Single-phase grounding judgment protection method and device based on three-phase current change of line

Abstract

The present invention discloses a single-phase grounding judgment and protection method and device based on the change of three-phase current of a line. The method is as follows: when it is judged that the change of the three-phase current of the line exceeds a set value, the increment of the three-phase current is obtained by vector operation, and the difference obtained by subtracting the intermediate value of the increment from the maximum value of the increment is then divided by the maximum value of the increment to obtain the unbalance degree. If the unbalance degree is greater than the set threshold, it is judged that an in-bound single-phase grounding fault has occurred at the rear end of the three-phase current measurement end of the line, and the grounding phase is the phase with the largest current increment. The device includes: a microcomputer measurement and control device and three-phase current transformers. The microcomputer measurement and control device includes a current sampling module, a single-chip microcomputer, and a relay output control module. The present invention is applicable to systems with ungrounded neutral points, neutral points with arc suppression coils, and neutral points grounded through small resistors, and can accurately judge various types of grounding faults, especially high-resistance grounding faults, improving the reliability and safety of power grid operation.

Description

Technical Field

[0001] The present invention belongs to the technical field of relay protection in power systems, and relates to a single-phase grounding judgment protection method and device based on the changes of three-phase line currents. Specifically, it is a judgment protection method and device that selects the fault line section and fault phase within the single-phase grounding fault based on the changes of three-phase line currents, and has the technical advantage of accurately discriminating various grounding faults, especially high-resistance grounding faults. It can be used for feeder branch nodes or terminals to timely select the line, determine the section, trip and isolate the fault, ensure the normal operation of the main line, and can also be used for segment determination and alarm of bus grounding faults. Background Art

[0002] In a power system, single-phase grounding faults account for more than 80% of the faults in distribution lines. Most of the neutral points of the distribution network systems in China adopt the small-current grounding method of non-grounding or grounding through an arc suppression coil. When a single-phase grounding fault occurs, since the fault current is small and the three-phase line voltages still remain symmetrical, it is allowed to operate with the fault for a period of time. However, with the expansion of the power grid and the increase of cable lines, the capacitive current of the system is getting larger and larger. Operating in the single-phase grounding fault state for a long time, overvoltage will cause different-phase grounding or phase-to-phase short circuit at other weak insulation points, resulting in the expansion of the fault into a major accident. Therefore, the technical guidelines for the distribution network promulgated by the state in recent years clearly state the principle for dealing with single-phase grounding: after avoiding instantaneous grounding faults, the principle of quickly isolating the fault nearby, that is, changing from "operating for 2 hours + grounding line selection" to "section selection and tripping". Accurately selecting the phase, selecting the line and determining the section for single-phase grounding faults are the necessary prerequisites for realizing the principle of "quickly isolating the fault nearby", which is related to the power supply reliability and safety, as well as the realization of power grid intelligence.

[0003] At present, the phase selection of single-phase grounding faults is mainly based on the change of phase voltage. The line selection mainly collects the zero-sequence current of the feeder, and uses methods such as zero-sequence active power, zero-sequence reactive power, and transient change of zero-sequence current. Using phase voltage to judge the grounding fault phase has certain deficiencies: Generally speaking, when the grounding transition resistance is low, the voltage of the grounding phase decreases, and the voltage of the non-grounding phase increases. However, in the case of high-resistance grounding, there may be a state where the voltage of a non-grounding phase decreases and is lower than the voltage of the grounding phase. If the system is unbalanced, the situation is more complex. Relying on phase voltage to select the fault phase is prone to misselection. For outdoor feeder terminals, if phase voltage is to be collected, additional equipment needs to be added, increasing costs and the difficulty of construction and maintenance. In addition, most of the common zero-sequence line selection methods have certain limitations. For example, the zero-sequence reactive power line selection method is not applicable to distribution network systems with arc suppression coils or small resistances grounded at the neutral point, and the zero-sequence active power line selection method is only applicable to systems with resistances grounded at the neutral point. Most of the existing fixed-section devices such as feeder terminal FTUs and fault indicators on the market cannot accurately judge single-phase grounding faults, and are even more blind to high-resistance grounding faults. Therefore, there is an urgent need for the emergence and popularization of a judgment protection method and device that can accurately identify various grounding faults, especially high-resistance grounding faults. Summary of the Invention

