoing line are basically equal at this time, and any outgoing line can be selected as the first outgoing line; for the case where the cable conditions are quite different and the current setting values of the first section overcurrent protection are not equal, the outgoing line with the smallest current setting value of the first section overcurrent protection can be selected. In this way, when the short-circuit current does not trigger the first section overcurrent protection of the first outgoing line, the overcurrent protection of the wire from the grounding point to the power supply in this short-circuit loop will definitely not be triggered.

[0028] Meanwhile, the duration of the short-circuit current is controlled by the on-off time of the high-voltage switch, so that the duration is not greater than the current duration of the overcurrent protection in Section II (this duration is greater than one cycle). Preferably, the current duration can be shortened to not exceed one cycle, such as less than half a cycle, equal to half a cycle, or greater than half a cycle and less than one cycle. In order to improve the detectability of the current waveform, the transient current waveform not greater than half a cycle can be a positive half-wave or a negative half-wave (including zero point), that is, all greater than or equal to zero, or all less than or equal to zero. The magnitude of the current is dozens of times that of the normal load current, and this feature is very obvious and can be easily detected by the fault indicator on the line, so that this characteristic current has great practical value, overcoming the technical deficiency in the prior art that it is impossible to generate as large a current as possible without tripping.

[0029] The following describes how to set the high-voltage switch so that it can generate an alternating current not exceeding one cycle during the on-off process, which can be illustrated by two embodiments:

[0030] 1. The high-voltage switch only includes one switch (such as only a vacuum AC contactor or a circuit breaker, etc. to switch the circuit on and off)

[0031] For a mechanical switch with a moving contact and a static contact, the key is to control the process from when the moving contact approaches the static contact and conducts the circuit to when the moving contact moves away from the static contact and cuts off the circuit to pass an alternating current not exceeding one cycle. The high-voltage switch can generally be adopted such as Figure 2 、 Figure 3The structure shown includes a vacuum bubble 50, in which a static contact 51 and a moving contact 52 are provided. One end of the static contact 51 extends out of the vacuum bubble 50 and is connected to the three-phase line. The moving contact 52 extends out of the vacuum bubble 50 and is connected to the moving contact driving mechanism and the moving contact reset mechanism, and is also connected to the grounding wire at the same time. 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 one 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 the 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 at the same time. When the electromagnetic coil 55 loses power, the magnetic force of the electromagnetic coil disappears, and the spring 56 presses the panel 541 downward, causing the connecting rod 53 to move downward and the moving contact 52 to separate from the static contact 51. There are also various other moving contact driving mechanisms and moving contact reset mechanisms for driving the moving contact to approach and move away from the static contact. The power source of the moving contact driving mechanism can be an electromagnetic coil or a spring, and the power source of the moving contact reset mechanism can be a spring or can be set as an electromagnetic coil.

[0032] Referring to the above high-voltage switch structure and referring to Figure 1 , the voltage transformers, namely PT 1, PT 3, and PT 5, collect the corresponding phase voltage signals and send them to the input terminals UA, UB, and UC of the control unit, that is, the secondary control part. Inside the control unit, they are transmitted to the single-chip microcomputer through the secondary PT and the analog-to-digital conversion unit. Finally, the electromagnetic coils of the moving contact driving mechanisms of two 10KV AC high-voltage vacuum contactors K1 and K2 are controlled through the relay. The moving contact reset mechanism of the high-voltage switch (AC high-voltage vacuum contactor) is a spring. After the electromagnetic coil of the moving contact driving mechanism loses power, the spring drives the moving contact to move in the direction of the original position. By setting the power of the electromagnetic coil of the moving contact driving mechanism, the speed of the moving contact approaching the static contact is controlled, and by adjusting the restoring force of the spring, the speed of the moving contact moving away from the static contact is controlled. When the electromagnetic coil of the moving contact driving mechanism is powered on, it drives the moving contact to move towards the static contact. At this time, the spring is stretched, and the moving contact overcomes the restoring force of the spring at the same time. When the electromagnetic coil of the moving contact driving mechanism loses power, the moving contact is only affected by the restoring force of the spring, and moves away from the static contact and cuts off the line under the action of the restoring force.

[0033] Specifically analyzing the process in which the moving contact approaches the static contact, conducts the circuit, and then leaves the static contact to cut off the circuit, it can include the following five stages: 1. The moving contact starts to move. That is, under the drive of the moving contact drive 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 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 phase of the voltage across the switch is the closing phase angle). 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 drive 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, 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 away from the static contact until it reaches the insulating distance from the static contact, at which 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.

