Accurate arc extinguishing method for single-phase ground fault of small current grounding system

Abstract

The application discloses a small-current grounding system single-phase grounding fault accurate arc extinguishing method, which comprises the following steps: step 1, when a grounding fault occurs, calculating a residual current Ig of a fault point; step 2, injecting a current or a voltage into the small-current grounding system and calculating the residual current Ig of the fault point in real time; step 3, adjusting the injection value of the current or the voltage according to the change of the residual current Ig of the fault point, if the Ig becomes smaller, continuously adjusting the injection value in the original direction, if the Ig becomes larger, adjusting the injection value in the opposite direction, setting a threshold value, if the Ig is larger than the threshold value, continuously adjusting the injection value, if the Ig is smaller than the threshold value, ending the adjustment and maintaining the current injection value. The arc extinguishing method can solve the problem that the single-phase grounding at the end of a heavy-load long line cannot be accurately compensated.

Description

Technical Field

[0001] This invention belongs to the field of relay protection of power systems, and specifically relates to a precise arc suppression method for single-phase grounding faults in low-current grounding systems. Background Technology

[0002] In 3kV-66kV low-current grounding systems (also known as neutral-point non-effectively grounded systems), single-phase grounding is a common type of fault. When a single-phase grounding occurs in a low-current grounding system, the voltage to ground of the faulty phase decreases, while the voltage to ground of the non-faulty phases increases, and the line voltages remain symmetrical. Under these conditions, because the grounding current is very small, operation can be allowed for 1-2 hours to ensure power supply reliability. However, due to arcing overvoltage in the non-faulty phases, problems such as insulation breakdown of weak parts, voltage transformer core saturation, system overvoltage, and arcing of the faulty phase burning cables and potentially causing electric shock accidents are easily caused. Therefore, after a single-phase grounding, it is necessary to isolate the faulty phase in a timely manner to ensure the safe and stable operation of the system and the reliability of power supply.

[0003] Load current and line impedance cause a ΔU difference between the busbar and the fault point. The flexible arc suppression system cannot know the value of ΔU without knowing the distance to the fault point and the load current. Therefore, it can only reduce the voltage of the fault phase at the busbar to near 0. For single-phase grounding at the end of a heavy-load long line, the voltage difference ΔU between the end of the line and the busbar may reach 600V. When a grounding fault occurs at the end through a 100Ω transition resistor, if the voltage at the busbar is only compensated to 0, the residual current at the fault point will reach 6A. Summary of the Invention

[0004] The purpose of this invention is to provide a precise arc suppression method for single-phase grounding faults in low-current grounding systems, which can solve the problem of inaccurate compensation for single-phase grounding faults at the end of heavy-load long lines.

[0005] To achieve the above objectives, the solution of the present invention is:

[0006] A precise arc suppression method for single-phase ground faults in a low-current grounding system includes the following steps:

[0007] Step 1: When a ground fault occurs, calculate the residual current Ig at the fault point;

[0008] Step 2: Inject current or voltage into the low-current grounding system and calculate the residual current Ig at the fault point in real time;

[0009] Step 3: Adjust the injection value of current or voltage according to the change of residual current Ig at the fault point to achieve precise arc extinguishing technology.

[0010] In step 1 above, the method for calculating the residual current Ig at the fault point is as follows: first, the zero-sequence voltage U0 of the bus and the zero-sequence current I0 of each branch when there is no fault are calculated to obtain the capacitance to ground of each branch; then, the zero-sequence capacitive reactance X at the power frequency of 50Hz is calculated. C When a ground fault occurs, according to the zero-sequence capacitive reactance X C The zero-sequence voltage of the bus and the zero-sequence current of each branch when there is no fault are calculated to obtain the zero-sequence capacitance current Ic of each branch; then the residual current Ig at the fault point is obtained by subtracting the capacitance current Ic from the zero-sequence current I0.

[0011] The zero-sequence voltage U0 of the bus and the zero-sequence current I0 of each branch when there is no fault are obtained in the following way: a non-power frequency voltage is injected into the neutral point of the low current grounding system, a zero-sequence voltage will be generated on the bus, and a zero-sequence current will appear in each branch due to the zero-sequence voltage. The zero-sequence voltage of the bus and the zero-sequence current of each branch are collected.