[0004] The object of the present invention is to address the common problem of blind areas in high-resistance fault judgment in current single-phase grounding protection technologies and products, and propose a judgment protection method and device for single-phase grounding phase selection, line selection, and section determination using the changes in the three-phase currents of the line. It has the technical advantage of accurately identifying various grounding faults, especially high-resistance grounding faults, and can be generally applied to systems with ungrounded neutral points, arc suppression coils, and small resistances grounded. It is suitable for both feeder branch lines or terminals without voltage transformers and zero-sequence current transformers, and for phase selection, line selection, section determination, and protection of grounding faults in conventional lines.

[0005] One of the technical solutions adopted by the present invention is: a single-phase grounding judgment protection method based on the changes in the three-phase currents of the line. When it is judged that the changes in the three-phase currents of the line exceed the set value, the increment of the three-phase currents is obtained by vector operation. The difference between the maximum increment value and the intermediate increment value is divided by the maximum increment value to obtain the unbalance degree. If the unbalance degree is greater than the set threshold, it is judged that a single-phase grounding fault has occurred at the rear position of the three-phase current measurement end of the line, and the grounding phase is the phase with the largest current increment.

[0006] Furthermore, this method specifically includes:

[0007] (1) When the system is powered on, the zero-sequence current I0 synthesized from the three-phase current is enabled. If I0 is less than the high-overlimit setting value I0H_set, it is judged to be in the normal operation state, and at the same time, the amplitudes, phases and calculated sampling point position information of the three-phase normal currents IA_normal, IB_normal, and IC_normal are updated;

[0008] (2) If the zero-sequence current I0 is greater than the high-overlimit setting value I0H_set, it is judged to be in an abnormal operation state, and the single-phase grounding fault judgment is entered.

[0009] Furthermore, the single-phase grounding fault judgment steps are as follows: First, align the sampling point positions and correct the phase value of the current, so that the sampling calculation position of the current is N cycles different from the current at the normal operation moment, where N is a positive integer; then perform a vector subtraction operation on the amplitude and phase of the current and the current phase at the normal operation moment recorded, and obtain the modulus after vector subtraction, which is the increment ΔIi of the three-phase current, where i is A, B or C; then subtract the intermediate value of the increment from the maximum value of the increment, divide by the maximum value of the increment, and multiply by 100 to obtain the percentage value of the phase current increment, which is the unbalance degree d of the phase current increment:

[0010] d = [max(ΔIi) - mid(ΔIi)] / max(ΔIi) × 100

[0011] Compare the unbalance degree d with the set threshold Dset. If d < Dset, it is a non-internal fault; if d ≥ Dset, it is an internal grounding fault, and the phase with the largest current vector increment is the grounding phase.

[0012] Furthermore, the high-overlimit setting value I0H_set is set according to the device sampling accuracy and system conditions, and is generally set to be greater than the zero-sequence current value in the normal state; the threshold Dset is greater than 25% of the three-phase load imbalance, and is generally set to be above 30%.

[0013] The second technical solution adopted by the present invention is: A single-phase grounding judgment and protection device based on the change of three-phase current of a line. The judgment and protection device is used to implement the method described in the above solution. The judgment and protection device includes a microcomputer measurement and control device and three-phase current transformers. The microcomputer measurement and control device includes a current sampling module, a single-chip microcomputer and a relay output control module; the three-phase current transformers are used to monitor the three-phase current of the line in real time and send the monitoring signals to the microcomputer measurement and control device.

[0014] Further, the current sampling module converts the analog signal into a digital signal through AD and outputs it to the single-chip microcomputer. The control output end of the single-chip microcomputer is connected to the input end of the relay output control module. The single-chip microcomputer samples, calculates, processes, and judges the currents of each phase. When it is determined that the changes in the three-phase currents of the line conform to the criterion for single-phase grounding faults within the boundary, it drives the relay to output a control node to trip and protect the line with a single-phase grounding fault within the boundary and send out an alarm signal.

