A method for selecting single-phase ground fault line in distribution network based on phase current phase difference

Abstract

The present invention discloses a method for selecting a line for a single-phase grounding fault in a distribution network based on a phase current phase difference, comprising the following steps: S01: determining whether a single-phase grounding fault occurs, and further determining the fault phase if a fault occurs; S02: calculating the sum of a first healthy phase current and a fault phase current, and the sum of a second healthy phase current and a fault phase current according to the determination result and the collected electrical parameters, and determining the fault line according to the phase difference between the two sums. The substantial effects of the present invention include: realizing line selection for a single-phase grounding fault in a distribution network by utilizing the phase current phase difference. After a single-phase grounding fault occurs in a distribution network, the fault line is accurately determined, the fault is promptly eliminated to avoid further development into a phase-to-phase fault, and power supply is restored as soon as possible to improve the reliability of power supply in the power grid.

Description

Technical Field

[0001] The invention relates to the field of relay protection of power systems, and in particular to a single-phase grounding fault line selection method for a distribution network based on phase current phase difference. Background Art

[0002] my country's distribution network is mainly at a voltage level of 35kV and below, and most of them adopt a non-direct grounding method in which the neutral point is not grounded or is grounded through an arc suppression coil. Since only a small value of capacitive current to the ground flows through the fault point after a single-phase grounding fault occurs, this type of power grid belongs to a small current grounding power grid. Distribution lines are prone to various types of faults due to their wide coverage, complex working environment, and numerous access devices. Among them, single-phase grounding faults account for more than 80% of the total number of distribution line faults. After a single-phase grounding fault occurs in a distribution line, since the line voltage remains symmetrical and has no effect on the power supply of the load, it is allowed to continue to operate with the fault for 1-2 hours. However, long-term operation with a fault is very likely to cause phase-to-phase faults, resulting in an expansion of the fault range and causing various problems that endanger the safe operation of the system. Therefore, it is necessary to promptly and accurately determine the line where the fault occurs after a single-phase grounding fault occurs.

[0003] However, the fault characteristics of single-phase grounding fault in distribution network are not obvious, which increases the difficulty of fault line selection. According to the selected fault information, the existing single-phase grounding fault line selection methods in distribution network can be divided into steady-state signal method, transient signal method and injection signal method.

[0004] In terms of fault line selection using steady-state signals, the article "Fault Line Selection Method Based on Zero-Sequence Current Amplitude Ratio Method [J]. Journal of North University of China (Natural Science Edition), 2014, 35(04): 473-478." proposed the power frequency zero-sequence current amplitude ratio method, which uses the characteristic that the zero-sequence current amplitude of the fault line is larger than that of the non-fault line to achieve fault line selection. Another document proposed the power frequency zero-sequence current phase ratio method, which uses the characteristic that the zero-sequence current on the fault line and the non-fault line has opposite directions to select the line. Based on the zero-sequence current amplitude ratio method and phase ratio method, another document proposed the group amplitude ratio and phase ratio method, which first selects multiple lines with larger zero-sequence current amplitudes as candidate lines, and then distinguishes the fault lines based on the phase. The principles of these three line selection methods are simple and easy to implement, but their accuracy is affected by the neutral point grounding method, transition resistance and system operation mode, and they are not universal. In view of the problem that the neutral point grounding method affects the accuracy of fault line selection, other literature proposes to use the active component and the 5th harmonic component of the zero-sequence current of the fault line for fault line selection. Since the active current and the 5th harmonic current will not be compensated by the arc suppression coil, this method is not limited by the neutral point grounding method, but the size of these two components in the fault current is closely related to factors such as system parameters and transition resistance, making the actual line selection effect unsatisfactory. In addition, other literature proposes to use the difference in negative sequence current amplitude and phase on the fault line and non-fault line for fault line selection, but the system asymmetry and load characteristics will affect the negative sequence current, thereby interfering with the line selection results.

[0005] In terms of using transient signals for fault line selection, the existing technology performs wavelet decomposition on the zero-sequence transient current of each line after the fault. After determining the characteristic frequency band of each current according to the principle of maximum energy, the amplitude, polarity and energy of the transient current in the characteristic frequency band are used to identify the fault line. The accuracy of these methods depends on the accurate selection of the characteristic frequency band, which places high demands on the signal analysis effect of wavelet decomposition. Another document proposes to perform similarity analysis on the zero-sequence transient current of each line, and use the characteristics that the similarity between the zero-sequence transient currents of non-fault lines is high and the similarity with the zero-sequence transient currents of fault lines is low to achieve fault line selection. This method is simple in principle and is not limited by the neutral point grounding method, but is susceptible to noise interference. Another document proposes to select the line by detecting the fault traveling wave generated by the fault point. This method places high demands on the accurate identification of the fault wave head.