[0012] The formulas for calculating the capacitance to ground of each of the above branches are: Among them, f zr The frequency of the current or voltage injected into the flexible source.

[0013] In step 3 above, the specific method for adjusting the injected current or voltage value based on the change of residual current Ig at the fault point is as follows: if Ig decreases, continue to adjust the injected value in the original direction; if Ig increases, adjust the injected value in the opposite direction.

[0014] In step 3 above, a threshold value is set. If the value is greater than the threshold value, the injection value is adjusted further; if the value is less than the threshold value, the adjustment ends and the current injection value is maintained.

[0015] By adopting the above scheme, the present invention can control the residual current and the voltage to ground at the fault point to be close to 0, realize precise arc suppression technology, prevent accidents such as arc overvoltage, cable burnout and personal injury during continued operation after grounding, and solve the problem that when the fault line has a large load and a small grounding resistance, the activation of the fault transfer device may increase the fault current and have an adverse impact on the power grid. Attached Figure Description

[0016] Figure 1 This is a wiring diagram of the flexible arc suppression system used in this invention;

[0017] Figure 2 This is a schematic diagram for calculating the grounding point current Ig during a single-phase grounding event;

[0018] Figure 3 This is a schematic diagram of the present invention for gradually approximating and compensating the voltage and current at the fault point when phase A is grounded. Detailed Implementation

[0019] The technical solution and beneficial effects of the present invention will be described in detail below with reference to the accompanying drawings.

[0020] This invention provides a precise arc suppression method for single-phase grounding faults in low-current grounding systems, solving the problem of inaccurate compensation of the grounding point current during single-phase grounding faults in current low-current grounding systems. It is specifically designed for 3kV-66kV low-current grounding systems. Figure 1 As shown, the flexible arc suppression system includes a line selection arc suppression device, a flexible power supply, and a step-up transformer. The line selection arc suppression device is connected to the flexible power supply, the bus voltage transformer, and the zero-sequence current transformers of each branch, and is used to collect the bus voltage and the zero-sequence current of each branch and send commands to the flexible power supply. The line selection arc suppression device determines the existence of a ground fault based on the collected bus voltage and zero-sequence current and selects the faulty line. The high-voltage side of the step-up transformer is connected between the system neutral point and ground, and the low-voltage side is connected to the flexible power supply. The flexible power supply receives the commands issued by the line selection arc suppression device and sends a specified voltage or current vector to the low-voltage side of the step-up transformer.

[0021] The system operates as follows: A single-phase ground fault is determined by whether the zero-sequence voltage at the power frequency exceeds the "start-up line selection setpoint." If no single-phase ground fault occurs, the zero-sequence voltage at the power frequency is less than the start-up line selection setpoint. The line selection and arc suppression device controls the flexible power supply to send a specified voltage vector to the low-voltage side of the step-up transformer, thereby injecting a voltage or current into the neutral point (a non-power frequency voltage can be injected to reduce interference to the system). A zero-sequence voltage is generated on the busbar, and each branch experiences a zero-sequence current due to this voltage. The capacitance to ground of each branch is calculated based on the collected busbar zero-sequence voltage and the zero-sequence current of each branch. Then, the zero-sequence capacitive reactance at 50Hz power frequency is calculated. When a single-phase ground fault occurs, the ground selection device calculates the zero-sequence capacitive current Ic of each branch based on the capacitive reactance. The zero-sequence current I0 minus the capacitive current equals the fault point's current to ground Ig. A current or voltage is then injected into the system through the flexible power supply. After injection, Ig is recalculated based on the changed values ​​of U0 and I0. If Ig decreases, the injection direction is correct, and the injection value is adjusted in the original direction. If Ig increases, the injection direction is incorrect, and the injection value is adjusted in the opposite direction. When Ig is greater than the threshold, the injection is adjusted repeatedly; when Ig is less than the threshold, the adjustment ends, and the existing injection amount is maintained.

[0022] In this embodiment, as Figure 2 As shown, the voltage transformer has a primary voltage of 10kV, a secondary rated voltage of 100V, an open delta secondary rated voltage of 57.74V, and a branch zero-sequence transformer ratio of 50 / 1.