[0015] Further, the judgment and protection device further includes a zero-sequence current transformer, and the zero-sequence current transformer is used to monitor the zero-sequence current of the line in real time and send the monitoring signal to the microcomputer measurement and control device.

[0016] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0017] The method and device of the present invention have functions such as single-phase grounding fault phase selection, line selection, section determination, and tripping protection, and can timely and accurately judge the occurrence of various single-phase grounding faults. In particular, it can accurately distinguish high-resistance grounding faults that cannot be judged by conventional grounding line selection devices or section determination devices. It can be applied to both the section protection of feeder branch lines or terminal grounding faults and the section alarm of busbar grounding faults, meeting the principle of quickly isolating faults nearby for single-phase grounding in modern power grids. It can also give early warnings for high-resistance grounding faults, expand the scope of single-phase grounding protection, and improve the reliability and safety of power grid operation.

[0018] The method and device of the present invention are applicable to systems with ungrounded neutral points, arc suppression coil grounded systems, and small resistance grounded systems. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 is a flowchart of the judgment and protection method of the present invention.

[0020] Figure 2 is a schematic circuit connection diagram of the device of the present invention, and NZFTU in the figure is the device of the present invention.

[0021] Figure 3 is a principle block diagram of the microcomputer measurement and control device of the present invention.

[0022] Figure 4 is a principle introduction diagram of the present invention.

[0023] Figure 5 is a true-type test circuit diagram of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0024] The present invention will be described in detail below with reference to the accompanying drawings.

[0025] Embodiment 1

[0026] As Figure 1As shown in the figure, a grounding judgment method based on the asymmetric change of the three-phase current of a line. When it is judged that the change of the three-phase current of the line exceeds the set value, the increment of the three-phase current is obtained by vector operation. The difference between the maximum value of the increment and the intermediate value of the increment is divided by the maximum value of the increment to obtain the unbalance degree. If the unbalance degree is greater than the set threshold, it is judged that a single-phase grounding fault has occurred at the rear end of the measurement end of the three-phase current of the line, and the grounding phase is the phase with the largest current increment.

[0027] The specific steps of this method are as follows:

[0028] (1) After the system is powered on, enable the synthesis of the three-phase current into the zero-sequence current I0. If I0 is less than the high-crossing limit set value I0H_set, it is judged to be in the normal operation state, and at the same time, update the amplitude, phase, and calculated sampling point position information of the three-phase normal currents IA_normal, IB_normal, and IC_normal.

[0029] (2) If the zero-sequence current I0 is greater than the high-crossing limit set value I0H_set, it is judged to be in an abnormal operation state, and enter the single-phase grounding fault judgment: First, align the sampling point positions and correct the phase value of the current, so that the sampling calculation position of the current is different from the current at the normal operation moment by N (N is a positive integer) cycles; then, perform a vector subtraction operation on the amplitude and phase of the current and the phase of the current at the normal operation moment recorded, and the modulus after the vector subtraction is obtained, which is the increment ΔIi (i is A, B, or C) of the three-phase current; then subtract the intermediate value of the increment from the maximum value of the increment, divide by the maximum value of the increment, and multiply by 100 to obtain the percentage value of the phase current increment, which is the unbalance degree d of the phase current increment:

[0030] d = [max(ΔIi) - mid(ΔIi)] / max(ΔIi) × 100

[0031] Compare the unbalance degree d with the set threshold Dset. If d < Dset, it is a non-grounding fault; if d ≥ Dset, it is an in-bound grounding fault, and the phase with the largest current vector increment is the grounding phase.

[0032] Among them, the high-crossing limit set value I0H_set is set according to the device sampling accuracy and system conditions, and is generally set to be greater than the zero-sequence current value in the normal state. The threshold Dset is set to be greater than 25% of the three-phase load unbalance, and is generally set to be more than 30%.