[0006] In terms of fault line selection using injected signals, the prior art proposes injecting a high-frequency signal into the system from a grounding transformer. The signal forms a loop through the grounding point of the faulty line, and determines whether the line is a faulty line by detecting whether the injected high-frequency signal exists in the line. The main disadvantage of this method is that it will cause an impact on the primary system.

[0007] It can be seen that the current line selection method for single-phase grounding fault in distribution network is not yet perfect, and the study of single-phase grounding fault line selection method has important theoretical and practical value. Summary of the invention

[0008] In view of the problem that the prior art lacks a suitable line selection method for single-phase grounding fault in distribution network, the present invention provides a line selection method for single-phase grounding fault in distribution network based on phase current phase difference, which realizes line selection for single-phase grounding fault in distribution network by using phase current phase difference. After a single-phase grounding fault occurs in the distribution network, the fault line can be accurately determined, the fault can be eliminated in time to avoid further development into phase-to-phase fault, and power supply can be restored as soon as possible to improve the power supply reliability of the power grid.

[0009] The following is the technical solution of the present invention.

[0010] A method for selecting a single-phase ground fault line in a distribution network based on phase current phase difference, comprising the following steps:

[0011] S01: Determine whether a single-phase grounding fault occurs, and if so, further determine the fault phase;

[0012] S02: Calculate the sum of the first healthy phase current and the faulty phase current, and the sum of the second healthy phase current and the faulty phase current according to the judgment result and the collected electrical parameters, and determine the faulty line according to the phase difference between the two sums.

[0013] The present invention is based on the electrical principle of phase current phase difference and can accurately determine the fault phase and fault line by calculating parameters such as voltage and current.

[0014] Preferably, step S01 includes: collecting the voltage of the voltage transformer on the bus, calculating the zero-sequence voltage, and when the zero-sequence voltage appears, determining that a single-phase grounding fault has occurred, wherein the phase with the smallest phase voltage is determined to be the faulty phase.

[0015] Preferably, the determining of the faulty line according to the phase difference between the two and includes: when the phase difference between the two and is 60 degrees, the line is a non-faulty line; when the phase difference between the two and is less than 60 degrees, the line is a faulty line.

[0016] Preferably, when judging whether the phase difference is less than 60 degrees, an error correction range is set, and when the phase difference is greater than or equal to 54 degrees, it is judged to be equal to 60 degrees, otherwise it is judged to be less than 60 degrees.

[0017] The phase difference judgment formula is:

[0018]

[0019] in is the sum of the healthy phase current with relatively advanced phase and the fault phase current on the i-th line, is the sum of the healthy phase current and the fault phase current with relative phase lag on the i-th line, θ set is the setting value of the criterion.

[0020] Preferably, the method is applicable to single-phase grounding faults in a distribution network with an ungrounded neutral point or grounded via an arc suppression coil and in a non-direct grounding mode.

[0021] The substantial effects of the present invention include: clear principle, judging the faulty line based on the phase difference between the sum of the two healthy phase currents and the faulty phase current on the faulty line after the fault is less than the set value; easy to implement, and can be implemented by only using the voltage of the installed voltage transformer and the current of the current transformer; and suitable for both ungrounded neutral point distribution networks and grounded distribution networks via arc suppression coils. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 This is a capacitive current distribution diagram when a single-phase grounding fault occurs in a distribution network with an ungrounded neutral point according to an embodiment of the present invention;

[0023] Figure 2 1. It is a phase diagram of each phase-to-ground voltage and zero-sequence voltage when phase A is grounded in an embodiment of the present invention;

[0024] Figure 3 is a current phasor diagram on a non-fault line when phase A is grounded in an embodiment of the present invention;

[0025] Figure 4 is a current phasor diagram on a fault line when phase A is grounded in a case where the neutral point is not grounded in an embodiment of the present invention;

[0026] Figure 5 This is a current distribution diagram of a single-phase grounding fault in a distribution network in which the neutral point is grounded via an arc suppression coil in an embodiment of the present invention;

[0027] Figure 6 It is a current phasor diagram on the fault line when phase A is grounded in the case where the neutral point is grounded via an arc suppression coil in an embodiment of the present invention. DETAILED DESCRIPTION

[0028] The technical scheme of the present application will be described below in conjunction with the embodiments. In addition, in order to better illustrate the present invention, numerous specific details are provided below. It should be understood by those skilled in the art that the present invention can be implemented without certain specific details. In some instances, methods, means, components and circuits well known to those skilled in the art are not described in detail in order to highlight the gist of the present invention.