[0023] When no single-phase ground fault occurs, the power frequency zero-sequence voltage is less than the starting line selection setting. At this time, by injecting a non-power frequency voltage into the neutral point, a zero-sequence voltage will be generated on the bus. Due to the zero-sequence voltage, each branch will have a zero-sequence current. The capacitance to ground of each branch can be calculated based on the collected bus zero-sequence voltage and the zero-sequence current of each branch. Then, the zero-sequence capacitive reactance at a power frequency of 50Hz was calculated. f zr The frequency of the current or voltage injected into the flexible source is generally non-power frequency in order not to affect the normal operation of the system.

[0024] Assuming the residual current of the system is 2A after compensation by the arc suppression coil, the single-phase ground fault line uses YJV22-120 cable with a resistance of 0.196Ω / km and a reactance of 0.102Ω / km. The line length is 10km, and the load current is 200A. The voltage difference ΔU from the busbar to the end of the line caused by the load current is 441.9V. At the end of the line, phase A is grounded through a 100Ω transition resistor. The current and voltage to ground at the fault point of phase A are 1.9988A and 199.88V, respectively. If the voltage at the busbar of phase A is directly compensated to 0, the voltage to ground at the fault point will be 441.9V, and the current will be 4.419A, which is higher than when no compensation is applied.

[0025] The high-voltage side power supply to ground of the step-up transformer is U0. After the flexible power supply is injected, the magnitude and direction of U0 are changed. Generally, an adjustment trajectory in the Ig direction is selected to adjust U0. The adjustment amplitude ΔU0 is generally taken as 100V per step. Figure 3 As shown, after the first adjustment, the Ig current decreases. The second adjustment continues along the trajectory until the direction of Ig changes to the opposite direction. At this point, the amplitude ΔU0 of the adjustment in the opposite direction along the trajectory drops to 50V. If the direction of Ig does not change, the adjustment continues. If the direction of Ig changes, the adjustment is carried out in the opposite direction with the amplitude ΔU0 halved to 25V until Ig is less than 0.1A. At this point, Ua is also close to 0, and the adjustment is stopped.

[0026] The above embodiments are merely illustrative of the technical concept of the present invention and should not be construed as limiting the scope of protection of the present invention. Any modifications made to the technical solutions based on the technical concept proposed in this invention shall fall within the scope of protection of this invention.

Claims

1. A precise arc suppression method for single-phase grounding faults in a low-current grounding system, characterized in that... Includes the following steps: Step 1: When a ground fault occurs, calculate the residual current Ig at the fault point; The method for calculating the residual current Ig at the fault point is as follows: based on the zero-sequence voltage of the bus when there is no fault. and zero-sequence current of each branch The capacitance to ground of each branch was calculated, and then the zero-sequence capacitive reactance X at 50Hz power frequency was calculated. C When a ground fault occurs, according to the zero-sequence capacitive reactance X C The zero-sequence capacitance current Ic of each branch is calculated from the bus zero-sequence voltage and the zero-sequence current of each branch when there is no fault; then the zero-sequence current is used... Subtracting the capacitor current Ic yields the residual current Ig at the fault point. Step 2: Inject current or voltage into the low-current grounding system and calculate the residual current Ig at the fault point in real time; Step 3: Adjust the injection value of current or voltage according to the change of residual current Ig at the fault point; the specific adjustment method is as follows: if Ig decreases, continue to adjust the injection value in the original direction; if Ig increases, adjust the injection value in the opposite direction; set a threshold value, and if Ig is greater than the threshold value, continue to adjust the injection value. If Ig is less than the threshold value, the adjustment ends and the current injection value is maintained.

2. The precise arc suppression method for single-phase grounding faults in low-current grounding systems as described in claim 1, characterized in that: The bus zero-sequence voltage when there is no fault and zero-sequence current of each branch The following method is used to obtain the zero-sequence voltage: inject a non-power frequency voltage into the neutral point of the low-current grounding system. A zero-sequence voltage will be generated on the bus, and a zero-sequence current will appear in each branch due to the zero-sequence voltage. Collect the zero-sequence voltage of the bus and the zero-sequence current of each branch.

3. The precise arc suppression method for single-phase grounding faults in low-current grounding systems as described in claim 1, characterized in that: The formula for calculating the capacitance to ground of each branch is: ,in, The frequency of the current or voltage injected into the flexible source.

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