[0033] Embodiment 2

[0034] As Figure 2 and 3, A grounding judgment and protection device based on the variation of three-phase line currents, which includes a microcomputer measurement and control device, three-phase current transformers, and a zero-sequence current transformer (optional), and monitors the three-phase currents and zero-sequence currents (which can be synthesized from three-phase currents) of the line in real time. The microcomputer measurement and control device NZFTU is its core component, which includes: a current sampling module, a single-chip microcomputer, and a relay output control module. The current sampling module converts the analog signal into a digital signal through AD and outputs it to the single-chip microcomputer. The single-chip microcomputer samples, calculates, processes, and judges the currents of each phase. When it is determined that the variation of the three-phase currents conforms to the criterion for single-phase grounding faults within the boundary, it drives the relay output control node to trip and protect the faulty line with a single-phase grounding fault and issue an alarm signal.

[0035] The present invention will be described in detail below in combination with specific applications.

[0036] As Figure 4 shown, the system has two feeders: Feeder 1 and Feeder 2. Two sets of devices of the present invention, simply referred to as Terminals i-i (i = 1, 2), are installed at different positions on each feeder. After a phase C grounding fault occurs at the position shown in Feeder 1#, the device in the substation can judge the grounding fault based on the increment of the three-phase currents of each feeder, and select the faulty phase and line; using the devices (Terminals i-i) on the feeder, the interval position of the fault can be judged.

[0037] Suppose that before the single-phase grounding, the normal moment of the system is t0, and a grounding fault occurs within the judgment period T (20 - 100 ms). The three-phase current increments are calculated as follows. In the formula: I A(t0) 、I B(t0) 、I C(t0) are the phase A currents at time t0, and I A(t0+T) 、I B(t0+T) 、I C(t0+T) are the phase A, B, and C currents at time t0 + T. ΔI A 、ΔI B 、ΔIc are the phase A, B, and C current increments respectively.

[0038] ΔI A =I A(t0+T) -I A(t0)

[0039] ΔI B =I B(t0+T) -I B(t0)

[0040] ΔI C =I C(t0+T) -I C(t0)

[0041] Then for Terminal 1-1, there is:

[0042] ΔIc>>ΔI A , ΔIc>>ΔI B , and ΔI A ≈ΔI B .

[0043] For terminal 1-2, terminal 2-1 and terminal 2-2, there are:

[0044] ΔI A ≈ΔI B ≈ΔIc.

[0045] Based on this, it can be determined that a C-phase grounding fault has occurred in feeder 1, and the fault location is between terminal 1-1 and terminal 1-2.

[0046] Take a 10kV real test platform (primary wiring diagram as shown Figure 5 As shown in the figure, ground fault segmentation cases of various neutral point grounding modes are carried out.

[0047] The capacitive current of this real test system is 10A, and the neutral point can be connected to ground without grounding, through arc suppression coil, or through small resistance. Among them, 01#~03# lines are multi-level feeder structures, which can simulate the segmentation and branching of distribution network lines. 04# is an independent single feeder.

[0048] A single-phase grounding fault is simulated at #01, #03 or #04 in the figure respectively, and the grounding fault within the boundary can be accurately determined by using the asymmetric phase current change method.

[0049] Specific examples are as follows:

[0050] Example 1.

[0051] The neutral point of the real test platform was configured as ungrounded, and single-phase grounding faults with different grounding resistance values ​​were made on phase B of 01# line, phase C of 04# line and phase C of 01# line. The normal voltage and the three-phase current increments of each line calculated by the device at the time of the fault are shown in Table 1.

[0052] Table 1 Current changes of grounding test circuit in neutral point ungrounded system

[0053]

[0054] Note: The voltage unit is V, and the current unit is mA, both are quadratic values.

