A method for handling a phase-to-phase short circuit in a three-phase power system

Abstract

The application discloses a kind of processing methods of three-phase power system phase short circuit, there are several switches on three-phase power system, switch can detect current signal and voltage signal, when phase short circuit fault occurs and thereby generates short circuit current, from short circuit current generation time, when certain the switch detects short circuit current continues to preset tripping duration, it is tripped, the switch that does not detect the short circuit current detects voltage signal, certain the switch does not detect the voltage signal again after the planned opening time of the first switch upstream of the switch, it is tripped.The method can cut off the switch closest to the phase short circuit fault point on both sides to remove short circuit fault and make the system self-healing, and it is convenient to operate, and practical.

Description

Technical Field

[0001] This invention relates to the field of power system protection, and specifically to a method for handling phase-to-phase short circuits in a three-phase power system. Background Technology

[0002] Regarding the handling methods for phase-to-phase short circuits in three-phase power systems, common practices include: 1. Using reclosing: First, disconnect the first circuit breaker on the line, then close it. If it is a transient phase-to-phase short circuit and is cleared after closing the first circuit breaker, normal power supply continues. If the phase-to-phase short circuit fault persists after closing the first circuit breaker, disconnect the first circuit breaker and wait for maintenance. 2. Trip the first circuit breaker first for overcurrent, then trip the last load switch without current (other load switches are closed). Then reclose the first circuit breaker. If the fault occurs below the last load switch, the fault can be eliminated. Otherwise, if there is still fault current after reclosing the first circuit breaker, trip the first circuit breaker again for overcurrent, then trip the second-to-last load switch without current, and then reclose the first circuit breaker again. If the phase-to-phase short circuit occurs between the second-to-last load switch and the last load switch, the fault can be eliminated. This process continues sequentially, disconnecting load switches upwards under no-current conditions until the fault is cleared. During this operation, the power supply system is repeatedly subjected to large short-circuit current surges. Excessive exposure to these surges can damage the lines, and the fault clearing time is also lengthy. 3. Circuit breakers with fault current tripping capabilities are installed on the lines. When a fault occurs, all circuit breakers are set to overcurrent tripping. The process begins with closing the first circuit breaker. If there is overcurrent, it trips, and the fault is cleared. If the first circuit breaker closes successfully without overcurrent, it is locked for overcurrent tripping for a period. During this time, the second circuit breaker closes. Since the first circuit breaker is already locked for overcurrent, the second circuit breaker trips due to overcurrent, and the fault is cleared. If there is no overcurrent, the next circuit breaker is closed, and so on. This scheme has complex logic and is relatively cumbersome to operate. 4. Using a time-differentiated tripping method: Circuit breakers on the same line are set with different overcurrent tripping times based on their distance from the power source. The closer to the power source, the longer the tripping time. Generally, the setting difference is 100ms. This time is determined by the mechanical action time of the switch and the algorithm's processing time. This method can isolate the fault area. However, for faults close to the power source, the power supply system has a longer short-circuit current tolerance time, resulting in a greater impact on the power grid. Furthermore, although the switch between the fault point and the power source is disconnected, the area after the fault point cannot be powered by the backup power source because the fault point has not yet been cleared; that is, the power grid cannot self-heal. Self-healing refers to disconnecting the switches on both sides of the fault point, thereby utilizing two power sources to supply power to the area outside the area between the disconnected switches on both sides of the fault point, thus controlling the power outage to a smaller area. Summary of the Invention

[0003] The purpose of this invention is to provide a method for handling phase-to-phase short circuits in a three-phase power system. This method can disconnect the nearest switches on both sides of the phase-to-phase short circuit fault point to remove the short circuit fault and enable the system to self-heal. It is also convenient to operate and highly practical.

[0004] To achieve the above objectives, the present invention adopts the following technical solution: A method for handling phase-to-phase short circuits in a three-phase power system, wherein the three-phase power system is equipped with several switches capable of detecting current and voltage signals, and when a phase-to-phase short circuit fault occurs, resulting in a short-circuit current, the following steps are taken: S1: From the moment the short-circuit current is generated, a certain switch will trip when it detects that the short-circuit current continues for a preset tripping time. The preset tripping time of the switch closer to the power supply side is set to be longer than the preset tripping time of the switch farther away from the power supply side. S2: The switch detects the voltage signal when the short-circuit current is not detected. A switch trips when the voltage signal is not detected again after the planned tripping time of the first switch upstream of the switch. The upstream is the direction towards the power supply side counting switch. The planned tripping time is the time when the first switch upstream of the switch should trip according to the preset tripping time, starting from the time the short-circuit current is generated.

[0005] Preferably, the voltage signal is the phase voltage signal between one phase of the three-phase line and the ground; Alternatively, the voltage signal may be the line voltage signal between two phases of the three-phase line. Alternatively, the voltage signal may be the 3U0 voltage signal of the three-phase line.