[0029] Example:

[0030] A method for selecting a single-phase ground fault line in a distribution network based on phase current phase difference, comprising the following steps:

[0031] S01: Determine whether a single-phase grounding fault occurs. If a fault occurs, further determine the fault phase; including: collecting the voltage of the voltage transformer on the bus, calculating the zero-sequence voltage, and when the zero-sequence voltage appears, determine that a single-phase grounding fault occurs, and the phase with the smallest phase voltage is determined to be the fault phase.

[0032] S02: According to the judgment result and the collected electrical parameters, the sum of the first healthy phase current and the fault phase current, as well as the sum of the second healthy phase current and the fault phase current are calculated, and the fault line is determined according to the phase difference between the two sums; including: when the phase difference between the two sums is 60 degrees, the line is a non-fault line, and when the phase difference between the two sums is less than 60 degrees, the line is a fault line. When judging whether the phase difference is less than 60 degrees, an error correction range is set, and when the phase difference is greater than or equal to 54 degrees, it is judged to be equal to 60 degrees, otherwise it is judged to be less than 60 degrees.

[0033] The present invention is based on the electrical principle of phase current phase difference and can accurately determine the fault phase and fault line by calculating parameters such as voltage and current.

[0034] The following takes a distribution network with an ungrounded neutral point and a distribution network with a neutral point grounded through an arc suppression coil as examples to illustrate the calculation principle of this embodiment and prove the effectiveness of the method:

[0035] In the attached Figure 1 In the neutral point ungrounded distribution network shown in the figure, the generator G has two distribution lines. The distributed capacitance to the ground of the generator G, line I and line II is represented by the concentrated capacitance C 0G , C 0I and C 0II After a single-phase grounding fault occurs on line II, if the influence of the voltage drop of the three-phase symmetrical load current and the capacitive current on the line impedance is not taken into account, the voltage to ground of the fault phase of the whole system is equal to zero, so the capacitive current of each component fault phase to ground is also equal to 0. The capacitive current flowing through the non-fault phase is analyzed below.

[0036] Taking the A phase grounding fault as an example, the distribution of the ground capacitance current in the system after the fault is shown in the attached figure. Figure 1 In the figure, “→” indicates the direction of the capacitive current. After phase A is grounded, the relative ground voltage and zero-sequence voltage are:

[0037]

[0038] and is the three-phase potential of the generator. The relationship between each voltage phasor is shown in the attached figure. Figure 2 as shown in .

[0039] On the non-fault line I, the current of phase A is zero, and there is capacitive current flowing to the fault point in phases B and C. and The capacitive current flowing through each phase is expressed as:

[0040]

[0041] ω is the angular frequency of the power frequency. The current phasors on the non-fault line I are also drawn in the attached figure. Figure 2 It can be seen from the figure that the sum of the currents between healthy phases B and C and fault phase A is the current of each healthy phase, that is:

[0042]

[0043] The two current phasors in formula (3) are and Shown in the attached Figure 3 The phase difference between them is:

[0044]

[0045] That is, the sum of the two healthy phase currents on the non-fault line I and the fault phase current and The phase difference between them is 60°.

[0046] On the fault line II, the grounding point will flow back the sum of the capacitance currents of phase B and phase C of the entire system, that is:

[0047] C 0Σ =C 0I +C 0II +C 0G is the sum of the distributed capacitance of each phase to ground in the whole system. This current flows back to the generator through phase A, so the current flowing out of phase A is Phase B and phase C have their own capacitive currents. The expression of the current flowing through each phase is:

[0048] The corresponding valid values ​​are:

[0049]

[0050] Therefore, the sum of the currents in healthy phases B and C and fault phase A is:

[0051]

[0052] Draw the current phasors on the fault line II on the attached Figure 4 In. Because:

[0053]

[0054] According to the cosine theorem, we know that:

[0055]

[0056] According to the sine theorem, we know that:

[0057]

[0058] thus,

[0059]

[0060] That is, the sum of the two healthy phase currents and the fault phase current on the fault line II and The phase difference between them is less than 60°.

[0061] In summary, the following conclusions can be drawn: in a distribution network with an ungrounded neutral point, the phase difference between the sum of the two healthy phase currents and the fault phase current on the non-fault line is 60°, and the phase difference between the sum of the two healthy phase currents and the fault phase current on the fault line is less than 60°.