[0034] For the present invention, the key lies in the process of the above-mentioned second to fourth stages. Because this process is the entire process from circuit conduction to cutting off, that is, the process in which the short-circuit current exists. The time of this process is the time of the short-circuit current. The present invention requires that the alternating current in this process does not exceed the current duration of the second-stage overcurrent protection, preferably does not exceed one cycle, and more preferably does not exceed half a cycle. For the power system in our country, one cycle is 20 milliseconds. On this premise, the on-off time of the high-voltage switch can be further reduced so that the current passes through for no more than half a cycle. As long as the power of the electromagnetic coil of the moving contact drive mechanism of the alternating-current high-voltage vacuum contactor and the elastic force of the spring of the moving contact reset mechanism (such as adjusting the spring constant) are adjusted, 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 on-off time of the circuit. By adjusting the combination of the power of the electromagnetic coil of the moving contact drive 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 current duration can also be controlled by adjusting the initial distance between the moving contact and the static contact.

[0035] When the above-mentioned asymmetrical current source is used for single-phase ground fault detection, when a ground fault occurs in one phase of a certain outgoing line, the potential transformer detects that the voltage of the faulty phase decreases, the voltage of the non-faulty phase increases, and transmits the signal to the control unit. The control unit controls the driving electromagnetic coil of the non-faulty phase AC high-voltage vacuum contactor to act through the relay, and starts to drive the moving contact to approach the static contact. When approaching to conduct the circuit, the alternating voltage across the high-voltage switch, that is, the voltage closing phase angle, can be controlled between 0 and 90 degrees, preferably 75 to 90 degrees. Since the magnitude of the voltage closing phase angle determines the magnitude of the current at conduction, when the voltage closing phase angle is between 75 and 90 degrees, 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 voltage closing phase angle being 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 ground fault occurs and is detected by the control unit, the control unit first obtains the moment when the alternating voltage of the fitting closing line where the high-voltage switch is located will cross zero in the future (which can be obtained through the zero-crossing and voltage period detection circuit, which is an existing technology), and can also determine the time △t required for the AC high-voltage vacuum contactor to drive the moving contact to move until the circuit is conducted through pre-tests. a , so that the control unit issues a command at a moment △t a before the voltage crosses zero to make the electromagnetic coil of the AC high-voltage vacuum contactor start to act, and it can be realized that when the moving contact and the static contact approach each other to conduct the circuit, the alternating voltage across the high-voltage switch just crosses zero, that is, the voltage closing phase angle is zero. This moment △t a before the zero-crossing is the preset moment.

[0036] The methods for controlling the voltage closing phase angle to be 75 degrees, 90 degrees, etc. are similar to this.

[0037] 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 line determines the duration of the short-circuit current. It can be set in the control unit that the electromagnetic coil of the moving contact driving mechanism loses power after a time △t b from when the closing command is issued, and then the spring immediately acts alone. This △t b time can be determined through off-line tests to ensure that the short-circuit current does not exceed one cycle, such as controlling an alternating current of half a cycle or less than half a cycle.

[0038] Figure 3The figure shows the oscillogram curves of multiple electrical parameters recorded by an oscillograph when a high-voltage switch conducts and cuts off a line under the conditions of small current and 10 kV voltage. 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 high-voltage switch, and curve c is the zero-crossing detection signal of the high-voltage switch voltage. The vertex of each wave peak on curve c is the voltage zero-crossing point. It can be seen that the voltage changes periodically before the high-voltage switch closes. When the high-voltage switch closes and the line just starts to conduct, the voltage at both ends of the high-voltage switch 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 high-voltage switch remains zero for some time thereafter, indicating that this process is a process in which the high-voltage switch starts to conduct continuously with current in the line. Curve e is the current curve passing through the high-voltage switch. The time corresponding to the abscissa of curve e at 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 high-voltage switch starts to change in waveform again (curve a). The part of curve e between the dashed line L and the dashed line M is an alternating current that does not exceed one cycle, specifically a half-cycle alternating current plus the decaying current after the zero-crossing point of this half-cycle alternating current. The reason for the generation of this decaying current may be that the moving contact does not move away from the static contact to a distance that can cut off the circuit within the second half-cycle after conduction. As the voltage reverses, a reverse current is generated, but it quickly decays to zero, making the entire current not exceed one cycle. If the spring restoring force increases so that the moving contact just leaves the static contact to cut off the circuit within the half-cycle time of conduction, the current can be exactly half a cycle. The half-cycle alternating current, or the half-cycle alternating current plus the decaying current, are all alternating currents that do not exceed one cycle. The current value is large and the characteristics are obvious, making it easy to detect. Figure 3 Curve b in it is the voltage curve of the electromagnetic coil of the moving contact drive mechanism of the high-voltage switch, and curve d is the voltage curve of the DC solid-state relay that controls the electromagnetic coil of the moving contact drive mechanism. Figure 5 The relevant circuit of this test is shown. Among them, the 5 channels of the acquisition card of the oscillograph 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 drive 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 high-voltage switch voltage, corresponding to curve a; the electromagnetic coil 87 of the moving contact drive 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.

[0039] 2. The high-voltage switch is composed of two switches cooperating with each other

[0040] The high-voltage switch is composed of a first high-voltage switch and a second high-voltage switch connected in series. The first high-voltage switch is in an open state, and the second high-voltage switch is in a closed state. The duration of the short-circuit current is controlled not to exceed one cycle by the time difference between closing the first high-voltage switch and opening the second high-voltage switch.