[0055] As can be seen from the table, when phase B of line 01# is grounded through a 1000Ω resistor, only the current increment of phase B of this line is much larger than the current increments of the ungrounded phases A and C, and the three-phase current increments of the ungrounded lines 03# and 04# are approximately equal; when phase C of line 04# is grounded through a 6000Ω resistor, the current increment of phase C of this line is more than 5 times the current increments of the ungrounded phases A and C, and the three-phase current increments of the ungrounded lines 01# and 03# are approximately equal; when phase C of line 01# is solidly grounded, the current increment of phase C of this line is more than 3 times the second largest increment (phase B), and there is no such relationship for the three-phase current increments of the ungrounded lines 03# and 04#.

[0056] Example 1 verifies that the ground fault phase selection and section determination method of the present invention is correct in a non-grounded neutral system.

[0057] Example 2.

[0058] Configure the neutral point of the full-scale test platform to be grounded through an arc suppression coil, and conduct single-phase ground faults on phase B of line 01#, phase C of line 04#, and phase C of line 01# respectively. The normal voltages and the three-phase current increments of each line calculated by the device at the fault moment are shown in Table 2.

[0059] Table 2 Current changes of grounding test lines in a system with the neutral point grounded through an arc suppression coil

[0060]

[0061] As can be seen from the table, when phase B of line 01# is grounded through a 1000Ω resistor, only the current increment of phase B of this line is much larger than the current increments of the ungrounded phases A and C, and the three-phase current increments of the ungrounded lines 03# and 04# are approximately equal; when phase C of line 04# is grounded through a 6000Ω resistor, the current increment of phase C of this line is more than 6 times the current increments of the ungrounded phases A and C, and the difference in the three-phase current increments of the ungrounded lines 01# and 03# does not exceed 3mA; when phase C of line 01# is solidly grounded, the current increment of phase C of this line is 3.6 times the second largest increment (phase B), and the three-phase current increments of the ungrounded lines 03# and 04# are approximately equal.

[0062] Example 2 verifies that the ground fault phase selection and section determination method of the present invention is correct in a system with the neutral point grounded through an arc suppression coil.

[0063] Example 3.

[0064] Configure the neutral point of the full-scale test system to be grounded through a small resistor, and conduct single-phase ground faults on phase B of line 01# and phase A of line 03# respectively. The normal voltages and the three-phase current increments of each line calculated by the device at the fault moment are shown in Table 3.

[0065] Table 3 Current Variation of the Grounding Test Circuit in the System with a Small Resistor Grounded at the Neutral Point

[0066]

[0067] As can be seen from Table 3, when phase B of line 01# is grounded through a 1000Ω resistor, only the current increment of phase B of this line is much larger than the current increments of the ungrounded phases A and C. The current increments of the three phases of line 03# after the grounding point (load side) on the same line and line 04# of the ungrounded line are very small and approximately equal; when phase A of line 03# is grounded through a wet sand resistor, the current increment of phase A of this line is much larger than the current increments of the ungrounded phases B and C. Line 01# on the power supply side of the same line shows the same characteristics: the current increment of the grounded phase A is much larger than the current increments of the ungrounded phases B and C, and the current increments of the three phases of the ungrounded line 04# are very small and approximately equal.

[0068] Example 3 proves that the method for selecting the grounding fault phase and determining the section of the present invention is correct in the system with a small resistor grounded at the neutral point.

[0069] Through Examples 1, 2, and 3, it is proved that the method of the present invention for selecting the phase, selecting the line, and determining the section by using the increment change characteristics of the three-phase current of the line before and after the grounding fault is correct, has the technical advantages of accurately discriminating various grounding faults, especially accurately judging high-resistance grounding faults, and is applicable to various neutral grounding methods of the distribution network.

[0070] The above shows and describes the basic principles, main features, and advantages of the present invention. Those of ordinary skill in the art should understand that the above embodiments do not limit the protection scope of the present invention in any form. Any technical solutions obtained by means of equivalent replacement and the like fall within the protection scope of the present invention. The parts not involved in the present invention are the same as or can be implemented by the prior art.