[0062] When the neutral point of the distribution network is grounded through an arc suppression coil, the distribution of the ground capacitance current and the arc suppression coil current is shown in the attached diagram. Figure 5 The sum of the two healthy phase currents and the fault phase current on the non-fault line I and The phase difference between them is the same as that of the neutral point ungrounded system, that is

[0063] On fault line II, in addition to the sum of the capacitive currents of phases B and C of the entire system flowing back to the ground point, the inductive compensation current of the arc suppression coil also flows back to the generator through phase A. When the power grid uses arc suppression coils for compensation, overcompensation is adopted, and the overcompensation degree is generally selected as P = 5% to 10%. Therefore, the current flowing out of phase A is

[0064] Phases B and C have their own capacitive currents, and their current expressions are the same as those in equation (6). The sum of the currents of healthy phases B and C and the fault phase A is expressed as

[0065]

[0066] Plot the two current phasors in equation (14) on the Figure 6 According to the exterior angle theorem of a triangle, and The phase difference between them satisfies

[0067]

[0068] That is, in a distribution network with a neutral point grounded through an arc suppression coil, the phase difference between the sum of the two healthy phase currents and the fault phase current on the non-fault line is also 60°, and the phase difference between the two healthy phase currents and the sum of the fault phase current on the fault line is also less than 60°. These characteristics are the same as those in a neutral point ungrounded power grid.

[0069] Based on the above conclusions, it can be proved that when zero-sequence voltage appears in the system, the phase with the smallest phase voltage is determined to be the fault phase; for each line, the fault line can be determined based on the phase difference between the sum of the two healthy phase currents and the fault phase current. The basis for determining the fault line is:

[0070]

[0071] in, is the sum of the healthy phase current with relatively advanced phase and the fault phase current on the i-th line, is the sum of the healthy phase current and the fault phase current with relative phase lag on the i-th line, θ set is the setting value of the criterion.

[0072] Through the description of the above implementation mode, the technical personnel in the relevant field can understand that the embodiment of the present application can be implemented in the form of hardware or in the form of a software functional unit. If it is implemented in the form of a software functional unit and sold or used as an independent product, it can be stored in a readable storage medium. Based on such an understanding, the technical solution of the embodiment of the present application is essentially or the part that contributes to the prior art or all or part of the technical solution can be embodied in the form of a software product, which is stored in a storage medium, including several instructions to enable a device (which can be a single-chip microcomputer, chip, etc.) or a processor (processor) to perform all or part of the steps of the various embodiments of the present application. The aforementioned storage medium includes: U disk, mobile hard disk, read-only memory (readonly memory, ROM), random access memory (random access memory, RAM), disk or optical disk and other media that can store program code.

[0073] The above contents are only specific implementation methods of the present application, but the protection scope of the present application is not limited thereto. Any technician familiar with the technical field can easily think of changes or substitutions within the technical scope disclosed in the present application, which should be included in the protection scope of the present application. Therefore, the protection scope of the present application should be based on the protection scope of the claims.

Claims

1. A method for selecting a single-phase ground fault line in a distribution network based on phase current phase difference, characterized in that: The following steps are involved: S01: Determine whether a single-phase grounding fault occurs, and if so, further determine the fault phase; S02: Calculate the sum of the first healthy phase current and the faulty phase current, and the sum of the second healthy phase current and the faulty phase current according to the judgment result and the collected electrical parameters, and determine the faulty line according to the phase difference between the two sums; Step S01 includes: collecting the voltage of the voltage transformer on the bus, calculating the zero-sequence voltage, and when the zero-sequence voltage appears, judging that a single-phase grounding fault has occurred, wherein the phase with the smallest phase voltage is judged as the fault phase; The method of determining the faulty line according to the phase difference between the two and includes: when the phase difference between the two and is 60 degrees, the line is a non-faulty line; when the phase difference between the two and is less than 60 degrees, the line is a faulty line.

2. A method for selecting a single-phase ground fault line in a distribution network based on phase current phase difference according to claim 1, characterized in that: When judging whether the phase difference is less than 60 degrees, an error correction range is set. When the phase difference is greater than or equal to 54 degrees, it is judged to be equal to 60 degrees, otherwise it is judged to be less than 60 degrees.

3. A method for selecting a single-phase ground fault line in a distribution network based on phase current phase difference according to claim 2, characterized in that: The phase difference judgment formula is: in is the sum of the healthy phase current and the fault phase current with relatively advanced phase on the i-th line, is the sum of the healthy phase current and the fault phase current with relative phase lag on the i-th line, and θset is the setting value of the criterion.

4. The method for selecting a single-phase ground fault line in a distribution network based on phase current phase difference according to claim 1, characterized in that: The method is applicable to single-phase grounding faults in distribution networks with non-direct grounding modes where the neutral point is not grounded or is grounded via an arc suppression coil.

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