[0041] In order to control the current duration, the utility model patent ZL201921412022.3 requests to protect a switch structure for controlling the short-time on-off of a single-phase line. The content of this patent is incorporated by reference in its entirety to solve the control of the short-circuit current duration in this patent, so that the duration of the short-circuit current does not exceed the current duration of the second-stage overcurrent protection, preferably does not exceed one cycle, more preferably half a cycle, and even shorter than half a cycle. When the voltage closing phase angle is exactly 90 degrees and the short-circuit current duration is one-quarter cycle, a short-circuit pulse current with all current values greater than or equal to zero or all less than or equal to zero and a large absolute value can be obtained, which further increases the possibility of detection.

[0042] In addition, if a high-voltage power electronic switch is used, since it has no mechanical movement of an AC high-voltage vacuum contactor, the response is very timely. At this time, the high-voltage power electronic switch can be directly turned on the line at the voltage zero-crossing point and the line can be cut off after one cycle (20 ms) or half a cycle (10 ms). That is, the best control method for the electronic switch is to conduct at the voltage zero-crossing and cut off at the current zero-crossing.

[0043] The above embodiments are only several descriptions of the concept and implementation of the present invention, and do not limit it. Under the concept of the present invention, the technical solutions without substantial transformation are still within the protection scope.

Claims

1. An asymmetric current source in a small current grounding system, comprising a high-voltage switch with two-phase independence or three-phase independence. Select one outgoing line in the small current grounding system as the first outgoing line. One side of the high-voltage switch intersects with any two phases or three phases in the first outgoing line at a connection point, and the other side is connected to the ground. It is characterized in that, When a single-phase grounding fault occurs in one phase of a certain outgoing line in the small current grounding system and the high-voltage switch of a non-faulty phase of the first outgoing line is closed and then opened to generate a short-circuit current, the impedance of the single-phase wire from the connection point position to the power supply outlet of the small current grounding system or the sum of the impedance of the single-phase wire from the connection point position to the power supply outlet of the small current grounding system and the current-limiting impedance between the connection point position and the ground can make the magnitude of the short-circuit current not trigger the first-stage overcurrent protection of the first outgoing line and make the short-circuit current maximum; the time when the high-voltage switch is closed and then opened can make the duration of the short-circuit current less than the current duration of the second-stage overcurrent protection of the first outgoing line.

2. The asymmetric current source according to claim 1, wherein Define the presettable impedance as the impedance of the single-phase wire from the connection point position to the power supply outlet of the small current grounding system or the sum of the impedance of the single-phase wire from the connection point position to the power supply outlet of the small current grounding system and the impedance value of the series-connected current-limiting impedance, then the connection point position is determined as follows: (a) Assume that a metallic two-phase direct short circuit occurs at the power supply outlet of the small current grounding system for the first outgoing line, and define the short-circuit current at this time as the limit short-circuit current; (b) Insert a hypothetical impedance into the short-circuit loop in step (a) to reduce the magnitude of the short-circuit current from the limit short-circuit current to less than or equal to 0.9 times the current setting value of the first-stage overcurrent protection of the first outgoing line. Then the impedance value of the hypothetical impedance is the impedance value of the presettable impedance. Then, calculate the length of the wire to be inserted according to the impedance value of the presettable impedance and the impedance of the wire per unit length, and thus determine the connection point position, or determine the connection point position according to the difference between the impedance value of the presettable impedance minus the impedance value of the current-limiting impedance and the impedance value of the wire per unit length.

3. The asymmetric current source according to claim 1 or 2, characterized in that, Among the outgoing lines of the small current grounding system, select the outgoing line with the smallest first-stage overcurrent protection current setting value as the first outgoing line.

4. The asymmetric current source according to claim 1, wherein The time when the high-voltage switch is closed and then opened can make the duration of the short-circuit current not exceed one cycle.

5. The asymmetric current source according to claim 4, wherein The duration of the short-circuit current is shorter than half a cycle, or equal to half a cycle, or longer than half a cycle and shorter than one cycle.

6. The asymmetric current source according to claim 5, wherein The high-voltage switch includes a vacuum interrupter. Inside the vacuum interrupter, there are static contacts and moving contacts. Two-phase or three-phase static contacts are connected to two-phase or three-phase of the small current grounding system, and the moving contact is connected to the ground. 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.

7. The asymmetric current source according to claim 6, wherein The power source of the moving contact drive mechanism is the first electromagnetic coil or the first spring, and the power source of the moving contact reset mechanism is the second spring or the second electromagnetic coil.

8. The asymmetric current source according to claim 6, wherein A voltage transformer is set to monitor the voltage of the first outgoing line. The voltage transformer is connected to the control unit, and the control unit controls the actions of the moving contact drive mechanism and the moving contact reset mechanism.

9. The asymmetric current source according to claim 4, wherein Each high-voltage switch is composed of a first high-voltage switch and a second high-voltage switch connected in series. The first high-voltage switch is in the open state, and the second high-voltage switch is in the closed state.

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