Claims

1. A single-phase grounding judgment and protection method based on the variation of three-phase line currents, characterized in that: When it is judged that the change of the three-phase current of the line exceeds the set value, the increment of the three-phase current is obtained by vector operation. The difference obtained by subtracting the middle value of the increment from the maximum value of the increment is then divided by the maximum value of the increment to obtain the unbalance degree. If the unbalance degree is greater than the set threshold, it is judged that a single-phase grounding fault has occurred at the rear position of the measurement end of the three-phase current of the line, and the grounded phase is the phase with the largest current increment. The judgment process is as follows: (1) After the system is powered on, the three-phase current is synthesized into the zero-sequence current I0. If I0 is less than the high-crossing limit set value I0H_set, it is judged as the normal operation state, and at the same time, the amplitudes, phases and calculated sampling point position information of the three-phase normal currents IA_normal, IB_normal, and IC_normal are updated; (2) If the zero-sequence current I0 is greater than the high-crossing limit set value I0H_set, it is judged as the abnormal operation state, and the single-phase grounding fault judgment is entered; The steps of the single-phase grounding fault judgment are as follows: First, align the sampling point positions and correct the phase value of the current at present so that the sampling calculation position of the current at present is different from the current at the normal operation moment by N cycles, where N is a positive integer; Then, perform a vector subtraction operation on the amplitude and phase of the current at present and the current phase at the normal operation moment recorded to obtain the modulus after vector subtraction, which is the increment ΔIi of the three-phase current, where i is A, B or C; then subtract the middle value of the increment from the maximum value of the increment, divide by the maximum value of the increment, and multiply by 100 to obtain the percentage value of the phase current increment, which is the unbalance degree d of the phase current increment: d = [max(ΔIi) - mid(ΔIi)] / max(ΔIi) × 100 Compare the unbalance degree d with the set threshold Dset. If d < Dset, it is a non-internal fault; if d ≥ Dset, it is an internal grounding fault, and the phase with the largest current vector increment is the grounded phase; The high-crossing limit set value I0H_set is set according to the sampling accuracy of the device and the system conditions, and its set value is greater than the zero-sequence current value in the normal state; the threshold Dset is set to be greater than 30% of the three-phase load unbalance.

2. A single-phase grounding judgment and protection device based on the variation of three-phase line currents, characterized in that, The judgment protection device is used to implement the method described in claim 1 above and execute the judgment steps described in claim 1. The judgment protection device includes a microcomputer measurement and control device and three-phase current transformers. The microcomputer measurement and control device includes a current sampling module, a single-chip microcomputer and a relay output control module; the three-phase current transformers are used to monitor the three-phase current of the line in real time and send the monitoring signals to the microcomputer measurement and control device; Two sets of the protection devices are installed at different positions of feeder 1 and feeder 2 respectively. The device judges the grounding fault according to the increment of the three-phase current of each feeder, and selects the fault phase and the line; the device on the feeder is used to judge the interval position of the fault; The protection device is based on a 10 kV full-scale test platform to perform phase discrimination, line selection, and section determination for grounding faults in various neutral grounding modes, and adopts grounding methods with the neutral point connected to be ungrounded, through an arc suppression coil, and through a small resistance; among them, lines 01# to 03# are multi-stage feeder structures, simulating the sectioning and branching forms of distribution network lines, and line 04# is an independent single feeder.

3. The single-phase grounding judgment and protection device based on the variation of three-phase line currents according to claim 2, characterized in that, The current sampling module converts analog signals into digital signals through AD and outputs them to the single-chip microcomputer. The control output end of the single-chip microcomputer is connected to the input end of the relay output control module. The single-chip microcomputer samples, calculates, processes, and judges the current of each phase. When it is determined that the change in the three-phase current of the line conforms to the criterion for single-phase grounding faults within the boundary, it drives the relay to output a control node to trip and protect the line with a single-phase grounding fault within the boundary and issue an alarm signal.

4. The single-phase grounding judgment and protection device based on the variation of three-phase line currents according to claim 2, characterized in that, The judgment protection device further includes a zero-sequence current transformer, and the zero-sequence current transformer is used to monitor the zero-sequence current of the line in real time and send the monitoring signal to the microcomputer measurement and control device.

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