[0006] In the above scheme, when a phase-to-phase short-circuit fault occurs, a fault current flows through the faulty phase conductor between the fault point and the power source. The corresponding switch on this line can detect the fault current. The closer the switch is to the fault point, the shorter the tripping time. Thus, the switch closest to the fault point carrying the fault current will inevitably trip and disconnect the current before other switches capable of detecting the fault current. Simultaneously with the generation of the fault current, the phase voltage relative to ground on a certain phase of the three-phase line, or the line voltage or 3U0 voltage between two lines, will also change, generating a unique voltage signal. This voltage signal will not disappear until the aforementioned switch trips and disconnects the fault current. Furthermore, based on the principle of equal conductor potential, this voltage signal will be distributed throughout the entire conductor until the fault current is interrupted. In this invention, timing begins from the moment a phase-to-phase short-circuit fault generates a fault current. Based on the current duration condition for triggering tripping, the pre-planned tripping time of each switch (which is a moment in the time sequence from the moment the phase-to-phase short-circuit fault generates a fault current signal) can be determined. Adding the mechanical action time of the switch from initial tripping to complete current interruption, the pre-planned tripping time of each switch can be obtained. When a switch trips according to the current condition, the first downstream switch (located on the side away from the power source from the fault point, so neither this switch nor any further downstream switches detect the fault current) can detect the aforementioned voltage signal from that moment onward. Tripping based on this signal precisely achieves the expected tripping of the nearest switch on the back side of the fault point. Furthermore, since each switch has a hypothetical tripping time planned in the time series (these tripping times are arranged sequentially in the time series), if the voltage condition for triggering a trip of a certain switch is set to the hypothetical tripping time of the first upstream switch, no voltage signal can be detected after that. Only that switch will meet the condition, while other switches downstream of that switch will not trip because voltage signals can be detected after the planned tripping time of their respective upstream first switches. Attached Figure Description

[0007] Figure 1 This is a schematic diagram of the relevant structure of the three-phase power system in the method of the present invention (the dashed lines omit the lines, switches and power supply to the right of the dividing switch 14). Figure 2 This is a schematic diagram of a phase-to-phase short-circuit simulation circuit; Figure 3 The simulated line voltage variation curves between phases (the three lines represent the line voltages between phases AB, BC, and CA, respectively). Figure 4 The simulated phase-to-phase short-circuit phase voltage variation curves (the three lines represent the phase voltages of A, B, and C to ground, respectively). Figure 5The voltage change curve of 3U0 is shown when a phase-to-phase short-circuit fault is simulated and the fault point is grounded. Detailed Implementation

[0008] The present invention will be further described below with reference to the accompanying drawings and specific embodiments: like Figure 1 As shown, the three-phase power system includes a power source and a three-phase line 20. Several switches, such as switches 10, 11, 12, and 13, are installed on the three-phase line 20. A dividing switch 14 is used to disconnect the connection between two power sources. When the dividing switch 14 is closed, the other power source can be used as a backup power source.

[0009] Assuming a phase-to-phase short-circuit fault occurs between phases B and C at point F, it should be handled as follows: When a phase-to-phase short circuit fault occurs, a short circuit current is generated. Because of the existence of this short circuit current signal, the phase voltage signal between a phase of the three-phase line and the ground, or the line voltage signal between two phases of the three-phase line, or the 3U0 voltage signal of the three-phase line changes.

[0010] The aforementioned switch has the ability to detect both current duration and voltage (simply by installing current and voltage sensors on the switch, a common technique in the field), and can be set to trip based on either current or voltage signals. The preset trip duration for switches closer to the power supply side is longer than that for switches farther from the power supply side. For example, the preset trip duration (current duration) for switches 10, 11, 12, and 13 is set to 0 (i.e., tripping upon detection of short-circuit current), 100, 200, and 300 milliseconds, respectively. When the current lasts for 200 milliseconds, switch 12 trips. Previously, switches 10 and 11 did not trip because no short-circuit current flowed. After switch 12 trips, switch 13 will not trip because there is no more current. This ensures that the switch closest to the fault point F (i.e., switch 12) trips. Those skilled in the art will understand that when tripping based on current duration, the difference in current duration triggered by the switches must be greater than the switch opening time.

[0011] When a short-circuit current is present, sectionalizing switches 10, 11, 12, and 13 will all detect a voltage signal. However, in terms of logic settings, only switches that do not detect a short-circuit current will trip based on the voltage condition. Switches with short-circuit current flowing through them will not trip based on the voltage condition.

[0012] In one embodiment, the voltage condition for switch tripping is set as follows: When a phase-to-phase short-circuit fault occurs, a fault current is generated (at this time, the line voltage between the two faulty phases will suddenly drop, so this can also be used as the starting point for the sensor to determine the occurrence of the short-circuit fault). The time when the fault current is generated is 0, and the timing starts. Assuming the current duration difference is 100 milliseconds, the time when switch 10 triggers tripping when the fault current signal flows is 0 milliseconds (that is, it trips simultaneously as soon as the fault current is detected). Assuming the mechanical action time for the switch to trip is 50 milliseconds (that is, it takes 50 milliseconds for the switch to trip completely and cut off the current from the start of tripping), the time when switch 10 trips is the 50th millisecond. Similarly, the time when switch 11 trips is the 150th millisecond, the time when switch 12 trips is the 250th millisecond, and the time when switch 13 trips is the 350th millisecond (these time points are planned tripping times). The tripping time, which is the time assumed to trip according to the plan (in reality, which switch trips depends on the random location of the fault point), etc. Assuming the fault point F is located between switches 12 and 11, based on the current condition, switch 12 trips at 250 milliseconds. After 250 milliseconds, switch 11 will no longer detect a voltage signal and can therefore trip. The voltage condition for switch 10 to trip is that no voltage signal is detected after 150 milliseconds (because its upstream first switch is switch 11, which trips at 150 milliseconds). However, given the current location of fault point F, switch 11 will not trip due to the current condition, so there will still be a voltage signal after 150 milliseconds. Therefore, switch 10 can detect the voltage signal and will not trip. The same logic applies to other switches. This condition ensures that only the current-controlled switch closest to the fault point F trips, while other current-controlled switches do not trip, and it is independent of the specific random location of the fault point F. Therefore, the voltage tripping condition can be summarized as follows: a switch detects a voltage signal without detecting a short-circuit current, and a switch trips after the planned tripping time of the first switch upstream of that switch. In this invention, "upstream" refers to the direction in which switches are counted sequentially towards the power source, and "downstream" refers to the opposite direction.

[0013] A phase-to-phase short circuit simulation was performed on a 10kV power supply system using simulation software. The simulation included line voltage, phase voltage during a simple phase-to-phase short circuit, and the 3U0 voltage during a phase-to-phase short circuit with associated grounding. It was observed that after the upstream switch trips, the voltage signals detected by the downstream switch are significantly different. Therefore, these voltage signals can be used as criteria for downstream switch tripping. The simulation line setup interface is shown below. Figure 2 As shown.

[0014] Simulation Example 1: 0.1s, phase-to-phase short circuit between B and C; 0.25s, switch 1 opens; 0.3s, switch 2 opens. The line voltage is as follows: Figure 3 As shown, by Figure 3 It is evident that the line voltage changes significantly before and after switch 1 is disconnected, which can be used as a criterion for switch 2 to trip.

[0015] Simulation Example 2: 0.1s, phase-to-phase short circuit between B and C; 0.25s, switch 1 opens; 0.3s, switch 2 opens. The phase voltages are as follows: Figure 4 As shown, by Figure 4 It is evident that the phase voltage changes significantly before and after switch 1 is disconnected, which can be used as a criterion for switch 2 to trip.

[0016] Simulation Example 3: 0.1s, phase-to-phase short circuit with grounding; 0.25s, switch 1 opens; 0.3s, switch 2 opens; voltage 3U0 is as follows... Figure 5 As shown, by Figure 5 It is evident that the 3U0 voltage changes significantly before and after switch 1 is disconnected, which can be used as a criterion for switch 2 to trip.

[0017] The above embodiments are merely illustrative of the concept and implementation of the present invention and are not intended to limit it. Under the concept of the present invention, technical solutions without substantial changes are still within the scope of protection.

Claims

1. A method for handling phase-to-phase short circuits in a three-phase power system, wherein the three-phase power system is equipped with a plurality of switches, the switches being capable of detecting current signals and voltage signals, characterized in that: When a phase-to-phase short-circuit fault occurs, resulting in a short-circuit current, the following steps should be taken: S1: From the moment the short-circuit current is generated, a certain switch will trip when it detects that the short-circuit current continues for a preset tripping time. The preset tripping time of the switch closer to the power supply side is set to be longer than the preset tripping time of the switch farther away from the power supply side. S2: The switch detection voltage signal is not detected when the short-circuit current is not detected. A certain switch trips after the planned tripping time of the first upstream switch of the switch. The upstream is the direction towards the power supply side counting switch. The planned tripping time is the time when the first upstream switch of the switch should trip according to the preset tripping time, starting from the time when the short-circuit current is generated. The voltage signal is the phase voltage signal between one phase of the three-phase line and the ground; Alternatively, the voltage signal may be the line voltage signal between two phases of the three-phase line. Alternatively, the voltage signal may be the 3U0 voltage signal of the three-phase line.

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